It is the process that enables plants, some species of bacteria and some protistans to use the energy from sun, carbon dioxide and water to manufacture their own food in the form of glucose. Oxygen is also produced as a result of this reaction
The glucose thus produced is converted into pyruvate which releases adenosine triphosphate abbreviated as ATP through cellular respiration.
The reaction of photosynthesis can be summarized as the equation:
carbon dioxide + water -> glucose + oxygen (Catalysed by sunlight and chlorophyll)
This reaction is made possible in the green pigment called chlorophyll, which is a complex molecule. Different variations are present among different species of plants and photosynthetic organisms.
All organisms capable of photosynthesis contains chlorophyll a. they also contain accessory pigments that are helpful to absorb energy that chlorophyll a is unable to absorb. Accessory pigments includes chlorophyll b (also c, d, and e in algae and protistans), xanthophylls, and carotenoids (such as beta-carotene).
The chlorophyll a pigment absorbs the energy from the wavelengths of orange-red, violet-blue and reddish lights.
All types of chlorophylls have:
• a lipid-soluble hydrocarbon tail (C20H39 -)
• a flat hydrophilic head with a magnesium ion at its centre; different chlorophylls have different side-groups on the head
The tail and head are linked by ester bond.
Among all the photosynthetic organisms only plants species have leaves. A leaf can be considered as a solar collector crammed full of photosynthetic cells.
The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the leaf, and the products of photosynthesis, sugar and oxygen, leave the leaf. Oxygen is released in the environment through stomata that is present in the epidermis of the leaf.
Water enters the root of the plant which is then transported up to the leaves through specialized plant cells known as xylem vessels. Land plants are needed to guard themselves against dehydration and hence have evolved specialized structures known as stomata to allow gas exchange with the environment through the leaf. Carbon dioxide can enter the leaf through stomata only. Similarly, the oxygen produced by the plants can exit only through stomata. During these processes the plants loses a considerable amount of water. For example, Cottonwood trees will lose 100 gallons (about 450 dm3) of water per hour during hot desert days.
The structure of the chloroplast and photosynthetic membranes:
The thylakoid is the structural and also the functional unit of photosynthesis for both photosynthetic prokaryotes and eukaryotes, which are flattened sacs/vesicles containing photosynthetic chemicals. Only eukaryotes have chloroplasts with a surrounding membrane.
Thylakoids are stacked like pancakes in stacks collectively known as grana. The areas between grana are referred to as stroma. While the mitochondrion has two membrane systems, the chloroplast has three, which forms three compartments.
Stages of photosynthesis
Photosynthesis is a chemical reaction where the electrons gain energy and get excited by absorbing sun light. It is called a light energized Oxidation reduction process. The energy produced in thisi reaction is utilised by the plants to oxidise the water to produce oxygen and hydrogen and electrons. The electrons and hydrogen atoms are used for the reduction of the nitrate and sulphate to amino and sulhydryl groups in amino acids.
The chlorophyll a molecule is photoactivated which results in the split of water molecules and reduction of nicotinamide adenine dinucleotide phosphate (NADP).
The chemical reactions involved include:
Condensation reactions – responsible for water molecules splitting out, including phosphorylation (the addition of a phosphate group to an organic compound).
Oxidation/reduction (redox) reactions – which involves electron transfer.
Photosynthesis is a two stage process.
The Light dependent reactions (or the “light” stage)
A light-dependent series of reactions which occur in the grana, and require the direct energy of light to make energy-carrier molecules that are used in the second process.
Light energy is trapped by chlorophyll to make ATP (photophosphorylation), it is also called the light capturing reactions. At the same time water is split into oxygen, hydrogen ions and free electrons:
2H2O –> 4H+ + O2 + 4e- (photolysis)
The electrons then react with a carrier molecule nicotinamide adenine dinucleotide phosphate (NADP), changing it from its oxidised state (NADP+) to its reduced state (NADPH):
NADP+ + 2e- + 2H+ –> NADPH + H+
The light-independent reactions (or the “dark” stage)
During this stage the ATP and NADPH are formed as a result of light capturing reactions. A light-independent series of reactions which occur in the stroma of the chloroplasts, when the products of the light reaction, ATP and NADPH, are used to make carbohydrates from carbon dioxide (reduction); initially glyceraldehyde 3-phosphate (a 3-carbon atom molecule) is formed.
Factors affecting the rate of photosynthesis:
The main factors are light intensity, carbon dioxide concentration and temperature, known as limiting factors.
1. As light intensity increases, the rate of the light-dependent reaction, and therefore photosynthesis, increases proportionately. As light intensity is increased however, the rate of photosynthesis is eventually limited by some other factor. Chlorophyll a is used in both photosystems.
2. The wavelength of light is also important. Photosystem I absorbs energy most efficiently at 700 nm and PhotosystemII at 680 nm. Light with a high proportion of energy concentrated in these wavelengths will produce a high rate of photosynthesis.
3. An increase in the carbon dioxide concentration increases the rate at which carbon is incorporated into carbohydrate in the light-independent reaction and so the rate of photosynthesis generally increases until limited by another factor.
4. Photosynthesis is dependent on temperature. It is a reaction catalysed by enzymes. As the enzymes approach their optimum temperatures the overall rate increases. Above the optimum temperature the rate begins to decrease until it stops.