Cellular Energy: Study Guide (Biology 3.3)

Why every living thing needs a supply of energy

A cell is busy all the time. It builds proteins and nucleic acids, pumps ions against their concentration gradients, moves organelles, copies its DNA and divides. Each of these jobs needs energy. Stop the energy supply and the jobs stop.

Organisms get energy in two main ways:

• Light energy. Plants, algae and some bacteria capture energy from sunlight and store it in sugars (Topic 3.4).
• Chemical energy. Animals, fungi and most bacteria take in energy-rich molecules made by other organisms, then release the energy in respiration (Topic 3.5).

Either way, the rule is the same: all living systems need a continuous input of energy. There is no organism that runs on no energy at all.

Order, energy and the laws of thermodynamics

Living things are highly ordered. A cell keeps its membranes intact, its proteins folded, its ions at very different concentrations inside and outside, and its organelles in the right places. Order like this does not last on its own. Proteins wear out, ions leak, and molecules drift apart. Keeping order needs energy all the time.

Two laws of physics describe energy. Life follows both.

First law: energy is conserved. Energy cannot be created or destroyed. It can only be converted from one form to another. A plant converts light energy into chemical energy in sugar. Your muscles convert chemical energy into movement and heat. An organism cannot make energy; it can only take it in and convert it.

Second law: every conversion increases disorder overall. No energy conversion is perfect. Each one releases some energy as heat, which spreads out into the surroundings. Disorder in a system is called entropy. In every real process, the total entropy of a system plus its surroundings increases.

So how can a growing organism become more ordered? Because an organism is an open system: it exchanges energy and matter with its surroundings.

• It takes in ordered, energy-rich molecules (food) or light.
• It uses some of that energy to build and maintain its own ordered structures.
• It releases heat and small, simple molecules such as CO₂ and H₂O into its surroundings.

The organism’s own order goes up, but the surroundings become much more disordered. The total entropy still increases. Life does not break the second law; it depends on it.

This leads to three conditions for staying alive:

1. Energy input must exceed energy loss. Some energy is always lost as heat. To keep order and power cell processes, an organism must take in more energy than it loses.
2. Energy-releasing processes are coupled to energy-requiring ones. The energy released by breaking down food is passed, through ATP, to the jobs that need it.
3. A large loss of order or of energy flow is fatal. If energy input falls well below losses for long enough, the organism uses up its stores, cannot maintain order, and dies.

Energy coupling and ATP

Reactions in cells fall into two groups:

• Energy-releasing (exergonic) reactions give out energy. Breaking down glucose and hydrolysing ATP are examples.
• Energy-requiring (endergonic) reactions need energy put in. Joining amino acids into a protein and pumping ions against a gradient are examples.

An energy-requiring reaction cannot run by itself. A cell makes it run by coupling it to an energy-releasing reaction, so that the energy released by one is used to drive the other. On these pages a + sign means energy is required and a − sign means energy is released, in kJ per mole of reaction.

The usual go-between is ATP (adenosine triphosphate). Hydrolysing its last phosphate group releases energy:

ATP + H₂O → ADP + Pᵢ, releasing about 30 kJ per mole under standard laboratory conditions

(Pᵢ means inorganic phosphate.) Inside living cells, where ATP is kept much more concentrated than ADP, the energy released is larger, roughly double. This guide uses the rounded laboratory value, 30 kJ mol⁻¹, for calculations.

ATP is then rebuilt from ADP and Pᵢ, using energy from energy-releasing processes such as respiration. ATP works like a rechargeable battery that is used and recharged many times.

Energy pathways run in steps

If you set fire to glucose, it reacts with oxygen in one fast burst. All the energy comes out at once as heat and light. A cell cannot use energy that way. A sudden burst would overheat it, and most of the energy would be wasted.

Instead, cells break down glucose through a metabolic pathway: a sequence of reactions, each catalysed by its own enzyme.

A → B → C → D → E

Two features matter:

• The product of one step is the reactant for the next. B is made from A and then used to make C, and so on. The steps are linked like a production line.
• Energy is released in small, controlled amounts. Each step releases only a little energy. That amount is small enough to be captured, for example in ATP or in an electron carrier, rather than lost as heat.

The total energy released from A to E is the same as in the one-step reaction (the first law). What changes is control. Because each step has its own enzyme, the cell can speed up or slow down a pathway by regulating one enzyme. If one enzyme is blocked, its substrate builds up and everything after it runs short.

Shared pathways and common ancestry

All known life falls into three domains: Bacteria, Archaea and Eukarya. Their members look very different. Yet at the level of energy metabolism, they share a great deal.

• ATP is the main energy carrier in cells of all three domains.
• Glycolysis, the pathway that splits glucose (Topic 3.5), is found in all three domains. Many archaea run a modified version in which some steps use different enzymes, but the overall route from glucose to pyruvate is shared.
• ATP synthase, an enzyme in a membrane that makes ATP as protons flow through it, is found in all three domains. In bacteria and archaea it sits in the plasma membrane; in eukaryotes it sits in mitochondria (and chloroplasts). Making ATP this way, using a proton gradient set up by electron transport, is called oxidative phosphorylation in respiration.

Why would such different organisms share the same detailed machinery? The simplest explanation is common ancestry: these pathways were present in an early common ancestor and were passed down to all its descendants. Because the pathways are so central, mutations that wrecked them were usually harmful, so they have been conserved with relatively little change. Shared, conserved features like these are among the strongest evidence that all living things are related.

Worked example 1: coupling a reaction to ATP

Question. In a fictional cell, an enzyme joins molecule P and molecule Q into PQ. The reaction requires +21 kJ per mole. Take ATP hydrolysis as releasing 30 kJ per mole (−30 kJ mol⁻¹).

(a) Explain why P + Q → PQ cannot run on its own, and calculate the overall energy change when it is coupled to the hydrolysis of one ATP. (b) A second reaction requires +50 kJ per mole. What is the smallest whole number of ATP that can drive it, and what is the overall energy change?

(a)

1. The reaction requires energy (+21 kJ mol⁻¹): the product holds more energy than the reactants. It cannot happen without an energy input.
2. Coupled reaction: P + Q + ATP → PQ + ADP + Pᵢ.
3. Overall change = (+21) + (−30) = −9 kJ mol⁻¹. The coupled process releases energy overall, so it can proceed.
4. About 21 ÷ 30 × 100 = 70% of the energy from ATP ends up stored in PQ. The other 9 kJ mol⁻¹ is released as heat.

(b)

1. One ATP: (+50) + (−30) = +20 kJ mol⁻¹. Still energy-requiring, so one ATP is not enough.
2. Two ATP: (+50) + 2 × (−30) = +50 − 60 = −10 kJ mol⁻¹. Energy is released overall, so two ATP can drive it.
3. You cannot hydrolyse a fraction of an ATP molecule, so “1.67 ATP” is not an answer.

Interpretation. In both cases the totals balance (first law), and some energy always becomes heat (second law). Coupling works because the energy-releasing reaction releases more energy than the energy-requiring one needs.

Worked example 2: an energy budget

Question. A fictional small mammal is studied in the wild. In mild weather, each day it takes in 75 kJ of chemical energy in food. Of this, 12 kJ leaves in faeces (not absorbed), 58 kJ is released as heat from respiration and 5 kJ is stored in new tissue.

(a) Show that the budget balances, and give the percentage of the intake released as heat. (b) In a cold spell, food intake falls to 50 kJ per day, faeces carry 8 kJ, and heat loss rises to 62 kJ per day. Find the daily energy balance, and how long a 400 kJ fat reserve would last at this rate. (c) Link your answers to the conditions for staying alive.

(a)

1. Total out = 12 + 58 + 5 = 75 kJ, equal to the 75 kJ in. Energy has been converted, not created or destroyed (first law).
2. Heat as a percentage of intake = 58 ÷ 75 × 100 = 77.3%. Only 5 ÷ 75 × 100 = 6.7% is stored.

(b)

1. Balance = input − losses = 50 − 8 − 62 = −20 kJ per day. The animal is losing more energy than it takes in.
2. The deficit is met from stores: 400 kJ ÷ 20 kJ per day = 20 days.

(c) In mild weather, input exceeds losses, so the animal keeps its body ordered and can even grow. In the cold spell, the deficit must come from stored fat. If it lasts longer than about 20 days, the animal can no longer power the processes that keep its cells ordered, such as ion pumping and protein repair, and it will die. Notice also how much energy is lost as heat: most of an organism’s energy intake ends up warming the surroundings, as the second law predicts.

Worked example 3: building an argument for common ancestry

Question. A student finds that a soil bacterium, a methane-producing archaeon and a yeast all make ATP with a membrane ATP synthase, and all carry out a version of glycolysis. Make a claim from this evidence and justify it.

1. Claim. The three organisms share a common ancestor that already used these pathways.
2. Evidence. Organisms from all three domains have the same core features: ATP as the energy carrier, the glycolysis pathway and an ATP synthase driven by a proton gradient.
3. Reasoning. These features are complex, with many enzymes working in a fixed order. It is very unlikely that three separate lineages invented the same detailed pathway independently. It is far more likely that the pathways arose once in an early ancestor and were inherited. Because they are essential, most changes to them were harmful and removed by natural selection, so they have been conserved.
4. Limit. One shared feature alone would be weak evidence. The argument gets stronger as more shared, conserved features are found, and stronger still when the DNA sequences of the genes for these enzymes are similar.

Common misconceptions

• “Living things break the second law because they become more ordered.” They become ordered only by releasing heat and simple molecules that increase the disorder of their surroundings by more.
• “Organisms make energy.” Energy cannot be created. Organisms capture it (from light or food) and convert it.
• “ATP stores energy for the long term.” ATP is a short-term carrier, used and remade constantly. Long-term stores are fats and carbohydrates such as starch and glycogen.
• “The bond in ATP releases energy when it breaks.” Breaking any bond needs energy. Energy is released by the whole hydrolysis reaction, because the products are more stable than the reactants.
• “A stepwise pathway releases more energy than burning.” The total is the same. Steps release it in smaller, controlled amounts that can be captured.
• “Coupling works if ATP supplies any energy.” ATP must supply more than the reaction needs, so that the overall change releases energy.
• “Similar pathways in all organisms just show that all cells need energy.” Needing energy does not explain why the same enzymes and steps are used. Shared detail points to shared ancestry.

Where this leads

You saw in Topic 3.2 that enzymes depend on their conditions; here those enzymes are organised into pathways that move energy. Next, Topic 3.4, Photosynthesis, shows how light energy enters living systems, and Topic 3.5 shows how cellular respiration releases it again as ATP. Test yourself with the practice questions, then use the revision notes and the checklist to consolidate.