Transport In Plants - Transpiration

Question 1

Define transpiration

Answer 1

Loss of water vapour from the upper parts of the plant, especially the leaves

Question 2

Define the transpiration stream

Answer 2

The flow of water through a plant from roots to leaves in the xylem vessels

Question 3

What is evaporation limited by

Answer 3

Waxy cuticle

Question 4

What are the three main processes that transpiration can be split into

Answer 4

1) Osmosis
2) Evaporation
3) Diffusion

Question 5

Describe the first stage of transpiration

Answer 5

• Water enters the leaves travelling in the xylem.
• It then passes into the mesophyll cells by osmosis.
• Water may move from cell to cell by the symplast and apoplast pathway

Question 6

Describe the second stage of transpiration

Answer 6

• the water evaporates from the surface of the mesophyll cells to form water vapour
• water vapour collects in the air spaces between mesophyll cells raising the water potential

Question 7

Describe the third stage of transpiration

Answer 7

• Once the water vapour potential is higher inside the leaf than outside water molecules will diffuse out of the leaf through the stomata
• water vapour is carried away from the leaf by air movements

Question 8

how is the water potential gradient maintained

Answer 8

the evaporation of water from cells lowers the water potential

Question 9

give adaptations of guard cells

Answer 9

• only epidermal cells containing chloroplasts
• in daylight, stomata opens
  chloroplasts make sugars
  guard cells actively pump in K+ ions
• thicker inner walls stretch less (cells curve)
• hoops of cellulose microfibrils prevent cells getting wider when they swell

Question 10

why do the stomata close if the plant is losing a lot of water

Answer 10

water potential outside guard cells is lower, so water leaves

Question 11

why do stomata close at nighttime

Answer 11

less photosynthesis, less sugars, higher water potential, water leaves guard cells

Question 12

how can you estimate the rate of transpiration

Answer 12

measure water uptake and estimate water loss (as around 99% is lost in transpiration)

Question 13

what do you use to measure water uptake

Answer 13

potometer

Question 14

briefly describe how to measure water uptake using a potometer

Answer 14

• fill apparatus with water, ensuring no air bubbles
• take healthy shoot and cut underwater at a slant
• keeping everything underwater, insert shoot into apparatus
• dry leaves and allow to acclimatise
• adjust water in the capillary tube to the start of the scale
• keep conditions constant and measure how far water moves in a set period of time

Question 15

how do you calculate volume of water uptake

Answer 15

area of capillary tube x distance water moved

Question 16

how do you calculate rate of water uptake

Answer 16

volume of water moved/time taken

Question 17

give all the precautions when using a potometer (7)

Answer 17

• ensure no air bubbles in the system
• make all joints watertight
• ensure shoot is fresh and healthy
• cut shoot underwater and don’t expose to air
• cut stem at an angle
• dry the leaves
• allow shoot to acclimatise to each condition before taking results

Question 18

why do we ensure no air bubbles

Answer 18

air bubbles would block the continuous column of water

Question 19

why do we make all joints watertight

Answer 19

stop water leaking, giving valid results

Question 20

why do we ensure shoot is fresh and healthy

Answer 20

ensures water will slow and transpiration rate is normal

Question 21

why do we cut shoot underwater and not expose to air

Answer 21

less chance air will enter the xylem and block continuous column of water

Question 22

why do we cut stem at an angle

Answer 22

increases surface area for water uptake

Question 23

why do we dry the leaves

Answer 23

maintain the water vapour potential gradient for transpiration

Question 24

why do we allow shoot to acclimatise

Answer 24

plant can adjust to new conditions, so readings are valid

Question 25

what are the factors affecting the transpiration rate (8)

Answer 25

- number of leaves - number, size and position of stomata - presence of cuticle - light - temperature - relative humidity - air movement/wind - water availability

Question 26

how does number of leaves affect transpiration rate

Answer 26

more leaves, more stomata, higher rate

Question 27

how does number, size and position of stomata affect transpiration rate

Answer 27

- more stomata, higher rate - if stomata are only on lower surface, reduces rate - larger size, higher rate

Question 28

how does presence of a cuticle affect transpiration rate

Answer 28

thicker cuticle reduces water loss and gives lower rate

Question 29

how does light affect transpiration rate

Answer 29

greater light intensity, more open stomata, more water vapour diffusing out, higher rate

Question 30

how does temperature affect transpiration rate

Answer 30

- greater temp, higher kinetic energy of water molecules, more movement and higher rate - greater temp, more evaporation of water, greater water vapour potential gradient, higher rate

Question 31

how does relative humidity affect transpiration rate

Answer 31

greater relative humidity, reduced water vapour potential gradient, lower rate

Question 32

how does air movement/wind affect transpiration rate

Answer 32

greater air movement, water vapour moved away more quickly, increases water vapour potential gradient, increases rate

Question 33

how does water availability affect transpiration rate

Answer 33

more water available, higher rate

Question 34

explain why the measurements obtained from a potometer are only an estimation of rate of transpiration

Answer 34

measures water uptake, some water may be used in photosynthesis

Question 35

state the cause of transpiration in plants

Answer 35

evaporation of water