All living cells require energy, and this energy isprovided by the oxidation of glucose – respiration.
glucose + oxygen → carbon dioxide + water
- The oxidation of glucose is strongly exothermic, but 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 (PO3-4, abbreviated to Pi).
- So all respiration really does is convert chemical energy stored in glucose into chemical energy stored in ATP.
- ATP is a nucleotide (one of the four found in DNA),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.
- This is a good thing, as these small packets of easily-released energy are more useful to cells and can be used to do simple common jobs.
What is the energy in ATP used for?
The processes in a cell that require energy can be put into three groups:
- Muscle contraction and other forms of movement, such as cilia, flagella, cytoplasmic streaming, etc. Each step of the muscle crossbridge cycle costs one ATP molecule.
- Active transport. Each shape change in an active transport protein pump costs one ATP molecule.
- Biosynthesis– building up large molecules from smaller ones, e.g. protein synthesis, DNA replication, starch synthesis, etc. Each monomer added to a growing polymer chain costs one ATP molecule.
Since these processes use ATP, they all involve ATPase enzymes. ATPases catalyse the hydrolysis of ATP to ADP + Pi, and do work with the energy released.
All the thousands of chemical reactions taking place in a cell are referred to as Metabolism.
To make the reactions easier to understand, biochemists arrange them into metabolic pathways.The intermediates in these metabolic pathways are called metabolites.
- Much of respiration takes place in the mitochondria.
- Mitochondria have a double membrane: the outer membrane contains many protein channels called porins, which let almost any small molecule through; while the inner membrane is more normal and is selectively permeable to solutes.
- The inner membrane is highly folded into projections called cristae, giving a larger surface area.
- The electron microscope reveals blobs on the inner membrane, called stalked particles.
- These blobs have now been identified as enzyme complexes that synthesise ATP, and are more correctly called ATP synthase enzymes (more later).
- 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 and expressed here.
Details of Respiration
The equation for cellular respiration is usually simplified to:
glucose + oxygen →carbon dioxide + water (+ energy)
- The different stages of respiration take place in different parts of the cell. This compartmentation 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.