Respiration

• Respiration


  The process of breakdown of food molecules to release energy, which is then trapped in the form of ATP
• All living organisms need energy for carrying out daily life activities
• Food


  The macromolecules that are oxidised to release energy
• Green plants and cyanobacteria can prepare their own food by the process of photosynthesis
• In green plants, only cells containing chloroplasts carry out photosynthesis
• Animals are heterotrophic and obtain food from plants
• Ultimately, all the food that is respired for life processes comes from photosynthesis
• Respiration

  The breaking of the C-C bonds of complex compounds through oxidation within the cells, leading to release of considerable amount of energy
• Respiratory substrates


  The compounds that are oxidised during respiration to release energy
• Usually carbohydrates are oxidised to release energy, but proteins, fats and even organic acids can also be used as respiratory substances in some plants, under certain conditions
• The energy released by oxidation in respiration is not used directly, but is used to synthesise ATP, which acts as the energy currency of the cell
• Plants do not have specialised organs for gaseous exchange, but have stomata and lenticels for this purpose
• Each living cell in a plant is located quite close to the surface of the plant, facilitating gas exchange
• The complete combustion of glucose produces CO2, H2O and energy, most of which is given out as heat
• Some cells can live in anaerobic conditions where oxygen may not be available
• Glycolysis

  The partial oxidation of glucose to form two molecules of pyruvic acid, which occurs in the cytoplasm and is present in all living organisms
• Glycolysis

  1. Glucose phosphorylation

  2. Glucose to fructose-6-phosphate conversion

  3. Fructose-6-phosphate to fructose-1,6-bisphosphate conversion

  4. Fructose-1,6-bisphosphate splitting

  5. 3-phosphoglyceraldehyde to 1,3-bisphosphoglycerate conversion

  6. 1,3-bisphosphoglycerate to 3-phosphoglyceric acid conversion

  7. 3-phosphoglyceric acid to phosphoenolpyruvate conversion

  8. Phosphoenolpyruvate to pyruvic acid conversion
• In glycolysis, 2 ATP molecules are directly synthesised from one glucose molecule
• Pyruvic acid


  The key product of glycolysis, whose metabolic fate depends on the cellular need
• Metabolic fates of pyruvate

  • Lactic acid fermentation
  • Alcoholic fermentation
  • Aerobic respiration
• Glycolysis pathway from one glucose molecule

  1. Glucose (6C)

  2. Glucose-6-phosphate (6C)

  3. Fructose-6-phosphate (6C)

  4. Fructose1, 6-bisphosphate (6C)

  5. Triose phosphate (glyceraldehyde-3-phosphate) (3C)

  6. Triose phosphate (Dihydroxy acetone phosphate) (3C)

  7. 2 × Triose bisphosphate (1,3 bisphosphoglyceric acid) (3C)

  8. 2 × Triose phosphate (3-phosphoglyceric acid) (3C)

  9. 2 × 2-phosphoglycerate

  10. 2 × phosphoenolpyruvate

  11. 2 × Pyruvic acid (3C)
• Pyruvic acid


  Key product of glycolysis
• Metabolic fate of pyruvate

  1. Lactic acid fermentation

  2. Alcoholic fermentation

  3. Aerobic respiration
• Fermentation takes place under anaerobic conditions in many prokaryotes and unicellular eukaryotes
• For the complete oxidation of glucose to CO2 and H2O, organisms adopt Krebs' cycle which is also called as aerobic respiration
• Aerobic respiration requires O2 supply
• Alcoholic fermentation


  Pyruvic acid is converted to CO2 and ethanol by enzymes pyruvic acid decarboxylase and alcohol dehydrogenase
• Lactic acid fermentation


  Pyruvic acid is converted to lactic acid
• In animal cells, when oxygen is inadequate for cellular respiration, pyruvic acid is reduced to lactic acid by lactate dehydrogenase</b>
• In both lactic acid and alcohol fermentation, less than seven per cent of the energy in glucose is released and not all of it is trapped as high energy bonds of ATP
• Aerobic respiration is the process that leads to a complete oxidation of organic substances in the presence of oxygen, and releases CO2, water and a large amount of energy present in the substrate
• Aerobic respiration

  1. Pyruvate is transported from the cytoplasm into the mitochondria

  2. Complete oxidation of pyruvate by the stepwise removal of all the hydrogen atoms, leaving three molecules of CO2

  3. Passing on of the electrons removed as part of the hydrogen atoms to molecular O2 with simultaneous synthesis of ATP
• The first process of aerobic respiration takes place in the matrix of the mitochondria while the second process is located on the inner membrane of the mitochondria
• Oxidative decarboxylation of pyruvate


  1. Catalysed by pyruvic dehydrogenase

  2. Produces acetyl CoA, CO2, NADH
• Tricarboxylic Acid (TCA) Cycle

  1. Acetyl CoA enters the cycle

  2. Citrate is formed from acetyl CoA and oxaloacetic acid

  3. Citrate is isomerised to isocitrate

  4. Two successive decarboxylation steps form α-ketoglutaric acid and then succinyl-CoA

  5. Succinyl-CoA is oxidised to oxaloacetic acid, allowing the cycle to continue

  6. GTP is synthesised from succinyl-CoA

  7. NAD+ is reduced to NADH + H+ at three points

  8. FAD+ is reduced to FADH2 at one point
• The TCA cycle requires the continued replenishment of oxaloacetic acid and the regeneration of NAD+ and FAD+ from NADH and FADH2 respectively
• Electron Transport System (ETS) and Oxidative Phosphorylation

  1. Electrons from NADH are oxidised by NADH dehydrogenase (complex I) and transferred to ubiquinone

  2. Ubiquinone also receives electrons from FADH2 (complex II)

  3. Reduced ubiquinone (ubiquinol) is oxidised, transferring electrons to cytochrome c via cytochrome bc 1 complex (complex III)

  4. Cytochrome c transfers electrons to cytochrome c oxidase complex (complex IV)

  5. As electrons pass through complexes I-IV, they are coupled to ATP synthase (complex V) for the production of ATP from ADP and inorganic phosphate
• Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, while that of one molecule of FADH2 produces 2 molecules of ATP
• ATP synthase

  Complex that consists of F1 headpiece (site for ATP synthesis) and F0 (integral membrane protein complex that forms the channel for protons to cross the inner membrane)
• The passage of protons through the F0 channel is coupled to the catalytic site of the F1 component for the production of ATP