3.4.P Mass Transport in Plants

• How is water a dipole molecule?

  It has both positive and negative poles
• What does hydrogen bonding occur between?

  It occurs between a slightly positively charged hydrogen atom in one molecule and a slightly negatively charged atom in another molecule
• Give three properties of water

  ○ Metabolite.

  ○ Solvent.

  ○ High specific heat capacity.

  ○ High latent heat of vaporisation.

  ○ Cohesive due to hydrogen bonding.
• What reactions are metabolites involved in? [4]

  ○ Condensation reactions.

  ○ Hydrolysis reactions.

  ○ Photosynthesis.

  ○ Respiration.
• Why is water a good solvent? [2]

  Water is a polar molecule, meaning it has a slightly negative end which is attracted to slightly positive ions and a slightly positive end which is attracted to slightly negative ions. This means that the water molecules can pull the ions apart in a molecule and dissolve it.
• How does water dissolve molecules? [2]

  Water molecules pull the ions apart breaking the bonds between then and then the Ions become surrounded by water molecules.
• What does it mean that water has a high specific heat capacity? [4]

  A lot of heat energy is required to break the hydrogen bonds between molecules and so less energy is available to increase the temperature of the water so it can buffer changes in temperature. Due to this the temperature of water and cells is likely to remain stable.
• What does it mean that water has a high latent heat of vaporisation? What does this help with? [3]

  Due to hydrogen bonding lots of heat energy is required to change waters state from liquid to gas. Helping to cool down organisms by evaporation.
• What is cohesion and how is it useful? [3]

  Cohesion is the hydrogen bonding between water molecules. It's useful for water transport in plants, supporting collums of water and in creating surface tension which supports smaller organisms.
• What is adhesion and what does it play a role in? [2]

  Adhesion is hydrogen bonding to other surfaces and also plays a role in water transport.
• Give two useful features of water.

  It's not easily compressed so can provide support. It's transparent which allows light to penetrate it.
• Why is water not being easily compressed a useful feature? Give examples.

  It provides support, e.g. turgidity in plants and supporting the fetus in the womb.
• Why is water being transparent a useful feature? Give examples.

  Light can penetrate it allowing for the photosynthesis of aquatic plants and vision for fish.
• Give 4 features of the xylem that show it's adapted for its function. [4]

  Long cells with no end walls.

  Hollow tubes with no organelles.

  Walls which contain thick lignin and there are pits in the walls.
• How does the xylem having walls with thick lignin show that it's adapted to its function?

  They provide strength, resist tension and are waterproof to prevent water loss.
• How does the xylem having hollow tubes with no organelles show that it's adapted to its function?

  It allows for an unrestricted and continuous flow of water
• How does the xylem having long cells with no end walls show that it's adapted to its function?

  This allows for a continuous water column
• How does the xylem having walls with thick lignin show that it's adapted to its function?

  This allows water to get around blocked vessels
• Describe the structure of the phloem.

  It's made of sieve tube elements which are long, living, cells arranged end to end. Their end walls are perforated with pores to form sieve plates. Companions cells carry out metabolic functions for the sieve tube elements. They actively transport sucrose into the sieve tube.
• What is translocation? [2]

  The transport of organic molecules (sucrose, amino acids, etc) from one part of the plant to another. This occurs in phloem vessels.
• Why does translocation have to occur in either direction? [2]

  Sugars produced during photosynthesis are transferred from the sites of production to the sites of use/storage. Sinks can be found any where in a plant, either below or above the source. So translocation has to occur in either direction.
• How does loading occur at the source? [4]

  Due to a high concentration of sucrose in sieve tubes which is due to active transport by companion cells, the water potential in the sieve tubes is lowered causing water to move from the xylem into the sieve tubes by osmosis. This results in a high hydrostatic pressure at the source.
• How is sucrose transported to the sink? [5]

  Sucrose inactively transported into respiring cells for use/storage resulting in a lowered water potential in respiring cells. Due to this water moves into respiring cells by osmosis causing the hydrostatic pressure at the sink to decrease, causing a mass flow of sucrose down the pressure gradient from the source to the sink
• What is transported in the xylem and what is transported in the phloem?

  Water and ions in the xylem. Organic solutes, sucrose and amino acids in the phloem.
• Where does the xylem transport water/ions to and where does the phloem transport organic solutes to?

  Roots to leaves in the xylem. Up and down the plant, source to sink in the phloem
• What processes are associated with the xylem and what processes are associated with the phloem?

  Transpiration and photosynthesis in the xylem. Translocation and respiration in the phloem.
• Is the xylem or phloem made of dead cells? What is the phloem made of?

  The xylem is made of dead cells and the phloem is made of living cells.
• What is the structure of the xylem? (short)

  Hollow tubes, no cytoplasm, contains lignin and has pitted walls
• What is the structure of the phloem? (short)

  Sieve plates, companion cells, sieve tube element, no nucleus and few organelles
• What transport mechanisms are used in the xylem and what transport mechanisms are used in the phloem?

  Diffusion and osmosis in the xylem. Active transport, osmosis and mass transport in the phloem.
• Fill in the blank:

  A) Endodermis

  B) Cortex

  C) Phloem

  D) Root Hair

  E) Epridermis

  F) Xylem

  
• How do you work out how many stomata are in a given area without counting?

  1. Place the specimen under a microscope.
  2. Select several random fields of view across the leaf surface and count the number of stomata in each of these fields.
  3. Calculate the stomata density using the equation : average number of stomata per field divided by the area of a field of view (mm^2)
  4. Estimate the total number of stomata with this equation: stomata density x total area (mm^2)
• When water leaves the xylem, this creates tension which pulls the xylem inwards, causing the xylem lumen to narrow.
• The xylem is strengthened to withstand pressure and waterproofed due to the presence of lignin.
• The process of evaporation from the leaf is called transpiration.
• A potometer is used to measure water uptake. Why is the rate of uptake in a plant, not the transpiration rate?

  Additional water is produced in respiration, water is used in photosynthesis, and water is used for support in the plant.
• Sieve plates allow sucrose to flow between cells.
• Sieve tube elements have thick cell walls to resist the pressure of mass flow.
• What insects are used to study mass flow?

  Aphids
• Removing a ring of bark from a step reduces the mass flow of sucrose from source to sink. This is because phloem vessels have been removed.