Respiration

In respiration the energy is released not as heat, but in the form of chemical energy in a compound called ATP (adenosine triphosphate). ATP is built up from ADP and phosphate. So all respiration really does is convert chemical energy stored in glucose into chemical energy stored in ATP.

ATP is a nucleotide, but it also has this other function as an energy storage molecule. So ATP is actually a bigger molecule than glucose, but it is very soluble and the energy it contains can be released very quickly and easily. ATP stores a much smaller amount of energy than glucose. These small packets of easily-released energy are more useful to cells without wasting a lot of energy.

Mitochondria

Much of respiration takes place in the mitochondria. Mitochondria have a double membrane: the outer membrane contains many protein channels called porins, which let small molecules through. While the inner membrane is more normal and is selectively permeable to solutes. The inner membrane is folded into cristae, giving a larger surface area.
There are also enzyme complexes that synthesise ATP. The space inside the inner membrane is called the matrix, and is where the Krebs cycle takes place. The matrix also contains DNA, tRNA and ribosomes, and some genes are replicated here.

Details of Respiration

1. Glucose enters cells from the tissue fluid by facilitated diffusion via a carrier. This carrier can be controlled by hormones such as insulin, so that the uptake of glucose can be regulated.

2. The first step is the phosphorylation of glucose to form glucose phosphate, using phosphate from ATP.

3. Glucose is phosphorylated again (using another ATP) and split into two triose phosphate (3 carbon) sugars.

4. The triose sugar is changed to form pyruvate, a 3-carbon compound. In these steps some energy is released to form ATP (the only ATP formed in glycolysis), and a hydrogen atom is also released. This hydrogen atom is important as it stores energy, which is later used by the respiratory chain to make more ATP. The hydrogen atom is taken up and carried to the respiratory chain by the coenzyme NAD, which becomes reduced in the process.
Pyruvate marks the end of glycolysis, the first stage of respiration.

5. In the absence of oxygen pyruvate is converted into lactate or ethanol in anaerobic respiration.

6. In the presence of oxygen