A Unity and Diversity

1 Molecules

A1.1 Water

• A1.1.1—Water as the medium for life (SL and HL)
• A1.1.2—Hydrogen bonds as a consequence of the polar covalent bonds within water molecules (SL and HL)
• A1.1.3—Cohesion of water molecules due to hydrogen bonding and consequences for organisms (SL and HL)
• A1.1.4 —Adhesion of water to materials that are polar or charged and impacts for organisms (SL and HL)
• A1.1.5 —Solvent properties of water linked to its role as a medium for metabolism and for transport in plants and animals (SL and HL)
• A1.1.6—Physical properties of water and the consequences for animals in aquatic habitats (SL and HL)

A1.2 Nucleic acids

• A1.2.1—DNA as the genetic material of all living organisms (SL and HL)
• A1.2.9—Diversity of possible DNA base sequences and the limitless capacity of DNA for storing information (SL and HL)
• A1.2.10—Conservation of the genetic code across all life forms as evidence of universal common ancestry (SL and HL)

2 Cells

A2.3 Viruses

• A2.3.1—Structural features common to viruses (AHL)

3 Organisms

A3.2 Classification and Cladistics

• A3.2.3—Advantages of classification corresponding to evolutionary relationships (AHL)
• A3.2.4—Clades as groups of organisms with common ancestry and shared characteristics (AHL)
• A3.2.5—Gradual accumulation of sequence differences as the basis for estimates of when clades diverged from a common ancestor (AHL)
• A3.2.6—Base sequences of genes or amino acid sequences of proteins as the basis for constructing cladograms (AHL)
• A3.2.7—Analysing cladograms (AHL)
• A3.2.8—Using cladistics to investigate whether the classification of groups corresponds to evolutionary relationships (AHL)

4 Ecosystems

A4.1 Evolution and speciation

• A4.1.2—Evidence for evolution from base sequences in DNA or RNA and amino acid sequences in proteins (SL and HL)
• A4.1.3—Evidence for evolution from selective breeding of domesticated animals and crop plants (SL and HL)
• A4.1.9—Adaptive radiation as a source of biodiversity (AHL)

A4.2 Conservation of biodiversity

• A4.2.3—Causes of anthropogenic species extinction (SL and HL)
• A4.2.4—Causes of ecosystem loss (SL and HL)
• A4.2.5—Evidence for a biodiversity crisis (SL and HL)
• A4.2.6—Causes of the current biodiversity crisis (SL and HL)
• A4.2.7—Need for several approaches to conservation of biodiversity (SL and HL)

B Form and function

1 Molecules

B1.1 Carbohydrates and lipids

• B1.1.3—Digestion of polymers into monomers by hydrolysis reactions (SL and HL)
• B1.1.4—Form and function of monosaccharides (SL and HL)
• B1.1.5—Polysaccharides as energy storage compounds (SL and HL)
• B1.1.6—Structure of cellulose related to its function as a structural polysaccharide in plants (SL and HL)

B1.2 Proteins

• B1.2.1—Generalized structure of an amino acid (SL and HL)
• B1.2.2—Condensation reactions forming dipeptides and longer chains of amino acids (SL and HL)
• B1.2.5—Effect of pH and temperature on protein structure (SL and HL)
• B1.2.6—Chemical diversity in the R-groups of amino acids as a basis for the immense diversity in protein form and function (AHL)
• B1.2.7—Impact of primary structure on the conformation of proteins (AHL)
• B1.2.8—Pleating and coiling of secondary structure of proteins (AHL)

2 Cells

B2.1 Membranes and membrane transport

• B2.1.1—Lipid bilayers as the basis of cell membranes (SL and HL)
• B2.1.2—Lipid bilayers as barriers (SL and HL)
• B2.1.3—Simple diffusion across membranes (SL and HL)
• B2.1.5—Movement of water molecules across membranes by osmosis and the role of aquaporins (SL and HL)

B2.2 Organelles and compartmentalization

• B2.2.4—Adaptations of the mitochondrion for production of ATP by aerobic cell respiration (AHL)
• B2.2.5—Adaptations of the chloroplast for photosynthesis (AHL)

B2.3 Cell specialization

• B2.3.5—Cell size as an aspect of specialization (SL and HL)
• B2.3.6—Surface area-to-volume ratios and constraints on cell size (SL and HL)
• B2.3.7—Adaptations to increase surface area-to-volume ratios of cells (AHL)

3 Organisms

B3.1 Gas Exchange

• B3.1.5—Ventilation of the lungs (SL and HL)
• B3.1.6—Measurement of lung volumes (SL and HL)
• B3.1.7—Adaptations for gas exchange in leaves (SL and HL)
• B3.1.8—Distribution of tissues in a leaf (SL and HL)
• B3.1.9—Transpiration as a consequence of gas exchange in a leaf (SL and HL)
• B3.1.10—Stomatal density (SL and HL)

B3.2 Transport

• B3.2.3—Adaptations of arteries for the transport of blood away from the heart (SL and HL)
• B3.2.4—Measurement of pulse rates (SL and HL)
• B3.2.7—Transport of water from roots to leaves during transpiration (SL and HL)
• B3.2.8—Adaptations of xylem vessels for transport of water (SL and HL)
• B3.2.15—Adaptations of the mammalian heart for delivering pressurized blood to the arteries (AHL)
• B3.2.16—Stages in the cardiac cycle (AHL)
• B3.2.17—Generation of root pressure in xylem vessels by active transport of mineral ions (AHL)

B3.3 Muscle and motility

• B3.3.2—Sliding filament model of muscle contraction (SL and HL)
• B3.3.4—Structure and function of motor units in skeletal muscle (SL and HL)
• B3.3.7—Range of motion of a joint (SL and HL)
• B3.3.8—Internal and external intercostal muscles as an example of antagonistic muscle action to facilitate internal body movements (SL and HL)
• B3.3.10—Adaptations for swimming in marine mammals (SL and HL)

4 Ecosystems

B4.1 Adaptation to environment

• B4.1.1—Habitat as the place in which a community, species, population or organism lives (SL and HL)
• B4.1.2—Adaptations of organisms to the abiotic environment of their habitat (SL and HL)
• B4.1.3—Abiotic variables affecting species distribution (SL and HL)
• B4.1.4—Range of tolerance of a limiting factor (SL and HL)
• B4.2.11—Adaptations of plant form for harvesting light (SL and HL)

B4.2 Ecological niches

• B4.2.1—Ecological niche as the role of a species in an ecosystem (SL and HL)
• B4.2.2—Differences between organisms that are obligate anaerobes, facultative anaerobes and obligate aerobes (SL and HL)
• B4.2.3—Photosynthesis as the mode of nutrition in plants, algae and several groups of photosynthetic prokaryotes (SL and HL)
• B4.2.6—Saprotrophic nutrition in some fungi and bacteria (SL and HL)
• B4.2.12—Fundamental and realized niches (SL and HL)

C Interactions and interdependence

1 Molecules

C1.1 Enzymes and metabolism

• C1.1.1—Enzymes as catalysts (SL and HL)
• C1.1.2—Role of enzymes in metabolism (SL and HL)
• C1.1.3—Anabolic and catabolic reactions (SL and HL)
• C1.1.4—Enzymes as globular proteins with an active site for catalysis (SL and HL)
• C1.1.5—Interactions between substrate and active site to allow induced-fit binding (SL and HL)
• C1.1.6—Role of molecular motion and substrate-active site collisions in enzyme catalysis (SL and HL)
• C1.1.7—Relationships between the structure of the active site, enzyme–substrate specificity and denaturation (SL and HL)
• C1.1.8—Effects of temperature, pH and substrate concentration on the rate of enzyme activity (SL and HL)
• C1.1.9—Measurements in enzyme-catalysed reactions (SL and HL)
• C1.1.10—Effect of enzymes on activation energy (SL and HL)
• C1.1.11—Intracellular and extracellular enzyme-catalysed reactions (AHL)
• C1.1.14—Allosteric sites and non-competitive inhibition (AHL)

C1.2 Cell respiration

• C1.2.4—Cell respiration as a system for producing ATP within the cell using energy released from carbon compounds (SL and HL)
• C1.2.6—Variables affecting the rate of cell respiration (SL and HL)
• C1.2.10—Anaerobic cell respiration in yeast and its use in brewing and baking (AHL)

C1.3 Photosynthesis

• C1.3.1—Transformation of light energy to chemical energy when carbon compounds are produced in photosynthesis (SL and HL)
• C1.3.2—Conversion of carbon dioxide to glucose in photosynthesis using hydrogen obtained by splitting water (SL and HL)
• C1.3.3—Oxygen as a by-product of photosynthesis in plants, algae and cyanobacteria (SL and HL)
• C1.3.4—Separation and identification of photosynthetic pigments by chromatography (SL and HL)
• C1.3.5—Absorption of specific wavelengths of light by photosynthetic pigments (SL and HL)
• C1.3.6—Similarities and differences of absorption and action spectra (SL and HL)
• C1.3.7—Techniques for varying concentrations of carbon dioxide, light intensity or temperature experimentally to investigate the effects of limiting factors on the rate of photosynthesis (SL and HL)
• C1.3.9—Photosystems as arrays of pigment molecules that can generate and emit excited electrons (AHL)
• C1.3.10—Advantages of the structured array of different types of pigment molecules in a photosystem (AHL)
• C1.3.12—ATP production by chemiosmosis in thylakoids (AHL)
• C1.3.13—Reduction of NADP by photosystem I (AHL)
• C1.3.14—Thylakoids as systems for performing the light-dependent reactions of photosynthesis (AHL)

2 Cells

C2.1 Chemical signalling

• C2.1.3—Hormones, neurotransmitters, cytokines and calcium ions as examples of functional categories of signalling chemicals in animals (SL and HL)

C2.2 Neural signalling

• C2.2.1—Neurons as cells within the nervous system that carry electrical impulses (SL and HL)
• C2.2.3—Nerve impulses as action potentials that are propagated along nerve fibres (SL and HL)
• C2.2.4—Variation in the speed of nerve impulses (SL and HL)

3 Organisms

C3.1 Integration of body systems

• C3.1.1—System integration (SL and HL)
• C3.1.4—The brain as a central information integration organ (SL and HL)
• C3.1.5—The spinal cord as an integrating centre for unconscious processes (SL and HL)
• C3.1.6—Input to the spinal cord and cerebral hemispheres through sensory neurons (SL and HL)
• C3.1.7—Output from the cerebral hemispheres to muscles through motor neurons (SL and HL)
• C3.1.8—Nerves as bundles of nerve fibres of both sensory and motor neurons (SL and HL)
• C3.1.10—Role of the cerebellum in coordinating skeletal muscle contraction and balance (SL and HL)
• C3.1.14—Feedback control of heart rate following sensory input from baroreceptors and chemoreceptors (SL and HL)
• C3.1.15—Feedback control of ventilation rate following sensory input from chemoreceptors (SL and HL)

4 Ecosystems

C41.1 Populations and communities

• C4.1.5—Carrying capacity and competition for limited resources (SL and HL)
• C4.1.6—Negative feedback control of population size by density-dependent factors (SL and HL)
• C4.1.7—Population growth curves (SL and HL)
• C4.1.8—Modelling of the sigmoid population growth curve (SL and HL)
• C4.1.9—Competition versus cooperation in intraspecific relationships (SL and HL)
• C4.1.10—A community as all of the interacting organisms in an ecosystem (SL and HL)
• C4.1.11—Herbivory, predation, interspecific competition, mutualism, parasitism and pathogenicity as categories of interspecific relationship within communities (SL and HL)
• C4.1.16—Predator–prey relationships as an example of density-dependent control of animal populations (SL and HL)

C4.2 Transfers of energy and matter

• C4.2.1—Ecosystems as open systems in which both energy and matter can enter and exit (SL and HL)
• C4.2.2—Sunlight as the principal source of energy that sustains most ecosystems (SL and HL)
• C4.2.3—Flow of chemical energy through food chains (SL and HL)
• C4.2.5—Supply of energy to decomposers as carbon compounds in organic matter coming from dead organisms (SL and HL)
• C4.2.6—Autotrophs as organisms that use external energy sources to synthesize carbon compounds from simple inorganic substances (SL and HL)
• C4.2.7—Use of light as the external energy source in photoautotrophs and oxidation reactions as the energy source in chemoautotrophs (SL and HL)
• C4.2.8—Heterotrophs as organisms that use carbon compounds obtained from other organisms to synthesize the carbon compounds that they require (SL and HL)
• C4.2.9—Release of energy in both autotrophs and heterotrophs by oxidation of carbon compounds in cell respiration (SL and HL)
• C4.2.15—Primary production as accumulation of carbon compounds in biomass by autotrophs (SL and HL)
• C4.2.16—Secondary production as accumulation of carbon compounds in biomass by heterotrophs (SL and HL)
• C4.2.19—Release of carbon dioxide into the atmosphere during combustion of biomass, peat, coal, oil and natural gas (SL and HL)
• C4.2.21—Dependence of aerobic respiration on atmospheric oxygen produced by photosynthesis, and of photosynthesis on atmospheric carbon dioxide produced by respiration (SL and HL)

D Continuity and change

1 Molecules

D1.1 DNA replication

• D1.1.4—Polymerase chain reaction and gel electrophoresis as tools for amplifying and separating DNA (SL and HL)
• D1.1.5—Applications of polymerase chain reaction and gel electrophoresis (SL and HL)

2 Cells

D2.1 Cell and nuclear division

• D2.1.4—Roles of mitosis and meiosis in eukaryotes (SL and HL)
• D2.1.5—DNA replication as a prerequisite for both mitosis and meiosis (SL and HL)
• D2.1.6—Condensation and movement of chromosomes as shared features of mitosis and meiosis (SL and HL)
• D2.1.7—Phases of mitosis (SL and HL)
• D2.1.8—Identification of phases of mitosis (SL and HL)
• D2.1.9—Meiosis as a reduction division (SL and HL)
• D2.1.11—Meiosis as a source of variation (SL and HL)

D2.2 Gene expression

• D2.2.8—Examples of environmental effects on gene expression in cells and organisms (AHL)

D2.3 Water potential

• D2.3.1—Solvation with water as the solvent (SL and HL)
• D2.3.2—Water movement from less concentrated to more concentrated solutions (SL and HL)
• D2.3.3—Water movement by osmosis into or out of cells (SL and HL)
• D2.3.4—Changes due to water movement in plant tissue bathed in hypotonic and those bathed in hypertonic solutions (SL and HL)
• D2.3.6—Effects of water movement on cells with a cell wall (SL and HL)
• D2.3.11—Water potential and water movements in plant tissue (AHL)

3 Organisms

D3.1 Reproduction

• D3.1.12—Dispersal and germination of seeds (SL and HL)

D3.3 Homeostasis

• D3.3.1—Homeostasis as maintenance of the internal environment of an organism (SL and HL)
• D3.3.2—Negative feedback loops in homeostasis (SL and HL)
• D3.3.5—Thermoregulation as an example of negative feedback control (SL and HL)
• D3.3.6—Thermoregulation mechanisms in humans (SL and HL)
• D3.3.11—Changes in blood supply to organs in response to changes in activity (AHL)

4 Ecosystems

D4.1 Natural selection

• D4.1.1—Natural selection as the mechanism driving evolutionary change (SL and HL)
• D4.1.4—Abiotic factors as selection pressures (SL and HL)
• D4.1.9—Concept of the gene pool (AHL)
• D4.1.10—Allele frequencies of geographically isolated populations (AHL)
• D4.1.11—Changes in allele frequency in the gene pool as a consequence of natural selection between individuals according to differences in their heritable traits (AHL)
• D4.1.12—Differences between directional, disruptive and stabilizing selection (AHL)
• D4.1.13—Hardy–Weinberg equation and calculations of allele or genotype frequencies (AHL)
• D4.1.14—Hardy–Weinberg conditions that must be maintained for a population to be in genetic equilibrium (AHL)
• D4.1.15—Artificial selection by deliberate choice of traits (AHL)

D4.2 Stability and change

• D4.2.2—Requirements for stability in ecosystems (SL and HL)
• D4.2.8—Eutrophication of aquatic and marine ecosystems due to leaching (SL and HL)

D4.3 Climate change

• D4.3.1—Anthropogenic causes of climate change (SL and HL)
• D4.3.2—Positive feedback cycles in global warming (SL and HL)