Chapter 15

🔑 Plasma Membrane — Definition

The plasma membrane is a selectively permeable barrier that surrounds the cell and regulates the movement of substances in and out while allowing communication with the environment.

🧱 1. Lipid Bilayer

Phospholipid — Definition

A phospholipid is an amphipathic molecule composed of:

  • A hydrophilic (polar) phosphate head

  • Two hydrophobic (nonpolar) fatty acid tails

➡ Because of this, they spontaneously form a bilayer in water

Amphipathic — Definition

A molecule that contains both:

  • Hydrophilic (water-loving)

  • Hydrophobic (water-fearing) regions

Bilayer Structure

  • Heads face outside (toward water)

  • Tails face inside (away from water)

🧊 2. Membrane Fluidity

Fluid Mosaic Model — Definition

The membrane is a dynamic structure where lipids and proteins can move laterally, forming a “mosaic” of components.

Factors Affecting Fluidity:

1. Fatty Acid Composition

  • Unsaturated tails → more fluid (kinks prevent tight packing)

  • Saturated tails → less fluid

2. Cholesterol — Definition

A lipid molecule that:

  • Stabilizes membrane fluidity

  • Prevents membrane from becoming too rigid or too fluid

3. Temperature

  • High temp → increased movement

  • Low temp → decreased movement

🧬 3. Membrane Proteins

Integral Proteins — Definition

Proteins that are embedded in the lipid bilayer, often spanning the membrane (transmembrane proteins).

Peripheral Proteins — Definition

Proteins that are loosely attached to the membrane surface (do NOT span the bilayer).

Functions of Membrane Proteins:

  • Transport (channels/carriers)

  • Signal reception (receptors)

  • Enzymatic activity

  • Cell adhesion

  • Cytoskeleton attachment

🍬 4. Carbohydrates on Membrane

Glycoprotein — Definition

Protein with a carbohydrate chain attached

Glycolipid — Definition

Lipid with a carbohydrate chain attached

➡ Form the cell surface marker system

Glycocalyx — Definition

A carbohydrate coating on the cell surface involved in:

  • Cell recognition

  • Protection

  • Cell-cell interactions

🚪 5. Membrane Permeability

Selective Permeability — Definition

The ability of the membrane to allow some substances to pass while restricting others

What passes easily:

  • Small nonpolar molecules (O₂, CO₂)

  • Lipid-soluble molecules

What does NOT:

  • Ions (Na⁺, Cl⁻)

  • Large polar molecules (glucose)

🚚 6. Passive Transport (No Energy)

Diffusion — Definition

Movement of molecules from high → low concentration

Facilitated Diffusion — Definition

Movement of molecules across a membrane via transport proteins down their concentration gradient

Channel Proteins — Definition

Proteins that form pores allowing specific ions/molecules to pass

Carrier Proteins — Definition

Proteins that bind and change shape to transport molecules

Osmosis — Definition

Movement of water across a membrane toward the area of higher solute concentration

Aquaporins — Definition

Specialized channel proteins that transport water

⚡ 7. Active Transport (Requires ATP)

Active Transport — Definition

Movement of molecules against their concentration gradient (low → high) using energy (ATP)

Ion Pump — Definition

A membrane protein that uses ATP to transport ions

Na⁺/K⁺ Pump — Definition

An ATP-powered pump that:

  • Moves 3 Na⁺ out

  • Moves 2 K⁺ in

➡ Maintains membrane potential

🔋 8. Electrochemical Gradient

Electrochemical Gradient — Definition

A gradient formed by:

  • Difference in concentration

  • Difference in electrical charge across the membrane

➡ Drives ion movement

🔁 9. Secondary Active Transport

Secondary Active Transport — Definition

Transport that uses the energy stored in ion gradients (NOT directly ATP)

Types:

  • Symport → same direction

  • Antiport → opposite directions

📦 10. Vesicular Transport

Endocytosis — Definition

Process where the cell engulfs material by forming vesicles

Types:

  • Phagocytosis → “cell eating” (large particles)

  • Pinocytosis → “cell drinking” (fluid)

  • Receptor-mediated endocytosis → specific uptake

Exocytosis — Definition

Process where vesicles fuse with membrane to release contents outside the cell

🧲 11. Membrane Domains

Lipid Rafts — Definition

Regions of the membrane rich in:

  • Cholesterol

  • Sphingolipids

➡ Important for cell signaling

🧠 12. Key Concepts to Remember

  • Membrane = fluid + dynamic

  • Built from phospholipid bilayer

  • Proteins = functional components

  • Transport = passive, active, vesicular

  • Carbohydrates = cell identity + communication