Living organisms need a continuous supply of energy to remain alive. This energy is used by organisms to perform a variety of work. This includes:

1. Anabolic Reaction

Synthesis of new compounds such as proteins and DNA requires energy.

2. Movement

Movement of ions and molecules against the concentration gradient. 

3. Transport Of  Substances

Organisms move as a whole with the help of muscles. Muscles consume a large amount of energy to generate the pulling force.

4. Maintenance Of Temperature

Thermoregulation involves a set of different process which all works with the help of energy.

Photosynthesis As A Energy Harvesting Process

Living organisms trap energy from sunlight through the process of photosynthesis. This energy is then stored in chemical compounds. The energy from chemical compounds can be released through the process of respiration. Both of the reactions involve an intermediate compound called ATP.


The cell uses energy in the form of a water-soluble molecule called ATP. These molecules are produced regularly by the break down of compounds that are used in different anabolic reactions for synthesis.


ATP consist of a nucleotide that is made up of three components.

1) A nitrogenous base called adenine.
2) Pentose ribose sugar.
3) phosphate group which are attached to the ribose sugar.

The phosphate group of ATP is where energy is stored.

Formation Of ATP

ATP is formed regularly in the cell by phosphorylation of ADP.


The ATP can then be dephosphorylated to form ADP and energy is released.


ATP is a water-soluble compound. It is an unstable molecule and is therefore formed only when cells need energy after that it is broken down back to ADP.

Thus, cells store energy in the form of glucose and fats. These molecules are broken down to form ATP which can be used for different reactions.

ATP As Universal Energy Currency

ATP is used in almost every reaction in the body. The use of a single molecule for energy helps in controlling reactions. Every cell has to produce its ATP it can be imported from somewhere else. Thus, after some time cells have to recycle its energy store through respiration.

Generation Of ATP In Respiration

In eukaryotic cells, ATP is generated in large numbers by oxidation of molecules during respiration. Most of the time cell uses glucose as the respiratory substance. Complete oxidation of glucose releases a total of 36 Atp molecules. This takes place in the following steps:

1) Substrate Level Phosphorylation

Substrate-level phosphorylation occurs in glycolysis and Kreb cycle. It is the transfer of high energy phosphate group from one intermediate substrate to ADP resulting in the formation of ATP.


Glycolysis occurs in the cytosol. It is partial oxidation of glucose and takes place in the absence of oxygen.

Kreb Cycle

After glycolysis, the substrate enters mitochondria where it is further broken down. This step takes place only in the presence of oxygen.

2) Electron Transport Chain

The maximum amount of ATP is produced through an electron transport chain. The membrane of mitochondria and chloroplast are used for chemiosmosis. That is combining chemical bonds formation and movement of molecules across the cell membrane. This movement of molecules creates an osmotic gradient that is used to synthesize ATP.

Role Of Coenzymes In Respiration

Coenzymes are molecules that help enzymes in catalyzing reactions.


NAD or nicotinamide dinucleotide has a ring structure. This allows it to accept electrons. When it has accepted new electrons it is said to be reduced. Now it can easily transfer these electrons to other substrates. Transfer of electrons is an essential part of respiration.


Flavin adenine dinucleotide works in the same way as NAD.

Coenzyme A

It is involved in removing two carbon from the substrate like glucose and fats. It helps in the transfer of glucose from the cytosol to mitochondria.

Respiratory Substrate

The respiratory substrate is molecules from which energy is released during respiration. Most of the cells in the body use glucose as respiratory substrates.


Most of the cells use glucose as respiratory substrates. For example the brain, muscles, etc

Fatty Acids

Some cells like the liver can also oxidize fatty acids when glucose is not available.

Amino Acids

Amino acids are deaminated and its carbon part is used to release energy.

The relative energy value of these substances is not the same.

Energy Values Of Respiratory Substrate

The energy obtained by oxidation of a substrate depends upon the conversion of hydrogen bonds to water. The more hydrogen bond a substate has the greater will be its energy value. That is why the same amount of lipids produces more energy than carbohydrates. As lipids have a greater carbon-hydrogen bond.

Respiratory Quotients


The respiratory quotient is the ratio of carbon dioxide produced to oxygen consumed.

RQ=CO2 Produced / Oxygen Consumed

The respiratory quotient for different respiratory substrates are calculated as below


The RQ for carbohydrate can be calculated by the following equation:

C6H12O6 + 6 O2 → 6 CO2+ 6 H2O
Since CO2 produced and Oxygen used is the same in the equation the RQ for glucose is 1.


Fats have a chemical composition different from carbohydrates as they have more hydrogen bonds. The RQ for fats can be calculated from the equation for palmitic acid.

C16H32O2 + 23 O2 → 16 CO2 + 16 H2O



RQ for proteins is calculated as follows:

C72H112N18O22S + 77 O2 → 63 CO2 + 38 H2O + SO3 + 9 CO(NH2)2


Determining RQ Of Germinating Seeds Or Simple Invertebrate Using Respirometer

The organism is placed in one tube and a nonliving substance is placed in another. The manometer of both tubes is filled with a colored fluid. The fluid should reach the same level in both tubes. Two rubber bungs are taken and fitted with a glass tube. Close the spring clips and attach the manometer with the bent glass tubing. Place the bung on top of the tube. (as shown in the image). The spring-clip is removed so that equilibrium is reached with atmospheric pressure. Note the level of fluid in each tube. Close the clips. After every minute record the reading.

The first respirometer should contain soda lime as well. So that CO2 produced in it will be absorbed. This means the change of fluid level in the first tube will be due to oxygen consumption. In the second tube, there should be no soda lime. Thus, a fluid change will also be due to CO2.

When the organism will start respiring it will consume oxygen and produce CO2 due to which the level of the fluid will change.

Determination Of RQ

Distance moved by fluid in experimental tube = xmm

Distance moved by fluid in control tube = ymm

xmm shows oxygen consumed.

X-y shows CO2 produced



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