[Photo credit: Yvan Lindekens]
Photosynthesis is the process by which green plants manufacture their food, making use of carbon dioxide and water in the presence of sunlight.
It is carried out by chloroplast-containing plants in the presence of sunlight. It is a very vigorous process, especially in bright sunlight, thus actively photosynthesizing plants take in carbon dioxide and give out oxygen.
However, in the absence of photosynthesis, plants take in oxygen and give out carbon dioxide like animals. Besides the gases involved in these two processes, water vapor also escapes from plants during transpiration.
Cellular respiration is carried out by all plant cells all the time. Since plants are inactive, their energy requirements are low, so cellular respiration proceeds slowly.
Plants do not have special respiratory organs for gaseous exchange. However, gases move in and out through the:
- stomata
- lenticles
- root hairs
1. Stomata: Stomata (singular: stoma) are very thin openings found in the epidermis of leaves and stems of seedlings. Each stoma is surrounded by two bean-shaped cells called guard cells. Somata are more abundant in the lower epidermis of leaves and are very few in the upper epidermis. The closing and opening of the stomata are controlled by the guard cells.
2. Lenticles: Lenticles are breathing pores or tiny openings found in the bark of older stems. A lenticle consists of a loose mass of small thin-walled cells which permits easy diffusion of gases in and out of the plants.
3. Root hairs: Root hairs provide a large surface area for the absorption of water, mineral salts and oxygen. Oxygen present in the soil air dissolves in the soil moisture and diffuses into the root hairs. From here, it diffuses into the other root cells. The carbon dioxide produced by the root cells diffuses out of the root into the soil via the root hairs.
MECHANISM OF GASEOUS EXCHANGE IN PLANTS
The shoot system of flowering plants obtains oxygen from the atmosphere and gives out carbon dioxide and water vapor to the atmosphere through the stomata of the leaves and lenticles in the stem through the process of diffusion. Due to the differences in concentration gradient, oxygen is taken in through the stomata and lenticles especially during the night and carbon dioxide and water vapor are given out.
But during the day when photosynthesis is going on, oxygen and water vapor from photosynthesis diffuse out to the exterior through the stomata and lenticles. It will be recalled that the closing and opening of the stomata are controlled by the guard cells.
Turgidity of the guard cells quicken the opening of the stomata while flaccidity of the guard cells causes the closing of the guard cells. Gaseous exchange, especially through the stomata, is made possible when stomata open.
SUMMARY
- Gases enter and leave the plants through the stomata, lenticles and root hairs.
- Opening and closing of stomata regulate gaseous exchange, especially in leaves. This is brought about by changes in the solute concentration of guard cells, making them turgid (stoma is open) or flaccid (stoma is closed).
Differences Between Photosynthesis and Respiration
Respiration
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Photosynthesis
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It takes place in all living cells
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It takes place in plants only
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It takes place at all times
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It takes place in the presence of sunlight only
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Food substances are broken down
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Food substances are built up
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It releases energy
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It makes use of energy
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It results in a decrease in weight
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It results in increase in weight
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oxygen is used
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Oxygen is released
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Carbon dioxide and water are released
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Carbon dioxide and water are used
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