Photorespiration and factors affecting the rate of photosynthesis


Do you know what is photorespiration? The below article will provide you with a detailed mechanism of photorespiration. You will also find the factors which affect the rate of photosynthesis and significance of photorespiration.

Introduction

It was normally believed that the rate of respiration is the same for day as well as night. Recently it has been observed that light affects respiration and the rate of respiration during daylight maybe three to five times higher than the respiration in darkness. Such type of respiration is called photorespiration and is marked as one of the discoveries of plants physiology. Photorespiration may be defined as the process taking place in a green plant cell in which oxygen is consumed in the presence of light releasing carbon dioxide. In simple words, it can also be said that light-driven respiration is photorespiration. In photorespiration, temperature plays a very important role, its rate being very high in between 250C to 350C. Photorespiration also depends upon the concentration of oxygen and increases with increasing oxygen concentration even up to 100%. However, normal respiration is independent of oxygen concentration. In normal respiration, the respiratory substrate is sucrose which in photorespiration glycolic acid (2 carbon compound) serves as a substrate.

Characteristic features of C2 plants

The diagnostic features of photorespiration are as follows:

  • The respiratory substrate is glycolate a two-carbon (2C) compound.

  • The substrate is always recently formed.

  • The entire process of photorespiration occurs in between chloroplast, peroxisome and mitochondria.

  • Photorespiration shows a positive co-relationship with oxygen concentration.

  • The enzyme for glycolate metabolism are found in peroxisomes.

  • ATP is not produced in this process.

  • Rate of photorespiration is highly accelerated in between 250C to 350C.

  • It takes place only in green cells in the presence of bright sunlight.


  • Mechanism of photorespiration


    The chief metabolite in photorespiration is glycolate or glycolic acid which is a 2C compound. It is oxidized in photorespiration and the overall process takes place inside three organelles i.e. chloroplast, mitochondria and peroxisome.

    1. In chloroplast: The ribulose diphosphate reacts with oxygen to produce one molecule of 3 carbon, phosphoglyceric acid (PGA) and one molecule of 2 carbon, phosphoglycolic acid in the presence of RuDP-oxygenase enzyme. Phosphglycolic acid is immediately dephosphorylated into glycolic acid or glycolate in the presence of phosphatase enzyme.

    2. In peroxisomes: The glycolic acid enters the peroxisome where it is oxidized to produce glyoxylic acid (glyoxylate) and hydrogen peroxide (H2O2) in the presence of glycolic acid oxidase. Hydrogen peroxide (H2O2) is decomposed into water and oxygen with the help of catalase enzyme. The glycoxylic acid is then transaminated to glyceine in the presence of transaminase enzyme.

    3. In mitochondria: The glycine molecule formed in peroxisomes enters into the mitochondria and is decarboxylated to form serine. Serine diffuses into the peroxisome where it is converted into glycerate, which enters the chloroplast and the Calvin cycle to produce sugars.

    Significances of photorespiration

    Based on photorespiration we can divide plants into two groups. Plants showing photorespiration are found mainly in temperate regions while tropical plants do not show photorespiration. Photorespiration also helps in understanding the process of development in plants. Plants having the property of photorespiration shows the high rate of photosynthesis because oxygen released in photorespiration is stored in intercellular spaces of leaves.

    Factors affecting the rate of photosynthesis

    The factors influencing the rate of photosynthesis can be categorized into external and internal factors. The different types of internal and external factors which affect the rate of photosynthesis are as below:

    External factors

    The external factors or environmental factors affecting the rate of photosynthesis may be considered as follows:

    1. Carbon dioxide: Carbon dioxide is one of the basic raw materials of photosynthesis; therefore, its concentration affects the rate of photosynthesis. Because of its low concentration in the atmosphere, it becomes a limiting factor in natural photosynthesis. At optimum temperature and light intensity, if the supply of carbon dioxide is increased, the rate of photosynthesis increases markedly.

    2. Light: Light has a manifold effect on the rate of photosynthesis. When the intensity of light is increased, the rate of photosynthesis is also increased. The maximum photosynthesis occurs in the red light and the minimum photosynthesis takes place in the green light. Ultraviolet light has a lethal effect on plants if the exposure is for a prolonged period.

    3. Temperature: It has been observed that photosynthesis increases with temperature but it declines with time, it is called time factor. In cold regions it takes place between 00 C to 10 0C and in desert at 550 C. In hot water spring algae, it takes place at 750 C.

    4. Water: Water has an indirect effect on the rate of photosynthesis though it is one of its raw materials. It affects the water relations of the plant thus affecting the rate of photosynthesis.

    5. Oxygen: Oxygen is a by-product of photosynthesis. It has been suggested that oxygen accumulation retards the rate of photosynthesis.

    Internal factors

    The internal factors which affect the rate of photosynthesis are as below:

  • Chlorophyll contents: The amount of chlorophyll present has a direct relationship with the rate of photosynthesis because it is the pigment which is photoreceptive and is directly involved in trapping the light energy.

  • Protoplasm: According to Bridges (1922) protoplasm contains some unknown factors that affect the rate of photosynthesis. From researches, it becomes clear that protoplasm contains some enzymes that affect the dark reaction. The decrease in the rate of photosynthesis at 350C or in intense light shows enzymic nature of protoplasm.

  • Accumulation of end products: The rate of translocation of food manufactured in the leaves decreases in the afternoon and therefore it starts accumulating in the mesophyll cells. The accumulation of end product decreases the rate of photosynthesis in the afternoon.

  • Internal structure of leaf: The rate of photosynthesis depends on the size, shape and number of stomata. The rate of photosynthesis increases when the number of stomata is more and they are opened for a long duration. It is because of the uptake of more carbon dioxide through stomata for a long duration. The arrangement of mesophyll also affects the rate of photosynthesis.


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