Topic 9.2 Transport in Angiospermophytes

Question 1

9.2.1 Outline how the root system provides a large surface area for mineral ion and water uptake by means of branching and root hairs.

Answer 1

• Plant roots are important for uptake of mineral ions & water, as well as for providing stability
• To increase surface area:
  
  • roots have a branching pattern, which forms when new roots develop from the embryonic root (either fibrous, or lateral branches growing off taproot)
  • root hairs may grow on epidermis of these roots

Question 2

9.2.2 List ways in which mineral ions in the soil move to the root.

Answer 2

1. diffusion of mineral ions along a concentration gradient
2. mass flow of water carrying ions, when water drains through soil and into roots
3. fungal hyphae (grow around plant roots in a mutualistic relationship) absorb minerals and trade them for sugars in plants

Question 3

9.2.3 Explain the process of mineral ion absorption from the soil into roots by active transport.

Answer 3

1. active transport
   
   • ions pass through protein pumps (e.g. potassium ions through potassium channels)
2. indirect active transport
   
   • fertile soil contains negatively charged clay particles to which positively charged minerals may attach
   • proton pumps in plasma membranes of cells use ATP to pump hydrogen ions out → displaces the positively charged mineral ions for absorption (other negatively charged minerals may bind to the H⁺ ions) → concentration gradient causes H⁺ to be reabsorbed with minerals via diffusion

Question 4

9.2.4 State that terrestrial plants support themselves by means of thickened __________, cell _______ and ___________ xylem.

Answer 4

Terrestrial plants support themselves by means of thickened cellulose, cell turgor and lignified xylem.

Question 5

9.2.5 Define transpiration.

Answer 5

Transpiration: the loss of water vapour from leaves or other aerial parts of a plant.

Question 6

9.2.6 Explain how water is carried by the transpiration stream, including the structure of xylem vessels, transpiration pull, cohesion, adhesion and evaporation.

Answer 6

Xylem vessels are composed of many cell types, including:

• tracheids - hollow cell w/o cytoplasm or nucleus; has pits & modified pits to allow water to move between cells)
• and vessel elements - end walls have pores to allow rapid water transport; secondary walls are lignified to add strength.

Cohesion-tension theory

1. external heat provides energy for evaporation of water from cell walls of spongy mesophyll tissue (high water concentration in the air spaces)
2. evaporated water replaced with water from xylem vessels
   
   • water is pulled out of xylem vessels and through pores in spongy mesophyll cell walls by capillary action
3. low pressure/suction created inside xylem vessels when water is pulled out
   
   • called transpiration pull
4. transpiration pull extends down through columns of water in xylem vessels to the roots, pulling more water up
5. xylem vessel water column is maintained through cohesion (H₂O molecules form H-bonds with each other) and adhesion (H₂O molecules form H-bonds with sides of vessels, counteracting gravity)

Question 7

9.2.6 State that guard cells regulate ___________ by opening and closing stomata.

Answer 7

Guard cells regulate transpiration by opening and closing stomata.

Question 8

9.2.7 State that the plant hormone _______ acid causes the closing of stomata.

Answer 8

The plant hormone abscisic acid causes the closing of stomata.

Question 9

9.2.9 Explain how the abiotic factors light, temperature, wind and humidity, affect the rate of transpiration in a typical terrestrial plant.

Answer 9

• Light warms the leaf and opens the stomata, increasing rate of transpiration
• Water diffuses out of leaf when there’s a concentration gradient between air spaces inside leaf (always nearly saturated with water) and air outside. The lower the humidity outside the leaf, the steeper the gradient and therefore the faster the rate of transpiration
• Wind carries humid air away from stomata, increasing rate of transpiration
• Higher temperatures evaporate more water, thus increase rate of transpiration
  
  • also increase rate of diffusion through the air spaces in the spongy mesophyll, and reduce the relative humidity of the air outside the leaf

Question 10

9.2.10 Outline four adaptions of xerophytes that help to reduce transpiration.

Answer 10

Xerophytes: plants adapted to arid climates - have special modifications/adaptions to decrease transpirational water loss.

• small, thick leaves to decrease surface area
• reduced # of stomata
• stomata located in crypts or pits on leaf surface to increase surrounding humidity
• hair-like cells on surface of leaf trap water vapor to maintain high humidity near stomata
• thickened waxy cuticle
• alternative photosynthetic process
  
  • CAM photosynthesis closes stomata during day (most transpiration) and opens during night
• many desert plants shed leaves during driest months and become dormant

Question 11

9.2.11 Outline the role of phloem in active translocation of sugars (sucrose) and amino acids from source (photosynthetic tissue and storage organs) to sink (fruits, seeds, roots).

Answer 11

Phloem sap: dissolved organic minerals in water

1. Sugar is loaded into sieve tubes at the source. Osmosis occurs from surrounding cells b/c relative water concentration in sieve tube members is reduced.
2. Uptake of water causes positive pressure in sieve tubes, resulting in flow of phloem sap.
3. Sugars are removed from sieve tubes at the sink, diminishing pressure. Here, they’re turned into starch (insoluble and exerts no osmotic effect).
4. Relatively pure water is recycled by xylem, which carries it from the sink to source.

Note:

• loading & removal of sugars at source & sink = active transport
• transport in sieve tube = passive process
• Phloem tissue occurs in all parts of the plant, and leaves are source regions.