Photosynthesis

Chapter 11 

Photosynthesis

Autotrophic Nutrition

• Photosynthesis is not only carried out by plants by using chlorophyll, but in the plants which do not contain chlorophyll it is conducted by two types of bacteria known as Purple Sulphur bacteria and green Sulphur bacteria which prow in absence of oxygen (Anaerobic).

• Purple Sulphur bacteria develop a purple pigment (as the name indicates) as substitute to chlorophyll. These bacteria uses sunlight to decompose hydrogen-supplied (H2S) gas into hydrogen gas and Sulphur, wherein the gas hydrogen produced is used to reduce carbon- dioxide to carbohydrates through the series of reactions (Dark-reactions) as in green plants.

• Green Sulphur bacteria containing the pigment bacteriocidic by using sunlight energy oxidized hydrogen supplied (H2S) by liberating Hydrogen reduce carbon dioxide to carbohydrate in stepwise series of reactions.

• Photosynthetic Pigments:

• Plants manufacture their food for survival in their green leaves with the help of chlorophyll. Chlorophyll is not a single compound but is a complex of similar compounds and all these are taking active part in photosynthesis. They are namely chlorophyll (a & b), carotene and xanthophyll.

Chlorophyll

• Biological Source:

• Chlorophyll is the natural pigment obtained from the green leaves of higher plants and green algae. It is a mixture of four different pigments i.e. Chlorophyll a, Chlorophyll b, Carotene and Xanthophylls, i.e. blue—black, green—black, orange—red, yellow in color.

Description:

• Chlorophyll occurs as waxy, blue-black mass mixed with each other, in the green plasmids called chloroplast while carotene and xanthophyll occur without chloroplast in any part of plants.

Chemical Constituents:

• Chlorophyll is a mixture of four different pigments, which exist in the chloroplast.

Isolation of Chlorophyll (A) The upper layer appears deep green in color containing chlorophyll a and carotene which are separated by adding potassium hydroxide solution and methyl alcohol, again shaken for a few minutes and allowed to stand, immediately each liquid separates out into two layers, upper layer is deep green containing chlorophyll a, and the lower layer is yellowish brown containing carotene. (B) Lower layer appears yellowish brown light greenish yellow containing chlorophyll b and xanthophyll which are separated by adding potassium hydroxide solution and methyl alcohol, shake it for a few minutes. Allow it to stand for a few minutes, immediately each liquid separates out into two layers, upper layer is greenish containing chlorophyll b and lower is light yellow containing xanthophyll. Chemical Test: In transmitted light Chlorophyll solution appears deep green in color while in reflected light it appears blood red in color. Standards of Quality: Uses: Chlorophyll is used as a colorings agent for soaps, oils, confectionary and cosmetics. It is also used in toothpaste as additive and deodorant. Storage: Store in well-filled and well-closed containers away from sunlight. Carotenes Carotenes are the precursors of vitamin A and are equally important for photosynthetic process, they comprise the group of a carotene, § carotene and y carotene. pigments absorb solar energy of different wavelength and transfer it to chlorophyll-a which then initiates photosynthesis. The process of photosynthesis consists of two stages (1) Light reaction (Taking place in grana) (2) Dark reaction (Taking place in stroma of chloroplast) (1) Light Reaction: As the name indicates, it takes place in the presence of sunlight. Various pigments as mentioned earlier get involved in the process along with carriers like ferredoxin, plastocyanin, cytochromes and others. During this reaction chlorophyll-a absorbs solar energy and gets excited and as a result of which an energy rich electron is tossed from chlorophyll-a. While passing through the cyclic as well as non-cyclic paths energy is stepped down for the formation of ATP and electron gets back to its normal level. The formation of ATP in the presence of sunlight is known as photophosphorylation. While photolysis of water is its splitting into H* and OH- ions in the presence of light. This is non-cyclic electron transfer. During the process, electrons from the excited chlorophyll-a do not return to chlorophyll-a, but, are transferred to ferredoxin and then converted to NADP that gets ionized. H* ion liberated gets combined with NADP to form NADPH z- The OH gives electron to chlorophyll-a and forms water and oxygen. obtained from OH— produces adenosine triphosphate (ATP). Dark Reaction (Calvin-cycle): This reaction takes place in the absence of light and hence is called dark reaction. It takes place in stroma of the chloroplast. As the reaction gets carried in the absence of light it is not a photochemical, but it is an enzymic process. Most of the plants follow Calvin cycle of C3 pathway of fixing carbon-dioxide to carbohydrates. But certain plants of tropical area follow another pathway, which fix up carbon-dioxide even of lower concentration and is explained by Hatch and Slack pathway also known as Cm pathway since the first intermediate product is Cm acid.

Factors Affecting Photosynthesis

• Two different types of factors affect the rate of photosynthesis is plants (1) External factors and (2) Internal factors.

• External Factors Affecting Rate of Photosynthesis: They are as under

• (1) Light: It plays most important role in photosynthesis, in absence of sunlight plants can not undertake formation of food material their survival rate of photosynthesis depends upon the intensity of light. However very small amount of solar energy is used by plants for photo synthesis. Excessive sunlight is also harmful to the plants.

• (2) Water: Water makes the photosynthetic cells active and also turgid. The process of photosynthesis starts with the splitting of water into hydrogen and oxygen under the action of light and chlorophyll. However hardly 1.0% of water absorbed by roots and used for photosynthesis.

• (3) Chlorophyll: Not applicable to fungi or saprophytes as they are parasitic, but other plants can not produce their food in absence of chlorophyll. Chlorophyll is present in plastids in the cell of the plants.

• (4) Carbon dioxide: Carbon dioxide naturally found in air is the only source of carbon. By absorbing it plants produce sugar, starch for their livelihood. But if the concentration of carbon dioxide in the air increases it is also very harmful.

• (5) Temperature: Photosynthesis takes place at ambient temperature and even below the freezing point of water. Plants are growing under various conditions at different places; the optimum temperature requirement varies from place to place of location. However, plants can survive upto 45"C.

• (6) Accumulation of photosynthetic products in the assimilating cells reduces the rate of photosynthesis in plants.

• Internal Factors Affecting Rate of Photosynthesis:

• (1) Chlorophyll content: This is of prime importance because more the chlorophyll content in chloroplast, the rate of photosynthesis increases, where it gets reduced (may be due to any reasons) the rate of photosynthesis falls down. In case of saprophytic penner orgasm or fungi, there is no chlorophyll and hence these cannot prepare their food by means of photosynthesis.

• (2) Accumulation of Photosynthetic Products: Collection or storage of sugar or starch in the cells also reduces the process of photosynthesis in the cells of plants.

• (3) Protoplasmic factor is also responsible for slowdown of photosynthesis.

• (4) In absence of oxygen, photosynthesis does not take place.