Photosynthesis

Introduction

     The first photosynthetic organism probably appeared almost three billion years ago. With the evolution of photosynthesis, however, organisms began to change the face of our planet and, as a consequence, to exert strong influences on each other. Organisms have continued to change the environment, at an ever increasing rate, up to the present day.

Significance of Photosynthesis

     Photosynthesis is the only process which produces enormous quantities of organic matter for sustaining the life on this globe. It is the only known method of manufacture of organic food from inorganic raw materials. Animals including man are directly or indirectly dependent on photosynthetic plants for their food. All flesh is grass.

Site of Photosynthesis

     Photosynthesis occurs only in the green parts of the plant. For efficient photosynthesis the leaf should be thin and have a large surface area. This helps in absorption of light and gaseous diffusion, and a means of preventing excessive water loss through stomata and epidermis.

Photosynthetic Pigments

     Molecules of chlorophyll-a, chlorophyll-b, carotene and xanthopyhll are situated in the thylakoid membranes.

Photochemical and Biosynthetic Phases

     Light Phase consists of photochemical reactions which are carried out by two different photo systems, PS-I and PS-II. In the thylakoids, chlorophyll and other molecules are packed into units called photo systems.

Electron Transport System

     Electron transport chain refers to the light driven reactions of photosynthesis. They were first formulated in 1939 by Robert Hill.

Photolysis of Water (Photo Oxidation of Water)

     The PS-II reaction centre (P₆₈₀) by transferring electron to primary acceptor becomes oxidised.

Cyclic Electron Transport System

     Cylic reaction transport occurs only occasionally when synthesis of carbohydrate is curtailed due to limited supply of CO₂ and NADPH₂ starts accumulating. At this time, there would be no need for additional NADPH₂. Moreover, the cyclic electron transport serves the purpose of more production of ATP where needed by the chloroplast.

Photophosphorylation (Cyclic and Non-cyclic)

     Photosynthetic phosphorylation or photophosphorylation is the process of phosphate group transfer into ADP to synthesize energy rich ATP molecule making use of light as external energy source. According to chemi osmotic hypothesis (Mitchell 1961) the ATP is synthesized on ATPase complexes located on the non appressed portions of thylakoid membranes particularly towards margins.

C and C Pathway (Dark Reaction)

     ATP and NADPH₂, formed during the earlier phase are essential for the assimilation of CO₂ to carbohydrates. This assimilation reaction takes place in the stroma, where all the necessary enzymes are present. Collectively, these reactions are known as carbon reactions or dark reactions.

The Four - Carbon Pathway

     C₄ plants like maize, sugarcane; pearl millet, etc. have evolved a wonderful mechanism to avoid photorespiration, which is considered a wasteful process.

Photorespiration

     It has been observed that a high concentration of oxygen, inhibits photosynthesis. This is due to the reason that the CO₂ fixing enzyme RUBP carboxylase not only accepts CO₂ but can also combine with O₂. Since the reaction is an oxygenation reaction, the same enzyme is called RUBP oxygenase.

Factors Affecting Photosynthesis - External Factors

     Photosynthesis is influenced by two categories of factors - external or environmental and internal or plant factors. According to the law of limiting factors, put forward by F.F.Blackman in 1905, photosynthesis is limited by the most limiting factor.

Factors Affecting Photosynthesis - Internal Factors

     Anatomy which favours the amount of CO₂ diffusion into collenchyma cells the intensity dilution of light and the rate of photosynthesis. Thickness of epidermis, size structure, position and frequency of stomata, distribution of vascular tissue and intercellular spaces, position and number of chloroplasts.

Mode of Nutrition (Autotrophic, Heterotrophic and Saprophytic)

     Plants may be able to obtain raw materials and prepare their own organic food or can be dependent on others (dead or living) for the same. They are respectively known as autotrophic (autos = self, trophien = to nourish) and heterotrophic (heteros - other, trophien - to nourish).

Summary

     In photosynthesis, light energy is converted to chemical energy and carbon is fixed into organic compounds.

Did You Know

     Stephen Hales - Father of plant physiology. Lipmann - Father of ATP cycle.

Points to Remember

     Chlorophyll pigments - chl.a - porphyrin head with CH₃ group and phytol chain. Chl.b - porphyrin head with CHO group and phytol chain.