🧱 Cell Membrane and Transport

Fluid Mosaic Model

Structure:

• Phospholipid bilayer forms foundation
• Proteins embedded or attached
• Cholesterol maintains fluidity
• Carbohydrates attached (glycoproteins, glycolipids)

Properties:

• Fluid: phospholipids and proteins can move laterally
• Mosaic: varied composition of proteins and lipids
• Selectively permeable: controls what enters/exits

Components:

1. Phospholipids:

   • Hydrophilic heads face water
   • Hydrophobic tails face each other
   • Form bilayer spontaneously

2. Membrane proteins:

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

3. Cholesterol:

   • Maintains fluidity at different temperatures
   • Prevents tight packing at low temps
   • Restricts movement at high temps

4. Carbohydrates:

   • Attached to proteins (glycoproteins)
   • Attached to lipids (glycolipids)
   • Cell recognition, adhesion

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