Respiration: definition and glycolysis

In a nutshell

Living organisms must carry out many different processes in order to survive. Many of these cellular processes will require energy. Respiration breaks down organic molecules to release energy in the form of ATP, which can be used to drive many cellular processes.

Respiration

Definition

Respiration is the process by which cells break down organic molecules, such as glucose, to transfer energy for cellular processes. Respiration in the presence of oxygen is called aerobic respiration. This produces more ATP than anaerobic respiration, which is respiration in the absence of oxygen.

Glycolysis

Glycolysis takes place in the cytoplasm and occurs during both aerobic and anaerobic respiration as it does not require oxygen. During glycolysis, glucose (a six-carbon molecule) is broken down to produce two three-carbon molecules of pyruvate and a net of two molecules of ATP. The process of glycolysis can be seen below.

Note: Overall, four ATP molecules are produced by glycolysis. However, two ATP molecules are used in the first half of the process. Therefore, the net production of ATP is only two.

Biology; Energy for biological processes; KS5 Year 12; Respiration: definition and glycolysis

1.	Glucose is a relatively stable six-carbon molecule so it must be made reactive by phosphorylation. A molecule of ATP is hydrolysed to ADP and $P_i$ . The phosphate molecule is added to glucose to produce glucose phosphate.
2.	Another molecule of ATP is hydrolysed to ADP and $P_i$ . The phosphate molecule is added to glucose phosphate to produce hexose bisphosphate (another six-carbon molecule).
3.	Hexose bisphosphate is then split into two three-carbon molecules of triose phosphate.
4.	Triose phosphate is oxidised to produce pyruvate. The hydrogen that is lost during the oxidation is transferred to a carrier molecule called NAD. This produces reduced NAD (or NADH).
5.	ATP is produced during the conversion of triose phosphate. ADP and $P_i$ combine to reform two molecules of ATP.