Reproduction is how living things make new individuals, ensuring that a species continues from one generation to the next. In this topic you will compare two very different strategies — asexual reproduction, which produces genetically identical offspring quickly, and sexual reproduction, which mixes genes to create variation. You will also study the detailed structures of flowering plants and humans, including how gametes meet at fertilisation and how new life is supported and protected.
Asexual reproduction is the process resulting in the production of genetically identical offspring from one parent. There is no fusion of gametes, so the offspring are clones. It is fast and needs only one organism, which is useful in a stable environment, but it produces no variation, so a single disease or change could wipe out the whole population. Examples include bacteria dividing, potato tubers and strawberry runners.
Sexual reproduction is the process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote and the production of offspring that are genetically different from each other. It introduces variation, which helps a species survive environmental change and is the raw material for natural selection.
A flower is the reproductive organ of a plant. You should be able to identify and state the functions of its parts. The sepals protect the flower in bud. The petals attract pollinators (often large and brightly coloured in insect-pollinated flowers). The stamen is the male part, made of the anther (which makes pollen) and the filament. The carpel is the female part, made of the stigma (which receives pollen), the style and the ovary (which contains ovules).
Pollination is the transfer of pollen grains from the anther to the stigma. In self-pollination the pollen lands on the stigma of the same flower or another flower on the same plant; in cross-pollination it is transferred to a different plant of the same species, giving more variation.
After pollination the pollen grain grows a pollen tube down the style to the ovule. Fertilisation is the fusion of the pollen nucleus with the nucleus of an ovule. The fertilised ovule becomes a seed and the ovary becomes a fruit.
In the male system the testes make sperm and the hormone testosterone; the scrotum holds the testes outside the body to keep them slightly cooler. Sperm pass along the sperm ducts, mix with fluid from glands to form semen, and leave through the urethra in the penis. In the female system the ovaries contain eggs (ova) and make oestrogen and progesterone. An egg travels along the oviduct, where fertilisation usually happens, into the uterus, whose muscular wall and lining (endometrium) support a developing fetus. The cervix is the ring of muscle at the base of the uterus.
After fertilisation the zygote divides to form an embryo that implants in the uterus lining. The placenta allows exchange of dissolved substances such as glucose, oxygen and antibodies between mother and fetus, while keeping the two blood supplies separate. The umbilical cord connects the fetus to the placenta and the amniotic fluid cushions and protects the fetus.
The menstrual cycle is a roughly 28-day cycle controlled by hormones. Around day 14 ovulation releases an egg. Oestrogen, made by the ovaries, repairs and thickens the uterus lining and stimulates a surge that triggers ovulation. After ovulation, progesterone maintains the thick lining ready for a possible pregnancy. If the egg is not fertilised, progesterone falls, the lining breaks down and menstruation (the period) occurs, marking day 1 of the next cycle.
A sexually transmitted infection (STI) is an infection that is transmitted via body fluids through sexual contact. HIV is an example; it is a virus that may lead to AIDS by attacking the immune system. HIV is spread through infected blood and other body fluids, including during unprotected sex, by sharing needles and from mother to baby. Spread can be controlled by using condoms, screening blood and avoiding sharing needles.
Practise exam-style questions on this topic.