Biology 1120 bios 1120
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1 BIOS 1120 UNIT 1 EXAM STUDY GUIDE Chapter 1 1.1 Living organisms: Acquire and use materials and energy Actively maintain organized complexity Perceive and respond to stimuli Grow Reproduce Have the capacity to evolve, collectively 1.2 Evolution is the process by which new species arise 1. Genetic variation among members of a population due to differences in their DNA 2. Inheritance of those variations by offspring of parents carrying the variation 3. Natural selection of individuals whose survival and enhanced reproduction are due to the favorable variations they carry 1.3 How do Scientists Study Life? Scientists study living processes at different levels (atoms, molecules, cells, tissues, organs, organ systems, organisms, populations, communities, biota, and the biosphere) There are three major domains of organisms: Archaea, Bacteria and Eukarya Archaea and Bacteria are Prokaryotes Eukarya includes Animals, Plants, Fungi and Protists 1.4 What is Science? Science is one way of understanding the world of experience It is characterized by systematic inquiry using observation and experiments There is a method to science that distinguishes it from other disciplines. Science does not make reference to supernatural causes Chapter 2 2.1 Atoms: Are the basic structural units of the 92 known elements, and all matter Certain atoms (e.g. carbon, oxygen, hydrogen) are particularly common in living organisms Atoms are composed of still smaller particles (protons, neutrons and electrons) 2.2 Atoms interact to form molecules There are three basic types of bonds within molecules: ionic bonds (one atom loses and one atom gains an electron), covalent bonds (the atoms share electrons) polar (unequally) non-polar (equally) 2 hydrogen bonds (attraction between different polar molecules) 2.3 Why is Water so Important to Life? Water has several unique properties Cohesion- between water molecules due to hydrogen bonds accounts for how molecules of water can go up the height of a tree Polarity – the polarity of water molecules makes it easy for many substances to dissolve within it Water forms an unusual solid: Ice, which is less dense than liquid water Water-based solutions can be acidic, neutral or basic depending upon concentrations of H+ and OH- ions Chapter 3 3.1 Why is Carbon So Important to Life? Carbon has four electrons in its outermost shell, and as such can form multiple bonds Organic molecules have repeating C-H units; inorganic molecules are simpler Functional groups account for many of the properties of organic molecules 3.2 How are Organic Molecules Synthesized? Small organic molecules (called monomers) are joined to form longer molecules (called polymers) Formed by dehydration synthesis (removal of water) Broken by hydrolysis (addition of water) All organic molecules are either carbohydrates, lipids, proteins or nucleic acids 3.3-6 What are Carbohydrates, Lipids, Proteins and Nucleic Acids? • Carbohydrate molecules are composed of C, H, and O in the ratio of 1:2:1; source of energy – Glucose, deoxyribose, ribose, cellulose • Lipids are large chains of nonpolar hydrocarbons, mostly hydrophobic – Fats, oils, waxes and hormones – Phospholipids: basic components of cell membranes – Proteins are molecules composed of chains of amino acids; have many functions: structural and enzymes – Nucleic acids (RNA and DNA) are composed of a phosphate group, a sugar and a nitrogen base; serve as the molecules of heredity. Chapter 4 4.1 What is the Cell Theory? Every organism is made up of one or more cells The smallest organisms are single cells; cells are the functional units of multicellular organisms 3 All cells arise from pre-existing cells 4.2 What are the basic attributes of cells? All cells have a plasma membrane that separates the inside from the outside, regulates the flow of materials and may serve to communicate with other cells All cells contain cytoplasm, site of most metabolic processes All cells use DNA as the hereditary molecule, and RNA to copy this information to construct proteins Eukaryotic cells have membrane bound organelles (e.g. the nucleus; prokaryotic cells are smaller and do not 4.3 What are the Major Features of Eukaryotic Cells? Eukaryotic cells form the bodies of animals, plants, fungi and protists The cytoskeleton provides shape, support and movement The nucleus houses DNA, the nucleolus (ribosome production) The endomembrane system is associated with the creation and movement of many cell substances, e.g. proteins, hormones Plant cells have several unique structures: cell walls, chloroplasts, plastids, Central Vacuole 4.4 What are the Major Features of Prokaryotic Cells? Prokaryotic cells lack membrane bound organelles, have specialized surface features The nucleoid is region similar in function to nucleus Most prokaryotic cells also have plasmids (small rings of DNA with specific functions) Chapter 5 5.1 How is the Structure of the Cell Membrane Related to Its Function? Cell membranes are composed of phospholipid bilayers, which isolate the cell’s contents, regulate the exchange of materials, allow for communication, and may create attachments Membranes are fluid mosaics – the phospholipid molecules are not bound to one another, allowing proteins to move freely A variety of proteins suspended in the membrane account for its properties 5.2 How Do Substances Move Across Membranes? Transport may be passive or energy requiring Passive transport may occur by means of simple diffusion, facilitated diffusion or osmosis Energy requiring transport may be active transport (individual small molecules), endocytosis (liquids) or exocytosis (large molecules) 5.3 How Do Specialized Junctions Allow Cells to Connect and Communicate? Desmosomes attach cells together in tissues where cells are repeatedly stretched (e.g. skin) Tight junctions are used to prevent leakage 4 Gap junctions are used to coordinate metabolic activities of adjacent cells Plasmodesmata are channels that link adjacent plant cells Chapter 6 6.1 What is Energy? Energy is the capacity to do work, where work is the transfer of energy to an object causing it to move Chemical energy is the energy within the bonds of molecules Potential energy = stored energy; Kinetic energy is energy of movement First Law of Themodynamics: Energy can neither be created nor destroyed Second Law of Themodynamics: Whenever energy is converted from one form to another, the amount of useful energy decreases 6.2 How is Energy Transformed During Chemical Reactions? A chemical reaction either forms or breaks chemical bonds between atoms, converting one set of substances (reactants) to another (products) Exergonic: products have less energy than reactants; endergonic, products have more energy 6.3 How Is Energy Transported Within Cells? Energy carrier molecules (e.g. ATP) are synthesized at the site of exergonic reactions and release energy at site of endergonic reactions Coupled reactions are reactions in which one reaction provides the energy for another 6.4 How Do Enzymes Promote Biochemical Reactions? Enzymes (and Catalysts in general) reduce the activation energy of the reaction 6.5 How Are Enzymes Regulated? Cells regulate metabolic pathways by controlling both enzyme synthesis and also enzyme activity (competitive or non- competitive inhibition) BIOS 1120 UNIT 2 EXAM STUDY GUIDE Chapter 9 9.1 Cells divide to replace cells and perpetuate the species: Cell division involves the transmission of hereditary information contained in DNA to daughter cells Cell division is the basis for both asexual and sexual reproduction 9.2 The prokaryotic cell cycle is very simple The DNA of prokaryotes consists of a single, circular chromosome The replicated DNA attaches to the plasma membrane Cell division occurs by means of prokaryotic fission 5 9.3 Eukaryotic chromosomes are organized differently Multiple chromosomes are contained within the nucleus Usually exist as non-identical pairs of linear DNA double helices bound to proteins (diploid = 2 versions of each chromosome) Not all cells are diploid; haploid cells containing one member of each pair (gametes) play role in sexual reproduction 9.4 The eukaryotic cell cycle consists of interphase and mitosis Interphase: period of growth, differentiation and replication of DNA Mitotic cell division involves mitosis and cytokinesis 9.5 Mitotic cell division leads to creation of identical daughter cells Prophase: chromosomes condense and spindle fibers form Metaphase: chromosomes line up along cell equator Anaphase: sister chromatids representing same chromosome split Telophase: chromatids reach poles, nuclear envelope reforms Cytokinesis: cytoplasm divides between sister cells 9.6 The Cell Cycle is controlled by multiple mechanisms Hormones (growth factors) stimulate cell division by controlling the synthesis of proteins (cyclins); checkpoints also regulate the process 9.7 Sexual reproduction produces genetically unique offspring Occurs by means of shuffling genes contained in parents, mutations 9.8 Meiosis leads to the production of haploid cells (gametes) The fusion of haploid gametes (fertilization) restores the diploid no. Meiosis is divided into two stages: Meiosis I: separates homologous chromosomes, sending one homologue into each of two haploid daughter cells Meiosis II: separates identical chromatids from one another, creating four haploid daughter cells 9.9 Mitotic & Meiotic cell division occur at different stages Life cycle of eukaryotic organisms begins with fertilization (union of haploid gametes); resulting diploid zygote undergoes mitotic cell division and subsequent differentiation Reproduction of new individuals may involve haploid cells Eukaryotic organisms differ in how much of their life cycle is spent primary diploid (most animals) and haploid (most fungi), many plant species alternate between two 9.10 Genetic variability is the result of the shuffling of homologues, crossing over and the fusion of gametes Chapter 10 6 10.1 The physical basis of inheritance is the process by which information coding for traits (genes) is passed from parent to offspring Genes are located at particular locations (loci) on chromosomes Different forms of genes (alleles) differ in how they code traits Offspring inherit two alleles, one from each parent If these alleles are identical, the offspring is homozygous If these alleles differ, the offspring is heterozygous 10.2 Gregor Mendel discovered the principles of inheritance Chose right organism, designed appropriate experiment, analyzed results properly Worked with edible pea that self-fertilizes Mated different pea plants by means of cross fertilization 10.3 Mendel first worked on single traits True breeding organisms always give rise to offspring that look like parents (homologous) Crossing true breeding organisms that have different versions of the trait allows one to determine which allele is dominant and which is recessive (dominant alleles mask the presence of the recessive allele) Law of segregation: each individual has two copies of a gene (one from each parent), and the gametes that individual creates contain only one copy. Genotype: which two alleles the individual has vs. o Phenotype: what the individual looks like Punnett Square Method: way to keep track of genotypes and resulting phenotypes of offspring that can be used to predict outcomes of crosses 10.4 Mendel then considered how multiple traits are inherited Law of independent assortment: when considering multiple traits, the allele the offspring receives from its parent for one trait is inherited independently from the allele the offspring will receive from that parent for other traits. The significance of Mendel’s research was not recognized until decades later 10.5 Mendelian rules do not apply to all traits Incomplete dominance: the presence of one allele may not completely hide the presence of the other A single trait may have multiple alleles Codominance: the presence of both alleles is expressed Polygenetic inheritance: many traits are result of many genes Pleiotropy: a single gene may influence more than one trait The environment may also influence the expression of genes 7 10.6 Gene linkage: genes on same chromosome tend to be inherited together BUT crossing over between sister chromosomes during meiosis can lead to the exchange of material (genetic recombination). The greater the distance between genes on the chromosome, the greater chance of crossing over. 10.7 Sex and sex-linked traits have different patterns of inheritance The sex of an offspring is determined by the sex chromosome inherited from the male parent (XX = female, XY = male). Sex-linked traits are due to genes located on the sex chromosomes. 10.8 Human genetic disorders are inherited by means of a variety of mechanisms o Including single genes, and inheritance of abnormal nos. of chromosomes Chapter 11 11.1 Scientists discovered genes are made of DNA by series of elegant investigations Griffith established that something contained in a dead disease causing strain of bacteria could be transferred to a weakened strain Avery, MacLeod and McCarty discovered the transformative agent is DNA 11.2 The structure of DNA allows it to encode genetic information DNA consists of long chains of subunits (nucleotides) Nucleotides consist of a sugar, a phosphate and a base There are only four bases in DNA (Adenine (A), Thymine (T), Guanine (G) and Cytosine (C)) Chargraff discovered that regardless of the species, the amount of A = T, and the amount of G = C Watson and Crick determined DNA is a double helix of two nucleotide strands held together by hydrogen bonds 11.3 Genetic information is encoded by the sequence of bases 11.4 The replication of DNA ensures genetic constancy The process of DNA replication involves the separation of the two strands by DNA helicases and the formation of two new strands using the original strands as templates Semiconservative replication: DNA replication conserves each of the parental strands 11.5 Mutations occur as a result of errors in replication 8 While the process of DNA replication includes proof reading mechanisms, it is not entirely perfect Changes may include nucleotide substitutions, point mutations, insertion mutations and deletion mutations Chapter 31 31.1 Animals must regulate their internal environments owing to internal and external changes The external environment may change (e.g. temperature) The internal environment may change (e.g. the build up of wastes or the depletion of nutrients) Homeostasis refers to the ability of an organism to maintain its internal environment despite these changes Feedback mechanisms regulate internal conditions Positive feedback mechanism: exacerbates change Negative feedback mechanism (most common): returns system back to normal All feedback mechanisms involve a sensor, a control center and an effector 31.2 The animal body is composed of cells, tissues, organs and organ systems Cells are the basic units of life Tissues are composed of cells similar in structure and function Epithelial tissues: sheets of cells that line body cavities, glands Connective tissues: tissues that include extracellular matrix – tendons, bone, blood Muscle tissue: tissues with ability to contract – skeletal, smooth and cardiac Nerve tissue: tissues that can transmit electrical signals – brain, neurons Organs are two or more interacting tissues- e.g. skin Organ systems are two or more interacting organs Chapter 41 41.1 Animals reproduce both asexually and sexually Asexual reproduction (budding, regeneration, fission, parthenogenesis) results in clones Sexual reproduction involves union of sperm and egg May involve one individual, but usually two External fertilization: union of egg and sperm is external, with sperm and egg being deposited in water by means of spawning Internal fertilization: union of egg and sperm is within the body fo female 41.2 Human reproductive systems play role in both reproduction and health of organism 9 Puberty – age at which individual becomes capable of reproducing In both sexes begins in hypothalamus with release of GnRH, which stimulates the pituitary to produce LH and FSH In males, sperm are produced by means of spermatogenesis In females, eggs are produced by means of oogenesis 41.3 Pregnancies can be prevented by a variety of means Sterilization (blocking the pathway by which sperm or egg travels) Temporary birth control Prevent ovulation – e.g. the pill Prevent fertilization – e.g. condoms Prevent implantation – e.g. IUD Some methods of birth control are NOT effective against STDs (e.g. the pill); other are (e.g. condoms) The only complete protection against pregnancy and STDs is absolute abstinence Chapter 43 43.1 Plant bodies, like animal bodies, are composed of cells, tissues, organs and organ systems Root System – under the ground, serves to: anchor plant; absorb water and minerals; transports water, minerals and hormones; stores surplus sugars and starches; and interacts with soil fungi and bacteria that help plant acquire nutrients Shoot System – above ground, serves to: capture sunlight energy; synthesize sugars during photosynthesis; transports materials to all parts of plant; stores surplus sugars and starches; reproduce; and produce hormones. The plant kingdom is divided into monocots (e.g. grasses) and dicots (e.g. trees, bushes, vegetables and flowers) Monocots: single cotyledon (seed leaf); Dicots (two cotyledons) 43.2 Plants undergo both primary (e.g. height ) and secondary growth (e.g. diameter) 43.3 Plants are composed of multiple tissue types The Dermal tissue system covers the plant body, protects it from pathogens and prevents the loss of moisture The Vascular tissue system is composed of xylem (one way directional flow of water and minerals upwards) and phloem (two way directional flow of water and sugars) The Ground tissue system represents all other tissues of the plant body, including tissues used for storage, photosynthesis and the secretion of hormones 43.4 Leaves play a primary role in photosynthesis and the exchange of gases (CO a2d O ) wi2h the atmosphere 10 43.5 Stems are composed primarily of vascular tissue and are the site of primary & secondary growth and the support of other structures 43.6 Roots transport minerals and water from the soil 43.7 Plants need only inorganic nutrients (CO and O 2rom the2 atmosphere), trace minerals from the soil 1.Trace minerals are absorbed from soil water into a living cell by active transport against a concentration gradient 2.These minerals are then transported into interconnected living cells by means of plasmodesmata 3.Minerals then exit the living cell by means of active transport or diffusion into the xylem Water enters the root hair passively via osmosis Nitrogen fixing bacteria contained in the roots of some plants are a source of usable nitrogen by plants 43.8 The flow of water in xylem within the body of the plant is due to the cohesion-tension mechanism Transpiration (the evaporation of water from the leaves of the plants) is the basic driving force for movement According to the cohension-tension mechanism, water is pulled up as a result of two processes: cohesion between water molecules owing to hydrogen bonding and tension produced by the evaporation of water from the leaves Guard cells responsible for opening and closing the stomata regulate the escape of both water vapor and gas exchange 43.9 The transport of sugars in the plant occurs via the pressure-flow mechanism Differences in water pressure, owing to the creation of sugars in sources (e.g. leaves) are the basic driving force for movement of materials towards sinks (e.g. fruits) Chapter 44 44.1 Plants reproduce both asexually and sexually Alternation of Generations describes the sexual reproductive cycle of plants because they alternate between multicellular diploid (sporophyte) and multicellular haploid (gametophyte) generations 44.2 Angiosperms have flowers that typically contain both male (stamen) and female (carpel) reproductive structures Flowers represent a mechanism by which flowers attract (via colors, scent and food) animal pollinators Pollination is the process by which the male spore (pollen) is transferred from the male reproductive organ of one flower to the female organ of another flower 11 Successful pollination results in the release of a sperm that fertilizes an egg produced by the female spore. The gametophyte in most familiar plant species is microscopic and exists within the female reproductive organ 44.3 Fruits develop from the ovary; seeds from the ovule The fruit is a means by which plants entice animals to distribute their seeds away from the parental plant The seed contains stored energy for use by the developing embryo. It initially protects the embryo 44.3 Seeds germinate after a period of dormancy, often involving cold and drying. 44.4 Plants attract pollinators by means of a variety of mechanisms, including color, smell and food. 44.5 Fruits aid plants in the dispersal of seeds Some fruits directly disperse seeds (e.g. carried by wind) Other fruits indirectly disperse seeds by attaching to animals; or in the case of edible fruits, enticing them to carry the seeds away BIOS 1120 UNIT 3 EXAM STUDY GUIDE Chapter 28 28.1 Matter cycles and energy are continually exchanged between the living and non-living components of the environment All ecosystems consist of biotic (living) components (e.g. bacteria, plants, animal and fungi) and abiotic (non-living) components (e.g. climate, temperature, light, water and minerals in the soil) 28.2 Energy flows in ecosystems Energy makes a one way journey through ecosystems. Radiant energy from the sun is captured by the process of photosynthesis into chemical energy for use by plant cells, and thereby indirectly made available as a source of energy for other organisms. Ecologists study ecosystems in terms of trophic levels. Autotrophs (e.g. plants) make their own food and are referred to as producers because their activities make energy available for other organisms in the ecosystem. Heterotrophs (e.g. animals, bacteria) are organisms that get their energy from other organisms, and are referred to as consumers because they use energy taken from producers or other consumers. Food chains depict the flow of energy linearly from producers to consumers; food webs draw attention to the fact that organisms may be part of more than one food chain and have different roles. Decomposers and detritovores play a particularly important role in ecosystems, because they breakdown the remains of organisms 12 (which they use as a source of food), returning nutrients back to the abiotic environment The flow of energy between trophic levels is very inefficient, because with each conversion, part is converted to heat energy that is no longer usable. Biological magnification refers to the fact that because consumers at higher trophic levels must eat many times their own weight in order to live, trace toxic substances such as mercury tend to accumulate in their tissues 28.3 Matter cycles in ecosystems In contrast to energy, matter endlessly cycles between biotic and abiotic components of the environment. Atoms enter and leave associations with other atoms as molecules are formed and broken down, but at a fundamental level the atom is never changed by these interactions. The living world is composed primarily of macronutrients (oxygen, carbon, hydrogen, nitrogen, phosphorus), and micronutrients (trace amounts of other minerals, e.g. iron). The hydrological (water) cycle is driven by solar energy, only a small fraction of water is part of the living world. The movement of other macronutrient cycles is driven largely by the activities of living organisms. 28.4 Human activities disrupt nutrient cycles Human activities, particularly the burning of fossil fuels and the large scale use of fertilizers, play a particularly disruptive role. Chapter 26 26.1 Ecology is the study of the interactions between a population and the abiotic and biotic components of the environment. Populations consist of all members of a particular species within an ecosystem; communities consist of multiple species that cohabitate in a given area. Changes in the size of a population may reflect its natural rate of increase (r), immigration, emigration, birth and death rates. Exponential growth occurs whenever a constant rate of growth occurs and is depicted by a J-curve. 26.2 Population growth is limited by environmental resistance Environmental resistance refers to all curbs on a population’s growth, including biotic factors (e.g. predators, competitors) and also abiotic factors (e.g. freezing weather, droughts). Density-independent: limits population regardless of its density (e.g. weather); Density-dependent: effectiveness increases as density of population rises (e.g limited availability of nutrients, predators, communicable diseases) 26.3 Populations vary in how they are distributed in space and age. Spatial distributions can be clumped, uniform or random. Age distributions can be characterized as late-loss, constant loss or early loss. 26.4 Human population growth continues to be rapid 13 During the past few hundred years, human population growth has been exponential, owing to advances in technology, agriculture and medicine. This rapid growth is not uniform. In developed countries, birth rates are declining; developing countries, in contrast, have much higher birth rates The age structure of a population is useful in predicting future growth The demands human population growth are placing on the planet are unsustainable Chapter 14 14.1 Early biological thought did not include the concept of evolution. Evolution refers to a change in the characteristics of a population over time. Species were once believed to be created simultaneously by God at one time. Voyages of discovery and a growing recognition that the earth is ancient and fossils represent organisms that no longer exist led scientists to speculate that live had evolved. Darwin and Wallace proposed a plausible mechanism for how evolution has occurred: natural selection. 14.2 Natural selection requires four factors 1. Organisms in a population vary in ways that affect their ability to survive and reproduce; 2. Some of these differences are inheritable; 3. Not all who are born survive to adulthood and reproduce; and 4. The likelihood an organism survives and reproduces is not due to chance alone. 14.3 There is abundant evidence that evolution has occurred. The fossil record includes evidence of intermediate forms for those organisms with hard body parts. Comparative anatomy indicates similarities among modern organisms that represent modified versions of more basic ancestral plans. Homologous structures (similar structures that may have different functions) are evidence of common ancestry (e.g the arm of a man and the front leg of a dog); analogous structures are structures that have similar functions in the absence of shared ancestry that have arose via convergence (e.g the wing of a bird and a butterfly). Vestigial structures, embryological similarities and biochemical and genetic tests provide additional evidence. 14.4 There is evidence that natural selection has and continues to operate Artificial selection refers to breeding by humans, field experiments on a variety of organisms document natural selection. Chapter 16 16.1 Species are groups of populations that evolve independently. 14 According to the biological species concept, a species is defined in terms of reproductive isolation (the ability of individuals in a population to successfully mate). While it is useful for many familiar species, it is not useful for asexually reproducing organisms or extinct groups. Similarity per se can be misleading. Organisms that look similar to one another might be distinct species; organisms that look quite different might nevertheless be part of the same species. 16.2 Isolating mechanisms prevent interbreeding Premating isolating mechanisms (geographic, ecological, temporal, behavior isolation and mechanical incompatibility) prevent mating from occurring at all. Postmating isolating mechanisms (genetic incompatibility, hybrid inviability and hybrid infertility) prevent production of fertile offspring after mating. 16.3 Speciation is the process by which new species form. It may occur by a variety of means, but invariably it involves a two step process: isolation of populations and genetic divergence (the build up of genetic differences between them) Allopatric speciation occurs when a geographic barrier of some sort prevents interbreeding; sympatric speciation occurs when genetic divergence occurs in the absence of a geographic barrier. Adaptive radiation may occur when many new species arise in a short period of time- usually when a species colonizes a new area with unoccupied habitats. 16.4 Extinction occurs as a result of a variety of different causes Localized distribution, overspecialization, interactions with other species, and habitat change. Chapter 17 17.1 Life on our planet began over four billion years ago. Historically many thought that new members of species might on a regular basis by means of spontaneous generation. This was disproven by Redi and later Pasteur and Tyndall. Modern scientific ideas on the subject begin with Oparin and Haldane, who drew attention to the fact that the early atmosphere of earth was quite different than it is now. Miller and Urey experimentally demonstrated that organic molecules could arise spontaneously in such an environment. Cech and Altman discovered a small RNA (ribonucleic acid) that can function as an enzyme (ribozyme). RNA was likely the first self-replicating molecule, the early earth might have been an RNA world of RNA molecules competing with one another There is also evidence that proteins and lipids shaken up to simulate the action of waves form vesicles. The first cell (protocell) might have consisted of a ribozyme enclosed by such a membrane. 17.2 The earliest organisms were anaerobic prokaryotes. 15 They were unicellular, lived in the ocean and obtained energy by absorbing organic molecules in their environment. The evolution of photosynthesis about 3.5 billion years ago allowed some organisms to harness the energy of sunlight for the creation of complex, high energy molecules. This resulted in the release of large amounts of oxygen, a highly reactive molecule. The accumulation of oxygen in the atmosphere starting about 2.3 billion years ago led to massive extinctions of many anaerobic species and also the formation of the ozone layer, which protects life on earth from solar radiation. The dangers of oxygen led to the evolution of mechanisms to detoxify oxygen, and these mechanisms ultimately were coupled to reactions that form the basis of aerobic respiration According to the endosymbiont hypothesis, eukaryotes (cells with membrane bound organelles) arose by virtue of an anaerobic bacteria engulfing an aerobic bacteria. 17.3 The first multicellular organisms (algae) arose about 1.2 billion years ago. Multicellularity made them less vulnerable to predators, allowed individual cells to specialize. The first animals evolved 630 million years ago, resembled sponges and jellyfish. Predation amongst them favored the development of improved motility and senses, exoskeletons and later internal skeletons. 17.4 The first organisms to colonize land were plants, approximately 400 million years ago. Plants had to develop mechanisms to overcome the force of gravity in the absence of water to support them, the possibility of drying out, and also mechanisms to facilitate reproduction, including pollen, seeds, and later flowers and fruit. Arthropods were the first animals to colonize land, followed by lobefin fishes, the progenitors of modern amphibians, reptiles birds and mammals. 17.5 Mass extinctions have played a particularly important role in the evolution of live on our planet. The history of life on our planet can be seen in some ways as a series of succeeding dynasties. Mass extinction events have led one dynasty after another to fall, most of these have been the result of climate change. This includes changes due to plate tectonics, but also meteorites. 17.6 Humans evolved from primates (lemurs, monkeys and apes). Several key adaptations: binocular vision, grasping hands, a large brain, bipedal locomotion, made it possible for our hominoid ancestors to survive and thrive. Most branches of the hominid family have gone extinct. Modern humans emerged less than 200,000 years ago. Biological and cultural evolution continues in humans to this day. 16
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