ATP, coenzymes and glycolysis

Last time

Before we start: last time, in The immune response, vaccination and monoclonal antibodies, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Cells run on a currency

A cell cannot spend glucose directly. Burning it all at once would waste most of the energy as heat. So cells convert the energy in glucose into a small, standard currency, A T P, and spend that on work. This lesson covers the currency, the helper molecules, and the first stage of the production line.

Why cells need energy

Anabolic reactions, such as making proteins and D N A, need energy. So do active transport and endocytosis, and movement such as muscle contraction. Nerve impulses need energy to restore ion gradients, and cells need heat to keep enzymes working in mammals. A cell with a high energy demand, such as a muscle cell or a fully active jellyfish, has many mitochondria and makes a lot of A T P.

Why ATP is a good currency

A T P is soluble, so it moves easily in the cell. It is small, so it can pass to where energy is needed. Hydrolysis of A T P to A D P and phosphate releases a useful, small amount of energy in one step. That avoids wasting energy as heat. The terminal phosphate bond is easily broken but the molecule is stable at normal cell pH, so it does not break down by accident. A T P is made quickly from A D P and phosphate, so the supply is renewed constantly. It is not stored: a cell makes it as it needs it.

A past-paper example

May June 2025, Paper 41, question 1 part b. Explain the features of A T P that make it suitable to be the universal energy currency of cells. Four marks. It is small and soluble, so it moves easily. Hydrolysis of the terminal phosphate releases a small, useful amount of energy in one step. It is stable at the cell's pH. It can be rapidly remade from A D P and phosphate. Any four of these points earn the marks.