Chemosynthesis ;-Generally, green plants make use of light energy in the synthesis of organic substances in photosynthesis. But there are some aerobic bacteria (e.g., chemoautotrophic cells) which do not require light.

Such bacteria synthesize all their organic cell requirements from CO2, water and salts at the expense of oxidation of various inorganic compounds (viz., H2, NO3, SO4 or carbonate). Such reactions in which reduction of CO2 to carbohydrates does not require light and occurs in presence of energy ob- tained from oxidations of inorganic substances is called chemosynthesis.

Chemosynthetising forms:

Chemosynthetising forms

There are several types of chemoautotrophic bacteria which are commonly named after the chemical compound they use as source of energy.

(a) Nitrifying bacteria.

These bacteria derive energy by oxidizing ammonia into nitrate. Examples of nitrifying bacteria are – Nitrosomonas, Nitrobacter, Nitrosospira, Nitrosocystis, Nitrosogloea, etc. Conver- sion of ammonia into nitrate occurs in two steps and each step is carried by specialized group of bacteria –

The bacteria Nitrosomonas and Nitrosococ- cus obtain their energy by oxidizing ammonia to nitrite

2 NH3 + 3O22 ———————–>HNO₂+ 4 H+ + 2 NO2 + 132

The bacteria Nitrobacter and Nitrocystisoxidize nitrite to nitrate

2 NaNO2 + O2————————->2 NaNO +36

The nitrifying bacteria carry out reactions that help in the nitrogen cycle in nature.

(b) Sulphur bacteria.

These bacteria derive energy by oxidizing hydrogen sulphide or molecular sulphur. Beggiatoa, a colourless sulphur bacterium, oxidises hydrogen sulphide (HS) to water and sulphur. The energy released is used up and the sulphur granules are deposited inside or outside thebody of bacteria cell

2 H₂SO+O2———->2H2O+2S+ Energy

The elemental sulphur is oxidized to sulphuric acid by denitrifying sulphur bacteria (e.g. Thiobacillus denitrificans) and the energy released during the process is utilized in reproduction, growth and synthesis of other chemical substances

2S+2H2O+3O2—————>2H2SO4 + Energy

These bacteria usually live at the mid-oceanic ridge system (2-5 km below sea level). They generally live both freely and within the bodies of giant tube worms. They can even survive under extremely acidic conditions. Examples of sulphur bacteria are Beggiatoa, Thiobacillus, Thiothrix, etc. They participate in the sulphur cycle in nature.

(c) Iron bacteria.

Some chemoautotrophic bacteria such as Gallionella, Sphaerotilus, Ferrobacillus, etc. inhabit environments where iron compounds are in excess. They convert Ferrous ions to Ferric form. The Ferric ions are deposited in the form of insoluble Ferric hydroxide and the energy released during the conversion is used in the production of carbohydrates-

4 FeCO3 + O2 + 6 H₂O——-> 4 Fe (OH)+4CO₂+ Energy (81 kcal).

(d) Hydrogen bacteria.

These bacteria utilize free molecular hydrogen and oxidize it into water with the help of either oxygen or oxidized salts, e.g., Hydrogenomonas2 H₂+O₂ 2HO+ Energy (56 kcal).

(e) Carbon bacteria.

These bacteria oxidize carbon monoxide into carbon dioxide and use the liberated energy, eg, Bacillus oligocarbophilus

2 CO2 + 022 CO₂+ Energy

(f) Methane bacteria.

(f)The bacterium Methanomonas utilizes methane as source of carbon and energy

CH4 +2O2→ CO₂ +2H2O+Energy

(g) Methane-producing bacteria.

Thesespherical or rod-shaped bacteria which produce methane (CH4) from hydrogen gas and carbon dioxide –

CO2 + 4 H2 →CH4+ 2 H₂O

The synthesis of ATP and reduction of carbon dioxide are linked reactions and used as source of energy by methanogens (e.g., Methanobacterium). Methane (swamp gas) is produced under anaerobic conditions and can be used as a “biogas”, otherwise it is a pollutant that contributes to the green house effect and global warming.


The synthesis of carbohydrate food materials, mainly through the process of photosynthesis, occurs in green cells of plant. The non-green cells are therefore, dependent on photosynthetic cells for their carbohydrate supply. The organic carbon mainly from the leaves, is transported to the non-green parts where it is needed for respiration and biosynthesis.

The transport of organic solutes from one place to another in higher plants is referred to as the translocation of organic solutes.It has now been well established that ‘carbohydrates are translocated from leaves to roots and storage organs (tubers, bulbs, fruits, etc.) along the phloem in the form of sucrose. They are transported through living sieve elements of phloem (chiefly sieve tube members in seed plants).

The process of translocation requires the expenditure of metabolic energy and the solute moves at the rate of 100 cm/hr.Here are some evidences which organic solutes are translocated through phloem-

(1) ‘Ringing’ or girdling’ experiments.

The experiments in which the continuity of vascular tissues is disturbed by their removal are known as ringing or girdling experiments. Girdling usually employed for the removal of all the tissues outer to vascular cambium (bark, cortex and phloem) from the woody stems. In woody stems, with single ring of collateral vascular bundles, it is easily possible to remove a broad band of bark, including phloem and cambium ). Under such conditions the upper part of plant is attached to the lower part by only the central xylem cylinder and pith.

It results in the accumulation of food material just above the ring, which is evident by swelling and some times formation of adventitious roots Furthermore, it can be observed that the roots die first in a girdled plant. This can be explained because the transport of water and minerals occur through xylem.

The upper part gets ample amount of water supply, the supply of minerals and all the important factors for metabolism. It is therefore, the upper part continues to grow. The girdling results removal of phloem and since the phloem is supposed to translocate organic food materials, it cuts the food supply to the roots. The roots cannot synthesize there own carbohydrate and thus, they die first

(2) Isotopic studies.

Isotopic studies

If a leaf of a potted plant is illuminated in presence of radioactive 14CO2, it forms radioactive products of photosynthesis. These products, with radioactivity, are then transported to stem. It was detected by autoradiography that these substances are translocated through phloem and particularly through the seive tubes.pain

(3) Chemical analysis of phloem sap.

Chemical analysis of phloem sap

Chemical analysis of sieve tube sap proves to be a sweet, concentrated solution of sugar. This analysis support the evidence from the ringing experiments in pointing to the phloem as the main channel for the translocation of organic solutes.

Mechanism of Phloem Translocationfusion theory,

Electro-osmotic theory,Several theories have been proposed to explain the mechanism of phloem translocation (e.g., Diffusion hypothesis, Activated dif- hypothesis, etc.). Among them Munch’s mass flow hypothesis is the most widely ac cepted, which is briefly discussed below-Interfacial flow hypothesis,

Munch’s mass flow or pressure flow hypothesis.

The mass flow or pressure flow theory was first proposed by Munch (1930)) and later elaborated by Crafts (1938) and others. The Munch’s hypothesis postulated the movement of protoplasm en mass along a turgor pressure gradient, induced by a maintained gradient of water potential. The mass flow of organic solute takes place from the site of higher concentration to the site of lower concentration.tno gaz ads, balling the principle of mass flow hypothesis is illustrated in the model .

The two chambers A and B with semiperme- able membranes are connected by a tube. These chambers are immersed in two reservoirs connected through a tube T. The reservoirs are filled with water. The chamber A contains concentrated sugar solution and chamber B contains dilute sugar solution. The OP of chamber A is high as compared to B. The water enters into chamber A and its turgor pressure is increased. This increase in turgor pressure causes mass flow of sugar solution to chamber B under the influence of turgor pressure gradient. The movement of solute will continue till the solution in both the chambers attain the same concentration.

In the living plants, carbohydrates are continuously synthesized in mesophyll cells of leaves. Consequently, the OP of these cells is increased which then absorbs water from neighbouring cells. The turgor pressure of mesophyll cells is also increased. This allows some of the cell contents to pass into the sieve tubes. On the other end, in those parts like roots and storage organs, cells either consume food material or they convert it into insoluble storage forms. This transformation results decrease in OP. With the result their turgor pressure is also decreased. In this way, a turgor pressure gradient is formed between the supply site and consumption site and along with this gradient a mass flow of solute takes place.

Munch’s Hypothesis has been supported further by the following

(a) When a woody or herbaceous plant is girdled, the sap containing high sugar content exudates from the cut end.

(b) Positive concentration gradient dis- appears when the plants are defoliated

(c) Movement of viruses and growth hormones is fast in illuminated leaves as compared to shaded leaves.

Objections to the Munch’s Hypothesis

(a) The hypothesis fails to explain the bidirectional movement of metabolites which is common in plants.

(b) Osmotic pressure of mesophyll cells and that of root hair do not confirm the requirements.

(c) Munch hypothesis gives a passive role F to the sieve tube elements and the protoplasm.

Photosynthesis is the process by which green plants transform solar energy into chemical energy of carbohydrates. In addition, the process purifies atmospheric air by consuming carbon dioxide and liberating

2. Photosynthesis is the only source of energy for all living organisms. It provides food either directly or indirectly for most living things.

3. There are two steps in photosynthesis. The first step directly utilizes light energy and occurs in the thylakoids of chloroplasts. It is called light reaction or photochemical reaction. The second step utilizes the assimilatory power of light reaction and occurs in the stroma of chloroplasts. It is called dark reaction or thermochemical reaction.

4. Light reaction involves the participation of a large number of pigment molecules which are distributed in two separate complexes-PS I and PS II (Pigment System I and Pigment System II).

5. Each pigment system consists of a reaction centre, core complex and light harvesting antenna complex

The reaction centre of PS II is P680 (or chlorophyll asso) and that of PS I is P700 (or chlorophyll (700)

7. Visible light of different wavelengths is absorbed by pigments of antenna complex which transfer their absorbed energy to core complex and finally to reaction centres.

The reaction centres become photoexcited and transfer their outer valance electrons to acceptor molecules resulting in the production of reducing power.

9. PSI generates strong reductant NADPH. PS II produces a strong oxidant that forms oxygen from water. The initial steps of PS II result in the photo-oxidation (photolysis) of water involving participation of Mn CI and an unknown enzyme-water oxidizing enzyme.

10. Electrons accepted from reaction centres by the acceptors are passed down hill through the electron transportchain.

11. Non-cyclic electron transport occurs when PS II transfers its electrons to PS I, electrons from water move toPS II and NADP+ is reduced from electrons coming from PS L. Non cyclic electron transport, thus, involves the participation of both PS II and PS I and is responsible for reduction of NADP+, production evolution of molecular oxygen.

12. Cyclic electron transport occurs when the electrons released from PS I are returned back to PSI through electron transport pathway without reducing NADP. This pathway serves to generate extra ATP.

13. Hydrogen protons accumulate in thylakoid space as a result of photo-oxidation of water and electron transport through plastoquinone. An increase in the number of protons in thylakoid space creates an electrochemical gradient. When hydrogen protons flow down this gradient through ATPase complexes, ATP is synthesized from ADP and inorganic phosphate by ATP synthase.

14. The energy yield of the light reaction is stored in ATP and NADPH. These two molecules together are called assimilatory power which are used to fix CO, into carbohydrates in dark reaction.

15. Carbon dioxide fixation in dark reaction was thoroughly worked out by M. Calvin. Therefore, the complete pathway is called Calvin cycle. Since the first stable product of Calvin cycle is a three carbon compound(3-phosphoglyceric acid), the cycle is called C, cycle.

16. Calvin cycle or C, cycle primarily consists of three major steps: (i) carboxylation, (ii) reversal of glycolysis and (iii) regeneration of RuBP.

17. Carboxylation occurs in presence of enzyme ribulose bisphosphate carboxylase (RUBISCO). In presence of RUBISCO, carbon dioxide combines with ribulose-1, 5-bisphosphate (RuBP) to form a six-carbon compound which splits into two molecules of three carbon compounds-3-phosphoglyceric acid (PGA).

18. Reversal of glycolysis involves conversion of phosphoglyceric acid to phosphoglyceraldehyde, utilizing ATPand NADPH. Further reactions lead to the synthesis of fructose-1, 6-diphosphate (hexose).

19. Ribulose-1, 5-bisphosphate is regenerated through a complex series of reactions. Thus, six turns of Calvin cycle result in the formation of one molecule of glucose (C6H12O6)-

20. Under high temperature and oxygen concentration, the enzyme RUBISCO acts as ribulose bisphosphate oxygenase and catalyses photorespiration rather than Calvin cycle.

21 Photorespiration interferes with the successful functioning of Calvin cycle. Photorespiration is quite different from respiration. It is harmful to plants because as much as half the photosynthetically fixed carbon dioxide (in the form of RuBP) may be lost into atmosphere through this process.

22. Plants such as sugarcane and sorghum fix atmospheric carbon dioxide into phosphoenol pyruvic acid to produce oxaloacetic acid (a C acid). Such plants are called C, plants.

23, In C, plants, a four-carbon oxaloacetic acid is formed within mesophyll cells. This acid may get converted into malic acid or aspartic acid. These molecules enter into bundle sheath cells where they get converted to pyruvic acid releasing CO₂. The CO2 is used up by C, cycle to synthesize carbohydrates

.24 In C plants, CO, fixation occurs in mesophyll cells and Calvin cycle occurs in bundle sheath cells avoiding the disadvantage of photorespiration.

25. The rate of photosynthesis is influenced by various external and internal factors. The external factors are light quality and quantity, CO₂, water and temperature. The internal factors are age of leaf, chlorophyll contentand histology of leaf.

26. The sugars, synthesized in leaves, are translocated downwards, upwards and laterally to storage organs mainly through phloem. They are translocated in the form of sucrase.

27. Translocation of organic solutes is a metabolic process and occurs due to differences in hydrostatic pressure between the source (leaf) and sink (storage organs).


Q Succulents have their stomata closed during the day time. How do they get CO₂ for photosynthesis?

Ans. Succulents (e.g., Bryophyllum, Kalanchoe, Sedum, Calycinum, etc.) are xerophytes and grow under semi-arid conditions. They have their stomata closed during the day time and open at night (in dark). This helps these plants to conserve their water content which otherwise would be lost under extremely dry conditions. These plants fix atmospheric CO2 into organic acids (such as malic acid, oxalo aloacetic acid, etc.) at night when the stomata are open. Conversion of CO2 into organic acids is called (lacidification. The fixed CO2 is released during the day by the process of deacidification which is used into photosynthesis.

2. Can photosynthesis occur in a land plant if it is totally submerged in water?

Ans. Land plants get gaseous form of CO2 from the atmosphere which is utilized in photosynthesis. The atmospheric CO2 enters into the leaf through stomata. The land plants close their stomata if they are Submerged in water. As soon as the stomata get closed the entry of CO2 is also stopped. Therefore, photosynthesis can not occur in a land plant if it is totally submerged in more

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