Respiration
In all cases, the first step is the break-down of glucose, a six-carbon molecule, into a three-carbon
molecule called pyruvate. This process takes place in the cytoplasm.
Further, the pyruvate may be converted into ethanol (2 carbon molecule) and carbon dioxide. This
process takes place in yeast during fermentation. Since this process takes place in the absence of air
(oxygen), it is called anaerobic respiration.
Breakdown of pyruvate using oxygen takes place in the mitochondria. This process breaks up the
three-carbon pyruvate molecule to give three molecules of carbon dioxide. The other product is
water. Since this process takes place in the presence of air (oxygen), it is called aerobic
respiration .The release of energy in this aerobic process is a lot greater than in the anaerobic
process.
Sometimes, when there is a lack of oxygen in our muscle cells, another pathway for the break-down
of pyruvate is taken. Here the pyruvate is converted into lactic acid which is also a three-carbon
molecule. This build-up of lactic acid in our muscles during sudden activity causes cramps.
The energy released during cellular respiration is immediately used to synthesise a molecule called
ATP which is used to fuel all other activities in the cell. In these processes, ATP is broken down giving
rise to a fixed amount of energy which can drive the endothermic reactions taking place in the cell.
Since the aerobic respiration pathway depends on oxygen, aerobic organisms need to ensure that
there is sufficient intake of oxygen.
Plants
We have seen that plants exchange gases through stomata, and the large inter-cellular
spaces ensure that all cells are in contact with air.
Carbon dioxide and oxygen are exchanged by diffusion here. They can go into cells, or away
from them and out into the air.
The direction of diffusion depends upon the environmental conditions and the requirements
of the plant.
At night, when there is no photosynthesis occurring, CO2 elimination is the major exchange
activity going on. During the day, CO2 generated during respiration is used up for
photosynthesis, hence there is no CO2 release. Instead, oxygen release is the major event at
this time.
Animals have evolved different organs for the uptake of oxygen from the environment and
for getting rid of the carbon dioxide produced. Terrestrial animals can breathe the oxygen in
the atmosphere, but animals that live in water need to use the oxygen dissolved in water.
Since the amount of dissolved oxygen is fairly low compared to the amount of oxygen in the
air, the rate of breathing in aquatic organisms is much faster than that seen in terrestrial
organisms. Fishes take in water through their mouths and force it past the gills where the
dissolved oxygen is taken up by blood.
Terrestrial organisms use the oxygen in the atmosphere for respiration. This oxygen is
absorbed by different organs in different animals.
1. All these organs have a structure that increases the surface area which is in contact with the
oxygen-rich atmosphere.
2. Since the exchange of oxygen and carbon dioxide has to take place across this surface, this
surface is very fine and delicate.
3. In order to protect this surface, it is usually placed within the body, so there have to be
passages that will take air to this area.
4. In addition, there is a mechanism for moving the air in and out of this area where the oxygen
is absorbed.
In human beings, air is taken into the body through the nostrils.
Nostrils
The air passing through the nostrils is filtered by fine hairs that line the passage. The passage
is also lined with mucus which helps in this process.
From here, the air passes through the throat and into the lungs. Rings of cartilage are
present in the throat. These ensure that the air-passage does not collapse
