3.1.3 : Transport in plants - Biology

• Why do plants need a transport system?

  For metabolic demands : Underground plants don't photosynthesise so cannot make their own glucose. Need to remove waste

  Cannot rely on diffusion alone to supply the cells with what they need ( small SA:V)
• What do the transport systems in plants consist of?

  Xylem tissue which moves water and soluble mineral ions upwards only.

  Phloem tissue which moves assimilates (sugars) up OR down.
• What do the xylem consist of?

  Xylem walls are impregnated with lignin which makes them waterproof, strengths and kills them.

  Lignin thickening patterns in the walls may create a spiral, annular, reticulate or broken rings. Allows xylem to stretch.

  Allows water to move sideways.

  Lignification is not complete in some places. This leave gaps called bordered pits.

  Xylem parenchyma stores food and tannin deposits around the xylem.

  Tannin is a chemical that protects plants from being eaten.
• What do the phloem consist of?

  Made up of sieve tube elements and companion cells.

  Sieve tube elements are elongated cells lined end to end to form sieve tubes.

  Ends have perforated walls called sieve plates that allow sap to move through the vessel.

  Sieve plates support the lumen and keep it open.

  Contain no nucleus and very little cytoplasm, more space for sap and assimilates.
• What is transpiration?

  Transpiration is the loss of water from a plant by evaporation (driving force).

  Conditions are determined by the water potential outside the leaf.
• What is the structure of the leaf?

  Cuticle

  Upper epidermis cells

  Palisade mesophyll cells

  Spongy mesophyll cells

  Vein (vascular bundle)

  Lower epidermis cells
• How does humidity affect transpiration rates?

  Lower the humidity outside the leaf, faster that rate of transpiration as the water potential gradient is steeper.
• How does air movement affect transpiration rates?

  Increased air movement increases rate of transpiration as it moves water around the leaf.
• How does temperature affect transpiration rates?

  Increases temperature, increased rate of transpiration as there is increased kinetic energy and warms the air up so a lower water potential gradient in the air.
• How does light intensity affect transpiration rates?

  Greater light intensity increases rate of transpiration as it leaves the stomata open, increasing evaporation.
• Why is water important?

  Turgor pressure

  Cooling

  Minerals are transported in aqueous solutions

  Photosynthesis.
• How is water transported into the roots?

  Water is absorbed from soil by osmosis.

  Moves down water potential gradient due to active transport of minerals into the root hair cells, lowering the water potential in the cells.

  Enters the root by the tip as there are root tips, increasing the surface area from osmosis.
• Root hair cells

  Penetrate through soil easily

  Each hair has a cellulose wall for diffusion and osmosis to happen quickly.
• What is the apoplast route?

  Water moves through cellulose cells walls and intercellular spaces.

  Fibres of cellulose don't resist water flow.

  Water cannot pass the endodermis due to the casparian strip due to the water proof band of Suberin so must go through cytoplasm.
• What is the symplast route?

  Through cytoplasm of cells.

  Water passes through cells via the plasmodesmata (gaps in cell walls)
• Why is the casparian strip important?

  Prevents harmful substances from entering the xylem.

  Prevents water leakage from xylem vessels

  Aids development of root pressure.
• What is root pressure?

  Water is pushed up the xylem by hydrostatic pressure.
• What could happen if the roots were derived of 02?

  The 'pumping' of the ions would stop as it requires ATP produced in aerobic respiration and 02 is required for aerobic respiration.
• What is the evidence for root pressure?

  Cut stumps of plants exude water.

  Guttation: Some leaves exude water.

  Pressures recorded that cut stumps could push water in the xylem up to 30m.
• What are the limitations of root pressure?

  Pressure measured is not enough to get water to the top of trees.

  Relies on ATP for transport.
• What is capillarity?

  Water rises up narrow tubes due to the adhesive forces between water molecules and the wall of tube.

  Water rises up higher in narrower tubes (xylem).
• What is the cohesion-tension theory?

  Water molecules form hydrogen bonds with each other, causing them to 'stick' together (cohesion). The surface tension of the water also creates this sticking effect. Therefore as water is lost through transpiration, more can be drawn up the stem from the roots.
• What are the adaptations of xerophytes (dry habitat) ?

  Thick waxy cuticle: prevent evaporation.

  Reduced leaf area

  Hairy leaves: trapped a layer of moist air

  Sunken stomata: pits above stomata become saturated

  Rolled leaves: keeps stomata on the inside, reduce evaporation.

  Succulents: store water in specialised tissue.

  Leaf loss: losing their leaves when water isn't available.
• What are the adaptations of hydrophytes (wet habitat)?

  Very thin or no waxy cuticle.

  Stomata on the upper surface so they are in contact with air.

  No supporting structure as water supports leaves

  Air spaces enable leaves to float.

  Smaller roots, less need for uptake.
• What is translocation?

  Products are transported from source to sink.
• What happens during phloem loading?

  H+ ions are pumped out of companion cells, creating a concentration gradient.

  H+ ions enter the companion cells again, co-transporting sucrose.

  Concentration of sucrose in the cells increase and diffuses into sieve tube elements.

  The sucrose lowers the water potential in sieve tube elements.

  Water moves into sieve tube elements via osmosis from surrounding cells.

  Increases hydrostatic pressure at the source, causing mass flow to areas of lower pressure.
• What happens during phloem unloading?


  Sucrose is unloaded anywhere its needs.

  Sucrose diffuses into sieve tubes into the companion cells then other cells by osmosis.

  Sucrose is constantly being used, maintaining a concentration gradient.

  Loss of sucrose from sieve tubes, raises water potential so water moves out via osmosis.

  Reduces hydrostatic pressure at the sink.
• What are sources of assimilates?

  Green leaves and stems

  Storage organs

  Food stores in seeds.
• What is the transport system in dicotyledonous plants?

  Series of plants that run through stems, roots and leaves.

  This is the vascular system.

  The transport tissues are the xylem and the phloem which are arranged in vascular bundles.
• Where does xylem transport?

  From roots to leaves, against gravity.
• What does end walls being absent in xylem result in?

  Uninterrupted flow water.
• Where does the phloem transport to?

  All around the plant.
• What do companion cells do?

  Carry out metabolic activities for sieve cells as they contain a nucleus.
• What bonds are between water molecules?

  Hydrogen bonds.
• Where does water evaporate from?

  Surface of leaves.
• How do you estimate transpiration rates?

  Using a potometer.

  Measuring the rate of water uptake by a plant by timing how long it takes for a bubble to move a certain distance along a capillary tube of known diameter.
• How to set up a potometer

  1. Cut a leafy shoot from a plant and immediately put it under water

  2. Cut a small section from the stem whilst under water to prevent air getting into the xylem and breaking the water column

  3. DO NOT get the leaves wet as this will affect the transpiration rate

  4. Put the potometer under the water and move it from side to side to remove the air from the system

  5. Attach the leaf shoot into the potometer

  6. Seal all joints with water proofing substance (petroleum jelly) so that water loss is from the leaves only

  7. Ensure the environmental conditions don't change around the leaves as this will heavily influence the transpiration rate
• What must we assume when using a potometer?

  All water taken up is lost in transpiration but some could be used during photosynthesis, maintaining turgidity, hydrolysis reactions or respiration.
• Equation for volume of water uptake?

  volume of water uptake by shoot +πr2 *distance travelled
• Equation of rate of transpiration.

  Distance moved by bubble / time taken