Source: Introduction to Biology I, University of Washington
Difficulty: Introductory | Prerequisites: Basic chemistry (atoms, molecules, chemical bonds)
Metabolism is one of the foundational concepts in introductory biology. It sits at the intersection of chemistry and cell biology, and everything from genetics to ecology rests on it in some way. Before tackling this material, you should be comfortable with basic chemical bonds, the structure of molecules like glucose, and the general idea that energy is neither created nor destroyed. If you have covered the cell structure unit, you will recognise where these reactions physically take place.
Metabolism is the collection of chemical reactions that keep cells alive: breaking down nutrients for energy (catabolism) and building new molecules for growth and repair (anabolism). Aerobic metabolism uses oxygen and is far more efficient; anaerobic metabolism works without oxygen but yields less energy and produces waste products like lactic acid or ethanol. Enzymes speed up all of these reactions, and ATP is the energy currency that cells spend to do work.
Metabolism
The set of chemical reactions that occur within an organism to sustain life, including the breakdown of nutrients for energy and the synthesis of new molecules.
In simple terms, this means every chemical process your cells run to keep you alive.
Catabolism
The metabolic pathways that break down complex molecules into simpler ones, releasing energy in the process.
Think of it as the "demolition" side of metabolism.
Anabolism
The metabolic pathways that build complex molecules from simpler ones, consuming energy.
Think of it as the "construction" side of metabolism.
Aerobic metabolism
Metabolism that requires oxygen. It is the most efficient route for cells to extract energy from glucose, via cellular respiration.
In simple terms, this is what your cells do when oxygen is available, which is most of the time.
Anaerobic metabolism
Metabolism that occurs without oxygen, typically through fermentation. Less efficient than aerobic metabolism.
Think of it as the backup generator your cells switch to when oxygen runs low.
Cellular respiration
A series of metabolic reactions that break down glucose in the presence of oxygen to produce ATP, carbon dioxide, and water.
In simple terms, this is how your cells turn food into usable energy.
Fermentation
An anaerobic process that breaks down glucose without oxygen, producing small amounts of ATP plus waste products (lactic acid or ethanol, depending on the organism).
Think of it as the shortcut your muscles use during a sprint, or what yeast uses to make bread rise and beer brew.
ATP (adenosine triphosphate)
A molecule that stores and releases energy for cellular work. Often called the "energy currency" of the cell.
In simple terms, ATP is like a rechargeable battery that powers almost everything a cell does.
Enzyme
A protein that catalyses (speeds up) a specific chemical reaction without being consumed in the process.
Think of it as a biological machine that makes one particular reaction happen faster.
Active site
The specific region on an enzyme where the substrate binds and the reaction takes place.
In simple terms, this is the enzyme's "docking station" shaped to fit only the right molecule.
Substrate
The molecule that an enzyme acts upon. The substrate binds to the enzyme's active site.
Think of it as the raw material that the enzyme processes.
Metabolism encompasses all chemical reactions in a living organism
Two broad categories:
Catabolism breaks complex molecules down, releasing energy
Anabolism builds complex molecules up, consuming energy
Both categories run simultaneously in every living cell
Occurs in the presence of oxygen
Most efficient energy-extraction pathway
Cellular respiration breaks glucose (C₆H₁₂O₆) down step by step:
Glycolysis (in the cytoplasm): glucose splits into two pyruvate molecules, yielding a small amount of ATP
The Krebs cycle (in the mitochondrial matrix): pyruvate is further oxidised, releasing CO₂ and transferring energy to electron carriers
The electron transport chain (inner mitochondrial membrane): the bulk of ATP is produced here, using oxygen as the final electron acceptor
Net yield: approximately 36 to 38 ATP per glucose molecule
Byproducts: carbon dioxide and water
Occurs without oxygen
Much less efficient than aerobic metabolism
Two common types:
Lactic acid fermentation: pyruvate is converted to lactic acid. Happens in human muscle cells during intense exercise when oxygen supply cannot keep up
Alcoholic fermentation: pyruvate is converted to ethanol and CO₂. Used by yeast and some bacteria
Net yield: only about 2 ATP per glucose molecule
Waste products (lactic acid or ethanol) still contain a large amount of chemical energy that the cell cannot access without oxygen
ATP consists of adenine, ribose, and three phosphate groups
Energy is stored in the bonds between the phosphate groups
When a cell needs energy, it breaks the terminal phosphate bond, converting ATP to ADP (adenosine diphosphate) and releasing energy
ATP is constantly recycled: cells re-attach a phosphate group to ADP during cellular respiration to regenerate ATP
Used for muscle contraction, active transport across membranes, biosynthesis, and signalling
Enzymes are biological catalysts, almost always proteins
They lower the activation energy of a reaction, making it proceed faster without being used up themselves
Without enzymes, most metabolic reactions would be too slow to sustain life
Each enzyme has a uniquely shaped active site
The substrate (reactant) fits into the active site, much like a key into a lock (the "lock-and-key" model)
A more nuanced view is the "induced-fit" model, where the active site adjusts its shape slightly when the substrate binds
This specificity means each enzyme typically catalyses only one reaction or a small family of closely related reactions
Temperature: enzyme activity increases with temperature up to an optimum, then drops sharply as the protein denatures (unfolds)
pH: each enzyme has an optimal pH range. Moving outside it distorts the active site and reduces activity
Substrate concentration: activity rises with more substrate until all active sites are occupied (saturation)
Inhibitors: molecules that reduce enzyme activity. Competitive inhibitors block the active site; non-competitive inhibitors bind elsewhere and change the enzyme's shape
They catalyse every step of glycolysis, the Krebs cycle, and the electron transport chain
They are involved in the synthesis of new molecules (anabolism) as well as the breakdown of nutrients (catabolism)
They also metabolise drugs and foreign substances in the body, which is why drug interactions and dosing matter clinically
Summary equation for aerobic cellular respiration:
C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP (approx. 36--38)}Summary equation for alcoholic fermentation:
C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + \text{2 ATP}Summary equation for lactic acid fermentation:
C_6H_{12}O_6 \rightarrow 2C_3H_6O_3 + \text{2 ATP}ATP hydrolysis (energy release):
ATP \rightarrow ADP + P_i + \text{energy}Fermentation is the basis of brewing, winemaking, and bread baking. Yeast carries out alcoholic fermentation, producing the ethanol in beer and wine and the CO₂ that makes bread dough rise.
Lactic acid fermentation is why your muscles burn during a hard sprint. When oxygen delivery cannot match demand, muscle cells switch to anaerobic metabolism and lactic acid accumulates.
Enzyme specificity is the principle behind many medical diagnostic tests. For example, elevated levels of certain enzymes in the blood (such as troponin after a heart attack) indicate damage to specific tissues.
Students often think anaerobic metabolism produces no ATP at all. It does produce ATP, just far less than aerobic metabolism (2 versus roughly 36 to 38 per glucose).
Students often confuse "fermentation" with "cellular respiration." Fermentation is only the anaerobic pathway. Cellular respiration specifically refers to the aerobic process.
Students often believe enzymes are "used up" in reactions. They are not. An enzyme emerges unchanged and can catalyse the same reaction again.
Students often think that raising temperature always increases enzyme activity. Beyond the optimum temperature, the enzyme denatures and activity drops to zero.
⚠️ Be able to write and balance the summary equation for cellular respiration. This appears on nearly every introductory biology exam.
⚠️ Know the difference between aerobic and anaerobic metabolism, including the relative ATP yields and the waste products of each.
⚠️ Understand enzyme specificity: the lock-and-key model, the role of the active site, and how temperature and pH affect enzyme activity.
⚠️ Be ready to explain what happens to ATP when energy is needed (hydrolysis to ADP + Pᵢ) and how ATP is regenerated.
True or false: Anaerobic metabolism produces more ATP per glucose molecule than aerobic metabolism.
Fill in the blank: The molecule that stores and releases energy for cellular work is called ___.
True or false: Enzymes are consumed during the reactions they catalyse.
Fill in the blank: The region of an enzyme where the substrate binds is called the ___.
True or false: Lactic acid fermentation occurs in human muscle cells during intense exercise.
Answers: 1. False. 2. ATP. 3. False. 4. Active site. 5. True.
Q: What is the overall equation for aerobic cellular respiration?
A: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + approximately 36 to 38 ATP.
Q: Compare the ATP yield of aerobic and anaerobic metabolism. Why is there such a large difference?
A: Aerobic metabolism yields roughly 36 to 38 ATP per glucose; anaerobic metabolism yields only 2. The difference exists because fermentation does not fully oxidise glucose, so much of the chemical energy remains trapped in the waste products (lactic acid or ethanol).
Q: Explain what would happen to metabolic reactions in a cell if all enzymes were suddenly removed.
A: The reactions would slow to a rate far too low to sustain life. Enzymes lower the activation energy of reactions, so without them, essential processes like nutrient breakdown and molecule synthesis would essentially stop.
Q: A student heats an enzyme solution to 80 °C and finds the reaction rate has dropped to nearly zero. Explain why.
A: At 80 °C, most enzymes denature: the heat disrupts the bonds that maintain the protein's three-dimensional shape, distorting the active site so the substrate can no longer bind effectively.
Q: Name two types of fermentation and state a waste product of each.
A: Lactic acid fermentation produces lactic acid. Alcoholic fermentation produces ethanol (and CO₂).
This material connects directly to photosynthesis, which is essentially the reverse of cellular respiration: plants capture light energy and store it in glucose, which is then broken down via the pathways described here. Understanding metabolism also underpins later units on cell division (cells need ATP to replicate DNA and divide) and ecology (energy flow through ecosystems depends on metabolic processes at every trophic level).
Metabolism, catabolism, anabolism, aerobic respiration, anaerobic respiration, cellular respiration, glycolysis, Krebs cycle, citric acid cycle, TCA cycle, electron transport chain, oxidative phosphorylation, ATP, adenosine triphosphate, ADP, fermentation, lactic acid fermentation, alcoholic fermentation, enzyme, catalyst, active site, substrate, lock and key model, induced fit model, denaturation, activation energy, energy currency of the cell, powerhouse of the cell, mitochondria, BIOL 180, intro biology energy, UW biology