Autotrophic organisms such as green plants, algae and cyanobacteria (photosynthetic bacteria) make their own organic nutrients from simple inorganic nutrients (water, carbon dioxide and minerals) and radiant energy in a process called photosynthesis.

The radiant energy is converted into chemical energy of specific organic compounds, mainly sugars and starch, amino acids and lipids. Oxygen is released as by product.

Mechanism of Photosynthesis

In green plants, the photosynthesis takes place in the cells containing chloroplasts. Most chloroplasts occur in the mesophyll cells of green leaves, but some are found in parenchyma cells below the epidermis of herbaceous stems. In the chloroplasts, the energy of sunlight is trapped in chlorophyll. The absorption of light energy by chlorophyll induces a rearrangement of the molecules electronic structure to an excited state, a process called photochemical excitation. The structure excitation in chloroplasts is responsible for oxidative change of water, a process called photo-oxidation; the reduction of nicotinamide adenine dinucleotide phosphate (NADP+) to NADPH, termed photo-reduction and phosphorylation of adenosine diphosphate (ADP) to adenosine triphosphate (ATP), a process known as photophosphorylation. All these reactions are commonly termed light phase of photosynthesis. The chemical reactions taking place during light phase of photosynthesis were studied by Robert Hill, therefore also known as Hill’s reaction.

ATP and NADPH are utilized in the reactions of carbon dioxide fixation that are commonly referred the dark phase of photosynthesis since it does not require the presence of sunlight. In many plants, the carbon dioxide is almost exclusively fixed in the chloroplasts by combination with ribulose-1,5-bisphosphate to produce 3- phosphoglyceric acid (3-PGA), which is converted to 4-C, 5-C, 6-C and 7-C phosphorylated sugars via a series of reactions collectively known as Calvin-Benson Pathway (Calvin Cycle).

Mechanism of Photosynthesis

Mechanism of Photosynthesis

Origin of Oxygen in Photosynthesis

The raw material for photosynthesis is carbon dioxide and water and both contain oxygen. It has been observed that oxygen is released during photosynthesis as by-product. Whether the source of oxygen released is carbon dioxide or water was a mystery.

  1. B. van Niel, a microbiologist, showed the importance of photosynthetic bacteria in photosynthesis research. He pointed out the similarity of overall photosynthetic reaction between the green plants and photosynthetic bacteria, whereas on the other hand, showed that hydrogen donor in case of green Sulphur bacteria is H2S and Sulphur is released as by-product (other bacteria use acetic acid or succinic acid as electron sources). The overall photosynthetic equation for these bacteria is:

n CO2+ 2n H2S + light ———- (CH2O)n + H2O + 2S

when this equation was compared with that for the green plants:

n CO2 + H2O + light ———- (CH2O)n + n O2

Niel observed an analogy between the role of hydrogen and water, and of oxygen and Sulphur. This led to conclude that oxygen released by plants is derived from water, and not from carbon dioxide.

Site of Photosynthesis

In eukaryotes, the photosynthesis takes place in the sub-cellular organelles known as chloroplasts. The chloroplasts vary in shape, size from species to species. These arise from tiny immature, small, nearly colorless bodies called protoplastids found in the unfertilized egg cells. These are with few or no internal membranes. They develop into chloroplasts when the leaves and stems are formed.

Photosynthetic Pigments

Any substance that absorbs light energy is called pigment. All pigments active in photosynthesis are found in the chloroplast. These include chlorophylls (bacteriochlorophylls in bacteria), carotenoids and phycobiliproteins. Each serves a specific function.


Two kinds of chlorophylls are found in higher plants, chlorophyll a and chlorophyll b. The chlorophylls have a complex ring structure that is chemically related to the porphyrin-like groups found in hemoglobin and cytochromes. In addition, a long hydrocarbon tail is attached to the ring structure. The tail anchors the chlorophyll to the hydrophobic portion of the membrane.


Carotenoids are lipid compounds that range in color from yellow to purple. These are found in nearly all higher plants. There are two kinds of carotenoids.

Carotenes: These are pure hydrocarbons (C40 H56 – beta-carotene) composed of eight isoprene-like residues:

The major carotenoid found in the plant tissue is the orange-yellow pigment beta-carotene which is generally accompanied by alpha-carotene.


These are oxygen containing hydrocarbons (C40 H56 O2 – lutien).

The carotenoids are embedded within protein molecules by non-covalent bonds:

The carotenoids play two important roles in plants. The first being that they protect photosynthetic membranes against damage by the large amounts of energy absorbed by the pigments (photoprotection) and second, they absorb and transfer light energy to chlorophyll a, therefore, the carotenoids are also called accessory pigments.


In red and blue0green algae, and in photosynthetic bacteria, a protein-pigment complex called biliproteins are found. These are phycocyanin and phycoerythrins. The pigment part of this complex is called phycobilin. It is strongly attached to protein and absorbs light in the range of wavelengths not absorbed by chlorophylls. The absorbed light energy is transferred to chlorophyll. Thus, like carotenoids the phycobilins are also accessory pigments.

Light Reactions in Chloroplast

Light Reactions in Chloroplast

The chloroplast is involved in both stages of photosynthesis. The light reactions take place in the thylakoid. There, water (H2O) is oxidized and oxygen (O2) is released. The electrons that freed from the water are transferred to ATP and NADPH. The dark reactions then occur outside the thylakoid. In these reactions, the energy from ATP and NADPH is used to fix carbon dioxide (CO2). The product of this reaction are sugar molecules and various other organic molecules necessary for cell function and metabolism. Note that the dark reaction takes place in the stroma (the aqueous fluid surrounding the stacks of thylakoids) and in the cytoplasm.



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