Gas Exchange in Plants

Plants obtain the gases they need through their leaves. They require oxygen for respiration and carbon dioxide for photosynthesis.

The gases diffuse into the intercellular spaces of the leaf through pores, which are normally on the underside of the leaf - stomata. From these spaces they will diffuse into the cells that require them.

Stomatal opening and closing depends on changes in the turgor of the guard cells. Water flows into the guard cells by osmosis, their turgor increases and they expand. Due to the relatively inelastic inner wall, the guard cells bend and draw away from each other, so the pore opens. Opposite would happen if water left the plant.

Gas Exchange in Insects

Insects, being larger and having a hard, chitinous and therefore impermeable exoskeleton, have a more specialised gas exchange system.Insects have no transport system so gases need to be transported directly to the respiring tissues.There are tiny holes called spiracles along the side of the insect.The spiracles are openings of small tubes running into the insect's body, the larger ones being called tracheae and the smaller ones being called tracheoles. The ends of these tubes, which are in contact with individual cells, contain a small amount of fluid in which the gases are dissolved. The fluid is drawn into the muscle tissue during exercise. This increases the surface area of air in contact with the cells. Gases diffuse in through the spiracles and down the tracheae and tracheoles.

Ventilation movements of the body during exercise may help this diffusion.The spiracles can be closed by valves and may be surrounded by tiny hairs. These help keep humidity around the opening, ensure there is a lower concentration gradient of water vapour, and so less is lost from the insect by evaporation.

Gas Exchange in Plants

The guard cells lower their water potential to draw in water from the surrounding epidermal cells, by actively accumulating potassium ions. This requires energy in the form of ATP which, is supplied by the chloroplasts in the guard cells. Respiration occurs throughout the day and night, providing the plant with a supply of energy. Photosynthesis can only occur during sunlight hours so it stops at night. A product of respiration is carbon dioxide. This can be used directly by the plant in photosynthesis. However, during the day, photosynthesis can be going 10 or even 20 times faster than respiration (depending on light intensity), so the stomata must stay open so that the plant has enough carbon dioxide, most of which diffuses in from the external atmosphere.