Difficulty: Introductory | Prerequisites: Basic chemistry (atoms, bonds, molecular structure)
Metabolism is the umbrella term for every chemical reaction happening inside a living organism, and energy is the thread running through all of it. This topic sits early in an introductory biology course because nearly everything that follows (cellular respiration, photosynthesis, DNA replication) depends on understanding how energy flows and how cells pay for their work. You should already be comfortable with the idea that atoms form bonds and that breaking or forming those bonds involves energy changes. If thermodynamics sounds intimidating, this is the gentler, biology-flavoured introduction.
Cells need energy to do work, and they get it by breaking down molecules (catabolism) or spend it building molecules up (anabolism). The laws of thermodynamics set the rules: energy is conserved but every transfer wastes some as heat. ATP is the cell's universal energy currency, shuttling energy from exergonic reactions to endergonic ones.
Energy
The capacity to cause change. In biology, this means driving reactions, moving molecules, and building structures.
In simple terms: anything that makes something happen in a cell requires energy.
Kinetic energy
Energy associated with the motion of objects, from whole organisms down to vibrating molecules.
Think of it as the energy of movement at any scale.
Thermal energy
The kinetic energy of atoms and molecules moving randomly. When thermal energy is transferred from one body to another, we call it heat.
In simple terms: thermal energy is the jiggle of particles, and heat is that jiggle moving between things.
Potential energy
Energy stored by virtue of an object's position or structure. A boulder on a cliff has gravitational potential energy; a glucose molecule has chemical potential energy locked in its bonds.
Chemical energy
A form of potential energy stored in the arrangement of atoms within molecules. Released or absorbed when chemical bonds are broken or formed.
Think of it as a molecular savings account the cell can draw from.
First law of thermodynamics (conservation of energy)
Energy cannot be created or destroyed, only transferred or transformed. The total energy of the universe is constant.
In simple terms: you never make or lose energy, you just move it around or change its form.
Second law of thermodynamics
Every energy transfer or transformation increases the entropy (disorder) of the universe. Some energy is always lost as heat.
Think of it as the universe's one-way trend toward messiness.
Entropy
A measure of disorder or randomness in a system. Processes that increase entropy are thermodynamically favoured.
Gibbs free energy (ΔG)
The portion of a system's energy that is available to do work at constant temperature and pressure. The sign of ΔG tells you whether a reaction releases or absorbs usable energy.
In simple terms: ΔG is the answer to "does this reaction have energy left over to do something useful?"
Exergonic reaction
A reaction that proceeds with a net release of free energy. ΔG is negative. These reactions are spontaneous in the thermodynamic sense (they do not necessarily happen quickly).
Endergonic reaction
A reaction that absorbs free energy from its surroundings. ΔG is positive. These reactions are non-spontaneous and require an energy input to proceed.
Metabolism
The totality of an organism's chemical reactions, both building up and breaking down molecules.
Metabolic pathway
A series of linked chemical steps, each catalysed by a specific enzyme, converting a starting molecule into a final product through intermediates.
Catabolic pathway (catabolism)
A metabolic pathway that breaks down complex molecules into simpler ones, releasing energy. Cellular respiration is the classic example.
Think of it as dismantling something and harvesting the energy stored in its structure.
Anabolic pathway (anabolism)
A metabolic pathway that builds complex molecules from simpler precursors, consuming energy. Photosynthesis and protein synthesis are examples.
ATP (adenosine triphosphate)
The cell's primary energy-carrying molecule. It stores energy in the bonds between its three phosphate groups and releases it when hydrolysed to ADP and inorganic phosphate (Pi).
In simple terms: ATP is the rechargeable battery that powers almost everything in the cell.
ATP hydrolysis
The exergonic reaction in which ATP loses a phosphate group to become ADP + Pi, releasing energy the cell couples to endergonic work.
Energy coupling
The use of an exergonic reaction (such as ATP hydrolysis) to drive an endergonic reaction. The cell links the two so the energy released by one pays for the other.
Kinetic energy is energy of motion. At the molecular level this is thermal energy: the random vibration and movement of atoms and molecules.
When thermal energy moves from a warmer body to a cooler one, we call it heat.
Potential energy is stored energy. In biology, the important form is chemical energy, locked in the bonds of molecules like glucose and ATP.
A molecule with high chemical energy (e.g. glucose) can release that energy when its bonds are rearranged during a reaction.
First law (conservation): Energy is neither created nor destroyed. Every joule of energy that enters a biological system must leave it or be stored in another form.
Example: the chemical energy in food is converted into kinetic energy (muscle movement), thermal energy (body heat), and chemical energy (ATP, fat stores). Nothing is lost from the universe's total.
Second law (entropy): Every energy conversion increases the entropy of the universe. Some usable energy is always degraded to heat.
This is why no metabolic process is 100% efficient. Cells always produce waste heat.
Life appears to decrease entropy locally (building ordered structures), but it more than compensates by increasing entropy in its surroundings.
ΔG (the change in Gibbs free energy) tells you whether a reaction can do useful work.
Exergonic reactions (ΔG < 0): Release free energy. Products have less free energy than reactants. These reactions are spontaneous in the thermodynamic sense.
"Spontaneous" does not mean "fast." It means the reaction is energetically favourable. It may still need a push (activation energy) to get started.
Example: cellular respiration (glucose breakdown) is exergonic.
Endergonic reactions (ΔG > 0): Absorb free energy. Products have more free energy than reactants. These require an energy input.
Example: photosynthesis, protein synthesis.
Cells couple exergonic and endergonic reactions so that one pays for the other. ATP hydrolysis is the most common exergonic partner.
Metabolism is organised into pathways: chains of enzyme-catalysed steps where the product of one reaction becomes the substrate of the next.
Catabolic pathways break large molecules into smaller ones and release energy.
Cellular respiration: glucose → CO₂ + H₂O + energy (captured as ATP).
Anabolic pathways build large molecules from small ones and consume energy.
Photosynthesis: CO₂ + H₂O + light energy → glucose.
Protein synthesis: amino acids → polypeptides (driven by ATP).
The two are linked: catabolism generates the ATP and building blocks that anabolism consumes.
Cells use the energy released by ATP hydrolysis to perform three categories of work:
Chemical work: Driving endergonic reactions, such as linking amino acids into proteins or assembling DNA strands.
Transport work: Pumping ions and molecules across membranes against their concentration gradient (e.g. the sodium-potassium pump).
Mechanical work: Powering physical movement, including muscle contraction, beating of cilia, and movement of chromosomes during cell division.
ATP carries energy in the covalent bonds between its three phosphate groups. The bond between the second and third phosphate is particularly unstable.
During ATP hydrolysis, the terminal phosphate is cleaved off, producing ADP + Pi and releasing roughly 30.5 kJ/mol of free energy under standard conditions.
The cell then recycles ADP back to ATP (via cellular respiration or photophosphorylation), completing the ATP cycle.
ATP is not a long-term energy store. A typical human cell turns over its entire supply of ATP roughly every one to two minutes, constantly regenerating it.
Gibbs free energy change:
\Delta G = \Delta H - T\Delta SWhere ΔH is the change in enthalpy (total energy), T is absolute temperature in Kelvin, and ΔS is the change in entropy.
If ΔG < 0, the reaction is exergonic (spontaneous, releases free energy).
If ΔG > 0, the reaction is endergonic (non-spontaneous, requires energy input).
If ΔG = 0, the reaction is at equilibrium.
ATP hydrolysis:
\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy (}\approx 30.5 \text{ kJ/mol)}Your body temperature is a direct consequence of the second law of thermodynamics: metabolic reactions are never perfectly efficient, and the "waste" heat is what keeps you warm. This is also why you feel hot during exercise, when ATP turnover spikes.
Metabolic pathways are the basis of how your body extracts energy from food. Counting calories is, at its core, measuring the chemical energy in food that your cells can capture as ATP.
Students often think "spontaneous" means "fast." It does not. A spontaneous reaction (ΔG < 0) is energetically favourable but may proceed extremely slowly without a catalyst. Diamond turning into graphite is spontaneous and takes millions of years.
Students frequently confuse catabolism and anabolism. Catabolism breaks down (think "catastrophe" for destruction); anabolism builds up (think "anabolic steroids" for building muscle).
ATP is sometimes described as "storing" energy, which can suggest it is a long-term fuel reserve. ATP is a short-lived energy shuttle, not a storage molecule. Fat and glycogen are storage molecules.
The second law of thermodynamics does not mean living things violate physics by being orderly. Organisms create local order at the expense of greater disorder in their surroundings. The overall entropy of the universe still increases.
⚠️ Know the difference between exergonic and endergonic reactions, and be able to identify which is which from a ΔG value.
⚠️ Be ready to explain why ATP hydrolysis is exergonic and how the cell uses energy coupling to drive endergonic processes.
⚠️ The distinction between catabolic and anabolic pathways is a staple exam question. Know an example of each and which direction energy flows.
⚠️ Expect a question on the second law of thermodynamics and how living organisms seem to defy it (they do not).
True or false: An exergonic reaction always proceeds quickly without any help.
False. "Spontaneous" refers to thermodynamic favourability, not speed.
Fill in the blank: ATP is hydrolysed into ______ and ______, releasing energy.
ADP and inorganic phosphate (Pi).
True or false: Living organisms violate the second law of thermodynamics by maintaining internal order.
False. They increase entropy in their surroundings more than they decrease it locally.
Fill in the blank: A metabolic pathway that breaks down complex molecules and releases energy is called a ______ pathway.
Catabolic.
True or false: ΔG must be positive for a reaction to release free energy.
False. ΔG must be negative.
Q: A reaction has a ΔG of -15 kJ/mol. Is this reaction exergonic or endergonic, and will it release or absorb energy?
A: Exergonic. A negative ΔG means the reaction releases 15 kJ/mol of free energy and is thermodynamically spontaneous.
Q: Explain how ATP hydrolysis drives an endergonic reaction inside a cell.
A: The cell couples ATP hydrolysis (exergonic, ΔG negative) to the endergonic reaction through energy coupling. The phosphate group released from ATP is often transferred to a reactant, raising its free energy and enabling it to participate in the otherwise unfavourable reaction. The combined ΔG of the two reactions must be negative for the process to proceed.
Q: Why does the second law of thermodynamics not prevent organisms from building complex, ordered structures?
A: The second law applies to the universe as a whole, not to an individual system. Organisms build order locally by using energy (from food or sunlight), but in doing so they release heat and increase entropy in their surroundings by a greater amount than the local decrease. The net entropy of the universe still increases.
Q: Give one example each of a catabolic and an anabolic pathway, and state the direction of energy flow in each.
A: Catabolic: cellular respiration breaks glucose into CO₂ and H₂O, releasing energy captured as ATP. Anabolic: photosynthesis uses light energy to build glucose from CO₂ and H₂O, consuming energy.
Q: Name the three types of cellular work and give one example of each.
A: Chemical work (synthesising polymers such as proteins), transport work (pumping ions across a membrane via the sodium-potassium pump), and mechanical work (muscle contraction or chromosome movement during mitosis).
This material connects directly to cellular respiration and photosynthesis, which are the two major metabolic pathways you will study in depth next. Everything about how cells harvest or store energy builds on the thermodynamic principles and ATP cycling covered here.
Enzyme function (covered in the companion study notes) is tightly linked: enzymes are what make metabolic pathways possible by lowering the activation energy at each step. Without enzymes, the reactions described here would be too slow to sustain life.
The concept of entropy also reappears in ecology (energy flow through ecosystems) and in molecular biology (protein folding, DNA stability).
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