Photosynthesis
IGCSE Biology Topic 6 7:36 English narration · English + 中文 subtitles burned in
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A plant cannot hunt and it cannot move.
植物不能捕食，也不能移动。
So it builds its own food out of almost nothing: a gas from the air, water from the soil, and light.
所以它用几乎什么都没有的东西造出自己的食物： 空气中的一种气体、土壤中的水，还有光。
Every meal you have ever eaten started here.
你吃过的每一顿饭都从这里开始。
So did the oxygen in the breath you just took.
你刚才吸的那口氧气也是。
Welcome to photosynthesis.
欢迎来到光合作用。
The equation, what the plant does with the sugar, limiting factors, and how a leaf is built.
方程式、植物如何使用糖、限制因素，还有叶片的结构。
Let's begin.
让我们开始吧。
Here is the whole process.
这就是整个过程。
Light arrives.
光到达。
A gas goes in from the air, water comes up from the roots — and sugar and oxygen come out.
一种气体从空气进入，水从根部上来—— 然后糖和氧气出来。
Learn this equation exactly, because it is worth marks on its own.
要准确记住这个方程式，因为它本身就值分。
The two raw materials are carbon dioxide and water.
两种原料是二氧化碳和水。
Carbon dioxide plus water gives glucose plus oxygen.
二氧化碳加水生成葡萄糖加氧气。
Say it again: carbon dioxide and water go in; glucose and oxygen come out.
再说一遍：二氧化碳和水进去；葡萄糖和氧气出来。
But the equation is not complete without two conditions written above the arrow.
但这个方程式还不完整， 箭头上方必须写两个条件。
The reaction only happens in the light, and only where there is chlorophyll.
这个反应只在有光的时候发生，而且只在有叶绿素的地方发生。
Leave those out and you lose the mark.
漏掉它们就会失分。
For the supplement paper, learn the balanced equation too — six carbon dioxide and six water give one glucose and six oxygen.
补充部分还要记住配平方程式—— 六个二氧化碳和六个水生成一个葡萄糖和六个氧气。
Now chlorophyll.
现在说叶绿素。
Chlorophyll is a green pigment, and it is found inside the chloroplasts.
叶绿素是一种绿色色素，存在于叶绿体内。
Here is the sentence the mark scheme wants: chlorophyll traps light, and it transfers the energy from light into chemical energy stored in the carbohydrate.
评分标准想要这样一句话：叶绿素捕捉光，并把光的能量转变成储存在碳水化合物中的化学能。
Notice the verb — transfers.
注意这个动词——转变。
Energy is not created here, it is moved from one form to another.
能量不是在这里被创造的，而是从一种形式转移到另一种形式。
And notice something else: chlorophyll is not used up.
还要注意一点：叶绿素不会被消耗。
It does the same job over and over, which is why a leaf stays green while it works.
它一次又一次做同样的工作， 这就是为什么叶子在工作时一直是绿色的。
The plant does not just leave the glucose sitting there.
植物不会把葡萄糖就放在那里。
It has five uses, and questions love them.
它有五种用途，题目很喜欢考。
First, storage: glucose is changed into starch, which is an energy store that does not dissolve — and that matters, because a dissolved store would upset the cell by osmosis.
第一，储存：葡萄糖被转变成淀粉，这是一种不溶解的能量储存物—— 这很重要，因为溶解的储存物会通过渗透扰乱细胞。
Second, building: glucose is made into cellulose to build cell walls.
第二，建造： 葡萄糖被制成纤维素来建造细胞壁。
Third, glucose is broken down in respiration to release energy.
第三，葡萄糖在呼吸作用中被分解以释放能量。
Fourth, transport: it is changed into sucrose and moved around the plant in the phloem.
第四，运输：它被转变成蔗糖，通过韧皮部在植物体内运送。
And fifth, it is made into nectar, to attract insects for pollination.
第五，它被制成花蜜，吸引昆虫传粉。
Light, water and carbon dioxide are not quite everything.
光、水和二氧化碳还不是全部。
A plant also takes up mineral ions from the soil through its roots, and you need exactly two.
植物还通过根从土壤中吸收矿物离子， 你需要记住恰好两种。
Nitrate ions are needed to make amino acids, and therefore proteins.
硝酸根离子用来制造氨基酸，从而制造蛋白质。
Magnesium ions are needed to make chlorophyll.
镁离子用来制造叶绿素。
Now learn what a short plant looks like, because that is the exam question.
现在记住缺乏时植物是什么样子，因为这就是考题。
A plant short of nitrate grows poorly with weak stems.
缺硝酸根的植物长得差，茎很弱。
A plant short of magnesium has yellow leaves — it cannot make enough chlorophyll, and chlorophyll is what makes leaves green.
缺镁的植物叶子发黄—— 它做不出足够的叶绿素，而叶绿素正是让叶子变绿的东西。
Three things change the rate of photosynthesis.
有三样东西改变光合作用的速率。
More light gives a faster rate.
光越多，速率越快。
More carbon dioxide gives a faster rate.
二氧化碳越多，速率越快。
And a warmer temperature gives a faster rate too — but temperature is different from the other two, because it can go too far.
温度越高速率也越快——但温度和另外两个不同，因为它可能过头。
Keep heating, and the enzymes are denatured, and the rate crashes.
继续加热，酶就会变性，速率会骤降。
So light and carbon dioxide level off, while temperature rises to a peak and then falls.
所以光和二氧化碳会趋于平稳，而温度是升到一个峰值然后下降。
Here is the supplement idea, and the graph question that comes with it.
这是补充部分的概念，还有随之而来的图表题。
At any moment, the one factor in shortest supply holds the rate back.
在任何时刻， 供应最少的那一个因素限制着速率。
That factor is called the limiting factor.
这个因素叫做限制因素。
Now look at a graph of rate against light intensity: it rises steeply, then levels off flat.
现在看速率对光照强度的图：它先陡峭上升，然后变平。
On the rising part, adding light raises the rate — so light intensity is the limiting factor there.
在上升的部分，增加光会提高速率——所以光照强度是那里的限制因素。
On the flat part, adding more light changes nothing, so light is no longer limiting; something else is, usually carbon dioxide concentration or temperature.
在平的部分，再加光也没有变化，所以光不再是限制因素了； 是别的东西，通常是二氧化碳浓度或温度。
The test is always the same.
判断方法总是一样的。
If adding more of a factor speeds the reaction up, that factor was the one holding it back.
如果增加某个因素能让反应加快，那个因素就是原来在限制它的。
How do you prove a plant needs light, or chlorophyll, or carbon dioxide?
你怎么证明植物需要光、叶绿素或二氧化碳？
You take one of them away, then test a leaf for starch with iodine.
你把其中一个拿走， 然后用碘液检验叶片里的淀粉。
Two details earn the marks.
有两个细节能得分。
First, you must leave the plant in the dark for a day or two beforehand, to use up the starch it already has — otherwise you cannot tell old starch from new.
第一，你必须事先把植物放在黑暗中一两天，用掉它已有的淀粉—— 否则你分不清旧淀粉和新淀粉。
Second, you need a control: everything else kept the same, so the test is fair, and only the one factor differs.
第二，你需要一个对照： 其他条件都保持相同，这样实验才公平，只有那一个因素不同。
You can also watch gas exchange in a water plant with hydrogencarbonate indicator, which changes colour as the carbon dioxide level changes: in the light the plant takes carbon dioxide in, in the dark it gives it out.
你也可以用碳酸氢盐指示剂观察水生植物的气体交换，它会随二氧化碳浓度变化而变色： 在光下植物吸收二氧化碳，在黑暗中放出二氧化碳。
The neatest proof is a leaf with green and white parts.
最漂亮的证明是一片有绿色和白色部分的叶子。
Test it, and starch shows up only in the green parts — the parts that had chlorophyll.
检验它， 淀粉只出现在绿色部分——也就是有叶绿素的部分。
Finally, the leaf itself — well adapted for photosynthesis, and the rule here is to always link the part to its job.
最后是叶片本身——它很好地适应了光合作用，这里的规则是永远把结构和它的功能联系起来。
Start with the whole leaf: it is broad and thin.
先看整片叶子：它又宽又薄。
Broad gives a large surface area to catch plenty of light; thin means light and gases reach every cell quickly.
宽提供大的表面积来捕捉充足的光； 薄意味着光和气体能快速到达每一个细胞。
Now go down through the layers.
现在往下看各层。
On top is a clear waxy cuticle, which reduces water loss but still lets light through.
最上面是透明的蜡质角质层，它减少水分流失但仍然让光通过。
Below it, the upper epidermis is clear and has no chloroplasts, so light passes straight down to the palisade mesophyll — tall column-shaped cells packed with chloroplasts, sitting near the top where the light is strongest.
下面，上表皮是透明的、没有叶绿体，所以光直接向下传到栅栏叶肉—— 那是高高的柱状细胞，充满叶绿体，位于靠近顶部、光最强的地方。
They do most of the photosynthesis.
大部分光合作用由它们完成。
Under them, the spongy mesophyll has rounded cells with air spaces between them, so gases move easily.
再下面，海绵叶肉是圆形细胞， 细胞之间有气腔，所以气体容易移动。
The stomata, mostly on the lower surface, let carbon dioxide in and oxygen out, and each one is opened and closed by a pair of guard cells.
气孔大多在下表面， 让二氧化碳进入、氧气出去，每个气孔都由一对保卫细胞开合。
And running through it all, the vascular bundles carry water in through the xylem, and sugars away through the phloem.
贯穿其中的维管束通过木质部把水送进来，通过韧皮部把糖运走。
Three marks to lock in.
锁住三个分。
First: write the conditions above the arrow — light and chlorophyll — or the equation is incomplete.
第一：在箭头上方写出条件——光和叶绿素——否则方程式不完整。
Second: nitrate for amino acids, magnesium for chlorophyll; short of nitrate means poor growth, short of magnesium means yellow leaves.
第二：硝酸根用于氨基酸，镁用于叶绿素；缺硝酸根表示生长不良，缺镁表示叶子发黄。
Third: in any leaf question, link the part to its job — broad and thin to catch light, palisade cells full of chloroplasts, stomata and air spaces for gas exchange.
第三：任何关于叶片的题目，都要把结构和功能联系起来—— 又宽又薄以捕捉光，栅栏细胞充满叶绿体，气孔和气腔用于气体交换。
Master these, and this topic is yours.
掌握这些，这个专题就是你的了。
Five leftover facts.
还剩五件事实。
First, photosynthesis is how plants feed themselves: they make carbohydrates from simple raw materials, using energy from light.
第一，光合作用就是植物自己养活自己的方式：用光能，从简单原料制造碳水化合物。
Second, on a dull day light intensity is usually the limiting factor.
第二，阴天时，光照强度通常是限制因素。
Third, on a bright day carbon dioxide may be the limiting factor.
第三，晴天时，二氧化碳可能是限制因素。
Fourth, the lower epidermis is a thin layer that holds the stomata.
第四，下表皮是一层薄层，气孔长在上面。
Fifth, the cuticle is a clear waxy layer on top: it reduces water loss but still lets light through.
第五，角质层是顶上透明的蜡层：减少失水，但仍让光通过。