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CSEC Biology: Gaseous Exchange in Flowering Plants

Plants have different needs in gaseous exchange than humans due to the fact that they photosynthesize. Therefore, they need to take in a constant stream of carbon dioxide and release oxygen. But, just because the plant photosynthesizes doesn't mean that respiration doesn't occur. As a matter of fact, cellular respiration occurs in every living cell- and the same goes for plants.

So, they also need to be able to be able to take in oxygen and dispose of carbon dioxide. However plants do not have specialized organs for gaseous exchange, but instead, each part of the plant satisfies its own gaseous exchange requirements. So, the gaseous exchange surfaces of the plant are the cell walls and cell membranes of the living cells.

How can they survive without specialized gaseous exchange organs?

  1. As stated before, each part of the plant takes care of its own gaseous exchange. So, even though liquid transport occurs in plants, there is no need for any transport of gases like in humans.

  2. The parts of the plant also respire at very low rates (since they are sessile/immobile organisms). The only time that gaseous exchange occurs at large volumes is during photosynthesis.

  3. Gases do not have to diffuse far, as the cells involved in gaseous exchange are near the surface of the plant. It is true for leaves, and though it may not seem like it, it is also true for stems. The only living cells in the stem are in layers near the bark. The inner cells die at maturity, and are only there to provide mechanical support and to transport materials.

  4. The living cells in a plant are positioned so to have at least part of their surfaces exposed to air. The parenchyma cells in leaves, stems, and roots are packed loosely, with air spaces in between. Gases diffuse through air exponentially faster than they do through water. Once oxygen and carbon dioxide reach the network of intercellular air spaces, they diffuse rapidly through them.

  5. The two gases, oxygen and carbon dioxide can also pass through the cell walls and plasma membranes of the plant cells by diffusion. (read more: source at libretexts)

Gases will usually diffuse between the atmosphere and the air spaces through the stomata in the leaves and the lenticels (small areas of loosely packed cells) of the stems and roots.

The direction of the movement of oxygen and carbon dioxide depends on if the organ in question is photosynthesizing or not.

Gaseous Exchange in Photosynthesizing Organs of the Plant

The exchange of oxygen and carbon dioxide (as well as water vapour in transpiration) in the leaf occurs through the stomata.

The stomata are controlled by the guard cells, which open during the day and close more at night. So, the gaseous exchange in photosynthesizing organs depends on the time of day.

The rate of respiration of the cells of the organ remain the same throughout the day, but the rate of photosynthesis will fluctuate depending on whether the stomata are open or not.

You will see at dawn and dusk, the compensation point is reached. This is where respiration and photosynthesis are occurring at the same rate. So, there is no net movement of gases into or out of the leaf.

Gaseous Exchange in Photosynthesizing Organs of the Plant

Since these organs don't photosynthesize, only respiration occurs. So, oxygen diffuses in across the cell wall and membrane and carbon dioxide diffuses out.

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