Transpiration + Effect of Environmental Conditions

Question 1

how does water move through the leaves?

Answer 1

via osmosis

Question 2

why is the epidermis covered in a waxy cuticle?

Answer 2

so it is impermeable to water

Question 3

what does most water pass out of the stomata as?

Answer 3

water vapour

Question 4

how does water leave the stomata?

Answer 4

• water leaves the cells of the mesophyll and evaporates into the air spaces between the spongy mesophyll
• the water vapour then diffuses out through the stomata

Question 5

what does transpiration cause?

Answer 5

• the loss of water vapour from the leaves
• transpiration causes water to be pulled up the xylem in the stem and roots in a continuous flow known as the transpiration stream

Question 6

what is the definition of transpiration?

Answer 6

transpiration is the evaporation of water from the surface of a plant

Question 7

what are four functions of the transpiration stream?

Answer 7

• to carry mineral ions to the leaves for the synthesis of amino acid and chlorophyll
• to keep turgor pressure high in leaf cells, holding the leaves up
• evaporation cools the leaves
• supplies water for photosynthesis

Question 8

how does temperature affect transpiration?

Answer 8

• on a hot day, water will evaporate quickly from the leaves of a plant as the water molecules have more kinetic energy
• transpiration will therefore increase as temperature increases

Question 9

how does humidity affect transpiration?

Answer 9

• very humid air contains a great deal of water vapour - there is a smaller concentration gradient, so transpiration slows down
• in dry air, the diffusion of water vapour from the leaf to the atmosphere will be rapid
• transpiration therefore increases if humidity decreases

Question 10

how does wind speed affect transpiration?

Answer 10

• in still air, the region around a transpiring leaf will become saturated (full) with water vapour so that no more can escape from the leaf - causing transpiration to slow down
• in moving air, the water vapour will be blown away from the leaf as fast as it diffuses out - this will speed up transpiration
• transpiration therefore increases as wind speed increases

Question 11

how does light intensity affect transpiration?

Answer 11

• light itself does not affect evaporation, but in daylight the stomata of the leaves are open to supply carbon dioxide for photosynthesis
• this allows more water to diffuse out of the leaves and into the atmosphere

Question 12

how does water supply affect transpiration?

Answer 12

• if water is in short supply, and the plant is losing water by transpiration faster than it is being taken up by the roots
• then the plant may start to wilt
• before this happens the guard cells become flaccid and stomata start to close, therefore reducing transpiration and delaying wilting

Question 13

what are adaptations of the stomata?

Answer 13

• the stomata are holes in the surface of the leaf
• they are mainly found on the bottom surface
• there are two guard cells on either side of the stomata - these are sausage shaped cells which control the opening and closing of the stomata
• each guard cell has a thickened, inflexible inner cell wall (on the stomatal side)

Question 14

what happens to guard cells in light?

Answer 14

they photosynthesis

Question 15

how do guard cells photosynthesis?

Answer 15

• the concentration of sugars increases, the water potential in the guard cells falls and so water moves into the guard cells by osmosis
• they become turgid (swollen) - this causes the guard cells to become banana shaped, due to the inflexible inner cell wall, and opens the stomata

Question 16

what happens to guard cells in the dark?

Answer 16

• photosynthesis stops
• as the sugar concentration falls (due to respiration), water potential increases and water moves out of the guard cells - they become flaccid and the stomata close

Question 17

what is the translocation of sugars?

Answer 17

the phloem move sugars from the leaves to other parts of the plant, where they are needed for growth

Question 18

what is the purpose of the sieve tube?

Answer 18

• the sieve tube is living
• it has a thin layer of cytoplasm at the edge of the cell - but it is mainly filled with sap (a concentrated solution of sucrose and amino acids)
• the sap flows through the sieve plates and through the phloem
• it does not have a nucleus - it is controlled and supported by the neighbouring companion cells

Question 19

what do the companion cells of the sieve tube do?

Answer 19

• these have lots of mitochondria and actively transport sucrose into the phloem
• this produces a lower water potential (due to the high sucrose concentration) and so water moves into the sieve tube by osmosis from nearby cells
• this creates a high pressure and pushes the sap towards areas where sucrose (and water) are removed from the phloem