Topic 9: Plant Biology

Question 1

What is transpiration?

Answer 1

Transpiration is the loss of water from the leaves and stems of plants. Xylem vessels transport water through the plant (remember water has cohesive properties due to H-bonds). Water is heated in the mesophyll by sunlight and becomes vapor. This vapor transpires out of the stomata - pores in the leaf. Loss of water generates negative pressure and transpiration pull on water molecules in the xylem. More water is drawn into the leaf. Cohesion between water molecules means that the transpiration pull has a knock-on effect through the plant. Higher rates of transpiration lead to a faster transpiration stream and higher rates of water uptake. Transpiration flow is controlled by the rate of water loss through stomata.

Question 2

Outline the main factors that affect transpiration.

Answer 2

Temperature, humidity, air movement, and light intensity all affect transpiration.

Question 3

Identify adaptions of desert and saline plants for water conservation.

Answer 3

Xerophytes: survive in dry conditions by reducing transpiration. Where water is at a premium, plants need to adapt to reducing wastage through transpiration.

Life cycle adaptions:
- perennial plants bloom in wet seasons
- dormant seeds can survive for many years until conditions are ideal for growth

Metabolic adaptions:
- CAM plants (Crassulacean acid metabolism): CO2 is absorbed at night and stored as a C4 compound. During the day, photosynthesis can occur with the stomata closed by using these carbon stores.

Physical adaptions:
- fewer leaves or stomata
- rolled leaves or spines
- stomata in pits with hairs
- deeper roots to reach the water
- waxy cuticle reduces evaporation

Question 4

Identify the xylem and phloem.

Answer 4

Phloem transports water and solutes along hydrostatic pressure gradients.
- Relatively high concentration of sucrose and water in phloem sieve tubes
- Water is incompressible, i.e. it occupies a fixed volume
- The walls of the sieve tubes are rigid
- These two factors cause a build-up of hydrostatic pressure at the source
- water and the solutes (sucrose and amino acids) flow down the hydrostatic pressure at the source
- water and the solutes (sucrose and amino acids) flow down the hydrostatic gradient to the sink where pressure is relatively low
- this is due to the active unloading of sucrose and hence loss of water by osmosis at the sink

Question 5

Explain the role of auxin in cell growth.

Question 6

Outline the process of micropropagation.

Question 7

Outline seed dispersal.

Question 8

Explain the process of fertilization.

Question 9

Describe the role of phytochrome in flowering.

Question 10

Explain how different factors affect germination.