Gas exchange in plants

What are the parts of an angiosperm leaf?

Cuticle, upper epidermis, lower epidermis, palisade mesophyll, air spaces, spongy mesophyll, guard cells, stomata

What is the cuticle?

A waxy waterproof layer which reduces water loss in the plant. It is transparent to enable light to penetrate the mesophyll cells

What is the upper and lower epidermis?

Protects the tissue layers inside the leaf, and is transparent to enable light to penetrate

What is the palisade mesophyll?

Many cells fit in this layer to maximise absorption of light, and they have a large number of chloroplasts to increase absorption of light and the rate of photosynthesis

What is the spongy mesophyll?

Cells contain some chloroplasts, and there are a large number of air spaces for gas exchange within the leaf. Diffusion is entirely required in the leaf as there is no ventilation mechanism

What are the air spaces?

These allow gas exchange of carbon dioxide and oxygen for respiration and photosynthesis to occur. Water vapour evaporates into these spaces to maintain transport of water from the roots to the leaves

What is the xylem?

Transports water and mineral ions from the roots to all other parts of the plant

What is the phloem?

Transports carbon compounds (sucrose and amino acids) from the site of photosynthesis and storage organs (the roots) to the rest of the plant, including growing regions

What are bundle sheath cells?

These provide support and protection to the vascular bundles

What are the stomatal pores?

The site of gas exchange to the external environment. Water vapour is lost when the leaves are open

What are the guard cells?

These control the opening and closing of the stomata, and contain chloroplasts

What are chloroplasts?

The site of photosynthesis and are involved in the mechanism for stomatal opening and closing

How does exchange in plants occur?

Plants rely on diffusion for gas exchange. During the day, plants photosynthesise and respire. Most of the carbon dioxide for photosynthesis diffuses into the leaves from the atmosphere, but some comes from respiration. Most of the oxygen produced in photosynthesis diffuses out of the leaf.

What is a dicotyledon?

These plants grow two leaves (dicots) during their baby stage - monocots only grow one

How are dicotyledons adapted for gas exchange?

Large surface area provided by dicot leaves - the large differences in concentration for O2 and CO2 between the leaf interior and exterior environment produces a high value for Fick’s Law to increase the rate of diffusion. The thinness of the leaf provides a short diffusion distance for gases from the environment to the photosynthetic cells of the palisade layer allows for a rapid rate of diffusion of CO2 gas to the photosynthesising cell (essential for efficient photosynthesis). The extensive network of air spaces in the spongy mesophyll layer provides an easy passage of gases to and from the palisade cells and an efficient exchange system. Stomata are where gases enter and leave the leaf - their opening and closing is controlled by guard cells to reduce water loss and maximise efficient gas exchange

Where are stomata found?

Mainly on the lower surface of the leaf (except for water lilies)

When do stomata open?

During the day since there is light for photosynthesis - they close at night to reduce water loss and when there is no light to photosynthesise by. During periods of drought and stress, stomata might close during the day

What is malate theory?

In light, chloroplasts in the guard cells photosynthesise and the stomata opens by this mechanism; ATP from photosynthesise enables potassium ions (K+) to be pumped from the epidermal cells into the guard cells by active transport. Starch previously stored in the guard cells is converted to the organic ion ‘malate’ (C4H6O5 2-). Water potential in guard cells lowers during these ions dissolving. Guard cells become turgid. The inner wall is thicker than the outer wall, making it inelastic, so pairs of cells curve away from each other and the pore opens. In the dark, this process is reversed