Plant organ system

• Transpiration: the loss of water vapour from a plant’s surface by evaporation and diffusion
• Translocation: the movement of food (dissolved sugars) and other substances through a plant’s phloem tissue
• Transpiration at a plant’s leaf

  1. Water evaporates from spongy mesophyll cells into the air spaces that surround the cells
  2. The water vapour then diffuses through open stomata into the air around the leaf
• Transpiration stream: the continuous movement of water into a plant through its roots, through its xylem tissue and out of its leaves
• How does the transpiration stream work?

  1. Water is lost at the leaves by transpiration
  2. To replace the lost water, water is drawn out of the xylem into the leaves
  3. Root hair cells draw in water from the soil by osmosis to replace water in the xylem
• Effect of changing light intensity on the rate of transpiration

  As light intensity increases, the rate of transpiration increases. This is because stomata open as light intensity rises, increasing the amount of water lost by transpiration.
• Effect of changing temperature on the rate of transpiration

  As temperature increases, the rate of transpiration increases. This is because water molecules have more kinetic energy at higher temperatures, so more water molecules evaporate and diffuse out of a plant in a given time compared to at lower temperatures
• Effect of changing humidity on the rate of transpiration

  As humidity decreases, the rate of transpiration increases. This is because at low humidity, the concentration gradient of water between a plant and the air around it is greater than at high humidity so the rate of diffusion is higher.
• Effect of changing air flow on the rate of transpiration

  As air flow increases, the rate of transpiration increases. This is because when more air flows around a plant, more water vapour is carried away from around the plant. This keeps a concentration gradient of water between a plant and the air around it, so the rate of diffusion is higher than if the air was still.
• Stomata: little openings in a plant’s epidermal tissue
• Plants have stomata so they can exchange gases with their environment for photosynthesis.
• Guard cells: cells that surround stomata in a plant’s epidermal tissue
• A plant’s guard cells open and close the stomata to control gas exchange and water loss from the plant
• • To open a plant’s stomata, guard cells fill with water and become turgid, which makes them bend apart.
  • To close a plant’s stomata guard cells lose water and become flaccid, which makes them come together