Human Nutrition
IGCSE Biology Topic 7 15:52 English narration · English + 中文 subtitles burned in
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The food on your plate is made of molecules far too big to enter your blood.
你盘子里的食物是由分子构成的，这些分子太大了，进不了你的血液。
So your body takes it apart — first by crushing it, then by cutting the molecules themselves into pieces small enough to cross a wall one cell thick.
所以你的身体把它拆开——先把它压碎，再把分子本身切成足够小的碎片， 小到能穿过只有一个细胞厚的壁。
That whole journey takes about a day, and the exam asks about every stage of it.
整个旅程大约需要一天， 考试会问到其中的每一个阶段。
Welcome to human nutrition.
欢迎来到人体营养。
A balanced diet, the digestive system, the enzymes, and how food gets into the blood.
均衡饮食、消化系统、酶，以及食物如何进入血液。
Let's begin.
让我们开始吧。
A balanced diet gives you all the substances you need, in the right amounts, and it has seven parts.
均衡饮食提供你需要的一切物质，而且数量合适，它有七个部分。
Read that definition carefully, because the amounts do real work in it: a diet can contain all seven and still be unbalanced if the amounts are wrong.
这个定义要仔细读，因为"数量"在里面是真正起作用的： 一份饮食就算七样俱全，只要数量不对，它仍然是不均衡的。
Two of the seven are worth pinning down before the rest.
七样里有两样值得先钉牢。
Carbohydrates are your main source of energy — and "main" is the word that earns the mark, because fats are a store rather than the everyday supply.
碳水化合物是你能量的主要来源—— "主要"这个词才是拿分的地方，因为脂肪是储备，而不是日常的供应。
And Iron is needed to make haemoglobin, the red pigment in red blood cells that carries oxygen; too little iron means too little haemoglobin, and less oxygen reaching your cells.
还有铁，它用来制造血红蛋白，那是红细胞里携带氧气的红色色素； 铁太少就意味着血红蛋白太少，到达细胞的氧也就更少。
Now the full seven, each with the job the exam asks for.
现在来看完整的七样，每一样都配上考试要的那个作用。
Seven parts, and the exam wants the JOB of each, not just the name.
共有七部分，而考试要的是每一部分的作用，不只是名字。
Carbohydrates, from rice, bread and potatoes, are the main source of energy.
碳水化合物来自米饭、面包和土豆，是能量的主要来源。
Fats and oils, from butter, oil and nuts, are an energy STORE and help keep you warm.
脂肪和油来自黄油、食用油和坚果，是能量的储备，还帮助保暖。
Proteins, from meat, fish, eggs and beans, are for growth and repair of cells.
蛋白质来自肉、鱼、蛋和豆类，用于细胞的生长和修复。
Vitamin C keeps skin and gums healthy; vitamin D helps the body take in calcium.
维生素 C 使皮肤和牙龈保持健康；维生素 D 帮助身体吸收钙。
Calcium builds strong bones and teeth; iron is needed to make haemoglobin, the red pigment that carries oxygen.
钙构建坚固的骨骼和牙齿；铁用来制造血红蛋白， 也就是运载氧气的那种红色色素。
Fibre keeps food moving through the gut.
膳食纤维让食物在消化道中持续推进。
And water is needed for all reactions.
而水是一切反应所必需的。
The exam also wants SOURCES, not just jobs.
考试还要来源，不只是作用。
Vitamin C comes from fresh fruit and vegetables — that is why a plate like this matters.
维生素丙来自新鲜水果和蔬菜—— 所以像这样的一盘菜很重要。
Vitamin D is unusual: your skin makes it in sunlight, and you also get it from oily fish and eggs.
维生素丁比较特别：皮肤在阳光下会制造它， 你也可以从油性鱼类和蛋里获得。
Calcium comes from milk and cheese.
钙来自牛奶和奶酪。
Iron comes from red meat and leafy greens.
铁来自红肉和绿叶菜。
Fibre, sometimes called roughage, comes from vegetables and whole grains, and keeps food moving through the gut.
膳食纤维，有时也叫粗粮纤维， 来自蔬菜和全谷物，让食物在肠道中持续移动。
Learn one source for each mineral ion and each vitamin — that is a mark on its own.
每一种矿物质和维生素都要记住一个来源——那本身就是一分。
Two diseases come from missing a vitamin, and you need both.
有两种疾病来自缺乏维生素，两个你都必须知道。
Too little vitamin C causes scurvy — bleeding gums and slow healing.
维生素C太少会引起坏血病—— 牙龈出血、伤口愈合慢。
Too little vitamin D causes rickets — soft, bent bones — because without it the body cannot take in enough calcium to harden them.
维生素D太少会引起佝偻病——骨头变软弯曲—— 因为没有它，身体就不能吸收足够的钙把骨头变硬。
Learn each one as a pair, substance then disease, because that pairing is exactly where the marks are won and lost.
每一组都要成对地记：先物质，后疾病，因为分数正是在这个配对上得失的。
Here is the case we will work through in a moment.
下面就是我们马上要做的那道题。
A child eats almost no fresh fruit or vegetables, and their gums bleed.
一个孩子几乎不吃新鲜水果和蔬菜，他的牙龈出血。
Before we solve it, know what the answer has to look like: you must name the substance and the disease separately.
在解它之前，先知道答案该长什么样：你必须分别说出物质和疾病的名称。
Writing only "a vitamin deficiency" does not earn the mark.
只写"缺乏某种维生素"是得不到分的。
A child eats almost no fresh fruit or vegetables.
一个孩子几乎不吃新鲜水果和蔬菜。
Their gums bleed and small cuts heal slowly.
他的牙龈出血，小伤口也愈合得很慢。
Which part of the diet is missing, and why does that explain the symptoms?
饮食中缺的是哪一部分，而这为什么能解释这些症状？
Fresh fruit and vegetables are the main source of vitamin C, and too little vitamin C causes scurvy — whose signs are exactly bleeding gums and slow healing.
新鲜水果和蔬菜是维生素 C 的主要来源， 而维生素 C 不足会引起坏血病——它的表现正是牙龈出血和愈合缓慢。
So the missing part is vitamin C.
所以缺的是维生素 C。
Here is the marking point: name the SUBSTANCE and the DISEASE separately.
这里是评分要点： 要分别说出物质的名称和疾病的名称。
An answer that says only "a vitamin deficiency" does not earn the mark.
只回答"缺乏某种维生素"是拿不到分的。
The other pair to know is vitamin D and rickets — soft, bent bones, because the body cannot take in enough calcium.
另一对要记住的是维生素 D 和佝偻病——骨头变软变弯， 因为身体无法吸收足够的钙。
Follow the path food takes, because questions often ask for the parts in ORDER.
跟着食物走过的路线看一遍，因为题目常常要求按顺序说出各个部分。
Mouth, then oesophagus, then stomach, then small intestine — the duodenum first, then the ileum — then large intestine, colon, rectum and anus.
口腔，然后是食道，然后是胃，然后是小肠——先是十二指肠，再是回肠—— 然后是大肠、结肠、直肠和肛门。
Two organs sit beside the canal rather than in it: the liver, which makes bile, and the pancreas, which makes enzymes.
有两个器官位于消化道旁边，而不在管道之内： 制造胆汁的肝脏，和制造酶的胰腺。
Food never passes through either.
食物从来不经过它们中的任何一个。
That distinction between the canal itself and the glands that feed into it is worth getting right.
消化道本身与向它分泌的腺体之间的这个区别，值得弄清楚。
That path has a name worth using: the alimentary canal, one long tube running from mouth to anus.
这条通路有个值得用上的名字：消化道，一条从口腔通到肛门的长管。
Two things about it catch people out.
关于它有两件事最容易把人绊倒。
The first is the naming.
第一件是名字。
The small intestine is called small because it is narrow, not because it is short — it is by far the longest part of the canal, which is exactly what you would want from the place where nearly all absorption happens.
小肠之所以叫"小"，是因为它细，而不是因为它短—— 它其实是整条管子里最长的一段，而这正是你希望"几乎全部吸收都在这里发生"的 地方所具备的。
The large intestine is shorter and wider.
大肠反而更短、更粗。
The second is the part questions love: four organs help with digestion, but food does not pass through any of them.
第二件就是题目最爱考的那点：有四个器官帮助消化，但食物并不经过它们中的任何一个。
They add their juices to the canal from the side — the salivary glands in the mouth, the pancreas into the duodenum, the liver, which makes bile, and the gall bladder, which stores that bile until it is needed.
它们是从旁边把消化液加进消化道——口腔里的唾液腺、 通向十二指肠的胰腺、制造胆汁的肝脏，以及把胆汁储存起来备用的胆囊。
So if a question asks for the parts of the alimentary canal, none of those four belong in the answer; if it asks which organs aid digestion, all four do.
所以，如果题目问消化道由哪些部分组成，这四个一个都不该写进答案； 而如果问哪些器官帮助消化，这四个全都算。
Look again at the organs that sit BESIDE the tube.
再看一遍那些位于管道旁边的器官。
The salivary glands in the mouth start chemical digestion of starch with amylase, before the food has even been swallowed.
口腔里的唾液腺用淀粉酶开始对淀粉的化学消化， 食物甚至还没有被吞下。
The pancreas pours enzymes into the duodenum — amylase, protease and lipase all leave from here.
胰腺把酶注入十二指肠—— 淀粉酶、蛋白酶和脂肪酶都从这里出来。
The liver makes bile; the gall bladder stores that bile and releases it when fatty food arrives.
肝脏制造胆汁； 胆囊储存胆汁，并在脂肪性食物到来时释放它。
Food never passes through any of these four.
食物从来不经过这四个器官中的任何一个。
They only add juices.
它们只是加入消化液。
If a question asks which organs the food itself travels through, leave them out.
如果题目问食物本身经过哪些器官，就把它们排除在外。
Five things happen to food, in this order.
食物会经历五件事，顺序是这样的。
Ingestion is taking food and drink into the body through the mouth.
摄入是通过口腔把食物和饮料带进身体。
Digestion is breaking food down into small molecules.
消化是把食物分解成小分子。
Absorption is nutrients moving from the intestine into the blood.
吸收是营养物质从肠道进入血液。
Assimilation is when your cells take in those nutrients and use them.
同化是你的细胞吸收这些营养物质并利用它们。
And egestion is undigested food leaving the body as faeces.
排遗是未消化的食物以粪便的形式离开身体。
Now the trap that catches everybody: egestion is not excretion.
现在说那个人人都会掉进去的陷阱： 排遗不是排泄。
Egestion removes food that was never absorbed — it never took part in your chemistry.
排遗排出的是从未被吸收的食物——它从来没有参与你的化学反应。
Excretion removes the waste products your own reactions made.
排泄排出的是你自己的反应产生的废物。
Different words, different meanings.
不同的词，不同的意思。
Digestion itself comes in two kinds, and mixing them up loses marks.
消化本身有两种，搞混它们会丢分。
Physical digestion breaks food into smaller pieces without changing the molecules — chewing with teeth, squeezing in the stomach, and emulsifying fat with bile.
物理消化把食物弄成更小的碎片， 但不改变分子——用牙齿咀嚼、胃里挤压、以及用胆汁乳化脂肪。
That raises the surface area so enzymes can work faster.
这样增大表面积，让酶能更快地工作。
Chemical digestion is the enzyme step.
化学消化是酶的步骤。
It turns large insoluble molecules into small soluble ones that can cross into the blood.
它把大的不溶性分子变成小的可溶性分子，这些分子才能进入血液。
Physical first, chemical second — pieces, then molecules.
先物理，后化学——先碎片，再分子。
Physical digestion breaks food into smaller pieces without changing the molecules, giving a much larger surface area for the enzymes that come later.
物理消化把食物弄成更小的碎片，但不改变分子， 这样后面的酶就有大得多的表面积可以作用。
It starts with your teeth.
它从你的牙齿开始。
There are four kinds, and the diagram after this matches each shape to the job it does.
牙齿有四种，接下来的图会把每一种的形状和它承担的工作对应起来。
Then we look inside a tooth in section, because the shape is only half the story — what a tooth is MADE of is what decides how it fails.
之后我们再看一颗牙的纵切面，因为形状只是一半的故事—— 一颗牙是由什么构成的，才决定了它会怎样坏掉。
Take just the outermost layer for now.
现在只看最外面那一层。
Enamel is the hard white outer layer, and the hardest material in your body, but it contains no living cells at all.
牙釉质是坚硬的白色外层， 也是你身体里最硬的物质，但它完全不含活细胞。
That is the fact worth carrying: enamel cannot repair itself, so damage to it is permanent, which is why a dentist drills and fills instead of waiting for it to heal.
这就是值得记住的那个事实：牙釉质无法自我修复， 所以它受到的损伤是永久性的——这正是牙医要钻孔补牙、 而不是等它自己长好的原因。
The section coming up labels the layers beneath it.
接下来的切面图会标出它下面的各层。
Four kinds, and each shape matches its job — that is the link the exam wants.
有四种，而每一种的形状都与它的功能相配——这正是考试想要的那条联系。
Incisors are the sharp, chisel-shaped front teeth, for biting and cutting.
门齿是前面锋利的、凿子形状的牙，用来咬断和切割。
Canines are pointed, for tearing.
犬齿是尖的，用来撕扯。
Premolars are flat-topped, for chewing and grinding.
前臼齿的顶面是平的，用来咀嚼和磨碎。
And molars are the flat back teeth, also for chewing and grinding.
而臼齿是后面平的牙，同样用来咀嚼和磨碎。
Look at the model: the flat tops are at the back where the crushing force is greatest, and the sharp edges are at the front where you bite.
看这个模型：平的顶面在后面，那里的挤压力最大； 而锋利的边缘在前面，也就是你咬东西的地方。
Now one tooth in section, with four parts to label.
现在看一颗牙的剖面，有四个部分要标注。
Enamel is the hard white outer layer — the hardest material in the body.
牙釉质是坚硬的白色外层——身体里最硬的物质。
Dentine is the softer, bone-like layer underneath it.
牙本质是它下面较软的、类似骨头的一层。
Pulp is the soft centre, holding nerves and blood vessels, which is why a deep cavity HURTS while a shallow one does not.
牙髓是柔软的中心部分，里面有神经和血管， 这正是深的龋洞会疼、而浅的不疼的原因。
And cement fixes the root into the jaw.
而牙骨质把牙根固定在颌骨里。
The tooth sits in the bone of the jaw and is held firm by the gums.
牙齿嵌在颌骨中，并由牙龈牢牢固定住。
Physical digestion continues in the stomach, where the wall of the stomach is made of muscle that squeezes and mixes the food into smaller pieces.
物理消化在胃里继续，胃壁由肌肉构成，它挤压并混合食物，把它弄成更小的碎片。
Then, for the supplement paper, there is bile.
然后，补充部分还有胆汁。
Bile is made in the liver and stored in the gall bladder.
胆汁在肝脏里制造，储存在胆囊里。
It emulsifies fats — that means it breaks large drops of fat into many tiny droplets.
它乳化脂肪——意思是它把大的脂肪滴打散成许多微小的液滴。
Many small droplets have a much larger surface area than one big drop, so the enzyme lipase can digest the fat far faster.
许多小液滴的表面积比一个大液滴大得多，所以脂肪酶能快得多地消化脂肪。
And here is the detail examiners look for: bile contains no enzymes at all.
考官会找的细节是：胆汁完全不含酶。
It is physical digestion, not chemical.
它是物理消化，不是化学消化。
Chemical digestion is the enzymes' work.
化学消化是酶的工作。
It turns large insoluble molecules into small soluble ones — and that matters, because only small soluble molecules can be absorbed into the blood.
它把大的不溶性分子变成小的可溶性分子—— 这很重要，因为只有小的可溶性分子才能被吸收进血液。
Three enzymes, and you need all three.
三种酶，三个你都要记住。
Amylase breaks down starch into reducing sugars.
淀粉酶把淀粉分解成还原糖。
Protease breaks down protein into amino acids.
蛋白酶把蛋白质分解成氨基酸。
Lipase breaks down fats and oils into fatty acids and glycerol.
脂肪酶把脂肪和油分解成脂肪酸和甘油。
Learn where each one is made too.
也要记住每一种在哪里制造。
Amylase comes from the salivary glands and the pancreas.
淀粉酶来自唾液腺和胰腺。
Protease comes from the stomach and the pancreas.
蛋白酶来自胃和胰腺。
Lipase comes from the pancreas.
脂肪酶来自胰腺。
Your stomach makes hydrochloric acid, as part of the gastric juice, and it does exactly two jobs.
你的胃制造盐酸，它是胃液的一部分，它恰好做两件事。
First, it kills harmful microorganisms in the food.
第一，它杀死食物中有害的微生物。
Second, it gives an acidic pH, which is the best pH for the stomach's protease to work in.
第二，它提供酸性的pH， 这正是胃里的蛋白酶工作的最佳pH。
Two jobs, two marks — write both.
两个作用，两分——两个都要写。
Now the supplement detail, and it lands better if you see the principle behind it first: an enzyme is specific, so one food group can need several of them, and pH decides where each one is able to work.
现在说补充部分的细节，而先看清它背后的原理会更好懂： 酶是有专一性的，所以一类食物可能需要好几种酶， 而由 pH 决定每一种酶能在哪里工作。
Starch is digested in two steps, not one, because the product of the first step is still a sugar too big to absorb — and the enzyme that finishes the job is placed exactly where absorption happens, on the membranes lining the small intestine.
淀粉的消化分两步，不是一步，因为第一步的产物仍然是一种太大、无法吸收的糖—— 而完成收尾的那个酶，恰好就安置在吸收发生的地方，也就是小肠内壁的膜上。
Protein needs two different proteases for a different reason: it is broken down in two compartments whose pH could hardly be less alike, the acidic stomach and the alkaline small intestine, and no single enzyme works well in both.
蛋白质需要两种不同的蛋白酶，理由却不同： 它在两个 pH 相差到不能再远的区室里被分解——酸性的胃和碱性的小肠—— 而没有哪一种酶能在两者中都好好工作。
Which raises the obvious question — how does acidic food leaving the stomach become alkaline in time?
这就带来一个显而易见的问题：离开胃的酸性食物，怎么来得及变成碱性的？
Bile is alkaline, and neutralising that acid as it enters the duodenum is a second job, quite separate from emulsifying fat.
胆汁是碱性的，而在食物进入十二指肠时中和这些酸， 是它的第二项工作，与乳化脂肪完全是两回事。
Learn these as enzyme, substrate, product.
按酶、底物、产物这三样来记。
Starch takes TWO steps: amylase breaks starch into maltose, then maltase — sitting on the membranes of the epithelium lining the small intestine — breaks maltose into glucose.
淀粉要分两步：淀粉酶把淀粉分解成麦芽糖， 然后麦芽糖酶——它位于小肠上皮细胞的膜上——把麦芽糖分解成葡萄糖。
Protein also takes two, by two different proteases: pepsin works in the ACIDIC stomach, and trypsin works in the ALKALINE small intestine.
蛋白质也要两步，由两种不同的蛋白酶完成： 胃蛋白酶在酸性的胃里起作用，胰蛋白酶在碱性的小肠里起作用。
That pH difference is the point, and it is why bile matters beyond emulsifying fat — bile is alkaline, so it neutralises the acidic food arriving from the stomach and gives the duodenum's enzymes the pH they need.
这个 pH 的差别正是重点所在， 而它也说明了胆汁的作用不止是乳化脂肪——胆汁是碱性的， 所以它中和从胃里来的酸性食物， 给十二指肠里的酶提供它们所需要的 pH。
A villus is adapted for FAST absorption, and each adaptation should be given with its reason.
绒毛是为快速吸收而适应的，而每一项适应都应该连同它的理由一起说出来。
Microvilli on its surface add even more surface area on top of the villi themselves.
表面上的微绒毛在绒毛本身之外又增加了更多的表面积。
The wall is just ONE cell thick, so molecules have only a short distance to travel.
管壁只有一个细胞那么厚，所以分子只需移动很短的距离。
A network of capillaries carries away absorbed glucose and amino acids, keeping the concentration gradient steep.
毛细血管网把吸收的葡萄糖和氨基酸运走，使浓度梯度保持陡峭。
And a lacteal in the centre carries away the fatty acids and glycerol.
而中央的乳糜管把脂肪酸和甘油运走。
Note that last split carefully: capillaries take sugars and amino acids, the lacteal takes fats.
最后这个分工要仔细记住：毛细血管运走糖和氨基酸，乳糜管运走脂肪。
Getting those the wrong way round is the classic lost mark.
把这两者搞反，正是经典的失分点。
Here is a single villus in cross-section — the structure behind those adaptations.
这是一根绒毛的横切面——那些适应特征背后的结构。
The outer surface is folded into microvilli.
外表面折叠成微绒毛。
Just inside the wall runs a dense capillary network, ready to take up glucose and amino acids as soon as they cross.
紧贴壁的内侧有密集的毛细血管网， 葡萄糖和氨基酸一穿过就可以被带走。
Down the middle sits the lacteal, a lymph vessel that takes up fatty acids and glycerol.
中央是乳糜管， 一条淋巴管，负责带走脂肪酸和甘油。
The whole projection is only one cell thick at the wall, so the path into either vessel is short.
整个突起在壁处只有一个细胞那么厚，所以进入任何一条管道的路径都很短。
Label the wall, the capillaries and the lacteal, and you have the three marks this diagram usually carries.
标出管壁、毛细血管和乳糜管，你就拿到了这张图通常给出的三分。
Digested nutrients are absorbed in the small intestine — and be precise about which intestine does what, because the two are not doing the same job in different places.
消化后的营养物质在小肠被吸收——而且要说清楚哪一段肠子做什么， 因为这两段并不是在不同的地方做同一件事。
The small intestine absorbs the digested food, and most of the water with it.
小肠吸收消化好的食物，连同其中大部分的水。
The colon absorbs the water that is left over, which is why what reaches the rectum is solid rather than liquid.
结肠吸收剩下的那部分水，这正是到达直肠的东西是固体而不是液体的原因。
One reclaims food, the other reclaims water.
一个回收食物，另一个回收水。
The lining that does the absorbing is folded into millions of tiny finger-shaped villi, and each villus carries the adaptations we have just been through.
负责吸收的那层内壁被折叠成数百万个细小的指状绒毛， 而每个绒毛都带着我们刚讲过的那些适应特征。
Only the last of them is worth repeating, because it is the one students reverse: the capillaries carry away glucose and amino acids, while the lacteal carries away fatty acids and glycerol.
其中只有最后一点值得再说一遍，因为学生最容易把它说反： 毛细血管带走葡萄糖和氨基酸，而乳糜管带走脂肪酸和甘油。
Absorption is not only about sugars, amino acids and fats.
吸收不只是关于糖、氨基酸和脂肪。
Most water is absorbed in the small intestine too.
大部分水也在小肠被吸收。
Whatever water is left is taken up by the colon in the large intestine, and what remains is solid faeces, ready for egestion through the rectum and anus.
剩下的水由大肠中的结肠吸收，留下来的就是固体粪便， 准备经直肠和肛门排出。
So the small intestine does the heavy work on nutrients and most of the water; the colon finishes the water and packs the undigested fibre into faeces.
所以小肠承担营养物质和大部分水的重活； 结肠完成剩余水分的吸收，并把未消化的纤维压成粪便。
That is why plenty of fibre in the diet keeps the gut moving — there is something solid left for the colon to push.
这就是为什么饮食中有充足的膳食纤维能让肠道保持推动—— 结肠还有固体的东西可以推送。
Three marks to lock in.
锁住三个分。
First: egestion is not excretion — egestion removes food that was never absorbed.
第一：排遗不是排泄——排遗排出的是从未被吸收的食物。
Second: bile has no enzymes; it emulsifies fat to give a larger surface area, and it neutralises stomach acid.
第二：胆汁没有酶；它乳化脂肪以提供更大的表面积，并中和胃酸。
Third: for villi, always give surface area and a short distance — microvilli for surface area, a wall one cell thick for the short distance.
第三：说到绒毛，永远要提表面积和短距离——微绒毛提供表面积， 一个细胞厚的壁提供短距离。
Master these, and this topic is yours.
掌握这些，这个专题就是你的了。