Factors Affecting Rate of Photosynthesis

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The process of photosynthesis is affected by two types of factors namely; internal and external.

Internal factors

Chlorophyll: The process of photosynthesis depends on the amount of chlorophyll present in the leaf. If chlorophyll is absent in a leaf, photosynthesis does not takes place.

Leaf age and anatomy: leaf anatomy also influences the photosynthesis. The important anatomic structures which influence photosynthesis are size, shape and distribution of intercellular spaces, position and frequency of stomata, number, structure and distribution stomata and thickness of cuticle etc.

Demand for photo synthate: Rapidly growing plants show increased rate of photosynthesis in comparison to nature plants. When demand for photosynthesis is lowered by removal of meristem the photosynthetic rate declines.

Hormones: The rate of photosynthesis is also influenced by certain hormones present in a plant such as cytokinins, gibberellins, and abscisic acid. Cytokinins and gibberellins increase the rate of photosynthesis, while the abscisic acid reduces it.

External Factors

The major external factors when affect the rate of photosynthesis are temperature, light, carbon dioxide, water, mineral elements etc.

Concept of limiting factors: when a process is affected by various factors, the rate of the process depends upon the pace of the slowest factor. For example, out of light carbon dioxide and temperature, it is seen that when all three factors even if one of the factors become suboptimal and the other factors remain optimal, the rate of the process decline substantially. This known as law of limiting factors or law of minimum shown by Blackman in 1905 and the principle of limiting factor, when a process is conditional as to its rapidly by a number of separate factors, the rate of the process is limited by the pace of the slowest factor.

Light: Photosynthesis does not take place in dark. If the intensity of sunlight is increased slowly, the rate of photosynthesis increases up to a certain point (called saturation point). If the intensity of sunlight increases beyond saturation point, then photosynthesis slows down. Wavelength of light affects rate of photosynthesis. Red light and to some extent blue light has an enhancing influence on photosynthesis.

Temperature: At low temperature the rate of photosynthesis is low. As the temperature is increased, the rate of photosynthesis also increases up to a certain point. But a high increase in temperature slows down the rate of photosynthesis. Between , with every rise in temperature rate of photosynthesis doubles or is 2 (Q = quotient).

Carbon dioxide: The amount of carbon dioxide in the atmosphere is if the amount of increases up to the rate of photosynthesis also increases. But, the rate of photosynthesis decreases, if the amount of increases beyond at optimum temperature and light intensity, if carbon dioxide supply is increased the rate of photosynthesis increases markedly.

Water: water is a very important raw material for photosynthesis. A light decrease in amount of water causes of stomata. This result in less absorption of in leaves, hence the process if also slows down.

Mineral elements: Some mineral elements like copper, manganese, chloride etc. which are components of photosynthetic enzymes or magnesium as a component of chlorophylls also affect the rate of photosynthesis indirectly by affecting the synthesis of photosynthetic enzyme and chlorophyll respectively.

Chemosynthesis

Chemosynthetic autotrophic bacteria (chemoautotrophic bacteria) are bacteria which are able to manufacture their organic food from inorganic raw materials with help of energy derived from exergonic chemical reactions involving oxidation of an inorganic substance present in the external medium. The chemical energy obtained from oxidation reaction is trapped in ATP molecules. They play a great role in recycling nutrients like nitrogen, phosphorous, iron and sulphur. There are several types of chemoautotrophic bacteria but the well-known examples are nitrifying bacteria, sulphur oxidizing bacteria, and ion bacteria. This is found in many colourless bacteria and because they use chemical energy to reduce carbon dioxide, this process of carbohydrate synthesis is known as chemosynthesis. Chemosynthesis is the method of carbon assimilation when the reduction of is carried out in darkness, utilizing the energy obtained from oxidation of inorganic substances, such as the common chemosynthetic forms are; (i) Nitrifying bacteria. Nitrosomonas oxidises (ii) Sulphur bacteria (iii) Iron bacteria (iv) Hydrogen and methane bacteria.

Chemiosmotic Synthesis

This is a process in which energy stored as a hydrogen ion gradient across a membrane is used to synthesize ATP from ADP and Pi. The enzyme which uses the energy is ATP synthase and the energy or power source is the difference in the concentration of H+ ions on opposite sides of the membrane. The membrane is the inner membrane of the mitochondrion or the chloroplast.

Chemiosmosis is one of the processes by which ATP is synthesized. In eukaryotes, it takes place in the mitochondria during cellular respiration and in the chloroplasts during photosynthesis. In prokaryotes, it occurs in the cell membrane and cannot use it for ATP synthesis.

Differences between Photosynthesis and Chemosynthesis

Table Showing Difference between Photosynthesis and Chemosynthesis.
Table showing difference between photosynthesis and chemosynthesis.

Chemosynthesis

Photosynthesis

It occurs only in colourless aerobic bacteria

This process occurs in green plants including green bacteria

During this process CO2 is reduced to carbohydrates without light and chlorophyll

CO2 and H2O are converted into carbohydrates in the presence of light and chlorophyll

Here chemical energy released during oxidation of inorganic substances is used up to synthesize carbohydrates

Light energy is converted into chemical energy and stored in the form of carbohydrates

No pigment molecule is involved and oxygen is not evolved

Several pigments are involved and oxygen is evolved as a by product

No photophosphorylation takes place

Photophosphorylation takes place i.e. ATP is produced

No significant contribution of energy to the total biospheric energy reserve

Major contribution of energy to the total biospheric energy reserve

Sunlight or solar energy are not necessary

Sunlight or solar energy is essential