The below mentioned article provides a study note on the respiration in plants.
Respiratory Substrates:
Respiratory substrates are those organic substances which are oxidized during respiration to liberate energy inside the living cells. The common respiratory substrates are carbohydrates, proteins, fats and organic acids.
The most common respiratory substrate is glucose. It is a hexose monosaccharide. Another related compound is fructose. Glucose is formed from storage carbohydrates like starch in most plants and glycogen in animals and fungi.
Fats are used as respiratory substrates by a number of organisms because they contain more energy as compared to carbohydrates. However, fats are not directly used in respiration. Instead they are first broken down to intermediates common to glucose oxidation, viz., acetyl CoA, glyceraldehyde phosphate.
Proteins are used in respiration only rarely, as during germination of protein rich seeds and spores. Proteins are hydrolysed to form amino acids from which organic acids are produced through deamination. In animals, excess amino acids are regularly delaminated to produce organic acids. Organic acids enter Krebs Cycle, e.g., aspartic acid, glutamic acid.
At other times, proteins are employed as respiratory substrates under starvation conditions only when carbohydrates and fats become unavailable.
Respiration involving proteins as respiratory substrate is called protoplasmic respiration as compared to floating respiration which uses carbohydrates and fats. Protoplasmic respiration cannot be continued for long as it depletes protoplasm of structural and functional proteins as well as liberates toxic ammonia.
Gaseous Exchange:
Lavoisier observed that in respiration of animals, oxygen is taken in from the air. In return they give out carbon dioxide and water. This O2-CO2 gaseous exchange occurs not only between the organism and its environment but also between every cell and its surrounding environment. An animal cell exchanges gases with the extracellular fluid while a plant cell does so with the air present in intercellular spaces.
An organism shows exchange of gases in a liquid or gaseous environment depending upon the habitat. However, some organisms, especially micro-organisms, do not require oxygen for their respiration. Some of them give out CO2 while a few do not do so. In these organisms there is no gaseous exchange.
Types of Respiration:
Respiration is of two main types, aerobic and anaerobic. Aerobic respiration is that type of respiration in which organic food is completely oxidized with the help of oxygen (as terminal oxidant) into carbon dioxide and water.
686 kcal or 2870 kJ of energy is liberated per mole of glucose. The value was previously calculated to be 673 kcal. One kcal is equal to 1000 calories. It is that amount of energy (as heat) which can raise the temperature of one litre of water by 1°C.
Anaerobic respiration is a type of respiration where oxygen is not used as an oxidant and the organic food is broken down incompletely to liberate energy without oxygen being used as oxidant. Energy is liberated during breaking of bonds between various types of atoms. The common products of anaerobic respiration are CO2, ethyl alcohol and lactic acid.
Anaerobic respiration is the only mode of respiration in some micro-organisms. In higher organisms it occurs as a temporary measure.
Anaerobic respiration cannot continue for long in higher organisms because:
(i) It yields small amount of energy,
(ii) More substrate is decomposed so that little is left for growth and repair,
(iii) Some of the end products and intermediates of anaerobic respiration are toxic in higher concentration,
(iv) Several physiological processes of higher organisms are linked to aerobic respiration, e.g., active absorption of minerals, protoplasmic streaming, etc.
Compensation Point:
It is that value or point in light intensity and atmospheric CO2 concentration when the rate of photosynthesis is just equivalent to the rate of respiration in the photosynthetic organs so that there is no net gaseous exchange. The value of light compensation point is 2.5-100 ft. candles for shade plants and 100-400 ft. candles for sun plants.
The value of CO2 compensation point is 25-100 ppm (25-100 µl.l-1) in C3 plants and less than 5 ppm (5 µl.l-1) in C4 plants. A plant cannot survive for long at compensation point because there is net loss of organic matter due to respiration of non-green organs and dark respiration.
Respiratory Quotient (RQ):
Respiratory quotient is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed in respiration over a period of time. Its value can be one, zero, more than 1 or less than one.
RQ Equal to Unity:
Respiratory quotient is unity if carbohydrate is respiratory substrate and the respiration is aerobic.
RQ Less than Unity:
RQ is less than one when respiration is aerobic but the respiratory substrate is either fat or protein. RQ is about 0.7 for most of the common fats. It occurs during germination of fatty seeds.
RQ is about 0.9 in case of proteins, peptones, etc.
R.Q. Zero:
Succulents do not evolve carbon dioxide during night (when their stomata are open) as the same is used in carbon fixation. They also change carbohydrates to organic acids which utilise oxygen but do not evolve carbon dioxide.
RQ More than Unity:
(a) RQ slightly more than unity is found when organic acids are broken down as respiratory substrates under aerobic conditions, e.g.,
(b) In anaerobic respiration there is no consumption of oxygen. Carbon dioxide is produced in most of the cases. Therefore, respiratory quotient is infinity. Carbohydrate is the usual substrate.
An intermediate value is obtained where an organ is undergoing both aerobic and anaerobic modes of respiration.