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Respiration and its types

Respiration and its types
 

Introduction:

  • For performing different physiological processes such as absorption, transport, movement, reproduction or breathing, all living organisms need energy.
  • The energy needed for life processes is extracted by oxidation of some macromolecules.
  • In general, it is the food that acts as the fuel of the body.
  • Energy is stored in the chemical bonds of the food molecules and can be used to perform various functions by the body.
  • Cellular respiration is a biochemical process that helps in the release of energy from the nutrients obtained from the food such as carbohydrates, proteins and fats.
  • Definition: Cellular respiration is a catabolic biochemical process in which the stepwise enzymatic breakdown of the organic substances (carbohydrates, proteins and fats) takes place to release energy.
  • Energy is produced in the form of adenosine triphosphate (ATP) and hence ATP is known as the energy currency of the cell.

Topics covered
 

  • Salient features of cellular respiration
  • Respiratory substrate
  • Aerobic and anaerobic respiration
  • Factors affecting cellular respiration

Salient features of cellular respiration:

respiration
 

  • Respiration provides energy to perform body functions and also provides biochemical intermediates.
  • These biological intermediates can be used to make organic substances necessary for the growth, repair and metabolism.
  • On cellular respiration, one glucose molecule generally yields 38 ATP molecules or 1292 kJ of energy (one ATP molecule contains 34 kJ of energy).
  • Although one glucose molecule on complete oxidation can release 2870 kJ of energy. Hence, efficiency of cellular respiration is only 45%.
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  • Thus, only some part of the energy released by the breakdown of the glucose molecule is used to generate energy and the rest is lost in the form of heat.
  • In cellular respiration, oxidation of the organic molecules takes place using oxygen inhaled from the surrounding to generate energy along with the release of carbon dioxide and formation of water molecules.
  • Carbohydrates are the basic substrate of cellular respiration and hence the equation of cellular respiration can be given as:

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Respiratory substrate
 

Introduction:

  • The organic molecules used to generate energy in the body of an organism through cellular respiration are called respiratory substrates.
  • Glucose is the most common respiratory substrate.
  • When carbohydrates and fats are used as the respiratory substrate, the process is known as floating respiration and when proteins are used as respiratory substrate, the process is called protoplasmic respiration.

Floating respiration:
 

Glucose:

  • Glucose is the first preferred respiratory substrate.
  • Glucose is obtained in plants as the product of photosynthesis, or from sucrose.
  • It can also be obtained from the stored carbohydrate as starch in plants and glycogen in animals.
  • Sucrose on hydrolysis forms glucose and fructose by an invertase enzyme and both of them can enter into the catabolic pathway of cellular respiration to release energy.

Fats:

  • Fats are the second preferred respiratory substrates.
  • Fat is hydrolysed to form fatty acids and glycerol.
  • Glycerol is converted into glyceraldehyde-3-phosphate to enter the glycolysis step of respiration.
  • Fatty acids are the long chain of carboxylic acid which are fragmented to form 2 carbon compounds called acetyl coenzyme-A in the mitochondria.
  • These acetyl coenzyme-A fragments can now enter into respiration through the Krebs cycle.
  • The excess carbohydrates and proteins that we eat in our food are stored in the form of fats.
  • Fats are the major stored fuel inside the body.
  • Excess food can also be stored as glycogen, but the ability of the body to store food in the form of fat is higher compared to its ability to store in the form of glycogen.
  • 1 gram of fat provides around 9 kcal of energy, whereas 1 gram of carbohydrate provides 4 kcal of energy.
  • Hence, for a given mass, a person can store more energy in the form of fat compared to glycogen and hence formation of fat is prefered over glycogen.

Protoplasmic respiration:
 

Protein:

  • Proteins are used as the last preferred respiratory substrate.
  • It is used as a respiratory substrate only under the starved conditions after the depletion of carbohydrates and fats.
  • Proteins in contrast to carbohydrates and fats, also contain nitrogen in them and hence it cannot be completely oxidized.
  • The monomers of proteins are amino acids that can be used in the respiration after the removal of nitrogen from them.
  • Nitrogen is removed in 2 ways:

1. Transamination: Replacement of the amino group with a keto group to form keto acid.
 

transamination
 

2. Oxidative deamination: amino groups are removed from amino acids, resulting in the formation of corresponding keto acids and ammonia.The ammonia eventually goes into the urea cycle.

oxidative-deamination
 

  • Proteins enter into cellular respiration as organic acids through the Krebs cycle.

Aerobic and anaerobic respiration

  • Respiration is of two types:
    1. Aerobic respiration
    2. Anaerobic respiration

Aerobic respiration:

  • In aerobic respiration, respiratory substances are completely oxidized to form carbon dioxide and water in the presence of oxygen.
  • Aerobic respiration helps in releasing large amounts of energy in the form of ATP.
  • One glucose molecule produces 38 ATP after complete oxidation through aerobic respiration.
  • It occurs in both plant and animal cells.

Anaerobic respiration:

  • In anaerobic respiration, the respiratory substrate is incompletely broken down in the absence of oxygen.
  • It does not produce enough ATP.
  • Anaerobic respiration produces different products in different organisms such as ethyl alcohol along with carbon dioxide is produced in yeasts and lactic acid is produced in animal cells.
  • It occurs in the anaerobic microorganisms such as bacteria.
  • It also takes place in higher animals under certain conditions for a temporary period.
  • In the absence of enough oxygen, plant cells undergo anaerobic respiration to form alcohol.

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  • Whereas, in animal cells, anaerobic respiration takes place to form lactic acid.

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Factors affecting cellular respiration:
 

1. Light:

  • Increase in the intensity of the light increases the rate of cellular respiration.
  • It is due to the synthesis of food due to photosynthesis and hence higher availability of the respiratory substrate.
  • Also, the rate of gaseous exchange increases due to opening of the stomata due to rise in temperature.

2. Oxygen:

  • Increase in the concentration of the oxygen increases the rate of respiration upto a particular limit.
  • At higher concentration, oxygen is converted into reactive radicals as superoxide (O2-).
  • This superoxide can damage DNA, proteins and membrane and lead to aging.
  • Vitamin C and E can be used to detoxify superoxide.
  • The minimum concentration of oxygen below which aerobic respiration cannot take place is called the extinction point. It is around 3-10%.

3. Mineral salts:

  • The minerals that take part in the enzymatic reaction as an activator or are part of the chemical structure of enzymes, increase the rate of respiration.
  • Such minerals are Fe, Cu, K, Mg, etc.
  • Nitrogen and sulfur also increase the rate of respiration as they are the basic element used in the formation of the enzyme.
  • Boron and molybdenum inhibit respiration at higher concentrations.
  • The rate of respiration increases during the salt absorption and hence such respiration is called salt respiration.

4. Stage of development:

  • Different rates of respiration are observed at different stages of development of the plants.
  • The rate of respiration is very high during seed germination.
  • Rate of respiration is also higher during the formation of the flower.
  • The rate of respiration increases during the formation of the fruit, reduces after that and increases during the ripening of the fruit.
  • The increase in the rate of respiration during ripening is called climacteric rise, during which ethylene is also released.

Frequently Asked Questions: FAQs

Q1: How is cellular respiration affected by light?
Ans :

  • Increase in the intensity of the light increases the rate of cellular respiration.
  • It is due to the synthesis of food due to photosynthesis and hence higher availability of the respiratory substrate.

Q2: What is extinction point?
Ans :

  • The minimum concentration of oxygen below which aerobic respiration cannot take place is called the extinction point. It is around 3-10%.

Q3: What is salt respiration?
Ans :

  • When a plant is kept in a salt solution, its rate of respiration increases as compared to its respiration when kept in water. This is called salt respiration. The effect is seen because anions are absorbed via the cytochrome chain, which is directly involved in the respiration process.

Q4: What is floating respiration?
Ans :

  • When carbohydrates and fats are used as the respiratory substrate, the process is known as floating respiration.

Q5: What is protoplasmic respiration?
Ans :

  • When proteins are used as a respiratory substrate, the process is called protoplasmic respiration.

Q6: How is nitrogen removed from the amino acid during protoplasmic respiration?
Ans :

  • During protoplasmic respiration, the nitrogen from the amino group is removed by transamination (conversion of amino group into keto group) and oxidative deamination (conversion of amino group into ammonia. The ammonia eventually goes into the urea cycle.).

Q7: What is the efficiency of cellular respiration?
Ans :

  • On cellular respiration, one glucose molecule generally yields 38 ATP molecules or 1292 kJ of energy (one ATP molecule contains 34 kJ of energy).
  • Although one glucose molecule on complete oxidation can release 2870 kJ of energy. Hence, efficiency of cellular respiration is only 45%.

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Q8: Why ATP is known as the energy currency of the cell?
Ans :

  • Energy produced through cellular respiration is stored in the form of adenosine triphosphate (ATP) and hence ATP is known as the energy currency of the cell.
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