MASS TRANSPORT IN PLANTS

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  • MASS TRANSPORT IN PLANTS
    • Xylem and Phloem are mass transport systems
    • xylem and phloem tissue and other tissues, in vascular bundles. The location of the vascular bundles is dependent on which organ they’re in as the different organs are under different stresses
  • XYLEM: a mass transport system - water and mineral ions in solution from roots to the leaves by transpiration
  • PHLOEM: mass transport systems - organic substances eg. Sugars in solution up and down the plant by translocation
  • VASCULAR BUNDLES
    Xylem and phloem tissue and other tissues, in vascular bundles. The location of the vascular bundles is dependent on which organ they’re in as the different organs are under different stresses:
    1. In roots
    2. in stems
    3. in leaves
  • 1, VASCULAR BUNDLES - ROOTS
    • in the roots the vascular bundle is found in the centre and the centre core of this is xylem tissue.
    • this helps the roots withstand the pulling strains they’re subjected to as the plant transports water upwards and grows.
    • phloem tissue is on the edges of the centre core.
  • 2. VASCULAR BUNDLE - STEMS
    • in the stem the vascular bundles are located around the outside and the xylem tissue is on the inside (closest to the centre of the stem) to help support the plant and the phloem tissue is found on the outside (closest to the epidermis)
  • 3. VASCULAR BUNDLES - LEAVES
    • In the leaves the vascular bundles form the midrib and veins and therefore spread from the centre of the leaf in a parallel line.
    • the xylem tissue is found on the upper side of the bundles (closest to the upper epidermis) the phloem tissue is found on the lower side of the bundles (closest to the lower epidermis)
  • XYLEM STRUCTURE
    Xylem vessels are very long, tube-like structures formed from dead cells and are lined with lignin.
  • XYLEM STRUCTURE ADAPTATIONS
    1. ELONGATED CELLS - arranged end to end (no end walls) - continuous, uninterrupted column
    2. LIGNIFIED CELL WALLS - adds strength to withstand hydrostatic pressure so they don’t collapse
    3. RIGID - less likely to collapse under low pressure
    4. HOLLOW DUE TO LIGNIFICATION - no cytoplasm / nucleus to slow water flow
    5. WATERPROOF - preventing water loss
    6. PITS - allow lateral water movements
    7. NARROW LUMEN - assists capillary action and prevents water column from breaking
  • MOVEMENT OF WATER
    ROOT PRESSURE
    • water in roots pushes water up the xylem
    -but not all the way up, so only in small plants
    CAPILLARY ACTION
    • water has a tendency to move up into small tubes
    -but this is too slow so plants require another way of transporting water
    COHESION-TENSION AND EVAPORATION
    • this is the method which plants use to transport water from the roots and out of the leaves by transpiration
  • FLOW OF WATER FROM SOIL TO ROOTS
    OSMOSIS
    1. Water potential is higher within the soil than the root hair cell
    2. this is because of the dissolved substances in the cell sap
    3. water is taken up by the roots of a plant and through the endodermis, before being moved into xylem tissue which is in the centre of the root
    • root hair cells function to increase the surface area in order for water to be absorbed and for nutrients to be pumped across against the concentration gradient by active transport
  • FLOW OF WATER FROM ROOTS TO XYLEM: 1
    APOPLASTIC
    • water + dissolved mineral ions + salts move from root hair cells to xylem by travelling from cell to cell directly through cell walls + intercellular spaces
    • water moves through cell walls through spaces between cellulose molecules down water potential gradient
    • when water + mineral ions are near xylem, the casparian strip forms an impenetrable barrier to water in cell walls
    • means that it must move into cytoplasm to get to xylem cortex, now moving by symplastic flow
  • FLOW OF WATER FROM ROOTS TO XYLEM: 2
    APOPLASTIC
    • this give the plant control over ions that enter its xylem vessels since water must cross plasma membrane to get there
  • FLOW OF WATER FROM ROOTS TO XYLEM
    SYMPLASTIC
    • water moves from root hair cells to xylem by travelling from cell to cell through cytoplasm
    • water + minerals are immediately filtered bc they cross the cell surface membrane of the root hair cell which is partially permeable
    • water moves through plasmodesmata
    • as water + minerals have already been filtered, they bypass the casparian strip + move straight into the xylem
    • the water moves down the water potential gradient between cells
    • as soon as water reaches xylem its rapidly transported away, meaning water potential gradient is maintained
  • PLASMODESMATA: small channels in cell walls that connect neighbouring cells
  • TRANSPIRATION: the evaporation of water from a plants surface, especially the leaves
  • TRANSPIRATION AND MOVEMENT ACROSS LEAF: 1
    • water evaporates from the moist cell walls and accumulates in the space between cells in the leaf
    • the humidity of the atmosphere is usually less than that of the air spaces next to the stomata
    • as a result there is a water potential gradient from the spaces through the stomata to the air
    • provided the stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air
  • TRANSPIRATION AND MOVEMENT ACROSS LEAF: 2
    • water is lost by diffusion from air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells. This water reaching the mesophyll cells is from the xylem
    -mesophyll cells lose water to air spaces by evaporation due to heat supplied by the sun
    -these cells now have a lower water potential + so water enters by osmosis from neighbouring cells
    -the loss of water from these neighbouring cells lowers their water potential
    -they, in turn, take in water from their neighbours by osmosis
  • TRANSPIRATION AND MOVEMENT ACROSS LEAF: 3
    • in this way, a water potential gradient is established that pulls water from the xylem, across the leaf mesophyll, and finally out into the atmosphere
  • TRANSPORTATION AND ENERGY
    The transpiration pull is a passive process + therefore doesn’t require metabolic energy to take place
    1. The xylem vessels through which the water passes are dead + so can’t actively move the water
    2. xylem vessels have no end walls which means that xylem forms a series of continuous, unbroken tubes from root to leaves, which is essential to the cohesion-tension theory of water flow up the stem
    3. energy is nevertheless needed to drive the process of transpiration
    4. this energy is in the form of heat that evaporates water from the leaves + it ultimately comes from the sun
  • WHAT TYPE OF PROCESS IS TRANSPIRATION PULL AND WHAT DOES IT NOT REQUIRE TO TAKE PLACE?
    1. Its a massive process
    2. so doesn’t require metabolic energy
  • COHESION-TENSION THEORY: 1
    This is how water moves up the xylem against gravity via the transpiration stream
    1. Water evaporates from leaves via (open) stomata due to transpiration
    • atmosphere is less humid, water moves down water potential gradient + diffuses out of the leaf
    2. Reducing water potential in the cell, increasing water potential gradient
    3. Water is drawn out of xylem to the mesophyll cells
    4. Tension in the xylem is created due to the transpiration pull
    5. Water is cohesive due to hydrogen bonding (polar molecules) so they have a tendency to stick together
  • COHESION-TENSION THEORY: 2
    6. The cohesive forces between water molecules pull water up as a column
    7. As water moves up, there is a lower water potential in the roots so more enters the roots via osmosis, via down water potential gradient
    8. Water is therefore moving up the xylem, against gravity
  • EVIDENCE THAT SUPPORTS COHESION-TENSION THEORY
    1. Change in the diameter of tree trunks
    2. if a xylem vessel is broken and air enters it, the tree can no longer draw up water
    3. when a xylem vessel is broken, water doesn’t leak out
  • EVIDENCE THAT SUPPORTS COHESION-TENSION THEORY
    1, Change in the diameter of tree trunks according to the rate of transpiration. During the day, when transpiration is at its greatest, there is more tension (more negative pressure) in the xylem. This pulls the walls of the xylem vessels inwards and causes the trunk to shrink in diameter. At night when transpiration is at its lowest, there’s less tension in the xylem and so the diameter of the trunk increases.
  • EVIDENCE THAT SUPPORTS COHESION-TENSION THEORY
    2. If a xylem vessel is broken and air enters it, the tree can no longer draw up water. This is because the continuous column of water is broken and so the water molecules can no longer stick together
  • EVIDENCE THAT SUPPORTS COHESION-TENSION THEORY
    3. When a xylem vessel is broken, water doesn’t leak out, as would be the case if it were under pressure. Instead air is drawn in, which is consistent with it being under tension.
  • FACTORS AFFECTING TRANSPIRATION RATE
    1. Light intensity
    2. temperature
    3. humidity
    4. wind
  • FACTORS AFFECTING TRANSPIRATON RATE
    1, LIGHT INTENSITY: the lighter it is the faster the transpiration rate - positive correlation
    • this is because the stomata open when it gets light to let in carbon dioxide for photosynthesis
    • When it’s dark the stomata are usually closed, so there’s little transpiration
  • FACTORS AFFECTING TRANSPIRATION RATE
    2. TEMPERATURE: the higher the temperature the faster the transpiration rate - positive correlation
    • warmer water molecules have more kinetic energy so they evaporate by diffusion from the cells inside the leaf faster
    • this increases the water potential gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster
  • FACTORS AFFECTING TRANSPIRATION RATE
    3. HUMIDITY: the lower the humidity, the faster the transpiration rate - negative correlation
    • if the air around the plant is dry, the water potential gradient between the leaf and the air is increased, which increases transpiration rate
    • if the air is moist, there is a lower water potential gradient so diffusion is slower out of the leaf
  • FACTORS AFFECTING TRANSPIRATION RATE 

    4. WIND: the windier it is, the faster the transpiration rate - positive correlation
    • lots of air movement blows away water molecules from around the stomata
    • this increases the water potential gradient, which increases the rate of transpiration
  • ESTIMATING TRANSPIRATION RATE - POTOMETRY
    A potometer is a special piece of apparatus used to estimate transpiration rates. It actually measures water uptake by a plant, but it’s assumed that water uptake by the plant is directly related to water loss by the leaves. you can use it to estimate how different factors affect the transpiration rate
  • ESTIMATING TRANSPIRATION RATE - POTOMETRY: 1
    1. Cut a shoot underwater to prevent air from entering the xylem. Cut it at a slant to increase the surface area available for water uptake
    2. assemble the POTOMETER under the water and insert the shoot with the apparatus still under the water so no air can enter
    3. remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water
    4. check that the apparatus is watertight and airtight
    5. dry the leaves, allow time for the shoot to acclimatise and then shut the tap
  • ESTIMATING TRANSPIRATION RATE - POTOMETRY: 2
    6. remove end of capillary tube from beaker of water until one air bubble has formed, then put the end of the tube back into water
    7. Record starting position of air bubble
    8. Start stopwatch + record distance moved by bubble per unit time eg. Per hour. Rate of air bubble movement is an estimate of transpiration rate
    9. Remember, only change one variable eg. Temperature at a time. All other conditions eg. Humidity, light intensity must be kept constant
  • STRUCTURE OF PHLOEM
    • living cells joined end to end forming tubes
    • Living cells are sieve tube elements
    -but have no nucleus and few organelles
    • there is a companion cell for each sieve tube element
    -carry out the living functions and provide energy for active transport of solutes
  • TRANSLOCATION: movement of solutes / organic substances through the plant bidirectionally from the source to the sink
    • by mass flow hypothesis
    • hypothesis as not enough evidence for a theory
  • MASS FLOW MECHANISM OF TRANSLOCATION
    1. at the source, sucrose moved into phloem by active transport
    2. this increases sucrose conc + decreases water potential in phloem
    3. water moves into phloem from xylem + surrounding tissues by osmosis down water potential gradient
    4. = results in increase in hydrostatic / turgor pressure in sieve tube elements
    5. solutes + water move from high to low pressure moving away from the source
    6. sucrose is absorbed (assimilated) at the sink
    7. sucrose conc in phloem decreases + water potential increases
    8. water moves back into xylem by osmosis down water potential gradient
  • RINGING EXPERIMENTS
    WHAT ARE RINGING EXPERIMENTS?
    A section of the outer layers (protective layer and phloem) is removed around the whole circumference of the stem
  • WHAT DOES RINGING DO TO THE PLANT?
    1. removal of phloem interrupts continuity of phloem tubes, preventing downward movement of sugars + other organic compounds
    2. however, upward flow of sap in xylem vessels=unaffected
    3. as result sugars produced in leaves+transported through phloem accumulate above ringed area, leads to swelling of region. This swelling known as ring of callus, a bulge
    4. fluid from bulge has higher conc of sugars than fluid from below ring. This is bc sugars can’t move past area where barks been removed.
    5. also causes death of tissues below ring as sugars not provided