Describe: Tricarboxylic acid cycle.

(N/A) The $TCA$ cycle starts with the condensation of an acetyl group with oxaloacetic acid $(OAA)$ and water to yield citric acid. The reaction is catalysed by the enzyme citrate synthase and a molecule of $CoA$ is released.
$OAA (4C) + \text{Acetyl } CoA + H_2O \xrightarrow{\text{Citrate synthase}} \text{Citric acid } (6C) + CoA$
Citrate is then isomerised to isocitrate.
It is followed by two successive steps of decarboxylation, leading to the formation of $\alpha$-ketoglutaric acid and then succinyl-$CoA$.
$\text{Citric acid} \rightarrow \text{Isocitrate} \rightarrow \alpha\text{-ketoglutaric acid}$
Remaining steps of the citric acid cycle:
Succinyl-$CoA$ is oxidised to $OAA$, allowing the cycle to continue.
During the conversion of succinyl-$CoA$ to succinic acid, a molecule of $GTP$ is synthesized. This is a substrate-level phosphorylation.
In a coupled reaction, $GTP$ is converted to $GDP$ with the simultaneous synthesis of $ATP$ from $ADP$.
$GTP + ADP \rightarrow GDP + ATP$
Also, there are three points in the cycle where $NAD^+$ is reduced to $NADH + H^+$ and one point where $FAD^+$ is reduced to $FADH_2$. The continued oxidation of acetyl $CoA$ via the $TCA$ cycle requires the continued replenishment of oxaloacetic acid.
It is the first member of the cycle. In addition, it also requires the regeneration of $NAD^+$ and $FAD$ from $NADH$ and $FADH_2$ respectively.
The summary equation for this phase of respiration may be written as follows:
$\text{Pyruvic acid} + 4NAD^+ + FAD^+ + 2H_2O + ADP + Pi \xrightarrow{\text{Mitochondrial Matrix}} 3CO_2 + 4NADH + 4H^+ + FADH_2 + ATP$
Glucose has been broken down to release $CO_2$ and eight molecules of $NADH + H^+$; two of $FADH_2$ have been synthesised besides just two molecules of $ATP$ in the $TCA$ cycle.