Bio111 Exam 2- Module 5- TAMU- Fletcher
🧠 CELL MEMBRANES & TRANSPORT – STUDY NOTES
Part 1: Cell Membrane Structure & Fluid Mosaic Model
Fluid Mosaic Model:
Describes the cell membrane as a fluid (flexible) layer made of phospholipids, with a mosaic of proteins, cholesterol, and carbohydrates embedded within.
Fluid → components move sideways within the layer.
Mosaic → diverse molecules like proteins and lipids make up the structure.
Membrane Components:
Phospholipids: Form the bilayer; hydrophilic (water-loving) heads face outward, hydrophobic (water-fearing) tails face inward.
Cholesterol: Scattered within phospholipids; stabilizes membrane and affects fluidity.
Integral proteins: Span the membrane; involved in transport and signaling.
Peripheral proteins: On surface; support and communication roles.
Asymmetry:
The inner and outer layers differ in composition and function.
Glycolipids/glycoproteins are mainly on the outer layer; different proteins face different directions.
Maintains selective interactions and signaling.
Factors Affecting Fluidity:
Saturated vs. Unsaturated fats:
Saturated = less fluid (straight tails).
Unsaturated = more fluid (kinks prevent tight packing).
Cholesterol:
Adds stability; prevents too much movement or freezing.
Temperature:
Higher temp → more fluid; lower temp → less fluid.
Part 2: Movement Across Membranes
Key Terms:
Diffusion: Movement of molecules from high → low concentration.
Osmosis: Diffusion of water through a selectively permeable membrane.
Amphipathic: Molecule with both hydrophobic and hydrophilic parts (ex: phospholipids).
Electrogenic: Generates an electrical charge difference (like ion pumps).
Molecules That Pass Easily:
Small, nonpolar molecules: O₂, CO₂, N₂ → diffuse easily.
Small polar molecules (H₂O) → slower or through channels.
Large or charged molecules → need transport proteins.
Tonicity Effects on Animal Cells:
Hypotonic solution: Water enters cell → cell swells or bursts.
Isotonic solution: Water moves equally → cell stays the same.
Hypertonic solution: Water leaves cell → cell shrinks.
Simple vs. Facilitated Diffusion:
Simple: Molecules pass directly through bilayer (no proteins, no energy).
Facilitated: Uses transport proteins (channel or carrier) to move molecules down their gradient.
Part 3: Passive vs. Active Transport
Passive Transport:
No energy needed; moves down concentration gradient (high → low).
Includes: diffusion, osmosis, facilitated diffusion.
Active Transport:
Requires energy (ATP); moves substances against gradient (low → high).
Needs carrier proteins (pumps).
Conditions for Active Transport:
Requires ATP and a specific membrane protein.
Often used to maintain ion gradients.
Sodium-Potassium Pump:
Moves 3 Na⁺ out, 2 K⁺ in using ATP.
Maintains charge difference (negative inside).
Example of electrogenic pump → creates electrochemical gradient.
Electrochemical Gradient:
Combination of ion concentration difference and electrical charge across membrane.
Cells are more negative inside.
Provides stored energy for cellular processes.
Co-Transport (Secondary Active Transport):
Uses energy stored in one gradient (like H⁺ or Na⁺) to move another molecule against its gradient.
Example: glucose absorption in intestines with Na⁺ ions.
Part 4: Bulk Transport
Bulk Transport:
Moves large molecules or materials using vesicles.
Requires energy → active transport.
Endocytosis Types:
Phagocytosis: “Cell eating” – engulfing solids or large particles (ex: white blood cells eating bacteria).
Pinocytosis: “Cell drinking” – taking in fluids and dissolved substances.
Receptor-Mediated Endocytosis: Specific molecules bind to receptors and get internalized (ex: LDL cholesterol uptake).
Endocytosis vs. Exocytosis:
Endocytosis: Imports material into the cell (enter).
Exocytosis: Exports material out of the cell (exit).
Both use vesicles and energy.