10.8-10.10

• Hormones that help control water reabsorption

  • Antidiuretic hormone (ADH)
  • Others
• Antidiuretic hormone (ADH)


  The most important hormone for controlling the body’s reabsorption of water
• When an animal in a hot dry environment has lost a lot of water


  The amount of intercellular and extracellular fluid decreases
• Low intercellular and extracellular fluid

  Causes low blood pressure
• This is detected by the osmoreceptor cells in the hypothalamus of the brain
• When the water level of blood is low


  It stimulates the pituitary to release ADH
• ADH action


  1. Released ADH carried by blood

  2. Increases permeability of distal convoluted tubules

  3. Increases permeability of collecting ducts

  4. Retains water until normal balance is attained
• If fluid bathing osmoreceptor cells has higher-than-normal water content

  Less ADH will be released
• Less ADH released


  Decreases permeability of distal convoluted tubules and collecting ducts to water
• Decreased permeability


  Causes less water to be reabsorbed and more dilute urine to be excreted
• The release of extra water


  Removes the initial stimulus of higher-than-normal water levels
• Factors affecting transpiration rate

  • Light
  • Temperature
  • Humidity
  • Wind
  • Soil water
• Light


  Stomata usually open in the light and close in the dark
• Increase in temperature


  Increases the rate of evaporation and therefore increases transpiration
• Increase in humidity


  Causes a decrease in transpiration
• Decrease in transpiration is due to a decreased diffusion gradient between the cells and the atmosphere, which reduces evaporation
• Wind increases transpiration


  By the removal of water vapour around the stomatal pore
• Water vapour removal


  Maintains concentration gradient, allowing water to continue evaporating
• If the supply of soil water is reduced


  Less water is taken up by the roots and the transpiration rate falls
• Stomata


  Facilitate gas exchange
• Guard cells


  Border stomata and regulate the size of the opening
• The air spaces inside the plant cells are saturated with water vapor
• For CO2 to be taken in

  Water vapor must be released
• When guard cells are turgid


  The pore opens
• When guard cells lose water

  The pore closes
• Contractile vacuoles


  Regulatory organelles of spherical shape found in freshwater protozoa and lower metazoans
• Contractile vacuole's job


  To pump water out of the cell through osmoregulation
• Diastole


  When the contractile vacuole expands and takes in large amounts of water
• Systole


  When the vacuole contracts, carrying water and wastes outside of the cell
• Cuticle


  The outermost layer of land plants interacting with the environment
• Cuticle properties


  Extracellular and hydrophobic
• Cuticle forms a physical barrier to protect plants from desiccation and diverse stresses
• Homeostatic mechanisms


  Used by both animals and plants to keep a stable, consistent, and optimal level to fit their environment
• When an environment is at stress

  ABA is released in plants
• Abscisic acid (ABA)


  A sesquiterpene with important roles in seed development and maturation
• ABA controls


  The response of organ size and stomata closure
• Adaptations of xerophytes


  • Survive with little liquid water
  • Found in hot and dry habitats
  • Tissues capable of extreme heat/dry conditions
  • Protoplasm can be almost dried out and still function
  • Produce ABA to control stomata
  • Small life cycle completed after a wet season
• Halophytes


  A plant that grows in water of high cell salinity
• Halophytes can suffer water loss due to the hypertonic environment
• Halophyte adaptations


  Root cells containing high levels of salt<|>Epidermis of root hairs excludes salt<|>Salt excluders block uptake of sodium ions<|>Salt excretors secrete salt from leaves