Biology · Energy, Enzymes & Metabolism

Energy transformations, matter conservation and living systems

A seedling becomes heavier while its leaves receive light. Light supplies energy, but the plant does not turn light into carbon atoms. To explain growth, ask two questions: where did the material come from, and what transformations supplied the capacity to build and maintain it?

Read the Chemistry of Life and Cells & Membranes chapters first if atoms, molecular interactions and membrane transport are unfamiliar. The necessary ideas are also introduced here. You do not need to memorise complete biochemical pathways or calculate thermodynamic quantities to follow the first reading.

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By the end, you can

Trace atoms and energy separately across a living system's boundary.

Keep two accounts

In ordinary biological chemical reactions, atoms are rearranged into new molecules. Carbon in a sugar molecule can later become carbon in carbon dioxide or in another organic molecule. An atom has not disappeared because we have stopped following its product. A complete material account includes inputs, outputs and changes in what remains inside the chosen boundary.

Energy is transferred and transformed too. Light absorbed by a photosynthetic cell can contribute to chemical changes; chemical reactions can support transport or movement and transfer energy to the surroundings as heat. A cell does not create energy. Its energy content can change, because it is an open system exchanging energy with its surroundings. The conservation account must include both.

Follow matter and energy along different routes

Carbon atoms can cycle; usable energy is progressively dispersed. This is a plant-based route, not a complete ecosystem balance.

Read the diagram in words

Two schematic rows separate a material route from an energy route. Carbon dioxide contributes carbon to organic molecules and can receive that carbon again through oxidation. Light contributes energy to photosynthesis; chemical changes support ATP-coupled work and heat transfer. The arrows show different kinds of relationships, not equal amounts or universal rates.

Carbon in CO2

Material input

Material account

Carbon in organic molecules

Atoms incorporated and rearranged

Material account

Carbon in CO2

One possible material output

Material account

Light energy

A photosynthetic input

Energy account

Chemical transformations

Energy changes depend on reactants and products

Energy account

ATP-coupled work

Building, transport and movement

Energy account

Heat transferred

Energy reaches the surroundings

Energy account

Material account
Description	Process
Carbon in CO2 → Carbon in organic molecules	Carbon fixation
Carbon in organic molecules → Carbon in CO2	Complete oxidation

Energy account
Description	Process
Light energy → Chemical transformations	Photosynthesis
Chemical transformations → ATP-coupled work	Energy coupling
Chemical transformations → Heat transferred	Heat transfer
ATP-coupled work → Heat transferred	Further transformations

Material account
Energy account

Consider the whole chemical change

Breaking an isolated chemical bond requires energy. Forming bonds releases energy. Whether a complete reaction releases energy depends on the entire change, including new bonds and interactions with the surroundings. 'The bond snapped, releasing energy' is therefore a misleading account of how food or ATP supports cellular work.

Under specified conditions, an exergonic reaction has a negative change in Gibbs free energy, written ΔG < 0. An endergonic reaction has ΔG > 0. The symbol Δ means a change: products compared with reactants. Free energy helps judge which direction is thermodynamically favourable at those conditions. It is not a promise that the reaction will be fast. We return to that distinction when considering enzymes.

Organisation needs an ongoing exchange

Cells can build organised structures without contradicting thermodynamics. Their activity is coupled to exchanges and transformations that increase the total entropy of the cell and surroundings. Entropy concerns the distribution of energy and possible physical arrangements; a tidy-looking cell is not a complete entropy measurement.

Many food webs depend on photosynthesis, but sunlight is not every organism's immediate energy source. Some microbes obtain usable energy from reactions involving inorganic substances. At deep-sea vents, such microbes can support food webs through chemosynthesis. The common principle is an energy transformation, not one universal fuel or one universal habitat.

Worked example

Follow one glucose molecule

The net equation for complete aerobic oxidation of glucose is C6H12O6 + 6 O2 → 6 CO2 + 6 H2O. Count the atoms: both sides have six carbon atoms, twelve hydrogen atoms and eighteen oxygen atoms. Energy is transferred during the overall process; it is not an extra atom that needs balancing.

This equation is a net account across many reactions. It does not depict a glucose molecule colliding with six oxygen molecules in one cellular step, and it does not show where every particular oxygen atom ends up. The pathways and ATP-coupling mechanisms must be considered separately.

Check your understanding

A cell receives 100 illustrative energy units over an interval, exports 70 as heat and 10 through work. If these are all its energy transfers, how much does its energy content increase?
Show reasoning
By 20 units: 100 − 70 − 10 = 20. This is a constructed conservation example, not an efficiency measurement for a living cell.
Does the phrase 'the plant gets energy from light' explain the source of the carbon in new plant tissue?
Show reasoning
No. The energy source and material source are different accounts. Photosynthetic carbon fixation incorporates carbon from carbon dioxide into organic molecules.
If a chemical reaction is thermodynamically favourable, must it happen rapidly?
Show reasoning
No. A substantial activation barrier can make a favourable reaction very slow under the conditions considered.

What to carry forward

Trace the material and energy accounts together, without treating atoms and energy as interchangeable entries.

References

Resources supporting this lesson’s explanations and examples.