AP Bio Unit 3: Cellular Energetics

Cellular energetics

The study of how cells obtain, transform, and use energy to perform life processes, like growth, movement, and making molecules.

Enzyme

A protein that speeds up a chemical reaction in a cell without being used up (catalyst).

Substrate

The reactant molecule that binds to an enzyme’s active site and is transformed into a product during a chemical reaction.

Active Site

The specific region on an enzyme where the substrate binds and the chemical reaction happens.

Activation Energy

The minimum amount of energy needed to start a chemical reaction.

chemical reaction

A process where substances (reactants) change into new substances (products) by breaking and forming chemical bonds.

Example: Hydrogen + Oxygen → Water
(H₂ + O₂ → H₂O)

Denature

When a protein (like an enzyme) loses its shape and can no longer work properly, usually due to heat, pH changes, or chemicals.

Inhibitors

A molecule that binds to an enzyme and decreases or stops its activity, preventing the enzyme from converting its substrate into a product.

• Competitive inhibitors compete directly with the substrate for the enzyme’s active site.

• Non-competitive inhibitors bind elsewhere on the enzyme, changing its shape so the substrate can’t bind effectively

• Feedback inhibitor is a product of a chemical reaction that stops an enzyme early in the pathway so the cell doesn’t make too much.

Entropy

Entropy: A measure of disorder or randomness in a system.

Higher entropy means more chaos or less organization.

pH

A scale that measures how acidic or basic a solution is, from 0 (very acidic) to 14 (very basic), with 7 being neutral.

Gibbs free energy (G)

The energy in a system that can be used to do work.

1st Law of Thermodynamics (Law of Energy Conservation)

• Energy cannot be created or destroyed, only transformed. Ex:

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    • When you eat food, chemical energy in the food doesn’t disappear—it’s turned into ATP (usable energy) or heat.

2nd Law of Thermodynamics (Entropy Law)

• Every energy transfer increases disorder (entropy) in the universe. Energy transformations are never 100% efficient.

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  • When your body turns food into energy, some energy is lost as heat—you can’t perfectly convert all energy into work.

Work

The use of energy to move or change something.

For example, cells do work when they:

• Move molecules across membranes

• Contract muscles

• Build or break down molecules

Endergonic reaction

A reaction that requires energy to happen. Think of it as “energy going in.”

Example: photosynthesis.

Exergonic reaction

A reaction that releases energy as it happens. Think of it as “energy going out.” Example: cellular respiration.

Cellular respiration

The process by which cells break down glucose (sugar) to release energy (ATP) that the cell can use to do work.

Glycolysis

The first step of cellular respiration where glucose is broken down into two molecules of pyruvate, producing a small amount of ATP and NADH.

Krebs cycle

A process in the mitochondria that breaks down molecules from food to make energy-carrying molecules (like ATP, NADH, and FADH₂) and carbon dioxide.

Oxidative phosphorylation

The final stage of cellular respiration, where most ATP is made by using energy from electrons passed through the electron transport chain in the mitochondria, with oxygen as the final electron acceptor.

Electron Transport Chain (ETC)

A series of proteins in the inner mitochondrial membrane that pass electrons along, releasing energy that is used to make ATP. Think of it like a conveyor belt passing electrons down the line.

Chemiosmosis

The process in the mitochondria where hydrogen ions (H⁺) flow through ATP synthase, and this flow is used to make ATP during oxidative phosphorylation.

Photosynthesis

The process by which plants, algae, and some bacteria use sunlight, carbon dioxide, and water to make glucose (sugar) and oxygen.

ATP

The main energy currency of the cell that powers most cellular work.

Light independent reaction

The first stage of photosynthesis where light energy is captured by chlorophyll in the thylakoid membranes to produce ATP, NADPH, and oxygen from water.

Location: Thylakoid membrane 

Starting material: water (electrons), photons (energy)

Products: ATP, NADPH

Linear electron flow

The main pathway of electrons during the light reactions of photosynthesis, where electrons move from water → photosystem II → photosystem I → NADP⁺, producing ATP, NADPH, and oxygen.

NADPH

An energy-carrying molecule that provides electrons and hydrogen for the Calvin cycle in photosynthesis.

Photosystem I (PS I)

A protein-pigment complex in the thylakoid membrane that uses light energy to make NADPH.

Photosystem II (PS II)

A protein-pigment complex in the thylakoid membrane that uses light energy to split water, releasing oxygen and electrons.

Calvin cycle

The light-independent stage of photosynthesis in the stroma, where ATP and NADPH from light reactions are used to make glucose from CO₂.

Stroma

The fluid-filled space inside a chloroplast, surrounding the thylakoids, where the Calvin cycle takes place. Think of it like the “cytoplasm” of the chloroplast—it’s where the sugar-making reactions happen.

Thylakoid

A flattened sac inside a chloroplast where the light reactions of photosynthesis happen.