AP Biology Unit 2: Cell Structure and
Function
1. Cell Types
Prokaryotic Cells
● No nucleus or membrane-bound organelles
● DNA in the nucleoid region
● Smaller and simpler
● Examples: Bacteria, Archaea
Eukaryotic Cells
● Have a nucleus and membrane-bound organelles
● Larger and more complex
● Examples: Plants, animals, fungi, protists
2. Organelles and Their Functions
Organelle
Function
Nucleus
Stores DNA, controls cell activities
Ribosomes
Make proteins (can be free or bound to ER)
Rough ER
Protein synthesis and modification
Smooth ER
Lipid synthesis, detoxification
Golgi Apparatus
Modifies, packages, and ships proteins
Mitochondria
Site of cellular respiration, produces ATP
Chloroplast (plants)
Site of photosynthesis, converts light to sugar
Lysosome (animals)
Digests waste and damaged cell parts
Vacuole
Storage (large central vacuole in plants)
Cell membrane
Controls what enters/leaves the cell
Cell wall (plants)
Provides structure and protection
Cytoskeleton
Maintains shape, helps with movement and
transport
3. Endosymbiotic Theory
● Mitochondria and chloroplasts were once free-living bacteria.
● Evidence: Double membranes, own DNA, reproduce independently
4. Membrane Structure
Fluid Mosaic Model
● Membrane is flexible and made of many parts:
○ Phospholipid bilayer (hydrophilic heads, hydrophobic tails)
○ Proteins (transport, communication, enzymes)
○ Cholesterol (maintains fluidity)
○ Carbohydrates (cell recognition)
5. Transport Across Membranes
Passive Transport
(no energy required)
● Diffusion: movement from high to low concentration
● Facilitated diffusion: diffusion through a transport protein
● Osmosis: diffusion of water
Active Transport
(requires energy/ATP)
● Moves substances against the gradient (low to high)
● Uses protein pumps or bulk transport (endocytosis/exocytosis)
Tonicity Terms
● Isotonic: equal solute, no net water movement
● Hypotonic: lower solute outside → water moves into cell (cell may burst)
● Hypertonic: higher solute outside → water moves out of cell (cell shrinks)
6. Surface Area to Volume Ratio
● Cells need a high surface area to volume ratio to exchange materials efficiently.
● As cells grow larger, the ratio decreases, slowing exchange.
● Smaller cells or cells with folds (like microvilli) are more efficient.
7. Compartmentalization
● Eukaryotic cells use organelles to increase efficiency and separate incompatible
processes.
● Membranes create specialized environments (e.g., pH, enzyme concentration).
0
