Notes-Class-11-Science-Biology-Chapter-13-Respiration and Energy Transfer-Maharashtra Board

Topics to be Learn : Introduction Formation of ATP Anaerobic respiration Aerobic respiration Utility of stepwise oxidation

Introduction :

• Maintenance of life requires continuous supply of energy.
• Respiration fulfills the continuous need of energy.
• Nutrients like carbohydrates, fats and proteins are used for energy production
• At cellular level, organisms require energy to carry out different metabolic activities.
• The energy is made available by oxidizing/breaking the food.
• Oxygen is required by aerobic organisms for breaking the food and carbon dioxide is released as a byproduct of oxidation.

Formation of ATP :

ATP :

• ATP (Adenosine triphosphate) is an energy rich organic compound.
• Whenever energy is released, ATP synthesis takes place from ADP and Pi.
• Whenever energy is required for any metabolic reactions, ATP is hydrolysed and energy released as the high energy phosphate bond is broken.
• Therefore, ATP is called energy currency of the cell.

Phosphorylation : Formation of ATP is called as phosphorylation. ATP is formed by addition inorganic phosphate to ADP

ADP + Pi ⇌ ATP

Different ways of Phosphorylation : Phosphorylation occurs in three different ways as - photophosphorylation, Substrate level phosphorylation and oxidative phosphorylation.

Substrate-level phosphorylation : It is a direct phosphorylation of ADP by transfer of a phosphate group from any suitable substrate. It occurs in cytoplasm of the cells and matrix of mitochondria.

Oxidative phosphorylation : In oxidative phosphorylation ATP is synthesize by using the energy released during the oxidation of substrates like NADH + H⁺ and FADH₂. This occurs on the inner mitochondrial membrane only.

Anabolic and catabolic process :

• Anabolic process— Photosynthesis (Biosynthetic process).
• Catabolic process— Respiration (Breakdown process).

Respiration : Respiration is a catabolic process wherein complex organic substrate is oxidized to simple components to generate biological energy. i.e. ATP.

Cellular respiration occurs in two different ways as anaerobic and aerobic respiration.

Anaerobic respiration :

• Anaerobic respiration is the cellular respiration that does not involve the oxygen at all. It is also called as fermentation.
• It involves glycolysis where the product of glycolysis i.e. pyruvate is converted to either lactic acid or ethanol.

Reactions of glycolysis :

Irreversible chemical reactions: '

• Some chemical reactions can occur in only one direction i.e. these reactions are irreversible reactions.
• The reactants can change to the products, but the products cannot change back to the reactants.
• Enzymes that catalyse the irreversible reactions : Hexokinase, Phosphofructokinase, Phosphoglycerate kinase and Pyruvate kinase

Reversible chemical reactions:

• Some chemical reactions can occur in both directions i.e. these reactions are reversible reactions. In this case the reactants change to the products and the products can change back to the reactants, at least under specific conditions.
• Out of ten, four are irreversible reactions which involves the enzyme kinase that is required for phosphorylation reactions, these reactions involve large negative energy AG, hence the reactions are irreversible. Other reversible reactions do not involve high negative energy hence are reversible.

Remember :

• Dihydroxy acetone phosphate (DHAP) and. 3-phosphoglyceraldehyde (3-PGAL) are the products of cleavage in glycolysis.
• In glycolysis, dehydration occurs when 2-Phosphoglyceric acid loses a water molecule (dehydration) to form phosphoenol pyruvic acid in presence of the enzyme enolase.
• Glycolysis is only source of energy production in erythrocytes, renal medulla, brain and sperm
• Some plant tissues which are modified to store starch (like potato) mainly depend upon glycolysis for energy production.

 Regulation of glycolysis:

• Glycolysis is under tight control. Its rate depends upon the requirement of ATP and many other factors.
• Glycolytic rate control is achieved by complex interplay between ATP consumption, NADH₂ regeneration and regulation of various glycolytic enzymes like hexokinase, PFK-1, pyruvate kinase, etc.
• Besides, it is also controlled by hormones like glucagon, epinephrine and insulin.

Role of Mg⁺⁺, Zn⁺⁺ in various steps of glycolysis :

• Mg⁺⁺ and Zn⁺⁺ are the cofactors that are tightly bound to enzymes and helps the enzymes to perform their functions.
• They regulate the activity of the most important enzymes like Hexokinase, Phosphofmctokinase, Triose phosphate dehydrogenase, Phosphoglycerate kinase, Enolase, Pymvate kinase.

Q. Why is glycolysis considered as biochemical proof of evolution?

Anaerobic respiration in man and yeast :

(i) Anaerobic respiration in man :

In absence of oxygen anaerobic respiration takes place in muscles of man.

Lactic acid Fermentation :In muscles, the NADH+H⁺ produced during glycolysis is reoxidized to NAD⁺ by donating one proton and two electrons to pyruvic acid which yields lactic acid.

• Skeletal muscles usually derive their energy by anaerobic respiration.
• After vigorous exercise lactic acid accumulates, leading to muscle fatigue.
• During rest, however, the lactic acid is reconverted to pyruvic acid and is channeled back into the aerobic respiration pathway.
• White muscle fibres mainly performs anaerobic respiration

Anaerobic respiration in yeast :

• Yeast shows both aerobic and anaerobic respiration depending upon the presence or absence of oxygen.
• In absence of oxygen anaerobic respiration takes place in cytoplasm of the yeast.
• In absence of oxygen, the pyruvate undergoes anaerobic respiration where it is decarboxylated to acetaldehyde. The acetaldehyde is then reduced by NADH+H⁺ to ethanol and carbon dioxide. This type of anaerobic respiration is termed alcoholic fermentation.
• Accumulation of ethanol by fermentation in a culture of yeast may stop further multiplication and lead to the death of cells..
• In the presence of oxygen however, it can respire aerobically to produce CO₂ and H₂O.

Q. Why do athletes like sprinters have higher proportion of white muscle fibers?

Remember :

• In anaerobic respiration, incomplete oxidation takes place which leads to release of less amount of energy.
• Some of the products of anaerobic respiration can be oxidised further to release energy which shows that anaerobic respiration does not liberate the whole energy contained in the respiratory substrate.
• NADH₂ does not produce ATP, as electron transport is absent.
• Only 2 ATP molecules are generated from one molecule of glucose during anaerobic respiration.

Aerobic Respiration :

• Aerobic respiration involves molecular oxygen during oxidation of glucose, which acts as a final electron acceptor, liberated during oxidation of glucose.
• In this type of respiration, the glucose is completely oxidized to CO₂ and H₂O with release of large amount of energy.
• It involves glycolysis, acetyl CoA formation (connecting link reaction), Krebs cycle, electron transfer chain reaction and terminal oxidation.
• Aerobic respiration occurs in the mitochorldria in eukaryotes

Conversion of pyruvic acid to Acetyl CoA :

• Conversion of pyruvic acid to Acetyl CoA is an oxidative decarboxylation reaction.
• It is catalyzed by multi enzyme complex-pyruvate dehydrogenase complex (PDH). This enzyme is present in mitochondria of eukaryotes and cytosol of prokaryotes.
• This reaction is called as ‘connecting link‘ reaction between glycolysis and Krebs cycle.

Know This : Vitamin B1 is important for maintaining good health. Pyruvate dehydrogenase complex needs thiamin (vitamin B1) as a co-enzyme. It cannot function in absence of vitamin B1. Hence, thiamin deficiency causes pyruvic acidosis and lactic acidosis, the life threatening conditions. Hence balanced diet is very important in maintenance of health.

Significance of Krebs cycle:

• It is a common pathway for breakdown of carbohydrates, proteins and fats, during respiration.
• It forms reduced co-enzymes like NADH₂ and FADH₂.
• Krebs cycle has the potential to produce 24 ATP from one molecule of glucose.
• The CO₂ which is liberated is useful in photosynthesis.
• It forms a number of intermediate products which serve as building blocks for the synthesis of other complex organic compounds

Amphibolic Pathway :

• Respiration is considered as a catabolic process; however, it is not entirely correct in case of Krebs cycle.
• Many reactions of Krebs cycle involve oxidation of acetyl CoA to release energy and CO₂.
• However, the breakdown of respiratory substrates provides intermediates like α-ketoglutarate, oxaloacetate are used as precursors for synthesis of fatty acids, glutamic acid and aspartic acid respectively.
• Thus, as the same respiratory process acts as catabolic as well as anabolic pathway for synthesis of various intermediate metabolic products, it is called amphibolic pathway.

Significance of ETS :

• The electron transport system (ETS) or terminal oxidation generates major amount of energy in the form of ATP molecules, 34 ATP molecules out of total 38 ATP molecules are produced through ETS.
• It regenerates oxidized coenzymes such as NAD⁺ and FAD⁺ from their reduced forms (NADH+H⁺ and FADH²) for recycling.
• It also produces water molecules.
• It releases energy in a stepwise manner to prevent damage of cells.

Know This : Oxidative phosphorylation : Oxidative phosphorylation: It is a metabolic pathway that uses energy released by the oxidation of substrates to produce ATP. . Oxidative phosphorylation takes place in the mitochondrial membrane. Many intermediate products during respiration are oxidised and release 2H+. The released hydrogen is trapped by NAD+ or FAD+. Electrons pass through electron transport system to produce ATP and metabolic water.

 Balance sheet for ATP by aerobic oxidation of 1 glucose molecule.

Glycolysis, TCA cycle and electron transport chain link :

• The coenzymes are initially present in the form of NAD⁺ and FAD+ which latter get reduced to NADH+H⁺ and FADH+H⁺ by accepting the hydrogen from organic substrate during glycolysis, link reaction and Kerbs cycle.
• During glycolysis, glucose is oxidised to two molecules of pyruvic acid with net gain 2 molecules of NADH+H⁺.
• This pyruvic acid undergoes link reaction to form two molecules of acetyl CoA and two molecules of NADH+H⁺.
• Acetyl CoA, thus formed enters into the Krebs cycle and it gets completely oxidised to CO₂ and H₂O with a net gain of 6 NADH+H⁺ and 2 FADH+H⁺ are formed.
• During ETS, reduced coenzymes are reoxidized to NAD⁺ and FAD⁺ with a net gain of 34 ATPs.
• The ATPs thus formed are used during glycolysis.
• The oxidized NAD⁺ and FAD⁺ will again accept the hydrogen from organic substrate. Thus, reduced coenzymes are converted back to their oxidized forms by dehydrogenation to keep the process going.

Utility of stepwise oxidation :

Advantage of step wise energy release in respiration :

• In ETS energy is released in step wise manner to prevent damage of cells.
• A stepwise release of the chemical bond energy facilitates the utilization of a relatively higher proportion of that energy m ATP synthesis.
• Activities of enzymes for the different steps may be enhanced or inhibited by specific compounds. This provides a means of controlling the rate of the pathway and the energy output according to need of the cell.
• The same pathway may be utilized for forming intermediates used in the synthesis of other biomolecules like amino acids.

Remember : Removal of Hydrogen from respiratory materials is the primary process in respiration : The fact that during respiration oxygen is taken in and carbon dioxide is given out may give a false impression that respiratory materials directly unite with oxygen. It must be remembered that oxygen does not play such a primary role in the process of respiration. The primary process in respiration consists in removal of hydrogen from the respiratory materials. The reactions in which hydrogen is removed are catalyzed by enzymes called dehydrogenases free hydrogen cannot exists in the cell. As soon as it is removed from respiratory material it is picked up by substances known as acceptors. In aerobic respiration this hydrogen is ultimately handed over to oxygen. These two combine with each other and form water.

Respiratory substrates : Respiratory substrates are the molecules that are oxidized during respiration to release energy which can be used for ATP synthesis. Carbohydrates, fats and proteins are the common respiratory substrate.

Respiratory Quotient :

Ratio of volume of CO₂ released to the volume of O₂ consumed in respiration is called the respiratory quotient (RQ) or respiratory ratio. It depends on the type of respiratory substrate.

R.Q. = \(\frac{volume\,of\,CO_2\,released}{volume\,of\,O_2\,consumed}\)

Significance of Respiration :

• Respiration provides energy for synthesis of biomolecules.
• It is also a source of energy for cell division, growth, repairs and replacement of worn out parts, movements, locomotion etc.
• Various intermediates of Krebs cycle are used as building blocks for synthesis of other complex compounds.
• Coupled with photosynthesis, it helps to maintain the balance between CO₂ and O₂ in the atmosphere.
• Anaerobic respiration (fermentation) is used in various industries such as dairies, bakeries, distilleries, leather industries, paper industries etc. It is used in the commercial production of alcohol, organic acids, vitamins, antibiotics etc.