人体的营养

A balanced diet 均衡饮食 gives you all the substances you need, in the right amounts. There are seven parts.

Two diseases come from missing a vitamin:

• too little vitamin C → scurvy 坏血病 (bleeding gums, slow healing).
• too little vitamin D → rickets 佝偻病 (soft, bent bones), because the body cannot take in enough calcium.

Worked example. A child eats almost no fresh fruit or vegetables. Their gums bleed and small cuts heal slowly. Which part of the diet is missing, and why does that explain the symptoms? 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. So the missing part is vitamin C. Name the substance and the disease separately: an answer that says only "a vitamin deficiency" does not earn the mark.

The digestive system 消化系统 breaks food down. Food passes along one long tube, the alimentary canal 消化道:

mouth → oesophagus 食道 → stomach 胃 → small intestine 小肠 (the duodenum 十二指肠 then the ileum 回肠) → large intestine 大肠 (the colon 结肠, rectum 直肠 and anus 肛门).

Some organs help with digestion but food does not pass through them: the salivary glands 唾液腺 in the mouth, the pancreas 胰腺, the liver 肝脏 and the gall bladder 胆囊.

Five things happen to food, in order:

Physical digestion breaks food into smaller pieces without changing the food molecules. This gives a larger surface area 表面积 for the enzymes 酶 to act on later.

Teeth

You have four kinds of teeth 牙齿:

A tooth is built from these parts:

Teeth are set into the bone of the jaw and held firm by the gums 牙龈.

The stomach

The wall of the stomach is made of muscle 肌肉 that squeezes and mixes the food, breaking it into smaller pieces.

Bile (Supplement)

Bile 胆汁 is made in the liver and stored in the gall bladder. It emulsifies 乳化 fats and oils — it breaks large drops of fat into many tiny droplets. This gives a much larger surface area for the enzyme lipase 脂肪酶 to digest the fat. Bile itself contains no enzymes.

Chemical digestion breaks large insoluble 不溶性 molecules into small soluble 可溶性 molecules. Only small soluble molecules can be absorbed into the blood. Enzymes carry out this work.

Acid in the stomach

The stomach makes hydrochloric acid 盐酸, which is part of the gastric juice 胃液. This acid:

• kills harmful microorganisms 微生物 in the food, and
• gives an acidic 酸性 pH, the best pH for the stomach's protease to work.

Digesting starch and protein (Supplement)

Starch is digested in two steps:

• amylase breaks starch into maltose 麦芽糖.
• maltase 麦芽糖酶, on the membranes of the epithelium 上皮 lining the small intestine, breaks maltose into glucose 葡萄糖.

Protein is digested by two proteases:

• pepsin 胃蛋白酶 breaks protein down in the acidic stomach.
• trypsin 胰蛋白酶 breaks protein down in the alkaline 碱性 small intestine.

Bile is alkaline. It neutralises 中和 the acidic food and gastric juice as they enter the duodenum, giving a suitable pH for the enzymes there.

Digested nutrients are absorbed into the blood in the small intestine. Most water is absorbed here too; the colon absorbs the rest, leaving solid faeces.

Villi (Supplement)

The inside of the small intestine is covered with millions of tiny finger-shaped villi 绒毛. Each villus is itself covered in much smaller microvilli 微绒毛. Together these give a huge surface area, so absorption is fast.

A single villus has:

• a wall just one cell thick, so molecules have only a short distance to travel.
• a network of capillaries 毛细血管 that carry away absorbed glucose and amino acids.
• a lacteal 乳糜管 in the centre that carries away absorbed fatty acids and glycerol.

Common mistakes.

• Saying bile digests fat. Bile has no enzymes. It breaks fat into small droplets (emulsifies it), which is physical digestion; lipase then digests the fat.
• Saying the stomach acid digests protein. The acid kills microorganisms and gives the low pH that the stomach protease needs; the protease digests the protein.
• (Supplement) Saying amylase turns starch straight into glucose. Amylase makes maltose; maltase on the small intestine lining then turns maltose into glucose.
• Saying an enzyme is "used up" in digestion. Each enzyme molecule is not changed, so it can digest many food molecules.
• Saying villi help absorption because they are "long". Say what they give: a large surface area, a wall one cell thick (a short distance) and a good blood supply.

• A balanced diet has the right amount of each part. Learn one source and one use for each.
• Vitamin C ↔ scurvy; vitamin D ↔ rickets.
• The five processes in order: ingestion → digestion → absorption → assimilation → egestion. Egestion (faeces) is not excretion.
• Physical digestion makes pieces smaller (more surface area); chemical digestion uses enzymes to make molecules smaller and soluble.
• amylase → sugars, protease → amino acids, lipase → fatty acids + glycerol. Stomach acid kills microorganisms and sets a low pH.
• Villi and microvilli give a large surface area, and a thin wall gives a short distance — both make absorption faster.