Difficulty: Introductory | Prerequisites: Basic chemistry (energy, bonds)
Metabolism is the sum of all the chemical reactions in a living organism. This topic covers the rules energy follows (thermodynamics), the two broad categories of metabolic pathways (catabolic and anabolic), the three types of work a cell performs, and how enzymes speed up and regulate those reactions. You need this before tackling cellular respiration or photosynthesis, because those are specific metabolic pathways built on the principles here.
Cells do three types of work (chemical, mechanical, transport) powered by ATP. Energy obeys two laws: it cannot be created or destroyed, and every transfer increases disorder (entropy). Catabolic pathways break molecules down and release energy; anabolic pathways build molecules up and consume energy. Enzymes speed reactions by lowering activation energy, and their activity is regulated by inhibitors (competitive and noncompetitive) and allosteric regulators.
Metabolism
The totality of an organism's chemical reactions. In simple terms, everything your cells do to convert food into energy and raw materials.
Catabolism
Metabolic pathways that break down complex molecules into simpler ones, releasing energy (exergonic). Cellular respiration is the main catabolic pathway. Think of it as demolition that recovers usable materials.
Anabolism
Metabolic pathways that build complex molecules from simpler ones, consuming energy (endergonic). Protein synthesis and dehydration reactions are examples. Think of it as construction that requires an energy investment.
Exergonic reaction
A reaction that releases free energy (negative delta G). Products have less energy than reactants. In simple terms, the reaction gives off energy.
Endergonic reaction
A reaction that absorbs free energy (positive delta G). Products have more energy than reactants. In simple terms, the reaction needs an energy input to proceed.
First Law of Thermodynamics
Energy can be transferred and transformed, but it cannot be created or destroyed. Also called the Principle of Conservation of Energy.
Second Law of Thermodynamics
Every energy transfer or transformation increases the entropy (disorder) of the universe. In simple terms, some usable energy is always lost as heat, and systems naturally move towards disorder.
Activation energy (Ea)
The minimum amount of energy required to start a chemical reaction, to reach the transition state. In simple terms, the energy "hump" reactants must get over before a reaction can proceed.
Enzyme
A biological catalyst, usually a protein, that speeds up a chemical reaction by lowering its activation energy. The enzyme is not consumed or permanently altered by the reaction.
Substrate
The molecule(s) an enzyme acts upon. The substrate binds to the enzyme's active site.
Active site
The specific region of an enzyme where the substrate binds and the catalytic reaction occurs.
Competitive inhibition
A type of enzyme inhibition where a molecule resembling the substrate competes for binding at the active site. Can be overcome by increasing substrate concentration.
Noncompetitive inhibition
A type of enzyme inhibition where the inhibitor binds to a site other than the active site (an allosteric site), causing a conformational change that distorts the active site and reduces catalytic activity.
Allosteric regulation
Regulation of an enzyme by the binding of a molecule at a site other than the active site (the allosteric site). This can either inhibit the enzyme (stabilising the inactive form) or stimulate it (stabilising the active form).
ATP (adenosine triphosphate)
The cell's primary energy currency. Energy is released when a phosphate group is removed (hydrolysis), converting ATP to ADP.
Chemical work
The synthesis of polymers from monomers (e.g. dehydration synthesis to build proteins). One of the three types of cellular work.
Mechanical work
Movement within the cell or of the cell itself, such as chromosome movement during cell division or muscle contraction.
Transport work
Pumping substances across membranes against their concentration gradient, e.g. the sodium-potassium pump.
Cells use ATP to perform three categories of work:
Chemical work: building polymers from monomers (e.g. linking amino acids into proteins via dehydration synthesis)
Mechanical work: physical movement (e.g. chromosomes being pulled apart during mitosis, muscle fibres contracting)
Transport work: moving substances across membranes against their gradient (e.g. the sodium-potassium pump)
First Law (Conservation of Energy): Energy is neither created nor destroyed, only transformed. When glucose is broken down in cellular respiration, the chemical energy in its bonds is converted into ATP, heat and other forms, but the total energy is conserved.
Second Law (Entropy): Every energy conversion increases the overall entropy of the universe. Biological systems maintain their own order (low entropy) only by increasing disorder in their surroundings, primarily by releasing heat. This is why metabolic processes are never 100% efficient.
Catabolic pathways break large molecules into smaller ones and release energy (exergonic). Example: cellular respiration breaks glucose into CO2 and water, yielding ATP.
Anabolic pathways build large molecules from smaller ones and require energy (endergonic). Example: dehydration synthesis joins amino acids into polypeptide chains.
The two are coupled: the energy released by catabolic reactions (stored temporarily in ATP) drives anabolic reactions.
Enzymes lower the activation energy (Ea) of a reaction, allowing it to proceed faster at body temperature. They do not change the direction of a reaction or the amount of free energy released; they only reduce the energy barrier.
The substrate binds to the enzyme's active site. The enzyme-substrate complex forms, the reaction occurs, and the products are released. The enzyme is recycled.
Competitive inhibition: the inhibitor molecule resembles the substrate and binds directly to the active site, blocking the substrate. This can be overcome by flooding the system with more substrate.
Noncompetitive inhibition: the inhibitor binds to an allosteric site (not the active site), causing a conformational change in the enzyme that distorts the active site. The substrate can no longer bind effectively. Increasing substrate concentration does not overcome this.
Allosteric regulation is how cells fine-tune enzyme activity:
Allosteric inhibition: a regulatory molecule binds to the allosteric site and stabilises the inactive conformation of the enzyme, reducing or stopping catalytic activity.
Allosteric activation (stimulation): a regulatory molecule binds to the allosteric site and stabilises the active conformation, enhancing catalytic activity.
This mechanism allows metabolic pathways to respond to the cell's needs. For example, feedback inhibition occurs when the end product of a pathway acts as an allosteric inhibitor of an enzyme early in that pathway, preventing overproduction.
Many drugs work as enzyme inhibitors. Aspirin irreversibly inhibits cyclooxygenase (COX), reducing inflammation. Statins competitively inhibit HMG-CoA reductase, lowering cholesterol production. Understanding enzyme kinetics is central to pharmacology.
Students often think enzymes add energy to a reaction. They do not. They lower the activation energy barrier, making it easier for the reaction to start, but the overall energy change of the reaction stays the same.
Students sometimes confuse competitive and noncompetitive inhibition. The key: competitive inhibitors bind the active site (can be overcome by more substrate); noncompetitive inhibitors bind elsewhere and change the enzyme's shape (more substrate will not help).
Students often think the Second Law of Thermodynamics means living organisms violate it by being highly ordered. They do not. Organisms increase entropy in their surroundings (releasing heat) by more than enough to compensate.
Students sometimes think catabolism is "bad" and anabolism is "good." Both are essential. Catabolism provides the energy that anabolism requires.
Expect a question on which metabolic process breaks down polymers into monomers with energy release. The answer is catabolism.
The mechanism of enzyme catalysis (lowering activation energy) is a staple multiple-choice question.
Know the difference between competitive and noncompetitive inhibition cold. Noncompetitive inhibition involves binding at an allosteric site and a shape change.
Be ready to define allosteric regulation and explain how it can both inhibit and stimulate enzyme activity.
The three types of cellular work (chemical, mechanical, transport) with an example for each is a classic short-answer question.
True or False: Anabolism breaks down complex molecules into simpler ones. (False, that is catabolism.)
Fill in the blank: Enzymes speed up reactions by lowering the ________ energy. (Activation.)
True or False: Noncompetitive inhibitors bind to the active site. (False, they bind to an allosteric site.)
Fill in the blank: The First Law of Thermodynamics states that energy cannot be ________ or ________. (Created, destroyed.)
True or False: Allosteric regulation can only inhibit enzyme activity. (False, it can also stimulate it.)
Q: Describe the three main types of work performed by a cell and provide an example for one of them.
A: The three types are chemical work (synthesis of polymers, e.g. dehydration reactions building proteins), mechanical work (physical movement, e.g. chromosome movement during cell division) and transport work (pumping substances against their gradient, e.g. the sodium-potassium pump moving ions across the membrane).
Q: What are the two laws of thermodynamics as they apply to biological systems?
A: The First Law states that energy can be transferred or transformed but never created or destroyed. The Second Law states that every energy transfer increases the entropy (disorder) of the universe. In biology, this means metabolic processes always produce some heat, and organisms maintain internal order only by increasing disorder in their surroundings.
Q: How do enzymes catalyse chemical reactions in a metabolic pathway?
A: Enzymes lower the activation energy (Ea) required to reach the transition state. This allows the reaction to proceed much faster at normal cellular temperatures. The enzyme binds its substrate at the active site, facilitates the reaction and releases the products unchanged.
Q: Define allosteric regulation and explain how it can either inhibit or stimulate enzyme activity.
A: Allosteric regulation occurs when a regulatory molecule binds to a site other than the active site (the allosteric site). If the molecule stabilises the inactive conformation, the enzyme is inhibited. If it stabilises the active conformation, the enzyme is stimulated and catalytic activity increases.
This material underpins cellular respiration (a specific catabolic pathway producing ATP, covered in the next set of notes). Enzyme regulation connects to genetics and gene expression, because cells also control enzyme levels through transcription. The three types of cellular work reappear throughout the course whenever ATP usage is discussed.
metabolism, catabolism, anabolism, exergonic, endergonic, free energy, delta G, activation energy, enzyme, catalyst, substrate, active site, enzyme-substrate complex, competitive inhibition, noncompetitive inhibition, allosteric regulation, allosteric site, feedback inhibition, first law of thermodynamics, second law of thermodynamics, entropy, conservation of energy, ATP, ADP, phosphorylation, chemical work, mechanical work, transport work, dehydration synthesis, hydrolysis, BIOL 110, intro to biology, Penn State