Many small steps

Section 2 of 4

A flame gives out all of glucose's energy in one go. A cell takes glucose apart in a long series of small steps, and many of them give out an amount of energy small enough for the cell to catch.

Each step is a small reaction with an enzyme of its own. As you have seen, an enzyme holds its molecule in its active site and lowers the energy hill, so the reaction runs fast at body temperature. A step changes the molecule a little: it moves a group, adds water, splits a bond, or takes off two hydrogen atoms.

The same drop in energy, all at once or in small steps A graph of energy. A dashed line near the top is labelled glucose plus oxygen, and a dashed line near the bottom is labelled carbon dioxide plus water. On the left, in a flame, one long amber arrow drops from the top line to the bottom line: the energy comes out all at once, as heat. On the right, in a cell, a staircase of 8 small steps goes down between the same two lines, with each small drop in amber: the energy comes out a little at a time. energy glucose + oxygen carbon dioxide + water in a flame all at once, as heat in a cell a little at a time
Burning and respiration have the same total drop in energy. A flame gives it out all at once, as heat. A cell goes down in many small steps, and at many of them it catches some of the energy.

Glycolysis: splitting glucose

The first stage happens in the watery fluid that fills the cell. Ten steps split one glucose molecule, with 6 carbon atoms, into two molecules of pyruvate, with 3 carbon atoms each. Pyruvate is the ion of pyruvic acid, CH₃COCOOH. This stage is called glycolysis, from the Greek words for sweet and splitting. It needs no oxygen, and it gives the cell only a small share of glucose's energy.

The citric acid cycle

Each pyruvate then moves into a mitochondrion (plural mitochondria), one of the hundreds of tiny oval parts inside most of your cells. There it gives off one carbon atom as carbon dioxide. The two carbon atoms left make an ethanoyl group, -⁠COCH₃, the group that ethanoic acid carries, called acetyl in living things. The acetyl group joins a four-carbon molecule to make citric acid, the acid that makes lemons sour.

Citric acid, C₆H₈O₇A central carbon atom carries an -OH group, a -COOH group and two CH₂ groups, and each CH₂ carries another -COOH group, three -COOH groups in all.OHOHOOHOHOO
Citric acid, C₆H₈O₇. In the cycle, a two-carbon acetyl group and a four-carbon molecule join to make it, and seven more steps take it apart again.

Seven more steps take citric acid apart again. They give off two carbon dioxide molecules and leave the four-carbon molecule ready to pick up the next acetyl group. The steps go round and round, so this stage is called the citric acid cycle. Along the way, carrier molecules collect the hydrogen atoms taken off. At the inner wall of the mitochondrion they pass them on to the oxygen you breathe in, which becomes water, and this last stage gives out most of the energy.

Quick check

Why do your cells break glucose down in many small steps?