生物分子

Living things are built from a few kinds of molecule 分子. The most important are carbohydrates, fats and proteins. Each is made from a small number of chemical elements 元素.

• Carbohydrates 碳水化合物 and fats 脂肪 contain three elements: carbon 碳 (C), hydrogen 氢 (H) and oxygen (O).
• Proteins 蛋白质 contain carbon, hydrogen and oxygen too, plus nitrogen 氮 (N). (Some proteins also contain sulfur, S.)

Large molecules are made by joining many small molecules together, like beads on a string.

Starch and glycogen are energy stores; cellulose makes plant cell walls. Proteins are built from about 20 different kinds of amino acid joined in a chain. One fat molecule is made from three fatty acids joined to one glycerol.

You can test a piece of food to find out which substances it contains. Add the test chemical and watch for a colour change.

Two things to remember: only the Benedict's test needs heat; and DCPIP loses its colour (goes colourless), while the other tests gain a new colour.

Worked example. A student tests an unknown food. Iodine solution stays orange-brown. Benedict's solution, after heating, turns brick-red. Biuret solution turns purple. What does the food contain? Iodine staying orange-brown is a negative result, so there is no starch. Brick-red is a positive Benedict's result, so reducing sugar is present. Purple is a positive biuret result, so protein is present. The food contains reducing sugar and protein, but no starch. "No colour change" is a real result - it tells you the substance is absent.

DNA is the genetic material 遗传物质 — it carries the instructions for building and running an organism. Its structure has four key points:

• two strands 链 are coiled together to form a double helix 双螺旋 (like a twisted ladder).
• each strand carries chemicals called bases 碱基.
• bonds 化学键 between pairs of bases hold the two strands together.
• the bases always pair in the same way: A with T, and C with G.

Common mistakes.

• Saying iodine turns "black" or "blue". Write the full change: orange-brown → blue-black.
• Forgetting to heat in the Benedict's test, or heating in the other tests. Only Benedict's needs heating, in a water bath.
• Saying Benedict's tests for "sugar" or for starch. It tests for reducing sugars such as glucose; starch gives no brick-red colour.
• Writing "turns clear" for DCPIP. Clear means see-through and may still be coloured; DCPIP turns colourless.
• Saying proteins are made of glucose, or that fats are made of amino acids. Proteins are built from amino acids; a fat is three fatty acids and one glycerol.
• Pairing A with G, or A with C. The bases always pair A–T and C–G.

• Carbohydrates and fats are made of C, H and O. Proteins also contain N (nitrogen).
• Learn each food test and its colour change. For reducing sugars you must heat the Benedict's solution.
• DCPIP goes from blue to colourless; the other four tests each give a new colour.
• In DNA, the bases pair only A–T and C–G. So if you know the bases on one strand, you can work out the other.