Transport in Plants

Question 1

what is transpiration

Answer 1

the evaporation of water from a plants surface

Question 1

how does temperature affect the rate of transpiration

Answer 1

• increase in temperature increase the rate transpiration by providing the water molecules with more kinetic energy allowing them to evaporate more readily

Question 1

what environmental factors affect the rate of transpiration

Answer 1

• light
• temperature
• humidity
• air movement (wind)

Question 2

how does light affect the rate of transpiration

Answer 2

• during daylight, stomata open to allow carbon dioxide in for photosynthesis
• this increases the rate of transpiration as water evaporates from the mesophyll cells and diffuses out of the leaf

Question 3

how does humidity affect the rate of transpiration

Answer 3

• increase in humidity means the air has a higher water potential which decreases the water potential gradient
• this decreases the rate of transpiration

Question 4

how does air movement affect the rate of transpiration

Answer 4

• air movement removes water vapour from the leaf surface increasing the water potential gradient and the rate of transpiration
• in still air, water vapour builds up around the leaf decreasing the water potential gradient and the rate of transpiration

Question 5

What are xerophytes?

Answer 5

plants adapted to limit water loss (living in dry habitats)

Question 6

how does a thick cuticle limit water loss

Answer 6

• a thick cuticle provides a long diffusion pathway reducing the rate of evaporation

Question 6

adaptations of xerophytes to reduce water loss

Answer 6

• thick waxy cuticle
• sunken stomata
• reduced leaf area
• hairy leaves
• curled leaves

Question 7

what evidence is there for the movement of ions in the xylem?

Answer 7

radioactive isotopes are used as tracers for evidence of ion transport in the xylem

• the xylem and the phloem are separated using a wax cylinder to prevent lateral transport
• the roots are supplied with ⁴²K and the plant is left for a few hours
• when measured, the concentration of ⁴²K in the xylem is considerably higher than in the phloem therefore ions are transported in the xylem.

Question 7

how do hairy leaves surface limit water loss

Answer 7

• the hairs on the leaf’s surface traps a layer of still air which becomes saturated with water vapour reducing the water potential gradient for water loss

Question 8

how do curled leaves limit water loss

Answer 8

• rolling up of leaves traps a layer of still air which becomes saturated with water vapour
• this reduces the water potential gradient for water loss, reducing the rate of transpiration

Question 8

how does reduced leaf area limit water loss

Answer 8

• reduced surface area to volume ratio of leaves reduces the surface area for water loss

Question 9

what is the structure of a xylem

Answer 9

hollow tubes strengthened by lignin

• lignin makes the cell walls more rigid and provides support

Question 9

how does a sunken stomata limit water loss

Answer 9

• stomate positioned in epidermal pits or grooves reduce their exposure to air currents
• the trapped air becomes saturated with water vapour and reduces the water potential gradient for evaporation

Question 10

what are some examples of sinks in plants?

Answer 10

• growing areas such as young leaves, shoot tips and root tips
• the roots
• developing fruits or storage areas

Question 10

what is the cohesion-tension theory

Answer 10

• a theory used to explain the movement of water and mineral ions from roots to leaves in the xylem

Question 11

how does the cohesion-tension theory work

Answer 11

1 -solar heat energy causes the evaporation of water from leaves
2 -water moves from cell to cell across the leaf by osmosis down a water potential gradient
3 - water is drawn from the xylem creating a tension in the xylem vessels effectively pulling up the water and dissolved ions
4 - the water column is maintained in the xylem by cohesive and adhesive forces. (cohesion between the water and adhesion between the water and the xylem walls)
5 - the upward movement of the water from the xylem in the roots maintains the water potential gradient across the root cells, providing the mechanism for water uptake from the soil

Question 12

what evidence is there for the Cohesion-Tension theory?

Answer 12

• as the rate of transpiration increases the diameter of the tree trunk and branches decreases
• evaporation of water from the leaves draws water from the xylem by osmosis, water is pulled up the xylem creating a tension
• the tension pulls in the walls of the xylem vessels so the overall diameter of the trunks and branches decreases

Question 12

describe the process of translocation in the phloem (mass flow hypothesis)

Answer 12

• photosynthesis in the leaf produces glucose that is turned into sucrose via a condensation reaction and sucrose is actively transported into the sieve tubes by companion cells
• this lowers the water potential of the sieve tubes = water enters from the xylem via osmosis = high hydrostatic pressure in the sieve tubes
• in the sink, sugars are being used and so sucrose diffuses from the phloem to the sink = higher water potential in the sieve tubes
• water moves out via osmosis = lower hydrostatic pressure at the sink therefore photosynthetic products are transported down the pressure gradient from source to sink by mass flow

Question 12

describe the structure of the phloem

Answer 12

• sieve elements are joined end to end to form sieve tubes
• the end walls of sieve elements are known as sieve plates as they possess pores
• next to each sieve elements is a companion cell with dense cytoplasm and many mitochondria

Question 13

how do mature sieve elements differ from normal sieve elements

Answer 13

mature sieve elements do not possess a nucleus and the cytoplasm which is around the edge of the cell contains few organelles

Question 13

what is a source and a sink in plants?

Answer 13

source - where organic molecules ( sucrose , amino acids, fatty acids, etc) are made

sink - where the organic molecules are transported to

Question 14

what are some pieces of evidence supporting the mass flow hypothesis

Answer 14

• cutting the stem of a plant results in the phloem sap being released indicating hydrostatic pressure in sieve tubes
• lowering the temperature or the use of respiratory inhibitors reduce the rate of translocation indicating an active transport mechanism is involved

Question 14

how is ringing used as evidence for the transport of photosynthetic products in the phloem

Answer 14

• a complete ring of the phloem is removed from the trunk of a tree
• after ringing a tree trunk, a slight swelling develops above the ring
• the swelling was linked to the build up of photosynthetic product which were presented from being transported past the ring due to the lack of phloem

Question 14

what are some pieces of evidence against the mass flow hypothesis

Answer 14

• the specific structure of sieve tubes and sieve plates is not required for mass flow
• in young phloem tissue , substances have been observed moving in opposite directions in the same sieve tube