Gas Exchange in Plants

In order to carry on photosynthesis, green plants need a supply of carbon dioxide and a means of disposing of oxygen. In order to carry on cellular respiration, plant cells need oxygen and a means of disposing of carbon dioxide (just as animal cells do).

Unlike animals, plants have no specialized organs for gas exchange (with the few inevitable exceptions!). The are several reasons they can get along without them:

Each part of the plant takes care of its own gas exchange needs. Although plants have an elaborate liquid transport system, it does not participate in gas transport.
Roots, stems, and leaves respire at rates much lower than are characteristic of animals. Only during photosynthesis are large volumes of gases exchanged and each leaf is well adapted to take care of its own needs.
The distance that gases must diffuse in even a large plant is not great. Each living cell in the plant is located close to the surface. While obvious for leaves, it is also true for stems. The only living cells in the stem are organized in thin layers just beneath the bark. The cells in the interior are dead and serve only to provide mechanical support.
Most of the living cells in a plant have at least part of their surface exposed to air. The loose packing of parenchyma cells in leaves, stems, and roots provides an interconnecting system of air spaces. Gases diffuse through air several thousand times faster than through water. Once oxygen and carbon dioxide reach the network of intercellular air spaces (arrows), they diffuse rapidly through them.

Leaves

The exchange of oxygen and carbon dioxide in the leaf (as well as the loss of water vapor in transpiration) occurs through pores called stomata (singular = stoma).

Normally stomata open when the light strikes the leaf in the morning and close during the night.

The immediate cause is a change in the turgor of the guard cells. The inner wall of each guard cell is thick and elastic. When turgor develops within the two guard cells flanking each stoma, the thin outer walls bulge out and force the inner walls into a crescent shape. This opens the stoma. When the guard cells lose turgor, the elastic inner walls regain their original shape and the stoma closes.

Time	Osmotic Pressure, lb/in²
7 A.M.	212
11 A.M.	456
5 P.M.	272
12 midnight	191

The table shows the osmotic pressure measured at different times of day in typical guard cells. The osmotic pressure within the other cells of the lower epidermis remained constant at 150 lb/in². When the osmotic pressure of the guard cells became greater than that of the surrounding cells, the stomata opened. In the evening, when the osmotic pressure of the guard cells dropped to nearly that of the surrounding cells, the stomata closed.

Opening stomata

The increase in osmotic pressure in the guard cells is caused by an uptake of potassium ions (K⁺). The concentration of K⁺ in open guard cells far exceeds that in the surrounding cells. This is how it accumulates:

ATP, generated by the light reactions of photosynthesis, drives a proton pump in the plasma membrane of the guard cell.
As protons (H⁺) are pumped out of the cell, its interior becomes increasingly negative.
This attracts additional potassium ions into the cell, raising its osmotic pressure.

Closing stomata

Although open stomata are essential for photosynthesis, they also expose the plant to the risk of losing water through transpiration. Some 90% of the water taken up by a plant is lost in transpiration.

ABA is the hormone that triggers closing of the stomata when soil water is insufficient to keep up with transpiration (which often occurs around mid-day).

The mechanism:

ABA binds to receptors at the surface of the plasma membrane of the guard cells.
This initiates
- a rise in pH in the cytosol
- the formation of the "second messenger", cyclic ADP ribose (cADPR)
Increased pH stimulates the loss of K⁺ and organic ions from the cell
Rising levels of cADPR cause Ca²⁺ to move from the vacuole to the cytosol which blocks the uptake of K⁺ into the guard cell
The combined effects result in a loss of solutes in the cytosol.
This reduces the osmotic pressure of the cell and thus turgor.
The stomata close.

Roots and Stems

Woody stems and mature roots are sheathed in layers of dead cork cells impregnated with suberin - a waxy, waterproof (and airproof) substance. So cork is as impervious to oxygen and carbon dioxide as it is to water.

However, the cork of both mature roots and woody stems is perforated by nonsuberized pores called lenticels. These enable oxygen to reach the intercellular spaces of the interior tissues and carbon dioxide to be released to the atmosphere.

The photo shows the lenticels in the bark of a young stem.

In many annual plants, the stems are green and almost as important for photosynthesis as the leaves. These stems use stomata rather than lenticels for gas exchange.

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15 March 1999