Atlas MCAT Course
Biochemical Integration → Metabolism
Metabolism
How cells manage matter and energy.
01The idea
Cells are open systems. They must extract energy, build molecules, maintain order, and respond to changing conditions.
They do this through metabolism — a coordinated network of chemical reactions governed by thermodynamics, kinetics, and regulation.
THREAD · Regulation vs. capacity
If you track those three things, metabolism becomes predictable.
02Build the model
AThe two faces of metabolism
A.1Catabolism: breaking down to release energy
- Breaks large molecules into smaller ones
- Releases free energy
- Generates ATP and reduced electron carriers
Primary goals: capture usable energy; provide building blocks.
A.2Anabolism: building up using energy
- Synthesizes complex molecules
- Requires energy input
- Uses ATP and reducing power
Primary goals: growth, repair, storage.
BWhy metabolism is stepwise
Cells do not extract all energy at once. Instead, they break reactions into small steps, capture energy incrementally, and avoid uncontrolled energy release.
Why this matters: it prevents heat loss, enables regulation, and allows coupling to ATP synthesis.
CATP: the energy currency (conceptual)
ATP is not “stored energy” in isolation. It is a transfer molecule.
ATP hydrolysis is thermodynamically favorable and can be coupled to unfavorable reactions. ATP connects catabolism to work, and energy release to energy use.
DElectron carriers: energy in transit
Energy released during oxidation is first captured as NADH and FADH₂. These carriers store high-energy electrons and deliver them to later stages of metabolism.
This explains why glycolysis produces little ATP directly, and why the electron transport chain is essential.
EMetabolic pathways are regulated, not maximized
Cells do not run pathways at maximum speed. They regulate entry points, irreversible steps, and branch points.
Regulation ensures balance between supply and demand, prevention of waste, and rapid response to change.
FCommitment and control points
Certain steps are energetically irreversible and commit substrates to a pathway. These steps are tightly regulated and are common sites of allosteric control.
This logic repeats across all metabolic pathways.
GCompartmentalization: why location matters
Eukaryotic cells separate pathways by location (cytosol, mitochondria). This allows simultaneous but opposing processes, independent regulation, and increased efficiency.
HMetabolism as a network, not a line
Pathways intersect, share intermediates, and branch and converge. This creates metabolic flexibility and context-dependent behavior.
03See the connections
The lesson names these links itself. Read the map left to right.
Track three things
- Carbon
- Electrons
- Energy
Through
- Catabolism and anabolism, coupled
- ATP (transfer)
- NADH and FADH₂ (electrons in transit)
Controlled by
- Commitment steps
- Allosteric control
- Compartments
- A network of shared intermediates
Next: glycolysis.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
Why do cells break glucose oxidation into many small steps?
Think first.
Where is most ATP made, relative to where electrons are removed from fuel?
Think first.
Where does metabolic regulation concentrate?
Think first.
How are catabolism and anabolism related?
05MCAT lens
The MCAT emphasizes
Flow of energy and electrons
Regulatory logic
System-level reasoning
You are rarely asked to memorize every step or recall enzyme names in isolation. You are often asked to predict what happens when conditions change.
06Common traps
Trap 1: Metabolism is just memorization
Misconception
It follows rules.
Correct model
Track carbon, electrons, and energy.
Trap 2: ATP is made everywhere equally
Misconception
Most ATP comes later.
Correct model
Electron carriers mediate energy transfer.
Trap 3: Pathways run independently
Misconception
They are integrated.
Correct model
Regulation coordinates flow.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
Why metabolism is stepwise
Check against 02 · B: incremental capture, control, and coupling.
Why ATP and electron carriers are both required
Check against 02 · C and D: ATP transfers energy locally; carriers move electrons to where most ATP is made.
Why regulation exists
Check against 02 · E and F: to balance supply and demand at costly-to-reverse steps.
Why location matters
Check against 02 · G: compartments allow opposing processes and independent regulation.
If something feels unclear, the missing idea is usually thisMetabolism is controlled energy flow through enzymes.
08Remember
REMEMBER · if you keep only a few things
- Track carbon, electrons, and energy.
- Catabolism releases energy; anabolism spends it. They’re coupled.
- Stepwise pathways make energy capture controllable.
- ATP is a transfer molecule. NADH and FADH₂ carry electrons to where most ATP is made.
- Regulation tunes flux at committing, irreversible steps.
- Compartments and shared intermediates make metabolism a network.
09Forward hook
Now that the rules are established, the next question is:
How does the cell begin extracting energy from glucose without oxygen?
That leads directly to glycolysis.
Next lessonGlycolysis→