· Respiration is the process of breaking down complex organic compounds into simpler forms with the release of energy.
· Catabolic (Destructive) and energy-releasing process.
· Decrease in dry weight.
· It is the cellular oxidation process to releases energy.
· Respiratory substrates are carbohydrates, fats, proteins etc.
· Carbohydrate is a respiratory substrate for floating respiration.
· Fat/lipids – Respiratory substrate for accessory respiration
· Protein – Respiratory substrate for protoplasmic respiration.
Respiratory Quotient= `\frac{Volume \ of \ CO2 \ RELEASED}{Volume \ of \ O2 \ CONSUMED}`
· Respiratory Quotient (RQ) for
  1. Carbohydrate = 1 (Unity)
  2. Fats/proteins = < 1 (less than unity)
  3. Organic acid = > 1
  4. Anaerobic respiration = (infinity)
· ATP – Universal energy currency or carrier of the cell where energy is stored in terminal phosphate bond.

I. Aerobic Respiration
· In Aerobic respiration, there is complete oxidation of glucose under aerobic respiration.
· Complete breakdown of glucose requires four steps.
(1) Glycolysis or EMP pathway
(2) Oxidative Decarboxylation
(3) Kreb's cycle or TCA cycle or Critic acid cycle
(4) Oxidative phosphorylation or ETC (Electron Transport Chain)

· Glucose (C6H12O6) + 6O2 → 6CO2 + 6H2O + 686 Kcal
· Free oxygen is essential.
· Occurs in cytoplasm and mitochondria.
· Release of 38 ATP.
· Complete oxidation of glucose results in the formation of CO2 and H2O accompanied by the release of energy.
· In most forms of life, respiration occurs aerobically.

II. Anaerobic Respiration
· In this process, the complex organic compound is incompletely oxidised into simplest organic compounds like ethanol, lactic acid.
· Often called fermentation
· C6H12O6 → C2H5OH + CO2 + 50 kcal.
· Takes place in absence of oxygen.
· Incomplete oxidation of glucose into organic compounds like ethyl alcohol, accompanied by the release of energy.
· Water is not a product of this reaction.
· Common in germinating seeds, fruits, and yeast.
1. Obligate anaerobes – Truly anaerobic organisms, e.g. bacteria, if they are exposed to oxygen they die.
2. Facultative anaerobes – can respire both in the presence and absence of oxygen.

III. Lactic Acid Fermentation
(Lactic acid) + 36 Kcal 
· Occurs in bacteria and in muscles during high exercise.
· Anaerobic respiration releases only 2 ATP.

Mechanism of Respiration

· Common metabolic pathway for both aerobic and anaerobic respiration.
· Occurs in the cytoplasm.
· Glycolysis is a process of breakdown of one molecule of glucose (6C) into two molecules of pyruvic acid (3C).
· Does not require the presence of oxygen.
· Net gain of ATP in glycolysis = 4 – 2 = 2 ATP.
· Net gain of NADH2 = 2
· In aerobic glycolysis, 2NADH2 yields → 2×3 = 6 ATP
· Note: Net gain of ATP in anaerobic glycolysis is 2.
· Total production of ATP in glycolysis of aerobic respiration = 2 + 6 = 8 ATP
· Product of glycolysis is pyruvic acid.
· Net production of ATP from one molecule of pyruvic acid is 15.

Fig: Graphical Representation of Glycolysis

· In this process, pyruvic acid is oxidatively decarboxylated to form Acetyl CoA.
· It is the biochemical process linking glycolysis and the Krebs cycle.
· 2CH3COCOOH + 2NAD+ + 2CoA → 2Acetyl CoA + 2NADH2 + 2CO2
· No direct gain of ATP but 2NADH2 yields 2 ✖ 3 = 6 ATP through electron transport system (ETS).
· Organic compound linking glycolysis and Krebs cycle is Acetyl CoA.
· Occurs inside mitochondria.
· 1 Molecule of Acetyl CoA gives 12 ATP from its complete oxidation.

· It is an amphibolic pathway which is also called Tricarboxylic Acid Cycle (TCA) or Citric Acid Cycle.
· It was given by Hans Kreb.
· Amphibolic pathway means both constructive and destructive changes occur in the cycle.
· Common pathway for the interconversion of fats, proteins and carbohydrates.
· Acetyl CoA (2C) produces not only from pyruvic acid but also from the β-oxidation of fatty acids.
· Enzymes required for the Krebs cycle are found in the matrix of mitochondria.
· Opening door of Krebs cycle is Oxaloacetic acid (4C)
· 1st stable product of the Krebs cycle is Citric acid (a 6-carbon containing compound).
· No direct ATP formation.
· 6NADH2, 2FADH2, and 2GTP are produced in the Krebs cycle.
· Decarboxylation takes place at 2 places and 4CO2 are produced from one molecule of glucose in the Krebs cycle.
· NADH is the Universal Hydrogen acceptor.
· 4+2 = 6CO2 are released in aerobic respiration from one molecule of glucose.
  1. 6 NADH2 → 3 ✖ 6 = 18 ATP
  2. 2 FADH2 → 2 ✖ 2 = 4 ATP
  3. 2 GTP → 2 ✖ 1 = 2 ATP
  4. In Total, 18 + 4 + 2 = 24 ATP.
· 2 molecules of acetyl CoA yields 12 ATP on complete oxidation.
· Similarly, one molecule of pyruvic acid yields (12+3) = 15 ATP molecules on complete oxidation.

Note: 2NADH2 molecules are produced in the oxidative decarboxylation of 2 molecules of pyruvic acid. So one molecule of pyruvic acid yields 3 ATP during oxidative decarboxylation and 12 ATP during the Krebs cycle. Hence 1 molecule of pyruvic acid yields 15 ATP and 1 molecule of Acetyl CoA yields only 12 ATP from its complete oxidation.

Fig: Showing Krebs cycle

· Last step of respiration
· Occurs in oxysomes or cristae of the inner mitochondrial membrane.
· Enzymes for ETS are located in the inner mitochondrial membrane.
· The electrons donated by glucose are carried by NADH and transferred from one electron carrier to another. Ultimately, they are passed to a molecule of oxygen.
· Final electron acceptor in ETS is oxygen.
· In ETS electron passes through cytochrome b → cytochrome a3 → O2.
· During ETS
  • 1 NADH2 → 3 ATP
  • 1 FADH2  2 ATP
· Inhibitor of oxidative phosphorylation is 24 
DNP, Oligomycin B and DCCD.

High Yeilding Tips from RESPIRATION

a. Net gain of ATP in aerobic respiration is = 38 ATP

b. Net gain of ATP in anaerobic respiration or fermentation = 2 ATP

. Net gain of ATP in aerobic respiration of Prokaryotes = 38 ATP

. Net gain of ATP in aerobic respiration of Eukaryotes = 36 ATP
Note: In most eukaryotic cells, 2 ATP molecules are expended in transporting 2NADH molecules produced in glycolysis to the mitochondria. In such cells, there is a net gain of 36 ATP molecules instead of 38 ATP.

e. Aerobic respiration in kidney, liver and heart cells of eukaryotes= 38 ATP

f. Aerobic respiration in muscle, and nerve cells of eukaryotes= 36 ATP

g. Rate of respiration is more in the younger life of plants than in the older stage.

h. Mercury is used in anaerobic respiration because it does not react with CO2.

i. Respirometer measures the rate of respiration.

j. Cytochrome is an iron-containing protein common to both photosynthesis and respiration.

k. Fruits and seeds are stored at low temperatures to reduce the rate of respiration.

l. Abnormal rise in respiratory rate during ripening of fruits is called climacteric.

m. A sudden change from anaerobic to aerobic respiration due to the availability of oxygen is the Pasteur effect.

Fig: Graphical Representation

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