Biology

Core Declarative Knowledge
What should students know?

A2.2 Cell Structure (5 Hours)
A2.2.1—Cells as the basic structural unit of all living organisms NOS: Students should be aware that deductive reason can be used to generate predictions from theories. Based on cell theory, a newly discovered organism can be predicted to consist of one or more cells.
A2.2.3—Developments in microscopy: Include the advantages of electron microscopy, freeze fracture, cryogenic electron microscopy, and the use of fluorescent stains and immunofluorescence in light microscopy.
A2.2.4—Structures common to cells in all living organisms: Typical cells have DNA as genetic material and a cytoplasm composed mainly of water, which is enclosed by a plasma membrane composed of lipids. Students should understand the reasons for these structures.
A2.2.5—Prokaryote cell structure; Include these cell components: cell wall, plasma membrane, cytoplasm, naked DNA in a loop and 70S ribosomes. The type of prokaryotic cell structure required is that of Gram-positive eubacteria such as Bacillus and Staphylococcus. Students should appreciate that prokaryote cell structure varies. However, students are not required to know details of the variations such as the lack of cell walls in phytoplasmas and mycoplasmas.
A2.2.6—Eukaryote cell structure: Students should be familiar with features common to eukaryote cells: a plasma membrane enclosing a compartmentalized cytoplasm with 80S ribosomes; a nucleus with chromosomes made of DNA bound to histones, contained in a double membrane with pores; membrane-bound cytoplasmic organelles including mitochondria, endoplasmic reticulum, Golgi apparatus and a variety of vesicles or vacuoles including lysosomes; and a cytoskeleton of microtubules and microfilaments.
A2.2.7—Processes of life in unicellular organisms include these functions: homeostasis, metabolism, nutrition, movement, excretion, growth, response to stimuli and reproduction.
A2.2.8—Differences in eukaryotic cell structure between animals, fungi and plants: include presence and composition of cell walls, differences in size and function of vacuoles, presence of chloroplasts and other plastids, and presence of centrioles, cilia and flagella.
A2.2.9—Atypical cell structure in eukaryotes: Use numbers of nuclei to illustrate one type of atypical cell structure in aseptate fungal hyphae, skeletal muscle, red blood cells and phloem sieve tube elements.
A2.2.12—Origin of eukaryotic cells by endosymbiosis: Evidence suggests that all eukaryotes evolved from a common unicellular ancestor that had a nucleus and reproduced sexually. Mitochondria then evolved by endosymbiosis. In some eukaryotes, chloroplasts subsequently also had an endosymbiotic origin. Evidence should include the presence in mitochondria and chloroplasts of 70S ribosomes, naked circular DNA and the ability to replicate. NOS: Students should recognize that the strength of a theory comes from the observations the theory explains and the predictions it supports. A wide range of observations are accounted for by the theory of
endosymbiosis.
A2.2.13—Cell differentiation as the process for developing specialized tissues in multicellular organisms Students should be aware that the basis for differentiation is different patterns of gene expression often triggered by changes in the environment.
A2.2.14—Evolution of multicellularity: Students should be aware that multicellularity has evolved repeatedly. Many fungi and eukaryotic algae and all plants and animals are multicellular. Multicellularity has the advantages of allowing larger body size and cell specialization.
A2.1 – Origins of Cells (2 Hours)
A2.1.1 – Conditions on early Earth and the pre-biotic formation of carbon compounds: include the lack of free oxygen and therefore ozone, higher concentrations of carbon dioxide and methane, resulting in higher temperatures and ultraviolet light penetration. The conditions may have caused a variety of carbon compounds to form spontaneously by chemical processes that do not now occur.
A2.1.2 – Cells as the smallest units of self-sustaining life:Discuss the differences between something that is living and something that is non-living. Include reasons that viruses are considered to be non-living.
A2.1.3 – Challenge of explaining the spontaneous origin of cells: Cells are highly complex structures that can currently only be produced by division of pre-existing cells. Students should be aware that catalysis, self-replication of molecules, self-assembly and the emergence of compartmentalization were necessary requirements for the evolution of the first cells.
NOS: Students should appreciate that claims in science, including hypotheses and theories, must be testable. In some cases, scientists have to struggle with hypotheses that are difficult to test. In this case the exact conditions on pre-biotic Earth cannot be replicated and the first protocells did not fossilize.
A2.1.4—Evidence for the origin of carbon compounds Evaluate the Miller–Urey experiment.
A2.1.5—Spontaneous formation of vesicles by coalescence of fatty acids into spherical bilayers: Formation of a membrane-bound compartment is needed to allow internal chemistry to become different from that outside the compartment.
A2.1.6—RNA as a presumed first genetic material: RNA can be replicated and has some catalytic activity so it may have acted initially as both the genetic material and the enzymes of the earliest cells. Ribozymes in the ribosome are still used to catalyse peptide bond formation during protein synthesis.
A2.1.7—Evidence for a last universal common ancestor: Include the universal genetic code and shared genes across all organisms. Include the likelihood of other forms of life having evolved but becoming extinct due to competition from the last universal common ancestor (LUCA) and descendants of LUCA.
A2.1.8—Approaches used to estimate dates of the first living cells and the last universal common ancestor: Students should develop an appreciation of the immense length of time over which life has been evolving on Earth.
A2.1.9—Evidence for the evolution of the last universal common ancestor in the vicinity of hydrothermal
vents Include fossilized evidence of life from ancient seafloor hydrothermal vent precipitates and evidence of conserved sequences from genomic analysis.
B2.1 Membranes and Membrane Transport (6 Hours)
B2.1.1—Lipid bilayers as the basis of cell membranes: Phospholipids and other amphipathic lipids naturally form continuous sheet-like bilayers in water.
B2.1.2—Lipid bilayers as barriers: Students should understand that the hydrophobic hydrocarbon chains that form the core of a membrane have low permeability to large molecules and hydrophilic particles, including ions and polar molecules, so membranes function as effective barriers between aqueous solutions.
B2.1.3—Simple diffusion across membranes: Use movement of oxygen and carbon dioxide molecules between phospholipids as an example of simple diffusion across membranes.
B2.1.4—Integral and peripheral proteins in membranes: Emphasize that membrane proteins have diverse structures, locations and functions. Integral proteins are embedded in one or both of the lipid layers of a membrane. Peripheral proteins are attached to one or other surface of the bilayer.
B2.1.5—Movement of water molecules across membranes by osmosis and the role of aquaporins: Include an explanation in terms of random movement of particles, impermeability of membranes to solutes and differences in solute concentration.
B2.1.6—Channel proteins for facilitated diffusion: Students should understand how the structure of channel proteins makes membranes selectively permeable by allowing specific ions to diffuse through when channels are open but not when they are closed.
B2.1.7—Pump proteins for active transport: Students should appreciate that pumps use energy from adenosine triphosphate (ATP) to transfer specific particles across membranes and therefore that they can move particles against a concentration gradient.
B2.1.8—Selectivity in membrane permeability: Facilitated diffusion and active transport allow selective permeability in membranes. Permeability by simple diffusion is not selective and depends only on the size and hydrophilic or hydrophobic properties of particles.
B.2.1.9—Structure and function of glycoproteins and glycolipids: Limit to carbohydrate structures linked to proteins or lipids in membranes, location of carbohydrates on the extracellular side of membranes, and roles in cell adhesion and cell recognition.
B2.1.10—Fluid mosaic model of membrane structure: Students should be able to draw a two-dimensional representation of the model and include peripheral and integral proteins, glycoproteins, phospholipids and cholesterol. They should also be able to indicate hydrophobic and hydrophilic regions.
B2.1.11—Relationships between fatty acid composition of lipid bilayers and their fluidity Unsaturated fatty acids in lipid bilayers have lower melting points, so membranes are fluid and therefore flexible at temperatures experienced by a cell. Saturated fatty acids have higher melting points and make membranes stronger at higher temperatures. Students should be familiar with an example of adaptations in membrane composition in relation to habitat.
B2.1.12—Cholesterol and membrane fluidity in animal cells: Students should understand the position of cholesterol molecules in membranes and also that cholesterol acts as a modulator (adjustor) of membrane fluidity, stabilizing membranes at higher temperatures and preventing stiffening at lower temperatures.
B2.1.13—Membrane fluidity and the fusion and formation of vesicles: Include the terms “endocytosis” and “exocytosis”, and examples of each process.
B2.1.14—Gated ion channels in neurons: Include nicotinic acetylcholine receptors as an example of a neurotransmitter-gated ion channel and sodium and potassium channels as examples of voltage-gated channels.
B2.1.15—Sodium–potassium pumps as an example of exchange transporters: Include the importance of these pumps in generating membrane potentials.
B2.1.16—Sodium-dependent glucose cotransporters as an example of indirect active transport: Include the importance of these co-transporters in glucose absorption by cells in the small intestine and glucose reabsorption by cells in the nephron.
B2.1.17—Adhesion of cells to form tissues: Include the term “cell-adhesion molecules” (CAMs) and the understanding that different forms of CAM are used for different types of cell–cell junction. Students are not required to have detailed knowledge of the different CAMs or junctions.
B2.2 Organelles and Compartmentalization (3 Hours)
B2.2.1—Organelles as discrete subunits of cells that are adapted to perform specific functions: Students should understand that the cell wall, cytoskeleton and cytoplasm are not considered organelles, and that nuclei, vesicles, ribosomes and the plasma membrane are. NOS: Students should recognize that progress in science often follows development of new techniques. For example, study of the function of individual organelles became possible when ultracentrifuges had been invented and methods of using them for cell fractionation had been developed.
B2.2.2—Advantage of the separation of the nucleus and cytoplasm into separate compartments: Limit to separation of the activities of gene transcription and translation—post-transcriptional modification of mRNA can happen before the mRNA meets ribosomes in the cytoplasm. In prokaryotes this is not possible—mRNA may immediately meet ribosomes.
B2.2.3—Advantages of compartmentalization in the cytoplasm of cells: Include concentration of metabolites and enzymes and the separation of incompatible biochemical processes. Include lysosomes and phagocytic vacuoles as examples.
B2.2.4—Adaptations of the mitochondrion for production of ATP by aerobic cell respiration: Include these adaptations: a double membrane with a small volume of intermembrane space, large surface area of cristae and compartmentalization of enzymes and substrates of the Krebs cycle in the matrix.
B2.2.5—Adaptations of the chloroplast for photosynthesis: Include these adaptations: the large surface area of thylakoid membranes with photosystems, small volumes of fluid inside thylakoids, and compartmentalization of enzymes and substrates of the Calvin cycle in the stroma.
B2.2.6—Functional benefits of the double membrane of the nucleus: Include the need for pores in the nuclear membrane and for the nucleus membrane to break into vesicles during mitosis and meiosis.
B2.2.7—Structure and function of free ribosomes and of the rough endoplasmic reticulum: Contrast the synthesis by free ribosomes of proteins for retention in the cell with synthesis by membrane- bound ribosomes on the rough endoplasmic reticulum of proteins for transport within the cell and secretion.
B2.2.8—Structure and function of the Golgi apparatus: Limit to the roles of the Golgi apparatus in processing and secretion of protein.
B2.2.9—Structure and function of vesicles in cells: Include the role of clathrin in the formation of vesicles.

Core Procedural Knowledge
What should students be able to do?

A2.2 Cell Structure
A2.2.2—Microscopy skills: Students should have experience of making temporary mounts of cells and tissues, staining, measuring sizes using an eyepiece graticule, focusing with coarse and fine adjustments, calculating actual size and magnification, producing a scale bar and taking photographs.NOS: Students should appreciate that measurement using instruments is a form of quantitative observation.
A2.2.10—Cell types and cell structures viewed in light and electron micrographs: Students should be able to identify cells in light and electron micrographs as prokaryote, plant or animal. In electron micrographs, students should be able to identify these structures: nucleoid region, prokaryotic cell wall, nucleus, mitochondrion, chloroplast, sap vacuole, Golgi apparatus, rough and smooth endoplasmic reticulum, chromosomes, ribosomes, cell wall, plasma membrane and microvilli.
A2.2.11—Drawing and annotation based on electron micrographs: Students should be able to draw and annotate diagrams of organelles (nucleus, mitochondria, chloroplasts, sap vacuole, Golgi apparatus, rough and smooth endoplasmic reticulum and chromosomes) as well as other cell structures (cell wall, plasma membrane, secretory vesicles and microvilli) shown in electron micrographs. Students are required to include the functions in their annotations.

Core Declarative Knowledge
What should students know?

A1.1 Water (3 hours)
A1.1.1—Water as the medium for life: Students should appreciate that the first cells originated in water and that water remains the medium in which most processes of life occur.
A1.1.2—Hydrogen bonds as a consequence of the polar covalent bonds within water molecules: Students should understand that polarity of covalent bonding within water molecules is due to unequal sharing of electrons and that hydrogen bonding due to this polarity occurs between water molecules. Students should be able to represent two or more water molecules and hydrogen bonds between them with the notation shown below to indicate polarity.
A1.1.3—Cohesion of water molecules due to hydrogen bonding and consequences for organisms: Include transport of water under tension in xylem and the use of water surfaces as habitats due to the effect known as surface tension.
A1.1.4—Adhesion of water to materials that are polar or charged and impacts for organisms: Include capillary action in soil and in plant cell walls.
A1.1.5—Solvent properties of water linked to its role as a medium for metabolism and for transport in plants and animals: Emphasize that a wide variety of hydrophilic molecules dissolve in water and that most enzymes catalyse reactions in aqueous solution. Students should also understand that the functions of some molecules in cells depend on them being hydrophobic and insoluble.
A1.1.6—Physical properties of water and the consequences for animals in aquatic habitats: Include buoyancy, viscosity, thermal conductivity and specific heat capacity. Contrast the physical properties of water with those of air and illustrate the consequences using examples of animals that live in water and in air or on land, such as the black-throated loon (Gavia arctica) and the ringed seal (Pusa hispida). Note: When students are referring to an organism in an examination, either the common name or the scientific name is acceptable.
A1.1.7—Extraplanetary origin of water on Earth and reasons for its retention: The abundance of water over billions of years of Earth’s history has allowed life to evolve. Limit hypotheses for the origin of water on Earth to asteroids and reasons for retention to gravity and temperatures low enough to condense water.
A1.1.8—Relationship between the search for extraterrestrial life and the presence of water: Include the idea of the “Goldilocks zone”.
A1.2 Nucleic Acid (5 Hours)
A1.2.1—DNA as the genetic material of all living organisms: Some viruses use RNA as their genetic material but viruses are not considered to be living.
A1.2.3—Sugar–phosphate bonding and the sugar–phosphate “backbone” of DNA and RNA Sugar–phosphate bonding makes a continuous chain of covalently bonded atoms in each strand of DNA or RNA nucleotides, which forms a strong “backbone” in the molecule.
A1.2.4—Bases in each nucleic acid that form the basis of a code: Students should know the names of the nitrogenous bases.
A1.2.7—Differences between DNA and RNA: Include the number of strands present, the types of nitrogenous bases and the type of pentose sugar. Students should be able to sketch the difference between ribose and deoxyribose. Students should be familiar with examples of nucleic acids.
A1.2.8—Role of complementary base pairing in allowing genetic information to be replicated and expressed: Students should understand that complementarity is based on hydrogen bonding.
A1.2.9—Diversity of possible DNA base sequences and the limitless capacity of DNA for storing information: Explain that diversity by any length of DNA molecule and any base sequence is possible. Emphasize the enormous capacity of DNA for storing data with great economy.
A1.2.10—Conservation of the genetic code across all life forms as evidence of universal common ancestry: Students are not required to memorize any specific examples.
A1.2.11—Directionality of RNA and DNA: Include 5′ to 3′ linkages in the sugar–phosphate backbone and their significance for replication, transcription and translation.
A1.2.12—Purine-to-pyrimidine bonding as a component of DNA helix stability: Adenine–thymine (A–T) and cytosine–guanine (C–G) pairs have equal length, so the DNA helix has the same three-dimensional structure, regardless of the base sequence.
A1.2.13—Structure of a nucleosome: Limit to a DNA molecule wrapped around a core of eight histone proteins held together by an additional histone protein attached to linker DNA.
A1.2.14—Evidence from the Hershey–Chase experiment for DNA as the genetic material: Students should understand how the results of the experiment support the conclusion that DNA is the genetic material. NOS: Students should appreciate that technological developments can open up new possibilities for experiments. When radioisotopes were made available to scientists as research tools, the Hershey–Chase experiment became possible.
A1.2.15—Chargaff’s data on the relative amounts of pyrimidine and purine bases across diverse life forms: NOS: Students should understand how the “problem of induction” is addressed by the “certainty of falsification”. In this case, Chargaff’s data falsified the tetranucleotide hypothesis that there was a repeating sequence of the four bases in DNA.
B1.1 Carbohydrates and Lipids (4 hours)
B1.1.1—Chemical properties of a carbon atom allowing for the formation of diverse compounds upon which life is based: Students should understand the nature of a covalent bond. Students should also understand that a carbon
atom can form up to four single bonds or a combination of single and double bonds with other carbon atoms or atoms of other non-metallic elements. Include among the diversity of carbon compounds examples of molecules with branched or unbranched chains and single or multiple rings. NOS: Students should understand that scientific conventions are based on international agreement (SI metric unit prefixes “kilo”, “centi”, “milli”, “micro” and “nano”).
B1.1.2—Production of macromolecules by condensation reactions that link monomers to form a polymer: Students should be familiar with examples of polysaccharides, polypeptides and nucleic acids.
B1.1.3—Digestion of polymers into monomers by hydrolysis reactions: Water molecules are split to provide the -H and -OH groups that are incorporated to produce monomers, hence the name of this type of reaction.
B1.1.4—Form and function of monosaccharides: Students should be able to recognize pentoses and hexoses as monosaccharides from molecular diagrams showing them in the ring forms. Use glucose as an example of the link between the properties of a monosaccharide and how it is used, emphasizing solubility, transportability, chemical stability and the yield of energy from oxidation as properties.
B1.1.5—Polysaccharides as energy storage compounds: Include the compact nature of starch in plants and glycogen in animals due to coiling and branching during polymerization, the relative insolubility of these compounds due to large molecular size and the relative ease of adding or removing alpha-glucose monomers by condensation and hydrolysis to build or mobilize energy stores.
B1.1.6—Structure of cellulose related to its function as a structural polysaccharide in plants: Include the alternating orientation of beta-glucose monomers, giving straight chains that can be grouped in bundles and cross-linked with hydrogen bonds.
B1.1.7—Role of glycoproteins in cell–cell recognition: Include ABO antigens as an example.
B1.1.8—Hydrophobic properties of lipids: Lipids are substances in living organisms that dissolve in non-polar solvents but are only sparingly soluble in aqueous solvents. Lipids include fats, oils, waxes and steroids.
B1.1.9—Formation of triglycerides and phospholipids by condensation reactions: One glycerol molecule can link three fatty acid molecules or two fatty acid molecules and one phosphate group.
B1.1.10—Difference between saturated, monounsaturated and polyunsaturated fatty acids Include the number of double carbon (C=C) bonds and how this affects melting point. Relate this to the prevalence of different types of fatty acids in oils and fats used for energy storage in plants and endotherms respectively.
B1.1.11—Triglycerides in adipose tissues for energy storage and thermal insulation: Students should understand that the properties of triglycerides make them suited to long-term energy storage functions. Students should be able to relate the use of triglycerides as thermal insulators to body temperature and habitat.
B1.1.12—Formation of phospholipid bilayers as a consequence of the hydrophobic and hydrophilic regions: Students should use and understand the term “amphipathic”.
B1.1.13—Ability of non-polar steroids to pass through the phospholipid bilayer: Include oestradiol and testosterone as examples. Students should be able to identify compounds as steroids from molecular diagrams.
B1.2 Proteins (4 hours)
B1.2.3—Dietary requirements for amino acids: Essential amino acids cannot be synthesized and must be obtained from food. Non-essential amino acids can be made from other amino acids. Students are not required to give examples of essential and non- essential amino acids. Vegan diets require attention to ensure essential amino acids are consumed.
B1.2.4—Infinite variety of possible peptide chains: Include the ideas that 20 amino acids are coded for in the genetic code, that peptide chains can have any number of amino acids, from a few to thousands, and that amino acids can be in any order. Students should be familiar with examples of polypeptides.
B1.2.5—Effect of pH and temperature on protein structure: Include the term “denaturation”.
B1.2.6—Chemical diversity in the R-groups of amino acids as a basis for the immense diversity in protein form and function: Students are not required to give specific examples of R-groups. However, students should understand that R-groups determine the properties of assembled polypeptides. Students should appreciate that R- groups are hydrophobic or hydrophilic and that hydrophilic R-groups are polar or charged, acidic or basic.
B1.2.7—Impact of primary structure on the conformation of proteins: Students should understand that the sequence of amino acids and the precise position of each amino acid within a structure determines the three-dimensional shape of proteins. Proteins therefore have precise, predictable and repeatable structures, despite their complexity.
B1.2.8—Pleating and coiling of secondary structure of proteins: Include hydrogen bonding in regular positions to stabilize alpha helices and beta-pleated sheets.
B1.2.9—Dependence of tertiary structure on hydrogen bonds, ionic bonds, disulfide covalent bonds and hydrophobic interactions: Students are not required to name examples of amino acids that participate in these types of bonding, apart from pairs of cysteines forming disulfide bonds. Students should understand that amine and carboxyl groups in R-groups can become positively or negatively charged by binding or dissociation of hydrogen ions and that they can then participate in ionic bonding.
B1.2.10—Effect of polar and non-polar amino acids on tertiary structure of proteins: In proteins that are soluble in water, hydrophobic amino acids are clustered in the core of globular proteins. Integral proteins have regions with hydrophobic amino acids, helping them to embed in membranes.
B1.2.11—Quaternary structure of non-conjugated and conjugated proteins: Include insulin and collagen as examples of non-conjugated proteins and haemoglobin as an example of a conjugated protein. NOS: Technology allows imaging of structures that would be impossible to observe with the unaided senses. For example, cryogenic electron microscopy has allowed imaging of single-protein molecules and their interactions with other molecules.
B1.2.12—Relationship of form and function in globular and fibrous proteins: Students should know the difference in shape between globular and fibrous proteins and understand that their shapes make them suitable for specific functions. Use insulin and collagen to exemplify how form and function are related.
A2.3 Viruses (2 hours)
A2.3.1—Structural features common to viruses: Relatively few features are shared by all viruses: small, fixed size; nucleic acid (DNA or RNA) as genetic material; a capsid made of protein; no cytoplasm; and few or no enzymes.
A2.3.2—Diversity of structure in viruses: Students should understand that viruses are highly diverse in their shape and structure. Genetic material may be RNA or DNA, which can be either single- or double-stranded. Some viruses are enveloped in host cell membrane and others are not enveloped. Virus examples include bacteriophage lambda, coronaviruses and HIV.
A2.3.3—Lytic cycle of a virus: Students should appreciate that viruses rely on a host cell for energy supply, nutrition, protein synthesis and other life functions. Use bacteriophage lambda as an example of the phases in a lytic cycle.
A.2.3.4—Lysogenic cycle of a virus: Use bacteriophage lambda as an example.
A2.3.5—Evidence for several origins of viruses from other organisms: The diversity of viruses suggests several possible origins. Viruses share an extreme form of obligate parasitism as a mode of existence, so the structural features that they have in common could be regarded as convergent evolution. The genetic code is shared between viruses and living organisms.
A2.3.6—Rapid evolution in viruses: Include reasons for very rapid rates of evolution in some viruses. Use two examples of rapid evolution: evolution of influenza viruses and of HIV. Consider the consequences for treating diseases caused by rapidly evolving viruses.

Core Procedural Knowledge
What should students be able to do?

A1.2 Nucleic Acid
A1.2.2—Components of a nucleotide: In diagrams of nucleotides use circles, pentagons and rectangles to represent relative positions of phosphates, pentose sugars and bases.
A1.2.5—RNA as a polymer formed by condensation of nucleotide monomers: Students should be able to draw and recognize diagrams of the structure of single nucleotides and RNA polymers.
A1.2.6—DNA as a double helix made of two antiparallel strands of nucleotides with two strands linked by hydrogen bonding between complementary base pairs: In diagrams of DNA structure, students should draw the two strands antiparallel, but are not required to draw the helical shape. Students should show adenine (A) paired with thymine (T), and guanine (G) paired with cytosine (C). Students are not required to memorize the relative lengths of the purine and pyrimidine bases, or the numbers of hydrogen bonds.
A1.2.13—Structure of a nucleosome: Students are required to use molecular visualization software to study the association between the proteins and DNA within a nucleosome.
B1.2 Proteins
B1.2.1—Generalized structure of an amino acid: Students should be able to draw a diagram of a generalized amino acid showing the alpha carbon atom with amine group, carboxyl group, R-group and hydrogen attached.
B1.2.2—Condensation reactions forming dipeptides and longer chains of amino acids: Students should be able to write the word equation for this reaction and draw a generalized dipeptide after modelling the reaction with molecular models.

Core Declarative Knowledge
What should students know?

A3.1 Diversity of Organisms (5 hours)
A3.1.1—Variation between organisms as a defining feature of life: Students should understand that no two individuals are identical in all their traits. The patterns of variation are complex and are the basis for naming and classifying organisms.
A3.1.2—Species as groups of organisms with shared traits: This is the original morphological concept of the species as used by Linnaeus.
A3.1.4—Biological species concept: According to the biological species concept, a species is a group of organisms that can breed and produce fertile offspring. Include possible challenges associated with this definition of a species and that competing species definitions exist.
A3.1.5—Difficulties distinguishing between populations and species due to divergence of non-interbreeding populations during speciation: Students should understand that speciation is the splitting of one species into two or more. It usually happens gradually rather than by a single act, with populations becoming more and more different in their traits. It can therefore be an arbitrary decision whether two populations are regarded as the same or different species.
A3.1.6—Diversity in chromosome numbers of plant and animal species: Students should know in general that diversity exists. As an example, students should know that humans have 46 chromosomes and chimpanzees have 48. Students are not required to know other specific chromosome numbers but should appreciate that diploid cells have an even number of chromosomes.
A3.1.8—Unity and diversity of genomes within species: Students should understand that the genome is all the genetic information of an organism. Organisms in the same species share most of their genome but variations such as single-nucleotide polymorphisms give some diversity.
A3.1.9—Diversity of eukaryote genomes: Genomes vary in overall size, which is determined by the total amount of DNA. Genomes also vary in base sequence. Variation between species is much larger than variation within a species.
A3.1.11—Current and potential future uses of whole genome sequencing Include the increasing speed and decreasing costs. For current uses, include research into evolutionary relationships and for potential future uses, include personalized medicine.
A3.1.12—Difficulties applying the biological species concept to asexually reproducing species and to bacteria that have horizontal gene transfer: The biological species concept does not work well with groups of organisms that do not breed sexually or where genes can be transferred from one species to another.
A3.1.13—Chromosome number as a shared trait within a species: Cross-breeding between closely related species is unlikely to produce fertile offspring if parent chromosome numbers are different.
A3.1.15—Identification of species from environmental DNA in a habitat using barcodes: Using barcodes and environmental DNA allows the biodiversity of habitats to be investigated rapidly.

A3.2 Classification and Cladistics (3 hours)
A3.2.1—Need for classification of organisms: Classification is needed because of the immense diversity of species. After classification is completed, a broad range of further study is facilitated.
A3.2.2—Difficulties classifying organisms into the traditional hierarchy of taxa: The traditional hierarchy of kingdom, phylum, class, order, family, genus and species does not always correspond to patterns of divergence generated by evolution. NOS: A fixed ranking of taxa (kingdom, phylum and so on) is arbitrary because it does not reflect the gradation of variation. Cladistics offers an alternative approach to classification using unranked clades. This is an example of the paradigm shift that sometimes occurs in scientific theories.
A3.2.3—Advantages of classification corresponding to evolutionary relationships: The ideal classification follows evolutionary relationships, so all the members of a taxonomic group have evolved from a common ancestor. Characteristics of organisms within such a group can be predicted because they are shared within a clade.
A3.2.4—Clades as groups of organisms with common ancestry and shared characteristics: The most objective evidence for placing organisms in the same clade comes from base sequences of genes or amino acid sequences of proteins. Morphological traits can be used to assign organisms to clades.
A3.2.5—Gradual accumulation of sequence differences as the basis for estimates of when clades diverged from a common ancestor: This method of estimating times is known as the “molecular clock”. The molecular clock can only give estimates because mutation rates are affected by the length of the generation time, the size of a population, the intensity of selective pressure and other factors.
A3.2.6—Base sequences of genes or amino acid sequences of proteins as the basis for constructing cladograms: Examples can be simple and based on sample data to illustrate the tool. NOS: Students should recognize that different criteria for judgement can lead to different hypotheses. Here, parsimony analysis is used to select the most probable cladogram, in which observed sequence variation between clades is accounted for with the smallest number of sequence changes
A3.2.9—Classification of all organisms into three domains using evidence from rRNA base sequences: This is the revolutionary reclassification with an extra taxonomic level above kingdoms that was proposed in 1977..

B3.1 Gas Exchange (4 Hours)
B3.1.1—Gas exchange as a vital function in all organisms: Students should appreciate that the challenges become greater as organisms increase in size because surface area-to-volume ratio decreases with increasing size, and the distance from the centre of an organism to its exterior increases.
B3.1.2—Properties of gas-exchange surfaces: Include permeability, thin tissue layer, moisture and large surface area.
B3.1.3—Maintenance of concentration gradients at exchange surfaces in animals: Include dense networks of blood vessels, continuous blood flow, and ventilation with air for lungs and with water for gills.
B3.1.4—Adaptations of mammalian lungs for gas exchange: Limit to the alveolar lungs of a mammal. Adaptations should include the presence of surfactant, a branched network of bronchioles, extensive capillary beds and a high surface area.
B3.1.5—Ventilation of the lungs: Students should understand the role of the diaphragm, intercostal muscles, abdominal muscles and ribs.
B3.1.7—Adaptations for gas exchange in leaves: Leaf structure adaptations should include the waxy cuticle, epidermis, air spaces, spongy mesophyll, stomatal guard cells and veins.
B3.1.9—Transpiration as a consequence of gas exchange in a leaf Students should be aware of the factors affecting the rate of transpiration
B3.1.11—Adaptations of foetal and adult haemoglobin for the transport of oxygen: Include cooperative binding of oxygen to haem groups and allosteric binding of carbon dioxide.
B3.1.12—Bohr shift: Students should understand how an increase in carbon dioxide causes increased dissociation of oxygen and the benefits of this for actively respiring tissues.
B3.1.13—Oxygen dissociation curves as a means of representing the affinity of haemoglobin for oxygen at different oxygen concentrations: Explain the S-shaped form of the curve in terms of cooperative binding.

B3.1 Transport (5 hours)
B3.2.1—Adaptations of capillaries for exchange of materials between blood and the internal or external environment: Adaptations should include a large surface area due to branching and narrow diameters, thin walls, and fenestrations in some capillaries where exchange needs to be particularly rapid.
B3.2.3—Adaptations of arteries for the transport of blood away from the heart: Students should understand how the layers of muscle and elastic tissue in the walls of arteries help them to withstand and maintain high blood pressures.
B3.2.5—Adaptations of veins for the return of blood to the heart: Include valves to prevent backflow and the flexibility of the wall to allow it to be compressed by muscle action.
B3.2.7—Transport of water from roots to leaves during transpiration: Students should understand that loss of water by transpiration from cell walls in leaf cells causes water to be drawn out of xylem vessels and through cell walls by capillary action, generating tension (negative pressure potentials). It is this tension that draws water up in the xylem. Cohesion ensures a continuous column of water.
B3.2.8—Adaptations of xylem vessels for transport of water Include the lack of cell contents and incomplete or absent end walls for unimpeded flow, lignified walls to withstand tensions, and pits for entry and exit of water.
B3.2.11—Release and reuptake of tissue fluid in capillaries: Tissue fluid is formed by pressure filtration of plasma in capillaries. This is promoted by the higher pressure of blood from arterioles. Lower pressure in venules allows tissue fluid to drain back into capillaries.
B3.2.12—Exchange of substances between tissue fluid and cells in tissues: Discuss the composition of plasma and tissue fluid.
B3.2.13—Drainage of excess tissue fluid into lymph ducts: Limit to the presence of valves and thin walls with gaps in lymph ducts and return of lymph to the blood circulation.
B3.2.14—Differences between the single circulation of bony fish and the double circulation of mammals: Simple circuit diagrams are sufficient to show the sequence of organs through which blood passes.
B3.2.15—Adaptations of the mammalian heart for delivering pressurized blood to the arteries: Include form–function adaptations of these structures: cardiac muscle, pacemaker, atria, ventricles, atrioventricular and semilunar valves, septum and coronary vessels. Students should be able to identify these features on a diagram of the heart in the frontal plane and trace the unidirectional flow of blood from named veins to arteries.
B3.2.17—Generation of root pressure in xylem vessels by active transport of mineral ions: Root pressure is positive pressure potential, generated to cause water movement in roots and stems when transport in xylem due to transpiration is insufficient, for example when high humidity prevents transpiration or in spring, before leaves on deciduous plants have opened.
B3.2.18—Adaptations of phloem sieve tubes and companion cells for translocation of sap: Include sieve plates, reduced cytoplasm and organelles, no nucleus for sieve tube elements and presence of many mitochondria for companion cells and plasmodesmata between them. Students should appreciate how these adaptations ease the flow of sap and enhance loading of carbon compounds into phloem sieve tubes at sources and unloading of them at sinks.

B3.3 Muscle and Motility (3 hours)
B3.2.17—Generation of root pressure in xylem vessels by active transport of mineral ions: Root pressure is positive pressure potential, generated to cause water movement in roots and stems when transport in xylem due to transpiration is insufficient, for example when high humidity prevents transpiration or in spring, before leaves on deciduous plants have opened.
B3.2.18—Adaptations of phloem sieve tubes and companion cells for translocation of sap: Include sieve plates, reduced cytoplasm and organelles, no nucleus for sieve tube elements and presence of many mitochondria for companion cells and plasmodesmata between them. Students should appreciate how these adaptations ease the flow of sap and enhance loading of carbon compounds into phloem sieve tubes at sources and unloading of them at sinks.
B3.3.8—Internal and external intercostal muscles as an example of antagonistic muscle action to facilitate internal body movements: Students should appreciate that the different orientations of muscle fibres in the internal and external layers of intercostal muscles mean that they move the ribcage in opposite directions and that, when one of these layers contracts, it stretches the other, storing potential energy in the sarcomere protein titin.
B3.3.9—Reasons for locomotion: Include foraging for food, escaping from danger, searching for a mate and migration, with at least one example of each.
B3.3.10—Adaptations for swimming in marine mammals: Include streamlining, adaptation of limbs to form flippers and of the tail to form a fluke with up-and-down movement, and changes to the airways to allow periodic breathing between dives.

Core Procedural Knowledge
What should students be able to do?

A3.1 Diversity of Organisms
A3.1.3—Binomial system for naming organisms: Students should know that the first part of the name identifies the genus, with the second part of the name distinguishing the species. Species in the same genus have similar traits. The genus name is given an initial capital letter but the species name is lowercase.
A3.1.7—Karyotyping and karyograms Application of skills: Students should be able to classify chromosomes by banding patterns, length and centromere position. Students should evaluate the evidence for the hypothesis that chromosome 2 in humans arose from the fusion of chromosomes 12 and 13 with a shared primate ancestor. NOS: Students should be able to distinguish between testable hypotheses such as the origin of chromosome 2 and non-testable statements.
A3.1.10—Comparison of genome sizes: Application of skills: Students should extract information about genome size for different taxonomic groups from a database to compare genome size to organism complexity.
A3.1.14—Engagement with local plant or animal species to develop a dichotomous key Application of skills: Students should engage with local plant or animal species to develop a dichotomous key.
A3.2 Classification and Cladistics (3 hours)
A3.2.7—Analysing cladograms: Students should be able to deduce evolutionary relationships, common ancestors and clades from a cladogram. They should understand the terms “root”, “node” and “terminal branch” and also that a node represents a hypothetical common ancestor.
A3.2.8—Using cladistics to investigate whether the classification of groups corresponds to evolutionary relationships: A case study of transfer of plant species between families could be used to develop understanding, for example the reclassification of the figwort family (Scrophulariaceae). However, students are not required to memorize the details of the case study. NOS: Students should appreciate that theories and other scientific knowledge claims may eventually be falsified. In this example, similarities in morphology due to convergent evolution rather than common ancestry suggested a classification that by cladistics has been shown to be false.
B3.1 Gas Exchange
B3.1.6—Measurement of lung volumes: Application of skills: Students should make measurements to determine tidal volume, vital capacity, and inspiratory and expiratory reserves.
B3.1.8—Distribution of tissues in a leaf: Students should be able to draw and label a plan diagram to show the distribution of tissues in a transverse section of a dicotyledonous leaf.
B3.1.10—Stomatal density Application of skills: Students should use micrographs or perform leaf casts to determine stomatal density. NOS: Reliability of quantitative data is increased by repeating measurements. In this case, repeated counts of the number of stomata visible in the field of view at high power illustrate the variability of biological material and the need for replicate trials.

B3.2 Transport
B3.2.2—Structure of arteries and veins: Application of skills: Students should be able to distinguish arteries and veins in micrographs from the structure of a vessel wall and its thickness relative to the diameter of the lumen.
B3.2.4—Measurement of pulse rates: Application of skills: Students should be able to determine heart rate by feeling the carotid or radial pulse with fingertips. Traditional methods could be compared with digital ones.
B3.2.6—Causes and consequences of occlusion of the coronary arteries: Application of skills: Students should be able to evaluate epidemiological data relating to the incidence of coronary heart disease. NOS: Students should understand that correlation coefficients quantify correlations between variables and allow the strength of the relationship to be assessed. Low correlation coefficients or lack of any correlation could provide evidence against a hypothesis, but even strong correlations such as that between saturated fat intake and coronary heart disease do not prove a causal link.
B3.2.9—Distribution of tissues in a transverse section of the stem of a dicotyledonous plant: Application of skills: Students should be able to draw plan diagrams from micrographs to identify the relative positions of vascular bundles, xylem, phloem, cortex and epidermis. Students should annotate the diagram with the main functions of these structures.
B3.2.10—Distribution of tissues in a transverse section of the root of a dicotyledonous plant: Application of skills: Students should be able to draw diagrams from micrographs to identify vascular bundles, xylem and phloem, cortex and epidermis.
B3.2.16—Stages in the cardiac cycle: Application of skills: Students should understand the sequence of events in the left side of the heart that follow the initiation of the heartbeat by the sinoatrial node (the “pacemaker”). Students should be able to interpret systolic and diastolic blood pressure measurements from data and graphs.

B3.3 Muscle and Motility
B3.3.7—Range of motion of a joint: Application of skills: Students should compare the range of motion of a joint in a number of dimensions. Students should measure joint angles using computer analysis of images or a goniometer.

Core Declarative Knowledge
What should students know?

A4.1 Evolution and Speciation (5 Hours)
A4.1.1—Evolution as change in the heritable characteristics of a population: This definition helps to distinguish Darwinian evolution from Lamarckism. Acquired changes that are not genetic in origin are not regarded as evolution. NOS: The theory of evolution by natural selection predicts and explains a broad range of observations and is unlikely ever to be falsified. However, the nature of science makes it impossible to formally prove that it is true by correspondence. It is a pragmatic truth and is therefore referred to as a theory, despite all the supporting evidence.
A4.1.2—Evidence for evolution from base sequences in DNA or RNA and amino acid sequences in proteins: Sequence data gives powerful evidence of common ancestry.
A4.1.3—Evidence for evolution from selective breeding of domesticated animals and crop plants: Variation between different domesticated animal breeds and varieties of crop plant, and between them and the original wild species, shows how rapidly evolutionary changes can occur.
A4.1.4—Evidence for evolution from homologous structures: Include the example of pentadactyl limbs.
A4.1.5—Convergent evolution as the origin of analogous structures: Students should understand that analogous structures have the same function but different evolutionary origins. Students should know at least one example of analogous features.
A4.1.6—Speciation by splitting of pre-existing species: Students should appreciate that this is the only way in which new species have appeared. Students should also understand that speciation increases the total number of species on Earth, and extinction decreases it. Students should also understand that gradual evolutionary change in a species is not speciation.
A4.1.7—Roles of reproductive isolation and differential selection in speciation: Include geographical isolation as a means of achieving reproductive isolation. Use the separation of bonobos and common chimpanzees by the Congo River as a specific example of divergence due to differential selection.
A4.1.8—Differences and similarities between sympatric and allopatric speciation: Students should understand that reproductive isolation can be geographic, behavioural or temporal.
A4.1.9—Adaptive radiation as a source of biodiversity: Adaptive radiation allows closely related species to coexist without competing, thereby increasing biodiversity in ecosystems where there are vacant niches.
A4.1.10—Barriers to hybridization and sterility of interspecific hybrids as mechanisms for of preventing the mixing of alleles between species: Courtship behaviour often prevents hybridization in animal species. A mule is an example of a sterile hybrid.
A4.1.11—Abrupt speciation in plants by hybridization and polyploidy: Use knotweed or smartweed (genus Persicaria) as an example because it contains many species that have been formed by these processes.
A4.2 Conservation of Biology (3 Hours)
A4.2.1—Biodiversity as the variety of life in all its forms, levels and combinations: Include ecosystem diversity, species diversity and genetic diversity.
A4.2.2—Comparisons between current number of species on Earth and past levels of biodiversity: Millions of species have been discovered, named and described but there are many more species to be discovered. Evidence from fossils suggests that there are currently more species alive on Earth today than at any time in the past. NOS: Classification is an example of pattern recognition but the same observations can be classified in different ways. For example, “splitters” recognize more species than “lumpers” in a taxonomic group.
A4.2.3—Causes of anthropogenic species extinction: This should be a study of the causes of the current sixth mass extinction, rather than of non-anthropogenic causes of previous mass extinctions. To give a range of causes, carry out three or more brief case studies of species extinction: North Island
giant moas (Dinornis novaezealandiae) as an example of the loss of terrestrial megafauna, Caribbean monk seals (Neomonachus tropicalis) as an example of the loss of a marine species, and one other species that has gone extinct from an area that is familiar to students.
A4.2.4—Causes of ecosystem loss
Students should study only causes that are directly or indirectly anthropogenic. Include two case studies
of ecosystem loss. One should be the loss of mixed dipterocarp forest in Southeast Asia, and the other
should, if possible, be of a lost ecosystem from an area that is familiar to students.
A4.2.5—Evidence for a biodiversity crisis: Evidence can be drawn from Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services reports and other sources. Results from reliable surveys of biodiversity in a wide range of habitats around the world are required. Students should understand that surveys need to be repeated to provide evidence of change in species richness and evenness. Note that there are opportunities for contributions from both expert scientists and citizen scientists. NOS: To be verifiable, evidence usually has to come from a published source, which has been peer- reviewed and allows methodology to be checked. Data recorded by citizens rather than scientists brings not only benefits but also unique methodological concerns.
A4.2.6—Causes of the current biodiversity crisis: Include human population growth as an overarching cause, together with these specific causes: hunting and other forms of over-exploitation; urbanization; deforestation and clearance of land for agriculture with consequent loss of natural habitat; pollution and spread of pests, diseases and invasive alien species due to global transport.
A4.2.7—Need for several approaches to conservation of biodiversity: No single approach by itself is sufficient, and different species require different measures. Include in situ conservation of species in natural habitats, management of nature reserves, rewilding and reclamation of degraded ecosystems, ex situ conservation in zoos and botanic gardens and storage of germ plasm in seed or tissue banks.
A4.2.8—Selection of evolutionarily distinct and globally endangered species for conservation prioritization in the EDGE of Existence programme
Students should understand the rationale behind focusing conservation efforts on evolutionarily distinct and globally endangered species (EDGE). NOS: Issues such as which species should be prioritized for conservation efforts have complex ethical, environmental, political, social, cultural and economic implications and therefore need to be debated.

B4.1 Adaptation to Environment (3 Hours)
B4.1.1—Habitat as the place in which a community, species, population or organism lives: A description of the habitat of a species can include both geographical and physical locations, and the type of ecosystem.
B4.1.2—Adaptations of organisms to the abiotic environment of their habitat: Include a grass species adapted to sand dunes and a tree species adapted to mangrove swamps.
B4.1.3—Abiotic variables affecting species distribution: Include examples of abiotic variables for both plants and animals. Students should understand that the adaptations of a species give it a range of tolerance.
B4.1.5—Conditions required for coral reef formation: Coral reefs are used here as an example of a marine ecosystem. Factors should include water depth, pH, salinity, clarity and temperature.
B4.1.6—Abiotic factors as the determinants of terrestrial biome distribution: Students should understand that, for any given temperature and rainfall pattern, one natural ecosystem type is likely to develop. Illustrate this using a graph showing the distribution of biomes with these two climatic variables on the horizontal and vertical axes.
B4.1.7—Biomes as groups of ecosystems with similar communities due to similar abiotic conditions and convergent evolution: Students should be familiar with the climate conditions that characterize the tropical forest, temperate forest, taiga, grassland, tundra and hot desert biomes.
B4.1.8—Adaptations to life in hot deserts and tropical rainforest: Include examples of adaptations in named species of plants and animals

B4.2 Ecological Niches (4 Hours)
B4.2.1—Ecological niche as the role of a species in an ecosystem: Include the biotic and abiotic interactions that influence growth, survival and reproduction, including how a species obtains food.
B4.2.2—Differences between organisms that are obligate anaerobes, facultative anaerobes and obligate aerobes: Limit to the tolerance of these groups of organisms to the presence or absence of oxygen gas in their environment.
B4.2.3—Photosynthesis as the mode of nutrition in plants, algae and several groups of photosynthetic prokaryotes: Details of different types of photosynthesis in prokaryotes are not required.
B4.2.4—Holozoic nutrition in animals: Students should understand that all animals are heterotrophic. In holozoic nutrition food is ingested, digested internally, absorbed and assimilated.
B4.2.5—Mixotrophic nutrition in some protists: Euglena is a well-known freshwater example of a protist that is both autotrophic and heterotrophic, but many other mixotrophic species are part of oceanic plankton. Students should understand that some mixotrophs are obligate and others are facultative.
B4.2.6—Saprotrophic nutrition in some fungi and bacteria: Fungi and bacteria with this mode of heterotrophic nutrition can be referred to as decomposers.
B4.2.7—Diversity of nutrition in archaea: Students should understand that archaea are one of the three domains of life and appreciate that they are metabolically very diverse. Archaea species use either light, oxidation of inorganic chemicals or oxidation of carbon compounds to provide energy for ATP production. Students are not required to name examples.
B4.2.9—Adaptations of herbivores for feeding on plants and of plants for resisting herbivory: For herbivore adaptations, include piercing and chewing mouthparts of leaf-eating insects. Plants resist herbivory using thorns and other physical structures. Plants also produce toxic secondary compounds in seeds and leaves. Some animals have metabolic adaptations for detoxifying these toxins.
B4.2.10—Adaptations of predators for finding, catching and killing prey and of prey animals for resisting predation Students should be aware of chemical, physical and behavioural adaptations in predators and prey.
B4.2.11—Adaptations of plant form for harvesting light Include examples from forest ecosystems to illustrate how plants in forests use different strategies to reach light sources, including trees that reach the canopy, lianas, epiphytes growing on branches of trees, strangler epiphytes, shade-tolerant shrubs and herbs growing on the forest floor.
B4.2.12—Fundamental and realized niches Students should appreciate that fundamental niche is the potential of a species based on adaptations and tolerance limits and that realized niche is the actual extent of a species niche when in competition with other species.
B4.2.13—Competitive exclusion and the uniqueness of ecological niches: Include elimination of one of the competing species or the restriction of both to a part of their fundamental niche as possible outcomes of competition between two species.

Core Procedural Knowledge
What should students be able to do?

B4.1 Adaptation to Environment
B4.1.4—Range of tolerance of a limiting factor: Application of skills: Students should use transect data to correlate the distribution of plant or animal species with an abiotic variable. Students should collect this data themselves from a natural or semi-natural habitat. Semi-natural habitats have been influenced by humans but are dominated by wild rather than cultivated species. Sensors could be used to measure abiotic variables such as temperature, light intensity and soil pH.

B4.2 Ecological Niches
B4.2.8—Relationship between dentition and the diet of omnivorous and herbivorous representative members of the family Hominidae. Application of skills: Students should examine models or digital collections of skulls to infer diet from the anatomical features. Examples may include Homo sapiens (humans), Homo floresiensis and Paranthropus robustus. NOS: Deductions can be made from theories. In this example, observation of living mammals led to theories relating dentition to herbivorous or carnivorous diets. These theories allowed the diet of extinct organisms to be deduced.