Reproductive System

(a) Types of fertilisation

• (i) External fertilisation: the fusion of the male and female gametes takes place outside the body of the female, usually in water. The female lays her eggs and the male sheds sperm over them in the surrounding water.
• (ii) Internal fertilisation: the fusion of the male and female gametes takes place inside the body of the female, the sperm being deposited into her reproductive tract during mating.

(b) Two groups of animals for each type

• External fertilisation: bony fishes and amphibians (frogs/toads).
• Internal fertilisation: mammals and birds (also reptiles).

(c)(i) Three effects on the foetus if the placenta detaches from the uterine wall

• The foetus is cut off from its supply of food and oxygen, so it may suffocate and die.
• Waste products (carbon dioxide, urea) cannot be removed from the foetus, so they accumulate and poison it.
• It causes bleeding (haemorrhage) and may lead to miscarriage or premature birth of the foetus.

(c)(ii) Three other features of the uterus useful for foetal development

• A thick, muscular wall that protects the foetus and contracts to expel it at birth.
• A rich supply of blood vessels in the uterine lining that supplies food and oxygen and removes wastes.
• The amnion containing amniotic fluid that surrounds and cushions the foetus against shock (and the umbilical cord linking foetus to placenta).

(d) How the activities of organisms bring about dynamic equilibrium in a habitat

In a stable habitat, producers, consumers and decomposers interact so that materials and numbers stay roughly balanced. Green plants make food and release oxygen while animals consume food and release carbon dioxide, so the gases are kept balanced through photosynthesis and respiration. Predators feed on prey and so control the number of prey, while the food supply controls the number of predators, keeping populations steady. Decomposers break down dead matter and return mineral nutrients to the soil for plants to reuse. These continuous, opposing activities balance one another, so the habitat stays in a steady state called dynamic equilibrium.

(e) Four problems of organisms in the intertidal zone of a marine habitat

• Danger of drying out (desiccation) when the tide goes out and they are exposed to air and sun.
• Strong wave action that can wash them away or break them off the rocks.
• Wide changes in temperature and salinity as pools warm up, cool down or are diluted by rain.
• Exposure to predators (birds at low tide, fish at high tide) and changing oxygen supply.

(f) Why the following are necessary for germination

• (i) Moisture (water): water is absorbed to soften the seed coat, activate the enzymes, and dissolve and transport the stored food to the growing embryo.
• (ii) Viable seeds: the seed must be alive with a healthy, living embryo; a dead (non-viable) embryo cannot divide and grow, so no germination can occur.

(g) Why light energy is a limiting factor in food production by autotrophs

Autotrophs (green plants) make food by photosynthesis, and light provides the energy for the process. When light is dim or absent, the rate of photosynthesis is low no matter how much carbon dioxide and water are available, so the amount of food made is restricted. Because a shortage of light holds back (limits) the rate of food production, light energy is described as a limiting factor.