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Photosynthesis
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Plants make their own food using light. Almost every living thing on Earth depends on this.
The reaction
In photosynthesis, plants use light energy to make glucose (a sugar) from carbon dioxide and water. Oxygen is given off.
carbon dioxide + water → glucose + oxygen (light energy, absorbed by chlorophyll)
It happens in chloroplasts, which contain the green pigment chlorophyll.
Plants use glucose for respiration, or turn it into starch to store and into cellulose to build cell walls. Iodine solution turns blue-black when starch is present.
carbon dioxide + water → glucose + oxygen (light energy, absorbed by chlorophyll)
It happens in chloroplasts, which contain the green pigment chlorophyll.
Plants use glucose for respiration, or turn it into starch to store and into cellulose to build cell walls. Iodine solution turns blue-black when starch is present.
Leaves are adapted
Broad and flat: a large surface area to absorb light.
Thin: gases do not have far to diffuse.
Lots of chloroplasts near the upper surface (in the palisade cells), where light is brightest.
Stomata on the underside let carbon dioxide in and oxygen out.
Veins bring water and carry sugars away.
Roots take in water and mineral nutrients from the soil through root hair cells, which have a large surface area. Minerals such as nitrates are needed for healthy growth.
Thin: gases do not have far to diffuse.
Lots of chloroplasts near the upper surface (in the palisade cells), where light is brightest.
Stomata on the underside let carbon dioxide in and oxygen out.
Veins bring water and carry sugars away.
Roots take in water and mineral nutrients from the soil through root hair cells, which have a large surface area. Minerals such as nitrates are needed for healthy growth.
Why it matters
Photosynthesis makes the food at the start of almost every food chain, so almost all life depends on it.
It also puts oxygen into the atmosphere and takes carbon dioxide out, helping to keep the levels of these gases in the air steady.
It also puts oxygen into the atmosphere and takes carbon dioxide out, helping to keep the levels of these gases in the air steady.
A leaf is kept in the dark for two days, then tested with iodine. It stays orange-brown. Explain why.
- Without light, the plant cannot photosynthesise.
- It uses up its stored starch in respiration.
- No starch means iodine stays orange-brown.
Answer: No light means no photosynthesis, so there is no starch to turn the iodine blue-black.
Carbon dioxide + water → glucose + oxygen, using light absorbed by chlorophyll in chloroplasts. Leaves are broad, thin, full of chloroplasts and have stomata. Roots absorb water and minerals. Almost all life depends on photosynthesis.
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