Reproduction

The correct answer is B. An individual can survive without reproducing, but reproduction produces offspring that replace organisms as they die and thereby maintain the continuity of the species.

The offspring needs a copy of the parent's genetic information to develop. Without copying, that information cannot be passed on to the new organism.

Small copying differences can create variation among offspring. If the environment changes, some variations may help individuals survive and reproduce; not every variation is beneficial.

Asexual reproduction involves one parent and no fusion of gametes. The offspring therefore receives its genetic information from that parent and is usually very similar, though small copying variations can occur.

In sexual reproduction, gametes from two parents combine. Their genetic information forms new combinations in the offspring, which can increase variation.

In binary fission, one parent cell divides to form two daughter cells. In multiple fission, it produces many daughter cells, so their number is the distinguishing feature.

Amoeba has an irregular shape, so it can divide in different planes. Leishmania has a definite body orientation, including a flagellum at one end, so its binary fission occurs along a definite plane.

Rhizopus spores have protective walls that help them withstand unfavourable conditions. When conditions become favourable, the spores can germinate and grow into new organisms.

In Hydra, a small outgrowth called a bud forms on the parent, grows, and can develop into a new individual. This is a form of asexual reproduction.

In fragmentation, a parent organism breaks into pieces, and a piece can grow into a new organism. Regeneration is the regrowth of lost or damaged parts by cell division and development; in some organisms, a piece can regenerate into a whole organism.

Planaria has specialised cells that can divide and develop into different tissues. These cells allow a cut piece, under suitable conditions, to regenerate the missing parts and form a complete organism.

Vegetative propagation produces a new plant from a part of one parent plant, such as a stem, root or leaf. Because this is a form of asexual reproduction, the offspring are usually genetically very similar to the parent and to one another.

Vegetative propagation allows farmers to grow new plants from parts such as stems, roots or leaves. It is therefore useful for plants that do not produce seeds, because new plants can be raised without seeds.

This process is based on totipotency: under suitable conditions, a plant cell can divide and develop into a complete plant. In tissue culture, a small piece of plant tissue is grown on a suitable nutrient medium to produce many plants.

Self-pollination involves pollen from the same flower or another flower on the same plant, so it usually introduces less genetic variation. Cross-pollination can combine genetic material from different plants of the same species and may therefore produce greater variation.

After a compatible pollen grain lands on the stigma, it can germinate and form a pollen tube. The male gametes travel through this tube towards the ovule, where fertilisation can occur.

A pollen grain landing on the stigma is pollination, but the male gametes must still reach the ovule for fertilisation. The pollen tube provides their route; without it, they are unlikely to reach the ovule.

After fertilisation, the ovule usually develops into a seed, while the ovary usually develops into a fruit. The developing embryo is contained within the seed.

The fruit develops from the ovary and can protect the seeds inside it. Many fruits also help disperse seeds, for example when animals carry or eat them. The ways seeds are dispersed vary among plants.

A seed absorbs water and becomes active; oxygen from the air is needed for respiration, which releases energy. A suitable temperature allows the chemical processes needed for germination to proceed.

The testes produce sperm and also secrete male sex hormones, especially testosterone. These hormones help regulate male reproductive functions and contribute to changes during puberty.

The testes produce sperm, and sperm formation works best at a temperature slightly below normal body temperature. The scrotum keeps the testes cooler than the rest of the body.

Cutting the sperm duct blocks the passage of sperm, but it does not remove the hormone-producing cells in the testes. Therefore, the procedure does not stop testicular hormone production.

Ovaries produce eggs and also secrete hormones such as oestrogen and progesterone. These hormones help regulate the reproductive cycle and contribute to changes during puberty.

After ovulation, the egg enters a fallopian tube. Sperm can travel through the reproductive tract to the tube, so the two gametes usually meet and fertilisation occurs there.