Energy, ATP, coenzymes and the mitochondrion

1Where does the energy go?

A plate of ugali and beans holds enough chemical energy to keep you going for hours, yet no cell can use glucose directly to contract a muscle or pump an ion. Cells first transfer the energy from glucose into a small, universal molecule, ATP, and then spend ATP wherever work is done. The process that makes ATP from food is cellular respiration. In Senior 1 you met the mitochondrion as the site of aerobic respiration; this unit follows respiration step by step, as an A-level biochemist does.

2Why organisms need energy

Every living cell does work that needs energy:

• Synthesis (anabolism): building proteins from amino acids, DNA from nucleotides, glycogen from glucose.
• Active transport: pumping ions and molecules against their concentration gradients, such as the sodium–potassium pump in every cell, or mineral uptake by root hairs.
• Movement: muscle contraction, the beating of cilia and flagella, and moving chromosomes in cell division.
• Nerve impulses: restoring the ion gradients across the membranes of neurones.
• Keeping warm: in birds and mammals, energy released as heat by respiration keeps the body at a constant temperature.

3ATP: the energy currency

Figure 1: The structure of ATP, ADP and AMP.

When the bond to the last phosphate is hydrolysed, ATP becomes ADP and inorganic phosphate (${\mathrm{P}}_{\mathrm{i}}\backslash mathrm\{P\_i\}$), and energy is released:

$$\mathrm{A}\mathrm{T}\mathrm{P}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{A}\mathrm{D}\mathrm{P}+{\mathrm{P}}_{\mathrm{i}}\mathrm{\Delta }G=-30.5\mathrm{k}\mathrm{J}\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1}\backslash mathrm\{ATP\}\; +\; \backslash mathrm\{H\_2O\}\; \backslash rightarrow\; \backslash mathrm\{ADP\}\; +\; \backslash mathrm\{P\_i\}\; \backslash qquad\; \backslash Delta\; G\; =\; -30.5\backslash ,\backslash mathrm\{kJ\backslash ,mol^\{-1\}\}$$

The reaction is catalysed by ATPase enzymes. The reverse reaction, phosphorylation of ADP by ATP synthase, needs the same amount of energy, supplied by respiration (or, in chloroplasts, by light). ATP suits its job because:

• it releases energy in small, manageable amounts, close to what a single reaction needs, so little is wasted as heat;
• it is hydrolysed in one quick step by one enzyme;
• it is soluble and moves easily within a cell;
• it is not stored but made as fast as it is used: the same ADP is recycled many times a minute.

ATP is made in two ways. In substrate-level phosphorylation, a phosphate group is transferred directly from a substrate molecule to ADP (in glycolysis and the Krebs cycle). In oxidative phosphorylation, ATP synthase uses the energy of a gradient of H⁺ ions across the inner mitochondrial membrane (Lesson 3); this makes most of the ATP.

4The coenzymes NAD, FAD and coenzyme A

Respiration is a series of oxidations: hydrogen atoms (each a proton and an electron) are removed from substrates by dehydrogenase enzymes. The hydrogen is carried to the mitochondrion's inner membrane by two coenzymes, and acetyl groups are carried by a third.

Because the body cannot make these vitamins, a lack of them slows respiration: a severe lack of niacin causes the disease pellagra.

5The mitochondrion

Figure 2: A mitochondrion, drawn from an electron micrograph (not to scale).

Cells that use a lot of energy, such as heart muscle, liver cells and sperm, contain many mitochondria with densely packed cristae.