Cellular Respiration, ATP, and Enzymes: General Biology Study Guide

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ATP (Adenosine Triphosphate)

Definition and Structure

ATP is the primary energy carrier in all living cells. It is composed of adenine (a nitrogenous base), ribose (a five-carbon sugar), and three phosphate groups.

Structure: Adenine + ribose + 3 phosphate groups
Energy Storage: Energy is stored in the bonds between phosphate groups.

ATP Cycle and Function

When the last phosphate bond is broken, ATP is converted to ADP (adenosine diphosphate) and inorganic phosphate ().
ATP is rechargeable: ADP can gain another phosphate to form ATP again.
Uses of ATP:
- Active transport (e.g., sodium-potassium pump)
- Muscle contraction
- Cell division
- Protein synthesis
- Nerve impulse transmission
- Any cellular process that requires energy

Electron Carriers in Cellular Respiration

NADH and FADH2

Electron carriers are molecules that transport high-energy electrons and hydrogen ions during cellular respiration.

NADH: Reduced from NAD+, carries high-energy electrons.
FADH2: Reduced from FAD, carries slightly lower-energy electrons.
They deliver electrons to the electron transport chain (ETC), where most ATP is produced.

Carrier	Reduced From	Function
NADH	NAD+	Carries high-energy electrons
FADH2	FAD	Carries slightly lower-energy electrons

Mitochondria Structure & Function

Key Features

Outer membrane: Allows small molecules in/out.
Inner membrane: Folded into cristae—site of ETC and ATP synthase.
Matrix: Fluid inside inner membrane—site of Krebs cycle.

Stage	Location
Glycolysis	Cytoplasm (outside mitochondria)
Krebs Cycle	Mitochondrial matrix
Electron Transport Chain (ETC)	Inner mitochondrial membrane (cristae)
ATP Synthase	Inner membrane (part of ETC complex)

Overview of Aerobic Cellular Respiration

General Equation

The overall process of aerobic cellular respiration can be summarized by the following equation:

Reactants (inputs): Glucose + oxygen
Products (outputs): Carbon dioxide + water + ATP

Three Major Phases of Aerobic Cellular Respiration

Stage	Starting Molecule(s)	Ending Products	Main Function
Glycolysis	Glucose	2 Pyruvate, 2 ATP, 2 NADH	Breaks glucose into smaller parts to start energy extraction
Krebs (Citric Acid) Cycle	2 Pyruvate → Acetyl-CoA	2 ATP, 6 CO2, NADH, FADH2	Produces electron carriers for ETC
Electron Transport Chain	NADH, FADH2, O2	~32–34 ATP, H2O	Uses electron energy to make ATP through oxidative phosphorylation

ATP Synthase: Enzyme that spins like a water turbine to make ATP as H+ ions flow down their concentration gradient across the inner membrane.

How the Electron Transport Chain Works

Electrons from NADH and FADH2 pass through membrane proteins.
Energy released pumps H+ ions into the intermembrane space.
This creates a concentration gradient (like stored potential energy).
H+ ions flow back through ATP synthase, powering ATP production.
This process connects to:
- Membrane structure: Selectively permeable
- Proteins: Electron carriers

Poisons and the Electron Transport Chain (ETC)

Poisons like cyanide or carbon monoxide block electron transport.
This prevents ATP formation, stopping cell energy production and leading to cell death.

Cellular Respiration and Food Molecules

Macromolecules as Energy Sources

Carbohydrates: Converted to glucose → enter glycolysis.
Lipids: Broken into glycerol and fatty acids → enter glycolysis or Krebs cycle.
Proteins: Broken into amino acids → converted into Krebs intermediates. All macromolecules can be used for energy production.

Fermentation (Anaerobic Respiration)

Overview

Occurs without oxygen
Only uses glycolysis
Produces 2 ATP per glucose (vs. 36–38 in aerobic respiration)

Type	Occurs In	Products
Lactic Acid Fermentation	Animals, some bacteria	Lactic acid + 2 ATP
Alcoholic Fermentation	Yeast	Ethanol + CO2 + 2 ATP

Energy Comparison

Process	O2 Required?	ATP Produced
Aerobic Respiration	Yes	~36–38
Fermentation	No	2

Cellular Respiration, Exercise, and Muscles

Low-intensity / High-oxygen: Aerobic respiration → sustained energy, lots of ATP.
High-intensity / Low-oxygen: Anaerobic respiration → lactic acid buildup → sore muscles.
Training: Increases mitochondrial density and oxygen efficiency → less lactic acid, faster recovery.

Diet, Energy, and Health Concepts

Term	Definition
Calories	Energy unit; 1 Calorie = 1000 calories
Metabolic Rate	How fast body uses energy
BMI	Body mass index (height/weight measure)
LDL	"Bad" cholesterol — clogs arteries
HDL	"Good" cholesterol — removes cholesterol
Diabetes	Blood glucose regulation disorder
Hypertension	High blood pressure
Systolic/Diastolic	Heartbeat pressure / resting pressure
Heart Attack/Stroke	Blocked blood flow to heart/brain
Obesity	Excess body fat
Anorexia/Bulimia	Eating disorders affecting metabolism and health
USDA Food Guides	Nutritional recommendations for a balanced diet

Enzymes

Concepts and Function

Concept	Explanation
Macromolecule Type	Protein
Function	Speed up (catalyze) reactions by lowering activation energy
Substrate	Molecule the enzyme acts on
Active Site	Region on enzyme where substrate binds
Induced Fit	Enzyme changes shape slightly to fit substrate perfectly
Specificity	Each enzyme fits only one type of substrate (like a lock and key)

Protein Structure Levels

Primary: Amino acid sequence
Secondary: Coils/folds (alpha helix, beta sheet)
Tertiary: 3D shape due to interactions between side chains
Quaternary: Multiple protein chains combined

Denaturation

When an enzyme loses its shape (and function).
Caused by extreme heat, pH, or salt concentration.
Example: Pineapple enzyme lab — heating or acidity destroyed enzyme activity.

Enzymes in Different Environments

Enzymes are adapted to their environment:
- Pepsin (stomach): Works best in acidic pH ~2
- Trypsin (small intestine): Works best in basic pH ~8
- Arctic/hot-spring organisms: Enzymes adapted to temperature extremes