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Section A


Respiration is an intracellular process of oxidation-reduction reactions by which cells release energy from organic compounds(e.g., sugars) to generate ATP ( to do other metabolic processes) through a series of chemical reactions involving the transfer of electrons.In respiration, glucose is oxidized and thus releases energy. Oxygen is reduced to form water.The carbon atoms of the sugar molecule are released as carbon dioxide (CO2).

This equation is essentially the opposite of photosynthesis. Photosynthesis is a building process, while respiration is a breaking-down process. Respiration takes place in the mitochondria of every cell in a organism

Difference between Photosynthesis and Respiration.

Photosynthesis Respiration
Produces food Uses food
Stores energy Releases energy
Uses water Produces water
Uses carbon dioxide Produces carbon dioxide
Releases oxygen Uses oxygen
Occurs in sunlight Occurs in the dark as well as light

It is basically an energy releasing and supplying process. The energy released in respiration is of two types :

(a) Chemical energy, i.e. ATP. It is utilized for the cellular activities.

(b) Heat energy. It is mostly lost. Depending upon the availability of oxygen as an oxidant, respiration is of two types : (1) aerobic respiration, in which oxygen in used in the respiratory break down of organic substrate, and (2) anaerobic respiration, in which oxygen is not used in the respiratory breakdown of organic substrate.

Respiration can be divided into the following stages (image 904):- The process of respiration is completed in two phases:-

In aerobic as well as anaerobic respiration, initial sequence of events are the same, therefore collectively known as glycolysis.

  1. Glycolysis Glycolysis is the breakdown of a 6-carbon glucose molecule into two molecules of 3-carbon pyruvate; it takes place in the cytoplasm (cytosol)of all living cells.and are common to both types of respiration.Therefore, glycolysis is also known as cytoplasmic respiration.

  1. Reactions in Phase II depend upon whether O2 is utilized or not. Respiration is of two types : -
    • Aerobic respiration :- When free or molecular oxygen participates in the respiratory breakdown of organic substrate, it is called aerobic respiration. (In this process, free molecular oxygen acts as the final electron acceptor.
    • Anaerobic respiration :- When the respiratory breakdown of organic substrate takes place without participation of free molecular oxygen, it is called anaerobic respiration. In anaerobic respiration, pyruvate is used in fermentation.

Chemical equation for anaerobic respiration:-

Glycolysis ( also known as E.M.P. pathway = Embden - Meyerhof - Parnas Pathway ):-Glycolysis (glyco = sugar; lysis = splitting) splits a 6-carbon sugar, glucose, into two molecules of 3-carbon pyruvate in a series of steps, each catalyzed by a particular enzyme.Glycolysis takes place in the cytoplasm (cytosol)of all living cells, not in mitochondria, and does not require the presence of oxygen.Therefore, glycolysis is also known as cytoplasmic respiration.

The various steps of glycolysis are:-

  1. Phosphorylation of glucose This is called the preparatory phase of glycolysis . The reactions are :-
    • In this step, 6-C glucose is converted into glucose-6-phosphate. One ATP is used in the reaction.
    • The next step involves isomerization of glucose 6-phosphate into Fructose 6-phosphate.
    • Fructose-6-phosphate is then phosphorylated to fructose-1, 6-diphosphate. This reaction utilizes one ATP.
    • Thus, two ATP are used up in the preparatory phase.The initial 6-carbon sugar( glucose) is transformed into an unstable fructose 1, 6-diphosphate for easy breakdown. This addition of phosphate groups is called phosphorylation. Phosphorylation activates the sugar and prevents it from getting out of the cell.
  2. Splitting of fructose-1, 6 - diphosphate:-
    • The molecule of 6-C fructose-1, 6-diphosphate is broken down into two molecules of 3-C triose phosphates. One is 3-phosphoglyceraldehyde (3-PGAL) and the other is dihydroxy acetone phosphate (DHAP). These are isomers of each other, with each molecule having the formula C3H4O2-phosphate.
    • However, further reactions in glycolysis utilize only PGAL. Therefore, DHAP is also converted into PGAL.
    • Thus, two molecules of 3-PGAL are formed from the splitting of one molecule fructose-1, 6- diphosphate.
  3. Formation of 3-C pyruvic acid:-
    • 3-PGAL in combination with one inorganic phosphate (derived from H3 Po4) and co-enzyme NAD (nicotinamide adenine dinucleotide) is converted into 1, 3 diphosphoglyceric acid (1, 3 diPGA). This is an oxidation and phosphorylation reaction. The reduced co-enzyme NADH2 is formed in the process.Hydrogen is then removed from the triose phosphate(PGAL) and transferred to the carrier molecule NAD (nicotinamide adenine dinucleotide) and the reduced NAD (NADH2) is used during oxidative phosphorylation . The NADH+ accepts 2 electrons and one proton [one hydrogen ion]; the second hydrogen ion remains free.
    • In the next step, 1, 3-diPGA undergoes dephosphorylation to form 3-phosphoglyceric acid (3-PGA). Phosphate removed from each diPGA is transferred to by ADP to form ATP. Two ATP molecules are formed at this step.
    • 3-PGA is transformed into 2-PGA.
    • In the next step, 2-PGA is converted into 2-phosphoenol pyruvic acid (2-PEP) with the loss of water (dehydration).
    • Substrate-level phosphorylation In the final step of glycolysis, 2-PEP is dephosphorylated to form 3-C pyruvic acid, which is the end product of glycolysis. The reaction produces 2 ATP.