Transpiration

• Transpiration: The loss of water vapour from plant leaves by evaporation of water at the surfaces of the mesophyll cells followed by diffusion of water vapour through the stomata. Most transpiration happens in the leaves
• • Water moves by osmosis and diffusion
  • Minerals move by active transport
• Plants continually lose water to the air because there is more water inside the leaves than in the air outside, water vapour diffuses out of the leaf through stomata that are open for gas exchange. This happens faster in hot, dry, and windy conditions.
• Most plants have a waxy layer called the cuticle on their leaves to prevent too much water loss from the leaves. The waxy cuticle is much thicker in plants that live in drier areas to prevent water loss by transpiration.
• Water lost through transpiration's mainly lost through the stomata.
• Guard cells control whether stomata are opened or closed depending on the conditions the plant is in, directly affecting how much transpiration can occur
• If plants are losing water faster than they are gaining it, the stomata close. Without this happening, the plants will wilt - and they may die
• Factors Affecting Transpiration:

  • Humidity
  • Temperature
  • Wind Speed
  • Light Intensity
• The rate of transpiration from a leaf is affected by anything that changes the concentration gradient of water molecules between the leaf and the air
• Humidity: The concentration of water molecules in the air. High humidity air feels damp because of wind and high concentration of water molecules in the air. The air feels dry when humidity is low.
• Humidity:

  There is a high concentration of water molecules in the air spaces of leaves, so when there is high humidity, there is a low concentration gradient between the leaf and the air, so transpiration rate is low. In low humidity there is a high concentration gradient, so the rate of transpiration is higher.

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  High humidity: Low transpiration rate

  Low humidity: High transpiration rate
• Wind:

  Increasing wind speed leads to increased rate of transpiration. Windy conditions maintain concentration gradient between air spaces and the air outside. Good air flow removes water from air surrounding the leaf which sets up a concentration gradient between leaf and air, increasing water loss

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  Fast/High wind: High transpiration rate

  Slow/Low wind: Low transpiration rate
• Temperature:

  Increasing temperature leads to an increased rate of transpiration. Molecules have more kinetic energy so evaporate from mesophyll and diffuse from stomata faster.

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  High temperature = High transpiration rate

  Low temperature = Low transpiration rate
• Light Intensity:

  Increasing light intensity leads to an increased rate of transpiration. Guard cells are responsive to light intensity; increased light intensity makes guard cells turgid, opening the stomata and allowing water to be lost

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  High light intensity = High transpiration rate

  Low light intensity = Low transpiration rate
• Factor - Transpiration relationship: + Order of limiting factor (number 1 is first to limit the reaction and to plateau)

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  1. Light Intensity - Transpiration: Direct
  2. Temperature - Transpiration: Direct
  3. Wind - Transpiration: Direct
  4. Humidity - Transpiration: Indirect
• Adaptations to minimise water lose:

  • Thick cuticle
  • Stomatal control
  • Reduced surface area
  • Hairy or small leaves
  • Sunken stomata
• Thick Cuticle: The cuticle is a waxy, waterproof layer covering the outer surface of leaves. A thicker cuticle helps to reduce water loss by providing a barrier that prevents water molecules from escaping easily through the leaf surface
• Stomatal Control: Stomata are small openings on the leaf surface that allow for gas exchange, but also result in water loss. Plants can regulate the opening and closing of stomata. Some plants have specialized cells, like guard cells, that control stomatal opening and closing, minimizing water loss during hot or dry periods by closing stomata
• Reduced Surface Area: Plants in dry environments might have reduced leaf size or fewer leaves to minimize surface area available for transpiration. This helps limit water loss while still allowing for photosynthesis.
• Hairy or Small Leaves: Some plants have adaptations such as hairy or small, needle-like leaves. These can create a microenvironment that reduces air movement around the leaf surface, thereby decreasing the rate of transpiration
• Sunken Stomata: In certain plants, stomata are located in small depressions or pits on the leaf surface. This positioning helps to trap moisture and reduce air movement around the stomatal openings, thereby minimizing water loss
• Potometer: An instrument for measuring the rate at which a plant absorbs water.

  • Bubble potometer
  • Mass potometer
• Potometer Practical: How it Works:

  As the plant transpires, water is drawn up through the stem and leaves and lastly to the atmosphere. This causes the water level in the capillary tube to decrease. Regularly measure and record the change in the water at fixed time intervals for a specific duration. The air bubble's position will change depending on the rate of transpiration. The movement of the air bubble is measured. This practical works because you can assume that the water uptake by the plant is directly related to water loss by the leaves (transpiration)
• Shortage of water from transpiration means more water is drawn up from the roots, so there is a constant transpiration stream
• Transpiration stream: Water constantly travels up xylem from the roots into the leaves of the plant to replace the water that has been lost due to transpiration, creating a transpiration stream through the plant
• Transpiration is just a side-effect of the way leaves are adapted for photosynthesis. They have stomata in them so that gases can be exchanged easily. Because there's more water inside the plant than in the air outside, the water escapes from the leaves through the stomata by diffusion
• Potometer Practical: Measuring Transpiration pt1

  1. Cut shoot underwater to prevent air entering xylem and place in tube. Cut at a slant to increase surface area available for water uptake
  2. Set up the apparatus as shown in diagram, make sure it is airtight, using vaseline to seal any gaps
  3. Dry leaves of the shoot (wet leaves affect results)
  4. Remove capillary tube from beaker of water to allow a single air bubble to form and place tube back into water
  5. Set up environmental factor you are investigating
  6. Use reservoir to position bubble at the end of the scale if needed
• Potometer Practical: Measuring Transpiration pt2:

  
  

  7. Allow plant to adapt to new environment for 5 min

  8. Record starting location of air bubble

  9. Leave for set period of time

  10. Record end location of air bubble

  11. Change light intensity or wind speed or level of humidity or temperature (only one - whichever factor is being investigated)

  12. Reset bubble by opening tap below the reservoir

  13. Repeat experiment

  14. The further the bubble travels in same time period, the faster transpiration is occurring and vice versa

  15. Repeat with altered independent variable
• Potometer: A device used to measure the rate of transpiration from a plant.
• Potometer Practical Variables:

  • Control: Setup (apparatus)
  • Independent: Environment (ex: temperature, humidity, etc.)
  • Dependent: Time taken for air bubble to travel certain distance
• Mass potometer: Measures transpiration through the loss of mass, Moving-bubble potometer: Measures water uptake by the shoot
• Water uptake in a cut stem is likely to be greater than that in a rooted plant. Which is why a shoot is used in the experiment
• Investigation Transpiration:

  • Independent Variable: Depends on factor affecting transpiration tested
  • Dependent Variable: Distance moved by bubble in a set time
  • Control Variable: Remaining factors that weren’t tested (if light intensity was tested it would be independent variable (distance between potometer and light source), and humidity, temperature and wind speed would be control variables)
• Environmental factors can be investigated in the following ways:

  • Airflow: Set up a fan or hairdryer
  • Humidity: Spray water in a plastic bag and wrap around the plant
  • Light intensity: Change the distance of a light source from the plant
  • Temperature: Temperature of room (cold room or warm room)
• When designing an investigation to ensure a fair test you must keep all factors the same other than the one you are investigating. If investigation light, then humidity, temperature, and wind speed are control variables
• 1. Plants gain water from soil via their roots
  2. Water travels up the xylem to the leaves
  3. Water enters the cells of the leaf from the xylem.
  4. Water evaporates from the surface of mesophyll cells into the air spaces of the leaf. 
  5. The water diffuses out of the stomata in the form of water vapour
• Water is needed in leaves for:

  • Photosynthesis
  • Transport of mineral ions
  • Structural support
• Always consider effect on concentration gradient when figuring out transpiration rate
• Investigation Transpiration:

  • It is essential to prevent air bubbles from getting into a potometer. 
  • Prevent stomata from getting blocked by drying the leaves
• The role of stomata and guard cells (found mostly on the underside of the leaf) is to control gas exchange and water loss