PLANT RESPIRATION

Waipa Post, Volume 49, Issue 3555, 8 December 1934, Page 10

PLANT RESPIRATION

INVESTIGATION BY STUDENTS

It is almost one hundred years since the great French chemist Lavoisier -discovered, the nature of the chemical reactions that go on when a candle burns, and showed their essential similarity to the reactions from which the energy, of living organisms is mainly derived. Candles are made of organic substances whose molecules contain carbon and hydrogen, and, when the candle burns, these molecules, are destroyed and the carbon and hydrogen unite with oxygen from the air to form a gas, carbon dioxide, and water. This process is called oxidisation, and in this process a. large amount of energy is set free; in the burning of the candle this energy takes the form of light and heat. In the cells of living- organisms an essentially similar process occurs, and complex organic substances are broken down into carbon dioxide and water, with the liberation of energy. The living cells of animals and of plants are specialised, however, so that they oxidise only certain substances. A motor car will not run on milk, any more than a man can derive nourishment from petrol. In animals and practically all plants the substance oxidised is a simple sugar .glucose. It forms the substrate for the respiratory reactions. Even the apparently complicated process of alcoholic fermentation which is carried out by the yeast plants is only a modified v form of oxidisation in which a simple sugar, glucose, is the substrate.

It has been pointed out that the essential feature in respiration is the production of energy which is ujtilised in movement, or in miaintaining the temperature of the organism), and, especially in plants, the energy is utilised to build up other substances, such as those which form the framework and living protpiplasm of the plant. PECULIAR BACTERIA. But while glucose forms the substrate in most plants, other substances are utilised, especially by certain bacteria. The bacteria are primitive, yet very specialised, plants. One of the commonest of these peculiar bacteria is Beggiatoa, which lives in waters containing sulphuretted hydrogen —an evil-smelling gas, the smell of which is like that of rotten eggs. The gas is fonnd in some mineral springs and in waters contaminated by sewage. The sulphuretted hydrogen is oxidised to free sulphur and ultimately to sulphuric acid by Beggiatoa. In these reactions onergiy is liberated and the bacteria utilise this energy to build up carbohydrates frdm cai'bon dioxide dissolved in the water. Another '£ sulphur bacterium," lives in a very specialised medium 1 , and oxidises sodium: thiosulphate—the " hypo " of photographers deriving energy from this reaction. Another groujp of bacteria carries out the oxidisation cif iron—another process liberating energ]y. These forms are not uncommon, and sometimes cause considerable trouble in water pipes, which become clogged with the particles of iron hydroxide formed by the bacteria.

Perhaps the strangest of all the bacteria, however, are certain forms which live in marshes or bogs. The suibptrfates for respiration in these cases are not solid compounds, but gases. One group ,is able to oxidise hydi-ogen to water, and another group converts marsh gas (methane) into carbon dioxide and water. IMPORTANT GROUP. But by far the most important group of bacteria whicli have unubual

types of respiration are the " nitrogen bacteria." These obtain the necessary energy for movement and growth from the oxidation of nitro-gen-containing compounds. They are of very great economic importance, for they play a part in the circulation of nitrogen in nature. The bulk of living mtaterial consists of complex nitrogen compounds called! proteins, which are built up ultimately from tlhje simple nitrates which are absorbed in solution by the roots of plants. In animals the waste products of nitrogen metabolism are excreted in the urine as urea, which cannot be absorbed by plants. When the plant or animal dies, the proteins of its bodies are also not available to plants. They represent great stores of nitrogen which are locked up, as it were, and not in circulation. But actually waste is averted, owing to the activities of bacteria. One set of bacteria breaks down tine urea or the proteins into ammonia. This amlmonia is used as a respiratory substrate by one group of soil bacteria and oxidised to nitrates yielding energy in the process which is utilised by the bacteria . Once nitrite has been formed yet another bacterium oxidises the nitrite into a nitrate, which is the usual form in which nitrogen is absorbed by the plant. Still another group of the nitrogen bacteria is able to " fix " atmospheric nitrogen—that is, to. oxidise the nitrogen of the air to a nitrate. This is done by certain bacteria (as Azotobacte), which live in the soil, and also by others which live in nodules on the roots of plants belonging to the pea family. The latter pass, on the nitrate to the pea plant in exchange for carbohydrates. The importance of this process is tremendous, since the nitrogen content of an impoverished soil may in large meiasure be restored by moans, cf these small plants. The process is a rather' remiarkable one, for nitrogen, which forms about foui'-fifths of the atmosphere, is a very inert gas from a chemical point of view. ATOM'S ADVE'NTUKOUS CAIR'EER. - From this outline one can see that a single nitrogen atom may have a very adventurous career. One can imagine it first forming part of the atmosphere, then being captured by one cf the nitrogen fixing bacteria, and being handed on to a green plant to be built up into a plant protein. This portion may pass into the bodiy of a herbivorous animal and ultimately pass to the soil as urea. It is there attacked by fermenting bacteria which form, ammonia from urea, and ultimately the amnnonia is oxidised to nitrate, which, again enters the plant. Alternatively, the nitrate might meet one of the bacteria which extract oxygen, in which case the nitrogen atclm would be set free hgain in the atmosphere, and the cycle would be complete. But, however important froni the economic point of view these various activities of bacteria may be, from the point of view cf the plant they are simply respix'atory processes; and the essential feature of respii'ation is a chemical reaction which liberates energy which is necessary for the vital processes of the'plant.—J.G.W. in the Melbourne Age.