9. plant biology

• Whilst water can travel across a membrane in the specific channel (referred to as an aquaporin), it does not carry ions with it via that channel because the aquaporin is only just wide enough for one water molecule at a time. Instead, separate transporter proteins or 'protein pumps' are adapted to carry individual ions across the membrane, by active transport. 
• Transpiration is the loss of water vapour from the leaves/stomata (and stems) of plants
• A potometer does not measure the true rate of transpiration in a plant because:

  • water is used by plant for support/turgidity
  • water is used for hydrolysis reactions
  • water is used during photosynthesis
  • water is produced in condensation reactions
  • water is produced during respiration
  • The potometer only measures transpiration in one shoot/section of the plant OR transpiration rates may vary in other parts of the plant
  • Leaking apparatus OR apparatus not sealed properly
• Plants with adaptations to conserve water, such as marram grasses or cacti, are described as xerophytic, or known as xerophytes.
• Marram grass leaves are well adapted to minimise water loss in the following ways:

  • Leaves are rolled up to reduce the exposure of leaf surfaces to the wind and so reduce water loss by evaporation
  • The stomata are sunken in pits to reduce water loss by evaporation
  • The inner surface of the leaf possesses (a large number of) hairs which shield the stomata and trap moist air, again reducing water loss by evaporation
  • The exposed surface has a thick waxy cuticle to reduce evaporation
• A plant replaces the water it loses via transpiration by the following mechanisms:

  • Transpiration/evaporation of water (at leaves) creates suction/tension
  • This causes water to be sucked/pulled out of the xylem (in leaves)
  • Water moves up xylem (in stem) due to suction/tension/pulling forces
  • (Pulling forces transmitted by) cohesion/hydrogen bonding between water molecules
  • This moves water from roots to leaves
  • Water enters root (cell) via osmosis due to higher solute concentration inside root (cell)
• A halophyte is a plant that is adapted to saline/salty conditions/soils OR a plant that lives in salt marshes/areas periodically flooded with sea water
• Adaptations of halophytes may include:

  • The ability to sequester/store away salts within their cell walls/vacuoles
  • The ability to concentrate absorbed salts in certain leaves, which then fall off the plant
  • Stems that can take over the role of photosynthesis when leaves are shed
  • Reduced leaf surface area to reduce water loss OR sunken stomata to reduce water loss
  • Salt glands that actively excrete salt to stop it from building up
  • Deep roots to reach fresh water underground
• Minerals are absorbed into the plant root by:

  1. minerals can move into root cells by facilitated diffusion
  2. from an area of high concentration to low concentration
  3. root hair cell has large surface area for absorption
  4. minerals can also move into root cells via active transport
  5. moving against the concentration gradient, requires ATP
  6. through protein pumps/carrier proteins
  7. proton pumps move H+ out of root cells, allowing mineral ions to enter
• Pressure changes in the xylem occurs during transpiration:

  • Pressure increases as water moves into the xylem at the root
  • Increase in active transport of mineral ions increases osmosis into the xylem and increases pressure
  • Pressure decreases / negative pressure occurs at the leaves as water evaporates from the spongy mesophyll cells
  • Increased transpiration results in a more negative pressure
  • (Caused by) conditions such as increased wind movement, increased temperature, increased sunlight or decreased humidity
•  The route of water movement can be described as follows:

  • Osmosis into the root from the soil; from a low to a high osmolarity
  • Apoplast route/pathway through the cell walls of the cells in the cortex
  • Symplast route/pathway through the cytoplasm of the cells in the cortex
  • Casparian strip is a water proof layer, so water diverts out of the cell walls into the cytoplasm / out of the apoplastic route into the symplastic route
  • Water moves into the xylem by osmosis
• More air movement leads to increased rates of transpiration

  • When the air is relatively still, water molecules can accumulate just outside the stomata, creating a local area of high humidity
  • Less water vapour will diffuse out into the air due to the reduced concentration gradient
  • Air currents, or wind, can carry water molecules away from the leaf surface, increasing the concentration gradient and causing more water vapour to diffuse out
• • Higher temperatures lead to higher rates of transpiration, up to a point at which transpiration rates will slow 
  • An increase in temperature results in an increase in the kinetic energy of molecules
  • This increases the rate of transpiration as water molecules evaporate out of the leaf at a faster rate
  • If the temperature gets too high the stomata close to prevent excess water loss
  • This dramatically reduces the rate of transpiration
• Higher light intensities will increase the rate of transpiration up to a point at which transpiration rates will level off 

  • Stomata close in the dark and their closure greatly reduces the rate of transpiration
  • Stomata open when it is light to enable gas exchange for photosynthesis; this increases the rate of transpiration
  • Once the stomata are all open any increase in light intensity has no effect on the rate of transpiration
• • Higher humidity levels reduce the rate of transpiration 
  • If the humidity is high that means the air surrounding the leaf surface is saturated with water vapour
  • This causes the rate of transpiration to decrease as there is no concentration gradient between the inside of the leaf and the outside
  • At a certain level of humidity, an equilibrium is reached; water vapour levels inside and outside the leaf are the same, so there is no net loss of water vapour from the leaves
• Xylem is closer to the centre of the stem, phloem is further
• The amount of water lost from the leaves (transpiration rate) is regulated by the opening and closing of stomata

  • Guard cells flank the stomata and can occlude the opening by becoming increasingly flaccid in response to cellular signals
  • When a plant begins to wilt from water stress, dehydrated mesophyll cells release the plant hormone abscisic acid (ABA)
  • Abscisic acid triggers the efflux of potassium from guard cells, decreasing water pressure within the cells (lose turgor)
  • A loss of turgor makes the stomatal pore close, as the guard cells become flaccid and block the opening
• The rate of phloem transport will principally be determined by the concentration of dissolved sugars in the phloem.

  The concentration of dissolved sugars will be affected by:

  • The rate of photosynthesis (which is affected by light intensity, CO2 concentration, temperature, etc.)
  • The rate of cellular respiration (this may be affected by any factor which physically stresses the plant)
  • The rate of transpiration (this will potentially determine how much water enters the phloem)
  • The diameter of the sieve tubes (will affect the hydrostatic pressure and may differ between plant species)
• Measuring translocation rates

  • A plant is grown with radioactively-labelled carbon dioxide
  • The leaves will convert the CO2 into radioactively-labelled sugars, which are transported by the phloem
  • Aphids are positioned along the plant’s length and encouraged to feed on the phloem sap
  • Aphid stylet is severed and sap continues to flow from the plant at the selected positions
  • The sap is then analysed for the presence of radioactively-labelled sugars
  • Translocation rate can be calculated based on the time taken for the radioisotope to be detected at different positions