Reproduction
IGCSE Biology Topic 16 10:39 English narration · English + 中文 subtitles burned in
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There are only two ways to make a new organism, and they trade off against each other.
制造新生物只有两种方式，而且它们互相权衡。
One is fast, needs a single parent, and makes perfect copies.
一种很快，只需要一个亲本，产生完全一样的复制品。
The other is slow, needs two parents, and makes offspring that are all slightly different.
另一种慢一些，需要两个亲本，产生的后代彼此都略有不同。
Neither is better — and understanding why is the key to this whole topic.
哪一种都不是更好的——理解为什么，就是掌握整个专题的关键。
Welcome to reproduction.
欢迎来到生殖。
Asexual and sexual, flowers, the human systems, and the menstrual cycle.
无性和有性、花、人体的生殖系统，还有月经周期。
Let's begin.
让我们开始吧。
Asexual reproduction needs only one parent, and the offspring have exactly the same genes — they are clones.
无性生殖只需要一个亲本，后代拥有完全相同的基因——它们是克隆体。
Fast and reliable when conditions are good, but there is no variation, so one new disease can wipe out every offspring.
在条件好的时候这又快又可靠，但没有变异，所以一种新疾病就能把所有后代全部消灭。
Sexual reproduction needs two parents.
有性生殖需要两个亲本。
It is the fusion of the nuclei of two gametes to make a zygote, and that fusion is called fertilisation.
它是两个配子的细胞核融合形成合子，这个融合叫做受精。
The offspring get a mix of genes, so they show variation — and some may survive when the environment changes.
后代获得混合的基因，所以表现出变异—— 而变异意味着当环境改变时，有些后代可能能活下来。
The cost is that it is slower.
代价是它比较慢，而且需要两个亲本。
For the supplement paper: gamete nuclei are haploid, one set of chromosomes, and the zygote nucleus is diploid, two sets.
补充部分还要加上染色体数目： 配子的细胞核是单倍体，一套染色体，合子的细胞核是二倍体，两套。
Asexual reproduction is useful when conditions are good and stable.
当条件好而且稳定时，无性生殖很有用。
The offspring are genetically identical clones, so a crop of identical plants gives an even harvest.
后代是基因完全相同的克隆， 所以一茬相同的植株能给出均匀的收成。
That same identity is the disadvantage: because they share every weakness, a new disease can wipe out the whole crop.
同一份相同也是缺点：因为它们共享每一种弱点， 一种新疾病就能毁掉整片作物。
Farmers like the even harvest, and that is also why one infection can kill it.
农民喜欢均匀的收成，这也是一次感染就能毁掉它的原因。
When the environment is good and stable, asexual reproduction is the faster route: one parent, genetically identical offspring, an even harvest.
当环境好而且稳定时，无性生殖是更快的路线： 一个亲本，基因相同的后代，均匀的收成。
When the environment changes — a new disease, a new climate — sexual reproduction's variation is what lets some offspring survive.
当环境改变——一种新疾病，一种新气候—— 有性生殖的变异才让部分后代活下来。
Neither method is better.
哪一种方法都不是更好。
The exam asks you which advantage matches which cost, not which type you prefer.
考试问的是哪个优点配哪个代价，不是你更喜欢哪一种。
Start with the flower, and sort the parts into outside, male and female.
从花开始，把各部分分成外部、雄性和雌性。
Sepals protect the bud before it opens.
萼片在花开之前保护花蕾。
Petals are large and colourful, to attract insects.
花瓣又大又鲜艳，用来吸引昆虫。
The male parts are the stamens: each stamen is a filament holding up an anther, and the anther makes the pollen grains.
雄性部分是雄蕊： 每个雄蕊是一根花丝托着一个花药，花药产生花粉粒。
The female part is the carpel, and it has three pieces — a stigma, which catches pollen; a style below it; and an ovary at the base, which holds the ovules that become seeds.
雌性部分是雌蕊，它有三个部分——柱头，负责接住花粉；下面是花柱； 基部是子房，里面有会变成种子的胚珠。
Stamen is male, carpel is female.
雄蕊是雄性，雌蕊是雌性。
Get that split right and the labelling questions are easy.
把这个区分弄对，标注题就很容易了。
Pollination is the transfer of pollen grains from an anther to a stigma — only the transfer; fertilisation comes later.
传粉是花粉粒从花药转移到柱头。 注意它只是转移——受精在后面。
Now compare the two kinds, because they are opposites.
现在逐个特征比较两种传粉方式，因为它们是相反的。
Insect-pollinated flowers have bright petals, scent and nectar, and sticky or spiky pollen that clings to the insect.
虫媒花有鲜艳的花瓣、香味和花蜜，花粉黏或有刺，所以能粘在昆虫身上。
Wind-pollinated flowers have small dull petals, anthers and stigmas hanging outside, and light smooth pollen that blows away.
风媒花的花瓣小而暗淡，花药和柱头悬在外面的空气中， 花粉又轻又光滑，所以容易被吹走。
Look at a pollen grain itself.
看花粉粒本身。
Insect-pollinated pollen is sticky or spiky, so it clings to the insect that visits the flower.
虫媒花的花粉黏或有刺， 所以能粘在来访的昆虫身上。
Wind-pollinated pollen is light and smooth, so it blows.
风媒花的花粉又轻又光滑，所以会被吹走。
Under an electron microscope each plant shows its own surface pattern of ridges and spikes.
在电子显微镜下，每种植物都有自己的表面纹路， 由脊和刺组成。
That surface is the adaptation, not decoration.
那层表面才是适应，不是装饰。
For the supplement paper, one more pair.
补充部分还有一对。
Self-pollination is pollen landing on the stigma of the same flower, or of another flower on the same plant.
自花传粉是花粉落在同一朵花的柱头上， 或同一株植物另一朵花的柱头上。
It is more reliable because it does not wait for an insect or the wind.
它更可靠，因为它不等昆虫或风。
Cross-pollination is pollen carried to a flower on a different plant of the same species.
异花传粉是花粉被带到同种的另一株植物上。
Cross-pollination gives more variation, helping the population cope with change, but it relies on pollinators or wind.
异花传粉带来更多变异，帮助种群应对变化， 但依赖传粉者或风。
Fertilisation in a plant happens when a pollen nucleus fuses with a nucleus in an ovule.
植物的受精发生在花粉的细胞核与胚珠中的细胞核融合的时候。
For the supplement paper, say how it gets there: a pollen tube grows down through the style into the ovary, carrying the pollen nucleus to the ovule.
补充部分要说清楚它是怎么过去的：花粉管沿着花柱向下长进子房， 把花粉的细胞核送到胚珠。
And once you have a seed, it will only germinate — begin to grow — when it has three things: water, oxygen and a suitable temperature.
有了种子之后，它只有在具备三样东西时才会萌发——开始生长： 水、氧气和合适的温度。
Notice what is not on that list.
注意这个清单上没有什么。
A seed does not need light to germinate, and it does not need soil.
种子萌发不需要光，也不需要土壤。
Fertilisation always means the same thing: the joining of the nuclei of two sex cells, the gametes, to make a zygote.
受精永远是同一件事： 两个性细胞也就是配子的细胞核接合，形成合子。
In a plant that joining happens in an ovule.
在植物里，那次接合发生在胚珠中。
In a human it happens in an oviduct.
在人体里，它发生在输卵管。
Now the human systems, each part with one job.
现在看人体的生殖系统，每个部分都有一个功能。
In the male: the testes make sperm and testosterone.
男性：睾丸产生精子和睾酮。
The scrotum holds them outside the body, kept slightly cool.
阴囊把睾丸悬在体外，保持略微凉爽。
Sperm ducts carry the sperm away, the prostate gland adds fluid to make semen, and the urethra carries it out through the penis.
输精管把精子送走，前列腺加入液体形成精液，尿道把精液经阴茎送出。
In the female: the ovaries make egg cells and oestrogen.
女性：卵巢产生卵细胞和雌激素。
The oviducts carry the egg towards the uterus — and this is the one to remember, because fertilisation happens in the oviduct, not in the uterus.
输卵管把卵子送向子宫—— 这一点要记住，因为受精发生在输卵管，不是在子宫。
The uterus is where the baby develops, the cervix is the ring of muscle at its base, and the vagina is also the birth canal.
子宫是胎儿发育的地方，子宫颈是它基部的肌肉环， 阴道接受阴茎，也是产道。
The table skipped two names students lose.
表格漏了两个学生会丢分的名称。
The penis passes sperm into the female.
阴茎把精子送入女性体内。
The urethra carries semen, and urine, out through the penis.
尿道把精液，还有尿液，经阴茎送出。
The vagina receives the penis, and it is also the birth canal.
阴道接受阴茎，也是产道。
The cervix is the ring of muscle at the base of the uterus.
子宫颈是子宫基部的肌肉环。
Those four names turn a labelling question into marks.
这四个名称能把标注题变成分数。
Compare the two gametes, because every difference is an adaptation.
比较两种配子，因为每一个差别都是一种适应。
The sperm is tiny, there are millions, and it can move — it swims using a flagellum.
精子很小，数量有数百万，而且能移动——它用鞭毛游动。
It carries many mitochondria to release the energy for that swimming, and an acrosome full of enzymes to break into the egg.
它带有许多线粒体，为游动释放能量，还有一个装满酶的顶体，用来钻进卵子。
The egg cell is large, there are few — about one a month — and it cannot move.
卵细胞很大，数量很少——大约一个月一个——而且它不能移动；它是被运送的。
It has a store of food for the early embryo, and a jelly coat that changes after fertilisation to keep every other sperm out.
它有为早期胚胎储存的养分，还有一层胶质层， 在受精后会改变，把其他所有精子挡在外面。 小而能动，对上大而有储备。
After fertilisation the zygote divides to form an embryo — a ball of cells that implants into the lining of the uterus.
受精后合子分裂形成胚胎——一团植入子宫内膜的细胞。
As it grows it becomes a fetus, and three structures support it.
它长大后成为胎儿，有三个结构支持它。
The placenta is where dissolved nutrients, gases and wastes are exchanged between the mother's blood and the fetus's blood.
胎盘是母亲的血液和胎儿的血液之间交换溶解的养分、气体和废物的地方。
Here is the detail that earns the mark: the two never mix — they come close enough to exchange substances but stay separate.
这里有一个能得分的细节：两者从不混合。 它们靠得足够近以交换物质，但保持分开。
The umbilical cord joins the fetus to the placenta, and the amniotic sac is a bag of fluid that cushions it.
脐带把胎儿连到胎盘，羊膜囊是装着羊水的袋子，缓冲保护胎儿。
Some pathogens and toxins can cross the placenta.
补充部分要注意，有些病原体和毒素能穿过胎盘伤害胎儿。
The amniotic sac is a bag around the fetus, filled with amniotic fluid that cushions it.
羊膜囊是包着胎儿的袋子，里面装满缓冲保护它的羊水。
The umbilical cord joins the fetus to the placenta.
脐带把胎儿连到胎盘。
Some pathogens and toxins can cross the placenta and harm the fetus.
有些病原体和毒素能穿过胎盘伤害胎儿。
Dissolved nutrients, gases and wastes still have to cross there — that is the exchange, and it is also the route those poisons take.
溶解的养分、气体和废物仍然必须在那里交换—— 那就是交换，也是那些毒物走的路。
At puberty, sex hormones make the body develop secondary sexual characteristics — things like body hair, a deeper voice in boys, and breasts in girls.
在青春期，性激素让身体发育出第二性征—— 比如体毛、男孩变粗的嗓音，还有女孩的乳房发育。
Testosterone does this in boys; oestrogen does it in girls.
睾酮在男孩身上起这个作用；雌激素在女孩身上起这个作用。
And notice where those hormones come from: they are made in the testes and the ovaries, the same organs that make the gametes.
注意这些激素从哪里来：它们在睾丸和卵巢中产生， 也就是产生配子的同一批器官。
So each organ has two jobs.
所以每个器官都有两项工作。
Watch one cycle.
看一个周期。
The lining builds up, an egg is released in the middle, and then it either holds or breaks down.
子宫内膜增厚，中间释放一个卵子，然后内膜要么保持，要么脱落。
The menstrual cycle takes about twenty-eight days.
月经周期大约二十八天，为可能的怀孕做准备。
Three things happen.
有三件事发生。
The lining of the uterus thickens.
子宫内膜增厚。
Then, around day fourteen, an ovary releases an egg — that is ovulation.
然后，大约在第十四天，卵巢释放一个卵子——这就是排卵。
And if the egg is not fertilised, the lining breaks down and leaves the body as a period.
如果卵子没有受精，内膜就崩解并离开身体，成为月经，然后周期重新开始。
Four hormones control the menstrual cycle.
四种激素控制月经周期。
F S H makes an egg mature and makes the ovary release oestrogen.
FSH让卵子成熟，并让卵巢释放雌激素。
L H triggers ovulation.
LH触发排卵。
Oestrogen repairs and thickens the lining of the uterus.
雌激素修复并增厚子宫内膜。
Progesterone keeps the lining thick.
孕酮保持内膜厚。
Oestrogen and progesterone are made in the ovaries; during pregnancy, progesterone keeps the lining in place so the embryo can stay.
雌激素和孕酮在卵巢中产生；怀孕期间， 孕酮让内膜保持在原位，胚胎才能留下。
Here is the calculation, and it has one trap.
这是计算题，里面有一个陷阱。
A woman has a regular twenty-eight day cycle, and her period starts on the first of the month.
一个女性有规律的二十八天周期， 她的月经在这个月一号开始。
On roughly which date is she most likely to conceive?
她最可能在大约哪一天受孕？
Ovulation happens about day fourteen, so counting fourteen days on from the start of the period points to about the fourteenth, and the days either side of that are the most fertile.
排卵大约在第十四天，所以从月经开始那天往后数十四天， 指向大约十四号，前后几天是最容易受孕的。
Now the trap: day one is the day the period starts, not the day it ends.
现在说陷阱：第一天是月经开始的那天，不是结束的那天。
Counting from the wrong day is what makes most cycle questions go wrong.
从错误的那天开始数，正是大多数周期题出错的原因。
Finally, a sexually transmitted infection is an infection spread through sexual contact.
最后，性传播感染是通过性接触传播的感染。 你必须知道的是HIV。
Be precise about two words: H I V is a virus, and it can lead to AIDS, a condition in which the immune system stops working properly.
要准确区分这两个词：HIV是一种病毒，它可能导致艾滋病， 也就是免疫系统不能正常工作的状况。
The virus is not the condition.
病毒不等于那个状况。
It is passed on in infected body fluids — sharing needles, sex, or mother to baby across the placenta or in breast milk.
HIV通过被感染的体液传播——共用针头、性行为， 或者从母亲传给婴儿，通过胎盘或母乳。
The spread is controlled by using condoms, having fewer partners, testing and treating infected people, and not sharing needles.
传播的控制方法是使用安全套、减少性伴侣、 检测并治疗感染者，以及不共用针头。
Three marks to lock in.
锁住三个分。
First: asexual gives no variation, sexual gives variation — and that trade-off explains every advantage and disadvantage.
第一：无性生殖没有变异，有性生殖有变异—— 这个权衡解释了所有的优点和缺点。
Second: fertilisation happens in the oviduct, and the mother's and fetus's blood never mix at the placenta.
第二：受精发生在输卵管，而且在胎盘处母亲和胎儿的血液从不混合。
Third: day one is when the period starts, and ovulation is about day fourteen.
第三：第一天是月经开始的那天，排卵大约在第十四天。
Master these, and this topic is yours.
掌握这些，这个专题就是你的了。