| Core | Supplement |
|---|---|
| 1 List the chemical elements that make up: carbohydrates, fats and proteins | |
| 2 State that large molecules are made from smaller molecules, limited to: (a) starch, glycogen and cellulose from glucose (b) proteins from amino acids (c) fats and oils from fatty acids and glycerol | |
| 3 Describe the use of: (a) iodine solution test for starch (b) Benedict’s solution test for reducing sugars (c) biuret test for proteins (d) ethanol emulsion test for fats and oils (e) DCPIP test for vitamin C | 4 Describe the structure of a DNA molecule: (a) two strands coiled together to form a double helix (b) each strand contains chemicals called bases (c) bonds between pairs of bases hold the strands together (d) the bases always pair up in the same way: A with T, and C with G (full names are not required) |
Biological molecules
IGCSE Biology · Topic 4
4.1
The chemical elements in food
Syllabus
Source: Cambridge International syllabus
Bread is rich in carbohydrates, one of the main food groups.
Carbohydrates and fats contain C, H and O; proteins also contain nitrogen
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.)
Explore
Biological molecules lab
Classify food molecules by their main job in cells.
Vocabulary
Train
| English | Chinese | Pinyin |
|---|---|---|
| molecule | 分子 | fèn zǐ |
| elements | 元素 | yuán sù |
| carbohydrates | 碳水化合物 | tàn shuǐ huà hé wù |
| fats | 脂肪 | zhī fáng |
| carbon | 碳 | tàn |
| hydrogen | 氢 | qīng |
| proteins | 蛋白质 | dàn bái zhì |
| nitrogen | 氮 | dàn |
4.1
Building large molecules from small ones
Large molecules are made by joining many small molecules together, like beads on a string.
| Large molecule | Built from |
|---|---|
| starch 淀粉, glycogen 糖原 and cellulose 纤维素 | many glucose 葡萄糖 units |
| proteins | amino acids 氨基酸 |
| fats and oils | fatty acids 脂肪酸 and glycerol 甘油 |
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.
Large molecules are built by joining many small units, like beads on a string
Starch grains from a potato, seen under a microscope — each grain is built from many glucose units
Vocabulary
Train
| English | Chinese | Pinyin |
|---|---|---|
| starch | 淀粉 | diàn fěn |
| glycogen | 糖原 | táng yuán |
| cellulose | 纤维素 | xiān wéi sù |
| glucose | 葡萄糖 | pú táo táng |
| amino acids | 氨基酸 | ān jī suān |
| fatty acids | 脂肪酸 | zhī fáng suān |
| glycerol | 甘油 | gān yóu |
4.1
Food tests
You can test a piece of food to find out which substances it contains. Add the test chemical and watch for a colour change.
| Food substance | Test | Positive result |
|---|---|---|
| starch | add iodine solution 碘液 | orange-brown → blue-black |
| reducing sugars 还原糖 (such as glucose) | add Benedict's solution 本尼迪特试剂 and heat | blue → brick-red / orange |
| protein | add biuret 双缩脲 solution | blue → purple |
| fats and oils | ethanol 乙醇 emulsion 乳浊液 test | a cloudy white layer forms |
| vitamin 维生素 C | add to blue DCPIP | blue → colourless |
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.
Each food test gives its own colour change for a positive result
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.
Explore
Food test lab
Follow how the test result reveals the nutrient in a sample.
Vocabulary
Train
| English | Chinese | Pinyin |
|---|---|---|
| iodine solution | 碘液 | diǎn yè |
| reducing sugars | 还原糖 | huán yuán táng |
| Benedict's solution | 本尼迪特试剂 | běn ní dí tè shì jì |
| biuret | 双缩脲 | shuāng suō niào |
| ethanol | 乙醇 | yǐ chún |
| emulsion | 乳浊液 | rǔ zhuó yè |
| vitamin | 维生素 | wéi shēng sù |
4.1
The structure of DNA (Supplement)
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.
DNA is a double helix; the bases always pair A–T and C–G
Vocabulary
Train
| English | Chinese | Pinyin |
|---|---|---|
| genetic material | 遗传物质 | yí chuán wù zhì |
| strands | 链 | liàn |
| double helix | 双螺旋 | shuāng luó xuán |
| bases | 碱基 | jiǎn jī |
| bonds | 化学键 | huà xué jiàn |
4.1
Exam tips
- 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.