169-412: Agricultural science (or agriscience for short) is a broad multidisciplinary field of biology that encompasses the parts of exact, natural, economic and social sciences that are used in the practice and understanding of agriculture . Professionals of the agricultural science are called agricultural scientists or agriculturists . In the 18th century, Johann Friedrich Mayer conducted experiments on
338-535: A last universal common ancestor that lived about 3.5 billion years ago . Geologists have developed a geologic time scale that divides the history of the Earth into major divisions, starting with four eons ( Hadean , Archean , Proterozoic , and Phanerozoic ), the first three of which are collectively known as the Precambrian , which lasted approximately 4 billion years. Each eon can be divided into eras, with
507-440: A lipid bilayer , including cholesterols that sit between phospholipids to maintain their fluidity at various temperatures. Cell membranes are semipermeable , allowing small molecules such as oxygen, carbon dioxide, and water to pass through while restricting the movement of larger molecules and charged particles such as ions . Cell membranes also contain membrane proteins , including integral membrane proteins that go across
676-443: A nucleus , and prokaryotic cells, which do not. Prokaryotes are single-celled organisms such as bacteria , whereas eukaryotes can be single-celled or multicellular. In multicellular organisms , every cell in the organism's body is derived ultimately from a single cell in a fertilized egg . Every cell is enclosed within a cell membrane that separates its cytoplasm from the extracellular space . A cell membrane consists of
845-508: A number of shapes , ranging from spheres to rods and spirals . Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats . Bacteria inhabit soil, water, acidic hot springs , radioactive waste , and the deep biosphere of the Earth's crust . Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about 27 percent of
1014-546: A 'strongman' was often the best solution. Athens fell under a tyranny in the second half of the 6th century BC. When this tyranny was ended, the Athenians founded the world's first democracy as a radical solution to prevent the aristocracy regaining power. A citizens' assembly (the Ecclesia ), for the discussion of city policy, had existed since the reforms of Draco in 621 BC; all citizens were permitted to attend after
1183-474: A cell's size, shape, membrane potential , metabolic activity , and responsiveness to signals, which are largely due to highly controlled modifications in gene expression and epigenetics . With a few exceptions, cellular differentiation almost never involves a change in the DNA sequence itself. Thus, different cells can have very different physical characteristics despite having the same genome . Morphogenesis, or
1352-408: A chemical (e.g., nitrous acid , benzopyrene ) or radiation (e.g., x-ray , gamma ray , ultraviolet radiation , particles emitted by unstable isotopes). Mutations can lead to phenotypic effects such as loss-of-function, gain-of-function , and conditional mutations. Some mutations are beneficial, as they are a source of genetic variation for evolution. Others are harmful if they were to result in
1521-500: A coalition of 31 Greek city states, including Athens and Sparta, determined to resist the Persian invaders. At the same time, Greek Sicily was invaded by a Carthaginian force. In 480 BC, the first major battle of the invasion was fought at Thermopylae , where a small rearguard of Greeks, led by three hundred Spartans, held a crucial pass guarding the heart of Greece for several days; at the same time Gelon , tyrant of Syracuse, defeated
1690-609: A council of elders (the Gerousia ) and magistrates specifically appointed to watch over the kings (the Ephors ). Only free, land-owning, native born men could be citizens entitled to the full protection of the law in a city-state. In most city-states, unlike the situation in Rome , social prominence did not allow special rights. Sometimes families controlled public religious functions, but this ordinarily did not give any extra power in
1859-528: A decisive victory, and in 447 lost Boeotia again. Athens and Sparta signed the Thirty Years' Peace in the winter of 446/5, ending the conflict. Despite the treaty, Athenian relations with Sparta declined again in the 430s, and in 431 BC the Peloponnesian War began. The first phase of the war saw a series of fruitless annual invasions of Attica by Sparta, while Athens successfully fought
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#17327728443612028-1233: A few archaea have very different shapes, such as the flat and square cells of Haloquadratum walsbyi . Despite this morphological similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes, notably for the enzymes involved in transcription and translation . Other aspects of archaeal biochemistry are unique, such as their reliance on ether lipids in their cell membranes , including archaeols . Archaea use more energy sources than eukaryotes: these range from organic compounds , such as sugars, to ammonia , metal ions or even hydrogen gas . Salt-tolerant archaea (the Haloarchaea ) use sunlight as an energy source, and other species of archaea fix carbon , but unlike plants and cyanobacteria , no known species of archaea does both. Archaea reproduce asexually by binary fission , fragmentation , or budding ; unlike bacteria, no known species of Archaea form endospores . The first observed archaea were extremophiles , living in extreme environments, such as hot springs and salt lakes with no other organisms. Improved molecular detection tools led to
2197-403: A final electron acceptor, which is usually the oxidized form of NADP , which is reduced to NADPH, a process that takes place in a protein complex called photosystem I (PSI). The transport of electrons is coupled to the movement of protons (or hydrogen) from the stroma to the thylakoid membrane, which forms a pH gradient across the membrane as hydrogen becomes more concentrated in the lumen than in
2366-436: A group of city-states allied themselves to defend Greece, the vast majority of poleis remained neutral, and after the Persian defeat, the allies quickly returned to infighting. Thus, the major peculiarities of the ancient Greek political system were its fragmented nature (and that this does not particularly seem to have tribal origin), and the particular focus on urban centers within otherwise tiny states. The peculiarities of
2535-466: A hollow sphere of cells , the blastula , during embryonic development . Over 1.5 million living animal species have been described —of which around 1 million are insects —but it has been estimated there are over 7 million animal species in total. They have complex interactions with each other and their environments, forming intricate food webs . Ancient Greece Ancient Greece ( Ancient Greek : Ἑλλάς , romanized : Hellás )
2704-769: A loss of function of genes needed for survival. Gene expression is the molecular process by which a genotype encoded in DNA gives rise to an observable phenotype in the proteins of an organism's body. This process is summarized by the central dogma of molecular biology , which was formulated by Francis Crick in 1958. According to the Central Dogma, genetic information flows from DNA to RNA to protein. There are two gene expression processes: transcription (DNA to RNA) and translation (RNA to protein). The regulation of gene expression by environmental factors and during different stages of development can occur at each step of
2873-417: A more successful evolutionary theory based on natural selection ; similar reasoning and evidence led Alfred Russel Wallace to independently reach the same conclusions. The basis for modern genetics began with the work of Gregor Mendel in 1865. This outlined the principles of biological inheritance. However, the significance of his work was not realized until the early 20th century when evolution became
3042-457: A new cycle. In contrast to mitosis, meiosis results in four haploid daughter cells by undergoing one round of DNA replication followed by two divisions. Homologous chromosomes are separated in the first division ( meiosis I ), and sister chromatids are separated in the second division ( meiosis II ). Both of these cell division cycles are used in the process of sexual reproduction at some point in their life cycle. Both are believed to be present in
3211-409: A process known as allopatric speciation . A phylogeny is an evolutionary history of a specific group of organisms or their genes. It can be represented using a phylogenetic tree , a diagram showing lines of descent among organisms or their genes. Each line drawn on the time axis of a tree represents a lineage of descendants of a particular species or population. When a lineage divides into two, it
3380-408: A region of deoxyribonucleic acid (DNA) that carries genetic information that controls form or function of an organism. DNA is composed of two polynucleotide chains that coil around each other to form a double helix . It is found as linear chromosomes in eukaryotes , and circular chromosomes in prokaryotes . The set of chromosomes in a cell is collectively known as its genome . In eukaryotes, DNA
3549-760: A result of Epaminondas ' liberation of Messenia from Spartan rule, the helot system there came to an end and the helots won their freedom. However, it did continue to persist in Laconia until the 2nd century BC. For most of Greek history, education was private, except in Sparta. During the Hellenistic period, some city-states established public schools . Only wealthy families could afford a teacher. Boys learned how to read, write and quote literature. They also learned to sing and play one musical instrument and were trained as athletes for military service. They studied not for
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#17327728443613718-541: A role in the synthesis and packaging of proteins, respectively. Biomolecules such as proteins can be engulfed by lysosomes , another specialized organelle. Plant cells have additional organelles that distinguish them from animal cells such as a cell wall that provides support for the plant cell, chloroplasts that harvest sunlight energy to produce sugar, and vacuoles that provide storage and structural support as well as being involved in reproduction and breakdown of plant seeds. Eukaryotic cells also have cytoskeleton that
3887-436: A separate clade as some protists may be more closely related to plants, fungi, or animals than they are to other protists. Like groupings such as algae , invertebrates , or protozoans , the protist grouping is not a formal taxonomic group but is used for convenience. Most protists are unicellular; these are called microbial eukaryotes. Plants are mainly multicellular organisms , predominantly photosynthetic eukaryotes of
4056-423: A single carbon atom can form four single covalent bonds such as in methane , two double covalent bonds such as in carbon dioxide (CO 2 ), or a triple covalent bond such as in carbon monoxide (CO). Moreover, carbon can form very long chains of interconnecting carbon–carbon bonds such as octane or ring-like structures such as glucose . The simplest form of an organic molecule is the hydrocarbon , which
4225-523: A special type of slaves called helots . Helots were Messenians enslaved en masse during the Messenian Wars by the state and assigned to families where they were forced to stay. Helots raised food and did household chores so that women could concentrate on raising strong children while men could devote their time to training as hoplites . Their masters treated them harshly, and helots revolted against their masters several times. In 370/69 BC, as
4394-446: A specific enzyme. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy that will not occur by themselves, by coupling them to spontaneous reactions that release energy. Enzymes act as catalysts —they allow a reaction to proceed more rapidly without being consumed by it—by reducing the amount of activation energy needed to convert reactants into products . Enzymes also allow
4563-399: A unified theory as the modern synthesis reconciled Darwinian evolution with classical genetics . In the 1940s and early 1950s, a series of experiments by Alfred Hershey and Martha Chase pointed to DNA as the component of chromosomes that held the trait-carrying units that had become known as genes . A focus on new kinds of model organisms such as viruses and bacteria, along with
4732-421: A varied mix of traits, and reproduction is able to increase any population, Darwin argued that in the natural world, it was nature that played the role of humans in selecting for specific traits. Darwin inferred that individuals who possessed heritable traits better adapted to their environments are more likely to survive and produce more offspring than other individuals. He further inferred that this would lead to
4901-722: A war with the Roman Republic in the late 3rd century. Although the First Macedonian War was inconclusive, the Romans, in typical fashion, continued to fight Macedon until it was completely absorbed into the Roman Republic (by 149 BC). In the east, the unwieldy Seleucid Empire gradually disintegrated, although a rump survived until 64 BC, whilst the Ptolemaic Kingdom continued in Egypt until 30 BC when it too
5070-492: A water molecule again. In pure water , the number of hydrogen ions balances (or equals) the number of hydroxyl ions, resulting in a pH that is neutral. Organic compounds are molecules that contain carbon bonded to another element such as hydrogen. With the exception of water, nearly all the molecules that make up each organism contain carbon. Carbon can form covalent bonds with up to four other atoms, enabling it to form diverse, large, and complex molecules. For example,
5239-684: A year, the Thirty had been overthrown. The first half of the fourth century saw the major Greek states attempt to dominate the mainland; none were successful, and their resulting weakness led to a power vacuum which would eventually be filled by Macedon under Philip II and then Alexander the Great. In the immediate aftermath of the Peloponnesian war, Sparta attempted to extend their own power, leading Argos, Athens, Corinth, and Thebes to join against them. Aiming to prevent any single Greek state gaining
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5408-444: Is a central feature of sexual reproduction in eukaryotes, and the most fundamental function of meiosis appears to be conservation of the integrity of the genome that is passed on to progeny by parents. Two aspects of sexual reproduction , meiotic recombination and outcrossing , are likely maintained respectively by the adaptive advantages of recombinational repair of genomic DNA damage and genetic complementation which masks
5577-641: Is a large family of organic compounds that are composed of hydrogen atoms bonded to a chain of carbon atoms. A hydrocarbon backbone can be substituted by other elements such as oxygen (O), hydrogen (H), phosphorus (P), and sulfur (S), which can change the chemical behavior of that compound. Groups of atoms that contain these elements (O-, H-, P-, and S-) and are bonded to a central carbon atom or skeleton are called functional groups . There are six prominent functional groups that can be found in organisms: amino group , carboxyl group , carbonyl group , hydroxyl group , phosphate group , and sulfhydryl group . In 1953,
5746-480: Is a small polar molecule with a bent shape formed by the polar covalent bonds of two hydrogen (H) atoms to one oxygen (O) atom (H 2 O). Because the O–H bonds are polar, the oxygen atom has a slight negative charge and the two hydrogen atoms have a slight positive charge. This polar property of water allows it to attract other water molecules via hydrogen bonds, which makes water cohesive . Surface tension results from
5915-918: Is a term of convenience as not all algae are closely related. Algae comprise several distinct clades such as glaucophytes , which are microscopic freshwater algae that may have resembled in form to the early unicellular ancestor of Plantae. Unlike glaucophytes, the other algal clades such as red and green algae are multicellular. Green algae comprise three major clades: chlorophytes , coleochaetophytes , and stoneworts . Fungi are eukaryotes that digest foods outside their bodies, secreting digestive enzymes that break down large food molecules before absorbing them through their cell membranes. Many fungi are also saprobes , feeding on dead organic matter, making them important decomposers in ecological systems. Animals are multicellular eukaryotes. With few exceptions, animals consume organic material , breathe oxygen , are able to move , can reproduce sexually , and grow from
6084-449: Is also important to life as it allows organisms to move , grow, and reproduce . Finally, all organisms are able to regulate their own internal environments . Biologists are able to study life at multiple levels of organization , from the molecular biology of a cell to the anatomy and physiology of plants and animals, and evolution of populations. Hence, there are multiple subdisciplines within biology , each defined by
6253-403: Is catalyzed by lactate dehydrogenase in a reversible reaction. Lactate can also be used as an indirect precursor for liver glycogen. During recovery, when oxygen becomes available, NAD attaches to hydrogen from lactate to form ATP. In yeast, the waste products are ethanol and carbon dioxide. This type of fermentation is known as alcoholic or ethanol fermentation . The ATP generated in this process
6422-465: Is debated. Herodotus was succeeded by authors such as Thucydides , Xenophon , Demosthenes , Plato and Aristotle . Most were either Athenian or pro-Athenian, which is why far more is known about the history and politics of Athens than of many other cities. Their scope is further limited by a focus on political, military and diplomatic history, ignoring economic and social history. The archaic period, lasting from approximately 800 to 500 BC, saw
6591-522: Is known from much more fragmentary documents such as annals, king lists, and pragmatic epigraphy . Herodotus is widely known as the "father of history": his Histories are eponymous of the entire field . Written between the 450s and 420s BC, Herodotus' work reaches about a century into the past, discussing 6th century BC historical figures such as Darius I of Persia , Cambyses II and Psamtik III , and alluding to some 8th century BC persons such as Candaules . The accuracy of Herodotus' works
6760-441: Is made by substrate-level phosphorylation , which does not require oxygen. Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organism's metabolic activities via cellular respiration. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water. In most cases, oxygen
6929-535: Is made up of microtubules , intermediate filaments , and microfilaments , all of which provide support for the cell and are involved in the movement of the cell and its organelles. In terms of their structural composition, the microtubules are made up of tubulin (e.g., α-tubulin and β-tubulin ) whereas intermediate filaments are made up of fibrous proteins. Microfilaments are made up of actin molecules that interact with other strands of proteins. All cells require energy to sustain cellular processes. Metabolism
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7098-526: Is mainly in the cell nucleus . In prokaryotes, the DNA is held within the nucleoid . The genetic information is held within genes, and the complete assemblage in an organism is called its genotype . DNA replication is a semiconservative process whereby each strand serves as a template for a new strand of DNA. Mutations are heritable changes in DNA. They can arise spontaneously as a result of replication errors that were not corrected by proofreading or can be induced by an environmental mutagen such as
7267-458: Is mountainous, and as a result, ancient Greece consisted of many smaller regions, each with its own dialect, cultural peculiarities, and identity. Regionalism and regional conflicts were prominent features of ancient Greece. Cities tended to be located in valleys between mountains, or on coastal plains, and dominated a certain area around them. In the south lay the Peloponnese , consisting of
7436-399: Is one of the key ways a cell releases chemical energy to fuel cellular activity. The overall reaction occurs in a series of biochemical steps, some of which are redox reactions. Although cellular respiration is technically a combustion reaction , it clearly does not resemble one when it occurs in a cell because of the slow, controlled release of energy from the series of reactions. Sugar in
7605-407: Is pulled to the separate poles of the bacterium as it increases the size to prepare for splitting. Growth of a new cell wall begins to separate the bacterium (triggered by FtsZ polymerization and "Z-ring" formation). The new cell wall ( septum ) fully develops, resulting in the complete split of the bacterium. The new daughter cells have tightly coiled DNA rods, ribosomes , and plasmids . Meiosis
7774-399: Is released as a waste product. Most plants, algae , and cyanobacteria perform photosynthesis, which is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies most of the energy necessary for life on Earth. Photosynthesis has four stages: Light absorption , electron transport, ATP synthesis, and carbon fixation . Light absorption is
7943-412: Is represented as a fork or split on the phylogenetic tree. Phylogenetic trees are the basis for comparing and grouping different species. Different species that share a feature inherited from a common ancestor are described as having homologous features (or synapomorphy ). Phylogeny provides the basis of biological classification. This classification system is rank-based, with the highest rank being
8112-468: Is the process by which genes and traits are passed on from parents to offspring. It has several principles. The first is that genetic characteristics, alleles , are discrete and have alternate forms (e.g., purple vs. white or tall vs. dwarf), each inherited from one of two parents. Based on the law of dominance and uniformity , which states that some alleles are dominant while others are recessive ; an organism with at least one dominant allele will display
8281-444: Is the scientific study of life . It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of at least one cell that processes hereditary information encoded in genes , which can be transmitted to future generations. Another major theme is evolution , which explains the unity and diversity of life. Energy processing
8450-485: Is the set of chemical reactions in an organism. The three main purposes of metabolism are: the conversion of food to energy to run cellular processes; the conversion of food/fuel to monomer building blocks; and the elimination of metabolic wastes . These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolic reactions may be categorized as catabolic —the breaking down of compounds (for example,
8619-410: Is the study of chemical processes within and relating to living organisms . Molecular biology is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including molecular synthesis, modification, mechanisms, and interactions. Life arose from the Earth's first ocean, which formed some 3.8 billion years ago. Since then, water continues to be
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#17327728443618788-493: Is to store, transmit, and express hereditary information. Cell theory states that cells are the fundamental units of life, that all living things are composed of one or more cells, and that all cells arise from preexisting cells through cell division . Most cells are very small, with diameters ranging from 1 to 100 micrometers and are therefore only visible under a light or electron microscope . There are generally two types of cells: eukaryotic cells, which contain
8957-479: Is unclear exactly how this change occurred. For instance, in Athens, the kingship had been reduced to a hereditary, lifelong chief magistracy ( archon ) by c. 1050 BC; by 753 BC this had become a decennial, elected archonship; and finally by 683 BC an annually elected archonship. Through each stage, more power would have been transferred to the aristocracy as a whole, and away from a single individual. Inevitably,
9126-762: The Archaic period , the Greek population grew beyond the capacity of the limited arable land of Greece proper, resulting in the large-scale establishment of colonies elsewhere: according to one estimate, the population of the widening area of Greek settlement increased roughly tenfold from 800 BC to 400 BC, from 800,000 to as many as 7 + 1 ⁄ 2 -10 million. This was not simply for trade, but also to found settlements. These Greek colonies were not, as Roman colonies were, dependent on their mother-city, but were independent city-states in their own right. Greeks settled outside of Greece in two distinct ways. The first
9295-699: The Battle of Aegospotami , and began to blockade Athens' harbour; driven by hunger, Athens sued for peace, agreeing to surrender their fleet and join the Spartan-led Peloponnesian League. Following the Athenian surrender, Sparta installed an oligarchic regime, the Thirty Tyrants , in Athens, one of a number of Spartan-backed oligarchies which rose to power after the Peloponnesian war. Spartan predominance did not last: after only
9464-644: The Cambrian explosion . During the Permian period, synapsids , including the ancestors of mammals , dominated the land, but most of this group became extinct in the Permian–Triassic extinction event 252 million years ago. During the recovery from this catastrophe, archosaurs became the most abundant land vertebrates; one archosaur group, the dinosaurs, dominated the Jurassic and Cretaceous periods. After
9633-495: The Cretaceous–Paleogene extinction event 66 million years ago killed off the non-avian dinosaurs, mammals increased rapidly in size and diversity . Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify. Bacteria are a type of cell that constitute a large domain of prokaryotic microorganisms . Typically a few micrometers in length, bacteria have
9802-576: The Delian League gradually transformed from a defensive alliance of Greek states into an Athenian empire, as Athens' growing naval power intimidated the other league states. Athens ended its campaigns against Persia in 450, after a disastrous defeat in Egypt in 454, and the death of Cimon in action against the Persians on Cyprus in 450. As the Athenian fight against the Persian empire waned, conflict grew between Athens and Sparta. Suspicious of
9971-654: The Greco-Persian Wars to the death of Alexander the Great in 323 BC, and which included the Golden Age of Athens and the Peloponnesian War . The unification of Greece by Macedon under Philip II and subsequent conquest of the Achaemenid Empire by Alexander the Great spread Hellenistic civilization across the Middle East. The Hellenistic Period is considered to have ended in 30 BC, when
10140-535: The Miller–Urey experiment showed that organic compounds could be synthesized abiotically within a closed system mimicking the conditions of early Earth , thus suggesting that complex organic molecules could have arisen spontaneously in early Earth (see abiogenesis ). Macromolecules are large molecules made up of smaller subunits or monomers . Monomers include sugars, amino acids, and nucleotides. Carbohydrates include monomers and polymers of sugars. Lipids are
10309-740: The Paeonians due north, the Thracians to the northeast, and the Illyrians , with whom the Macedonians were frequently in conflict, to the northwest. Chalcidice was settled early on by southern Greek colonists and was considered part of the Greek world, while from the late 2nd millennium BC substantial Greek settlement also occurred on the eastern shores of the Aegean , in Anatolia . During
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#173277284436110478-639: The Parthenon of Athens. Politically, the Classical Period was dominated by Athens and the Delian League during the 5th century, but displaced by Spartan hegemony during the early 4th century BC, before power shifted to Thebes and the Boeotian League and finally to the League of Corinth led by Macedon . This period was shaped by the Greco-Persian Wars , the Peloponnesian War , and
10647-492: The Precambrian about 1.5 billion years ago and can be classified into eight major clades : alveolates , excavates , stramenopiles , plants, rhizarians , amoebozoans , fungi , and animals. Five of these clades are collectively known as protists , which are mostly microscopic eukaryotic organisms that are not plants, fungi, or animals. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor ), protists by themselves do not constitute
10816-671: The Rise of Macedon . Following the Classical period was the Hellenistic period (323–146 BC), during which Greek culture and power expanded into the Near and Middle East from the death of Alexander until the Roman conquest. Roman Greece is usually counted from the Roman victory over the Corinthians at the Battle of Corinth in 146 BC to the establishment of Byzantium by Constantine as
10985-727: The activator is the transcription factor that stimulates transcription when it binds to the sequence near or at the promoter. Negative regulation occurs when another transcription factor called a repressor binds to a DNA sequence called an operator , which is part of an operon, to prevent transcription. Repressors can be inhibited by compounds called inducers (e.g., allolactose ), thereby allowing transcription to occur. Specific genes that can be activated by inducers are called inducible genes , in contrast to constitutive genes that are almost constantly active. In contrast to both, structural genes encode proteins that are not involved in gene regulation. In addition to regulatory events involving
11154-592: The bacterial phyla have species that can be grown in the laboratory. Archaea constitute the other domain of prokaryotic cells and were initially classified as bacteria, receiving the name archaebacteria (in the Archaebacteria kingdom ), a term that has fallen out of use. Archaeal cells have unique properties separating them from the other two domains , Bacteria and Eukaryota . Archaea are further divided into multiple recognized phyla . Archaea and bacteria are generally similar in size and shape, although
11323-404: The cell . In 1838, Schleiden and Schwann began promoting the now universal ideas that (1) the basic unit of organisms is the cell and (2) that individual cells have all the characteristics of life, although they opposed the idea that (3) all cells come from the division of other cells, continuing to support spontaneous generation . However, Robert Remak and Rudolf Virchow were able to reify
11492-403: The cell membrane of another cell or located deep inside a cell. There are generally four types of chemical signals: autocrine , paracrine , juxtacrine , and hormones . In autocrine signaling, the ligand affects the same cell that releases it. Tumor cells, for example, can reproduce uncontrollably because they release signals that initiate their own self-division. In paracrine signaling,
11661-416: The circulatory systems of animals or vascular systems of plants to reach their target cells. Once a ligand binds with a receptor, it can influence the behavior of another cell, depending on the type of receptor. For instance, neurotransmitters that bind with an inotropic receptor can alter the excitability of a target cell. Other types of receptors include protein kinase receptors (e.g., receptor for
11830-528: The domain followed by kingdom , phylum , class , order , family , genus , and species . All organisms can be classified as belonging to one of three domains : Archaea (originally Archaebacteria), bacteria (originally eubacteria), or eukarya (includes the fungi, plant, and animal kingdoms). The history of life on Earth traces how organisms have evolved from the earliest emergence of life to present day. Earth formed about 4.5 billion years ago and all life on Earth, both living and extinct, descended from
11999-481: The duplication of its DNA and some of its organelles , and the subsequent partitioning of its cytoplasm into two daughter cells in a process called cell division . In eukaryotes (i.e., animal, plant, fungal , and protist cells), there are two distinct types of cell division: mitosis and meiosis . Mitosis is part of the cell cycle, in which replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which
12168-403: The kingdom Plantae, which would exclude fungi and some algae . Plant cells were derived by endosymbiosis of a cyanobacterium into an early eukaryote about one billion years ago, which gave rise to chloroplasts. The first several clades that emerged following primary endosymbiosis were aquatic and most of the aquatic photosynthetic eukaryotic organisms are collectively described as algae, which
12337-423: The microscope . It was then that scholars discovered spermatozoa , bacteria, infusoria and the diversity of microscopic life. Investigations by Jan Swammerdam led to new interest in entomology and helped to develop techniques of microscopic dissection and staining . Advances in microscopy had a profound impact on biological thinking. In the early 19th century, biologists pointed to the central importance of
12506-412: The mitochondrial cristae . Oxidative phosphorylation comprises the electron transport chain, which is a series of four protein complexes that transfer electrons from one complex to another, thereby releasing energy from NADH and FADH 2 that is coupled to the pumping of protons (hydrogen ions) across the inner mitochondrial membrane ( chemiosmosis ), which generates a proton motive force . Energy from
12675-546: The phenotype of that dominant allele. During gamete formation, the alleles for each gene segregate, so that each gamete carries only one allele for each gene. Heterozygotic individuals produce gametes with an equal frequency of two alleles. Finally, the law of independent assortment , states that genes of different traits can segregate independently during the formation of gametes, i.e., genes are unlinked. An exception to this rule would include traits that are sex-linked . Test crosses can be performed to experimentally determine
12844-449: The poleis grouped themselves into leagues, membership of which was in a constant state of flux. Later in the Classical period, the leagues would become fewer and larger, be dominated by one city (particularly Athens , Sparta and Thebes ); and often poleis would be compelled to join under threat of war (or as part of a peace treaty). Even after Philip II of Macedon conquered the heartlands of ancient Greece, he did not attempt to annex
13013-497: The regulation of the rate of a metabolic reaction, for example in response to changes in the cell's environment or to signals from other cells. Cellular respiration is a set of metabolic reactions and processes that take place in cells to convert chemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. The reactions involved in respiration are catabolic reactions , which break large molecules into smaller ones, releasing energy. Respiration
13182-464: The 146 BC conquest of Greece after the Battle of Corinth. Macedonia became a Roman province while southern Greece came under the surveillance of Macedonia's prefect ; however, some Greek poleis managed to maintain a partial independence and avoid taxation. The Aegean Islands were added to this territory in 133 BC. Athens and other Greek cities revolted in 88 BC, and the peninsula was crushed by
13351-583: The Carthaginian invasion at the Battle of Himera . The Persians were decisively defeated at sea by a primarily Athenian naval force at the Battle of Salamis , and on land in 479 BC at the Battle of Plataea . The alliance against Persia continued, initially led by the Spartan Pausanias but from 477 by Athens, and by 460 Persia had been driven out of the Aegean. During this long campaign,
13520-525: The Corinthian empire in northwest Greece and defended its own empire, despite a plague which killed the leading Athenian statesman Pericles . The war turned after Athenian victories led by Cleon at Pylos and Sphakteria , and Sparta sued for peace, but the Athenians rejected the proposal. The Athenian failure to regain control of Boeotia at Delium and Brasidas ' successes in northern Greece in 424 improved Sparta's position after Sphakteria. After
13689-601: The Dark Ages was the Archaic Period , beginning around the 8th century BC, which saw early developments in Greek culture and society leading to the Classical Period from the Persian invasion of Greece in 480 BC until the death of Alexander the Great in 323 BC. The Classical Period is characterized by a "classical" style, i.e. one which was considered exemplary by later observers, most famously in
13858-668: The Greek city-states. It greatly widened the horizons of the Greeks and led to a steady emigration of the young and ambitious to the new Greek empires in the east. Many Greeks migrated to Alexandria, Antioch and the many other new Hellenistic cities founded in Alexander's wake, as far away as present-day Afghanistan and Pakistan , where the Greco-Bactrian Kingdom and the Indo-Greek Kingdom survived until
14027-460: The Greek colonies Syracusae ( Συράκουσαι ), Neapolis ( Νεάπολις ), Massalia ( Μασσαλία ) and Byzantion ( Βυζάντιον ). These colonies played an important role in the spread of Greek influence throughout Europe and also aided in the establishment of long-distance trading networks between the Greek city-states, boosting the economy of ancient Greece . Ancient Greece consisted of several hundred relatively independent city-states ( poleis ). This
14196-578: The Greek colony Sybaris in southern Italy, its allies, and the Serdaioi. In 499 BC, the Ionian city states under Persian rule rebelled against their Persian-supported tyrant rulers. Supported by troops sent from Athens and Eretria , they advanced as far as Sardis and burnt the city before being driven back by a Persian counterattack. The revolt continued until 494, when the rebelling Ionians were defeated. Darius did not forget that Athens had assisted
14365-483: The Greek system are further evidenced by the colonies that they set up throughout the Mediterranean , which, though they might count a certain Greek polis as their 'mother' (and remain sympathetic to her), were completely independent of the founding city. Inevitably smaller poleis might be dominated by larger neighbors, but conquest or direct rule by another city-state appears to have been quite rare. Instead
14534-815: The Greeks began 250 years of expansion, settling colonies in all directions. To the east, the Aegean coast of Asia Minor was colonized first, followed by Cyprus and the coasts of Thrace , the Sea of Marmara and south coast of the Black Sea . Eventually, Greek colonization reached as far northeast as present-day Ukraine and Russia ( Taganrog ). To the west the coasts of Illyria , Southern Italy (called " Magna Graecia ") were settled, followed by Southern France , Corsica , and even eastern Spain . Greek colonies were also founded in Egypt and Libya . Modern Syracuse , Naples , Marseille and Istanbul had their beginnings as
14703-423: The Greeks were very aware of their tribal origins; Herodotus was able to extensively categorise the city-states by tribe. Yet, although these higher-level relationships existed, they seem to have rarely had a major role in Greek politics. The independence of the poleis was fiercely defended; unification was something rarely contemplated by the ancient Greeks. Even when, during the second Persian invasion of Greece,
14872-535: The Ionian revolt, and in 490 he assembled an armada to retaliate. Though heavily outnumbered, the Athenians—supported by their Plataean allies—defeated the Persian hordes at the Battle of Marathon , and the Persian fleet turned tail. Ten years later, a second invasion was launched by Darius' son Xerxes . The city-states of northern and central Greece submitted to the Persian forces without resistance, but
15041-483: The League of Corinth following the death of Philip, Alexander began his campaign against Persia in 334 BC. He conquered Persia, defeating Darius III at the Battle of Issus in 333 BC, and after the Battle of Gaugamela in 331 BC proclaimed himself king of Asia. From 329 BC he led expeditions to Bactria and then India; further plans to invade Arabia and North Africa were halted by his death in 323 BC. The period from
15210-523: The Mediterranean region is commonly considered to have begun in the 8th century BC (around the time of the earliest recorded poetry of Homer) and ended in the 6th century AD. Classical antiquity in Greece was preceded by the Greek Dark Ages ( c. 1200 – c. 800 BC ), archaeologically characterised by the protogeometric and geometric styles of designs on pottery. Following
15379-481: The Phanerozoic eon that began 539 million years ago being subdivided into Paleozoic , Mesozoic , and Cenozoic eras. These three eras together comprise eleven periods ( Cambrian , Ordovician , Silurian , Devonian , Carboniferous , Permian , Triassic , Jurassic , Cretaceous , Tertiary , and Quaternary ). The similarities among all known present-day species indicate that they have diverged through
15548-685: The Roman general Sulla . The Roman civil wars devastated the land even further, until Augustus organized the peninsula as the province of Achaea in 27 BC. Greece was a key eastern province of the Roman Empire, as the Roman culture had long been in fact Greco-Roman . The Greek language served as a lingua franca in the East and in Italy , and many Greek intellectuals such as Galen would perform most of their work in Rome . The territory of Greece
15717-696: The accumulation of favorable traits over successive generations, thereby increasing the match between the organisms and their environment. A species is a group of organisms that mate with one another and speciation is the process by which one lineage splits into two lineages as a result of having evolved independently from each other. For speciation to occur, there has to be reproductive isolation . Reproductive isolation can result from incompatibilities between genes as described by Bateson–Dobzhansky–Muller model . Reproductive isolation also tends to increase with genetic divergence . Speciation can occur when there are physical barriers that divide an ancestral species,
15886-579: The ancient Greeks did not think in terms of race . Most families owned slaves as household servants and laborers, and even poor families might have owned a few slaves. Owners were not allowed to beat or kill their slaves. Owners often promised to free slaves in the future to encourage slaves to work hard. Unlike in Rome, freedmen did not become citizens. Instead, they were mixed into the population of metics , which included people from foreign countries or other city-states who were officially allowed to live in
16055-589: The body plan and the number, identity, and pattern of body parts. Among the most important toolkit genes are the Hox genes . Hox genes determine where repeating parts, such as the many vertebrae of snakes, will grow in a developing embryo or larva. Evolution is a central organizing concept in biology. It is the change in heritable characteristics of populations over successive generations . In artificial selection , animals were selectively bred for specific traits. Given that traits are inherited, populations contain
16224-450: The breaking down of glucose to pyruvate by cellular respiration ); or anabolic —the building up ( synthesis ) of compounds (such as proteins, carbohydrates, lipids, and nucleic acids). Usually, catabolism releases energy, and anabolism consumes energy. The chemical reactions of metabolism are organized into metabolic pathways , in which one chemical is transformed through a series of steps into another chemical, each step being facilitated by
16393-560: The capital of the Roman Empire in 330 AD. Finally, Late Antiquity refers to the period of Christianization during the later 4th to early 6th centuries AD, consummated by the closure of the Academy of Athens by Justinian I in 529. The historical period of ancient Greece is unique in world history as the first period attested directly in comprehensive, narrative historiography , while earlier ancient history or protohistory
16562-666: The center, while in the east lay Boeotia , Attica , and Megaris . Northeast lay Thessaly , while Epirus lay to the northwest. Epirus stretched from the Ambracian Gulf in the south to the Ceraunian Mountains and the Aoos river in the north, and consisted of Chaonia (north), Molossia (center), and Thesprotia (south). In the northeast corner was Macedonia , originally consisting Lower Macedonia and its regions, such as Elimeia , Pieria , and Orestis . Around
16731-410: The cohesive force due to the attraction between molecules at the surface of the liquid. Water is also adhesive as it is able to adhere to the surface of any polar or charged non-water molecules. Water is denser as a liquid than it is as a solid (or ice). This unique property of water allows ice to float above liquid water such as ponds, lakes, and oceans, thereby insulating the liquid below from
16900-433: The cold air above. Water has the capacity to absorb energy, giving it a higher specific heat capacity than other solvents such as ethanol . Thus, a large amount of energy is needed to break the hydrogen bonds between water molecules to convert liquid water into water vapor . As a molecule, water is not completely stable as each water molecule continuously dissociates into hydrogen and hydroxyl ions before reforming into
17069-455: The course of the archaic period. Already in the seventh century, the right of all citizen men to attend the assembly appears to have been established. After a failed coup led by Cylon of Athens around 636 BC, Draco was appointed to establish a code of laws in 621. This failed to reduce the political tension between the poor and the elites, and in 594 Solon was given the authority to enact another set of reforms, which attempted to balance
17238-473: The course of the eighth and seventh century. According to Spartan tradition, this constitution was established by the legendary lawgiver Lycurgus . Over the course of the first and second Messenian wars , Sparta subjugated the neighbouring region of Messenia , enserfing the population. In the sixth century, Greek city-states began to develop formal relationships with one another, where previously individual rulers had relied on personal relationships with
17407-574: The culmination of political and social developments which had begun in the Greek dark age, with the polis (city-state) becoming the most important unit of political organisation in Greece. The absence of powerful states in Greece after the collapse of Mycenaean power, and the geography of Greece, where many settlements were separated from their neighbours by mountainous terrain, encouraged the development of small independent city-states. Several Greek states saw tyrants rise to power in this period, most famously at Corinth from 657 BC. The period also saw
17576-501: The cytoplasm of a cell, there are many biomolecules such as proteins and nucleic acids . In addition to biomolecules, eukaryotic cells have specialized structures called organelles that have their own lipid bilayers or are spatially units. These organelles include the cell nucleus , which contains most of the cell's DNA, or mitochondria , which generate adenosine triphosphate (ATP) to power cellular processes. Other organelles such as endoplasmic reticulum and Golgi apparatus play
17745-455: The cytoplasm, where it is converted to waste products that may be removed from the cell. This serves the purpose of oxidizing the electron carriers so that they can perform glycolysis again and removing the excess pyruvate. Fermentation oxidizes NADH to NAD so it can be re-used in glycolysis. In the absence of oxygen, fermentation prevents the buildup of NADH in the cytoplasm and provides NAD for glycolysis. This waste product varies depending on
17914-461: The death of Alexander the Great in 323 BC until the death of Cleopatra , the last Macedonian ruler of Egypt, is known as the Hellenistic period. In the early part of this period, a new form of kingship developed based on Macedonian and Near Eastern traditions. The first Hellenistic kings were previously Alexander's generals, and took power in the period following his death, though they were not part of existing royal lineages and lacked historic claims to
18083-423: The deaths of Cleon and Brasidas, the strongest proponents of war on each side, a peace treaty was negoitiated in 421 by the Athenian general Nicias . The peace did not last, however. In 418 BC allied forces of Athens and Argos were defeated by Sparta at Mantinea . In 415 Athens launched an ambitious naval expedition to dominate Sicily; the expedition ended in disaster at the harbor of Syracuse , with almost
18252-625: The development of biological knowledge. He explored biological causation and the diversity of life. His successor, Theophrastus , began the scientific study of plants. Scholars of the medieval Islamic world who wrote on biology included al-Jahiz (781–869), Al-Dīnawarī (828–896), who wrote on botany, and Rhazes (865–925) who wrote on anatomy and physiology . Medicine was especially well studied by Islamic scholars working in Greek philosopher traditions, while natural history drew heavily on Aristotelian thought. Biology began to quickly develop with Anton van Leeuwenhoek 's dramatic improvement of
18421-418: The development of body form, is the result of spatial differences in gene expression. A small fraction of the genes in an organism's genome called the developmental-genetic toolkit control the development of that organism. These toolkit genes are highly conserved among phyla , meaning that they are ancient and very similar in widely separated groups of animals. Differences in deployment of toolkit genes affect
18590-440: The developmental fate of a cell, which becomes more restrictive during development. Differentiation is the process by which specialized cells arise from less specialized cells such as stem cells . Stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. Cellular differentiation dramatically changes
18759-411: The discovery of archaea in almost every habitat , including soil, oceans, and marshlands . Archaea are particularly numerous in the oceans, and the archaea in plankton may be one of the most abundant groups of organisms on the planet. Archaea are a major part of Earth's life . They are part of the microbiota of all organisms. In the human microbiome , they are important in the gut , mouth, and on
18928-420: The discovery of the double-helical structure of DNA by James Watson and Francis Crick in 1953, marked the transition to the era of molecular genetics . From the 1950s onwards, biology has been vastly extended in the molecular domain. The genetic code was cracked by Har Gobind Khorana , Robert W. Holley and Marshall Warren Nirenberg after DNA was understood to contain codons . The Human Genome Project
19097-674: The dominance that would allow it to challenge Persia, the Persian king initially joined the alliance against Sparta, before imposing the Peace of Antalcidas ("King's Peace") which restored Persia's control over the Anatolian Greeks. By 371 BC, Thebes was in the ascendancy, defeating Sparta at the Battle of Leuctra , killing the Spartan king Cleombrotus I , and invading Laconia. Further Theban successes against Sparta in 369 led to Messenia gaining independence; Sparta never recovered from
19266-409: The domination of politics and concomitant aggregation of wealth by small groups of families was apt to cause social unrest in many poleis . In many cities a tyrant (not in the modern sense of repressive autocracies), would at some point seize control and govern according to their own will; often a populist agenda would help sustain them in power. In a system wracked with class conflict , government by
19435-634: The earliest terrestrial ecosystems , at least 2.7 billion years ago. Microorganisms are thought to have paved the way for the inception of land plants in the Ordovician period. Land plants were so successful that they are thought to have contributed to the Late Devonian extinction event . Ediacara biota appear during the Ediacaran period, while vertebrates , along with most other modern phyla originated about 525 million years ago during
19604-726: The east to the Indian king Chandragupta Maurya in exchange for war elephants, and later lost large parts of Persia to the Parthian Empire . By the mid-third century, the kingdoms of Alexander's successors was mostly stable, though there continued to be disputes over border areas. The great capitals of Hellenistic culture were Alexandria in the Ptolemaic Kingdom and Antioch in the Seleucid Empire . The conquests of Alexander had numerous consequences for
19773-552: The elites of other cities. Towards the end of the archaic period, Sparta began to build a series of alliances, the Peloponnesian League , with cities including Corinth , Elis , and Megara , isolating Messenia and reinforcing Sparta's position against Argos , the other major power in the Peloponnese. Other alliances in the sixth century included those between Elis and Heraea in the Peloponnese; and between
19942-619: The end of the first century BC. The city-states within Greece formed themselves into two leagues; the Achaean League (including Corinth and Argos) and the Aetolian League (including Sparta and Athens). For much of the period until the Roman conquest, these leagues were at war, often participating in the conflicts between the Diadochi (the successor states to Alexander's empire). The Antigonid Kingdom became involved in
20111-609: The entire army killed, and the ships destroyed. Soon after the Athenian defeat in Syracuse, Athens' Ionian allies began to rebel against the Delian league, while Persia began to once again involve itself in Greek affairs on the Spartan side. Initially the Athenian position continued relatively strong, with important victories at Cyzicus in 410 and Arginusae in 406. However, in 405 the Spartan Lysander defeated Athens in
20280-666: The era of classical antiquity was immediately followed by the Early Middle Ages and the Byzantine period. Three centuries after the decline of Mycenaean Greece during the Bronze Age Collapse , Greek urban poleis began to form in the 8th century BC, ushering in the Archaic period and the colonization of the Mediterranean Basin. This was followed by the age of Classical Greece , from
20449-540: The exact borders of the Hellenistic kingdoms were not settled. Antigonus attempted to expand his territory by attacking the other successor kingdoms until they joined against him, and he was killed at the Battle of Ipsus in 301 BC. His son Demetrius spent many years in Seleucid captivity, and his son, Antigonus II , only reclaimed the Macedonian throne around 276. Meanwhile, the Seleucid kingdom gave up territory in
20618-481: The expression of deleterious recessive mutations . The beneficial effect of genetic complementation, derived from outcrossing (cross-fertilization) is also referred to as hybrid vigor or heterosis. Charles Darwin in his 1878 book The Effects of Cross and Self-Fertilization in the Vegetable Kingdom at the start of chapter XII noted “The first and most important of the conclusions which may be drawn from
20787-455: The form of glucose is the main nutrient used by animal and plant cells in respiration. Cellular respiration involving oxygen is called aerobic respiration, which has four stages: glycolysis , citric acid cycle (or Krebs cycle), electron transport chain , and oxidative phosphorylation . Glycolysis is a metabolic process that occurs in the cytoplasm whereby glucose is converted into two pyruvates , with two net molecules of ATP being produced at
20956-469: The founding of Greek colonies around the Mediterranean, with Euboean settlements at Al-Mina in the east as early as 800 BC, and Ischia in the west by 775. Increasing contact with non-Greek peoples in this period, especially in the Near East, inspired developments in art and architecture, the adoption of coinage, and the development of the Greek alphabet. Athens developed its democratic system over
21125-576: The government. In Athens, the population was divided into four social classes based on wealth. People could change classes if they made more money. In Sparta, all male citizens were called homoioi , meaning "peers". However, Spartan kings, who served as the city-state's dual military and religious leaders, came from two families. Women in Ancient Greece appear to have primarily performed domestic tasks, managed households, and borne and reared children. Slaves had no power or status. Slaves had
21294-426: The hormone insulin ) and G protein-coupled receptors . Activation of G protein-coupled receptors can initiate second messenger cascades. The process by which a chemical or physical signal is transmitted through a cell as a series of molecular events is called signal transduction . The cell cycle is a series of events that take place in a cell that cause it to divide into two daughter cells. These events include
21463-547: The increasing Athenian power funded by the Delian League, Sparta offered aid to reluctant members of the League to rebel against Athenian domination. These tensions were exacerbated in 462 BC when Athens sent a force to aid Sparta in overcoming a helot revolt, but this aid was rejected by the Spartans. In the 450s, Athens took control of Boeotia, and won victories over Aegina and Corinth. However, Athens failed to win
21632-414: The initial step of photosynthesis whereby light energy is absorbed by chlorophyll pigments attached to proteins in the thylakoid membranes . The absorbed light energy is used to remove electrons from a donor (water) to a primary electron acceptor, a quinone designated as Q. In the second stage, electrons move from the quinone primary electron acceptor through a series of electron carriers until they reach
21801-591: The last Hellenistic kingdom, Ptolemaic Egypt , was annexed by the Roman Republic . Classical Greek culture , especially philosophy, had a powerful influence on ancient Rome , which carried a version of it throughout the Mediterranean and much of Europe. For this reason, Classical Greece is generally considered the cradle of Western civilization , the seminal culture from which the modern West derives many of its founding archetypes and ideas in politics, philosophy, science, and art. Classical antiquity in
21970-406: The last eukaryotic common ancestor. Prokaryotes (i.e., archaea and bacteria) can also undergo cell division (or binary fission ). Unlike the processes of mitosis and meiosis in eukaryotes, binary fission in prokaryotes takes place without the formation of a spindle apparatus on the cell. Before binary fission, DNA in the bacterium is tightly coiled. After it has uncoiled and duplicated, it
22139-405: The ligand diffuses to nearby cells and affects them. For example, brain cells called neurons release ligands called neurotransmitters that diffuse across a synaptic cleft to bind with a receptor on an adjacent cell such as another neuron or muscle cell . In juxtacrine signaling, there is direct contact between the signaling and responding cells. Finally, hormones are ligands that travel through
22308-514: The loss of Messenia's fertile land and the helot workforce it provided. The rising power of Thebes led Sparta and Athens to join forces; in 362 they were defeated by Thebes at the Battle of Mantinea . In the aftermath of Mantinea, none of the major Greek states were able to dominate. Though Thebes had won the battle, their general Epaminondas was killed, and they spent the following decades embroiled in wars with their neighbours; Athens, meanwhile, saw its second naval alliance, formed in 377, collapse in
22477-444: The membrane serving as membrane transporters , and peripheral proteins that loosely attach to the outer side of the cell membrane, acting as enzymes shaping the cell. Cell membranes are involved in various cellular processes such as cell adhesion , storing electrical energy , and cell signalling and serve as the attachment surface for several extracellular structures such as a cell wall , glycocalyx , and cytoskeleton . Within
22646-495: The mid-350s. The power vacuum in Greece after the Battle of Mantinea was filled by Macedon, under Philip II . In 338 BC, he defeated a Greek alliance at the Battle of Chaeronea , and subsequently formed the League of Corinth . Philip planned to lead the League to invade Persia, but was murdered in 336 BC. His son Alexander the Great was left to fulfil his father's ambitions. After campaigns against Macedon's western and northern enemies, and those Greek states that had broken from
22815-403: The mitotic phase of an animal cell cycle—the division of the mother cell into two genetically identical daughter cells. The cell cycle is a vital process by which a single-celled fertilized egg develops into a mature organism, as well as the process by which hair, skin, blood cells , and some internal organs are renewed. After cell division, each of the daughter cells begin the interphase of
22984-437: The most abundant molecule in every organism. Water is important to life because it is an effective solvent , capable of dissolving solutes such as sodium and chloride ions or other small molecules to form an aqueous solution . Once dissolved in water, these solutes are more likely to come in contact with one another and therefore take part in chemical reactions that sustain life. In terms of its molecular structure , water
23153-403: The nature of their research questions and the tools that they use. Like other scientists, biologists use the scientific method to make observations , pose questions, generate hypotheses , perform experiments, and form conclusions about the world around them. Life on Earth, which emerged more than 3.7 billion years ago, is immensely diverse. Biologists have sought to study and classify
23322-441: The observations given in this volume, is that generally cross-fertilisation is beneficial and self-fertilisation often injurious, at least with the plants on which I experimented.” Genetic variation , often produced as a byproduct of sexual reproduction, may provide long-term advantages to those sexual lineages that engage in outcrossing . Genetics is the scientific study of inheritance. Mendelian inheritance , specifically,
23491-517: The only class of macromolecules that are not made up of polymers. They include steroids , phospholipids , and fats, largely nonpolar and hydrophobic (water-repelling) substances. Proteins are the most diverse of the macromolecules. They include enzymes , transport proteins , large signaling molecules, antibodies , and structural proteins . The basic unit (or monomer) of a protein is an amino acid . Twenty amino acids are used in proteins. Nucleic acids are polymers of nucleotides . Their function
23660-404: The organism. In skeletal muscles, the waste product is lactic acid . This type of fermentation is called lactic acid fermentation . In strenuous exercise, when energy demands exceed energy supply, the respiratory chain cannot process all of the hydrogen atoms joined by NADH. During anaerobic glycolysis, NAD regenerates when pairs of hydrogen combine with pyruvate to form lactate. Lactate formation
23829-428: The possibility of common descent . Serious evolutionary thinking originated with the works of Jean-Baptiste Lamarck , who presented a coherent theory of evolution. The British naturalist Charles Darwin , combining the biogeographical approach of Humboldt , the uniformitarian geology of Lyell , Malthus's writings on population growth, and his own morphological expertise and extensive natural observations, forged
23998-410: The power of the rich and the poor. In the middle of the sixth century, Pisistratus established himself as a tyrant, and after his death in 527 his son Hippias inherited his position; by the end of the sixth century he had been overthrown and Cleisthenes carried out further democratising reforms. In Sparta, a political system with two kings, a council of elders , and five ephors developed over
24167-907: The process of evolution from their common ancestor. Biologists regard the ubiquity of the genetic code as evidence of universal common descent for all bacteria , archaea , and eukaryotes . Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean eon and many of the major steps in early evolution are thought to have taken place in this environment. The earliest evidence of eukaryotes dates from 1.85 billion years ago, and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism . Later, around 1.7 billion years ago, multicellular organisms began to appear, with differentiated cells performing specialised functions. Algae-like multicellular land plants are dated back to about 1 billion years ago, although evidence suggests that microorganisms formed
24336-539: The process such as transcription , RNA splicing , translation , and post-translational modification of a protein. Gene expression can be influenced by positive or negative regulation, depending on which of the two types of regulatory proteins called transcription factors bind to the DNA sequence close to or at a promoter. A cluster of genes that share the same promoter is called an operon , found mainly in prokaryotes and some lower eukaryotes (e.g., Caenorhabditis elegans ). In positive regulation of gene expression,
24505-517: The promoter, gene expression can also be regulated by epigenetic changes to chromatin , which is a complex of DNA and protein found in eukaryotic cells. Development is the process by which a multicellular organism (plant or animal) goes through a series of changes, starting from a single cell, and taking on various forms that are characteristic of its life cycle. There are four key processes that underlie development: Determination , differentiation , morphogenesis , and growth. Determination sets
24674-406: The proton motive force drives the enzyme ATP synthase to synthesize more ATPs by phosphorylating ADPs . The transfer of electrons terminates with molecular oxygen being the final electron acceptor . If oxygen were not present, pyruvate would not be metabolized by cellular respiration but undergoes a process of fermentation . The pyruvate is not transported into the mitochondrion but remains in
24843-617: The reforms of Solon (early 6th century), but the poorest citizens could not address the assembly or run for office. With the establishment of the democracy, the assembly became the de jure mechanism of government; all citizens had equal privileges in the assembly. However, non-citizens, such as metics (foreigners living in Athens) or slaves , had no political rights at all. After the rise of democracy in Athens, other city-states founded democracies. However, many retained more traditional forms of government. As so often in other matters, Sparta
25012-420: The regions of Laconia (southeast), Messenia (southwest), Elis (west), Achaia (north), Korinthia (northeast), Argolis (east), and Arcadia (center). These names survive to the present day as regional units of modern Greece , though with somewhat different boundaries. Mainland Greece to the north, nowadays known as Central Greece , consisted of Aetolia and Acarnania in the west, Locris , Doris , and Phocis in
25181-518: The right to have a family and own property, subject to their master's goodwill and permission, but they had no political rights. By 600 BC, chattel slavery had spread in Greece. By the 5th century BC, slaves made up one-third of the total population in some city-states. Between 40–80% of the population of Classical Athens were slaves. Slaves outside of Sparta almost never revolted because they were made up of too many nationalities and were too scattered to organize. However, unlike later Western culture ,
25350-449: The same time. Each pyruvate is then oxidized into acetyl-CoA by the pyruvate dehydrogenase complex , which also generates NADH and carbon dioxide. Acetyl-CoA enters the citric acid cycle, which takes places inside the mitochondrial matrix. At the end of the cycle, the total yield from 1 glucose (or 2 pyruvates) is 6 NADH, 2 FADH 2 , and 2 ATP molecules. Finally, the next stage is oxidative phosphorylation, which in eukaryotes, occurs in
25519-680: The second and third stages, respectively, provide the energy and electrons to drive the synthesis of glucose by fixing atmospheric carbon dioxide into existing organic carbon compounds, such as ribulose bisphosphate (RuBP) in a sequence of light-independent (or dark) reactions called the Calvin cycle . Cell signaling (or communication) is the ability of cells to receive, process, and transmit signals with its environment and with itself. Signals can be non-chemical such as light, electrical impulses , and heat, or chemical signals (or ligands ) that interact with receptors , which can be found embedded in
25688-553: The skin. Their morphological, metabolic, and geographical diversity permits them to play multiple ecological roles: carbon fixation; nitrogen cycling; organic compound turnover; and maintaining microbial symbiotic and syntrophic communities, for example. Eukaryotes are hypothesized to have split from archaea, which was followed by their endosymbioses with bacteria (or symbiogenesis ) that gave rise to mitochondria and chloroplasts, both of which are now part of modern-day eukaryotic cells. The major lineages of eukaryotes diversified in
25857-448: The state. City-states legally owned slaves. These public slaves had a larger measure of independence than slaves owned by families, living on their own and performing specialized tasks. In Athens, public slaves were trained to look out for counterfeit coinage , while temple slaves acted as servants of the temple's deity and Scythian slaves were employed in Athens as a police force corralling citizens to political functions. Sparta had
26026-479: The stroma. This is analogous to the proton-motive force generated across the inner mitochondrial membrane in aerobic respiration. During the third stage of photosynthesis, the movement of protons down their concentration gradients from the thylakoid lumen to the stroma through the ATP synthase is coupled to the synthesis of ATP by that same ATP synthase. The NADPH and ATPs generated by the light-dependent reactions in
26195-695: The territories they controlled. The most important of these rulers in the decades after Alexander's death were Antigonus I and his son Demetrius in Macedonia and the rest of Greece, Ptolemy in Egypt, and Seleucus I in Syria and the former Persian empire; smaller Hellenistic kingdoms included the Attalids in Anatolia and the Greco-Bactrian kingdom . In the early part of the Hellenistic period,
26364-515: The territory or unify it into a new province, but compelled most of the poleis to join his own Corinthian League . Initially many Greek city-states seem to have been petty kingdoms; there was often a city official carrying some residual, ceremonial functions of the king ( basileus ), e.g., the archon basileus in Athens. However, by the Archaic period and the first historical consciousness, most had already become aristocratic oligarchies . It
26533-471: The third tenet, and by the 1860s most biologists accepted all three tenets which consolidated into cell theory . Meanwhile, taxonomy and classification became the focus of natural historians. Carl Linnaeus published a basic taxonomy for the natural world in 1735, and in the 1750s introduced scientific names for all his species. Georges-Louis Leclerc, Comte de Buffon , treated species as artificial categories and living forms as malleable—even suggesting
26702-626: The time of Alexander I of Macedon , the Argead kings of Macedon started to expand into Upper Macedonia , lands inhabited by independent Macedonian tribes like the Lyncestae , Orestae and the Elimiotae and to the west, beyond the Axius river , into Eordaia , Bottiaea , Mygdonia , and Almopia , regions settled by Thracian tribes. To the north of Macedonia lay various non-Greek peoples such as
26871-439: The total number of chromosomes is maintained. In general, mitosis (division of the nucleus) is preceded by the S stage of interphase (during which the DNA is replicated) and is often followed by telophase and cytokinesis ; which divides the cytoplasm , organelles and cell membrane of one cell into two new cells containing roughly equal shares of these cellular components. The different stages of mitosis all together define
27040-431: The underlying genotype of an organism with a dominant phenotype. A Punnett square can be used to predict the results of a test cross. The chromosome theory of inheritance , which states that genes are found on chromosomes, was supported by Thomas Morgans 's experiments with fruit flies , which established the sex linkage between eye color and sex in these insects. A gene is a unit of heredity that corresponds to
27209-580: The use of gypsum (hydrated calcium sulfate ) as a fertilizer . In 1843, John Bennet Lawes and Joseph Henry Gilbert began a set of long-term field experiments at Rothamsted Research in England, some of which are still running as of 2018. In the United States, a scientific revolution in agriculture began with the Hatch Act of 1887 , which used the term "agricultural science". The Hatch Act
27378-656: The various forms of life, from prokaryotic organisms such as archaea and bacteria to eukaryotic organisms such as protists , fungi, plants, and animals. These various organisms contribute to the biodiversity of an ecosystem , where they play specialized roles in the cycling of nutrients and energy through their biophysical environment . The earliest of roots of science, which included medicine, can be traced to ancient Egypt and Mesopotamia in around 3000 to 1200 BCE . Their contributions shaped ancient Greek natural philosophy . Ancient Greek philosophers such as Aristotle (384–322 BCE) contributed extensively to
27547-494: Was a northeastern Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of classical antiquity ( c. 600 AD ), that comprised a loose collection of culturally and linguistically related city-states and other territories. Prior to the Roman period , most of these regions were officially unified once under the Kingdom of Macedon from 338 to 323 BC. In Western history ,
27716-539: Was a notable exception to the rest of Greece, ruled through the whole period by not one, but two hereditary monarchs. This was a form of diarchy . The Kings of Sparta belonged to the Agiads and the Eurypontids, descendants respectively of Eurysthenes and Procles . Both dynasties' founders were believed to be twin sons of Aristodemus , a Heraclid ruler. However, the powers of these kings were held in check by both
27885-460: Was a situation unlike that in most other contemporary societies, which were either tribal or kingdoms ruling over relatively large territories. Undoubtedly, the geography of Greece —divided and sub-divided by hills, mountains, and rivers—contributed to the fragmentary nature of ancient Greece. On the one hand, the ancient Greeks had no doubt that they were "one people"; they had the same religion , same basic culture, and same language. Furthermore,
28054-752: Was conquered by the Romans. The Aetolian league grew wary of Roman involvement in Greece, and sided with the Seleucids in the Roman–Seleucid War ; when the Romans were victorious, the league was effectively absorbed into the Republic. Although the Achaean league outlasted both the Aetolian league and Macedon, it was also soon defeated and absorbed by the Romans in 146 BC, bringing Greek independence to an end. The Greek peninsula came under Roman rule during
28223-567: Was driven by farmers' interest in knowing the constituents of early artificial fertilizer. The Smith–Hughes Act of 1917 shifted agricultural education back to its vocational roots, but the scientific foundation had been built. For the next 44 years after 1906, federal expenditures on agricultural research in the United States outpaced private expenditures. Agriculture, agricultural science, and agronomy are closely related. However, they cover different concepts: Agricultural sciences include research and development on: Biology Biology
28392-417: Was in permanent settlements founded by Greeks, which formed as independent poleis. The second form was in what historians refer to as emporia ; trading posts which were occupied by both Greeks and non-Greeks and which were primarily concerned with the manufacture and sale of goods. Examples of this latter type of settlement are found at Al Mina in the east and Pithekoussai in the west. From about 750 BC
28561-434: Was launched in 1990 to map the human genome . All organisms are made up of chemical elements ; oxygen , carbon , hydrogen , and nitrogen account for most (96%) of the mass of all organisms, with calcium , phosphorus , sulfur , sodium , chlorine , and magnesium constituting essentially all the remainder. Different elements can combine to form compounds such as water, which is fundamental to life. Biochemistry
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