Respiration in Plants

Introduction:

Respiration is an amphibolic and exergonic cellular process as it takes place in various enzymatic reaction processes, which is also known as internal respiration/tissue respiration/dark respiration/cellular respiration/mitochondrial respiration. As in all living organisms one of the most important processes is respiration where there is a release of metabolic energy in the form of ATP. In case of plants there is a lack of specialised respiratory organs as each plant parts take care of its own gaseous exchange. They are very less dependent on the environment for respiratory gaseous exchange. In the plant body diffusing gases do not travel long distances.

Respiratory Substrate:

Carbohydrate-----fats-------proteins-----others

1. When respiratory substrates are carbohydrates like glycogen, starch, sucrose, hexose, or fats then respiration is known as floating respiration.

2. When protein is oxidised in respiration, then respiration is known as protoplasmic respiration in which protoplasmic components or cellular proteins may oxidise at the time of starvation and disease.

3. Exceptionally oxidation of protein in legume seeds is called floating respiration.

Types of Respiration:

1. Aerobic Respiration: 

The complete oxidation of food with use of oxygen and when the entire carbon is released as carbon dioxide is called aerobic respiration.

2. Anaerobic Respiration:

This is a complete oxidation. When food is oxidised into alcohol or organic acids without use of oxygen. During this process most of energy is lost in the form of heat. It occurs in cytoplasm and only 2ATP is produced.

It may take place in bacteria.

When oxygen is not available then in that  case the food is incompletely oxidised into some organic compound like ethanol,lactic acid, acetic acid.

In anaerobic respiration the final electron acceptor is a free oxygen molecule.

Steps in Aerobic Respiration:

1. Glycolysis: Occur in cytosol/cytoplasm.

2. Formation of acetyl coenzyme A: mitochondrial matrix.

3. TCA cycle or Kreb Cycle: Matrix of mitochondria.

4. ETS: Occurs in cristae or inner membrane of mitochondria and oxidative phosphorylation-occurs in oxysome heads.

1. Glycolysis: (Embden,Meyerhof, parnas)pathway

1.  It is the common phase in aerobic and anaerobic respiration.

2. In glycolysis neither consumption of oxygen nor liberation of carbon dioxide take place.

3. In glycolysis, 1 glucose produces 2 moles of pyruvic acid.

4. Glycolysis is also known as oxidative anabolism or catabolism resynthesis because it links with anabolism of fats and amino acids. An intermediate PGAL is used for the synthesis of glycerol that later forms fats or lipids. PGA is used for synthesis of serine, Glycine, Cystine, Alanine forms from pyruvate.

2. Formation of Acetyl Coenzyme A:

1. When respiration is aerobic, then pyruvic acid is oxidised to form 2C-compound-Acetyl CoA. It occurs in the presence of oxygen and carbon dioxide is released for the first time during it. 

2. Acetyl CoA is the connecting link between glycolysis and kreb cycle. Decarboxylation and dehydrogenation take place during formation of acetyl CoA.

3. Acetyl CoA is formed in the mitochondrial matrix by enzyme pyruvate dehydrogenase complex. 

3. Krebs Cycle or TCA:( tricarboxylic acid)

1. This cycle was discovered by H.A Krebs. 

2. TCA cycle in mitochondrial matrix or power house of the cell. All the enzymes of TCA cycle, Except marker enzymes succinic dehydrogenase.

3. Krebs cycle begins by formation of citric acid.

4. Oxidation occurs at 4 sites in the Kreb cycle. 3NADH2, 1FADH2, 1GTP.

4. ETS and oxidative phosphorylation:

(Terminal oxidation of NADH + H+ and FADH2)

1. It is associated with release and utilisation of the energy stored in NADH + H+ and FADH2. Oxygen acts as a final H2 acceptor.

2. UQ and Cyto. C are mobile carriers in mitochondrial ETS.

Fermentation:

1. Alcoholic Fermentation: 

This is the oldest and best known type of fermentation performed by yeast and some bacteria.

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2. Lactic Acid Fermentation:

It occurs during curd formation and also in muscles of animals (when oxygen is inadequate).

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Respiratory Quotient:

1. R.Q= Volume of carbon dioxide liberated/ Volume of oxygen liberated

2. Volume of R.Q depends upon the type of respiratory substrate used and measured by Ganong’s respirometer.

1. Carbohydrate: R.Q=1

2. Fat/oil= R.Q=0.70

3. Malic acid: R.Q= 1.33

4. Oxalic acid: R.Q= 4

5. Citric acid: R.Q=1.3

6. Protein: R.Q=0.8/0.9<1

7. Anaerobic bacteria: R.Q= Infinite 2 carbon dioxide/ 0 oxygen = Infinite

Factors Affecting Cell Respiration:

Temperature: 

1. Optimum temperature for respiration is between 20-35 degree celsius. Maximum temperature is about 45 degree celsius.

2. At low temperature respiration is low due to inactivation of enzymes while at very high temperature as enzymes denatured.

Oxygen:

1. The inhibition of anaerobic respiration by oxygen concentration is called Pasteur’s effect.

2. The amount of oxygen at which aerobic and anaerobic respiration take place simultaneously is called transition point.

3. The minimum amount of oxygen at which aerobic and anaerobic respiration become extinct is called extinction point.

Carbon Dioxide:

1. Carbon dioxide increases then the rate of respiration decreases in plants.

2. Preservation of fruits and vegetables at high concentration is called gas storage.

Salts:

1. If a plant is transferred from water to salt solution, it’s respiration increases this is known as salt respiration.

Light:

1. Rate of respiration increases with increase in light intensity. Light controls stomatal opening and influence on temperature and also produces respiratory substance.

Age:

1. Rate of respiration is more in young cells. Rate of respiration at the meristem apex is high.