生殖

Reproduction 生殖 makes new organisms of the same kind. There are two types.

Asexual reproduction

Asexual reproduction 无性生殖 needs only one parent 亲本. The offspring 后代 have exactly the same genes 基因 as the parent — they are genetically identical (clones).

• Advantage: it is fast and needs only one parent — useful when conditions are good and stable.
• Disadvantage: all the offspring are identical, so if the environment 环境 changes (for example a new disease) they may all die. (In crops, identical plants give an even harvest, but one disease can wipe out the whole crop.)

Sexual reproduction

Sexual reproduction 有性生殖 needs two parents. It involves the fusion 融合 (joining) of the nuclei 细胞核 of two gametes 配子 (sex cells) to make a zygote 合子. This joining is called fertilisation 受精. The offspring get a mix of genes from both parents, so they show variation 变异 (they are genetically different).

• Advantage: variation means some offspring may survive if the environment changes.
• Disadvantage: it is slower and needs two parents.

(Supplement) The nuclei of gametes are haploid 单倍体 (one set of chromosomes 染色体); the nucleus of the zygote is diploid 二倍体 (two sets).

Parts of an insect-pollinated flower

Pollination

Pollination 传粉 is the transfer of pollen grains from an anther to a stigma.

• insect-pollinated flowers have bright petals, scent and nectar, and sticky or spiky pollen, to attract insects and stick to them.
• wind-pollinated flowers have small dull petals, anthers and stigmas that hang outside, and light, smooth pollen that blows in the wind.

(Supplement) Self-pollination 自花传粉 is pollen landing on the stigma of the same flower (or another flower on the same plant). Cross-pollination 异花传粉 is pollen carried to a flower on a different plant of the same species. Cross-pollination gives more variation, helping the population cope with change, but it relies on pollinators 传粉者 or wind. Self-pollination is more reliable but gives less variation.

Fertilisation and germination

Fertilisation happens when a pollen nucleus fuses with a nucleus in an ovule. (Supplement) First a pollen tube 花粉管 grows down the style into the ovary, carrying the pollen nucleus to the ovule.

A seed 种子 will only germinate 萌发 (begin to grow) when it has water, oxygen 氧气 and a suitable temperature 温度.

The male reproductive system

The female reproductive system

Gametes and fertilisation

Fertilisation is the fusion of the nuclei of a male gamete (sperm) and a female gamete (egg cell). The two gametes are adapted to their jobs:

Pregnancy and the fetus

After fertilisation the zygote divides to form an embryo 胚胎 — a ball of cells that implants 植入 into the lining of the uterus. As it grows it becomes a fetus 胎儿. These structures support it:

• the placenta 胎盘 — where dissolved nutrients, gases and wastes are exchanged between the mother's blood and the fetus's blood (the two never mix).
• the umbilical cord 脐带 — joins the fetus to the placenta.
• the amniotic sac 羊膜囊 — a bag around the fetus, filled with amniotic fluid 羊水 that cushions it.

(Supplement) Some pathogens 病原体 and toxins 毒素 can cross the placenta and harm the fetus.

Puberty

At puberty 青春期, sex hormones 激素 make the body develop secondary sexual characteristics 第二性征 — such as body hair, a deeper voice in boys, and breasts in girls. Testosterone does this in boys; oestrogen does it in girls.

The menstrual cycle

The menstrual cycle 月经周期 (about 28 days) prepares the uterus for a possible pregnancy:

• the lining of the uterus thickens.
• around day 14 an ovary releases an egg (ovulation 排卵).
• if the egg is not fertilised, the lining breaks down and leaves the body (a period), and the cycle starts again.

(Supplement) Four hormones control the cycle: FSH (makes an egg mature and the ovary release oestrogen), LH (triggers ovulation), oestrogen (repairs and thickens the lining), and progesterone 孕酮 (keeps the lining thick). Oestrogen and progesterone are made in the ovaries; during pregnancy 怀孕, progesterone keeps the lining in place.

(Supplement) The hormones in order.

1. FSH from the pituitary gland 垂体 makes an egg mature inside a follicle 卵泡 in the ovary.
2. The follicle makes oestrogen. Oestrogen thickens the lining again after the period.
3. A sudden rise in LH, also from the pituitary, makes the follicle burst and release the egg (ovulation).
4. The empty follicle becomes the corpus luteum 黄体, which makes progesterone. Progesterone keeps the lining thick, ready for an embryo.
5. If no embryo implants, the corpus luteum breaks down, progesterone falls, and the lining is shed: the next period begins.

In pregnancy the corpus luteum keeps making progesterone at first. Later the placenta takes over and makes both progesterone and oestrogen, so the lining is never shed and no new eggs are released.

Common mistakes.

• Saying the ovary makes FSH or LH. They come from the pituitary gland; the ovary makes oestrogen and progesterone.
• Saying oestrogen releases the egg. The LH surge releases it.
• Saying a period happens because progesterone rises. It happens because progesterone falls.

Worked example. A woman has a regular 28-day cycle, and her period starts on the 1st of the month. On roughly which date is she most likely to conceive? Ovulation happens about day 14, so counting 14 days on from the start of the period points to about the 14th, and the days either side of it are the most fertile. Day 1 is the day the period starts, not the day it ends - counting from the wrong day is what makes most cycle questions go wrong.

A sexually transmitted infection 性传播感染 (STI) is an infection 感染 spread through sexual contact.

HIV is a virus 病毒 that causes an STI, and it can lead to AIDS, in which the immune system stops working properly. HIV is passed on in infected body fluids — for example by sharing needles, during sex, or from mother to baby (across the placenta or in breast milk).

The spread of STIs is controlled by using condoms, having fewer partners, testing and treating infected people, and not sharing needles.

• Asexual: one parent, identical offspring (no variation). Sexual: two parents, gametes fuse, offspring show variation.
• Fertilisation = fusion of two gamete nuclei → zygote. (Supplement: gametes haploid, zygote diploid.)
• Learn the flower parts and which are male (stamen) and female (carpel). Compare insect- and wind-pollinated flowers.
• Sperm: small, many, moving (flagellum). Egg: large, few, with a food store.
• Menstrual cycle: lining thickens → ovulation (day 14) → period if no fertilisation. (Supplement: FSH, LH, oestrogen, progesterone.)
• HIV is a virus that causes an STI and can lead to AIDS; it spreads through body fluids.