Introduction to Biology
Senior 1 · Biology
Introduction to Biology
Lesson 2 of 4
25 min
On a shelf in a farmer's store in Rulindo there is a sack of dry bean seeds from last season. Beside it lies a smooth stone from the bed of the Nyabarongo river, and a candle is burning on the table.
The candle flame moves, it "feeds" on wax, it takes in oxygen and gives out carbon dioxide, and it can even spread. The bean seeds have done nothing for six months. So is the flame alive and the seed dead? Biologists answer this with a checklist: a living thing shows all seven characteristics of life, not just one or two.
Part 1: growth. Put ten dry bean seeds on damp cotton wool in a dish. Boil ten more seeds for five minutes, cool them and put them in a second dish. Keep both damp and look at them every day for a week.
Part 2: respiration. Living cells release carbon dioxide. Limewater (calcium hydroxide solution) turns milky when carbon dioxide bubbles into it. Set up two jars as in Figure 1 and leave them for two days.
Figure 1: Testing whether bean seeds respire, after two days. The only difference between A and B is whether the seeds are alive, so B is the control.
Safety: wear eye protection when you pour limewater, because it irritates the eyes. Use seeds bought for eating, not seeds coated with pesticide, and wash your hands afterwards.
Typical results. Within three days most of the unboiled seeds swell and a small root breaks through the seed coat; a shoot follows. Their limewater turns milky. The boiled seeds do not change, and their limewater stays clear.
Conclusion. The dry seed was alive all along. It was dormant: its cells were respiring so slowly that nothing could be seen until it took in water.
Definition
Characteristics of living things (remember them as MRS GREN):
Figure 2: MRS GREN. Read the first letters clockwise from the top: M, R, S, G, R, E, N.
| Characteristic | In a goat | In a climbing bean plant |
|---|---|---|
| Movement | Walks to fresh grass | Its stem twines round a stake; leaves turn towards light |
| Respiration | Every cell releases energy from food, using oxygen | Every cell, even in the roots, respires day and night |
| Sensitivity | Runs away when a dog barks | The shoot grows towards light; roots grow down with gravity |
| Growth | A kid becomes an adult goat | A seedling becomes a plant more than 2 m tall |
| Reproduction | Gives birth to kids | Makes flowers, then pods full of seeds |
| Excretion | Urine; carbon dioxide breathed out | Oxygen made in photosynthesis; carbon dioxide; wastes stored in old leaves that fall |
| Nutrition | Eats plants: it cannot make its own food | Makes its own food by photosynthesis |
Plants do all seven too, only more slowly and less visibly than animals. Every living thing is also made of one or more cells, which you will study in Unit 5.
Warning
Respiration is not breathing. Breathing moves air in and out of the lungs. Respiration is a chemical process inside every cell. A bean seed and a banana leaf respire, but they do not breathe.
Excretion is not egestion. Passing faeces is egestion: most of the faeces is undigested food that was never part of the body's cells. Excretion removes wastes that cells made, such as carbon dioxide and urea.
Worked example 1: is a motorcycle alive?
A moto moves, takes in petrol, uses oxygen and gives out exhaust gases. Test it against every characteristic.
Conclusion: a motorcycle shows only movement, so it is non-living. One matching feature proves nothing; a living thing must show all seven.
Worked example 2: dead, or never alive?
Sort these into "living", "dead" and "never alive": a cow's horn, a stone, a dry bean seed, a piece of firewood.
Worked example 3: the germination test
Before planting, a cooperative in Rulindo tests its seed: 50 beans are put on damp cloth, and after seven days 42 have grown a root. What is the germination percentage?
Answer: 84% of the seed is alive and able to grow. The other 16% will leave gaps in the field unless the farmers plant a little extra seed.
Every planting season, farmers depend on seed being alive. A germination test like the one in Worked example 3 costs almost nothing and tells a cooperative whether stored seed is still worth planting. Seed dealers test certified seed in the same way before they sell it.
GCSE Biology: Characteristics of Living Things (Cognito). Watch for how each of the seven characteristics applies to plants as well as animals.
Tip
Memory hook: MRS GREN is an imaginary lady: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition. When a question asks "is it alive?", go through her name letter by letter.
Practice: characteristics of living things
Key points
Sources and credits
The investigation, Figures 1 and 2, the table, the worked examples and the practice questions are written by Ganzaa.
