Photosynthesis

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1Summary 重點摘要

Photosynthesis runs in the chloroplast: the photochemical reaction in the thylakoids converts light energy into ATP and NADPH and releases oxygen, while carbon fixation — the Calvin cycle — builds glucose from carbon dioxide in the stroma; light intensity, carbon dioxide concentration and temperature all set the pace, and greenhouse growing exploits exactly this.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: What photosynthesis meansGreen plants are autotrophs: using light energy, they build glucose from two simple inorganic substances, carbon dioxide and water, releasing oxygen at the same time. As an equation: carbon dioxide plus water, in the presence of light and chlorophyll, produces glucose plus oxygen. The sugars made store chemical energy that feeds the plant itself and, through food chains, every animal. For example, put a water plant in a beaker of water on a sunny windowsill and a stream of bubbles soon appears at the cut stem; a glowing splint relights in the gas, proving it is oxygen.
2. Step 2: How the leaf is built for the jobThe leaf is photosynthesis's main organ and every feature serves the function. Broad, thin blades catch plenty of light and shorten diffusion paths. Beneath the upper epidermis, tightly packed palisade mesophyll holds the most chloroplasts, intercepting sunlight first; the looser spongy mesophyll leaves large air spaces for gas movement. Stomata in the lower epidermis admit carbon dioxide, and veins deliver water to every cell. For example, hold a leaf up to the light and you can see vein networks dividing the blade into patches — plumbing that reaches everywhere.
3. Step 3: Chloroplasts and thylakoidsThe chloroplast is the site of photosynthesis, a flat oval containing thylakoid membranes stacked into grana and bathed in a fluid stroma. Chlorophyll and other photosynthetic pigments are embedded in the thylakoid membranes, absorbing mainly red and blue light while reflecting green — hence the colour of leaves. For example, in autumn an abscission layer forms at the leaf stalk, cutting off the water supply; chlorophyll breaks down first, and the yellow and orange pigments once masked now show through — proof of how dominant chlorophyll is during the growing season.
4. Step 4: The photochemical reactionThe photochemical reaction happens on the thylakoid membranes and needs light. Absorbing light energy, chlorophyll splits water into hydrogen and oxygen — the photolysis of water. The hydrogen is handed to carriers to form NADPH while light energy also converts ADP into ATP, and the oxygen escapes through the stomata. This stage turns light energy into chemical energy parked in ATP and NADPH for the next stage. For example, cover half of a leaf with aluminium foil, then test with iodine solution hours later: the illuminated half turns blue-black while the covered half does not — light is essential.
5. Step 5: Carbon fixation — the Calvin cycleCarbon fixation, the Calvin cycle, runs in the stroma and does not directly need light. Carbon dioxide combines with a five-carbon compound — the carbon dioxide acceptor — to form a three-carbon compound, which the ATP and NADPH from the photochemical reaction then reduce to glucose; the five-carbon acceptor is regenerated, and the cycle keeps turning. Carbon fixation thus depends completely on the products of the light reaction, the two stages dividing the work. For example, think of carbon dioxide as the raw material and ATP and NADPH as the power and the workers: once the light reaction stops turning up, the production line soon halts.
6. Step 6: Essentials and rate-limiting factorsPhotosynthesis needs four essentials — light, carbon dioxide, water and chlorophyll; remove any one and it stops. The main factors affecting the rate are light intensity, carbon dioxide concentration and temperature. When light or carbon dioxide is scarce, raising it lifts the rate until some level is reached, after which another factor becomes the bottleneck; temperature acts through the enzymes of carbon fixation, and excessive heat denatures them. For example, at midday in summer the light is strong, but leaves lose so much water that their stomata close, carbon dioxide runs short and the rate actually falls.
7. Step 7: Greenhouse gains and product conversionGlucose is only the beginning: it can be converted into starch for storage in chloroplasts or storage organs, into cellulose for cell walls, combined with nitrogen to make amino acids and proteins, or turned into lipids. On the farm, greenhouses pipe in carbon dioxide, add supplementary light and keep a steady temperature — all aimed at the limiting factors. For example, the grains of rice and wheat are packed with starch that leaves exported as sugar and converted in the fruits, a harvest that feeds more than half the world's population.

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5Worked example Example

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8Recap Recap

• Photosynthesis builds glucose from carbon dioxide and water using light energy and releases oxygen — feeding almost all life on Earth.
• Leaf structure serves the process: broad thin blades, palisade cells packed with chloroplasts, spongy tissue air spaces, stomata for gas exchange and veins to deliver water.
• The photochemical reaction on thylakoid membranes splits water under absorbed light, producing NADPH and ATP and releasing oxygen; it requires light.
• Carbon fixation — the Calvin cycle in the stroma — combines carbon dioxide with a five-carbon acceptor and reduces it to glucose with ATP and NADPH, so it depends on the light reaction.
• Light intensity, carbon dioxide concentration and temperature limit the rate; greenhouse growing lifts each in turn, and the products become starch, cellulose and amino acids.