Course 6.2Lesson 3 of 3
A cell holds thousands of small molecules that are made and broken down all the time. Chains of enzyme-catalysed steps are metabolic pathways; anabolic ones build molecules using energy, and catabolic ones break them down and release energy, stored in ATP. Material moves through a pathway at a flux. A closed system reaches equilibrium and can do no work. A living cell is an open system held in a steady state away from equilibrium.
Try the code and the simulations beside the lesson.
A single cell holds thousands of different small molecules. None of them sits still. Sugars are broken down, amino acids are joined into proteins, and fats are built and burned. All the chemical reactions of a living thing together are called metabolism.
The reactions of metabolism come in chains called metabolic pathways. Each step in a pathway is a reaction that an enzyme speeds up. The product of one step is the starting material of the next.
A famous pathway turns glucose into pyruvic acid in ten steps. In muscle, one more step turns pyruvic acid into lactic acid. Each step has its own enzyme.
Pathways are of two kinds.
The energy released in catabolism cannot be used as heat. Instead, the cell captures it in a molecule called ATP, which stands for adenosine triphosphate. ATP is a nucleotide: it is adenine, ribose and three phosphate groups.
When the last phosphate group is removed, about kJ of energy is released for every mole of ATP. The cell spends this energy on anabolic pathways, on moving molecules across membranes and on every other kind of work. Then the cell rebuilds ATP with energy from food. This is why ATP is called the energy currency of the cell.
Feed a cell with glucose that carries radioactive carbon. Soon the radioactive carbon turns up in amino acids, in fats and in nucleotides. The carbon moves from molecule to molecule all the time.
The molecules of a cell that are ready to be used form its metabolic pool. Material flows through a pathway at a definite rate and in a definite direction, like traffic on a road. The rate of this flow is the metabolic flux.
So every molecule of the body is in a dynamic state. It is made and broken down again and again, and yet the amount of each stays about the same.
Think of a sealed flask that holds sugar, enzymes and ATP. At first the reactions run fast. Then they slow down, and at last the amounts stop changing. The flask has reached equilibrium.
At equilibrium there is no net flow of material. No free energy is left, so the system can do no work. A closed system always ends this way.
A living cell is different. It is an open system. Food flows in, and waste flows out. The flow through every pathway never stops. The amount of each molecule stays about the same, like the level of water in a tub with the tap and the drain both open. This is called a steady state.
A steady state is far from equilibrium. It lasts only while the cell takes in energy and matter and keeps the flow going. Equilibrium is what a cell reaches when it dies.
Metabolism makes the free energy that keeps the cell away from equilibrium. So the living state and metabolism go together. There can be no living state without metabolism.
You should find that the supply is and the cell uses per cent of its pool each minute. The cell settles at a pool of thousand molecules, and the flow stays at . In the closed flask, the flow halves about every minutes. After minutes it is down to about .
The program follows the pool of a cell and of a closed flask, minute by minute.
You should see the cell settle near thousand molecules, while the flask runs down towards zero. With a supply of , the cell settles near . The flask does not change, because it has no supply.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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