Metabolism refers to all the chemical reactions taking place in a cell. There are thousands of these in a typical cell, and to make them easier to understand, biochemists arrange them into metabolic pathways. The intermediates in these metabolic pathways are called metabolites.

Reactions that release energy (usually breakdown reactions) are called catabolic reactions (e.g. respiration)
Reactions that use up energy (usually synthetic reactions) are called anabolic reactions (e.g. photosynthesis).

Photosynthesis and respiration are the reverse of each other, and you couldn’t have one without the other. The net result of all the photosynthesis and respiration by living organisms is the conversion of light energy to heat energy.



 
Cellular Respiration





The equation for cellular respiration is usually simplified to:

glucose + oxygen react to form carbon dioxide + water (+ energy)

But in fact respiration is a complex metabolic pathway, comprising at least 30 separate steps. To understand respiration in detail we can break it up into 3 stages:




Before we look at these stages in detail, there are a few points from the above summary:

The different stages of respiration take place in different parts of the cell. This allows the cell to keep the various metabolites separate, and to control the stages more easily.
The energy released by respiration is in the form of ATP.
Since this summarises so many separate steps (often involving H+ and OH- ions from the solvent water), it is meaningless to try to balance the summary equation.
The release of carbon dioxide takes place before oxygen is involved. It is therefore not true to say that respiration turns oxygen into carbon dioxide; it is more correct to say that respiration turns glucose into carbon dioxide, and oxygen into water.
Stage 1 (glycolysis) is anaerobic respiration, while stages 2 and 3 are the aerobic stages.
 

Mitochondria




Much of respiration takes place in the mitochondria. Mitochondria have a double membrane: the outer membrane contains many protein channels, which let almost any small molecule through; while the inner membrane is more normal and is impermeable to most materials. The inner membrane is highly folded into folds called cristae, giving a larger surface area. The electron microscope reveals blobs on the inner membrane, which were originally called stalked particles. These have now been identified as the enzyme complex that synthesises ATP, are is more correctly called ATP synthase. the space inside the inner membrane is called the matrix, and is where the Krebs cycle takes place (the matrix also contains DNA  and some genes are replicated and expressed here).



Details of Respiration


 
 
1. Glucose enters cells from the tissue fluid by facilitated diffusion using a specific glucose carrier protein. This carrier can be controlled (gated) by hormones such as insulin, so that uptake of glucose can be regulated.
2. The first step is the phosphorylation of glucose to form glucose phosphate, using phosphate from ATP. Glucose phosphate no longer fits the membrane carrier, so it can’t leave the cell. This ensures that pure glucose is kept at a very low concentration inside the cell, so it will always diffuse down its concentration gradient from the tissue fluid into the cell. Glucose phosphate is also the starting material for the synthesis of glycogen.
3. Glucose is phosphorylated again (using another ATP) and split into two triose phosphate (3 carbon) sugars. From now on everything happens twice per original glucose molecule.
4. The triose sugar is changed over several steps 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 very 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.
(oxidised form Õ) NAD + H Õ NADH (← reduced form)
Note: rather than write NADH examiners often simply refer to it as reduced NAD or reduced coenzyme
Pyruvate marks the end of glycolysis, the first stage of respiration. In the presence of oxygen pyruvate enters the mitochondrial matrix to proceed with aerobic respiration, but in the absence of oxygen it is converted into lactate (in animals and bacteria) or ethanol (in plants and fungi). These are both examples of anaerobic respiration.
5. Once pyruvate has entered the inside of the mitochondria (the matrix), it is converted to a compound called acetyl CoA. Since this step is between glycolysis and the Krebs Cycle, it is referred to as the link reaction. In this reaction pyruvate loses a CO2 and a hydrogen to form a 2-carbon acetyl compound, which is temporarily attached to another coenzyme called coenzyme A (or just coA), so the product is called acetyl coA. The CO2 diffuses through the mitochondrial and cell membranes by lipid diffusion, out into the tissue fluid and into the blood,