AP Biology Unit 3: Cellular Energetics Notes

Introduction

Cells couple energy-releasing and energy-requiring processes through controlled pathways, catalysts, electron carriers, gradients, and ATP.

Energy is transferred and transformed; it is never created by an enzyme, chloroplast, or mitochondrion. Enzymes lower activation barriers, pathways move matter through regulated steps, and membranes convert redox energy into proton gradients that can power ATP synthesis.

Keep a compartment ledger beside the energy story. In photosynthesis, water supplies electrons and oxygen while carbon dioxide supplies carbon. In respiration, carbon fuel is oxidized, electron carriers feed a membrane chain, and oxygen accepts electrons in aerobic systems.

Enzyme structure and environment

reaction rate

Exergonic step plus coupling

cellular work

Electron transfer

proton gradient

Proton return through ATP synthase

ATP production

Current exam guidance: Aligned to the current 2026–27 AP Biology course framework, including scientific practices, data analysis, and model-based reasoning.

3.1

Enzymes

How does an enzyme increase reaction rate without changing the reaction’s equilibrium?

Short answer

Reversible substrate binding positions reacting groups and stabilizes a lower-activation-energy pathway, accelerating approach to equilibrium without changing reactant/product energy difference.

Know firstProtein folding · Molecular collisions · Energy profiles

A catalyst changes the path, not the destination

A substrate encounters an enzyme’s active site through molecular motion. Compatible shape, charge, and chemical interactions favor a temporary enzyme–substrate complex. Binding can orient reacting groups, strain particular bonds, or stabilize a transition state. Products then leave, allowing the enzyme to participate again. The enzyme is not consumed in the overall reaction, although it can lose function under damaging conditions.

The catalyzed route has a lower activation-energy barrier. Reactant and product free-energy levels remain the same, so the enzyme does not make an unfavorable reaction favorable and does not shift equilibrium. It accelerates both directions available to the system and shortens the time required to approach equilibrium.

At fixed enzyme concentration, adding substrate usually increases initial rate until available active sites are occupied much of the time. The rate then approaches a maximum. An active-site diagram is a model: proteins fluctuate, binding is reversible, and a similar outline alone does not demonstrate molecular compatibility.

Teaching figure

Catalysis changes the route, not the endpoints

Both routes begin and end at the same free-energy levels. The enzyme provides a pathway with a lower activation barrier.

Use this visual to see: Separate activation energy, which affects rate, from net free-energy change, which describes the endpoints.
What it cannot show: The profile compresses a multidimensional reaction into one coordinate and does not show individual intermediates.

Test Ninjas · Original educational diagram

Rate curve rising steeply at low substrate and approaching a plateau as active sites become occupied — Finite active sites produce a rate plateau
At low substrate, added molecules create many more productive encounters. When active sites are occupied much of the time, the same addition produces a smaller rate gain.
Use this visual to see
Read the curve as changing enzyme availability rather than as substrate becoming ineffective.
What it cannot show
The curve is schematic and does not provide a real Km, Vmax, cooperativity, or inhibitor mechanism.
Test Ninjas · Original educational diagram

Two routes share endpoints

Enzyme–substrate interactions stabilize a lower-energy transition route, increasing the fraction of collisions that lead to product.

Two routes share endpointsBoth routes begin at energy 40 and end at 25. The uncatalyzed peak is 100, while the catalyzed peak is 66.

uncatalyzed

reactantEnergy: 40
barrierPeak: 100
productEnergy: 25

catalyzed

reactantEnergy: 40
barrierPeak: 66
productEnergy: 25

Entities

reactant energy level
product energy level
uncatalyzed transition barrier
catalyzed transition barrier

Rules

Reactant and product levels are identical in both routes.
Only the activation barrier changes.
Barrier height influences rate, not net reaction energy.

Two routes share endpoints states
State	id	reactantEnergy	barrierPeak	productEnergy
1	uncatalyzed	40	100	25
2	catalyzed	40	66	25

Limitations: The one-dimensional profile omits molecular coordinates, multiple intermediates, and rate constants.
Accessible summary: Both routes begin at energy 40 and end at 25. The uncatalyzed peak is 100, while the catalyzed peak is 66.

Recognize finite enzyme capacity

What pattern does the synthetic initial-rate series show as substrate rises?

Recognize finite enzyme capacity dataset
Relative substrate	Initial rate (units/min)	No-enzyme control
1	1.7	0.05
2	2.9	0.05
4	4.4	0.05
8	5.6	0.05

Analysis: Rate increases, but doubling substrate from four to eight raises rate only from 4.4 to 5.6. The flattening is consistent with finite enzyme availability.
Limitations: The synthetic data do not estimate a real Km or Vmax and do not prove a particular binding mechanism.

Provided accessible table

Recognize finite enzyme capacity accessible table
Substrate	Initial rate	No enzyme
1	1.7	0.05
2	2.9	0.05
4	4.4	0.05
8	5.6	0.05

Keep these three ideas

Active-site interactions create a lower-barrier route.
Catalysts change rate, not equilibrium or net energy difference.
Saturation reflects finite enzyme capacity.

Cellular Energetics