Process of glycolysis is summarised as follow

Glucose is the favourite sulphate for respiration as carbohydrates are first converted into glucose. Prior to used for respiration. Fats are acetyl CO-A are broken down into glycerol and fatty acid which is further degraded into acetyl Co-A, while protein is degraded into, smaller units anino acids. Respiratory process involves the breaking down of substrate is catabolic proceses. Sometime the fatty acid is required them synthesis of it occurs by withdrawing acetly Co-A. This synthesising phase is the anabolic process.

Thus, respiratory pathway involves the catabolic process (break down) and anabolic pathway synthesis of molecules using respiratory intermediates frant link, it is called as an amphibolic pathway.

An energy carrier is a highly specialised molecule that transfers, recieves and stores energy within the cell. This energy is then used to facilitate the chemical reactions within the cell. The three major types of energy carriers are ATP, NADPH and NADH.

Substrate-level phosphorylation is a type of metabolic reaction that results in the formation of Adenosine Triphosphate (ATP) or Guanosine Triphosphate (GTP) by the direct transfer and donation of a phosphoryl $\left(\mathrm{PO}_3\right)$ group to Adenosine Diphosphate (ADP) or Guanosine Diphosphate (GDP) from a phosphorylated reactive intermediate.

In glycolysis substrate level phosphorylation occurs in following two reactions

(i) 2 molecules of 1,3-diphosphoglyceric acid react with 2 molecules of ADP to form 2 molecules of 3-phosphoglyceric acid and 2 molecules of ATP .

(ii) 2 molecules of phosphoenolpyruvic acid reacts with 2 molecules of ADP to form 2 molecules of pyruvic acid and 2 ATP.

2 - phosphoenolpyruvic acid 2ADP $\xrightarrow[\text { (2 molecules) }]{\text { Pyruvic kinase }} \underset{\mathrm{Mg}^{2+}}{\text { Pyruvic acid }+2 \text { ATP }} \underset{\text { (2 molecules) }}{\text { Pyrule }}$

In an intermediate reaction of TCA cycle, succinyl Co-A is converted to succinic acid and one GTP molecule is synthesised through substrate level phosphorylation.

$$\begin{aligned} &\text { GTP formed in this reaction gives rise to ATP as follows }\\ &\text { GTP + ADP } \rightarrow \text { GDP + ATP } \end{aligned}$$