transpiration

Question 1

explain how light affects transpiration rate

Answer 1

• faster light = faster transpiration
• stomata open in light so CO2 can diffuse into the leaf for photosynthesis
• water is let out down water potential gradient
• stomata close in dark so there is little transpiration

Question 2

define transpiration

Answer 2

loss of water vapour from leaves as a result of water evaporation from cell surfaces inside the leaf and then diffusion down a concentration gradient out of the stomata

Question 3

define transpiration stream

Answer 3

movement of water through a plant from the roots until it is lost by evaporation from leaves

Question 4

define transpiration pull

Answer 4

force that aids drawing in water upward from roots to leaves to replace water lost by evaporation

Question 5

why is water loss inevitable for plants?

Answer 5

• CO2 moves in + O2 moves out of leaf by diffusion thru stomata
• when stomata are open, water vapour moves out by diffusion and is lost - transpiration
• stomata open/close to control amount of water lost, during the day must be open for photosynthesis but during night closed

Question 6

define stomata

Answer 6

pores on surface of leaf or stem that open or close by guard cells

Question 7

define guard cells

Answer 7

cells that can open/close stomatal pores - control gas exchange and water loss

Question 8

describe how poisons are evidence for the cohesion-tension theory

Answer 8

if cut shoot is placed in cyanide water, uptake and transpiration continue as usual
- shows transpiration does not rely on living cells and is not an active process

Question 9

describe how dyes are evidence for the cohesion-tension theory

Answer 9

if leafy shoot is cut and dipped in dye, dye is visibly taken into xylem vessels
- supports idea that major water pathway is through xylem

Question 10

describe how the diameter of tree trunks are evidence for the cohesion-tension theory

Answer 10

over 24 hour period diameter is narrowest at midday and widest at midnight
- cohesive forces in xylem cause negative pressure that draws tissues in and reduces diameter bc highest rate of photosynthesis is at midday & lowest around midnight

Question 11

explain how guard cells can open/close stomata

Answer 11

• when turgor is low, asymmetric configuration of guard cell walls close the pore
• when env. conditions favourable, guard cells pump solutes by AT, increasing turgor, opening the pore

Question 12

explain how temperature affects transpiration rate

Answer 12

• high temp = faster transpiration
• warmer water molecules have more energy so evaporate from cells inside leaf faster
• this increases water potential gradient between inside and outside leaf - water diffuses out faster

Question 13

explain how humidity affects transpiration rate

Answer 13

• lower humidity = faster transpiration
• if air around plant is dry, water potential gradient between leaf and air is inc. = increased transpiration

Question 14

explain how wind affect transpiration rate

Answer 14

• more wind = faster transpiration
• air movement blows water molecules away from stoma
• this increases water potential gradient = increased transpiration rate

Question 15

define xerophytes

Answer 15

a plant adapted to live in extreme dry climates where water availability is low
- eg. cacti

Question 16

explain the ways in which xerophytes have adapted to prevent water loss

Answer 16

• thick waxy cuticles: waterproof layer so water cannot move through
• sunken stomata: shelters from wind
• reduced stomata #s = reduced gas exchange and water loss
• hairy leaves: traps moist air around stomata

Question 17

define hydrophilic plants

Answer 17

plants that live in aquatic environments and are adapted to cope with low oxygen levels
- eg. water lilies

Question 18

explain some ways in which hydrophilic plants have adapted to help them cope with low oxygen levels and survive in water

Answer 18

• air spaces in tissues: help them float to max. amount of light they receive + increase photosynthesis
• stomata present on upper side of leaves = max. gas exchange
• very thin/no waxy cuticle: no need to conserve water