Transpiration

• Photosynthesis requires carbon dioxide which diffuses into the leaf from external air via the stomata.
• As gas exchange must occur during photosynthesis, when a plant is photosynthesising, the stomata are open to allow CO2 in.
• The surface of the cells within a leaf are covered by a thin layer of water. Which can evapourat from the surface giving the internal spaces of the leaf a high concentration of water vapour.
• Generally, the water vapour concentration of the external air is relatively low, which therefore forms a concentration gradient of water vapour between the air and the leaf, causing water vapour to move out of the leaf.
• This evaporation of water from the interior cell surface followed by the diffusion of water vapour out of the leaf is called transpiration.
• Due to transpiration, the water potential of cells in the leaf decrease, causing water to move by osmosis from adjacent cells, lowering the water potential of these cells and repeating the cycle. At a certain point, this reaches the xylem and water moves from the xylem to adjacent cells.
• Therefore, when transpiration is occuring, water is always being pulled out the xylem vessels. Scientists call this pulling effect tension.
• The movement of water from the soil, up the roots, up the xylem and out of the leaf is called the transpiration stream.
• Water molecules form hydrogen bonds with one another and this attraction is called cohesion.
• Water can also form hydrogen bonds with other molecules inside of the walls of the xylem vessels, such as carbohydrates. This is called adhesion.
• Due to adhesion and cohesion, water can move up very thin tubes against the force of gravity in a process called capillary action.
• So when water is removed from the top of the xylem vessels in transportation, more water moves up the xylem by capillary action to take its place. This process is called transpiration pull.
• Combining the effect of transpiration pull as well as adhesion and cohesion, water is drawn in from the roots and up the plant where it exits through the stomata. This whole process is called cohesion-tension theory.
• One piece of evidence for cohesion-tension theory is that, when a plant stem is cut, air is sucked into the xylem suggesting that the xylem vessels are under tension. However air prevents cohesion between water molecules so water movement stops.
• If we measured the diameter of a tree trunk, we can see it reduces when transpiration is at its maximum, this supports the idea that transpiration pull is generating a negative pressure or tension in the xylem.
• Using a bubble potometer, we can measure the rate of water uptake into the plant.
• a bubble potometer consists of:

  • A fine capillary tube which is filled with water.
  • the tube connected to a plant which has been cut at the stem.
  • The tube also connected to a syringe filled with water.
  • Use a needle to place a small air bubble at the end of the capillary tube
• As water evaporates from leaves, water is drawn into the stem, causing the air bubble in a potometer to move towards the plant. Therefore, we can calculate how Far the bubble moves in a given time.
• By measuring how far the air bubble moves in a given time, we can calculate the rate of water uptake into the plant. We can then change the conditions and see how the rate of water uptake changes with these conditions.
• Some conditions which may affect the rate of transpiration may be, wind, light intensity, temperature, humidity.
• a bubble potometer only measures the rate at which water is taken up by the plant, and not all of this water will take part in transpiration. For example some water taken up may be used in metabolic reactions.
• When setting up a bubble potometer, cut the stem of the plant and connect it to the capillary tube UNDERWATER to ensure that water will flow into the xylem, avoiding any air gaps.
• If possible, when setting up a potometer, you should avoid getting water on the underside of the leaves as this can decrease the rate of transpiration.
• To use a mass potometer, placed your plant, in it's pot, on a balance. As the plant transpires it loses water and so its mass will decrease. This directly measures transpiration.
• When making a mass potometer we cover the soil with plastic wrap so no water evaporates from the soil, which would appear to be lost via transpiration.
• Using a mass potometer is much less disruptive to the plant as it does not involve cutting the stem.
• For transpiration to occur, the stomata must be open which occurs when photosynthesising which happens in light conditions.
• As we increase light intensity, rate of transpiration increases as with a greater light intensity, the number of stomata open increases, allowing more water vapour to diffuse out of the leaf.
• At a certain point, increasing the light intensity has no further effect on rate of transpiration as almost all stomata are already open.
• Water moves out of the leaf, diffusing down the concentration gradient of water vapour from the low concentration in the inner spaces of the leaf to the high concentration in the atmosphere.
• At high humidity, the concentration of water vapour in the outside environment increases, and this decreases the concentration gradient for water to move out of the leaf, thus decreasing the rate of transpiration.
• The rate of transpiration increases with increasing temperature.
• At higher temperatures, water molecules have greater kinetic energy which causes a greater rate of evaporation of water from the internal surfaces of the leaf, meaning there is a higher concentration of water vapour in the leaf, steepening the concentration gradient.
• At higher temperatures, the relative humidity of external air decreases. This means the concentration gradient for the movement of water vapour out the leaf steepens thus increasing rate of transpiration.
• At higher temperatures, the rate of enzyme activity involved in photosynthesis increases, meaning the reaction is happening at a greater rate and so more stomata must be open to allow CO2 in, allowing more water vapour to evaporate.
• When water moves out of the stomata during transpiration, the water vapour can build up around the external surface of the leaf, thus increases the relative humidity outside the leaf, decreasing the concentration gradient.
• Air movement such as wind, removes water vapour from the leaf's external surface and so this maintains the concentration gradient for water, increasing the rate of transpiration.
• The stomata are controlled by two guard cells, the shape of which control whether the stoma is open or closed.
• The cellulose cell walls of guard cells are thicker on the inner side compared to the rest of the cell.
• Some of the cellulose microfibrils of the cell walls are arranged in rings.