The energy is stored in carbohydrates molecule during photosynthesis is released during cellular oxidation of carbohydrates into CO₂ and H₂O. This is known as respiration. In respiration, the oxidation of various organic food substances like carbohydrates, protein, fats etc. may takes place. Its oxidation proceed as shown in the following equation;

Respiration is a complex process which includes;

1. Absorption of CO₂.

2. Conversion of carbohydrates into CO₂ and H₂O.

3. Release of energy.

4. Liberation of CO₂ and H₂O.

5. Loss on weight in plant as a result of oxidation.

 The respiration is reverse process to photosynthesis.

Kinds of respiration:

On the basis of availability of O₂ during respiration it has been divided into two categories;

1. Aerobic respiration:

 It takes place in presence of O₂ and the stored food get completely oxidized into CO₂ and H₂O.

 This type of respiration is a common occurrence in plants and occurs in cytoplasm as well as in mitochondria.

2. Anaerobic respiration:

It takes place in absence of O₂ and the stored food is incompletely oxidized and instead of CO₂ and H₂O certain other substances are also found. These type of respiration is a rare occurrence but common among micro- organisms like yeast and can be represented by following equation;

C₆H₁₂O₆      →   2C₂H₅OH +2CO₂
                                      ethyl alcohol

Mechanism:

 It can be studied under following headings:

1. Glycolysis (EMP)

2. Anaerobic respiration (Fermentation)

3. Aerobic respiration (Kreb’s cycle)

1. Glycolysis (EMP):

                                     Fig. Glycolysis

The course of stepwise degradation from glucose to pyruvic acid is known as glycolysis. It is also known as EMP pathway. In the presence of oxygen, glycolysis is the first stage of cellular respiration.

Step 1
The enzyme hexokinase phosphorylates (adds a phosphate group to) glucose in the cell's cytoplasm.  In the process, a phosphate group from ATP is transferred to glucose producing glucose 6-phosphate . 

Glucose (C 6 H 12 O 6) + hexokinase + ATP → ADP + Glucose 6-phosphate (C 6 H11 O 6 P 1) 

Step 2
The enzyme phosphoglucoisomerase converts glucose 6-phosphate into its isomer fructose 6-phosphate. Isomers have the same molecular formula , but the atoms of each molecule are arranged differently. 
Glucose 6-phosphate (C 6 H 11 O 6 P 1) + Phosphoglucoisomerase → Fructose 6-phosphate

Step 3 
The enzyme phosphofructokinase uses another ATP molecule to transfer a phosphate group to fructose 6-phosphate to form fructose 1, 6-bisphosphate. 
Fructose 6-phosphate (C 6 H 11 O 6 P 1) + phosphofructokinase + ATP → ADP + Fructose 1, 6-bisphosphate (C 6 H 10 O 6 P 2 ) 

Step 4 

The enzyme aldolase splits fructose 1, 6-bisphosphate into two sugars that are isomers of each other. These two sugars are dihydroxyacetone phosphate and glyceraldehyde phosphate. 

Step 5 
The enzyme triose phosphate isomerase rapidly inter-converts the molecules dihydroxyacetone phosphate and glyceraldehyde phosphate. Glyceraldehyde phosphate is removed as soon as it is formed to be used in the next step of glycolysis. 

Step 6 
The enzyme triose phosphate dehydrogenase serves two functions in this step. First the enzyme transfers a hydrogen (H - ) from glyceraldehyde phosphate to the oxidizing agent nicotinamide adenine dinucleotide (NAD + ) to form NADH. Next triose phosphate dehydrogenase adds a phosphate (P) from the cytosol to the oxidized glyceraldehyde phosphate to form 1, 3-bisphosphoglycerate. This occurs for both molecules of glyceraldehyde phosphate produced in step 5.

Step 7 
The enzyme phosphoglycerokinase transfers a P from 1,3-bisphosphoglycerate to a molecule of ADP to form ATP. This happens for each molecule of 1,3-bisphosphoglycerate. The process yields two 3-phosphoglycerate molecules and two ATP molecules. 

Step 8 
The enzyme phosphoglyceromutase relocates the P from 3-phosphoglycerate from the third carbon to the second carbon to form 2-phosphoglycerate. 

Step 9 
The enzyme enolase removes a molecule of water from 2-phosphoglycerate to form phosphoenolpyruvic acid (PEP). This happens for each molecule of 2-phosphoglycerate. 

Step 10 
The enzyme pyruvate kinase transfers a P from PEP to ADP to form pyruvic acid and ATP. This happens for each molecule of PEP. This reaction yields 2 molecules of pyruvic acid and 2 ATP molecules. 

In the sequence of reaction of glycolysis there is a net gain of 2 molecules of ATP.

2. Anaerobic respiration (Fermentation):

In the absence of molecular oxygen, pyruvic acid undergoes anaerobic respiration or fermentation two types are common.

1. Alcoholic fermentation:

 Alcohol fermentation, also known as ethanol fermentation, is the anaerobic pathway carried out by yeasts in which simple sugars are converted to ethanol and carbon dioxide. Yeasts typically function under aerobic conditions, or in the presence of oxygen, but are also capable of anaerobic respiration, in which sugars are metabolized in the absence of oxygen. Alcohol fermentation occurs in the cytosol of yeast cells.

Equation

 Alcohol fermentation occurs as follows:

              C₆H₁₂O₆     →   2CH₃CH₂OH  + H

2. Lactic acid fermentation:

 Lactic acid fermentation is a biological process by which glucose and other six-carbon sugars (also, disaccharides of six-carbon sugars, e.g. sucrose or lactose) are converted into cellular energy and the metabolite lactate. It is an anaerobic fermentation reaction that occurs in some bacteria and animal cells, such as muscle cells.

Lactate dehydrogenase catalyzes the interconversion of pyruvate and lactate with concomitant interconversion of NADH and NAD+.

The overall equation for anerobic respiration involving Lactic acid fermentation is as follows;

            C₆H₁₂O₆     →   2CH₃CH₂OH   + 2CO₂

Aerobic respiration ( kreb’s cycle) :  

                                                   Fig. Citric acid cycle

Aerobic cycle is also known as citric acid cycle because citric acid is the starting compound of the cycle and citric acid as well as other intermediate acids contain 3 – carboxylic groups. If O₂ is available, aerobic respiration takes place. The pyruvic acid produced in glycolysis enters into kreb’s cycle for further oxidation. It is also known as Tri- carboxylic acid cycle (TCA). The important steps of kreb’s cycle are as follows;

Step 1: In the first step of the Krebs cycle, acetyl CoA is added to oxaloacetate to form citrate.

Note that coenzyme A (CoA-SH) is removed in the process.

Step 2: Citrate is isomerized forming isocitrate, which is less stable than citrate. During this step, one water molecule is removed and another water molecule is added.

Step 3: Isocitrate is converted into alphaketo glutarate. In this process, isocitrate is decarboxylated (carbondioxide is removed), and NAD⁺ is reduced, forming NADH + H⁺.

Step 4: Alpha-ketoglutarate is converted to succinyl CoA. During this step coenzyme A is added, carbon dioxide is lost, and NAD+ is reduced, forming NADH + H⁺.

Step 5: Succinyl CoA is converted to succinate. During this step, coenzyme A is released and GTP is made. GTP is then hydrolyzed to form ATP.

Step 6: Succinate is converted to fumarate. During this step FAD is reduced forming FADH₂.

Step 7: Fumarate is converted to malate with the addition of water.

Step 8: In the last step of the Krebs cycle, malate is converted to oxaloacetate. In the process, NAD+ is reduced to form NADH + H⁺ . Oxaloacetate can then accept another acetyl CoA and begin the Krebs cycle again.

Factors affecting respiration:

 External factors:

1. Temperature:

Optimum temperature is 20 - 30°C. At high temperature the rate of respiration declines with time and at very low temperature, the respiration rate is insignificant.

2. CO₂:

Increase in CO2 concentration and absence of O2 adversely affect the rate of aerobic respiration.

3. Light: Control respiration by raising the temperature of an organism.

4. Water: Very low water content in dry seeds and stored tubers is responsible for very feeble rate of respiration. In wilted tissues the stored starch converted into sugars which increase the rate of respiration while in well hydrated plants the rate of respiration is not likely to be affected much by slight changes in the water content.

Internal factors:

1.Respiratory substrates: Higher availability of respiratory substrates increases the rate of respiration upto a certain limit.

2.Protoplasmic factor: Young growing cells exhibit high rate of respiration as compared to mature cells.

3.Inhibitors: A number of chemicals inhibit respiration e.g., azide, cyanide, malonate, carbon monoxide etc.