Difficulty: Introductory | Prerequisites: Chapter 5 (macromolecules), Chapter 7 (membrane structure and transport).
Metabolism is the sum of all chemical reactions in a cell, and it is governed by the laws of thermodynamics. This chapter introduces the two branches of metabolism (catabolism and anabolism), explains how ATP serves as the cell's energy currency, and covers enzymes in depth: how they lower activation energy, how they are regulated, and what happens when they are inhibited. You need a basic grasp of macromolecule structure and membrane transport to follow this material.
Cells break down molecules (catabolism) to release energy and build molecules (anabolism) using that energy. ATP is the go-between. Enzymes speed up reactions by lowering activation energy, and their activity is tightly regulated by gene expression, inhibitors, cofactors, and allosteric molecules.
Metabolism
The totality of an organism's chemical reactions, transforming matter and energy according to the laws of thermodynamics. Think of it as everything the cell does chemically to stay alive.
Catabolism
The breakdown of complex molecules (polymers) into simpler ones (monomers) via hydrolysis, releasing energy. Cellular respiration is a catabolic pathway. These reactions are exergonic.
Anabolism
The building up of complex molecules from simpler ones via dehydration synthesis, consuming energy. Photosynthesis is an anabolic pathway. These reactions are endergonic.
Exergonic reaction
A reaction that releases free energy (delta G < 0). In simple terms, energy comes out, and the reaction happens spontaneously in thermodynamic terms.
Endergonic reaction
A reaction that absorbs free energy (delta G > 0). The reaction requires an energy input to proceed.
Energy
The capacity to cause change. Exists in two main forms: kinetic (energy of motion, e.g. a waterfall) and potential (stored energy, e.g. a battery or chemical bonds).
ATP (adenosine triphosphate)
The cell's primary energy currency. Energy is released when the terminal phosphate bond is hydrolysed (ATP to ADP + Pi).
Energy coupling
Using the energy released by an exergonic reaction to drive an endergonic one. This is how cells manage energy flow, linking catabolism to anabolism.
Catalyst
A chemical agent that speeds up a reaction without being consumed. In biological systems, most catalysts are enzymes (proteins), though some RNA molecules (ribozymes) also catalyse reactions.
Enzyme
A biological catalyst, typically a protein, that lowers the activation energy of a specific reaction. Each enzyme has an active site shaped for its particular substrate (lock-and-key model).
Activation energy (Ea)
The minimum energy required to start a chemical reaction, representing the transition state where bonds are breaking and forming. Enzymes work by lowering this barrier.
Competitive inhibitor
A molecule that binds directly to the enzyme's active site, competing with the substrate for access. In simple terms, it blocks the keyhole.
Noncompetitive inhibitor
A molecule that binds to a site other than the active site, changing the enzyme's shape and making the active site less effective. It bends the lock rather than blocking it.
Allosteric regulation
Regulation of enzyme activity by a molecule binding at a site other than the active site, which can either stimulate or inhibit the enzyme's function.
Feedback inhibition
A regulatory mechanism where the end product of a metabolic pathway inhibits an enzyme earlier in the pathway, preventing overproduction.
Cofactor
A non-protein helper that assists enzyme function. Can be inorganic (metal ions) or organic (called coenzymes, e.g. NAD, which is an organic coenzyme; FAD, which is derived from B-vitamins).
Irreversible inhibitor
An inhibitor that attaches to the enzyme via covalent bonds, permanently disabling it.
Reversible inhibitor
An inhibitor held by weak bonds (hydrogen bonds, van der Waals interactions, disulfide bridges), which can detach, restoring enzyme function.
A metabolic pathway is a series of chemical reactions, each catalysed by a specific enzyme, that converts a starting molecule into a product through intermediate steps.
Catabolism breaks polymers into monomers via hydrolysis. This releases energy (exergonic, delta G < 0). Example: cellular respiration breaks glucose into CO2 and water.
Anabolism builds monomers into polymers via dehydration synthesis. This consumes energy (endergonic, delta G > 0). Example: photosynthesis builds glucose from CO2 and water.
Chemical work: synthesis and breakdown of polymers and monomers
Mechanical work: pumping ions across concentration gradients (also covers muscle contraction and cytoplasmic streaming)
Transport work: movement of chromosomes during DNA replication and cell division
The cell manages its energy budget by coupling exergonic reactions (which release energy) to endergonic ones (which need energy). ATP acts as the intermediary, shuttling energy from catabolic to anabolic pathways. Photosynthesis and cellular respiration are the classic coupled pair.
Phosphorylation is the transfer of a phosphate group. Three types matter here:
Photophosphorylation: transfer of a phosphate group using energy from sunlight (occurs in photosynthesis)
Substrate-level phosphorylation: direct transfer of a phosphate group from a high-energy substrate to ADP, forming ATP (occurs in glycolysis and the Krebs cycle)
Oxidative phosphorylation: ATP production driven by redox reactions in the electron transport chain, using the energy of electrons passed from NADH and FADH2 to oxygen
Enzymes are biological catalysts (most are proteins) that lower the activation energy (Ea) of a reaction, allowing it to proceed faster without being consumed. The lock-and-key model describes how each enzyme has an active site precisely shaped for its specific substrate, making each enzyme-substrate pairing unique.
Activation energy is the energy barrier a reaction must overcome to proceed (the transition state where bonds break and form). Enzymes lower this barrier, so reactions that would otherwise require high temperatures can occur at body temperature.
Metabolic pathways are regulated at three levels:
Gene regulation: genes encode enzymes, controlling which reactions a cell can perform by controlling which enzymes are present
Cellular regulation: the cell sends internal signals to adjust pathway activity (G x P = E, where G is gene, P is protein, E is enzyme)
Biochemical regulation (competitive and noncompetitive inhibition):
Competitive inhibitors bind the active site, competing directly with the substrate
Noncompetitive inhibitors bind elsewhere on the enzyme, distorting the active site and reducing catalytic efficiency
The end product of a pathway inhibits an earlier enzyme in that same pathway. This prevents the cell from overproducing a substance it already has enough of.
Environmental factors: temperature and pH both affect enzyme shape and activity. Each enzyme has an optimal temperature and pH; outside that range, it denatures.
Chemicals: various molecules can enhance or inhibit enzyme function.
Enzymes sometimes need non-protein helpers to function:
Inorganic cofactors: metal ions (e.g. zinc, iron)
Organic cofactors (coenzymes): organic molecules. NAD is an organic coenzyme; FAD is derived from B-vitamins. Both are critical electron carriers in metabolism.
Irreversible inhibitors attach to the enzyme via covalent bonds and permanently disable it. They bind to the enzyme, not the substrate.
Reversible inhibitors use weak bonds (hydrogen bonds, van der Waals forces, R-group disulfide bridges) and can detach, restoring enzyme function.
Allosteric regulation occurs when a regulatory molecule binds to a protein at a site other than the active site, causing a conformational change that either activates or inhibits the enzyme. This provides fine-tuned control over metabolic pathways.
Students often think any reaction that uses a protein is "active." Facilitated diffusion uses proteins but is passive. The defining feature of active processes is the requirement for ATP.
Enzymes do not provide energy for a reaction. They lower the activation energy barrier so the reaction proceeds faster.
Competitive and noncompetitive inhibitors are frequently confused. Remember: competitive goes to the active site (competes with substrate); noncompetitive goes elsewhere (changes the enzyme's shape).
Cofactors are not enzymes. They are helpers that enzymes need to function properly.
⚠️ The essay asks you to define metabolic pathways and explain the role of enzymes in detail. Be ready to write about catabolism vs anabolism, exergonic vs endergonic, and how enzymes lower Ea.
⚠️ Know all three types of phosphorylation and be able to give an example of each.
⚠️ Competitive vs noncompetitive inhibition is a favourite test topic. Be able to explain and draw both.
⚠️ NAD is an organic coenzyme; FAD is derived from B-vitamins. This specific fact is highlighted in the source material.
⚠️ Understand the difference between reversible and irreversible inhibitors, including the types of bonds involved.
True or False: Anabolism releases energy. (False, anabolism consumes energy; catabolism releases it.)
Fill in the blank: Enzymes lower the ______ of a reaction. (Activation energy / Ea)
True or False: A noncompetitive inhibitor binds to the enzyme's active site. (False, it binds to a different site.)
Fill in the blank: The three types of cellular work are chemical, mechanical, and ______. (Transport)
True or False: Feedback inhibition activates the first enzyme in a pathway. (False, it inhibits it.)
Q: Define metabolic pathway and explain the difference between catabolism and anabolism.
A: A metabolic pathway is a series of enzyme-catalysed chemical reactions that transform a starting molecule into a product through intermediate steps. Catabolism breaks down complex molecules into simpler ones (e.g. glucose into CO2 and water during cellular respiration), releasing energy (exergonic, delta G < 0). Anabolism builds complex molecules from simpler ones (e.g. CO2 and water into glucose during photosynthesis), consuming energy (endergonic, delta G > 0).
Q: How do enzymes lower activation energy?
A: Enzymes bind their substrate at a specifically shaped active site (lock-and-key model). This binding stabilises the transition state, reduces the energy barrier the reaction must overcome, and allows the reaction to proceed at a much lower temperature than it would otherwise require. The enzyme is not consumed and can be reused.
Q: Compare competitive and noncompetitive inhibition.
A: A competitive inhibitor binds directly to the enzyme's active site, physically blocking the substrate from entering. Increasing substrate concentration can overcome competitive inhibition. A noncompetitive inhibitor binds to a site other than the active site, causing the enzyme to change shape so the active site becomes less effective. Increasing substrate concentration does not overcome noncompetitive inhibition.
Q: What is allosteric regulation and why does it matter?
A: Allosteric regulation occurs when a regulatory molecule binds to a protein at a site other than the active site, changing the enzyme's shape and either activating or inhibiting its function. It matters because it allows the cell to fine-tune metabolic pathways in response to changing conditions, turning enzymes up or down as needed.
This chapter provides the foundation for Chapter 9 (Cellular Respiration) and Chapter 10 (Photosynthesis), which are the two major metabolic pathways. The enzyme regulation concepts here also apply to gene expression (Chapter 17), where transcription factors regulate which enzymes are produced.
Energy coupling between photosynthesis (anabolic) and cellular respiration (catabolic) is a recurring theme throughout the rest of the course.
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