AP BIOLOGY UNIT 3

Energy

The ability to do work or bring about change; essential for all living systems.

Kinetic Energy

Energy of motion (e.g., moving molecules, muscle contraction).

Potential Energy

Stored energy due to position or structure (e.g., water behind a dam, bonds in molecules).

Chemical Energy

Potential energy stored in the chemical bonds of molecules such as glucose or ATP.

Mechanical Energy

Energy associated with motion or position of objects (e.g., muscle movement, flagella motion).

Thermodynamics

The study of energy transformations within physical and biological systems.

Heat

The least usable form of energy; released as a byproduct during energy transformations.

First Law of Thermodynamics

Energy cannot be created or destroyed, only transformed from one form to another.

Second Law of Thermodynamics

Energy transformations are inefficient; some energy is lost as heat, increasing disorder (entropy).

Entropy

A measure of disorder or randomness in a system; increases with every energy transformation.

Metabolism

The sum of all chemical reactions in a cell, including building and breaking down molecules.

Exergonic Reaction

A reaction that releases energy; products have less energy than reactants (e.g., cellular respiration).

Endergonic Reaction

A reaction that requires energy input; products have more energy than reactants (e.g., photosynthesis).

ATP (Adenosine Triphosphate)

The cell's main energy currency, composed of adenine, ribose, and three phosphate groups.

ADP (Adenosine Diphosphate)

Formed when ATP loses a phosphate group, releasing energy for cellular processes.

Functions of ATP

Powers chemical, transport, and mechanical work in cells.

Chemical Work

Supplies energy for synthesis of biomolecules.

Transport Work

Drives active transport of substances across membranes.

Mechanical Work

Provides energy for muscle contraction, cytoskeleton movement, and cilia/flagella motion.

Coupled Reactions

When energy released from an exergonic reaction (e.g., ATP hydrolysis) drives an endergonic one.

Enzyme

A biological catalyst (protein) that speeds up chemical reactions by lowering activation energy.

Ribozyme

An RNA molecule with enzymatic properties that catalyzes specific biochemical reactions.

Metabolic Pathway

A series of linked reactions where the product of one serves as the substrate for the next.

Catabolic Pathway

Breaks down molecules to release energy (exergonic).

Anabolic Pathway

Builds complex molecules from simpler ones using energy (endergonic).

Substrate

The specific reactant that binds to an enzyme's active site.

Product

The molecule produced after an enzyme acts on its substrate.

Enzyme-Substrate Complex

Temporary molecule formed when the enzyme binds its substrate.

Active Site

The region of an enzyme where the substrate binds and the reaction occurs.

Induced Fit Model

The enzyme slightly changes shape to better fit the substrate for catalysis.

Energy of Activation (Ea)

The amount of energy required to start a reaction; lowered by enzymes.

Denaturation

The loss of enzyme structure and function due to high temperature or extreme pH.

Substrate Concentration

Increasing substrate concentration increases enzyme activity until all active sites are occupied.

Optimal pH

The pH level where an enzyme functions most efficiently.

Optimal Temperature

The temperature at which enzyme activity is highest without denaturation.

Cofactor

A nonprotein helper (inorganic ion like Mg²⁺ or Zn²⁺) required for enzyme activity.

Coenzyme

An organic cofactor derived from vitamins that assists enzymes (e.g., NAD⁺, FAD).

Vitamins

Organic molecules required in small amounts for enzyme function and coenzyme synthesis.

Enzyme Activation

Enzymes can be activated or deactivated by adding/removing phosphate groups or activator molecules.

Inhibition

A process that decreases or stops enzyme activity.

Competitive Inhibition

Inhibitor competes with substrate for the active site.

Noncompetitive Inhibition

Inhibitor binds to an allosteric site, changing enzyme shape.

Allosteric Site

Secondary binding site on an enzyme that regulates its activity.

Redox Reaction

Chemical reaction involving the transfer of electrons between molecules.

Oxidation

Loss of electrons or hydrogen atoms (OIL = Oxidation Is Loss).

Reduction

Gain of electrons or hydrogen atoms (RIG = Reduction Is Gain).

Cellular Respiration

Glucose is oxidized to CO₂ and O₂ is reduced to H₂O, producing ATP.

Photosynthesis

CO₂ is reduced to glucose and H₂O is oxidized to O₂, storing energy.

Cellular Respiration Equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP

Photosynthesis Equation

6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂

Autotrophs

Organisms that produce their own food (e.g., plants, algae).

Heterotrophs

Organisms that consume others for energy (e.g., animals, fungi).

Stomata (Stoma)

Small leaf openings for gas exchange (CO₂ in, O₂ and water vapor out).

Chloroplast

Organelle where photosynthesis occurs.

Stroma

Fluid-filled interior of the chloroplast; site of the Calvin cycle.

Thylakoid

Flattened sac-like membranes that contain chlorophyll; site of light reactions.

Grana (Granum)

Stacks of thylakoids.

Chlorophyll

Green pigment that absorbs light energy for photosynthesis.

Carotenoids

Accessory pigments (orange/yellow) that absorb extra light and protect chlorophyll.

NADP⁺

Electron carrier molecule that accepts electrons during the light reactions.

NADPH

Reduced form of NADP⁺ that provides high-energy electrons to the Calvin cycle.

Light Reactions

Occur in thylakoids; convert light energy to ATP and NADPH; release O₂.

Photosystem

Protein-pigment complex that absorbs light energy.

PSII (Photosystem II)

Splits water, releasing O₂ and energizing electrons.

PSI (Photosystem I)

Energizes electrons to reduce NADP⁺ → NADPH.

Reaction Center

Specialized chlorophyll that donates excited electrons.

Electron Transport Chain (ETC)

Series of proteins transferring electrons to pump H⁺ and generate ATP.

Plastoquinone (Pq)

Electron carrier between PSII and the cytochrome complex.

Noncyclic Electron Flow

Electrons flow from water → PSII → PSI → NADP⁺; produces ATP and NADPH.

Cyclic Electron Flow

Electrons cycle through PSI → ETC → PSI; produces ATP only.

ATP Synthase

Enzyme that produces ATP using the proton (H⁺) gradient.

Chemiosmosis

Process where H⁺ diffusion through ATP synthase drives ATP formation.

Carbon Fixation

Incorporation of CO₂ into organic molecules during the Calvin cycle.

Calvin Cycle

Series of reactions in the stroma using ATP and NADPH to synthesize glucose.

RuBP (Ribulose-1,5-bisphosphate)

5-carbon molecule that combines with CO₂ in the Calvin cycle.

Rubisco (RuBP Carboxylase)

Enzyme that fixes CO₂ to RuBP; most abundant enzyme on Earth.

3PG (3-Phosphoglycerate)

First stable 3-carbon molecule formed in the Calvin cycle.

BPG (1,3-Bisphosphoglycerate)

Intermediate formed when 3PG is phosphorylated using ATP.

G3P (Glyceraldehyde-3-phosphate)

3-carbon sugar product used to form glucose and other molecules.

Carbon Dioxide Reduction

Step where ATP and NADPH reduce 3PG → G3P.

Regeneration of RuBP

Uses ATP to regenerate RuBP so the cycle can continue.

Glucose Phosphate

Carbohydrate derived from G3P; can form starch, sucrose, or cellulose.

Starch

Storage form of glucose in plants.

Cellulose

Structural polysaccharide that builds plant cell walls.

Sucrose

Transport form of sugar in plants.

Hydrocarbon Skeleton

Carbon backbone of G3P used to make lipids and amino acids.

Photorespiration

Occurs in C₃ plants when rubisco binds O₂ instead of CO₂, wasting energy.

C₃ Plants

Fix CO₂ directly into 3PG; efficient in cool, moist environments (e.g., wheat, rice).

C₄ Plants

Fix CO₂ into 4C compound (oxaloacetate) using PEP carboxylase; adapted to hot, dry climates (e.g., corn, sugarcane).

CAM Plants

Open stomata at night to fix CO₂ as malic acid; release it during the day for photosynthesis (e.g., cacti).

PEP (Phosphoenolpyruvate)

3C molecule that binds CO₂ in C₄ and CAM plants.

PEP Carboxylase (PEPCase)

Enzyme that fixes CO₂ to PEP; has high CO₂ affinity and doesn't bind O₂.

Oxaloacetate

4C compound formed when CO₂ binds PEP in C₄/CAM plants.

Malate

4C molecule derived from oxaloacetate that stores CO₂.

Bundle Sheath Cells

Specialized plant cells that form a protective sheath around the vascular bundles (veins) of leaves and play a crucial role in C4 photosynthesis

Mesophyll Cells

Leaf cells that perform photosynthesis and initial CO₂ fixation.

CAM Photosynthesis

Water-saving process in desert plants that separates CO₂ uptake (night) from Calvin cycle (day).