• Integral proteins: embedded in membrane (transmembrane)
• Peripheral proteins: attached to surface

Transport Mechanisms

Passive Transport (No ATP required)

1. Simple Diffusion

• Movement from high → low concentration
• Down concentration gradient
• Small, nonpolar molecules (O₂, CO₂)

2. Facilitated Diffusion

• Uses membrane proteins
• Down concentration gradient
• Channel proteins: form pores (ions)
• Carrier proteins: change shape (glucose)

3. Osmosis

• Diffusion of water across membrane
• Moves from high water → low water concentration
• From low solute → high solute concentration

Water potential (Ψ):

• Ψ = Ψₛ + Ψₚ
• Ψₛ = solute potential (negative)
• Ψₚ = pressure potential
• Water moves from high → low Ψ

Tonicity:

• Hypertonic: higher solute outside → cell shrinks (crenation/plasmolysis)
• Hypotonic: lower solute outside → cell swells (lysis/turgid)
• Isotonic: equal solute → no net movement

Active Transport (Requires ATP)

1. Primary Active Transport

• Directly uses ATP
• Moves against concentration gradient
• Example: Na⁺/K⁺ pump
  • Pumps 3 Na⁺ out, 2 K⁺ in
  • Maintains concentration gradients

2. Secondary Active Transport

• Uses electrochemical gradient
• No direct ATP use
• Cotransport: use one gradient to move another
  • Symport: same direction
  • Antiport: opposite directions

3. Bulk Transport

Endocytosis (into cell):

• Phagocytosis: "cell eating" (solid particles)
• Pinocytosis: "cell drinking" (fluid)
• Receptor-mediated: specific molecules bind receptors

Exocytosis (out of cell):

• Vesicles fuse with membrane
• Release contents outside
• Secretion of proteins, hormones

Key Concepts

1. Fluid mosaic model: phospholipid bilayer with proteins
2. Selectively permeable: controls what crosses
3. Passive transport: no energy, down gradient
4. Active transport: requires energy, against gradient
5. Osmosis: diffusion of water across membrane
6. Water moves from high → low water potential
7. Bulk transport: large molecules via vesicles

Rate factors:

• Concentration gradient
• Temperature
• Molecular size
• Lipid solubility

Examples:

• O₂, CO₂ (respiratory gases)
• N₂
• Small nonpolar molecules (ethanol, glycerol)
• Lipid-soluble substances (steroid hormones)

Equation (Fick's Law): $\text{Rate} \propto \frac{\Delta C \cdot A}{\Delta x}$

(b) Facilitated Diffusion:

Mechanism:

• Molecules move through channel proteins or carrier proteins
• Down concentration gradient (high → low)
• Protein-mediated

Requirements:

• ✗ No energy (ATP) needed - passive
• ✓ Requires specific transport protein
• Selective based on protein specificity

Types:

1. Channel proteins:

• Aquaporins (water)
• Ion channels (Na⁺, K⁺, Ca²⁺, Cl⁻)
• Can be gated (open/close in response to signal)

2. Carrier proteins:

• Bind substrate
• Change conformation
• Release on other side
• Example: GLUT1 (glucose transporter)

Characteristics:

• Shows saturation kinetics (max rate at high [S])
• Specific for certain molecules
• Faster than simple diffusion for large/polar molecules

Examples:

• Glucose into cells (GLUT transporters)
• Amino acids
• Ions through channels
• Water through aquaporins

(c) Active Transport:

Mechanism:

• Molecules pumped against concentration gradient (low → high)
• Requires energy input (ATP)
• Uses carrier proteins (pumps)

Requirements:

• ✓ Energy (ATP) required - active
• ✓ Requires specific pump protein
• Can create concentration gradients

Types:

1. Primary active transport:

• ATP directly powers transport
• Example: Na⁺/K⁺-ATPase pump
  • 3 Na⁺ out, 2 K⁺ in
  • Maintains electrochemical gradient
  • ~30% of cell's ATP used!

2. Secondary active transport (cotransport):

• Uses gradient created by primary transport
• Symport: both move same direction (Na⁺-glucose)
• Antiport: move opposite directions (Na⁺/Ca²⁺ exchanger)

Examples:

• Na⁺/K⁺ pump (all animal cells)
• Ca²⁺ pumps (muscle cells)
• H⁺ pumps (stomach acid, plant roots)
• Na⁺-glucose cotransporter (intestine)

Comparison Table:

$\boxed{\text{Passive (down gradient): simple/facilitated; Active (against gradient): requires ATP}}$

Energetics:

• Passive: $\Delta G < 0$ (spontaneous)
• Active: $\Delta G > 0$ (requires energy input from ATP hydrolysis)