Energy comes in two broad forms: kinetic (motion) and potential (stored). The laws of thermodynamics set the rules for every energy transformation in biology. Cells stay alive by coupling exergonic reactions (which release free energy) to endergonic ones (which require it), with ATP as the universal energy currency. Enzymes speed up reactions by lowering activation energy, and their activity is tightly regulated by inhibitors, activators, and feedback loops.
Difficulty: Introductory | Prerequisites: Basic chemistry (bonds, polarity). Helpful to have covered membrane transport first.
Every process in a living cell, from pumping ions across a membrane to building a protein, requires energy. This topic lays the thermodynamic foundation for understanding how cells harvest, store, and spend energy. It also introduces enzymes, the proteins that make biological reactions fast enough to sustain life. Without a solid grasp of free energy, ATP coupling, and enzyme regulation, later topics like cellular respiration and photosynthesis will not make sense. If you are behind, start here: this is the physics and chemistry that the rest of the course runs on.
Potential energy
Stored energy due to the position or arrangement of matter. Chemical energy in covalent bonds is a form of potential energy.
Think of it as energy waiting to be released, like a stretched rubber band.
Kinetic energy
The energy of motion. Thermal (heat) energy is kinetic energy at the molecular scale.
Metabolism
The sum of all chemical reactions in a living cell, carried out through enzyme-driven pathways.
Catabolism
Metabolic pathways that break down molecules and release energy.
Anabolism
Metabolic pathways that use energy to build molecules the cell needs.
Enthalpy (H)
The total energy of a system.
Entropy (S)
A measure of disorder or randomness in a system. Systems tend toward increased entropy.
Gibbs free energy (G)
The amount of energy in a system available to do work. The change in free energy (delta G) determines whether a reaction is spontaneous.
Exergonic reaction
A reaction with a negative delta G: energy flows from the system to the environment. Exergonic reactions are spontaneous (they proceed without added energy, though not necessarily quickly).
Endergonic reaction
A reaction with a positive delta G: energy flows from the environment into the system. Endergonic reactions are nonspontaneous and require an energy input.
Activation energy
The energy barrier that must be overcome for a reaction to proceed. Enzymes lower this barrier.
Enzyme
A protein that increases the rate of a chemical reaction by lowering the activation energy, without being consumed in the process.
Substrate
The molecule that binds to an enzyme's active site to undergo a reaction.
Product
The molecule released from an enzyme after the reaction is complete.
Active site
The specific region of an enzyme where the substrate binds. The active site's shape is complementary to the substrate.
Allosteric site
A site on the enzyme other than the active site where regulatory molecules can bind, changing the enzyme's shape and activity.
Competitive inhibitor
A molecule that competes with the substrate for the active site, blocking the substrate from binding.
Noncompetitive inhibitor
A molecule that binds elsewhere on the enzyme and changes its shape so the active site no longer works properly, even if the substrate is bound.
Allosteric activator
A molecule that binds to an allosteric site and increases the enzyme's affinity for its substrate.
Allosteric inhibitor
A molecule that binds to an allosteric site and decreases the enzyme's affinity for its substrate.
Feedback inhibition
A regulatory mechanism in which the end product of a metabolic pathway inhibits an enzyme earlier in the pathway.
Cofactor
An inorganic or organic molecule that binds to an enzyme and helps it function.
Coenzyme
An organic cofactor (often a vitamin derivative) that binds at the active site and assists with the reaction.
Kinetic energy is the energy of motion (thermal/heat energy is molecular-scale kinetic energy)
Potential energy is stored energy due to position or arrangement (chemical energy in covalent bonds is a key biological form)
Every energy transformation releases some heat to the surroundings
First law (conservation of energy): the total energy of the universe is constant. Energy changes form but is never created or destroyed.
Second law (entropy always increases): every energy transformation increases the entropy (disorder, randomness) of the system and its surroundings.
Free energy (G) is the portion of a system's energy available to do work
A reaction with negative delta G (products have less free energy than reactants) is exergonic: spontaneous, energy-releasing
A reaction with positive delta G is endergonic: nonspontaneous, requiring an energy input
"Spontaneous" means thermodynamically favourable; it says nothing about speed
Cells do three main types of work: transport work (moving substances across membranes), chemical work (building molecules), and mechanical work (muscle contraction, cell division).
Cells couple exergonic reactions to endergonic ones so the overall delta G is negative and the combined process is spontaneous
ATP is the universal energy currency: ATP hydrolysis is exergonic, and the energy it releases drives endergonic processes
The ATP/ADP cycle continuously regenerates ATP from ADP + inorganic phosphate, using energy from catabolic reactions
Enzymes are proteins that catalyse reactions by lowering the activation energy
The substrate binds to the enzyme's active site, forming a substrate-enzyme complex
The enzyme facilitates the reaction and releases the product, emerging unchanged and ready for another cycle
Competitive inhibition: the inhibitor binds to the active site, physically blocking the substrate
Noncompetitive inhibition: the inhibitor binds to a different site, distorting the enzyme's shape so the active site no longer functions properly
Allosteric sites are regulatory sites on the enzyme, separate from the active site
Allosteric activators bind to an allosteric site and increase the enzyme's affinity for its substrate
Allosteric inhibitors bind to an allosteric site and decrease the enzyme's affinity for its substrate
Feedback inhibition is a common regulatory strategy: the end product of a pathway binds to and inhibits an enzyme early in the pathway, preventing overproduction
Some enzymes need a non-protein helper to function
Cofactors can be inorganic (e.g. metal ions) or organic
Coenzymes are organic cofactors, often derived from vitamins, that bind at the active site
Phosphofructokinase (PFK) is a critical regulatory enzyme in glycolysis, and it illustrates allosteric regulation clearly:
When ATP levels are high (energy is abundant), ATP binds to PFK's allosteric site and inhibits it, slowing glycolysis
When AMP levels are high (energy is low), AMP activates PFK, allowing glycolysis to proceed
This means the cell adjusts the rate of glucose breakdown in direct response to its energy needs
High AMP / low ATP: glycolysis speeds up
Low AMP / high ATP: glycolysis slows down
Students often think "spontaneous" means "fast." A spontaneous (exergonic) reaction is thermodynamically favourable, but it may still be extremely slow without an enzyme. Spontaneity says nothing about rate.
Students confuse enzymes with reactants. Enzymes are catalysts: they speed up reactions but are not consumed. They emerge unchanged and are reused.
Students sometimes think competitive and noncompetitive inhibitors work the same way. A competitive inhibitor blocks the active site directly; a noncompetitive inhibitor binds elsewhere and distorts the enzyme's shape.
Students assume equilibrium means the system is at its most useful. In thermodynamics, equilibrium means no net work can be done (delta G = 0). Living cells stay alive precisely by staying away from equilibrium.
⚠️ Be able to classify a reaction as exergonic or endergonic from the sign of delta G.
⚠️ Explain how energy coupling works and why ATP hydrolysis makes an otherwise endergonic process spontaneous.
⚠️ Know the difference between competitive and noncompetitive inhibition and be able to identify each from a description.
⚠️ Understand allosteric regulation, including feedback inhibition, and be able to apply it to the PFK/glycolysis example.
⚠️ State the first and second laws of thermodynamics and explain what entropy means for biological systems.
True or False: An exergonic reaction has a positive delta G. (False: exergonic reactions have a negative delta G.)
Fill in the blank: Enzymes speed up reactions by lowering the ___. (activation energy)
True or False: A competitive inhibitor binds to the allosteric site. (False: it binds to the active site. Allosteric inhibitors bind to the allosteric site.)
Fill in the blank: When ATP levels are high, PFK is ___ and glycolysis slows down. (inhibited)
True or False: The second law of thermodynamics states that energy is conserved. (False: that is the first law. The second law states that entropy increases with every energy transformation.)
Q: What is the difference between an exergonic and an endergonic reaction?
A: An exergonic reaction releases free energy (negative delta G) and is spontaneous. An endergonic reaction absorbs free energy (positive delta G) and is nonspontaneous, requiring an energy input to proceed.
Q: How does ATP coupling allow endergonic reactions to occur in cells?
A: The hydrolysis of ATP is exergonic and releases energy. When this reaction is coupled to an endergonic reaction, the overall delta G becomes negative, making the combined process spontaneous. The energy from ATP hydrolysis effectively pays for the unfavourable reaction.
Q: Explain the difference between a competitive inhibitor and a noncompetitive inhibitor.
A: A competitive inhibitor binds directly to the enzyme's active site, preventing the substrate from entering. A noncompetitive inhibitor binds to a different location on the enzyme, changing its shape so the active site can no longer catalyse the reaction effectively, even if the substrate is present.
Q: How does feedback inhibition regulate a metabolic pathway?
A: The end product of the pathway accumulates and binds to an allosteric site on an enzyme early in the pathway, inhibiting it. This reduces the rate of the entire pathway, preventing overproduction of the end product. When the product is used up, inhibition is relieved and the pathway resumes.
Q: Why does high AMP activate PFK while high ATP inhibits it?
A: High AMP indicates the cell's energy reserves are low, so glycolysis should proceed to generate more ATP. High ATP indicates energy is abundant, so glycolysis should slow down to avoid wasting resources. PFK is allosterically activated by AMP and allosterically inhibited by ATP, making it a sensor of the cell's energy status.
ATP hydrolysis and energy coupling appear everywhere in biology. Active transport across membranes (Doc 1) is powered by ATP. Signal transduction cascades (Doc 2) rely on kinases that transfer phosphate groups from ATP. Cellular respiration and photosynthesis, which you will study next, are extended applications of the same thermodynamic principles: exergonic catabolic pathways that harvest energy from glucose and store it in ATP.
Enzyme regulation, especially feedback inhibition and allosteric control, will reappear in every metabolic pathway you encounter for the rest of the course.
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