Plant transport 7

• what is mass transport?

  Movement of materials over large distances due to pressure differences
• define transpiration

  the movement of water and dissolved materials through xylem tissue.
• define translocation?

  The movement of sugars and other organic molecules in plants through the phloem tissue
• transpiration is the process of water movement from the roots up the plant and its evaporation from leaves. transpiration is a consequence of gas exchange in leaves. when stomata open to allow exchange of O2 and CO2, water will be lost. Plants only use 1% of water entering roots.
• what do plants use water for?

  photosynthesis and maintaining turgidity
• what happens to the remaining 99% of water?

  it evaporates from the leaves.
• water is carried in xylem tissue. this is a series of continuous tubes called xylem vessels. these are modified dead cells arranged end to end
• Give ways in which xylem vessels are adapted to their function?

  Lignin in cell wall- strengthens xylem walls against tension within and makes them waterproof. Also causes cell contents to die which leaves a hollow lumen that offers little resistance to flow of water. Tiny holes called pits in walls of xylem- water can be diverted laterally if vessels become blocked or damaged. The vessels have no end walls so they form continuous columns for water movement from root to leaves.
• to understand the cohesion tension theory its important to understand that water molecules are polar so very many weak hydrogen bonds form making them stick together. this is called cohesion. water molecules also form hydrogen bonds with walls of the xylem- this is adhesion
• Cohesion tension theory:
• Water is evaporated from the leaves and diffuses through the stomata
• This causes the water potential in the leaves to reduce, creating a water potential gradient across the leaf
• Water leaves the xylem vessels in the leaf and diffuses into mesophyll cells by osmosis, replacing the lost water
• This process creates negative pressure or tension at the top of the xylem vessels
• The remaining water in the xylem is under tension and is pulled up towards the leaves
• Continuous columns of water are maintained due to cohesion between water molecules and adhesion
• This inward pull on vessel walls as water is pulled up causes the trunk to decrease in diameter
• the process is passive (no ATP needed). the energy needed to drive this process is the heat required to evaporate water from the leaves.
• why is the rate of transport in the xylem higher around midday?

  There's a higher rate of evaporation/ increased transpiration rate as higher temperature so molecules have more kinetic energy.
• what is the evidence for the cohesion tension theory?

  tension has been measured in xylem as plants transpire. if a column of water in xylem is broken, air bubbles in xylem form and this stops any further upward movement of water in that xylem vessel as air bubbles prevent cohesion. respiratory inhibitors like cyanide or lack of O2 don't inhibit this process. the diameter of trees decrease when they are transpiring - can be measured using a dendrometer
• when a xylem vessel is broken, water doesn't leak out, instead air is drawn in. how does this provide evidence for the theory?

  water must be under tension rather than under pressure
• why is the trunk diameter lowest at noon?

  higher rate of transpiration as stomata open due to higher temps/ light intensity. so increased tension and water is pulled up faster so walls of xylem are pulled in
• how can you measure rates of transpiration?

  Potometer- apparatus must be airtight so all joints sealed with vaseline. The shoot is cut underwater to prevent air entering the xylem vessels. the distance moved by the air bubble in known length of time can be used to estimate the rate of transpiration
• how to calculation rate of transpiration?


  volume/ time. volume calculated from diameter of capillary tube( area of a cylinder pi x r>2)
• The student assumed water uptake was equivalent to rate of transpiration. Give 2 reasons why this might be not a valid assumption

  Used in photosynthesis

  used to provide turgidity
• suggest how the reservoir allows repeat measurements to be made 


  Returns bubbles to start
• What are xerophytes?

  plants that have adapted to regions which have scarce water by reducing inevitable loss of water by transpiration. For example the leaves are covered in a waxy cuticle and their stomata can close
• Xerophytes have structural adaptations which allows them to survive in dry conditions by either reducing transpiration or by storing water
• How is marram grass a xerophyte?

  Leaf rolled up

  thick cuticles

  trapped humid air with high water potential

  hairs to trap moisture

  stomata in sunken pits

  longer root networks to reach more water

  curled leaves to trap moisture
• How does trapped humid air with high water potential help limit water loss?

  Water vapour is trapped within the rolled leaf, This reduces the water potential gradient between air spaces inside the leaf and the atmosphere. A lower rate of diffusion from stomata occurs
• How does hairs on lower epidermis of leaf help?

  Water vapour is trapped between the hairs so reducing the Water potential gradient between air spaces inside the leaf and the atmosphere. A lower rate of diffusion occurs and less transpiration occurs.
• Stomata sunken in pits?

  water vapour held above stomatal pore so reduces water potential gradient between air spaces inside leaf and the atmosphere.
• Thick waxy cuticles?

  increases length of diffusion pathway so decreases rate of diffusion of water through cuticle
• Some other adaptations of xerophytes?

  Reduction in surface area to volume ratio of leaves- slows rate of diffusion. Some cacti have spines rather than flat broad leaves.

  Stomata confined to underside of leaf- often cooler so less heat energy to evaporate water

  Daylight closure of stomata- reduces transpiration during hottest part of day

  Succulent stems or leaves- storage of water
• Define translocation


  The process by which soluble organic molecules like sucrose are transported around the plant as sap
• Why is it necessary to transport sucrose from one part of plant to another?

  Only leaves photosynthesise to produce sugars but all parts of plants require sugars for respiration like the root Cells
• why might non reducing sugars like sucrose be transported rather than reducing sugars like glucose?

  Reducing sugars are too reactive and would be chemically altered before arriving at their destination
• Translocation occurs in a tissue known as phloem. Phloem tissue is made up of sieve tube elements, long thin cells arranged end to end. Companion cells are associated with sieve tubes
• Describe structure of the phloem tissue

  • each sieve tube element links to the next via a sieve plate which is perforated with pores
  • the sieve tube has little cytoplasm, no nucleus, no vacuole and few organelles other than a small number of mitochondria
  • the sieve tubes are alive because of cytoplasmic connections (plasmodesmata) with the companion cell
  • each companion cell has a nucleus, many mitochondria and other organelles
• What is problem with the pores?

  They could become blocked by the sugary sweet sucrose Solution.