Respiration

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4.1

Respiration
Why organisms undergo respiration

What is respiration and why do we need it?
Respiration is the process whereby energy is released from food molecules (most usually glucose). It takes place inside living cells, both eukaryotic and prokaryotic. The process takes place inside the cytoplasm and the mitochondria. Energy is used to synthesise adenosine triphosphate (ATP), which acts as a short-term energy store in cells. All living organisms need energy to drive their metabolic reactions.

A molecule of ATP consists of one adenosine molecule and three phosphate groups. One molecule can be hydrolysed to produce one molecule of adenosine diphosphate (ADP) plus one inorganic phosphate (Pi). This hydrolysis reaction releases 30.6kJ of energy so energy is immediately available to all cells in small amounts that will not damage the cell or be wasted.

P
P
P
ATP is described as the universal energy currency of cells. This is because it is a high-energy molecule which can be used at any time to release energy for all metabolic reactions (biological reactions which take place in living organisms). It can be used for both anabolic reactions (building large molecules) and catabolic reactions (breaking large molecules into smaller ones).

Adenine

Ribose

Aerobic and anaerobic respiration
There are two very different versions of respiration which occur under different conditions. Eukaryotic organisms undergo aerobic respiration under conditions where oxygen is present. Where oxygen is not present, anaerobic respiration occurs. Both types of respiration begin with one stage, glycolysis, which is also called the common pathway (as both aerobic and anaerobic use it). Under aerobic conditions, there are a further three key stages wh ich you will meet in this chapter. Under anaerobic conditions, there is only an extension on the glycolysis pathway which you will again meet later.

Enzymes and coenzymes
During respiration (particularly under...
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