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Respiration: Overview, Introduction to Aerobic and Anaerobic Respiration, Respiration in Plants and Respiratory Substrate

What happens when we fast for too long? You must have noticed that a fasting person feels weak and lethargic with very little energy left in the body. Do you know why? That is because food gives us energy and all living organisms need energy to carry out daily life activities. But how is this energy obtained from the food we eat? It is obtained with the help of a process known as cellular respiration in which the absorbed food is broken down within the cell to release energy.

All animals and plants require O2 for respiration and they release CO2. Did you know that another process is closely coupled to the process of respiration? It is the process of breathing which helps organisms to exchange the respiratory gases with the atmosphere. But do all organisms breathe? We humans definitely do but do you think plants breathe too? Come let us find answers to these questions.

Table of Contents

Do plants breathe?

Yes, plants breathe or in other words they can exchange gases with the atmosphere. They do not have a dedicated respiratory organ and system for gaseous exchange like animals because they do not have a great demand for gaseous exchange as their energy requirements are less due to a sedentary lifestyle. The plant parts respire at a rate much less than that of animals. Also, the leaf cells are well equipped for the vigorous gaseous exchange required during photosynthesis and the oxygen produced during photosynthesis can be directly used for respiration. 

Respiratory gases cannot diffuse over long distances and hence each plant part takes care of its own gaseous exchange. Gases can directly diffuse from the plant surface. Living cells in plants are thus located close to the surface where tiny apertures facilitate gaseous exchange. Stomata and lenticels are the apertures through which gaseous exchange can take place.

Stomata are openings present on the surface of leaves which remain open during the day and closed during the night. The opening and closing of stomata is regulated by the guard cells. Lenticels are openings on roots and stems that remain open always and help in exchange of gases.

structure responsible for breathing in plants

Thus, most cells of the plant are in contact with air via at least some part of their surface. This is further facilitated by air spaces present between loosely packed parenchyma cells of the plant parts.

What are respiratory substrates? 

All living cells require energy to perform daily life processes. This energy is derived from the breakdown of food, obtained directly or indirectly from plants, through the process of respiration within the cells. The cytoplasm and the mitochondria are the sites of cellular respiration and the process might or might not involve oxygen. The energy is obtained in the form of ATP (also known as energy currency). 

The compounds, present in the food, that are broken down to produce energy during the process of cellular respiration are known as Respiratory substrates. 

Respiratory substrates are of following types - 

1. Carbohydrates
2. Fats 
3. Proteins 

Usually carbohydrates are oxidised to release energy, but proteins, fats and even organic acids can be used as respiratory substrates in some plants, under certain conditions. 

Calorific value

The amount of energy liberated from complete oxidation of one gram of the respiratory substrate in a bomb calorimeter (a closed metal chamber filled with oxygen) is its gross calorific value.

The actual amount of energy released by oxidation of one gram of the respiratory substrate in the body is the physiological value.

calorific values of respiratiory substance

Energy storage and energy transfer (ATP, NADH and FADH2)

The energy releasing process in a cell involves oxidation and reduction reactions. All the energy present in a respiratory substrate is not released freely into the cell, it is trapped in the bonds of some chemical compounds like ATP, NADH and FADH2, known as Energy carriers. The most important energy carrier is ATP.

ATP

Full form of ATP is Adenosine Triphosphate. It carries energy in two terminal phosphate bonds because the formation of these bonds requires a large amount of energy, hence these bonds are called high energy or energy rich bonds. 

formation of atp from adp

These bonds are very unstable and readily break down into Adenosine Diphosphate (ADP) and a phosphate group and release energy. Whenever the energy is required by a cell for any metabolic reaction, the same energy is utilised from ATP. Breaking down one mole of ATP generates 7.3 kcal of energy. This energy is released from the terminal phosphate bond.

realese of energy by breakdown of atp

NADH and FADH2

Full form of NAD is Nicotinamide Adenine Dinucleotide. Full form of FAD is Flavin Adenine Dinucleotide. NAD and FAD serve as oxidising agents and get reduced to NADH and FADH2. Both NADH and FADH2 are considered the reservoirs of ATP because they are not directly involved in energy production by breakdown of bonds but give rise to ATP by the process of Electron Transport Chain.

  • One NADH yields three ATP.
  • One FADH2 yields two ATP.

Floating and Protoplasmic respiration

On the basis of type of respiratory substrate , the respiration can be of two types

1. Floating respiration
2. Protoplasmic respiration

types of respiration based on repiratory substance

Floating respiration

In floating respiration the substrate that is broken down is either carbohydrate or fat. It is the most common respiration that occurs in the living cell. As no toxic products are formed in floating respiration, it keeps the cell healthy. As energy is required for every metabolic activity of a cell, floating respiration occurs indefinitely throughout the life of the cells.

Protoplasmic respiration

After using up all the carbohydrates and fats, the cell starts to utilise other protoplasmic substrates such as proteins for energy production. This type of respiration is called protoplasmic respiration. It is a rare mode of respiration because usually carbohydrates are available for energy production. Toxic byproducts are released during protoplasmic respiration.

Aerobic and Anaerobic respiration 

Based on the availability of oxygen, respiration can be of two types -

1. Aerobic respiration
2. Anaerobic respiration

types of respiration based on avalibility of oxygen

Aerobic respiration

Oxidative breakdown of respiratory substrate in the presence of oxygen is known as aerobic respiration. It occurs partially in the cytoplasm and primarily in the mitochondria. It occurs in five steps - glycolysis, oxidative decarboxylation, Krebs cycle, electron transport chain and oxidative phosphorylation. 

In aerobic respiration, one molecule of glucose is broken down to form six molecules of carbon dioxide, six molecules of oxygen and 38 molecules of ATP.

The overall equation for aerobic respiration can be represented as:

equation representing aerobic respiration

Glycolysis

It takes place in the cytoplasm and involves the breakdown of one molecule of glucose into two molecules of pyruvic acid in ten steps catalysed by different enzymes. This process does not require oxygen and occurs in both aerobic and anaerobic respiration.

glycolysis

Oxidative decarboxylation

The pyruvic acid/pyruvate molecules are brought to the mitochondria where they undergo oxidative decarboxylation to form acetyl coA which enters the Krebs cycle.

phases of arebic respiration till oxidation decarbxylation

Krebs cycle

The Krebs cycle is also known as the tricarboxylic acid cycle or the citric acid cycle and occurs in the mitochondria. In this process six molecules of carbon dioxide are produced along with ten molecules of NADH and two molecules of FADH2.

phases of arebic respiration till krebs cycle

Electron Transport Chain (ETC)

phases of arebic respiration till etc

This process is the precursor for oxidative phosphorylation which is the last step of aerobic respiration. It helps in building up the proton gradient in the inter membrane space of mitochondria which drives the production of ATP. The electrons released due to the oxidation of NADH and FADH2 are transported via a series of electron carriers in this process.

electron transport chain

Oxidative phosphorylation

all the phases of aerobic respiration

This process occurs across the inner mitochondrial membrane and helps in production of ATP from the oxidation of NADH and FADH2. As the proton gradient generated due ETC is broken, the energy is utilised for ATP synthesis from ADP and inorganic phosphate. Oxygen acts as the terminal electron acceptor in this process and is reduced to form water. Six molecules of water are generated by the complete oxidation of one molecule of glucose during aerobic respiration.

oxidative phosphorylation

Anaerobic respiration

Breakdown of respiratory substrate in the absence of oxygen is known as anaerobic respiration. It results in incomplete oxidation of food and mainly occurs in lower organisms like yeast, certain bacteria and fungi. Only two molecules of ATP are produced per molecule of glucose during anaerobic respiration. Glycolysis of glucose results in the formation of two molecules of pyruvate which can then enter into any one of the following pathways - alcoholic fermentation and lactic acid fermentation. 

Alcoholic fermentation 

In this process, pyruvic acid/pyruvate is broken down to ethyl alcohol (C2H5OH) and carbon dioxide. This occurs in yeast and certain bacteria.

This can be expressed as -

C6H12O6 → 2 Pyruvic acid → 2C2H5OH + 2CO2 + 2ATP

Lactic acid fermentation

In this process, pyruvate is broken down to lactic acid. This process occurs within lactic acid bacteria and in human muscle cells in the absence of oxygen.

C6H12O6 → 2 Pyruvic acid → Lactic acid + 2ATP

Aerobic respiration

Anaerobic respiration

It is a common type of respiration in higher plants.

It is mostly seen in lower organisms such as bacteria, yeast, etc.

Occurs in the presence of O2

Occurs in the absence of O2

End products released along with the energy are CO2 and H2O.

End products released along with the energy are either ethanol and CO2 or lactic acid.

The amount of energy released in the form of ATP will be 38 ATP.

The amount of energy released in the form of ATP will be 2 ATP.

Practice problems of Respiration

Question 1. Which of the following is the correct order of steps in aerobic respiration?

a) Krebs cycle, glycolysis, oxidative phosphorylation, oxidative decarboxylation
b) Glycolysis, oxidative decarboxylation, Krebs cycle, oxidative phosphorylation
c) Krebs cycle, oxidative phosphorylation, oxidative decarboxylation, glycolysis
d) Oxidative phosphorylation, Krebs cycle, glycolysis, oxidative decarboxylation

Answer: Aerobic respiration occurs in five steps - glycolysis, oxidative decarboxylation, Krebs cycle, electron transport chain and oxidative phosphorylation. 

1. Glycolysis - involves the breakdown of one molecule of glucose into two molecules of pyruvic acid in the cytoplasm.
2. Oxidative decarboxylation - The pyruvic acid/pyruvate molecules undergo oxidative decarboxylation to form acetyl coA in the mitochondria.
3. Krebs cycle - The acetyl coA enters the Krebs cycle and through a series of multiple steps, six molecules of carbon dioxide are produced along with ten molecules of NADH and two molecules of FADH2.
4. Oxidative phosphorylation - The electrons released due to the oxidation of NADH and FADH2 are transported via a series of electron carriers in this process. As the proton gradient generated due to ETC is broken, the energy is utilised for ATP synthesis from ADP and inorganic phosphate. 

Question 2. What are the differences between aerobic and anaerobic respiration?

Answer:  The differences between aerobic and anaerobic respiration are - 

Aerobic respiration

Anaerobic respiration

It is a common type of respiration in higher plants.

It is mostly seen in lower organisms such as bacteria, yeast, etc.

Occurs in the presence of O2

Occurs in the absence of O2

End products released along with the energy are CO2 and H2O.

End products released along with the energy are either ethanol and CO2 or lactic acid.

The amount of energy released in the form of ATP will be 38 ATP.

The amount of energy released in the form of ATP will be 2 ATP.

Question 3. Which of the following tissues are likely to undergo lactic acid fermentation in the absence of oxygen?

a) meristematic tissues
b) skeletal muscles
c) epidermal tissues
d) xylem tissues

Answer: In the scarcity of oxygen, the skeletal muscle tissues undergo anaerobic respiration by lactic acid fermentation. The pyruvic acid produced by glycolysis of glucose is converted to lactic acid and stored, which eventually leads to muscle fatigue. This is common during strenuous exercises.

Question 4. Which of the following is correct for gross calorific value?

a. The amount of energy liberated from complete oxidation of one gram of the respiratory substrate in a bomb calorimeter
b. The actual amount of energy released by reduction of one gram of the respiratory substrate in the body
c. The amount of energy liberated from partial oxidation of one gram of the respiratory substrate in a bomb calorimeter
d. The actual amount of energy released by the complete oxidation of one gram of the respiratory substrate in the body

Answer: Gross calorific value is the amount of energy liberated from complete oxidation of one gram of the respiratory substrate in a bomb calorimeter (a closed metal chamber filled with oxygen).

FAQs of Respiration

Question 1. What produces more energy - aerobic or anaerobic respiration?

Answer: Aerobic respiration produces 38 molecules of ATP from the breakdown of one molecule of glucose.

Anaerobic respiration produces only 2 molecules of ATP from the breakdown of one molecule of glucose.

Question 2. Which is the common phase between aerobic and anaerobic respiration?

Answer: Glycolysis is the common phase between aerobic and anaerobic respiration. It occurs in the cytoplasm and involves breaking down glucose into pyruvic acid.

Question 3. What will happen if cellular respiration machinery stops working?

Answer: Cellular respiration produces energy, which is used for various cellular metabolism. If the process stops working, the cells of an organism will not receive energy for essential life processes such as transport of metabolites, digestion of food, absorption of nutrients, etc. This can cause the cell to eventually die which will ultimately cause the organism to die.

Question 4. How does yeast respire?

Answer: Yeast respires aerobically in the presence of oxygen but it undergoes anaerobic respiration in the absence of oxygen. The type of anaerobic respiration undergone by yeast is alcoholic fermentation. The pyruvic acid generated by glycolysis is converted to ethanol and carbon dioxide in this reaction. This is why yeasts are wide used in the fermentation of sugary juices to form alcoholic beverages.

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