Biology (UP) - Limiting Water Loss in Organisms

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Overview of water loss in plants

  • Plants cannot have a small SA:V ratio to limit water loss because photosynthesis requires a large SA
  • Terrestrial plants have waterproof coverings and the ability to close their stomata
  • But plants have a restricted water supply. These plants are xerophytes. They have evolved to limit water loss through transpiration to stop them dying. They reduce the rate at which water is lost via evaporation.
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How do xerophytes limit water loss through evapora

  • Thick cuticles: Having a waxy cuticle is useful, but 10% of water is still lost. Having a thick cuticle means that less water is able to escape e.g. seen in holly
  • Rolled up leaves: Most leaves have stomata on the lower epidermis. Rolling protects them, and traps air within the roll. This area becomes saturated with water and how has high water potenion. There is no water potenial gradient between in/outside of lead, so no water is losts e.g. marram grass
  • Hairy leaves: Especially on lower epidermis, this traps still and moist air next to the surface. The water potential gradient decreases, therefore, less water is lost by evaporation e.g. type of heather plant
  • Stomata in pits/grooves: Traps still and moist air next to the lead, decreasing the water potential gradient and meaning less water is lost by evaporation e.g. pine leaves
  • Reduced SA:V ratio of leaves: The smaller the ratio, the slower the rate of diffusion. Small leaves that are roughly circular in cross section (like pine needles), rather than those that are broad and flat, means the rate of water loss is decreased. The decrease in SA balances against the need for a sufficient area for photosynthesis to meet plant requirements. (Caution - pointy spikes is to stop predators, and has nothing to do with aiding transpiration)
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How does water move through plants?

  • At the same time as water enters the plant, water leaves through evaporation in the leaves.
  • The water is absorbed by root hairs via osmosis. The xylem tubes bring the water upwards.
  • At the same time, water is lost by transpiration in the leaves. This creates a suction pressure to move more water up through the xylem against gravity.
  • Transpiration = the release of water vapour (not evaporation!) from a plant through the leaves, causing water to move up plant.
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What factors increase transpiration rate?

Atmospheric conditions affect the rate of transpiration

  • Wind: Moves water away, which increases conc. gradient between leaf interior and air outside. This stops the leaf becoming saturated, and increases diffusion and evaporation of water. No wind makes the leaves saturated, decreasing diffusion and transpiration.
  • High temperature: This gives the water vapour molecules more kinetic energy, so they move faster, increasing the rate of diffusion and therefore the rate of transpiration. Low temperatures gives a slower diffusion rate and transpiration rate.
  • Low humidity: Dry air increases concentration gradient, so more water vapour is moving out, speeding up diffusion and transpiration. High humitidies mean water can't evaporate, decreasing diffusion and evaporation.
  • High light intensity: Stomata open increases transpiration for gas exchange. Closing them means water can't exit and gases can't enter, so there is no diffusion -> no transpiration or gas exchange.

Water movement and transpiration is linked to gas exchange. Transpiration needs to happen to open the stomata for gas exchange. Vice versa, opening the stomata will increase transpiration.

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How do insects reduce water loss?

  • They have waterprood coverings all over their body. It is made of layers of chitin, which forms the barrier on their surface. This means that they cannot use their SA for gas exchange however
  • They have a small SA:V ratio to minimise area over which water is lost. They have fat bodies, giving them a low ratio. This means there is a smaller SA for evaporation to happen. However, this is not efficient for gas exchange.
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