Chapter no 13:Respiration and Energy transfer
Respiration and Energy Production – Grade 7
13.1 Formation of ATP
ATP (Adenosine Triphosphate) is the energy currency of the cell. The formation of ATP is called phosphorylation.
In nature, phosphorylation occurs in three different ways:
- Photophosphorylation – occurs during photosynthesis.
- Substrate-level phosphorylation
- Oxidative phosphorylation
Substrate-level phosphorylation is the direct transfer of a phosphate group from a substrate to ADP to form ATP. It occurs in the cytoplasm and in the mitochondrial matrix.
Oxidative phosphorylation occurs on the inner mitochondrial membrane. Here, ATP is formed using the energy released during oxidation of NADH+H+ and FADH2.
When energy is required, ATP is broken down (hydrolysed) into ADP and inorganic phosphate, releasing energy for metabolic activities.
Respiration is a catabolic process in which complex organic compounds are broken down to release energy.
Cellular respiration occurs in two ways:
- Anaerobic respiration
- Aerobic respiration
13.2 Anaerobic Respiration
Anaerobic respiration is respiration that occurs in the absence of oxygen. It is also called fermentation.
It includes glycolysis followed by incomplete breakdown of pyruvic acid.
Glycolysis
Glycolysis is the process of breakdown of glucose into two molecules of pyruvic acid.
- Occurs in cytoplasm
- Common to both aerobic and anaerobic respiration
- Consists of ten enzymatic steps
Glycolysis has two phases:
- Preparatory phase
- Pay-off phase
Preparatory Phase
In this phase, glucose is phosphorylated twice using two ATP molecules. It is converted into fructose 1,6-bisphosphate. This splits into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.
Dihydroxyacetone phosphate is converted into another molecule of glyceraldehyde-3-phosphate.
Pay-off Phase
Both molecules of glyceraldehyde-3-phosphate are oxidized and converted into pyruvic acid.
- 4 ATP molecules are formed
- Net gain of ATP = 2
- ATP formed by substrate-level phosphorylation
Glycolysis is controlled by enzymes such as hexokinase, PFK-1 and pyruvate kinase and hormones like insulin and glucagon.
Anaerobic Respiration in Muscles
In muscles, pyruvic acid is converted into lactic acid. This causes muscle fatigue after vigorous exercise. During rest, lactic acid is converted back to pyruvic acid.
Anaerobic Respiration in Yeast
In yeast, pyruvic acid is converted into ethanol and carbon dioxide. This is called alcoholic fermentation.
13.3 Aerobic Respiration
Aerobic respiration occurs in the presence of oxygen. Glucose is completely oxidized to carbon dioxide and water.
Steps of aerobic respiration:
- Glycolysis
- Formation of Acetyl-CoA
- Krebs cycle
- Electron transport chain
- Terminal oxidation
Conversion of Pyruvic Acid to Acetyl-CoA
Pyruvic acid is converted into acetyl-CoA by oxidative decarboxylation. This reaction is catalyzed by pyruvate dehydrogenase complex.
This reaction links glycolysis and Krebs cycle.
Krebs Cycle (Citric Acid Cycle)
Krebs cycle occurs in the mitochondrial matrix. It oxidizes acetyl-CoA to carbon dioxide.
- NADH+H+ and FADH2 are produced
- One ATP (GTP) is formed
- Oxaloacetic acid is regenerated
Amphibolic Pathway
Krebs cycle acts as both catabolic and anabolic pathway. Intermediates are used for synthesis of amino acids and fatty acids.
Electron Transport Chain (ETS)
ETS occurs on the inner mitochondrial membrane. It consists of complexes I to V.
- NADH+H+ produces 3 ATP
- FADH2 produces 2 ATP
Electrons finally combine with oxygen to form water. This is called terminal oxidation.
ATP synthesis occurs by chemiosmosis as proposed by Peter Mitchell.
Significance of ETS
- Produces maximum ATP
- Regenerates NAD+ and FAD
- Produces water
- Releases energy stepwise
13.4 Utility of Stepwise Oxidation
- Better energy utilization
- Control of metabolic rate
- Formation of intermediates
Always Remember
Removal of hydrogen is the primary step in respiration. Oxygen acts as a final hydrogen acceptor forming water.
Respiratory Quotient (RQ)
RQ is the ratio of volume of CO2 released to O2 consumed.
- Carbohydrates: RQ = 1
- Fats: RQ = 0.7
- Proteins: RQ = 0.9
- Anaerobic respiration: RQ = infinity
Significance of Respiration
- Provides energy
- Helps in growth and repair
- Provides intermediates
- Maintains atmospheric balance
- Used in industries like baking and brewing
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