Gas exchange in leaves

Aqa book p182-3

Homework

Green and Red q p.179Green and red p.183

Objectives

• How do plants exchange gases?• What is the structure of

dicotyledonous plant leaf?• How is the leaf adapted for efficient

gas exchange

Cell structure of a leafThe palisade cells are in theuppermost layers of the leaf

epidermis

palisade cell ( photosynthesis)

vessel (carries water)

stoma (admits air)

Label your diagramWaxy cuticle

Palisade mesophyll

Spongy mesophyll

What is the structure of dicotyledonous plant leaf and how do plants exchange

gases?1. Label your diagram with colour to

show the passage of gases from page 183

2. By which process does carbon dioxide move into a leaf?

3. List the features by which a leaf is adapted for efficient gas exchange.

4. The leaves are sometimes called the lungs of a plant. Give some reasons why this is true/false

Leaves like lungs?

• Yes• Site of gas exchange• Leaf flat, air spaces between spongy cells

so large surface area for gas exchange• Permeable so moist

• No• Leaves do not ventilate• No blood supply • Concentration gradient not maintained in

the same way

Compensation point…

Stomata

• Minute pores usually on lower epidermis.

• Why needed?• Why on lower leaf?• Name a plant where they are on the

upper epidermis…

Why are stomata on the

underside of leaves?• Less solar

radiation• Less wind• Therefore

less water loss

How do guard cells work?

• When the cell is turgid the uneven cellulose cell wall causes it to become curved so it opens the pore

• When the cell is flaccid the cell is not curved and the pore closes • Draw fig3 a and b page 183• Add to top - they close in dark/wilting and cells will be flaccid• Add to bottom – open in light for gas exchange cells turgid

How do guard cells work?

Light causes guard cell

chloroplasts to photosynthesise

Photosynthesis produces glucose

Lowers water potential so

water moves in by osmosis

Guard cells become turgid so opens stoma

This is not the whole story…

Observation of leaf sections

• Privet slides

Gas Exchange in The Leaf

The spongy mesophyll layer of the leaf, with its large air spaces and

thin-walled cells is the principal gas exchange surface within the leaf

The spongy mesophyll layer is in close contact with numerous pores or stomata, across which

gases enter and leave the leaf along steep concentration gradients

airspace

spongy mesophylllayer

waxy cuticleupper epidermis

palisade mesophylllayer

lower epidermis

guard cells

stoma

xylem vessel

Leaf blades display a large surface area and are very thin; the depth of the leaf is extremely small, providing a system that offers short diffusion paths for the

exchange of gases; leaves are held outwards from the parent plant

allowing for air movements to maintain the gaseous diffusion gradients between

the inside and outside of the leaves

The cells of the spongy mesophyll layer are loosely packed, creating

numerous air spaces that provide a large surface area for gas exchange

Gases dissolve in the film of water that covers the

saturated walls of the mesophyll cells, and are exchanged between the

cells and the intercellular air spaces

Gases diffuse into and out of the intercellular

spaces through numerous tiny pores

called stomata

loosely packed cells with many

air spaces

thin, permeablecell walls

air space

Oxygen and carbon dioxide diffuse into and out of the leaf

through the stomata along their concentration gradients

The concentration gradients for these gases is dependent upon their concentration within the

intercellular spaces of the mesophyll layer which, in turn, is dependent on the rates of cellular

respiration and photosynthesis

Gas exchange at night is concerned only with the process of respiration

As the mesophyll cells respire, oxygen is taken up into the cells

from the intercellular spaces, and carbon dioxide is released

Oxygen concentrations, within the intercellular spaces,

fall below that of the air surrounding the leaf thereby

creating a concentration gradient that maintains a

continual flow of oxygen into the leaf throughout the night

Carbon dioxide concentrations, within the intercellular spaces,

increase as respiration proceeds; carbon dioxide

diffuses out of the leaf along its concentration gradient

Gas exchange during the day is concerned with both photosynthesis and respiration

Under ideal conditions, the rate of photosynthesis exceeds the rate of respiration for most of

the daylight hoursCarbon dioxide gas, produced by the respiring cells, is used in the process of photosynthesis,

and the oxygen produced during photosynthesis is used in the process of respiration

As photosynthetic rates generally exceed respiratory rates, then the

demand for carbon dioxide gas by photosynthesising cells is greater than that supplied by respiration;

carbon dioxide levels in the intercellular spaces is lower than

that in the mesophyll cells and the gas diffuses into the cells

Gas exchange during the day is concerned with both photosynthesis and respiration

The continuous removal of carbon dioxide, from the

intercellular spaces into the photosynthesising cells,

maintains a concentration gradient that results in the

diffusion of this gas from the surrounding air into the leaf

In a similar way, the oxygen generated by photosynthesis

exceeds the respiratory demand of the mesophyll cells; the

oxygen concentration in the intercellular spaces exceeds

that of the surrounding air, and oxygen diffuses out of the leaf

along its concentration gradient

• Large surface area to volume ratio of the leaf blades, together with the large surface area provided by the numerous air spaces within the mesophyll layer, maximise diffusion rates

• Steep concentration gradients for oxygen and carbon dioxide are maintained by the diffusion of these gases into and out of the mesophyll cells as photosynthesis and respiration proceed; the large numbers of stomata provide a system that links the internal air spaces with the external atmosphere, allowing for gas exchange with the environment

• Leaves are held out into the atmosphere by their supporting stalks, allowing for air movements to increase diffusion gradients across the stomata

• The short diffusion path is the result of the thin, permeable cell walls of the spongy mesophyll cells together with the short depth of the mesophyll layer in contact with the external environment