BiologiBiology

Sclerenchyma cells have walls thickened with lignin and are dead at maturity, so they only support. Parenchyma and collenchyma are living, and companion cells help sieve tubes.

The light-dependent reaction makes ATP, NADPH and oxygen. ATP and NADPH reduce carbon dioxide to glucose in the stroma. Oxygen is released and glucose is a product of the light-independent reaction.

Iron is needed for chlorophyll synthesis and its shortage shows on the young leaves. Nitrogen and magnesium shortages show first on the mature leaves.

Rafflesia has no chlorophyll and lives on the roots of its host, so it is a parasite. Nepenthes is carnivorous, Platycerium (staghorn fern) is an epiphyte and Hydrilla is a free-living aquatic plant.

In the symplast pathway water moves through the cytoplasm of living cells, joined by plasmodesmata. A describes the apoplast pathway, C is movement inside the xylem and the Casparian strip blocks water.

Auxin stimulates the formation of adventitious roots on stem cuttings. Ethylene ripens fruits, abscisic acid inhibits growth and cytokinin stimulates cell division and lateral buds.

Auxin from the block moves down the left side only, so the cells on the left elongate more than those on the right. The longer left side pushes the coleoptile to curve towards the right. It does not need light.

The integument, the outer covering of the ovule, hardens into the testa (seed coat). The zygote becomes the embryo, and the funicle only attaches the ovule to the ovary wall.

Bright petals, scent and nectar attract insects, and sticky pollen sticks to their bodies. Wind-pollinated flowers are small and dull with light, dry pollen.

A long, flexible petiole keeps the leaf at the water surface to receive light and bends with water movement. Storing water and reducing evaporation are xerophyte features.

Pneumatophores grow up above the mud and have lenticels through which air enters, so the submerged roots can respire. Salt glands remove salt, prop roots give support and waxy leaves reduce water loss.

1b rules out the sparrow, 2b rules out the shark (it has gills) and 3b gives the animal that lays eggs and has scaly skin: the crocodile.

Dengue is spread by the Aedes mosquito. Malaria is spread by the female Anopheles mosquito. Cholera spreads through contaminated food and water, and tuberculosis through droplets in the air.

Total area sampled = 7 × (0.5 × 0.5) = 1.75 m². Density = 42 ÷ 1.75 = 24 per m². Option B uses 3.5 m², option A forgets that each quadrat is only 0.25 m², and D ignores the area.

Methane is a greenhouse gas. Trapping and burning it as biogas stops it from reaching the atmosphere and saves fossil fuel, so it reduces global warming. The practice does not reduce noise or acid rain.

Low dissolved oxygen means high BOD: microorganisms decomposing the organic waste use up the oxygen at station 2. Oxygen rises again downstream as the waste is decomposed and diluted, so the river recovers. Upstream water is cleaner, not warmer.

Woman I^AI^O × man I^AI^B gives I^AI^A, I^AI^B, I^AI^O and I^BI^O in equal numbers. Only I^BI^O is group B, so 1/4.

Cross X^BX^b × X^BY gives X^BX^B, X^BX^b, X^BY and X^bY. Only X^bY is a colour blind boy, so 1/4 of all children. 1/2 is the chance that a son (not any child) is colour blind.

One G is missing (GGA becomes GA), so the sequence is shorter by one base: a deletion. Every codon after that point is shifted and many amino acids change. Substitution would keep the same length, and inversion is a chromosomal mutation.

The DNA is extracted from the sample first, cut into fragments with restriction enzymes, separated by size by gel electrophoresis, then the bands are shown on X-ray film (textbook Figure 13.6). The fragments must be cut before they can be separated.