Plants and Photosynthesis
FoundationGreen plants use chlorophyll to turn carbon dioxide and water into nutrients and oxygen in sunlight — photosynthesis. Light, carbon dioxide, water and chlorophyll are all needed.
- Photosynthesis: carbon dioxide plus water, in light and with chlorophyll, becomes nutrients plus oxygen.
- The leaf is the factory: chlorophyll catches light, stomata exchange gases, veins carry water and food.
- Roots anchor the plant and absorb water; the stem supports and transports.
文體小知識：植物科學說明文解釋植物怎樣自製養分，用對照實驗一步一步驗證光合作用的條件。
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1Animals must eat to gain nutrition, but green plants can make their own. Using chlorophyll in the leaves to catch sunlight, plants combine carbon dioxide entering through the stomata with water taken up by the roots, making nutrients (starch) and releasing oxygen. This is photosynthesis: carbon dioxide plus water, in light and with chlorophyll, becomes nutrients plus oxygen.
2The leaf is the factory of photosynthesis: its broad, flat shape catches more sunlight; the cells inside hold plenty of chlorophyll; the stomata on the surface are doors for gases; and the veins carry water up from the roots and carry the nutrients away. Root, stem and leaf work together: roots anchor the plant and draw water, the stem supports and transports, and the leaf makes the food.
3How do scientists prove photosynthesis needs light? With a controlled experiment: keep a pot plant in the dark for a day or two to use up the starch stored in its leaves; cover half of one leaf with foil and set the plant in sunlight for hours; remove the leaf, dissolve the chlorophyll in hot alcohol, and drop on iodine — the sunlit half turns blue-black (starch present) while the covered half stays unchanged (no starch). This proves light is necessary for photosynthesis.
4Photosynthesis matters enormously to the Earth: it builds the plants' own food, feeds animals and gives out oxygen, and takes in carbon dioxide from the air. Forests and the algae of the sea are the planet's oxygen factories. Protecting plants and planting trees protect an Earth of fresh air and balanced nature.
1Summary 重點摘要
Green plants use chlorophyll to turn carbon dioxide and water into nutrients and oxygen in sunlight — photosynthesis. Light, carbon dioxide, water and chlorophyll are all needed.
2Vocabulary cards Vocabulary
The process by which green plants use light to make nutrients from carbon dioxide and water, releasing oxygen.
The green substance in leaves that absorbs sunlight.
Tiny pores on the leaf surface for gases to enter and leave.
A gas in the air, a raw material of photosynthesis.
The gas released by photosynthesis, needed by animals to breathe.
The food, such as starch, that plants make for growth.
An experiment changing one condition only, compared with an unchanged part.
The liquid used to test for starch, turning blue-black with it.
The pipelines in a leaf carrying water and nutrients.
3Step by step Steps
- Step 1: Learn the inputs and outputsPhotosynthesis is the process by which green plants use sunlight to make food. For example, the inputs are carbon dioxide (entering through the stomata on the leaf surface) and water (taken up by the roots); the outputs are nutrients (starch) and oxygen, powered by sunlight. Plants make their own food and also give animals oxygen and food — the very foundation of the natural world; without photosynthesis, neither people nor animals could easily survive.
- Step 2: Learn the parts of a leafThe leaf is the main place where photosynthesis happens, and it is cleverly built. For example, hold a leaf up to the light: the veins are its roads, carrying water and nutrients; the tiny pores on the surface are the stomata, letting air in and out; the green colour is where chlorophyll lies, catching the sunlight; and the broad, flat shape gathers more light. Each part does its own job and they work together perfectly — like a precise factory.
- Step 3: Darken the plant firstBefore a photosynthesis experiment, the plant must first be kept in the dark so the result is reliable. For example, keep the pot plant in a cupboard for a day or two so the leaves use up their stored starch, and only the newly made food will be measured. If old starch were still inside, we could not tell whether it was made during the experiment or was already there, and the conclusion would be unreliable. Darkening is the first step of success and must not be skipped.
- Step 4: Design the comparisonA scientific experiment needs a comparison before we can see clearly what each factor does. For example, cover the lower half of one leaf with foil so the same leaf has a lit half and a dark half, with light as the only variable. Since every other condition of the two halves is identical, any difference in the result can only come from the light — that makes the conclusion convincing. Changing only one condition at a time is an important principle of the scientific method.
- Step 5: Test for starchStarch turns blue-black when iodine is added, which is the usual way to test for it. For example, soften the leaf in hot water, dissolve the chlorophyll in hot alcohol (handled by adults — alcohol burns easily), and the leaf turns pale white; then drop on iodine and watch the colour change. Remember, the alcohol must be heated in a water bath, never directly over a flame; take care and stay safe during the experiment, with an adult present if possible.
- Step 6: Read the resultsAfter the experiment, compare the colour changes of the two halves of the leaf to draw a conclusion. For example, the lit half turns blue-black with iodine, showing starch; the dark half stays unchanged, showing none. Light is therefore necessary for photosynthesis; without light, a plant cannot make food. This also explains why plants grow towards the sunlight, and why a plant left long in the dark turns yellow and weak.
- Step 7: Link to life and the environmentPhotosynthesis is closely linked to our life and the environment. For example, the big trees in the park make oxygen and take in carbon dioxide every day, keeping our air fresh; growing green plants at home improves the air indoors too. Nearly all living things on Earth depend on the food and oxygen that photosynthesis provides, so caring for plants and planting more trees protects the very source of life — a concrete way of protecting the Earth.
4Diagrams Diagrams
5Worked example Example
Example: Ming keeps a pot plant in the dark for two days, then covers half of one leaf with foil and puts it in sunlight for four hours. After removing the chlorophyll he drops on iodine: the lit half turns blue-black, the covered half stays unchanged. Explain the reason for each step and the conclusion.
- Two days in the dark: to exhaust the stored starch, so old starch cannot spoil the result.
- Foil over half the leaf: to create lit and covered halves of the same leaf for comparison, with light as the only variable.
- Four hours in sunlight: to give the lit half ample time to photosynthesise and make starch.
- Reading the result: iodine turning blue-black on the lit half proves starch was made; the unchanged covered half proves none was made.
- Answer: The experiment proves light is necessary for photosynthesis: on the same leaf the lit half made starch and the covered half made none, and light was the only difference between them, so the conclusion is very reliable.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the raw materials and the products of photosynthesis?
Q2What is the green substance in leaves that absorbs sunlight?
Q3What colour does iodine turn with starch?
Q4Why cover half a leaf with foil in the experiment?
Q5Why does photosynthesis matter to animals?
8Recap Recap
- Photosynthesis: carbon dioxide plus water, in light and with chlorophyll, becomes nutrients plus oxygen.
- The leaf is the factory: chlorophyll catches light, stomata exchange gases, veins carry water and food.
- Roots anchor the plant and absorb water; the stem supports and transports.
- A controlled experiment changes one condition only; light is proven necessary for photosynthesis.
- Photosynthesis feeds animals, gives oxygen and removes carbon dioxide — protecting plants protects the Earth.