Photosynthesisis the process by which carbohydrates or sugars are manufactured in the green parts of the plants containing chlorophyll by combination of carbon (IV) oxide and water with the aid of radiant energy of sunlight while oxygen is given out as a by-product.

It is a very important process which provides food not only for green plants but also for all animals and other plants which lack green pigments. The green pigments called chlorophylls when concentrated into structures are known as chloroplasts. The chloroplasts are disc-shaped in most plants, spiral in Spirogyra and star-shaped in Zygnema. Chlorophylls are the only structures capable of trapping the sun energy or other form of light energy. Chloroplasts are also present in very young parts of stems and herbaceous plants enabling them to carry out photosynthesis. All organs of plants with chlorophylls are green in colour.

Carbon (IV) oxide and water are inorganic compounds (raw materials). During photosynthesis, these inorganic compounds are converted into organic products which are stored in plants and used as food. This type of nutrition in which inorganic compounds are converted into organic compounds is known as holophytic nutrition.

Holophytic nutrition is only possible in green plants.

The first stable product of photosynthesis is a simple sugar (glucose). The reaction is represented by a chemical equation.

Aquatic green plants obtain carbon (IV) oxide from dissolved air in water by diffusion. In terrestrial green plants, water is absorbed from the soil by the root hairs. The absorbed water moves up the xylem to the leaves. Carbon (IV) oxide from the atmosphere diffuses into the leaves through the stomata to the mesophyll layer.

The oxygen formed as a by-product diffuses out of the leaves through the stomata.


Mechanism of photosynthesis

The mechanism of photosynthesis is complex. It involves series of enzymes present in the plant. Two reactions called light and dark are involved:

A) Light reaction: It is called light reaction because it occurs only in the light. In the light reaction, four steps are involved:


Step 1

Chlorophyll is energized: The chlorophyll molecules absorb the sunlight energy and become energized.


Step 2

Photolysis of water: The energized chlorophyll supplies the energy (sunlight energy) that splits the water molecules into hydrogen (H) and hydroxyl (OH ) ions. This is called photolysis of water. The hydroxyl ion (OH ) gives up its negative electron to the chlorophyll molecules and forms water and oxygen. The oxygen is given out as a by-product.

Oxygen is given up as a by product


Step 3

Hydrogen is transferred by NADP (Nicotinamide-adenosine dinucleotide phosphate)

The hydrogen atom (H2) released above is immediately picked by a coenzyme called NADP (Nicotinamide-adenosine dinucleotide phosphate)in order to prevent its escape from the cell or recombining with oxygen to form water. NADP is now reduced to NADPH, having accepted hydrogen atoms (2H+) and serves as an electron carrier of hydrogen ions.


Step 4

Formation of ATP (Adenosine triphosphate)

Chloroplasts contain ADP (Adenosine diphosphate, a low energy carrier). The extra energy not used in the splitting of water molecules by the energized chlorophyll is released to ADP to form ATP (high energy carrier) which will be used in dark reaction. All these events take place in the grana (disc like structure chloroplasts) of the chloroplasts.


B) Dark reaction: (Calvin cycle)

It is called dark reaction because it does not require light energy. Carbon (IV) oxide is reduced by combining with two atoms of hydrogen provided by a coenzyme (NADP H2) to form sugars after undergoing a series of stepwise reactions. Each step is controlled by a specific enzyme. The energy needed for the formation of sugars is provided by ATP from light reaction. These events take place in the chloroplast surrounds the stroma of the chloroplasts. The production of glucose is not the end of photosynthetic activities. Some organic compounds at the 3-carbon sugar stage (phospho-glyceric acid) PGA, are set aside for the synthesis of fats, oil, proteins and numerous carbohydrate derived substances.


Factors which affect photosynthesis

Photosynthesis is affected by the following factors:

  1. Light intensity: Light is necessary for photosynthesis to take place. The rate of photosynthesis increases with moderately high light intensity while the rate is decreased with low light intensity. Very high light intensity reduces the rate of photosynthesis as chlorophyll is bleached and enzymes denatured.
  2. Carbon (IV) oxide: Carbon (IV) oxide is a necessary raw material in photosynthesis. The more carbon (IV) oxide in the photosynthetic cells the faster the rate of photosynthesis.
  3. Water: Water is an essential raw material for photosynthesis.
  4. Temperature: The light and dark reactions are controlled by enzymes. Low or high temperature will negatively affect enzyme action hence decrease in the rate of photosynthesis.
  5. Chlorophyll concentration: The reduction of chlorophyll level decreases the rate of photosynthesis. High light intensity, mineral deficiency diseases and ageing processes reduce chlorophyll concentration.
  6. High concentration of oxygen: High concentration of oxygen in the atmosphere decreases the rate of photosynthesis. It has been shown that oxygen competes with carbon (IV) oxide for active site in the carbon (IV)fixing enzyme (carboxylase) thus reducing the amount of carbon (IV) oxide, thereby decreasing the rate of photosynthesis.
  7. Inhibitors: Herbicides such as DCMU (dichlorophenyl diamethyl urea) inhibits the electron flow of the chloroplasts thereby decreasing the rate of photosynthesis.
  8. Pollution: Gases and materials of industrial wastes discharged into the atmosphere affect the rate of photosynthesis. Sulphur (IV) oxide and ozone for example damage the leaves of plants. Soot and dust particles block the stomata slowing down the rate of exchange of gases thus decreasing the photosynthetic rate. Soot and dust particles also reduce the intensity of sunlight, hence photosynthetic rate is reduced.


The fate of glucose after photosynthesis

Glucose is the first stable product of photosynthesis. It is used by all living cells for respiration during which energy is liberated. In the presence of sunlight, the glucose in the leaves is quickly converted to starch. This can be proved by testing a leaf for starch. In the dark (where there is no light) photosynthesis stops and starch in the leaves is converted to sucrose (a complex sugar) and translocated out of the leaves to other parts of the plant through the phloem vessels. If present in excess, the sucrose may be reconverted to starch and stored in some of the storage organs in the plant for later use (e.g. underground stems, root tubers and seeds). If not, the sucrose is converted into glucose and used by the living cells for respiration, or used as a starting material for the synthesis of cellulose, proteins, fats, oils and other structural components of the living cells. The synthesis of proteins, fats and oils, for instance, involves the absorption of mineral salts such as nitrates, sulphates and phosphates from the soil which are incorporated into organic compounds with the help of enzymes.

Starch, proteins, fats and oils are complex organic matters stored in the plant body as food.


The importance of photosynthesis in nature

  1. All living things require energy for growth and other metabolic activities. It is through photosynthesis that the energy of the sun is converted into useful energy stored in glucose which both plants and animals use during respiration.
  2. Glucose, a product of photosynthesis isa starting material for the synthesis of proteins, fats, oil and vitamins which are various forms of food for both plants and animals.
  3. Animals and non-green plants which cannot manufacture their own food depend on green plants for their food.
  4. Oxygen which is a by-product of photosynThesis is necessary for aerobic respiration.
  5. Photosynthesis purifies the atmosphere by the constant removal of carbon (IV) oxide.

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