Cellular respiration occurs in two aim stages involving series of chemical reactions and respiratory enzymes.

First stages Glycolysis

Glycolysis is a series of chemical reaction which involves the breaking down of glucoses to a 3-carbon molecule of pyruvic acid.

In this process, the glucose molecules is phosphorylated by addition of phosphate group to glucose to become glucose phosphate through series of oxidative enzymes, the glucose phosphate is converted to two molecules of triose sugar (a 3-carbon sugar). The triose sugar is converted to pyruvic acid by removal of four atoms of hydrogen by co-enzymes called NAD (Nicotinamide adenine dinueleotide) with the formation of 2ATP.

Second Stage: Aerobic Respiration

This stage occurs in the mitochondrion if sufficient oxygen is available, each molecule of pyruvic is oxidized to remove one molecule of carbon (iv) oxide (decarboxylation) and two atoms of hydrogen (dehydrogenation) forming one molecule of acetic acid a 2-carbon acid or acety co-enzymes.

A.Carbon (iv) oxide is released. Kreb cycle or citric acid cycle. The acetic acid entries into a kreb cycle where it is joined to a 4-carbon acid oxaloacetic acid present in the mitochondria to form citric acid a 6-carbon acid

One molecule of carbon (iv) oxide and one atom of hydrogen are removed by oxidative decarboxylation and NAD respectively to form a 5-carbon acid  ketoghitaric acid. Another molecule of carbon (iv) oxide and one atom of hydrogen are further removed from ketogluteric acid to form a 4-carbon succinic acid. One atom of hydrogen is removed from succinic acid to form a 4-carbon malic acid. Again another hydrogen atom is removed from malic acid to form a 4-carbon oxaloacotic acid. The oxaloacetic aid then combines with acetic acid again and the cycle is repeated. The hydrogen atoms removed in succeeding stages combine with molecular oxygen to form water. The overall reactions of glycolysis and kreb’s cycle is represented by an equation.

C₆H₁₂O₆+60₂ ———–>  6CO₂+6H₂0 + ENERGY (38ATP)

ATP is a water soluble energy carrying small molecules that is transported around cells.

The process of glycolysis is a catabolic process because it involves breaking down of complex organic molecule glucose to simple moleculses such as ATP. ### process realeses energy. In glycolysis one molecule of glucose releases 2ATP of energy. In kreb cycle 1 molecule of glucose releases 38ATP of energy.

Aerobic Respiration

This is the type of respiration which requires oxygen to break down glucose into water, carbondioxide and energy (ATP). This type of respiration is commonly carried out by most living thing

C₆H₁₂O₆+60₂—–> 6CO₂+6H₂0+Energy

Anaerobic Respiration is the type of respiration in which the glucose is broken down inside the cell without oxygen to produce carbon (iv) oxide and ethanol and 2ATP.

(a)Alcoholic Fermentation is the type of respiration in which glucose is first converted to pyruvic acid. The pyruvic acid is then reduced to ethanol by accepting the hydrogen of NADH. This type of respiration on is common to some organism that respire anaerobically such as yeast, bacteria, fungi and endoparasite such as tape worm and round warm

C₆H₁₂O₆   No   2CO_{2    —–>}     +2C₂H₅OH+  Energy

Glucose  O₂      Carbon(iv) oxide     (Alcohol)   2ATPS

(b)Lactic acid fermentation is the type of anaerobic respiration where glucose is broken down to

pyruvic aid. The pyruvic acid is then reduced to lactic acid by NADH without the production of

carbon (iv) oxide.

Lactic acid fermentation often occurs in the muscles of animal after a very fast race or strenuous exercise. In the absence of oxygen reaching the muscle cells, the pyruvic acid reduce to lactic acid whose accumulation in the muscle ells causes temporary muscle fatigue and aches. The athlete recovers when sufficient oxygen is carried to the muscle cell. Which oxidize the lactic acid to pyruvic acid and pyruvic acid is oxidized by the supplied oxygen to give carbon (iv) oxide, water and energy. The muscle cell then recovers

2CH3COCOOH  —->     CH3CHOHCOOH + 2NAD+ 2ATP

Pyruvic acid                           Lactic acid

Differences Between Aerobic and Anaerobic Respiration

Aerobic	Anaerobic
1. Occurs in most animals as well as Plant cells	as occurs in some bacteria, fungi and cells of the muscles of animals
2. Oxygen is needed	oxygen is not needed
3.  Produce greater energy (38ATP) per Molecule of glucose	Produce lesser energy 2ATPs per molecule of glucose
4.   Carbon (iv) oxide and water are the By-product	Carbon (iv) oxide and alcohole or lactic acid are the by- product
5.   It takes place in mitochondria	it takes place in the cypoplasm