Biology for AP Courses · Metabolism
Energy and Metabolism
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In 30 seconds
Every living cell is a chemical factory running thousands of reactions at once. Metabolism All chemical reactions occurring in an organism Full entry → is the term for all of those reactions — every molecule built, broken down, and rearranged to keep a cell alive. The reactions fall into two broad camps: Catabolism Reactions that break larger molecules into smaller ones Full entry →, which breaks larger molecules into smaller ones and typically releases Energy The capacity to do work or cause change Full entry →, and Anabolism Reactions that build larger molecules from smaller ones Full entry →, which builds larger molecules from smaller ones and typically requires energy. Cells organize these reactions into metabolic pathways — sequences in which the product of one reaction becomes the reactant of the next.
The thread running through all of metabolism is energy, the capacity to do work. A cell cannot build proteins, pump ions, or move without it. This topic sets up the vocabulary and framework; later topics in this chapter fill in the physics (free energy and thermodynamics), the currency (ATP Adenosine triphosphate, the cell's main energy-carrying molecule Full entry →), and the machines that control the pace (enzymes).
Why this matters
- Metabolism is life. Growth, repair, movement, reproduction — every life process is a metabolic process. When metabolism stops, the cell dies.
- Health and disease are metabolic stories. Diabetes is a failure of glucose handling; cancer cells rewire their metabolism to grow faster; metabolic syndrome links nutrition, energy balance, and disease risk.
- Food is fuel. Understanding catabolism explains why we eat, how the body extracts energy from food, and why excess intake is stored.
- Exam logic: AP questions ask you to classify reactions as catabolic or anabolic, predict whether a pathway releases or requires energy, and explain Energy coupling Using an exergonic reaction to drive an endergonic one Full entry →.
The college version
Core Concepts
Metabolism: the sum of a cell's chemical reactions
Metabolism (from the Greek metabolē, "change") includes every chemical reaction in an organism — organized, regulated, and often connected into sequences. Think of it as the cell's total chemical economy: some reactions generate products and energy, others consume them, and regulation keeps the whole system balanced.
Catabolism and anabolism: the two halves
Catabolic reactions break down complex molecules into simpler ones. Digestion of starch into glucose, the breakdown of glucose to carbon dioxide and water during cellular respiration, and the dismantling of proteins into amino acids are all catabolic. These reactions are generally exergonic — they release energy that the cell can capture.
Anabolic reactions do the opposite: they assemble complex molecules from simpler ones. Protein synthesis from amino acids, glycogen formation from glucose, and DNA replication are anabolic. These reactions are generally endergonic — they require an input of energy.
The two halves are inseparable: anabolism needs the energy and building blocks that catabolism supplies. A living cell is a cycle of building and breaking, not a one-way street.
Metabolic pathways: reactions in sequence
A Metabolic pathway A sequence of enzyme-catalyzed reactions, each step feeding the next Full entry → is a series of enzyme-catalyzed reactions, each with its own Intermediate A molecule produced in one pathway step and consumed in the next Full entry → product. Cellular respiration, for example, is a network of pathways — glycolysis, the citric acid cycle, and oxidative phosphorylation — each composed of many individual steps.
Three features of pathways are worth remembering:
- Each step is catalyzed by a specific enzyme, so a pathway can be controlled enzyme by enzyme.
- Pathways are regulated, often by feedback inhibition: the final product slows down an early enzyme, preventing overproduction (Topic 5).
- Pathways branch and interconnect. Glycolysis feeds into respiration but also supplies intermediates used to build amino acids and fats.
Energy: the capacity to do work
Energy exists in many forms — light, heat, electrical, mechanical, and chemical — and cells constantly convert one form into another. Photosynthetic cells convert light energy into chemical energy stored in glucose; muscle cells convert chemical energy into mechanical work. The rules governing these conversions are the laws of thermodynamics (Topic 3), and the amount of energy available to do work is called free energy (Topic 2).
In cells, chemical energy is stored in the bonds of molecules — carbohydrates, fats, ATP. Breaking those bonds in controlled, stepwise reactions (catabolism) releases energy gradually — one reason cells break glucose down through a dozen small steps instead of setting it on fire.
Energy coupling: spending to build
Cells frequently need to run endergonic reactions (ΔG > 0, requiring energy) that will not happen on their own. They do this by energy coupling: pairing the Endergonic reaction A reaction that requires an energy input (positive free-energy change) Full entry → with an exergonic one (ΔG < 0) whose released energy drives it. The most common coupler is ATP (Topic 4): cells spend ATP — hydrolysis is exergonic — to power anabolic reactions such as protein synthesis.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Catabolism | Anabolism | Catabolism breaks molecules down (releases energy); anabolism builds them up (consumes energy) |
| "Metabolism is just digestion" | Metabolism includes every cellular reaction | Digestion is one catabolic process; building proteins, replicating DNA, and pumping ions are also metabolism |
| Anabolic reactions only happen in plants | Anabolism happens in all organisms | All cells synthesize proteins, nucleic acids, and other molecules; plants simply do more of it (photosynthesis) |
| Breaking bonds always releases energy | Bond breaking requires energy input | It is the overall reaction (bonds broken and new bonds formed) that determines whether energy is released |
| A pathway is just a list of molecules | A pathway is a controlled, regulated process | Order, regulation, and interconnection matter — not just the ingredients |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Your body is like a Lego factory. Some workers take big Lego buildings apart into small bricks — that's catabolism, and it gives off energy, like releasing a spring. Other workers use the bricks to build new, bigger buildings — that's anabolism, and it needs energy. The factory never stops: it is always taking apart some buildings to get bricks and energy for building others.
Worked example
A runner eats a banana. In the small intestine, enzymes break the starch down into glucose — a catabolic step that begins releasing chemical energy. Glucose enters cells, where glycolysis and cellular respiration dismantle it step by step. At several steps, the released energy is captured by coupling reactions to ATP synthesis, so the runner's cells now hold energy in ATP.
Now the runner lifts weights. Muscle cells hydrolyze ATP to power the sliding filaments that shorten the muscle — mechanical work. Between sets, the cells use ATP (via energy coupling) to rebuild glycogen and repair muscle proteins — anabolic work. Every piece of this story — release, capture, spend, rebuild — is metabolism, and the coupling molecule at the center of it is ATP.
Key takeaways
- Metabolism = all chemical reactions in an organism; it has two halves: catabolism (breaking down, releases energy) and anabolism (building up, requires energy).
- Catabolism powers anabolism: energy and building blocks released by breakdown drive the synthesis reactions.
- Metabolic pathways are sequences of enzyme-catalyzed steps with intermediate products; they are regulated (often by feedback inhibition) and they interconnect.
- Energy is the capacity to do work; cells convert energy between forms (light → chemical → mechanical) under the laws of thermodynamics.
- Energy coupling joins an exergonic reaction (like ATP hydrolysis) to an endergonic one so the overall process is spontaneous.
- Terms to know cold: catabolism, anabolism, pathway, intermediate, exergonic, endergonic, energy coupling, ATP.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Define metabolism and name its two halves.
Show answer
Metabolism is the total of all chemical reactions in an organism. Its two halves are catabolism (breaking larger molecules into smaller ones, releasing energy) and anabolism (building larger molecules from smaller ones, requiring energy).
Is cellular respiration catabolic or anabolic? Explain your reasoning.
Show answer
Catabolic — it breaks glucose into carbon dioxide and water and releases energy the cell captures as ATP.
Why can't an endergonic reaction run on its own, and how does a cell make it run anyway?
Show answer
An endergonic reaction has a positive free-energy change; it needs energy input. The cell couples it to an exergonic reaction (typically ATP hydrolysis), making the overall coupled process exergonic so it can proceed.
What is a metabolic pathway, and why does organizing reactions into pathways help a cell control its chemistry?
Show answer
A pathway is a sequence of enzyme-catalyzed reactions in which each product feeds the next step. Pathways are regulated (e.g., feedback inhibition) at specific enzymes and branch to connect with other pathways.
Give one example of energy coupling in cell biology.
Show answer
ATP hydrolysis driving protein synthesis, muscle contraction, or active transport — an exergonic reaction powers an endergonic one.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Metabolism
- All chemical reactions occurring in an organism
- Catabolism
- Reactions that break larger molecules into smaller ones
- Anabolism
- Reactions that build larger molecules from smaller ones
- Metabolic pathway
- A sequence of enzyme-catalyzed reactions, each step feeding the next
- Intermediate
- A molecule produced in one pathway step and consumed in the next
- Energy
- The capacity to do work or cause change
- Exergonic reaction
- A reaction that releases energy (negative free-energy change)
- Endergonic reaction
- A reaction that requires an energy input (positive free-energy change)
- Energy coupling
- Using an exergonic reaction to drive an endergonic one
- ATP
- Adenosine triphosphate, the cell's main energy-carrying molecule
Sources & references
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