CHEMIOSMOTIC MECHANISM
Chemiosmosis is the movement of ions across a semipermeable membrane down their electrochemical gradient. It is a type of diffusion, where ions move across a membrane from a region of higher concentration to the region of lower concentration. ATP is generated by the movement of hydrogen ions across a membrane during cellular respiration or photosynthesis.
This hypothesis was first proposed by Peter Mitchell (1961). The thylakoid membranes within the chloroplast are involved in electron transport. ATP synthesis occurs through chemiosmotic mechanism in chloroplast. According to Peter Mitchell, the energy transducing membranes are impermeable to H+. The electron carriers are organized asymmetrically in the membranes. Beside the electron carriers, some proton carriers are also present, which translocate protons across the membrane against the proton gradient. The ion concentration differences and electric potential differences across the membranes provide free energy.
In thylakoids, water molecules split in the lumen side resulting in the accumulation of protons in the lumen of the thylakoids. As a result, the lumen becomes acidic (high concentration of positive hydrogen ions) and the stroma becomes more alkaline(low concentration of hydrogen ions). Thus a difference in the proton concentration across the membrane is developed. Since protons carry a positive charge, it also contributes to an electrical potential gradient across the membrane. These two gradients together constitute a proton motive force. It requires a certain amount of energy to pump the protons into the lumen against a proton motive force. This energy is made available by the electron transport.
The thylakoid membrane has a low conductance for protons. As a result, the protons cannot simply diffuse back to the stroma. The diffusion of the protons to the stroma takes place only with the help of highly specific protein lined channel that extends through the membranes. These channels are a part of the ATP synthesizing enzyme, ATP synthase.
The enzyme, ATP synthase consists of two multipeptide complexes CF0 and CF1. The CF0 is a hydrophobic complex (insoluble in water) is largely embedded in the membrane, it forms the transmembrane channel. The other complex CF1 is hydrophilic (soluble in water). CF1 is attached to CF0 on the stroma side. The CF1 complex contains active sites for ATP synthesis while the CF0 complex forms a proton channel across the membrane. The CF0 complex channels the energy of the proton gradient towards active sites of the enzyme. Thus, the requirements for chemiosmosis are a membrane, a proton gradient and an enzyme, ATP synthase. The protons are pumped into the lumen of the thylakoids from the stroma as a result of which, a proton gradient is developed. Simultaneously, the enzyme, ATP synthase allows the protons to return to the stroma, thereby, establishing a proton circuit. The energy conserved in the proton gradient is released. This energy activates the enzyme, ATP synthase for the synthesis of ATP.
Chemiosmosis
DIFFERENCE BETWEEN PHOTOSYNTHESIS AND RESPIRATION
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PHOTOSYNTHESIS |
RESPIRATION |
| 1. It takes place in the presence of light. | 1. It takes place during day as well as night. |
| 2. Photosynthesis occurs in the cell containing chlorophyll. | 2. Respiration occurs in all the living cells. |
| 3. The raw materials are carbon dioxide and water. | The raw materials are oxygen and carbohydrates. |
| 4. The end products are oxygen and carbohyrates. | 4. The end products are carbon dioxide and water. |
| 5. It absorbs sunlight and is an endothermic reaction. | 5. It gives out energy and is, therefore, exergonic. |
| 6. Photosynthesis is anabolic process. | 6. Respiration is a catabolic process. |
| 7. ATP is formed by the conversion of light energy (photophosphorylation). | 7. Oxidation of carbohydrates yields ATP (oxidative phosphorylation) |
| 8. Hydrogen released during the photolysis of water is accepted by NADP, which is reduced to NADPH2. | 8. Hydrogen released in oxidation of carbohydrates is trapped by a hydrogen acceptor (NAD), which is reduced to NADH2. |
| 9. Reactions of photosynthesis occur within the chloroplast. | 9. Glycolysis occurs in cytoplasm and oxidation of pyruvic acid occurs within mitochondria. |
| 10. ATP formed is utilized in the dark reaction of photosynthesis. | 10. ATP synthesized in respiration is utilized in various metabolic processes. |
| 11. Gaseous exchange involves release of oxygen and absorption of carbon dioxide. | 11. Gaseous exchange involves absorption of oxygen and evolution of carbon dioxide. |
| 12. Rate of photosynthesis is higher than the rate of respiration. | The rate of respiration is usually less than that of photosynthesis. |
DIFFERENCE BETWEEN GLYCOLYSIS AND KREBS CYCLE
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GLYCOLYSIS |
KREBS CYCLE |
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1. It occurs inside the cytoplasm. |
1. It occurs inside mitochondria. |
| 2. The process is common to both aerobic and anaerobic mode of respirations. | 2. It occurs in the presence of oxygen. |
| 3. It results in the breakdown of one molecule of glucose into two molecules of pyruvic acid. | 3. It results in the breakdown of pyruvic acid into inorganic substances, carbon dioxide and water. |
| 4. Glycolysis consumes two molecules of ATP for the initial phosphorylation of substrate molecule. | 4. It does not consume ATP. |
| 5. In glycolysis, one molecule of glucose liberates four molecules of ATP through substrate level phosphorylation. | 5. In Krebs cycle, two acetyl residues liberate two ATP molecules through substrate level phosphorylation. |
| 6. Gain is two molecules of NADH2 and two molecules of ATP for every molecule of glucose broken down. | 6. Krebs cycle produces six molecules of NADH2 and two molecules of FADH2 for every two molecules of acetyl CoA oxidised by it. Two molecules of NADH2 are liberated during conversation of two molecules of pyruvic acid to acetyl CoA. |
| 7. No carbon dioxide is evolved in glycolysis. | 7. Carbon dioxide is evolved in glycolysis. |
| 8. Oxygen is not required for glycolysis. | 8. Krebs cycle uses oxygen as terminal oxidant. |