Respiration

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Respiration is the stepwise oxidation of complex organic molecules and release of energy as ATP for various cellular metabolic activities. External respiration also known as breathing refers to a process of inhaling oxygen from the air and expelling carbon dioxide to the air. External respiration is a physical process and is occurs in green and non-green plants. Biochemical process takes place in all types of living cells and cells get their energy in the form of ATP is generally called cellular respiration. There are two types of cellular respiration, aerobic and anaerobic. Aerobic respiration can be utilized in the presence of oxygen, while anaerobic respiration does not require oxygen. The entire process of anaerobic respiration is takes place in the cytosol. In aerobic respiration the reactions of anaerobic respiration are followed by an oxygen requiring process that releases much larger quantity of energy in the form of ATP. This occurs in the mitochondria of the eukaryotes and in the folded plasma membrane of the prokaryotes. Starch and glycogen change into glucose by specialized enzymes for use in respiration. Initial steps, known as glycolysis are common in both aerobic and anaerobic respiration. Break down of glucose and formation of ATP molecules involves five major processes; glycolysis, oxidation of pyruvic acid, Krebs cycle (TCA cycle) or Citric acid cycle, electron transport chain or respiratory chain and Chemiosmotic ATP synthesis.

Differences between aerobic and anaerobic respiration

Table showing difference between aerobic and anaerobic respiration.
Table showing difference between aerobic and anaerobic respiration.

Aerobic (Aero = Air)

Anaerobic (Anaero = No Air)

Takes place in the presence of oxygen

Takes place in the complete absence of oxygen

Leads to complete oxidation of organic substrate

Incomplete oxidation of organic substrate takes place

It is most common in higher organisms (both plants and animals).

Takes place in lower organisms such as bacteria, fungi and in higher animals under limiting conditions of oxygen

Equation

Equation

OR

Equation

Takes place in cytoplasm and mitochondria in eukaryotes and plasma membrane in prokaryotes

Takes place in the cytoplasm

More energy is liberated. One molecule of glucose releases 6 73 kcal energy

Less energy is liberated. One molecule of glucose releases 21 kcal energy

Food is completely oxidized

Food is incompletely oxidized.

Coenzyme is a complex non-protein molecule which is temporarily bound to an enzyme and acts as a link between metabolic pathways, (series of biochemical reactions).

External Respiration/Gaseous Exchange

In plants, the atmospheric air moves in and out by simple diffusion that takes place through; the general body surface of the plant (stems, roots, fruits and seeds), lenticels (openings in the bark of the tree trunk) and stomata are openings generally present on the lower surface of the leaves through which the gases and water vapour diffuse in and out easily.

Image showing T.S. of lenticell.

Image Showing T.S. Of Lenticell.

Image showing T.S. of lenticell.

Plants do not need O2 carrier (in contrast to animals where O2 is carried by blood) because O2 requirement is less in plants than in animals because the plants have a large surface area (leaves) to absorb the required amount of O2 through diffusion.

Gases through the atmosphere enter the intercellular spaces inside the plants. More O2 utilized and more it diffuses into the plant. CO2 is being continuously formed and its concentration in tissue spaces becomes higher than in the surrounding air. So, it diffuses out of the plant, especially when it is not being used for photosynthesis.

During the day the rate of photosynthesis is usually much higher than the rate of respiration so plants absorb most of the carbon dioxide they need from the air by diffusion. The amount of oxygen produced is far greater than the amount needed for respiration of the cells, so it builds up inside the leaf and diffuses out in the air through the stomata. At night no photosynthesis takes place so the leaf absorbs the oxygen it requires from the atmosphere. Animals gives out CO2 at all times.

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