Lesson 5 - Gas exchange in plants

Question 1

function waxy cuticle

Answer 1

reduces water loss from the leaf surface

Question 2

function air spaces

Answer 2

interconnecting spaces that run throughout the mesophyll layer, which are adapted to provide a network of gases to allow gases to quickly diffuse in and out of leaf and to access photosynthesising cells

Question 3

function palisade mesophyll cells

Answer 3

cells located beneath the upper epidermis to carry out photosynthesis

Question 4

function spongy mesophyll cells

Answer 4

dispersed and irregular-shaped cells located beneath the palisade mesophyll layer to carry out photosynthesis. They are adapted by having a large surface area to maximise diffusion

Question 5

function stomata

Answer 5

small pores on the underside of the leaf which open and close via guard cells to allow air to enter into leaf. The stomata open and close when it’s suitable for photosynthesis, allowing the diffusion of oxygen and carbon dioxide, and to minimise water loss

Question 6

function lower epidermis

Answer 6

bottom layer of leaf which contains stomata and guard cells

Question 7

function vascular tissue

Answer 7

transports water and nutrients

Question 8

how do plants minimise water loss

Answer 8

-Have a waterproof waxy cuticle on their leaves
-Have guard cells which close the stomata when needed

Question 9

meaning xerophytes

Answer 9

plants adapted to living in dry environments with limited water availability

Question 10

describe 6 adaptations of xerophytes to reduce water loss

Answer 10

-Thick waxy cuticle reduces water loss through evaporation
-Folding of leaves closes the stomata on the lower surface to reduce air flow and the evaporation of water
-Hairs on leaves traps the moist air against the leaf surface to reduce the concentration gradient of water vapour
-Sunken stomata in pits reduces air flow and the evaporation of water
-Small, needle-like leaves reduces the surface area across which water can be lost
-Water storage organs conserves water when it is in low supply

Question 11

meaning lenticles

Answer 11

spongy areas with loosely packed cells that are the site of gas exchange in woody stems and roots

Question 12

how do the stomatal pores open

Answer 12

when conditions are favourable for photosynthesis and carbon dioxide is needed, solutes such as potassium ions are moved into the guard cells by active transport. Water then moves into the guard cells from the surrounding epidermal cells by osmosis, and the guard cells swell so the turgor pressure increases. The stomatal pores open because the uneven bendings of the inner cellulose cell wall

Question 13

how do stomatal pores close

Answer 13

when conditions are less favourable for photosynthesis, or when it is dark, the active pumping of potassium ions into the cell stops and potassium ions are excreted. So, water leaves the cell by osmosis, turgor is reduced and the guard cells become flaccid, closing the stomatal pores

Question 14

why do plants not have advanced adaptations for gas exchange

Answer 14

plants have low metabolic rates, so low rates of gas exchange. This is because plants do not move and do not need to maintain a high body temperature

Question 15

how do gases move in and out of plants

Answer 15

• when the concentration of carbon dioxide inside the plant is low, it will diffuse in from the air, through stomata, into the air spaces of the spongy mesophyll.
• The carbon dioxide will dissolve in the water films surrounding the spongy mesophyll cells and can then freely diffuse through the cell membrane into the photosynthesising tissues

Question 16

when does gas exchange occur the most in plants

Answer 16

during the day, when there’s light

Question 17

how are leaves adapted for gas exchange

Answer 17

• thin, to decreases diffusion distance
• lots of stomata
• spongy mesophyll cells have irregular shapes, to increase their surface area
• spongy mesophyll cells have air spaces between them
• the surface of the spongy mesophyll cells are moisy

Question 18

how is water lost

Answer 18

water evaporates from the moist cell walls in contact with the air spaced in the leaf and is lost by transpiration

Question 19

describe the structure of guard cells

Answer 19

guard cells are specialised epidermal cells that contain sap vacuole. They contain chloroplast. The cell walls have a uneven distribution of cellulose, with the inner wall being thicker

Question 20

how do plant cells use potassium ions to open the stomata

Answer 20

in the presence of light, or in low carbon dioxide concentrations, potassium ions are actively transported into the guard cells from the neighbouring epidermal cells. This leads to a higher internal potassium ion concentration, making the cells have a more negative water potential. This causes them to take up more water by osmosis from the neighbouring cells. They become turgid and change shape because the inner cell wall is thicker than the outer one, which causes the stomata to open.

Question 21

compare gas exchange in the leaf of a plant to insects

Answer 21

• both have short diffusion distances
• both have pores which gases diffuse through
• minimise water loss by opening and closing pores
• plants have a larger SA:V ratio
• the main site of gas exchange in a leaf is the spongy mesophyll cells, whilst in insects it’s the tracheoles