Topic 9.1: Transport in the Xylem of Plants Flashcards
Definition of transpiration
Loss of water vapor from stems and leaves of plants
Cause of water transport during transpiration (2)
a) Difference in water pressure between leaves (low) and roots (high)
b) Cohesive / Adhesive properties of water
What causes the loss of water in the leaves? (2)
a) Light absorbed by the leaves is converted into heat, which evaporates water
b) Opening of stomata for gas exchange in photosynthesis.
Result of the loss of water
Negative pressure due to transpiration creates a tension force in leaf cell walls dragging water from the xylem.
Role of cohesive properties of water in transpiration
H2O is dragged up by the xylem towards the leaves in a continuous stream
Role of adhesive properties of water in transpiration
Attraction between water and xylem walls creates further tension, pulling water upwards against gravity via capillary action
Control of opening / closing of stomata
Guard cells
a) Hypertonic due to K+ influx causes opening
b) Hypotonic due to K+
efflux causes closing
Xylem function
Conduct water and minerals to the leaves from the roots
Xylem structure (3)
a) Hollow dead cells
b) Pits in cell wall
c) Cell wall reinforced by cellulose and lignin
Types of xylem (2)
a) Tracheid = Exchange of water via pits
b) Vessel = Exchange of water via a continuous tube (Angiosperms)
Diagram of xylem (3)
a) Continuous tube
b) Xylem wall with pits
c) Lignin (Spiral / Rings)
Roots function
Absorb minerals and water from the soil
Roots structure / Adaptation
Branches + Root hairs
=> Increase in SA for absorption
Mechanism of roots to absorb minerals and water
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
Situation of xerophytes with respect to transpiration
High rates of transpiration due to high temperatures / low humidity
Adaptations of xerophytes
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
Situation of halophytes with respect to transpiration
Loss of water due to high intake of salt from the surrounding soils
Adaptations of halophytes
a) Cellular sequestration
b) Tissue partitioning
c) Root level exclusion
d) Salt excretion
e) Altered flowering schedule
Models to represent movement of water upwards in the xylem
a) Capillary tubing
b) Filter paper
c) Porous pots
Function of a potometer
Estimate transpiration rates by measuring the rate of water uptake
Mechanism of a potometer
Change in the movement of a water bubble in a tube connected to a plant shoot
Factors that affect transpiration
a) Light intensity (Direct)
b) Humidity (Inverse)
c) Wind (Direct)
d) Temperature (Direct)
e) Soil water (Direct)
f) CO2 level (Indirect)
