1Every living thing needs energy

A cell does work every second it is alive. It builds proteins and nucleic acids from smaller molecules, pumps ions against their concentration gradients, moves materials around inside itself, and, in many organisms, moves the whole body. All of this needs energy.

The energy comes from outside. Plants, algae and photosynthetic bacteria capture light energy and store it in sugars (Topic 3.4). Almost all other organisms take in chemical energy in the bonds of organic molecules made by others, and release it in cellular respiration (Topic 3.5). A few prokaryotes get chemical energy from inorganic substances instead. Either way, without a supply, the work stops.

2Order and the laws of thermodynamics

Two laws of physics govern all energy, in a cell or anywhere else.

• The first law of thermodynamics: energy cannot be created or destroyed. It can only be transferred or changed from one form to another: light to chemical, chemical to movement, chemical to heat.
• The second law of thermodynamics: every energy transfer or change increases the disorder of the universe. In practice, every time energy changes form, some of it becomes heat that spreads out and can no longer be used to do work.

A cell is a small, highly ordered system, and at first sight that seems to go against the second law. It does not, because the cell is not closed. It takes in energy-rich, ordered molecules (or light) and gives out heat and small, disordered molecules such as carbon dioxide and water. The order inside the cell increases or is maintained; the disorder of the cell plus its surroundings still increases. Life pays for its order by making its surroundings more disordered.

That is why the input has to be greater than the loss. Some of every input is lost as heat, so a cell must keep taking in energy just to stand still, and take in more to grow or reproduce. If the energy flow stops, or the order falls apart badly, the result is death: ion gradients run down, proteins are not replaced, membranes fail.

Worked example: an energy budget. An invented field study. A young lizard takes in 120 kJ of chemical energy in its food in a day. Of this, 18 kJ leaves in faeces without being absorbed, 78 kJ is used in respiration and lost as heat, and the rest is stored as new tissue.

Most of what the lizard eats is lost as heat. Only a fifth ends up as new, ordered tissue. That is the second law at work, and it is why energy flowing through a food chain shrinks at every level (Unit 8).

Life does not break the laws of thermodynamics: it stays ordered only by taking in more energy than it loses as heat.

3Energy coupling

Some reactions release energy: breaking glucose down into carbon dioxide and water is one. Others need energy: joining amino acids into a protein, or pumping an ion against its gradient. An energy-requiring reaction will not go on its own, so a cell couples it to an energy-releasing one. The energy released by one process is used to drive the other.

The usual go-between is ATP (adenosine triphosphate). Respiration releases energy from food and uses it to make ATP from ADP and inorganic phosphate. Elsewhere in the cell, ATP is split back to ADP and phosphate, releasing energy that drives the work.

Often the phosphate is passed to the molecule doing the work, for example a pump protein, changing its shape. You do not need the free-energy equation for this course; you need to recognise that one process gives out energy, another takes it in, and the two are linked.

Coupling is how you answer "predict" questions. Block ATP production with a poison, and everything that depends on ATP slows or stops: active transport, protein synthesis, muscle contraction. Passive processes such as diffusion carry on, because they need no energy input from the cell.

4Pathways in small steps

A cell does not release the energy in glucose in one burst. Burning glucose releases it all at once, almost entirely as heat, which a cell could not capture and which would damage it. Instead, the cell breaks glucose down in a metabolic pathway: a sequence of reactions, each catalysed by its own enzyme. Each step releases a small amount of energy, which can be captured in ATP or in carriers such as NADH. That is the point of the many steps: a more controlled transfer of energy.

In a pathway, the product of one reaction is the reactant of the next.

This makes pathways predictable. If enzyme 2 is inhibited:

• B builds up, because enzyme 1 is still making it but nothing is using it.
• C and D fall, because they are no longer being made.
• A may also build up a little later, as accumulating B slows enzyme 1 (Topic 3.2).

That one model answers a large share of pathway questions. Find the blocked step; everything before it accumulates and everything after it runs short.

5Shared pathways and common ancestry

Glycolysis, the first stage of breaking down glucose, is found in bacteria, archaea, fungi, plants and animals, with the same overall chemistry: glucose split into two pyruvate, a little ATP and NADH made. The use of an electron transport chain to build a proton gradient that drives ATP synthase, called oxidative phosphorylation in respiration (Topic 3.5), is also found across all three domains: Bacteria, Archaea and Eukarya.

These pathways are conserved: they have stayed largely the same across all the lineages that descended from the earliest cells. The most likely explanation is that they were already present in a common ancestor of all living things, and every descendant inherited them. If each group had evolved its own way of using glucose, there would be no reason for so many groups to share the same steps.

To use this in an argument, write it as claim, evidence and reasoning:

• Claim: all living organisms share a common ancestor.
• Evidence: core pathways such as glycolysis and oxidative phosphorylation are found in all three domains.
• Reasoning: a complex shared pathway is far more likely to have been inherited from one ancestor than to have arisen independently many times.