IBDP Biology Syllabus Statements 2025

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2	2025 Syllabus						2025 Syllabus
3	Theme	Organization Level		Content	Guidence	Application of Skills	Theme	Organization Level		Content	Guidence	Application of Skills

4	A. Unity & Diversity	A1.1 Water	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. 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.		C. Interaction and interdependence	C1.1 Enzymes and metabolism	C1.1.1	Enzymes as catalysts	Students should understand the benefit of increasing rates of reaction in cells.
5			A1.1.2	Hydrogen bonds as a consequence of the polar covalent bonds within 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.				C1.1.2	Role of enzymes in metabolism	Students should understand that metabolism is the complex network of interdependent and interacting chemical reactions occurring in living organisms. Because of enzyme specificity, many different enzymes are required by living organisms, and control over metabolism can be exerted through these enzymes.
6			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.				C1.1.3	Anabolic and catabolic reactions	Examples of anabolism should include the formation of macromolecules from monomers by condensation reactions including protein synthesis, glycogen formation and photosynthesis. Examples of catabolism should include hydrolysis of macromolecules into monomers in digestion and oxidation of substrates in respiration.
7			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.				C1.1.4	Enzymes as globular proteins with an active site for catalysis	Include that the active site is composed of a few amino acids only, but interactions between amino acids within the overall three-dimensional structure of the enzyme ensure that the active site has the necessary properties for catalysis.
8			A1.1.5	Solvent properties of water linked to its role as a medium for metabolism and for transport in	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.				C1.1.5	Interactions between substrate and active site to allow induced-fit binding	Students should recognize that both substrate and enzymes change shape when binding occurs.
9			A1.1.6	Physical properties of water and the consequences for animals in aquatic habitats	Include buoyancy, viscosity, thermal conductivity and specific heat. 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).				C1.1.6	Role of molecular motion and substrate-active site collisions in enzyme catalysis	Movement is needed for a substrate molecule and an active site to come together. Sometimes large substrate molecules are immobilized while sometimes enzymes can be immobilized by being embedded in membranes.
10			A1.1.7 AHL	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.				C1.1.7	Relationships between the structure of the active site, enzyme–substrate specificity and denaturation	Students should be able to explain these relationships.
11			A1.1.8 AHL	Relationship between the search for extraterrestrial life and the presence of water	Include the idea of the “Goldilocks zone”.				C1.1.8	Effects of temperature, pH and substrate concentration on the rate of enzyme activity	The effects should be explained with reference to collision theory and denaturation.	Students should be able to interpret graphs showing the effects.
12		A1.2 Nucleic Acids	A.1.2.1	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.				C1.1.9	Measurements in enzyme-catalysed reactions		Students should determine reaction rates through experimentation and using secondary data.
13			A.1.2.2	Components of a nucleotide	In diagrams of nucleotides use circles, pentagons and rectangles to represent relative positions of phosphates, pentose sugars and bases.				C1.1.10	Effect of enzymes on activation energy	Students should appreciate that energy is required to break bonds within the substrate and that there is an energy yield when bonds are made to form the products of an enzymecatalysed reaction. Students should be able to interpret graphs showing this effect.	Students should apprecaite that energy is required to break bonds within the substrate and that there is an energy yield when bonds are made to form the products of an enzyme catalysed reactions. Students should be able to interprret the graphs showing this effect.
14			A.1.2.3	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.				C1.1.11 AHL	Intracellular and extracellular enzyme-catalysed reactions	Include glycolysis and the Krebs cycle as intracellular examples and chemical digestion in the gut as an extracellular example.
15			A.1.2.4	Bases in each nucleic acid that form the basis of a code	Students should know the names of the nitrogenous bases.				C1.1.12 AHL	Generation of heat energy by the reactions of metabolism	Include the idea that heat generation is inevitable because metabolic reactions are not 100% efficient in energy transfer. Mammals, birds and some other animals depend on this heat production for maintenance of constant body temperature.
16			A.1.2.5	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.				C1.1.13 AHL	Cyclical and linear pathways in metabolism	Use glycolysis, the Krebs cycle and the Calvin cycle as examples.
17			A.1.2.6	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.				C1.1.14 AHL	Allosteric sites and non-competitive inhibition	Students should appreciate that only specific substances can bind to an allosteric site. Binding causes interactions within an enzyme that lead to conformational changes, which alter the active site enough to prevent catalysis. Binding is reversible.
18			A.1.2.7	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 distinction between ribose and deoxyribose. Students should be familiar with examples of nucleic acids.				C1.1.15 AHL	Competitive inhibition as a consequence of an inhibitor binding reversibly to an active site	Use statins as an example of competitive inhibitors. Include the difference between competitive and noncompetitive inhibition in the interactions between substrate and inhibitor and therefore in the effect of substrate concentration.
19			A.1.2.8	Role of complementary base pairing in allowing genetic information to be replicated and expressed	Students should understand that complementarity is based on hydrogen bonding.				C1.1.16 AHL	Regulation of metabolic pathways by feedback inhibition	Use the pathway that produces isoleucine as an example of an end product acting as an inhibitor.
20			A.1.2.9	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.				C1.1.17 AHL	Mechanism-based inhibition as a consequence of chemical changes to the active site caused by the irreversible binding of an inhibitor	Use penicillin as an example. Include the change to transpeptidases that confers resistance to penicillin.
21			A.1.2.10	Conservation of the genetic code across all life forms as evidence of universal common ancestry	Students are not required to memorize any specific examples of variations.			C1.2 Cell respiration	C1.2.1	ATP as the molecule that distributes energy within cells	Include the full name of ATP (adenosine triphosphate) and that it is a nucleotide. Students should appreciate the properties of ATP that make it suitable for use as the energy currency within cells.
22			A.1.2.11 AHL	directionality of RNA and DNA	Include 5' to 3' linkages in the sugar–phosphate backbone and their significance for replication, transcription and translation.				C1.2.2	Life processes within cells that ATP supplies with energy	Include active transport across membranes, synthesis of macromolecules (anabolism), movement of the whole cell or cell components such as chromosomes.
23			A.1.2.12 AHL	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.				C1.2.3	Energy transfers during interconversions between ATP and ADP	Students should know that energy is released by hydrolysis of ATP (adenosine triphosphate) to ADP (adenosine diphosphate) and phosphate, but energy is required to synthesize ATP from ADP and phosphate. Students are not required to know the quantity of energy in kilojoules, but students should appreciate that it is sufficient for many tasks in the cell.
24			A.1.2.13 AHL	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.	Students are required to use molecular visualization software to study the association between the proteins and DNA within a nucleosome.			C1.2.4	Cell respiration as a system for producing ATP within the cell using energy released from carbon compounds	Students should appreciate that glucose and fatty acids are the principal substrates for cell respiration but that a wide range of carbon/organic compounds can be used. Students should be able to distinguish between the processes of cell respiration and gas exchange.
25			A.1.2.14 AHL	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.				C1.2.5	Differences between anaerobic and aerobic cell respiration in humans	Include which respiratory substrates can be used, whether oxygen is required, relative yields of ATP, types of waste product and where the reactions occur in a cell. Students should be able to write simple word equations for both types of respiration, with glucose as the substrate. Students should appreciate that mitochondria are required for aerobic, but not anaerobic, respiration.
26			A.1.2.15 AHL	Chargaff’s data on the relative amounts of pyrimidine and purine bases across diverse life forms					C1.2.6	Variables affecting the rate of cell respiration		Students should make measurements allowing for the determination of the rate of cell respiration. Students should also be able to calculate the rate of cellular respiration from raw data that they have generated experimentally or from secondary data.
27		A2.1 Origins of Cells (AHL Only)	A2.1.1 AHL	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.				C1.2.7 AHL	Role of NAD as a carrier of hydrogen and oxidation by removal of hydrogen during cell respiration	Students should understand that oxidation is a process of electron loss, so when hydrogen with an electron is removed from a substrate (dehydrogenation) the substrate has been oxidized. They should appreciate that redox reactions involve both oxidation and reduction, and that NAD is reduced when it accepts hydrogen.
28			A2.1.2 AHL	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.				C1.2.8 AHL	Conversion of glucose to pyruvate by stepwise reactions in glycolysis with a net yield of ATP and reduced NAD	Include phosphorylation, lysis, oxidation and ATP formation. Students are not required to know the names of the intermediates, but students should know that each step in the pathway is catalysed by a different enzyme.
29			A2.1.3 AHL	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.				C1.2.9 AHL	Conversion of pyruvate to lactate as a means of regenerating NAD in anaerobic cell respiration	Regeneration of NAD allows glycolysis to continue, with a net yield of two ATP molecules per molecule of glucose.
30			A2.1.4 AHL	Evidence for the origin of carbon compounds	Evaluate the Miller–Urey experiment.				C1.2.10 AHL	Anaerobic cell respiration in yeast and its use in brewing and baking	Students should understand that the pathways of anaerobic respiration are the same in humans and yeasts apart from the regeneration of NAD using pyruvate and therefore the final products.
31			A2.1.5 AHL	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.				C1.2.11 AHL	Oxidation and decarboxylation of pyruvate as a link reaction in aerobic cell respiration	Students should understand that lipids and carbohydrates are metabolized to form acetyl groups (2C), which are transferred by coenzyme A to the Krebs cycle.
32			A2.1.6 AHL	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.				C1.2.12 AHL	Oxidation and decarboxylation of acetyl groups in the Krebs cycle with a yield of ATP and reduced NAD	Students are required to name only the intermediates citrate (6C) and oxaloacetate (4C). Students should appreciate that citrate is produced by transfer of an acetyl group to oxaloacetate and that oxaloacetate is regenerated by the reactions of the Krebs cycle, including four oxidations and two decarboxylations. They should also appreciate that the oxidations are dehydrogenation reactions.
33			A2.1.7 AHL	Evidence for a last universal common ancestor	Include the universal genetic code, several hundred types of genes. 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.				C1.2.13 AHL	Transfer of energy by reduced NAD to the electron transport chain in the mitochondrion	Energy is transferred when a pair of electrons is passed to the first carrier in the chain, converting reduced NAD back to NAD. Students should understand that reduced NAD comes from glycolysis, the link reaction and the Krebs cycle.
34			A2.1.8 AHL	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.				C1.2.14 AHL	Generation of a proton gradient by flow of electrons along the electron transport chain	Students are not required to know the names of protein complexes.
35			A2.1.9 AHL	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.				C1.2.15 AHL	Chemiosmosis and the synthesis of ATP in the mitochondrion	Students should understand how ATP synthase couples release of energy from the proton gradient with phosphorylation of ADP.
36		A2.2 Cell Structure	A2.2.1	Cells as the basic structural unit of all living organisms					C1.2.16 AHL	Role of oxygen as terminal electron acceptor in aerobic cell respiration	Oxygen accepts electrons from the electron transport chain and protons from the matrix of the mitochondrion, producing metabolic water and allowing continued flow of electrons along the chain.
37			A2.2.2	microscopy skills & calculating magnification		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.			C.1.2.17 AHL	Differences between lipids and carbohydrates as respiratory substrates	Include the higher yield of energy per gram of lipids, due to less oxygen and more oxidizable hydrogen and carbon. Also include glycolysis and anaerobic respiration occurring only if carbohydrate is the substrate, with 2C acetyl groups from the breakdown of fatty acids entering the pathway via acetyl-CoA (acetyl coenzyme A).
38			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.			C1.3 Photosynthesis	C1.3.1	Transformation of light energy to chemical energy when carbon compounds are produced in photosynthesis	This energy transformation supplies most of the chemical energy needed for life processes in ecosystems.
39			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.				C1.3.2	Conversion of carbon dioxide to glucose in photosynthesis using hydrogen obtained by splitting water	Students should be able to write a simple word equation for photosynthesis, with glucose as the product.
40			A2.2.5	prokaryote cell structures	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.				C1.3.3	Oxygen as a by-product of photosynthesis in plants, algae and cyanobacteria	Students should know the simple word equation for photosynthesis. They should know that the oxygen produced by photosynthesis comes from the splitting of water.
41			A2.2.6	Processes of life in unicellular organisms	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.				C1.3.4	Separation and identification of photosynthetic pigments by chromatography
42			A2.2.7		Include these functions: homeostasis, metabolism, nutrition, movement, excretion, growth, response to stimuli and reproduction.				C1.3.5	Absorption of specific wavelengths of light by photosynthetic pigments	Include excitation of electrons within a pigment molecule, transformation of light energy to chemical energy and the reason that only some wavelengths are absorbed. Students should be familiar with Syllabus content Biology guide 69 absorption spectra. Include both wavelengths and colours of light in the horizontal axis of absorption spectra.
43			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.				C1.3.6	Similarities and differences of absorption and action spectra		Students should be able to calculate Rf values from the results of chromatographic separation of photosynthetic pigments and identify them by colour and by values. Thin-layer chromatography or paper chromatography can be used.
44			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				C1.3.7	Techniques for varying concentrations of carbon dioxide, light intensity or temperature experimentally to investigate the effects of limiting factors on the rate of photosynthesis
45			A2.2.10	Cell types and cell structures viewed in light and electron micrographs		Students should be able to identify cells in light or 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.			C1.3.8	Carbon dioxide enrichment experiments as a means of predicting future rates of photosynthesis and plant growth	Include enclosed greenhouse experiments and free-air carbon dioxide enrichment experiments (FACE).	Students should be able to determine rates of photosynthesis from data for oxygen production and carbon dioxide consumption for varying wavelengths. They should also be able to plot this data to make an action spectrum.
46			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.			C1.3.9 AHL	Photosystems as arrays of pigment molecules that can generate and emit excited electrons	Students should know that photosystems are always located in membranes and that they occur in cyanobacteria and in the chloroplasts of photosynthetic eukaryotes. Photosystems should be described as molecular arrays of chlorophyll and accessory pigments with a special chlorophyll as the reaction centre from which an excited electron is emitted.	Students should be able to suggest hypotheses for the effects of these limiting factors and explore protocols based upon their understanding of photosynthesis, and test these by experimentation.
47			A2.2.12 AHL	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.				C1.3.10 AHL	Advantages of the structured array of different types of pigment molecules in a photosystem	Students should appreciate that a single molecule of chlorophyll or any other pigment would not be able to perform any part of photosynthesis.
48			A2.2.13 AHL	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.				C1.3.11 AHL	Generation of oxygen by the photolysis of water in photosystem II	Emphasize that the protons and electrons generated by photolysis are used in photosynthesis but oxygen is a waste product. The advent of oxygen generation by photolysis had immense consequences for living organisms and geological processes on Earth.
49			A2.2.14 AHL	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.				C1.3.12 AHL	ATP production by chemiosmosis in thylakoids	Include the proton gradient, ATP synthase, proton pumping by the chain of electron carriers and also the electrons sourced from photosystem I in cyclic photophosphorylation or photosystem II in non-cyclic photophosphorylation.
50		A.2.3 Viruses (AHL Only)	A2.3.1 AHL	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.				C1.3.13 AHL	Reduction of NADP by photosystem I	Students should appreciate that NADP is reduced by accepting two electrons that have come from photosystem I. It also accepts a hydrogen ion that has come from the stroma. The paired terms “NADP and reduced NADP” or “NADP+ and NADPH” should be paired consistently.
51			A2.3.2 AHL	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.				C1.3.14 AHL	Thylakoids as systems for performing the light-dependent reactions of photosynthesis	Students should appreciate where photolysis of water, synthesis of ATP by chemiosmosis and reduction of NADP occur in a thylakoid.
52			A2.3.3 AHL	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.				C1.3.15 AHL	Carbon fixation by Rubisco	Students should know the names of the substrates RuBP and CO2 and the product glycerate 3-phosphate. They should also know that Rubisco is the most abundant enzyme on Earth and that high concentrations of it are needed in the stroma of chloroplasts because it works relatively slowly and is not effective in low carbon dioxide concentrations.
53			A2.3.4 AHL	lysogenic cycle of a virus	Use bacteriophage lambda as an example.				C1.3.16 AHL	Synthesis of triose phosphate using reduced NADP and ATP	Reduced NADP supplies hydrogen for reducing NADP, and ATP supplies the necessary energy.
54			A2.3.5 AHL	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.				C1.3.17 AHL	Regeneration of RuBP in the Calvin cycle using ATP	Students are not required to know details of the individual reactions, but students should understand that five molecules of triose phosphate are converted to three molecules of RuBP, allowing the Calvin cycle to continue. If glucose is the product of photosynthesis, five-sixths of all the triose phosphate produced must be converted back to RuBP.
55			A2.3.6 AHL	rapid evolution in viruses	Consider 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.				C1.3.18 AHL	Synthesis of carbohydrates, amino acids and other carbon compounds using the products of the Calvin cycle and mineral nutrients	Students are not required to know details of metabolic pathways, but students should understand that all of the carbon in compounds in photosynthesizing organisms is fixed in the Calvin cycle and that carbon compounds other than glucose are made by metabolic pathways that can be traced back to an intermediate in the cycle.
56		A3.1 Diversity of Organisms	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.				C1.3.19 AHL	Interdependence of the light-dependent and light-independent reactions	Students should understand how a lack of light stops light-independent reactions and how a lack of CO2 prevents photosystem II from functioning.
57			A3.1.2	Species as groups of organisms with shared traits	This is the original morphological concept of the species as used by Linnaeus.			C2.1 Chemical signalling	C2.1.1 AHL	Receptors as proteins with binding sites for specific signalling chemicals	Students should use the term “ligand” for the signalling chemical.
58			A3.1.3	Binomial system for naming organisms	Students should know that the first name is the genus, the second name is the species and that species in the same genus have similar traits. The genus name is given an initial capital letter but the species name is lowercase.				C.2.1.2 AHL	Cell signalling by bacteria in quorum sensing	Include the example of bioluminescence in the marine bacterium Vibrio fischeri.
59			A3.1.4	Biological spcies 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.				C2.1.3 AHL	Hormones, neurotransmitters, cytokines and calcium ions as examples of functional categories of signalling chemicals in animals	Students should appreciate the differences between these categories.
60			A3.1.5	Difficulties distinguishing between populations and species due to divergence of noninterbreeding 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.				C2.1.4 AHL	Chemical diversity of hormones and neurotransmitters	Consider reasons for a wide range of chemical substances being used as signalling chemicals. Include amines, proteins and steroids as chemical groups of hormones. A range of substances can serve as neurotransmitters including amino acids, peptides, amines and nitrous oxide.
61			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.				C2.1.5 AHL	Localized and distant effects of signalling molecules	Contrasts can be drawn between hormones transported by the blood system and neurotransmitters that diffuse across a synaptic gap.
62			A3.1.7	Karyotyping and karyograms		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 in a shared ancestor.			C2.1.6 AHL	Differences between transmembrane receptors in a plasma membrane and intracellular receptors in the cytoplasm or nucleus	Include distribution of hydrophilic or hydrophobic amino acids in the receptor and whether the signalling chemical penetrates the cell or remains outside.
63			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.				C2.1.7 AHL	Initiation of signal transduction pathways by receptors	Students should understand that the binding of a signalling chemical to a receptor sets off a sequence of responses within the cell.
64			A3.1.9	Diversity of eukaryotic 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.				C2.1.8 AHL	Transmembrane receptors for neurotransmitters and changes to membrane potential	Use the acetylcholine receptor as an example. Binding to a receptor causes the opening of an ion channel in the receptor that allows positively charged ions to diffuse into the cell. This changes the voltage across the plasma membrane, which may cause other changes.
65			A3.1.10	Comparison of genome sizes		Students should extract information about genome size for different taxonomic groups from a database to compare genome size to organism complexity.			C2.1.9 AHL	Transmembrane receptors that activate G proteins	Students should understand how G protein-coupled receptors convey a signal into cells. They should appreciate that there are many such receptors in humans.
66			A3.1.11	Current and future use of 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.				C2.1.10 AHL	Mechanism of action of epinephrine (adrenaline) receptors	Include the roles of a G protein and cyclic AMP (cAMP) as the second messenger.
67			A3.1.12 AHL	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.				C2.1.11 AHL	Transmembrane receptors with tyrosine kinase activity	Use the protein hormone insulin as an example. Limit this to binding of insulin to a receptor in the plasma membrane, causing phosphorylation of tyrosine inside a cell. This leads to a sequence of reactions ending with movement of vesicles containing glucose transporters to the plasma membrane.
68	A3.1.13 AHL		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.		C2.1.12 AHL	Intracellular receptors that affect gene expression		Use the steroid hormones oestradiol, progesterone and testosterone as examples. Students should understand that the signalling chemical binds to a site on a receptor, activating it. The activated receptor binds to specific DNA sequences to promote gene transcription.
69	A3.1.14 AHL		Engagement with local plant or animal species to develop a dichotomous key		Students should engage with local plant or animal species to develop a dichotomous key.	C2.1.13 AHL	Effects of the hormones oestradiol and progesterone on target cells		For oestradiol, limit to cells in the hypothalamus that secrete gonadotropin-releasing hormone. For progesterone, limit to cells in the endometrium.
70	A3.1.15 AHL		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.		C2.1.14 AHL	Regulation of cell signalling pathways by positive and negative feedback		Limit to an understanding of the difference between these two forms of regulation and a brief outline of one example of each.
71	A3.2 Classification & Cladistics (AHL Only)	A3.2.1 AHL	Need for classification of organisms	Classification is needed because of the immense diversity of species. After classification is completed, a broad range further study is facilitated.		C2.2 Neural signalling	C2.2.1	Neurons as cells within the nervous system that carry electrical impulses	Students should understand that cytoplasm and a nucleus form the cell body of a neuron, with elongated nerve fibres of varying length projecting from it. An axon is a long single fibre. Dendrites are multiple shorter fibres. Electrical impulses are conducted along these fibres.
72		A3.2.2 AHL	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.			C2.2.2	Generation of the resting potential by pumping to establish and maintain concentration gradients of sodium and potassium ions	Students should understand how energy from ATP drives the pumping of sodium and potassium ions in opposite directions across the plasma membrane of neurons. They should understand the concept of a membrane polarization and a membrane potential and also reasons that the resting potential is negative.
73		A3.2.3 AHL	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.			C2.2.3	Nerve impulses as action potentials that are propagated along nerve fibres	Students should appreciate that a nerve impulse is electrical because it involves movement of positively charged ions.
74		A3.2.4 AHL	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.			C2.2.4	Variation in the speed of nerve impulses	Compare the speed of transmission in giant axons of squid and smaller non-myelinated nerve fibres. Also compare the speed in myelinated and non-myelinated fibres.
75		A3.2.5 AHL	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.			C2.2.5	Synapses as junctions between neurons and between neurons and effector cells	Limit to chemical synapses, not electrical, and these can simply be referred to as synapses. Students should understand that a signal can only pass in one direction across a typical synapse.
76		A3.2.6 AHL	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.			C2.2.6	Release of neurotransmitters from a presynaptic membrane	Include uptake of calcium in response to depolarization of a presynaptic membrane and its action as a signalling chemical inside a neuron.	Students should be able to describe negative and positive correlations and apply correlation coefficients as a mathematical tool to determine the strength of these correlations. Students should also be able to apply the coefficient of determination (R 2) to evaluate the degree to which variation in the independent variable explains the variation in the dependent variable. For example, conduction velocity of nerve impulses is negatively correlated with animal size, but positively correlated with axon diameter.
77		A3.2.7 AHL	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.			C2.2.7	Generation of an excitatory postsynaptic potential	Include diffusion of neurotransmitters across the synaptic cleft and binding to transmembrane receptors. Use acetylcholine as an example. Students should appreciate that this neurotransmitter exists in many types of synapse including neuromuscular junctions.
78		A3.2.8 AHL	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.			C2.2.8 AHL	Depolarization and repolarization during action potentials	Include the action of voltage-gated sodium and potassium channels and the need for a threshold potential to be reached for sodium channels to start to open.
79		A3.2.9 AHL	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.			C2.2.9 AHL	Propagation of an action potential along a nerve fibre/axon as a result of local currents	Students should understand how diffusion of sodium ions both inside and outside an axon can cause the threshold potential to be reached.
80	A4.1 Evolution & Speciation	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.			C2.2.10 AHL	Oscilloscope traces showing resting potentials and action potentials
81		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.			C2.2.11 AHL	Saltatory conduction in myelinated fibres to achieve faster impulses	Students should understand that ion pumps and channels are clustered at nodes of Ranvier and that an action potential is propagated from node to node.
82		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.			C2.2.12 AHL	Effects of exogenous chemicals on synaptic transmission (including cocaine)	Use neonicotinoids as an example of a pesticide that blocks synaptic transmission, and cocaine as an example of a drug that blocks reuptake of the neurotransmitter.	Students should interpret the oscilloscope trace in relation to cellular events. The number of impulses per second can be measured.
83		A4.1.4	Evidence for evolution from homologous structures	Include the example of pentadactyl limbs.			C2.2.13 AHL	Inhibitory neurotransmitters and generation of inhibitory postsynaptic potentials	Students should know that the postsynaptic membrane becomes hyperpolarized.
84		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.			C2.2.14 AHL	Summation of the effects of excitatory and inhibitory neurotransmitters in a postsynaptic neuron	Multiple presynaptic neurons interact with all-or-nothing consequences in terms of postsynaptic depolarization.
85		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.			C. Interaction and interdependence	C2.2.15 AHL	Perception of pain by neurons with free nerve endings in the skin	Students should know that these nerve endings have channels for positively charged ions, which open in response to a stimulus such as high temperature, acid, or certain chemicals such as capsaicin in chilli peppers. Entry of positively charged ions causes the threshold potential to be reached and nerve impulses then pass through the neurons to the brain, where pain is perceived.
86		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.				C2.2.16 AHL	Consciousness as a property that emerges from the interaction of individual neurons in the brain	Emergent properties such as consciousness are another example of the consequences of interaction.
87		A4.1.8 AHL	Differences and similarities between sympatric and allopatric speciation	Students should understand that reproductive isolation can be geographic, behavioural or temporal.		C3.1 Integration of body systems		C3.1.1	System integration	This is a necessary process in living systems. Coordination is needed for component parts of a system to collectively perform an overall function.
88		A4.1.9 AHL	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.				C3.1.2	Cells, tissues, organs and body systems as a hierarchy of subsystems that are integrated in a multicellular living organism	Students should appreciate that this integration is responsible for emergent properties. For example, a cheetah becomes an effective predator by integration of its body systems.
89		A4.1.10 AHL	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.				C3.1.3	Integration of organs in animal bodies by hormonal and nervous signalling and by transport of materials and energy	Distinguish between the roles of the nervous system and endocrine system in sending messages. Using examples, emphasize the role of the blood system in transporting materials between organs.
90		A4.1.11 AHL	Abrupt speciation in plants by hybridization and polyploidy (alium changed to knotweed)	Use knotweed or smartweed (genus Persicaria) as an example because it contains many species that have been formed by these processes.				C3.1.4	The brain as a central information integration organ	Limit to the role of the brain in processing information combined from several inputs and in learning and memory. Students are not required to know details such as the role of slow-acting neurotransmitters.
91	A4.2 Conservation of Biodiversity	A4.2.1	Biodiversity as the variety of life in all its forms, levels and combinations	Include ecosystem diversity, species diversity and genetic diversity.				C3.1.5	The spinal cord as an integrating centre for unconscious processes	Students should understand the difference between conscious and unconscious processes.
92		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.				C3.1.6	Input to the spinal cord and cerebral hemispheres through sensory neurons	Students should understand that sensory neurons convey messages from receptor cells to the central nervous system.
93		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.				C3.1.7	Output from the cerebral hemispheres to muscles through motor neurons	Students should understand that muscles are stimulated to contract.
94		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.				C3.1.8	Nerves as bundles of nerve fibres of both sensory and motor neurons	Use a transverse section of a nerve to show the protective sheath, and myelinated and unmyelinated nerve fibres.
95		A4.2.5	Evidence of 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.				C3.1.9	Pain reflex arcs as an example of involuntary responses with skeletal muscle as the effector	Use the example of a reflex arc with a single interneuron in the grey matter of the spinal cord and a free sensory nerve ending in a sensory neuron as a pain receptor in the hand.
96		A4.2.6	Causes of 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.				C3.1.10	Role of the cerebellum in coordinating skeletal muscle contraction and balance	Limit to a general understanding of the role of the cerebellum in the overall control of movements of the body.
97		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.				C3.1.11	Modulation of sleep patterns by melatonin secretion as a part of circadian rhythms	Students should understand the diurnal pattern of melatonin secretion by the pineal gland and how it helps to establish a cycle of sleeping and waking.
98		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).				C3.1.12	Epinephrine (adrenaline) secretion by the adrenal glands to prepare the body for vigorous activity	Consider the widespread effects of epinephrine in the body and how these effects facilitate intense muscle contraction.
99	B. Form & Function	B1.1 Carbohydrates & Lipids	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.				C3.1.13	Control of the endocrine system by the hypothalamus and pituitary gland	Students should have a general understanding, but are not required to know differences between mechanisms used in the anterior and posterior pituitary.
100			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.			Students should be able to draw / model bio molecules and be familiar with condensation + hydrolysis reactions.	C3.1.14	Feedback control of heart rate following sensory input from baroreceptors and chemoreceptors	Include the location of baroreceptors and chemoreceptors. Students should understand the role of the medulla in coordinating responses. Baroreceptors monitor blood pressure. Chemoreceptors monitor blood pH and concentrations of oxygen and carbon dioxide. Students should understand the role of the medulla in coordinating responses and sending nerve impulses to the heart to change the heart’s stroke volume and heart rate.