Gas exchange in plants
- Created by: Emily Cartwright
- Created on: 28-05-14 15:57
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- Gas exchange in Plants
- Respiration: Glucose + Oxygen -> CO2 + H2O + ATP
- Photosynthsis: CO2 + H2O ->(Light) Glucose + Oxygen
- Plants require CO2 as well as light for photosynthesis. They have leaves which are adapted for both light absorption and gas exchange
- Plants rely entirely on diffusion for the exchange of gases
- Structure of an (angiosperm) leaf
- The cuticle is impermeable to water, so reduces water loss (but also limits gas exchange)
- The epidermis prevents mechanical damage
- The palisade mesophyll is the main site of photosynthesis
- Spongy mesophyll allows the circulation of gases and is the main site of gas exchange
- Vascular bundle transports water and mineral ions in the xylem/sucrose in the phloem
- The air spaces allows circulation of water and gas needed in exchange
- The stomata permit the entry and exit of gases (and allow water through)
- Guard cells open and close the stomata
- Movement of Gases
- Day
- The rate of photosynthesis greater than the rate of respiration so overall, more oxygen is released
- Night
- The rate of respiration is smaller than the rate of photosynthesis so overall, more carbon dioxide is released by the plant
- Day
- Adaptations of the leaf for light absorption
- Leaves are flat and have a large surface area to absorb as much light as possible
- Leaves can grow towards the light and expose a greater surface area
- Leaves are thin to allow light to penetrate the lower layers of cells
- Cuticle and epidermis are transparent to allow light to penetrate the the mesophyll tissue
- Palisade cells are elongated and to reduce the total number of cell walls that would absorb the light and prevent it from getting to the chloroplasts
- Palisade cells contain many chloroplasts. These can move around within the cells to gain the best position for light absorption
- Adaptations for gas exchange/CO2 absorption
- Leaves are thin to reduce the absorption distance
- The spongy mesophyll;
- Has a large surface area for gas exchange
- Contains air spaces to allow the circulation of gases and reduce the diffusion pathway of CO2 into cells
- Is moist for the absorption of CO2
- Stomatal pores allow the entry of gases into the air spaces
- Adaptations to reduce water loss from gas exchange surfaces
- Leaves have a waxy cuticle on the upper epidermis to reduce water loss by evaporation
- Most water is lost from the plant via the stomatal pores
- The stomatal pores are located on the lower epidermis which helps reduce water loss via evaporation
- Guard cells surrounding the stomatal pores can change shape
- They open the stomatal pores during the day to allow gas exchange and close them at night to reduce water loss
- Mechanism of stomatal opening
- The stomata are bordered by two guard cells
- Guard cells are unusual in having chloroplasts and unevenly thickened walls with the inner wall being thick and the outer wall thin
- These can change shape to open and close the stomata to control the amount of water and gases that move in or out of the cell
- Guard cells are unusual in having chloroplasts and unevenly thickened walls with the inner wall being thick and the outer wall thin
- In the light, stomata open because:
- Photosynthesis occurs, which results in increased production of ATP and a lower CO2 concentration in the guard cells
- ATP is used to actively transport potassium ions (K+) into the guard cells surrounding the epidermal cells
- The lower CO2 concentration triggers the conversion of insoluble starch to soluble malate
- The K+ and malate lower the water potential of the guard cells
- Water flows into the guard cells by osmosis down the water potential gradient
- The guard cell expands and becomes turgid
- The inner wall of the guard cell is thick and inelastic, so the pairs of cells curve away from each other
- The pore opens
- The inner wall of the guard cell is thick and inelastic, so the pairs of cells curve away from each other
- The guard cell expands and becomes turgid
- Water flows into the guard cells by osmosis down the water potential gradient
- The K+ and malate lower the water potential of the guard cells
- The lower CO2 concentration triggers the conversion of insoluble starch to soluble malate
- ATP is used to actively transport potassium ions (K+) into the guard cells surrounding the epidermal cells
- Photosynthesis occurs, which results in increased production of ATP and a lower CO2 concentration in the guard cells
- The stomata are bordered by two guard cells
- Xerophytes
- These are plants adapted to dry or desert habitats
- Open stomata at night instead of the day to conserve water
- For the same reason, other plants may also close their stomata during the day in conditions of drought
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