Difficulty: Introductory | Prerequisites: Basic chemistry (atoms, bonds, electrons), Chapter 3 notes on macromolecules.
Big Picture
This unit is about how living things get and use energy. Every cell on the planet runs on the same basic currency (ATP), and the rules of thermodynamics determine which reactions can happen on their own and which need a push. If you understand energy flow at the molecular level, the next several units on cellular respiration and photosynthesis will make far more sense. You should already be comfortable with the idea that atoms share electrons in covalent bonds and that different bonds store different amounts of energy.
TL;DR
Organisms are classified by where they get energy (light vs. chemicals) and carbon (CO2 vs. organic molecules). All cells use ATP as an energy shuttle, and Gibbs free energy (ΔG) tells you whether a reaction releases or requires energy. Cells couple exergonic reactions to endergonic ones so they can drive unfavourable processes forward.
ATP (adenosine triphosphate)
A nucleotide consisting of adenine, ribose, and three phosphate groups. The cell's primary energy-transfer molecule.
In simple terms, ATP is the energy currency that pays for almost every job in the cell.
ADP (adenosine diphosphate)
The product left when ATP loses one phosphate group. Lower in chemical energy and more stable than ATP.
Think of it as a spent battery waiting to be recharged.
AMP (adenosine monophosphate)
ATP minus two phosphate groups, with only one phosphate remaining.
Catabolism
Metabolic pathways that break large molecules into smaller units, releasing energy and producing ATP.
In simple terms, catabolism is the "breaking down" side of metabolism.
Anabolism
Metabolic pathways that build large molecules from smaller units, consuming ATP.
Think of it as the "building up" side of metabolism.
Phototroph
An organism that harvests energy from sunlight (e.g. plants, cyanobacteria).
Chemotroph
An organism that obtains energy from chemical compounds rather than light (e.g. animals, fungi).
Autotroph
An organism that fixes inorganic carbon (CO2) into organic molecules. Makes its own food.
Heterotroph
An organism that obtains organic carbon by consuming molecules made by other organisms.
Kinetic energy
Energy of motion. Light (photon movement), electricity (electron movement), and heat (molecular movement) are all forms.
Potential energy
Stored energy that depends on an object's structure or position relative to a field. Chemical bonds and electrochemical gradients are biological examples.
Chemical energy
A form of potential energy held in the covalent bonds between atoms.
Gibbs free energy (G)
The portion of a system's energy that is available to do work. Defined by the equation ΔG = ΔH − TΔS.
Think of it as the answer to "can this reaction actually power something?"
Enthalpy (H)
The total energy content of a system, including chemical bond energy.
Entropy (S)
A measure of disorder or freedom of movement in a system.
Exergonic reaction
A reaction with a negative ΔG. Energy is released and available to do work. Spontaneous under standard conditions.
Endergonic reaction
A reaction with a positive ΔG. Energy input is required. Non-spontaneous on its own.
Energetic coupling
Using the energy released by a spontaneous (exergonic) reaction to drive a non-spontaneous (endergonic) reaction, so the net ΔG is negative.
In simple terms, pairing a "downhill" reaction with an "uphill" one so the overall process still goes forward.
Hydrolysis
A reaction in which water splits a bond. ATP hydrolysis (ATP + H2O → ADP + Pi) is the cell's most common way of releasing usable energy.
Organisms are grouped along two axes: where they get energy and where they get carbon.
Energy source
Phototrophs capture energy from sunlight. Plants are the classic example, using photosynthesis to build glucose. Even so, plants still break that glucose back down (just as animals do) to power most of their cellular work.
Chemotrophs extract energy from chemical compounds. Animals eat other organisms and harvest the chemical energy stored in organic molecules. Trace the energy back far enough and it almost always originated as sunlight.
Carbon source
Autotrophs fix CO2 into organic molecules themselves.
Heterotrophs consume organic molecules made by other organisms. Animals get both their energy and their carbon from the same glucose molecule.
Combining the two axes gives four categories (photoautotroph, photoheterotroph, chemoautotroph, chemoheterotroph). For this course, the most important pairing is photoautotrophs (plants) and chemoheterotrophs (animals).
Metabolism is the full set of chemical reactions that sustain life. It splits into two halves.
Catabolism breaks large molecules into smaller units and produces ATP.
Anabolism builds large molecules from smaller units and requires ATP.
The two halves are linked: catabolic reactions supply the ATP that anabolic reactions spend.
Carbon and oxidation state
Carbon-containing molecules sit on a spectrum of oxidation:
CH4 (methane) is the most reduced form of carbon, holding the most chemical energy.
Glucose (C6H12O6) sits in the middle.
CO2 is the most oxidised form, holding the least chemical energy.
Catabolism moves carbon toward the oxidised end (releasing energy). Anabolism moves it toward the reduced end (storing energy).
Kinetic energy is the energy of motion.
Light: movement of photons
Electricity: movement of electrons
Thermal energy (heat): movement of molecules
Potential energy is stored energy that depends on structure or position.
A stretched spring or a boulder on a cliff has gravitational potential energy.
An electrochemical gradient of ions across a membrane is potential energy in a biological context.
Molecules move down their concentration and electrical gradients from higher to lower potential energy.
Energy transformation is any conversion from one form of energy to another. Cells constantly transform potential energy (chemical bonds) into kinetic energy (molecular motion, transport, muscle contraction) and vice versa.
Chemical energy is a form of potential energy held in bonds between atoms.
Covalent bonds involve the sharing of electrons. Forming a covalent bond releases energy; breaking one requires energy.
Strong bonds (e.g. in CO2 and H2O) are stable, hard to break, and contain relatively little chemical energy.
Weak bonds (e.g. C–C and C–H) are less stable, easier to break, and store a lot of chemical energy.
This is the key insight: molecules with many weak bonds (like glucose) are energy-rich. When those weak bonds are broken and replaced by stronger bonds (like the ones in CO2 and H2O), the difference in energy is released.
ATP structure
ATP is built from adenosine (the base adenine + ribose, a five-carbon sugar) plus three phosphate groups.
ADP has two phosphate groups; AMP has one.
The energy that matters sits in the bonds between the phosphate groups. At body pH, each phosphate carries a negative charge, so the three phosphates repel each other. Holding them together takes a large input of energy, which means those bonds are weak and energy-rich.
How ATP releases energy
When ATP is hydrolysed to ADP + Pi, the weak phosphate-phosphate bond breaks and more stable bonds form in the products. The difference is released as usable energy for the cell. ATP is not energy itself; it is a molecule whose structure makes energy readily accessible.
Thermodynamics tells you whether a reaction will release or require energy.
Gibbs free energy (G) is the amount of energy in a system available to do work.
The change in free energy for a reaction is:
ΔG = ΔH − TΔS
where Δ = products minus reactants, H = enthalpy (total energy), S = entropy (disorder / freedom of movement), and T = temperature in kelvin.
Exergonic reactions (ΔG < 0)
Energy is released and available to do work.
Enthalpy decreases (−ΔH): products hold less chemical energy than reactants.
Entropy increases (+ΔS): products have more freedom of movement.
These reactions are spontaneous, meaning they are thermodynamically favourable. "Spontaneous" does not mean fast.
Endergonic reactions (ΔG > 0)
Energy is required to drive the reaction forward.
Enthalpy increases (+ΔH): products hold more chemical energy.
Entropy decreases (−ΔS): products are more ordered.
These reactions are non-spontaneous and will not proceed without an energy input.
A reaction that is spontaneous in the forward direction (A → B) is non-spontaneous in reverse (B → A).
ATP hydrolysis
ATP + H2O → ADP + Pi
This reaction is both spontaneous and exergonic.
−ΔH: ADP has fewer phosphate groups, is more stable, and holds less chemical energy, so enthalpy decreases.
+ΔS: one molecule (ATP) becomes two (ADP + Pi), so entropy increases.
The energy released comes from breaking weaker bonds in ATP and forming more stable bonds in the products.
ATP has an intermediate ΔG, which makes it useful as a universal energy shuttle.
Reactions with a more negative ΔG than ATP hydrolysis can transfer a phosphate to ADP, regenerating ATP.
Reactions with a less negative ΔG can accept a phosphate from ATP, driven by the coupling.
This ATP-ADP cycle sits at the core of energetic coupling between catabolic and anabolic reactions.
Energetic coupling in practice
Cells routinely pair an exergonic reaction with an endergonic one so the net ΔG is negative.
Example 1: ATP hydrolysis drives the formation of glucose 6-phosphate. The phosphate group released from ATP is transferred to glucose, acting as the physical intermediate that links the two reactions.
Example 2: hydrolysis of phosphoenolpyruvate (PEP) has a more negative ΔG than ATP formation. PEP hydrolysis drives the otherwise endergonic synthesis of ATP from ADP, coupled through the transfer of a phosphate group.
In both cases, the phosphate group is the molecular link between the exergonic donor and the endergonic acceptor.
Gibbs free energy equation
ΔG = ΔH − TΔS
ΔG < 0: exergonic (spontaneous, energy released) ΔG > 0: endergonic (non-spontaneous, energy required)
ATP hydrolysis
ATP + H2O → ADP + Pi (ΔG is negative, exergonic)
Oxidation spectrum of carbon
CH4 (most reduced, most energy) → C6H12O6 (intermediate) → CO2 (most oxidised, least energy)
Energetic coupling is why your muscles can contract: ATP hydrolysis (exergonic) drives the shape change in motor proteins (endergonic) that pulls actin filaments past myosin.
The oxidation spectrum of carbon explains why fossil fuels release energy when burned. Methane, petrol, and coal are all highly reduced carbon compounds; combustion oxidises them to CO2 and H2O, releasing the stored chemical energy as heat.
Students often say "ATP is energy." It is not. ATP is a molecule that stores energy in its phosphate bonds and releases it on hydrolysis.
Students often think "spontaneous" means "fast." Spontaneous only means thermodynamically favourable (negative ΔG). A spontaneous reaction can be extremely slow without an enzyme or catalyst.
Students confuse breaking bonds with releasing energy. Breaking a bond always requires energy input. Energy is released when new, more stable bonds form in the products.
Students sometimes think plants do not perform cellular respiration. Plants both photosynthesise and respire. They build glucose with light energy and then break it down for ATP, just as animals do.
Be able to classify any organism along both axes (energy source and carbon source) and give an example of each combination.
Know the Gibbs free energy equation and what each variable represents. Expect to be given ΔH and ΔS values and asked whether the reaction is exergonic or endergonic.
Understand why ATP hydrolysis releases energy (weak bonds broken, stronger bonds formed, entropy increases). This concept recurs in every unit on cellular respiration.
Be ready to explain energetic coupling with a specific example, including the role of the phosphate group as an intermediate.
True or false: breaking a covalent bond releases energy. (False. Breaking bonds requires energy; forming bonds releases it.)
Fill in the blank: an organism that gets energy from sunlight and carbon from CO2 is called a ________. (Photoautotroph.)
True or false: a reaction with a positive ΔG is spontaneous. (False. Positive ΔG means the reaction is endergonic and non-spontaneous.)
Fill in the blank: in ATP hydrolysis, entropy ________ because one molecule becomes two. (Increases.)
True or false: plants perform cellular respiration. (True. Plants both photosynthesise and respire.)
Q: A reaction has a negative ΔH and a positive ΔS. Is it exergonic or endergonic? Explain.
A: Exergonic. Plugging into ΔG = ΔH − TΔS, a negative ΔH and a positive ΔS both push ΔG negative, so the reaction releases free energy and is spontaneous.
Q: Explain why ATP is described as having an "intermediate" ΔG of hydrolysis and why that matters.
A: ATP's ΔG of hydrolysis is neither the most nor the least negative among phosphorylated compounds. This means reactions with a more negative ΔG can regenerate ATP (by transferring a phosphate to ADP), while ATP hydrolysis can in turn drive reactions with a less negative ΔG. This intermediate position makes ATP a universal energy shuttle between catabolic and anabolic pathways.
Q: A student claims that glucose has more chemical energy than CO2 because glucose has stronger bonds. Identify and correct the error.
A: The reasoning is backwards. Glucose has more chemical energy because it has weaker bonds (C-C, C-H), not stronger ones. Weak bonds are less stable and store more chemical energy. When glucose is oxidised to CO2 and H2O, those weak bonds are replaced by stronger, more stable bonds, and the difference in energy is released.
Q: Give a specific example of energetic coupling and identify the intermediate that links the two reactions.
A: ATP hydrolysis (exergonic) is coupled to the phosphorylation of glucose to form glucose 6-phosphate (endergonic). The phosphate group is the intermediate: it is released from ATP and transferred to glucose, so the net ΔG of the coupled reaction is negative.
Q: How does a chemoheterotroph differ from a photoautotroph?
A: A chemoheterotroph obtains both energy and carbon from organic molecules made by other organisms (e.g. animals eating food). A photoautotroph captures energy from sunlight and fixes carbon from CO2 into organic molecules (e.g. plants using photosynthesis).
This material connects directly to cellular respiration (glycolysis, the citric acid cycle, oxidative phosphorylation), where you will trace exactly how glucose is catabolised to CO2 and H2O and how each step couples to ATP production.
It also connects to photosynthesis, where light energy drives the endergonic reduction of CO2 to glucose, the reverse of the oxidation path described here.
The enzyme unit builds on this foundation as well: enzymes lower the activation energy of reactions but do not change ΔG, so understanding what makes a reaction spontaneous is essential before you study how enzymes speed them up.
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