2026 and 2025 paper questions

The Golgi apparatus (B) collects the secretory proteins that the rough ER makes, modifies them so that they can do their specific work (it also forms glycoproteins and lipoproteins), and packs them into vesicles for export. The ribosome (A) only makes the protein, the nucleus (C) holds the DNA, and the lysosome (D) digests material; none of these modifies and packages proteins.

Only about 10% of the energy of one trophic level reaches the next (Figure 15.3), so about 90% is lost, as heat, in respiration, growth, reproduction, non-predatory death and excretory products or faeces. 10% is the amount passed on, not lost, which is the tempting wrong choice; 50% and 100% do not fit the 10% rule.

Cellulose forms the microfibrils of the plant cell wall (C). The tempting option is B, because fungi also have a cell wall, but the fungal wall is made of chitin. Bacterial walls are made of peptidoglycan, and animal cells have no cell wall at all.

Detoxification of drugs and poisons is done by enzymes of the smooth ER, which has no ribosomes and forms branched tubules. In the figure that is Q, the tubular structures at the end of the ER. P is the nucleus, and S and R are flattened sheets studded with ribosomes, the rough ER, which makes secretory proteins. The tempting wrong choices are S and R, which lie next to the nucleus and look like membrane, but they are rough ER.

In muscle during strenuous exercise, oxygen runs short and the cell uses lactic acid fermentation: pyruvate is reduced by NADH to lactic acid, and NADH returns as $\mathrm{NAD}{}^{+}$ so that glycolysis can continue.

Ethanol (A) and $\mathrm{CO}{}_2$ (B) are the products of alcoholic fermentation in yeast, where pyruvate is first decarboxylated. Glucose (D) is the starting material of glycolysis, not a product.

Progesterone from the corpus luteum acts on the estrogen-primed endometrium and makes it glandular and secretory, so the main function is endometrium development (preparing it for implantation). Ovulation is triggered by the LH surge, follicle development is stimulated by FSH, and implantation is the embryo's attachment to the prepared endometrium, not a function of the hormone itself.

The energy that must be added to make molecules react is the energy of activation. In the book it is the energy needed to break the existing bonds and begin the reaction; an enzyme lowers it. Potential energy is energy stored in a system and kinetic energy is energy of motion, so neither names the starting energy of a reaction. Heat is only the way the energy of activation is supplied in a non-living system, not the name of the energy itself.

The unit that leaves the RNA polymerase once it is attached to the promoter is the **sigma ($\sigma$) factor**. It is needed only to bind the promoter; then it is removed and the core enzyme (two $\alpha$ and the $\beta$ subunits) carries on. The printed options have no sigma, so the intended letter is probably the one that looks closest in print, option C. The core subunits $\alpha$ and $\beta$ (A and B) stay on the DNA, and there is no $\theta$ subunit.

In the reduction phase of the Calvin cycle, 3-PGA takes a phosphate from ATP to become 1,3-bisphosphoglycerate, which NADPH then reduces (losing a phosphate) to G3P. So the sequence is 3PGA, 1,3-BPG, G3P (option A).

B leaves out 1,3-BPG; G3P to RuP is also only part of the regeneration, and 3PGA does not become RuP directly. C and D start from G3P and reverse the cycle: RuBP is the acceptor that combines with $\mathrm{CO}{}_2$ to make 3PGA, and G3P is not changed into RuBP in one step.

The book classifies viruses by the structure and composition of the virion: the capsid (shape: helical, polyhedral, enveloped or complex) and the genome (DNA or RNA, single or double stranded). Size alone (A) and colour (C) are not used. Habitat (D) is close to the classification by host (bacteriophage, plant and animal viruses), which is a separate scheme, not a structural one.

The book classification by potency puts oligopotent stem cells (D) at "a few different cell types", for example lymphoid or myeloid stem cells. The tempting option is B: unipotent cells make only one kind of cell, their own type. Totipotent cells (A) form all cell types and pluripotent cells (C) almost any cell.

The bacterial flora of the colon make vitamin K and B vitamins, which the body absorbs. Vitamins A, C and D are not made by gut bacteria: A and C come from the diet, and D is made in the skin in sunlight.

Elastin provides support for connective tissues such as tendons and ligaments (collagen is also a component of them). Keratin (B), the tempting option because it is also a fibrous structural protein, strengthens hair, nails, feathers, horns and beaks. Casein (A) is the milk protein and fibrinogen (D) is a blood plasma protein used in clotting.

Of the four plants, only Nepenthes (the pitcher plant) is a carnivorous plant that traps insects in modified leaves. Saltwort is a halophyte of saline soils, and sunflower and pine are ordinary photosynthetic plants that do not trap insects, so B, C and D are wrong.

A triacylglycerol (triglyceride) is the ester of one glycerol and three fatty acids: each of the three –OH groups of glycerol joins a fatty acid and three water molecules are released. Option A describes a monoacylglycerol (only one fatty acid). Option B is tempting but a phosphate group belongs to a phospholipid, which has glycerol, two fatty acids and a phosphate. Amino acids (D) are the units of proteins, not of lipids.

Malonate has a shape similar to succinate, the substrate of the enzyme succinate dehydrogenase (the book writes succinic dehydrogenase). It enters the active site, forms an enzyme-inhibitor complex and cannot be converted into product, so it competitively inhibits the enzyme. Succinate (B) is the substrate that malonate imitates, not the thing it inhibits, and fumarate (A) is the product of the reaction. Histidine decarboxylase (D) is a lyase and has nothing to do with malonate.

A mycorrhiza is a mutually beneficial partnership between soil fungi and the roots of plants. The fungal hyphae increase the absorbing surface of the root and the plant gives the fungus carbohydrates from photosynthesis.

Algae and fungi (B) is the tempting option, but that association is a lichen, not a mycorrhiza. Bacteria and roots (A) describes a different association, and plants and insects (C) is not a fungal association at all.

A centromere located slightly away from the centre gives two arms of unequal length (one slightly shorter): a submetacentric chromosome. In a metacentric chromosome (D) it is exactly in the centre; in an acrocentric (A) it is near the end; in a telocentric (C) it is at the end.

Amanita (for example Amanita virosa, the destroying angel, and Amanita phalloides, the death cap) is the poisonous genus; one mushroom can be fatal.

Agaricus (A) is the main edible mushroom genus and Morchella (B), the morel, is edible. Aspergillus (C) is a mold used, for example, in soy sauce, though some strains make aflatoxin. The poisonous mushroom is Amanita.

In holozoic nutrition an organism takes in (ingests) solid food and digests it inside its body. The book defines an animal as an eukaryotic, multicellular, ingestive heterotroph, so this is the mode of nutrition of animals. Plants make their own food (autotrophs), so they are not holozoic. Fungi and bacteria do not swallow food: they absorb dissolved nutrients from their surroundings.

The book names the only human disease known to be caused by a viroid as hepatitis D: a short, circular, single stranded RNA with no protein coat. The tempting answer is the virus (B): hepatitis A, B and C are caused by viruses, and hepatitis D needs the hepatitis B virus to supply its capsid, but the cause of hepatitis D is the viroid. Prions (C) cause kuru and mad cow disease.

A fern is a vascular plant: it has xylem and phloem, and its sporophyte has true roots, stems and leaves. Algae and fungi have no vascular tissue (they are not even in the plant groups with xylem and phloem), and a moss is a bryophyte, the non-vascular group, so options B, C and D are wrong.

The cork cambium (phellogen) is the meristem that adds cork to the outside and secondary cortex to the inside. The cork cells die and, with the epidermis, form the rough outer bark of a mature tree, so the bark is produced by the cork cambium.

Lateral meristem (B) is tempting because the cork cambium is a lateral meristem, but the vascular cambium is also a lateral meristem and it makes wood, so the general name does not identify the bark-producing tissue. Apical meristem (C) gives primary growth in length and intercalary meristem (D) gives growth at the nodes or bases of leaves, not bark.

Arthropoda means "animals with jointed legs" (joint-footed). The body is covered by a chitinous exoskeleton that is flexible at many places to allow articulation, and there are several pairs of appendages. Annelida are also segmented, but they move with circular and longitudinal muscles and setae, not jointed appendages.

Polygenic traits are controlled by two or more gene pairs at different loci, all acting on the same trait in an additive way. The intensity of the phenotype depends on the number of effect-causing alleles, so many small steps appear between the extremes and the trait shows continuous variation (height, skin colour, wheat grain colour). Single phenotype, dominant-only and mutation-only are not what polygenic inheritance means.

An X-linked recessive allele shows in a man with a single copy, because he has only one X, but a woman is affected only when she has two copies ($X^h{X}^h$). The trait is therefore more common in males (haemophilia, colour blindness). Autosomal traits are equally common in both sexes. A Y-linked trait occurs only in males, so it is 'only in males', not 'more common', and X-linked dominant traits are, if anything, more common in females.

Chlorophyll-a with the blue pigment phycocyanin (a phycobilin) and nitrogen-fixing heterocysts are the features of cyanobacteria. A plain bacterium (A) does not have these combined; fungi (C) are non-photosynthetic; algae (D) have chlorophyll but no phycocyanin-bearing heterocysts and are eukaryotes. Cyanobacteria are called blue-green algae but are bacteria.

Epistasis is an interaction between two genes at different loci in which one gene hides the effect of the other. The genotypes are formed exactly as in Mendel's dihybrid cross, but the classes that look alike merge, so the usual 9 : 3 : 3 : 1 phenotypic ratio is modified (to 9 : 3 : 4 in the Labrador retriever). The DNA itself and the proteins are not changed by the interaction, and the genotypic ratio is unchanged.

The notochord is one of the four basic features of every chordate, present at some stage, so it is found in every chordate embryo. The vertebral column is found only in vertebrates (it replaces the notochord), the placenta only in eutherian mammals, and lungs are not a chordate feature (gill slits are).

The menstrual cycle is controlled by hormones: the gonadotropins FSH and LH from the anterior pituitary, and estrogen and progesterone from the ovary. Enzymes, vitamins and minerals are not the chemical messengers that coordinate the ovarian and uterine cycles.

Intelligence, height and skin colour are polygenic traits with continuous variation. Tongue rolling is the book's example of a simple Mendelian trait of one gene pair (roller $T$ dominant, non-roller $t$) with two clear classes.

Molecular evidence comes from the common biomolecules in living organisms: DNA with a universal genetic code, ATP, the 20 amino acids and cytochrome c. Embryology and anatomy are different fields (comparative anatomy studies structures, not molecules), and fossils are evidence from palaeontology.

Griffith showed that a heritable factor passes from dead S-type to living R-type bacteria (transformation), and Hershey and Chase showed that only the DNA of a bacteriophage enters the bacterium to direct new viruses, while its protein coat stays outside. Together they point to DNA as the hereditary material. Protein (option A) was the older guess, which these experiments disproved; neither experiment involved RNA or lipids.

Homologous organs have the same basic internal pattern because of a common ancestor. The forelimb of a human and the wing of a bat are both mammalian forelimbs with the same bone pattern. The other pairs are analogous (insect wing and bird wing, insect wing and bat wing: same function, different origin) or unrelated (gills and lungs).

Cell secretion

• The cell secretions are mostly proteins. The Golgi complex collects them from the rough ER (through the smooth ER) in transport vesicles at the cis face.
• It modifies the proteins so that they can do their specific work, and forms glycoproteins and lipoproteins.
• At the trans face the cisternae break up into secretory vesicles that carry the finished products out of the cell. That is why the Golgi complex is abundant in secretory cells.

Cell division (plant cells)

• During cytokinesis the Golgi complex gives rise to vesicles that contain cell wall synthesising material.
• These vesicles line up on the cell equator and fuse to form the phragmoplast; the new cell wall between the daughter cells is made from it.

A cofactor is a non-protein part that some enzymes need to become active. There are three types (1 mark each):

Whisk ferns (Psilopsida) are the simplest vascular plants. Three general characteristics:

• No true roots: they have underground stems called rhizomes that bear rhizoids for anchorage.
• No true leaves: the aerial stems carry only very small green scales, and the stems themselves carry out photosynthesis.
• Sporangia are borne at the apical tips or along the sides of the aerial branches, and the gametophyte is a flat, colourless structure that lives beneath the soil with a fungus inside its cells.

The book classifies enzymes by the substrate they act upon into four groups. Any three, each with examples, earn the marks (1 mark per group with its examples).

Bryophytes (liverworts, hornworts and mosses) are the only non-vascular plants. Three main features:

• They lack xylem and phloem, so they have no true roots, stems or leaves (only rhizoids and root-like, stem-like and leaf-like parts).
• The gametophyte is the main plant body, green and independent. The sporophyte is smaller and depends on the gametophyte for food.
• They have multicellular sex organs that produce the embryo, and the sporophyte is homosporous (one kind of spore). Water is needed for fertilization because the sperms swim.

What the phase is (1 mark)

The preparatory (investment) phase is steps 1 to 5 of glycolysis in the cytoplasm. It uses 2 ATP and changes one glucose (6C) into two molecules of G3P (3C). No ATP is made and no NADH is formed in this phase.

The five steps (2 marks)

Because step 5 gives a second G3P, everything in the pay-off phase happens twice for each glucose.

1. Moist heat (steam, boiling, pasteurisation): the heat causes coagulation of protein in the microbe, so it dies. Milk is pasteurised at $71^{\circ }\mathrm{C}$ for 15 seconds.
2. Dry heat (hot air): heat oxidises the parts of the microbe and kills it. Both dry and moist heat are used in microbiology laboratories.
3. Radiation: electromagnetic radiation below 300 nm, for example gamma rays, kills micro-organisms and is used for sterilisation. (A membrane filter removes bacteria from heat-sensitive liquids such as antibiotics and sera.)

Polarity of water (1 mark)

In a water molecule the shared electrons are pulled towards oxygen, so oxygen is slightly negative ($\delta -$) and each hydrogen slightly positive ($\delta +$). Water is a polar molecule.

Dissolving ionic and polar substances (1 mark)

The charged ends of water molecules attract the ions of ionic compounds (electrolytes) and the polar groups of other molecules, surround them and separate them. Non-polar molecules that carry charged groups also dissolve because their charged regions attract water.

Importance in living things (1 mark)

On dissolving, substances dissociate into positive and negative ions, which are in a more favourable state to react. This is why all the chemical reactions of living organisms occur in an aqueous medium. Water is therefore called the universal solvent (for polar substances).

Phylum Porifera ("pore bearer") has these characteristics (any three, 1 mark each):

• Body organization: sponges are sessile, aquatic (mostly marine) animals. The body is multicellular but not organized into tissues or organs and has no symmetry.
• Pores and cavity: the body is perforated by many pores, the ostia, and has a single central cavity, the spongocoel. Water enters by the ostia and leaves by the osculum, bringing food with it.
• Skeleton: it is made of needle-like spicules (calcareous or siliceous) or of spongin fibre (bath sponge).
• Body wall and reproduction: three layers (pinacoderm, mesenchyme, choanoderm); asexual reproduction by budding or gemmules, sexual by egg and sperm; they regenerate from a small fragment.

Example: Spongilla, Sycon.

Why the bacterium forms a spore (1 mark)

When nutrients run short or temperature and dryness become extreme, bacteria such as Bacillus and Clostridium undergo sporulation instead of dying: the cell forms one endospore inside itself.

How the spore resists (1 mark)

1. The core holds the DNA, a little cytoplasm and ribosomes, with very little water.
2. Several membranes, a peptidoglycan cortex and a thick keratin-like protein coat surround it.
3. So it withstands high temperature, drying, chemicals and lack of nutrients.

Return to active life (1 mark)

The spore stays dormant until the mother cell ruptures and releases it. When conditions are favourable again it takes in water, germinates and grows into a vegetative cell. Since one cell makes one spore, this is survival, not reproduction.

Griffith injected mice with different bacterial samples. The control experiments (live R-type alone, heat-killed S-type alone, and the mixture) lead to three conclusions.

The sequence of amino acids is the primary structure of a protein and it decides how the protein works. In haemoglobin the change of one amino acid out of 146 in the β-chain (HbA to HbS) has three effects.

Effect 1: the haemoglobin molecule itself changes (1 mark)

Glutamic acid at position 6 of the β-chain is replaced by valine, so normal haemoglobin (HbA) becomes abnormal sickle haemoglobin (HbS).

Effect 2: the red blood cells change shape (1 mark)

Normal red blood cells are discs, like doughnuts without holes, and move easily through vessels. Cells with HbS become sickle or crescent shaped, stiff and sticky.

Effect 3: blood flow is blocked and the disease sickle cell anaemia results (1 mark)

Sickle cells block blood flow in the vessels of the limbs and organs, which causes pain and organ damage.

Their secretions make up the liquid part of the semen.

HIV has twelve numbered stages in the book. Any six, in order (half a mark each):

Definition (1 mark)

Incomplete dominance is a dominance relation in which neither allele of a pair is completely dominant, so the heterozygote shows a phenotype intermediate between those of its two homozygous parents.

Example: flower colour of the 4 O'clock plant (2 marks)

1. Red ($R_1{R}_1$) x white ($R_2{R}_2$) gives all pink ($R_1{R}_2$) F1 plants, as Carl Correns found in 1899.
2. F1 x F1 gives red : pink : white in the ratio 1 : 2 : 1, which is also the genotypic ratio ($R_1{R}_1:R_1{R}_2:R_2{R}_2$).

Why it occurs (1 mark)

Non-cyclic flow makes NADPH and ATP, but the Calvin cycle needs more ATP than NADPH. When NADPH rises and ATP is short, the chloroplast shifts for a while to cyclic flow, which makes only ATP, until the supply of ATP catches up with the demand.

How it occurs (2 marks)

Plants cannot move from place to place, but their organs move by differential growth in response to an external stimulus. These growth movements are called tropisms (tropic movements). Three of them are:

1. Phototropism (1 mark)

Movement of a plant part towards (positive) or away from (negative) light. Shoots grow towards light (positive); roots grow away from light (negative).

2. Geotropism (1 mark)

Movement of a plant part in response to gravity. Shoots grow upward, away from gravity (negative); roots grow downward, towards gravity (positive).

3. Thigmotropism (1 mark)

Movement in response to touch. Climbing plants coil round a wall, rod or rope that supports them.

(Chemotropism, the growth of fungal hyphae towards nutrients, may be written instead of any one of these.)

The lytic cycle of a bacteriophage (virulent phage) has these steps, in order:

Both groups are autotrophic: they build carbohydrate from carbon dioxide. They differ only in the source of energy used to do it.

Definition (1 mark)

A mycorrhiza is a mutually beneficial partnership between soil fungi and the roots of plants. The hyphae increase soil contact and surface area and help the root absorb phosphorus, zinc and copper; the plant gives the fungus carbohydrates made by photosynthesis.

Types (2 marks)

Bryophytes (liverworts, hornworts and mosses) are the non-vascular plants. Three characteristics:

• They lack specialized vascular tissue (no xylem or phloem), so they have no true roots, stems or leaves.
• The gametophyte is the dominant, green, independent plant body, while the sporophyte is smaller and obtains its food from the gametophyte.
• They form multicellular sex organs (antheridia and archegonia) and a multicellular embryo, and the sporophyte is homosporous; their swimming sperms need water for fertilization.

1. Moist heat (steam, boiling, pasteurisation): the heat causes coagulation of protein in the microbe, so it dies. Milk is pasteurised at $71^{\circ }\mathrm{C}$ for 15 seconds.
2. Dry heat (hot air): heat oxidises the parts of the microbe and kills it. Both dry and moist heat are used in microbiology laboratories.
3. Radiation: electromagnetic radiation below 300 nm, for example gamma rays, kills micro-organisms and is used for sterilisation. (A membrane filter removes bacteria from heat-sensitive liquids such as antibiotics and sera.)

Point mutation (1 mark)

A point (gene) mutation is a change in a single gene produced by a change of one or a few bases of the DNA. It may be a base substitution (one base replaced by another), an insertion or a deletion of bases.

How it causes sickle cell anaemia (2 marks)

Meaning of minimum temperature (1 mark)

The minimum temperature is the temperature at which an enzyme that has been inactivated by cold becomes active again when it is warmed.

Effect on enzymatic action (2 marks)

• Near or below freezing point enzymes are inactivated, not denatured: molecular motion is too slow for enzyme and substrate to react, but the enzyme structure is not damaged.
• When the temperature is raised the activity returns, and the rate then rises (about double for every 10 degrees Celsius) up to the optimum, 37 to 38 degrees Celsius for human enzymes.

Water entering the root hairs reaches the xylem by the apoplast, symplast or vacuolar pathway. The first two compare as follows:

Event 1: engulfment (1 mark)

Mitochondria and chloroplasts show many similarities with bacteria, so evolutionists believe that they were once independent organisms. An early ancestor of the eukaryotic cell (the host) engulfed an aerobic bacterium, which became the mitochondrion. In the same way a host cell engulfed a photosynthetic bacterium (a cyanobacterium), which became the chloroplast.

Event 2: the engulfed cell stays alive and forms a relationship (1 mark)

The engulfed cell was not digested. It formed a relationship with the host cell in which it was enclosed and became an endosymbiont, a cell living within another cell. The host gave it protection and raw materials, and the endosymbiont gave the host energy (ATP from the aerobic bacterium; food from the photosynthetic one).

Event 3: the endosymbiont becomes an organelle (1 mark)

Over time the endosymbiont became a permanent part of the host, an organism within an organism. It still divides by itself (fission) and keeps its own small circular DNA, all kinds of RNA and 70S ribosomes, so its number doubles before cell division and it makes some of its own proteins.

Molecular biology shows biochemical homology: the same kinds of molecules are present in a complete range of living organisms, just as organs and tissues show anatomical homology. Even taxonomically very remote organisms share them, so they reflect an evolutionary relationship and a common ancestor. Three examples:

1. DNA: the hereditary material of all living organisms is DNA made of the same kind of nucleotides, and the genetic code is universal.
2. ATP: the energy currency molecule is the same from prokaryotes to the most advanced plants and animals.
3. Amino acids: all living organisms make their proteins from the same 20 types of amino acid.

Cytochrome c, a respiratory protein of the respiratory chain of all aerobic organisms, is a fourth example. Because such molecules are shared even by remote species, they point to the origin of living organisms from a common ancestor.

Nitrogen gas ($\mathrm{N}{}_2$) has a very strong triple bond between its two atoms, so it is chemically unreactive and a large amount of energy is needed to break the bond. It must be fixed (combined into compounds) before organisms can use it. It is fixed in three ways.

1. Atmospheric fixation (1 mark)

Lightning supplies the energy for nitrogen and oxygen of the air to combine and form various oxides of nitrogen. Rain carries these oxides into the soil, where plants can use them. This is spontaneous and fixes only a small amount (5 to 8%).

2. Industrial fixation (1 mark)

Man makes nitrogen-containing fertilizers in industry, for example by combining nitrogen and hydrogen to form ammonia.

3. Biological fixation (1 mark)

Nitrogen-fixing bacteria convert atmospheric nitrogen to ammonia. This is the largest share (about 60%). Free-living bacteria such as Azotobacter (aerobic) and Clostridium (anaerobic) make fixed nitrogen available to plants when they die and lyse. Rhizobium is symbiotic and lives in the root nodules of legume plants.

Denitrification (1.5 marks)

In the absence of oxygen, certain soil bacteria (denitrifying bacteria, e.g. Pseudomonas) break down nitrates and use the oxygen for their own respiration. Nitrogen is released back into the atmosphere as a gas, so this is a loss of nitrogen from the soil. It completes the cycle.

Assimilation (1.5 marks)

Assimilation is the utilisation of nitrogenous compounds in living bodies. Plants take up nitrogen from the soil as inorganic compounds such as ammonia and nitrates and build amino acids, proteins and nucleic acids from it. Animals get their nitrogen by eating plants or other animals. (Many micro-organisms can use free nitrogen directly from the atmosphere.)

1. Survival of extreme conditions: the spore resists high temperature, drying (desiccation), and exposure to chemicals and most poisons, so the bacterium outlives shortage of water and nutrients.
2. Protection of the genetic material: the DNA, a little cytoplasm and ribosomes lie in the core, shielded by several membranes, a peptidoglycan cortex and a thick protein (keratin-like) coat.
3. Dormancy and recovery: the dehydrated spore stays dormant for a long time and, when conditions improve, germinates into an active vegetative cell that can grow and reproduce.

Structure (2 marks)

Functions (1 mark)

• Ovary: produces ova (eggs) by oogenesis and releases hormones (estrogen and progesterone).
• Oviduct: receives the oocyte, is the usual site of fertilization, and transfers the developing ovum towards the uterus.

Any three of these are accepted. GnRH (from the hypothalamus, makes the anterior pituitary release FSH and LH) and inhibin (from the Sertoli cells, inhibits GnRH and FSH when the sperm count is too high) are the other two.

Y-linked inheritance (1 mark)

Y-linked (holandric) traits are controlled by genes on the non-homologous region of the Y chromosome, which have no allele on the X chromosome.

Transmission (1 mark)

• They are expressed only in males, as only males have a Y chromosome.
• A son always receives the Y from his father, so the trait passes from father to all his sons and never to daughters.

Examples (1 mark)

Hypertrichosis (hair on the rim of the pinna), porcupine man (straight hair on the body) and webbing of the toes.

Events of the menstrual phase (days 1 to 5), 1 mark each

1. The uterus sheds all but the deepest part of its endometrium: the thick, hormone-dependent functional layer detaches from the uterine wall.
2. The detached tissue and blood pass out through the vagina as the menstrual flow, with bleeding for 3 to 5 days.
3. The ovarian hormones are at their lowest normal levels, and the FSH level begins to rise (a new follicle starts to grow).

Use the six groups of enzymes classified by reaction type. Each row is worth 1 mark.

Mendel's monohybrid cross gives a 3 : 1 phenotypic ratio in F2 because one allele is completely dominant and hides the other in the heterozygote. In incomplete dominance neither allele is completely dominant, so the ratio changes. The 4 O'clock plant (Mirabilis jalapa) shows this.

Cross (1 mark for parents and F1)

• Parents: pure red ($R_1{R}_1$) x pure white ($R_2{R}_2$). Gametes: $R_1$ and $R_2$.
• F1: all $R_1{R}_2$ with pink flowers. The heterozygote is intermediate and resembles neither parent, whereas Mendel's F1 resembled one parent.

F2 (1 mark)

Genotypic ratio 1 : 2 : 1 and phenotypic ratio 1 : 2 : 1 (red : pink : white).

Why it is a deviation (1 mark)

The alleles still separate (segregate) into different gametes, but because dominance is incomplete the heterozygote can be told apart from both homozygotes. The F2 phenotypic ratio is therefore 1 : 2 : 1 instead of Mendel's 3 : 1, and the genotypic and phenotypic ratios become the same.

(a) Genotypes and phenotypes (2 marks)

Parents: black $BbEe$ x chocolate $bbEe$. Gametes of $BbEe$: $BE$, $Be$, $bE$, $be$. Gametes of $bbEe$: $bE$, $be$.

Phenotypic ratio black : chocolate : yellow $=3:3:2$. Yellow appears because $ee$ is epistatic: it hides both $B$ and $b$.

(b) Probabilities (1 mark)

Black $=3/8$ (37.5%). Yellow $=2/8=1/4$ (25%).

Polypeptide elongation is repeated in three steps while the ribosome moves along the mRNA from the 5′ end towards the 3′ end.

Pyruvate made in the cytosol by glycolysis enters the mitochondrion by active transport through pyruvate translocase. There the pyruvate dehydrogenase complex (three enzymes) converts it to acetyl CoA, which enters the Krebs cycle. This is why it is the link reaction.

For one glucose (2 pyruvate) this gives 2 acetyl CoA, 2 NADH and 2 $\mathrm{CO}{}_2$. No ATP is made directly.

All chordates possess four basic characteristics (any three, 1 mark each):

• Notochord: a solid, unjointed rod in the mid-dorsal line between the gut and the nerve cord; it is the axial endoskeleton.
• Dorsal hollow nerve cord: a single, tubular, fluid-filled central nervous system lying above the notochord. Non-chordates have a ventral, solid nerve cord.
• Gill slits (pharyngeal pouches): paired openings from the pharynx to the outside, present at some stage of life.
• Post-anal tail (the fourth characteristic): a tail extending beyond the anus, present at least in the embryo.

Any three rows earn full marks: one mark for each disease named with its cause and a short explanation.

Bacteria were grown for many generations in ^{15}\mathrm{N} (heavy) medium, then moved to ^{14}\mathrm{N} (light) medium. DNA was extracted at intervals and spun in a CsCl density gradient; heavier DNA settles lower in the tube.

One middle band after one generation rules out the conservative model (it predicts a heavy and a light band), and the light band at 40 minutes rules out the dispersive model (it predicts a single band that only moves up). The results fit semi-conservative replication.

Differences between dicots and monocots in flower, stem and seed:

Lamarck was right that the environment can change an individual, but his mechanism fails for three main reasons:

1. No experimental proof: there is no experimental evidence that acquired characters are inherited.
2. Use and disuse cannot alter the genotype: the changes are non-genetic, so the organism's genes stay the same and nothing is passed on.
3. Somatic changes are not heritable: changes of the body made during life (such as a blacksmith's biceps) are not carried to the offspring. Organs are also not modified by the wish or requirements of an organism.

Angiosperms (flowering plants) are the dominant land plants today. Three characteristics that make them successful:

• Flowers and fruits: the ovules are enclosed in the ovary, which becomes the fruit after fertilization. The seed is protected, and the fruit helps to protect and spread it, which enhances reproductive success.
• Well-developed vascular system: vessels and fibres in the xylem and companion cells in the phloem conduct water and food efficiently, so the plant can grow large on land.
• Dominant sporophyte with heteromorphic alternation of generations, and a pollen tube that carries the male gametes to the ovule, so fertilization does not depend on water.

Three enzymes of DNA replication and their functions (any three):

The theory says that chloroplasts and mitochondria were once free-living bacteria that came to live inside an early host cell.

Evidence from the book

• Both organelles have a small circular DNA, all kinds of RNA and 70S ribosomes, like a bacterium.
• Both are self-replicating organelles and divide by fission.
• Both have their own metabolic machinery and can survive outside the cell in an artificial medium if carefully fractionated.
• Both are enclosed by a double membrane.

Convergent and divergent evolution are both ways in which species change, but they differ in ancestry, habitat and the structures produced:

Both are disaccharides (\ce{C12H22O11}) formed by a condensation reaction that releases one water molecule, but they differ as follows.

ATP synthetase (ATP synthase) sits in the inner mitochondrial membrane (and in the thylakoid membrane in chloroplasts). In chemiosmosis it does three jobs.

1. Proton channel: its F_0 part, embedded in the membrane, lets protons flow back down the gradient from the intermembrane space (high \ce{H+}) to the matrix (low \ce{H+}).
2. Uses the proton gradient: it is activated by the gradient built by the electron transport chain, and changes the energy of the flowing protons into chemical energy.
3. Makes ATP: its F_1 head, projecting into the matrix, joins ADP and inorganic phosphate (Pi) by a high-energy bond to form ATP.

The limitations (drawbacks) of Lamarckism are:

1. No experimental proof: there is no experimental evidence that acquired characters are inherited.
2. Use and disuse cannot alter the genotype: the changes are non-genetic, so the organism's genes stay the same and nothing is passed on.
3. Somatic changes are not heritable: changes of the body made during life (such as a blacksmith's biceps) are not carried to the offspring. Organs are also not modified by the wish or requirements of an organism.

In short, the environment can change an individual, but such acquired traits are non-genetic and cannot be inherited.

Angiosperms are flowering plants whose seeds are enclosed in fruits.

Vascular tissues (1.5 marks)

• The vascular system is well developed: the xylem has vessels and fibres (gymnosperms have only tracheids) and the phloem has companion cells, which gymnosperms lack.
• This allows efficient conduction of water, minerals and food, so angiosperms can grow large on land.

Alternation of generations (1.5 marks)

• They show heteromorphic alternation of generations: the sporophyte and gametophyte look different.
• The sporophyte is dominant and independent (the plant we see, with the flowers), while the gametophyte is small, less prominent and dependent on the sporophyte.

Identification (1 mark)

The figure shows the lytic cycle of a bacteriophage. The infected bacterium is filled with new phages and ends as a burst cell with many free phages. The phage is a lytic (virulent) phage and has a master-slave relationship with its host.

Infection process (3 marks)

Replication in the host cell (2 marks)

HIV is specific to CD4 cells: its gp 120 spikes bind the CD4 receptors on helper T cells, macrophages and certain brain cells (section 5.4.2). Its life cycle has twelve numbered stages, shown in the figure in the order of the book.

Steps of the life cycle (5 marks)

Life cycle diagram (2 marks)

The drawing shows a replication fork moving to the right: the unopened parental DNA is at the right, and the two daughter duplexes trail away at the left. Each part is named below, with its work (1 mark for naming and 1 mark for the explanation in each of the two halves, 3+3).

Identification and explanation (3 + 3 marks)

Xerarch succession is primary succession that begins on bare rock where water is deficient. It passes through seral stages and ends in a climax community.

In 1953 James Watson and Francis Crick built a model of DNA from the X-ray diffraction pictures of Wilkins and Franklin (a duplex 2 nm wide, with one turn every 3.4 nm) and Chargaff's rule (A = T and G = C). Their model says:

Structure (4 marks)

• DNA is made of two polynucleotide chains wound round each other as a right-handed double helix and held together by base pairs.
• The two chains are antiparallel: one runs 5′ to 3′ downward and the other 5′ to 3′ upward. The sugar-phosphate backbones are on the outside and the bases are inside.
• The helix has a constant width of 2 nm; each turn is 3.4 nm long and contains 10 base pairs.
• Pairing is specific: adenine pairs with thymine (two hydrogen bonds) and guanine pairs with cytosine (three hydrogen bonds), so a purine always pairs with a pyrimidine and the two chains are complementary.
• There is no restriction on the order of nucleotides along one strand; the sequence is specific for each species and individual and carries the genetic information.

Diagram (2 marks)

Translation (the second step of gene expression) is the decoding of the mRNA by the ribosome into a polypeptide. The amino acids have already been activated, that is joined to their own tRNAs by aminoacyl-tRNA synthetase (aminoacyl-tRNA complexes). The three phases that follow are given below.

1. Initiation complex (2 marks)

2. Elongation (2 marks)

3. Termination (1 mark)

When a stop codon (UAA, UAG or UGA) is exposed at the A site, no tRNA binds. Release factors recognise it, the polypeptide is released from the tRNA, the tRNA leaves, and the ribosomal subunits separate from the mRNA.

Diagrams (2 marks)

Passage of electrons (5 marks)

After glycolysis, the link reaction and the Krebs cycle, most of the energy of glucose is held in **NADH and \ce{FADH2}. They are oxidised in the electron transport chain (ETC) in the inner mitochondrial membrane, and their electrons fall down a series of carriers to the final acceptor, oxygen. The passage is a downhill journey**: energy is released in small steps instead of all at once.

The energy released pumps protons from the matrix into the intermembrane space at three sites: between FMN and CoQ (complex I), between Cyt b and Cyt c_1 (complex III) and between Cyt a and Cyt a_3 (complex IV). The resulting gradient is used by ATP synthetase to make ATP, so the chain is also called oxidative phosphorylation.

Diagram (2 marks)

Kinds of succession (3 marks)

Ecological succession is the process by which species are replaced over time. There are two kinds.

Succession is also named by the habitat. Hydrarch succession begins in water (ponds, lakes, marshes) and its stages are hydroseres. Xerarch succession begins on bare rock, sand dunes and rocky slopes where water is deficient, and its stages are xeroseres.

General stages of succession (3 marks)

Structure (2 marks)

Mitochondria are cylindrical or rod-shaped organelles present in all eukaryotic cells. Each is enclosed by a double membrane.

• Outer membrane: smooth and sieve-like because of the porins, special proteins that let molecules pass freely into the intermembrane space.
• Inner membrane: selectively permeable and folded inwards into cristae, which increase the surface area. Its inner surface carries the granular F0-F1 particles (ATP synthase) and the electron carrier complexes of the electron transport chain.
• Intermembrane space: the narrow region between the two membranes.
• Matrix: the jelly-like material enclosed by the inner membrane. It holds a small circular DNA, all kinds of RNA, 70S ribosomes and enzymes.

Functions (2 marks)

Mitochondria are the sites of cellular respiration: they use oxygen to make ATP by extracting energy from sugars, fats and other organic compounds.

Labelled figure (2 marks)

Pathways (4 marks)

The path depends on the ligand. A signal is passed through three stages: reception, transduction and response. The book describes two pathways.

1. Protein (peptide) signal

2. Steroid signal

Diagrams (2 marks)

Steroids (3 marks)

Structure. Steroids are lipids of high molecular weight which can be crystalline. The steroid nucleus has 17 carbon atoms arranged in four attached rings: three rings (A, B and C) of six carbons and a fourth ring (D) of five carbons. The side chains that extend from these rings distinguish one steroid from another. Steroids are synthesised from isoprene units.

Roles. Cholesterol is a structural component of the cell membrane and the precursor of other steroids: bile acids and bile salts (they emulsify fats), the male sex hormone testosterone and the female sex hormones progesterone and oestrogen. Vitamin D, which helps to regulate calcium metabolism, is also a steroid.

Prostaglandins (3 marks)

Structure. Prostaglandins are lipids derived from arachidonic acid, a fatty acid. They occur in virtually every mammalian tissue and act as local hormones.

Roles. Their functions depend on the tissue: some lower blood pressure while others raise it; they help to induce fever and inflammation and intensify pain; and they regulate the aggregation of platelets, an early step in blood clotting. Aspirin reduces fever and pain because it inhibits prostaglandin synthesis.

Stomata open when their guard cells are turgid and close when the guard cells are flaccid. According to the \ce{K+} theory the movement of potassium ions in and out of the guard cells changes their water potential.

Opening of stomata in the morning (3 marks)

Closing of stomata in the evening (3 marks)

Events of non-cyclic photophosphorylation (2 marks)

Non-cyclic photophosphorylation is the main pathway of the light reaction. Both photosystems and two electron transport chains are used, and the electrons do not return: they leave PS-II, pass to PS-I and end up in NADPH. The zigzag path of the electrons is called the Z-scheme.

Products: **ATP, NADPH and \ce{O2}**. Order of carriers: PS-II (P680), primary acceptor, PQ, cytochrome b_6f, PC, PS-I (P700), primary acceptor, Fd, NADP reductase, \ce{NADP+}.

Absorption of light by PS-II (2 marks)

Chemiosmotic synthesis of ATP (2 marks)

Photoperiodism (1 mark)

The effect of the photoperiod (the total duration of light in the day) on the development of flowers is photoperiodism. Short-day plants flower when the day is shorter than a critical length, long-day plants when it is longer, and day-neutral plants flower in any day length.

Dark period is the real stimulus (1 mark)

Night-interruption experiments show that the uninterrupted dark period is critical. A short-day plant (cocklebur) will not flower if its long night is interrupted by light, so short-day plants are really long-night plants. A long-day plant (henbane) behaves in the opposite way: it is a short-night plant. Red light (660 nm) is the effective light and far-red light (730 nm) reverses it; the last treatment decides the response.

Phytochrome and its two forms (2 marks)

The mechanism depends on the phytochromes, blue-green protein pigments that exist in two interconvertible forms: Pr (\mathrm{P_{660}}), which absorbs red light and changes into Pfr (\mathrm{P_{730}}), and Pfr, which absorbs far-red light and reverts to Pr. Pr changes into Pfr in the day; Pfr changes slowly back into Pr during the night, and this rate of conversion works as a biological clock that measures the length of the night.

Florigen (2 marks)

Phylum Cnidaria (2 marks)

• Diploblastic with mesoglea between ectoderm and endoderm; radial or biradial symmetry.
• Sac-like gastrovascular cavity with one opening (mouth) surrounded by tentacles.
• Stinging cells (cnidocytes with nematocysts) on tentacles and body.

Example: Hydra, Obelia, jellyfish.

Phylum Arthropoda (2 marks)

• Segmented body with several pairs of jointed appendages, bilateral symmetry.
• Body covered by a chitinous exoskeleton; the coelom is reduced to a haemocoel.
• Sexes separate; fertilization mostly internal; development through metamorphosis.

Example: prawn, butterfly, spider, cockroach.

Phylum Mollusca (2 marks)

• Soft, unsegmented, bilateral, coelomate animals; body of head, ventral muscular foot and dorsal visceral mass.
• A mantle covers the body and secretes the shell (external, internal or absent); a radula may be present.
• Development through trochophore larvae.

Example: snail, oyster, freshwater mussel, octopus.

What the disease is

Haemophilia is a hereditary disease in which blood does not clot properly after injury, because a clotting factor is reduced, faulty or absent. Haemophilia A (factor VIII, 80%) and B (factor IX, 20%) are X-linked recessive; haemophilia C is autosomal recessive.

Genotypes

Pattern of inheritance

• It is carried on the X chromosome and is recessive, so a man with one X^h is affected (he has no second X to mask it), while a woman must be X^hX^h. It is therefore much more common in men.
• A carrier mother passes X^h to half of her eggs, so half of her sons are affected and half of her daughters are carriers.
• An affected father gives his X only to daughters (all carriers) and his Y to sons, so it never passes directly from father to son. It 'zigzags' from the maternal grandfather through a carrier daughter to a grandson.

Yes, but only when the mother is a carrier. The haemophilic son gets his single X from his mother, and the allele is recessive, so a normal-looking mother can be X^HX^h while the father is X^HY (normal).

So the probability of a haemophilic child is 1/4 (25%); all the affected children are sons, and half of the sons (50%) are affected. No daughter can be haemophilic, because the normal father gives every daughter X^H. If the mother were X^HX^H, no child could be affected.

The smooth ER makes lipids (oil, cholesterol, phospholipids and steroids), so the answer is C. Protein synthesis (A) is the job of the ribosomes on the rough ER, DNA replication (B) happens in the nucleus, and respiration (D) takes place in the mitochondria.

The middle loop of a tRNA is the anticodon loop. It is made of 7 bases, the middle three of which are the anticodon, which pairs with the complementary codon on the mRNA. The amino acid is attached to the CCA at the 3′ end of the molecule (option D), not by the middle loop.

Glycogen is the storage carbohydrate of animals, kept mainly in the liver and muscles, so it is called animal's starch. Starch (A) is the tempting wrong answer: it is also a storage polysaccharide of α-glucose, but it is stored in plants. Cellulose (C) and chitin (D) are structural, not storage, polysaccharides.

Prostaglandins are derived from arachidonic acid, a fatty acid (spelled "Archidonic" in the paper). Steroids (B) are another group of lipids, but they are synthesised from isoprene units, not the source of prostaglandins. Amino acids and nucleic acids are the units of proteins and the nucleic acids, not lipid precursors.

Protein and peptide signals are water soluble (I) and cannot cross the membrane, so they bind a receptor on the plasma membrane (II). The answer is A. Statements III and IV describe the steroid signal: steroids are lipophilic and act without a second messenger, while the protein signal does use a second messenger such as cAMP.

Yeast carries out alcoholic fermentation: pyruvate is decarboxylated to acetaldehyde (releasing \ce{CO2}), and acetaldehyde is reduced by NADH to ethanol. Ethanol (with \ce{CO2}) is the end product.

Lactic acid (A) is the end product in muscles and some bacteria. Water (B) and oxygen (C) are not products of fermentation: water is formed only when oxygen accepts electrons in aerobic respiration, and oxygen is not made at all.

Ribosomes are assembled in the nucleolus, the dense, non-membrane-bound body in the nucleoplasm, so the answer is S (D). Its granular area holds ribosomal subunits and its fibrillar area holds rRNA and rDNA. P is the nuclear envelope, and Q and R mark the chromatin, which holds the DNA but does not build ribosomes.

Enzymes are biological catalysts. They speed up a reaction by lowering the energy of activation (Figure 3.6 of the book): the reactants and products stay at the same energy levels, but the hill between them is lower. A is the opposite of what happens. An enzyme is not consumed (it comes out unchanged, so C is wrong) and it does not change what the products are (D is wrong).

Lyases break specific covalent bonds and remove groups without hydrolysis (for example histidine decarboxylase). Hydrolases (B) break bonds too, but they do it by adding water. Oxidoreductases (A) add or remove electrons or H^+, and transferases (C) move a functional group from one substrate to another.

A virus that infects bacteria is a bacteriophage (phage), the tadpole-shaped virus of the book. Plant viruses (A) infect plants (for example tobacco mosaic virus) and animal viruses (B) infect animals. Retroviruses (D), such as HIV, have an RNA genome and infect animal cells (HIV attacks CD4 cells).

The book names the only human disease caused by a viroid as hepatitis D (a short, circular RNA with no coat). Hepatitis A is caused by the hepatitis A virus (A), hepatitis B by the hepatitis B virus (B) and hepatitis C by the hepatitis C virus (C). Hepatitis D needs the hepatitis B virus to enter a liver cell, which is why B can look right.

The bacterial flora of the gut make vitamin K (and B vitamins). Proteins, lipids and DNA are made by the body's own cells from the digested food; gut bacteria help digestion and vitamin supply but are not the source of these.

The drawing shows flagella spread all over the surface of the cell. That is the peritrichous arrangement (peri = around). Lophotrichous has a tuft of flagella at a pole, amphitrichous has one flagellum at each end, and monotrichous has a single flagellum.

The Venus flytrap (Dionaea muscipula) has modified leaves that act as traps. When an insect touches the sensitive hairs on the leaf, the trap closes, so it responds to a touch stimulus.

Heat (B), light (C) and gravity (D) are not the stimulus that closes the trap. Light and gravity are the stimuli for phototropism and geotropism of shoots and roots.

A phospholipid is built from glycerol, two fatty acids and a phosphate group (with a compound such as choline attached to the phosphate). Option A, three fatty acids only, describes a triacylglycerol, which has no phosphate. Glucose units only (C) describes a polysaccharide and amino acids only (D) a protein.

In conjugation a plasmid (sometimes part of the chromosome) passes directly from a donor to a recipient cell through a bridge-like connection. Binary fission (A) only copies DNA into daughter cells and moves nothing between cells. Transduction (C) and transformation (D) also transfer DNA, but by a bacteriophage and by free DNA taken up from the surroundings; the question describes direct donor-to-recipient transfer, which is conjugation.

In a lichen the fungal hyphae form the body, wrap round the algal cells and give them protection from drying out (moisture). The alga or cyanobacterium makes the food by photosynthesis and the fungus gains nutrients from it.

Food (A) is the tempting option, but it is supplied by the photosynthetic partner, not by the fungus. Oxygen (C) and carbon dioxide (D) are exchanged gases, not what the fungus is said to provide.

Ectomycorrhiza forms a sheath outside the root and its hyphae grow between the cell walls without entering the cells.

Endomycorrhiza (B) is the tempting option, but it penetrates the cortex cells. Lichens (A) are fungus and alga associations and have nothing to do with roots. Epiphytes (D) are plants that grow on other plants.

Organisms that feed on dead organic matter are saprophytes (saprotrophs). Fungi are the standard example: they secrete enzymes onto dead plants and animals and absorb the digested food.

Parasites (B) feed on a living host and harm it, so they are the tempting wrong choice. Autotrophs (A) make their own food and predators (D) kill and eat living prey.

Thigmotropism is the growth movement in response to touch. Climbing plants coil round a wall, rod or rope that supports them because they touch it.

Phototropism (A) is a response to light, geotropism (B) to gravity and chemotropism (D) to a chemical such as nutrients, so none of them is the response to a supporting structure.

Angiosperms are the plants whose seeds are enclosed in fruits (the name means 'enclosed seed'). Gymnosperms have naked seeds on cone scales, and ferns and bryophytes produce no seeds at all, so A, B and C are wrong.

DNA helicase opens the double helix by breaking the hydrogen bonds between base pairs. Gyrase (D) only works ahead of it to reduce the tension made by unwinding; primase (B) makes RNA primers and polymerase (C) builds new strands.

The water vascular system is a system of water-filled canals in the coelom of echinoderms (starfish, sea urchin). It works the tube feet and is used for locomotion, attachment, gas exchange and feeding. Annelids and molluscs have a coelom but no such canal system, and arthropods have a haemocoel.

Adult echinoderms (starfish, sea urchin) show radial symmetry and have a water vascular system of canals and tube feet. Hemichordata (acorn worm) and Chordata have neither; Vertebrata is a subphylum, not a phylum.

Menstruation begins when the corpus luteum degenerates and the progesterone level falls, depriving the endometrium of hormonal support so that its cells die. Estrogen is also low then, but progesterone is the hormone the book names as the one whose fall sets off the breakdown; LH falls too, but it acts on the corpus luteum, not on the lining directly, and FSH rises at this time.

Polygenic traits are controlled by several gene pairs whose effects add up, giving many small steps between the extremes: continuous variation. Discrete variation (clear separate classes) is what single-gene Mendelian traits and epistasis give, which is why it is the tempting wrong option.

Two genes at different loci decide the coat: B/b (black or chocolate) and E/e. A dog that is ee is yellow whatever its B gene, so one gene hides the effect of another, which is epistasis and gives a 9 : 3 : 4 ratio. In incomplete dominance and codominance only one gene (two alleles at one locus) is involved.

The fire destroyed the vegetation but the soil was left, so the habitat was already modified by a previous community. That is secondary succession. It is xerarch because it is a land (dry-habitat) succession; hydrarch succession begins in water. Primary succession needs a habitat never inhabited before (bare rock, lava), so A and C are wrong.

In a 28-day cycle the sudden release of LH at the end of the proliferative phase causes ovulation on about day 14. Day 1 is the first day of menstrual bleeding, day 7 falls in the middle of the proliferative phase while the follicle is still maturing, and day 28 is the end of the secretory phase when menstruation is about to begin.

Holandric traits are Y-linked: the Y goes from father to every son, so sons of an affected father are affected, and porcupine man is a book example. A daughter receives her father's X, not his Y, so she gets no holandric gene and cannot be a carrier. Carrier daughters of an affected father occur in X-linked recessive traits, not Y-linked ones.

In the vascular bundle of a dicot stem the phloem lies on the outer side and the xylem on the inner side (towards the pith). P is the outer part, the phloem, which carries organic food such as sucrose and amino acids. Q is the inner part, the xylem, which carries water and minerals.

So the order is sucrose then water (C). Option A reverses the two tissues. Options B and D are wrong because B names no water-carrying tissue and D names no sucrose-carrying tissue.

DNA is found in all organisms, is built of the same kind of nucleotides and uses a universal genetic code, so its similarity among species shows common ancestry. Lipids, carbohydrates and minerals are not named in the book as molecular evidence of evolution (the book names DNA, ATP, the 20 amino acids and cytochrome c).

Endospores are the survival structures of bacteria such as Bacillus and Clostridium. Of the pairs offered, C is the best match: it names endospores and plasmids (which carry extra genes). A is wrong because flagella and pili are for movement and attachment or gene transfer; B pairs pili (not a survival structure) with akinetes (resting cells of cyanobacteria); D pairs flagella with heterocysts, which fix nitrogen.

Griffith found that something from heat-killed S-type bacteria transformed living R-type bacteria, and Hershey and Chase showed that the DNA of the bacteriophage (labelled with ^{32}\mathrm{P}) entered the bacteria while its protein coat (^{35}\mathrm{S}) stayed outside. Protein (option B) was the tempting older idea, but it was shown not to enter the host cell and not to carry the information.

Club mosses (Lycopsida, for example Lycopodium). Three general characteristics:

• The plant body has a branching rhizome that produces aerial stems, usually not more than 30 cm tall.
• The stems are densely covered with scale-like leaves called microphylls, each with only a single strand of vascular tissue. Club mosses are the only living plants with microphylls.
• The sporangia are borne on terminal clusters of leaves called strobili, which look club-shaped, and the gametophyte is mainly underground.

(a) Identify A and B (1 mark)

• A: the outer coat (covering) of the testis (the tunica), which encloses the lobules.
• B: the testis lobules, each containing one to four tightly coiled seminiferous tubules (250 to 300 lobules in each testis).

(b) Functions of C and D (1 mark)

• C: rete testis, the tubular network that receives the sperms from the seminiferous tubules and in which they undergo further maturation; the efferent ductules then carry them out of the testis.
• D: epididymis, coiled on the outer surface of the testis; it is the place for transport and storage of the sperms, and it opens into the vas deferens.

(c) Inhibin (1 mark)

Inhibin is made by the Sertoli cells and released into the blood when the sperm count is too high. It acts on the hypothalamus and anterior pituitary and inhibits the release of GnRH and FSH, so spermatogenesis slows down (negative feedback). When the sperm count falls to 20 million per ml, inhibin is no longer released and the sperm count rises again.

Double membrane (1 mark)

The outer membrane is smooth and sieve-like because of porins. The inner membrane is selectively permeable and folded into cristae that increase its surface area.

F0-F1 particles and two compartments (1 mark)

The inner surface of the cristae has granular F0-F1 particles (ATP synthase). The inner membrane divides the organelle into the narrow intermembrane space and the matrix.

Matrix with its own genetic system (1 mark)

The jelly-like matrix contains a small circular DNA, all kinds of RNA, 70S ribosomes and enzymes.

In The Origin of Species Darwin explained natural selection through four observations (any three are enough for 3 marks):

1. Over production: each species produces more offspring than will survive to maturity, so populations can increase exponentially.
2. Variations: the individuals of a population differ in their traits, and some traits improve the chances of survival and reproduction.
3. Struggle for existence: there are more individuals than the limited food, water, light and growing space can support, so they compete (interspecific, intraspecific and environmental struggle).
4. Survival of the fittest: individuals with the most favourable combination of characteristics survive and reproduce, passing their heritable traits on. This is natural selection.

Result: favourable alleles increase, unfavourable alleles decrease, and over generations the population becomes better adapted and new species can arise.

Each enzyme has a specific substrate, because its active site is shaped for that substrate. Any three of these (1 mark each):

Hydrophytes live in water (hypotonic conditions, risk of flooding); mesophytes live where water is moderate (nearly isotonic). Their adaptations differ as follows:

Any three rows give the three marks.

What the phase is (1 mark)

The pay-off (oxidative) phase is steps 6 to 10. It starts with 2 G3P and ends with 2 pyruvate. ATP is made by substrate level phosphorylation and NADH is made. Each step happens twice for one glucose.

The five steps (2 marks)

Why (1 mark)

In eukaryotes transcription occurs in the nucleus but translation in the cytoplasm. The newly made pre-mRNA must be changed into mature, functional mRNA and protected from enzymes while it travels to the cytoplasm. (In prokaryotes there is no nucleus and no such delay.)

Changes during mRNA processing (2 marks)

Structure of prions (2 marks)

1. A prion is an infectious protein particle with no nucleic acid: it contains neither DNA nor RNA.
2. It consists of a single protein encoded by a single cellular gene. It is smaller than a virus and much more resistant than a virus to inactivation by UV light and heat.

Two diseases (1 mark)

Kuru in humans and mad cow disease in cattle: fatal neurodegenerative diseases with clumping of proteins in the brain, memory loss and permanent damage. There is no treatment or cure.

Phylum Cnidaria ("stinging celled organisms") (any three, 1 mark each):

• They are diploblastic: the body wall has ectoderm and endoderm with a jelly-like mesoglea between them, and the symmetry is radial or biradial.
• They have a sac-like gastrovascular cavity with a single opening, the mouth, usually surrounded by tentacles.
• The tentacles and body carry stinging cells (cnidocytes) containing nematocysts, which give the phylum its name.
• They occur as polyps (sessile, feeding) and medusae (free swimming, reproductive); the nervous system is a net; reproduction is by budding and by gametes.

Examples: Hydra, Obelia, jellyfish.

Vascular plants (tracheophytes) are distinguished by:

• Xylem and phloem present for conducting water, minerals and food, with the sporophyte as the main plant body bearing true roots, stems and leaves.
• Protected sporangia, which ultimately led to the evolution of seeds, and a pollen tube in seed plants that allows water-independent transfer of male gametes.
• Heteromorphic alternation of generations, and the evolution of flowers and fruits, which gives flexibility and reproductive success on land.

Hershey and Chase (1952) labelled the DNA of bacteriophages with ^{32}\mathrm{P} and their protein coats with ^{35}\mathrm{S}, let them infect bacteria, then blended and centrifuged the mixture. Their findings were:

Both are haemoglobin, the iron-rich protein of red blood cells that carries oxygen; they differ in one amino acid of the β-chain.

Homologous organs (structures) look externally different and may do different jobs, but are internally alike because they were inherited from a common ancestor.

1. Example: the forelimbs of man, horse, whale and bat have the same basic pattern of bones, although they serve as arm, leg, flipper and wing.
2. Type of evolution: they are caused by divergent evolution, in which different species have evolved from a common ancestor at different habitats.
3. Other homologous structures: sepals, petals, stamens and carpels (all modified leaves) and vestigial organs such as the human appendix.

Draw the twelve numbered stages of Figure 5.5 in order and label each with its stage name (the key enzymes are shown on the stages they act in). 1 mark for the diagram, 1 for correct order and 1 for labels.

• Stages 1 to 2: gp 120 binds the CD4 receptor; fusion by gp 41; entry by endocytosis; uncoating.
• Stages 3 to 7: reverse transcriptase (RNA to cDNA), RNA broken down by RNAase, double stranded DNA, integrase inserts it into the host DNA (provirus).
• Stages 8 to 12: transcription by host RNA polymerase, translation on host ribosomes, protease cuts the large proteins, maturation, release by budding.

1. Disinfectants: kill or inhibit bacteria on surfaces and non-living objects in hospitals, laboratories and homes. Examples: bleach (sodium hypochlorite), hydrogen peroxide, quaternary ammonium compounds.
2. Antiseptics: chemicals used on living tissues such as skin and mucous membranes to prevent or treat infection. Examples: alcohol-based hand sanitisers, iodine-based antiseptics.
3. Preservatives: added to food and cosmetics to inhibit the growth of bacteria so that the product lasts longer. Examples: sodium benzoate, potassium sorbate, parabens. (Chlorine and chloramine disinfect drinking water; sulphonamides and nitrofurans are used in chemotherapy.)

Each row gives one clue, and the other two cells follow from it.

Row a: bryophytes are non-vascular. Row b: seedless plants that have vascular tissue are the pteridophytes. Row c: the rose is a dicot flowering plant, so it belongs to the angiosperms.

Plant organs move by differential growth in response to external stimuli. Three important stimuli are:

A fourth stimulus is a chemical (nutrients), which causes chemotropism in fungal hyphae.

Any three differences are enough. In both modes the organism is heterotrophic and cannot make its own food.

Type of pattern (1 mark)

Humans follow the XX-XY type of sex determination: the female is XX (homogametic) and the male is XY (heterogametic).

Mechanism (2 marks)

1. Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes.
2. All eggs of a woman carry one X chromosome. Half of the sperms of a man carry X and half carry Y, with equal chance.
3. An X sperm fertilising the egg gives XX (a girl); a Y sperm gives XY (a boy). The sperm therefore decides the sex of the child, and the sex ratio is 1 : 1.

Bryophytes are called amphibious plants because they live on land but depend on water for reproduction, like amphibians. Three reasons:

• They are classified as land plants, with land adaptations such as a cuticle, rhizoids and multicellular embryos.
• Their male gametes (sperms) are motile and swim through a film of water to reach the egg in the archegonium, so fertilization cannot occur without water.
• They lack vascular tissue and so mostly live in wet or moist places, though some can survive in dry environments.

Differences between monocots and dicots:

1. Lipid synthesis (1 mark)

Enzymes of the smooth ER make lipids: oil, cholesterol, phospholipids and steroids.

2. Steroid hormones (1 mark)

It makes the sex hormones of vertebrates and the steroid hormones of the adrenal glands.

3. Detoxification (1 mark)

Its enzymes detoxify drugs and poisons, especially in liver cells. In skeletal muscle cells, where it is well developed, it is called the sarcoplasmic reticulum.

1. Disinfectants: kill or inhibit bacteria on surfaces and non-living objects in hospitals, laboratories and homes. Examples: bleach (sodium hypochlorite), hydrogen peroxide, quaternary ammonium compounds.
2. Antiseptics: chemicals used on living tissues such as skin and mucous membranes to prevent or treat infection. Examples: alcohol-based hand sanitisers, iodine-based antiseptics.
3. Preservatives: added to food and cosmetics to inhibit the growth of bacteria so that the product lasts longer. Examples: sodium benzoate, potassium sorbate, parabens. (Chlorine and chloramine disinfect drinking water; sulphonamides and nitrofurans are used in chemotherapy.)

Liver detoxifies alcohol (1 mark)

Peroxisomes are oxidative organelles whose main task is the detoxification of alcohol, and the liver is the organ where this is done.

Two-enzyme pathway (1 mark)

Number adapts to need (1 mark)

Peroxisomes increase in number and size when there is a lot of alcohol to break down. Liver cells handle such a heavy load of toxins, so they contain many peroxisomes.

Meaning of maximum temperature (1 mark)

The maximum temperature is the temperature that causes denaturation of the enzyme.

Effect on enzymatic action (2 marks)

• Above the optimum (37 to 38 degrees Celsius for human enzymes) the hydrogen bonds break and the secondary and tertiary structures are disrupted. The enzyme unfolds and the precise shape of the active site is lost.
• The substrate can no longer bind, so the rate falls steeply even though collisions are more frequent. At the maximum temperature the enzyme is fully denatured and the reaction stops. This cannot be reversed.

Origin of mitochondria (1.5 marks)

An early ancestor of the eukaryotic cell engulfed a free-living aerobic bacterium. It was not digested but lived on inside the host as an endosymbiont and, over time, became the mitochondrion, the organelle that makes ATP by respiration.

Origin of chloroplasts (1.5 marks)

A host cell engulfed a photosynthetic bacterium (a cyanobacterium) in the same way. It became a permanent endosymbiont and developed into the chloroplast, the organelle of photosynthesis.

Evidence from the book

• Both organelles have a small circular DNA, all kinds of RNA and 70S ribosomes, like a bacterium.
• Both are self-replicating organelles and divide by fission.
• Both have their own metabolic machinery and can survive outside the cell in an artificial medium if carefully fractionated.
• Both are enclosed by a double membrane.

Non-cyclic photophosphorylation uses both photosystems and two electron transport chains. Electrons flow from water to \ce{NADP+} and do not return.

Products: ATP, NADPH and oxygen.

Lecithin (phosphatidylcholine) is the commonest phospholipid. It is built from glycerol, two fatty acids (one saturated, one unsaturated) and a phosphate group with choline attached.

1. Initiation: under unfavourable conditions (lack of nutrients, extreme temperature) sporulation begins and the DNA replicates, giving two identical copies.
2. Asymmetric division: the cell divides unequally into a larger mother cell and a smaller forespore.
3. Membrane development: several protective membranes form around the forespore.
4. Peptidoglycan layer: the mother cell lays down layers of peptidoglycan around the forespore (the cortex).
5. Spore coat formation: the mother cell makes a tough protein coat around the spore.
6. Maturation: the endospore dehydrates, so its water content falls greatly.

The book lists a seventh stage, release: the mother cell ruptures and frees the mature endospore, which stays dormant until conditions improve. Write this too if the question allows seven.