Transpiration

• What is the turgor pressure in a typical leaf cell?

  Around 1.5 MPa
• What are some reasons that water is so important for plants?

  1. Its evaporation helps keep them cool

  2. Raw material for photosynthesis

  3. Mineral ions and products of photosynthesis are transported in aqueous solution

  4. Turgor drives cell expansion

  5. Turgor (a.k.a. hydrostatic) pressure in plants helps provide a hydrostatic 'skeleton' to keep the plant rigid
• Why are mineral ions actively moved into the cytoplasm of root hair cells?

  To keep the concentration of them higher than in the soil so that water moves in down a concentration gradient
• What are the two pathways that water can move along the root in?

  Apoplastic and symplastic
• Where does water move along in the symplastic pathway?

  Along the interconnected (via plasmodesmata) cytoplasm of root cells (passing around the vacuole)
• How does water move along the symplastic pathway?

  The cells on the very edge and end of the root have high water potentials due to the absorption of water from the soil. This water potential is higher than the water potential of the next cell along, so water moves along the root by osmosis this way.
• Where does water move along in the apoplastic pathway?

  Along the cell walls and intercellular spaces
• How does water move along the apoplastic pathway?

  It fills the open spaces between fibres in the cellulose cell wall. As water moves into the xylem, the cohesive forces between it and water further back in the cell walls creates tension that causes water molecules to be pulled along, unencumbered by the loose network of cellulose fibres in the cell walls.
• What is the Casparian Strip made of?

  A waxy material called suberin
• Why is water forced from the apoplastic to the symplastic pathway?

  Because water travelling along the apoplastic can carry unwanted contaminants from the soil with it, and forcing it into the symplastic pathway allows the cell surface membrane to remove these contaminants
• What is the Casparian strip and what is its purpose?

  A waxy, impermeable layer in the cell wall of the endodermis, whose purpose is to force water from the apoplastic pathway to the symplastic pathway
• What is the endodermis?

  The layer of cells surrounding the vascular tissue of the roots
• How is water moved from the endodermis to the xylem?

  Mineral ions are actively pumped into the xylem, which creates a concentration gradient which causes water to move into the xylem via osmosis
• What causes root pressure?

  The active pumping of mineral ions into the xylem
• What does root pressure do?

  Gives water a 'push' up the xylem, although this is not usually the major factor in moving water up to the leaves
• What are 4 pieces of evidence for the role of active transport in creating root pressure?

  1. Xylem sap can exude from cut ends of sap at certain times- links to guttation

  2. If levels of oxygen or respiratory substrates fall, root pressure falls

  3. Cyanide prevents ATP production. If it is applied to root cells, root pressure disappears

  4. Root pressure increases with a rise in temperature and falls with a drop, suggesting that chemical reactions occur
• What is guttation?


  Where xylem sap is forced out of pores at the end of the leaves in some conditions, such as at night when transpiration is low.
• Where is water lost from leaf cells to?

  The air spaces in the spongy mesophyll
• What is transpiration?


  The loss of water vapour from the stems and leaves of plants via stomata
• Why do some stomata need to be open at night?

  To allow oxygen in for cellular reactions such as aerobic respiration, as none is being produced by photosynthesis
• What is the transpiration stream?

  The movement of water from the moment it is absorbed in the roots of the leaf until it leaves as water vapour from stomata in the leaves
• What is the cohesion-tension theory?

  The model of water moving as a continuous stream from the roots all the way up to and across the leaves
• According to the cohesion-tension theory, how does water move inside the leaf?

  Water evaporates from cells in the leaves, causing their water potential to fall. This causes water to move in from an adjacent cell. This repeats all the way back to the xylem, where water moves into the first leaf cell by osmosis
• According to the cohesion-tension theory, how does water move up the xylem?

  Water molecules form hydrogen bonds with the carbohydrates in the walls of the xylem (known as adhesion), as well as with each other (known as cohesion), the combined effects of which cause water to move up the xylem via capillary action
• What is capillary action?

  Where water can be drawn up a narrow tube against gravity
• What is the transpiration pull?


  Where water is drawn up the xylem in a continuous stream to replace that lost in the leaves via evaporation
• According to the cohesion-tension theory, how does water move along the roots?

  The transpiration pull results in a tension in the xylem, helping move move water across the roots.
• What are some pieces of evidence to support the cohesion-tension theory?

  1. When a xylem vessel is broken, in most cases air is drawn in rather than water leaking out

  2. In the case above, water can no longer be drawn up the xylem vessel, as the continuous stream of water molecules is broken

  3. Changes in the diameter of trees; when transpiration is at its highest in the day, the tension in the xylem is the highest, so the diameter of the tree shrinks. The opposite occurs at night when transpiration is at its lowest.
• What is a common piece of equipment used to indirectly measure transpiration in plants, and what does it measure instead?

  A potometer, which measures water uptake instead
• How do guard cells close the stomata?

  When turgor is low due to low water content, the asymmetrically thick inner cell walls of the guard cells close the stomatal aperture
• How do plants cause guard cells to close the stomata when water levels are low?

  Hormonal signals from the plant can trigger turgor loss from the guard cells, which closes the stomatal pore
• How do guard cells open the stomatal aperture?

  They pump in solutes by active transport, which increases their turgor. Because of cellulose hoops the cells cannot expand in width so they do so in length- because the inner wall of the cells are thicker and less flexible, the cells change shape asymmetrically and open the pore
• What are 5 factors which affect the rate of transpiration?

  1. Temperature

  2. Humidity

  3. Wind speed

  4. Availability of water in the soil

  5. Light intensity
• How does temperature affect the rate of transpiration?

  1. By increasing the kinetic energy of molecules, speeding up osmosis, diffusion and evaporation

  2. It increases the concentration of water vapour that external air can hold before it becomes saturated
• How does humidity affect the rate of transpiration?

  More humid air has a higher concentration of water molecules, so there is a lower concentration gradient and the air accepts less molecules through evaporation before becoming saturated
• How does wind speed affect the rate of transpiration?

  Increasing wind speed blows more water vapour away from the stomata, so there is a higher concentration gradient and more water will leave the leaf
• How does soil water availability affect the rate of transpiration?

  If there is not enough water then the plant will be under water stress and will lose less water through transpiration- the opposite is true if it is very wet
• How does light intensity affect the rate of transpiration?

  In higher light intensities more photosynthesis will take place, so more gaseous exchange needs to take place through the stomata and more water will be lost through them