Biology

Core Declarative Knowledge
What should students know?

A2.2 Cell Structure (4 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.
B2.1 Membranes and Membrane Transport (4 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.
B2.2 Organelles and Compartmentalization (1 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.

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?

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 cotransporters 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 (2 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.
A1.2 Nucleic Acid (3 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.
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 (2 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”.

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.
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?

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.