AP Biology - Unit 2 Study Guide
AP Biology • Unit 2

Cell Structure & Function

Exam Weight: 10–13% • Topics 2.1 – 2.10
2.1

Cell Structure and Function

All living things are made of cells—the basic unit of life. Cells come in two fundamental types: prokaryotic and eukaryotic.

Cell Theory (The Foundation)
  1. All living things are composed of one or more cells
  2. The cell is the basic unit of life
  3. All cells arise from pre-existing cells

🦠 Prokaryotic Cells

  • No membrane-bound nucleus
  • No membrane-bound organelles
  • Smaller (1-10 μm)
  • Circular DNA in nucleoid region
  • Has ribosomes (70S)
  • Examples: Bacteria, Archaea

🧬 Eukaryotic Cells

  • Has membrane-bound nucleus
  • Has membrane-bound organelles
  • Larger (10-100 μm)
  • Linear DNA in chromosomes
  • Has ribosomes (80S)
  • Examples: Animals, Plants, Fungi, Protists
Key Organelles and Their Functions
Organelle Structure Function Found In
Nucleus Double membrane with pores Contains DNA; controls cell activities Eukaryotes only
Ribosome RNA + protein (no membrane) Protein synthesis All cells
Mitochondria Double membrane; inner folds (cristae) Cellular respiration → ATP Eukaryotes only
Chloroplast Double membrane; thylakoids Photosynthesis Plants & algae
ER (Rough) Membrane with ribosomes Protein synthesis & modification Eukaryotes only
ER (Smooth) Membrane without ribosomes Lipid synthesis; detoxification Eukaryotes only
Golgi Apparatus Stacked membrane sacs Modify, sort, package proteins Eukaryotes only
Lysosome Membrane-bound vesicle Digestion; contains enzymes Animal cells
Vacuole Large membrane-bound sac Storage; turgor pressure Plants (large central)
Cell Wall Rigid (cellulose in plants) Support & protection Plants, fungi, bacteria
AP Exam Tip: Be able to explain how structure relates to function for EACH organelle. For example: "Mitochondria have folded inner membranes (cristae) to increase surface area for ATP production during cellular respiration."
2.2

Cell Size

Cells are microscopic for a reason: the surface area-to-volume ratio limits how large a cell can be while still functioning efficiently.

The SA:V Problem

As a cell grows larger:

  • Volume increases faster than surface area (V grows as r³, SA grows as r²)
  • Less surface area per unit volume = harder to exchange materials
  • Diffusion becomes too slow to supply the cell's needs
1 × 1 × 1 SA = 6 V = 1 SA:V = 6:1 2 × 2 × 2 SA = 24 V = 8 SA:V = 3:1 3 × 3 × 3 SA = 54 V = 27 SA:V = 2:1
As the cell gets bigger, the SA:V ratio decreases. Smaller cells have more surface area relative to their volume, making material exchange more efficient.
✓ High SA:V (Small Cells)
• Efficient nutrient/waste exchange
• Faster diffusion throughout cell
• Better communication with environment
✗ Low SA:V (Large Cells)
• Not enough surface for exchange
• Center of cell "starves"
• Waste builds up inside
FRQ Strategy: When asked why cells are small, always connect SA:V ratio to diffusion efficiency. Larger cells compensate by having elongated/flat shapes, internal membranes (like ER), or dividing.
2.3

Plasma Membrane

The plasma membrane is the selectively permeable boundary between the cell and its environment. Its structure is described by the Fluid Mosaic Model.

Fluid Mosaic Model

Fluid: Phospholipids and proteins can move laterally within the membrane.

Mosaic: Various proteins are embedded in or attached to the lipid bilayer, creating a "mosaic" pattern.

EXTRACELLULAR FLUID (outside) CHANNEL Peripheral CYTOPLASM (inside) Channel Protein Carrier Protein Glycolipid Phospholipid
Phospholipid head Fatty acid tails Channel protein Carrier protein Carbohydrate chain Cholesterol
Membrane Components & Functions
Component Function
Phospholipid Bilayer Forms the basic barrier; hydrophobic core blocks most molecules
Cholesterol Maintains membrane fluidity (buffers temperature changes)
Integral Proteins Embedded in membrane; act as channels, carriers, receptors
Peripheral Proteins Attached to surface; cell signaling, cytoskeleton attachment
Glycoproteins/Glycolipids Cell recognition, immune response (ID tags)
Temperature & Fluidity: At high temps, membranes become too fluid. At low temps, they become too rigid. Cholesterol acts as a "fluidity buffer" — it restrains movement at high temps and prevents solidification at low temps.
2.4

Membrane Permeability

The membrane is selectively permeable—it allows some substances to pass freely while blocking others based on size, charge, and polarity.

What Can Cross the Membrane Directly?
✓ CAN Cross (No Help Needed)
  • Small nonpolar molecules (O₂, CO₂, N₂)
  • Small polar molecules (H₂O — slowly)
  • Lipid-soluble molecules (steroids)
✗ CANNOT Cross Directly
  • Large molecules (glucose, proteins)
  • Ions (Na⁺, K⁺, Cl⁻, Ca²⁺)
  • Polar molecules (amino acids)
PHOSPHOLIPID BILAYER O₂ Passes freely H₂O Slowly (or via aquaporin) Glucose ✗ Needs carrier Na⁺ ✗ Needs channel
Key Principle: The hydrophobic core of the membrane acts as a barrier. Anything charged (ions) or large/polar needs a protein to help it cross!
2.5

Membrane Transport

Transport across membranes falls into two main categories based on whether energy (ATP) is required.

Passive Transport

No ATP required — moves DOWN concentration gradient (high → low)

  • Simple diffusion
  • Osmosis (water)
  • Facilitated diffusion

Active Transport

ATP required — moves AGAINST concentration gradient (low → high)

  • Protein pumps (Na⁺/K⁺ pump)
  • Endocytosis
  • Exocytosis
Diffusion Basics

Diffusion is the net movement of molecules from an area of HIGH concentration to an area of LOW concentration until equilibrium is reached.

HIGH [conc] →→→ LOW [conc]

No energy needed — molecules move randomly until evenly distributed

Factors Affecting Diffusion Rate
Temperature

↑ Temp = ↑ Rate

Concentration Gradient

↑ Gradient = ↑ Rate

Molecule Size

Smaller = Faster

Surface Area

↑ SA = ↑ Rate

2.6

Facilitated Diffusion

When molecules can't cross the membrane directly, they need help from transport proteins. If no ATP is used, it's called facilitated diffusion.

Channel Proteins
• Form a hydrophilic tunnel through membrane
• Allow specific ions or water to pass
• Aquaporins = water channels
• Some are gated (open/close in response to signals)
Carrier Proteins
• Bind to specific molecules
• Change shape to move molecule across
• Slower than channels
• Example: Glucose transporters (GLUT)
CHANNEL PROTEIN K⁺ Ions flow through open tunnel CARRIER PROTEIN Glu Glu Protein changes shape to release molecule
Channel protein Carrier protein Ion (K⁺) Glucose
Remember: Facilitated diffusion is still PASSIVE — it moves substances DOWN their gradient. The protein just provides a path, not energy!
2.7

Tonicity and Osmoregulation

Osmosis is the diffusion of water across a selectively permeable membrane. Tonicity describes the relative solute concentration of two solutions.

The Three Tonicity Conditions
Condition Solute Comparison Water Movement Effect on Cell
Hypertonic Solution has MORE solute than cell Water leaves cell Cell shrinks (crenation/plasmolysis)
Hypotonic Solution has LESS solute than cell Water enters cell Cell swells (may lyse)
Isotonic Solution has SAME solute as cell No net movement Cell stays same
HYPERTONIC ISOTONIC HYPOTONIC Animal Cell Plant Cell Crenation (shriveled) Normal Lysis (bursts!) Plasmolysis Flaccid Turgid (ideal for plants!)
Key difference: Plant cells have a cell wall that prevents them from bursting in hypotonic solutions. Instead, they become turgid (firm), which is actually healthy for plants!
Common Mistake: Water moves toward the HIGHER solute concentration (where there's LESS water). Think: "Water follows solute" or "Water dilutes."
Water Potential (ψ): Water moves from HIGH water potential to LOW water potential. Pure water has ψ = 0. Adding solute makes ψ negative. Water always flows toward the more negative ψ.
2.8

Mechanisms of Transport

Active transport uses ATP to move substances against their concentration gradient (from low to high concentration).

The Na⁺/K⁺ Pump (Classic Example)

This pump maintains the electrochemical gradient essential for nerve impulses and cell function:

3 Na⁺ OUT ⟵ ATP ⟶ 2 K⁺ IN

Uses 1 ATP per cycle • Creates voltage difference across membrane

Bulk Transport (Moving Large Items)
Endocytosis (Into Cell)
Phagocytosis: "Cell eating" — engulfs solids (e.g., white blood cells eating bacteria)

Pinocytosis: "Cell drinking" — takes in fluids/dissolved solutes

Receptor-mediated: Specific molecules bind to receptors, triggering uptake (e.g., cholesterol via LDL receptors)
Exocytosis (Out of Cell)
Vesicles fuse with plasma membrane and release contents outside the cell.

Examples:
• Neurotransmitter release at synapses
• Hormone secretion
• Waste removal
Energy Comparison:
• Passive transport: No ATP — diffusion, osmosis, facilitated diffusion
• Active transport: Uses ATP — pumps, endocytosis, exocytosis
2.9

Cell Compartmentalization

Eukaryotic cells are divided into compartments by internal membranes. This allows different chemical environments and specialized functions.

Why Compartmentalize?
Efficiency

Concentrate enzymes and substrates in one location

Protection

Keep dangerous reactions (like in lysosomes) isolated

Specialization

Different pH, ion concentrations for different processes

The Endomembrane System

A network of connected membranes that work together to produce, process, and transport proteins and lipids:

Rough ER
Protein synthesis → Golgi
Modify & package → Vesicles
Transport → Membrane/Export
Final destination

Lysosomes

Contain digestive enzymes at pH ~5 (acidic). Break down old organelles, food particles, and invaders. If they rupture → cell death (apoptosis).

Mitochondria & Chloroplasts

Have their own DNA and double membranes. Create specific internal environments for energy production (proton gradients for ATP synthesis).

2.10

Origins of Cell Compartmentalization

The Endosymbiotic Theory explains how eukaryotic cells evolved their membrane-bound organelles—particularly mitochondria and chloroplasts.

Endosymbiotic Theory

Key Idea: Mitochondria and chloroplasts were once free-living prokaryotes that were engulfed by a larger host cell. Instead of being digested, they formed a mutually beneficial (symbiotic) relationship.

Evolutionary Time → Ancestral Prokaryote Large host cell Aerobic Engulfs aerobic bacterium Mitochondria established Some engulf cyanobacteria → Chloroplasts Evidence for Endosymbiosis: • Own circular DNA (like bacteria) • Own ribosomes (70S, bacterial type) • Double membrane (from engulfing)
Evidence Supporting Endosymbiotic Theory
Evidence Explanation
Double membrane Outer membrane from host cell engulfing; inner membrane from original bacterium
Own circular DNA Like bacterial chromosomes, not linear like eukaryotic nuclear DNA
Own ribosomes (70S) Same size as bacterial ribosomes, smaller than eukaryotic (80S)
Binary fission Divide independently of the cell, similar to bacteria
Size Similar size to bacteria
AP Exam Connection: Endosymbiotic theory is a great example of how structure provides evidence for evolutionary relationships. Be ready to explain how organelle features (DNA, ribosomes, membranes) support their bacterial origins.
Summary: Mitochondria vs Chloroplasts
Mitochondria

From aerobic bacteria

Found in ALL eukaryotes

Chloroplasts

From cyanobacteria (photosynthetic)

Found only in plants & algae

← Unit 1 Unit 3: Cellular Energetics →