In a natural classification, the genus and accompanying higher taxa consist of all the species that have evolved from one common ancestral species. Taxonomists sometimes reclassify groups of species when new evidence shows that a previous taxon contains species that have evolved from different ancestral species.
Natural classifications help in identification of species and allow the prediction of characteristics shared by species within a group. Cladistics 5. Evidence for which species are part of a clade can be obtained from the base sequences of a gene or the corresponding amino acid sequence of a protein.
Sequence differences accumulate gradually so there is a positive correlation between the number of differences between two species and the time since they diverged from a common ancestor.
Traits can be analogous or homologous. Cladograms are tree diagrams that show the most probable sequence of divergence in clades. Evidence from cladistics has shown that classifications of some groups based on structure did not correspond with the evolutionary origins of a group or species.
The pancreas secretes enzymes into the lumen of the small intestine. Enzymes digest most macromolecules in food into monomers in the small intestine. Villi increase the surface area of epithelium over which absorption is carried out. Villi absorb monomers formed by digestion as well as mineral ions and vitamins. Different methods of membrane transport are required to absorb different nutrients.
The blood system 6. Arteries have muscle cells and elastic fibres in their walls. The muscle and elastic fibres assist in maintaining blood pressure between pump cycles. Blood flows through tissues in capillaries.
Capillaries have permeable walls that allow exchange of materials between cells in the tissue and the blood in the capillary. Veins collect blood at low pressure from the tissues of the body and return it to the atria of the heart. Valves in veins and the heart ensure circulation of blood by preventing backflow.
There is a separate circulation for the lungs. The heart beat is initiated by a group of specialized muscle cells in the right atrium called the sinoatrial node.
The sinoatrial node acts as a pacemaker. The sinoatrial node sends out an electrical signal that stimulates contraction as it is propagated through the walls of the atria and then the walls of the ventricles. The heart rate can be increased or decreased by impulses brought to the heart through two nerves from the medulla of the brain.
Epinephrine increases the heart rate to prepare for vigorous physical activity. Defense against infectious disease 6. Cuts in the skin are sealed by blood clotting. Clotting factors are released from platelets. The cascade results in the rapid conversion of fibrinogen to fibrin by thrombin.
Ingestion of pathogens by phagocytic white blood cells gives non-specific immunity to diseases. Production of antibodies by lymphocytes in response to particular pathogens gives specific immunity. Antibiotics block processes that occur in prokaryotic cells but not in eukaryotic cells. Viruses lack a metabolism and cannot therefore be treated with antibiotics. Some strains of bacteria have evolved with genes that confer resistance to antibiotics and some strains of bacteria have multiple resistance.
Gas exchange 6. Type I pneumocytes are extremely thin alveolar cells that are adapted to carry out gas exchange. Type II pneumocytes secrete a solution containing surfactant that creates a moist surface inside the alveoli to prevent the sides of the alveolus adhering to each other by reducing surface tension.
Air is carried to the lungs in the trachea and bronchi and then to the alveoli in bronchioles. Muscle contractions cause the pressure changes inside the thorax that force air in and out of the lungs to ventilate them.
Different muscles are required for inspiration and expiration because muscles only do work when they contract. Neurons and synapses 6. The myelination of nerve fibres allows for saltatory conduction.
Neurons pump sodium and potassium ions across their membranes to generate a resting potential. An action potential consists of depolarization and repolarization of the neuron. Nerve impulses are action potentials propagated along the axons of neurons. Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential.
Synapses are junctions between neurons and between neurons and receptor or effector cells. When presynaptic neurons are depolarized they release a neurotransmitter into the synapse. A nerve impulse is only initiated if the threshold potential is reached. Hormones, homeostasis and reproduction 6. Thyroxin is secreted by the thyroid gland to regulate the metabolic rate and help control body temperature.
Leptin is secreted by cells in adipose tissue and acts on the hypothalamus of the brain to inhibit appetite. Melatonin is secreted by the pineal gland to control circadian rhythms. A gene on the Y chromosome causes embryonic gonads to develop as testes and secrete testosterone. Testosterone causes pre-natal development of male genitalia and both sperm production and development of male secondary sexual characteristics during puberty.
Estrogen and progesterone cause pre-natal development of female reproductive organs and female secondary sexual characteristics during puberty.
The menstrual cycle is controlled by negative and positive feedback mechanisms involving ovarian and pituitary hormones. DNA polymerases can only add nucleotides to the 3' end of a primer. DNA replication is continuous on the leading strand and discontinuous on the lagging strand. DNA replication is carried out by a complex system of enzymes. Some regions of DNA do not code for proteins but have other important functions.
Nucleosomes help to regulate transcription in eukaryotes. Eukaryotic cells modify mRNA after transcription. Splicing of mRNA increases the number of different proteins an organism can produce. Gene expression is regulated by proteins that bind to specific base sequences in DNA.
The environment of a cell and of an organism has an impact on gene expression. Synthesis of the polypeptide involves a repeated cycle of events. Disassembly of the components follows termination of translation.
Free ribosomes synthesize proteins for use primarily within the cell. Bound ribosomes synthesize proteins primarily for secretion or for use in lysosomes. Translation can occur immediately after transcription in prokaryotes due to the absence of a nuclear membrane. The sequence and number of amino acids in the polypeptide is the primary structure.
The secondary structure is the formation of alpha helices and beta pleated sheets stabilized by hydrogen bonding. The tertiary structure is the further folding of the polypeptide stabilized by interactions between R groups.
The quaternary structure exists in proteins with more than one polypeptide chain. Want to build the best possible college application? Dora Seigel. About the Author.
Search the Blog Search. Find Out How. Get the latest articles and test prep tips! Looking for Graduate School Test Prep? According to the cell theory , living organisms are composed of cells.
Prokaryotes have a simple cell structure without compartmentalization. Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules. Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport. Cells can only be formed by division of pre-existing cells. Mitosis is division of the nucleus into two genetically identical daughter nuclei.
Molecular biology explains living processes in terms of the chemical substances involved. Water molecules are polar and hydrogen bonds form between them. Monosaccharide monomers are linked together by condensation reactions to form disaccharides and polysaccharide polymers.
Amino acids are linked together by condensation to form polypeptides. Enzymes have an active site to which specific substrates bind. The replication of DNA is semi-conservative and depends on complementary base pairing.
Cell respiration is the controlled release of energy from organic compounds to produce ATP. Photosynthesis is the production of carbon compounds in cells using light energy. A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic.
Prokaryotes have one chromosome consisting of a circular DNA molecule. One diploid nucleus divides by meiosis to produce four haploid nuclei. Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed.
Gel electrophoresis is used to separate proteins or fragments of DNA according to size. Species are groups of organisms that can potentially interbreed to produce fertile offspring. Most ecosystems rely on a supply of energy from sunlight.
Autotrophs convert carbon dioxide into carbohydrates and other carbon compounds. Carbon dioxide and water vapour are the most significant greenhouse gases.
Evolution occurs when heritable characteristics of a species change. Natural selection can only occur if there is variation among members of the same species. The binomial system of names for species is universal among biologists and has been agreed and developed at a series of congresses. A clade is a group of organisms that have evolved from a common ancestor. The contraction of circular and longitudinal muscle of the small intestine mixes the food with enzymes and moves it along the gut.
Arteries convey blood at high pressure from the ventricles to the tissues of the body. The skin and mucous membranes form a primary defense against pathogens that cause infectious disease. Ventilation maintains concentration gradients of oxygen and carbon dioxide between air in alveoli and blood flowing in adjacent capillaries. Neurons transmit electrical impulses.
Nucleosomes help to supercoil the DNA. Transcription occurs in a 5' to 3' direction. Initiation of translation involves assembly of the components that carry out the process. Metabolic pathways consist of chains and cycles of enzyme-catalysed reactions. Enzymes lower the activation energy of the chemical reactions that they catalyse. Enzyme inhibitors can be competitive or non-competitive. Metabolic pathways can be controlled by end-product inhibition.
Cell respiration involves the oxidation and reduction of electron carriers. Phosphorylation of molecules makes them less stable. In glycolysis, glucose is converted to pyruvate in the cytoplasm. Glycolysis gives a small net gain of ATP without the use of oxygen. In aerobic cell respiration pyruvate is decarboxylated and oxidized, and converted into acetyl compound and attached to coenzyme A to form acetyl coenzyme A in the link reaction.
In the Krebs cycle, the oxidation of acetyl groups is coupled to the reduction of hydrogen carriers, liberating carbon dioxide. Transfer of electrons between carriers in the electron transport chain in the membrane of the cristae is coupled to proton pumping. Oxygen is needed to bind with the free protons to maintain the hydrogen gradient, resulting in the formation of water. The structure of the mitochondrion is adapted to the function it performs.
Light-dependent reactions take place in the intermembrane space of the thylakoids. Light-independent reactions take place in the stroma. Absorption of light by photosystems generates excited electrons.
Photolysis of water generates electrons for use in the light-dependent reactions. Transfer of excited electrons occurs between carriers in thylakoid membranes. Excited electrons from Photosystem II are used to contribute to generate a proton gradient.
In the light-independent reactions a carboxylase catalyses the carboxylation of ribulose bisphosphate. Triose phosphate is used to regenerate RuBP and produce carbohydrates.
Ribulose bisphosphate is reformed using ATP. The structure of the chloroplast is adapted to its function in photosynthesis. Transpiration is the inevitable consequence of gas exchange in the leaf. Plants transport water from the roots to the leaves to replace losses from transpiration.
The cohesive property of water and the structure of the xylem vessels allow transport under tension. The adhesive property of water and evaporation generate tension forces in leaf cell walls. Active uptake of mineral ions in the roots causes absorption of water by osmosis.
Plants transport organic compounds from sources to sinks. Incompressibility of water allows transport along hydrostatic pressure gradients. Active transport is used to load organic compounds into phloem sieve tubes at the source. High concentrations of solutes in the phloem at the source lead to water uptake by osmosis.
Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks. Undifferentiated cells in the meristems of plants allow indeterminate growth.
Mitosis and cell division in the shoot apex provide cells needed for extension of the stem and development of leaves. Plant hormones control growth in the shoot apex. Plant shoots respond to the environment by tropisms. Auxin efflux pumps can set up concentration gradients of auxin in plant tissue. Auxin influences cell growth rates by changing the pattern of gene expression. Flowering involves a change in gene expression in the shoot apex. The switch to flowering is a response to the length of light and dark periods in many plants.
Success in plant reproduction depends on pollination, fertilization and seed dispersal. Most flowering plants use mutualistic relationships with pollinators in sexual reproduction.
Chromosomes replicate in interphase before meiosis. Crossing over is the exchange of DNA material between non-sister homologous chromatids. Crossing over produces new combinations of alleles on the chromosomes of the haploid cells. Chiasmata formation between non-sister chromatids can result in an exchange of alleles. Homologous chromosomes separate in meiosis I. Sister chromatids separate in meiosis II.
Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I. Gene loci are said to be linked if on the same chromosome. Unlinked genes segregate independently as a result of meiosis.
Variation can be discrete or continuous. The phenotypes of polygenic characteristics tend to show continuous variation. Chi-squared tests are used to determine whether the difference between an observed and expected frequency distribution is statistically significant.
A gene pool consists of all the genes and their different alleles, present in an interbreeding population. Evolution requires that allele frequencies change with time in populations. Reproductive isolation of populations can be temporal, behavioural or geographic. Speciation due to divergence of isolated populations can be gradual.
Speciation can occur abruptly. Every organism has unique molecules on the surface of its cells. Pathogens can be species-specific although others can cross species barriers. B lymphocytes are activated by T lymphocytes in mammals. Activated B cells multiply to form clones of plasma cells and memory cells. Plasma cells secrete antibodies. Antibodies aid the destruction of pathogens. White cells release histamine in response to allergens.
Histamines cause allergic symptoms. Immunity depends upon the persistence of memory cells. Vaccines contain antigens that trigger immunity but do not cause the disease. Fusion of a tumour cell with an antibody-producing plasma cell creates a hybridoma cell.
Monoclonal antibodies are produced by hybridoma cells. Bones and exoskeletons provide anchorage for muscles and act as levers. Synovial joints allow certain movements but not others. Movement of the body requires muscles to work in antagonistic pairs. Skeletal muscle fibres are multinucleate and contain specialized endoplasmic reticulum.
Muscle fibres contain many myofibrils. Each myofibril is made up of contractile sarcomeres. The contraction of the skeletal muscle is achieved by the sliding of actin and myosin filaments. Buy, rent or sell. Rutherford Williams. Oxford University Press. ISBN 0 19 5. Disk Save to Yandex. Disk Download all. Oxford IB Download your answers. We ve updated a few pages in the second printing of this text. Download the addendum to the first printing. Create beautiful designs with your team Use Canva's drag-and-drop feature and layouts to design, share and print business cards, logos, presentations and more.
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