Process of Photosynthesis Step by Step

Process of photosynthesis is utilized by autotrophic organisms such as green plants, algae and cyanobacteria (photosynthetic bacteria). They use the photosynthesis process step by step in order to make their own organic nutrients from simple inorganic nutrients (water, carbon dioxide and minerals) and radiant energy. 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.

Photosynthesis Definition: In simpler terms, the process of photosynthesis is used by plants and other organisms to convert the radiant energy/light energy into chemical energy, that can be used to perform daily tasks.

Chemical Equation for Photosynthesis: The photosynthesis formula explains that combination of light energy, carbon dioxide and water produces a oxygen and a carbohydrate (glucose). The photosynthesis equation is stated below

6CO₂ + 6H₂O + Radiant Energy/Light Energy ————-> C₆H₁₂O₆ + 6O₂

Photosynthesis Reaction

Photosynthesis Process Step by Step – Mechanism of Photosynthesis

The process of photosynthesis in plants (green plants) takes place inside 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 returns to ground state in approximately 10 – 9 seconds. 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.

Hills Reaction – Light Reaction of Photosynthesis

All the above mentioned reactions are commonly termed as phase of light reaction of photosynthesis. The chemical reactions taking place during light phase of photosynthesis were studied by Robert Hill, therefore also known as Hills reaction.

Dark Reaction of Photosynthesis – Calvin Benson Pathway (Calvin Cycle)

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).

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.

B. van Niel and Importance of Photosynthetic Bacteria: C. 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 photosynthesis equation for these bacteria is:

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

when this photosynthesis 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 – Chloroplast

In eukaryotes, the site of photosynthesis are 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.

Pigments  – Explain and Describe Three Main Photosynthetic Pigments

Those substance which absorbs light energy are called pigments. All active photosynthetic pigments that are involved in the process of photosynthesis are found in the chloroplast. These include chlorophylls (bacteriochlorophylls in bacteria), carotenoids and phycobiliproteins. Each of the pigments serves a specific function.

Chlorophyll In Plants

The green pigment chlorophyll is present in leaves of the plants. 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 in plants to the hydrophobic portion of the membrane.

Carotenoids

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.

1. 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.
2. Xanthophyll’s: These are oxygen containing hydrocarbons (C₄₀ H₅₆ O₂ – lutein).

Role of Carotenoids: The carotenoids are embedded within protein molecules by non-covalent bonds: The carotenoids play two important roles in plants.

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

   Beta Carotene Structural Formula

Carotenoid Pigments

There are two common cartenoid pigments, Beta carotenen and xanthophyll.

Simplified Representation

Vertebrate animals oxidize the molecules of beta carotene taken in with food, each molecule yielding two molecules of vitamin A.

Xanthophyll has a very similar structure. It is described as an oxygenated hydrocarbon.

Phycobilins

In red and blue-green 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.

Also Read:- Factors Affecting Photosynthesis