Topic 9.1: Transport in the Xylem of Plants

Question 1

Definition of transpiration

Answer 1

Loss of water vapor from stems and leaves of plants

Question 2

Cause of water transport during transpiration (2)

Answer 2

a) Difference in water pressure between leaves (low) and roots (high)
b) Cohesive / Adhesive properties of water

Question 3

What causes the loss of water in the leaves? (2)

Answer 3

a) Light absorbed by the leaves is converted into heat, which evaporates water
b) Opening of stomata for gas exchange in photosynthesis.

Question 4

Result of the loss of water

Answer 4

Negative pressure due to transpiration creates a tension force in leaf cell walls dragging water from the xylem.

Question 5

Role of cohesive properties of water in transpiration

Answer 5

H2O is dragged up by the xylem towards the leaves in a continuous stream

Question 6

Role of adhesive properties of water in transpiration

Answer 6

Attraction between water and xylem walls creates further tension, pulling water upwards against gravity via capillary action

Question 7

Control of opening / closing of stomata

Answer 7

Guard cells
a) Hypertonic due to K+ influx causes opening
b) Hypotonic due to K+
efflux causes closing

Question 8

Xylem function

Answer 8

Conduct water and minerals to the leaves from the roots

Question 9

Xylem structure (3)

Answer 9

a) Hollow dead cells
b) Pits in cell wall
c) Cell wall reinforced by cellulose and lignin

Question 10

Types of xylem (2)

Answer 10

a) Tracheid = Exchange of water via pits

b) Vessel = Exchange of water via a continuous tube (Angiosperms)

Question 11

Diagram of xylem (3)

Answer 11

a) Continuous tube
b) Xylem wall with pits
c) Lignin (Spiral / Rings)

Question 12

Roots function

Answer 12

Absorb minerals and water from the soil

Question 13

Roots structure / Adaptation

Answer 13

Branches + Root hairs

=> Increase in SA for absorption

Question 14

Mechanism of roots to absorb minerals and water

Answer 14

Active uptake of mineral ions into the roots causes absorption of water by osmosis
a) Use of H+ pumps to diffuse cations with H+ via cotransport

Question 15

Situation of xerophytes with respect to transpiration

Answer 15

High rates of transpiration due to high temperatures / low humidity

Question 16

Adaptations of xerophytes

Answer 16

a) Reduced leaves (Reduce SA)
b) Rolled leave (Reduce exposure of stomata to the air)
c) Thick, waxy cuticle
d) Stomata in pits
e) Low growth
f) CAM physiology

Question 17

Situation of halophytes with respect to transpiration

Answer 17

Loss of water due to high intake of salt from the surrounding soils

Question 18

Adaptations of halophytes

Answer 18

a) Cellular sequestration
b) Tissue partitioning
c) Root level exclusion
d) Salt excretion
e) Altered flowering schedule

Question 19

Models to represent movement of water upwards in the xylem

Answer 19

a) Capillary tubing
b) Filter paper
c) Porous pots

Question 20

Function of a potometer

Answer 20

Estimate transpiration rates by measuring the rate of water uptake

Question 21

Mechanism of a potometer

Answer 21

Change in the movement of a water bubble in a tube connected to a plant shoot

Question 22

Factors that affect transpiration

Answer 22

a) Light intensity (Direct)
b) Humidity (Inverse)
c) Wind (Direct)
d) Temperature (Direct)
e) Soil water (Direct)
f) CO2 level (Indirect)