14.1 Do Plants Breathe?

Well, the answer to this question is not quite so direct. Yes, plants require O2 for respiration to occur and they also give out CO2.

Hence, plants have systems in place that ensure the availability of O2. Plants, unlike animals, have no specialised organs for gaseous exchange but they have stomata and lenticels for this purpose.

There are several reasons why plants can get along without respiratory organs. First, each plant part takes care of its own gas-exchange needs.

There is very little transport of gases from one plant part to another. Second, plants do not present great demands for gas exchange. Roots, stems and leaves respire at rates far lower than animals do.

Only during photosynthesis are large volumes of gases exchanged and, each leaf is well adapted to take care of its own needs during these periods. When cells photosynthesise, availability of O2 is not a problem in these cells since O2 is released within the cell.

Third, thedistance that gases must diffuse even in large, bulky plants is not great. Each living cell in a plant is located quite close to the surface of the plant. ‘This is true for leaves’, you may ask, ‘but what about thick, woody stems and roots?’ In stems, the ‘living’ cells are organised in thin layers inside and beneath the bark.

They also have openings called lenticels. The cells in the interior are dead and provide only mechanical support.

Thus, most cells of a plant have at least a part of their surface in contact with air. This is also facilitated by the loose packing of parenchyma cells in leaves, stems and roots, which provide an interconnected network of air spaces.

The complete combustion of glucose, which produces CO2 and H2O as end products, yields energy most of which is given out as heat.

If this energy is to be useful to the cell, it should be able to utilise it to synthesise other molecules that the cell requires. The strategy that the plant cell uses is to catabolise the glucose molecule in such a way that not all the liberated energy goes out as heat.

The key is to oxidise glucose not in one step but in several small steps enabling some steps to be just large enough such that the energy released can be coupled to ATP synthesis. How this is done is, essentially, the story of respiration.

During the process of respiration, oxygen is utilised, and carbon dioxide, water and energy are released as products. The combustion reaction requires oxygen. But some cells live where oxygen may or may not be available.

Can you think of such situations (and organisms) where O2 is not available? There are sufficient reasons to believe that the first cells on this planet lived in an atmosphere that lacked oxygen.

Even among present-day living organisms, we know of several that are adapted to anaerobic conditions. Some of these organisms are facultative anaerobes, while in others the requirement for anaerobic condition is obligate.

In any case, all living organisms retain the enzymatic machinery to partially oxidise glucose without the help of oxygen. This breakdown of glucose to pyruvic acid is called glycolysis.

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