Citric Acid Cycle

This diagram, of the use and regeneration of citric acid and all its attendant molecules, is perfectly typical of very very many molecules that have similarly configured lifecyles in biology, in all the different departments of biological function: metabolic, neurologic, reproductive, etc.

Citric Acid is a valuable molecule, it participates in metabolism, it is one of many valuable molecules in biochemistry studies, it happens to be a flavor molecule found in lemons, limes, grapefruit, tomatoes, pineapples, kiwifruit, lemon candy, et cetera. Dehydrated lemons and limes are up to 8% citric acid!

Citric acid is an important nutrient, and when it "does what it does", it changes, into cis-Acotinate, according to the diagram's second step going clockwise, and continues changing step by step until it is regenerated into another citric acid molecule. It's like a circular assembly line.

There are 10 step molecules in this citric acid circular diagram, there are other molecules that exist during this cycle. There are also molecules that participate in making each step, and breathed oxygen and glucose from food are used along the way.

There are many different sugars and starches that we eat:

Glucose - Grape sugar
Sucrose - Cane sugar
Lactose - Milk sugar
Maltose - Malt sugar
Mannose - Manna sugar
and more.

Starches are long chains of sugars. All the different sugars and starches are changed to glucose - grape sugar - for the use of the body.

The other main component of energy is oxygen, from breathing.

The amount of change that the big molecules can accomplish (per step) is more or less defined, determined, limited, by what the two main energy units, glucose and oxygen, are able to do for them. And, plant and animal biologies have evolved very clever, involved and ingenious methods to get all their work done.