Cell Membrane and Transport

The Cell Membrane

• Composed of a phospholipid bilayer with embedded transmembrane proteins and cholesterol.

Cell Membrane Functions

• Regulate molecular movement: import raw materials, export wastes, transfer products, prevent entry of unwanted molecules, prevent escape of needed molecules.

• All cell organelles are also surrounded by a phospholipid bilayer membrane.

Cell Membrane Structure

• Phospholipid bilayer with hydrophobic tails facing inwards and hydrophilic heads facing extracellular (EC) and intracellular (IC) fluids.

• The bilayer is selectively permeable.

• Membrane fluidity is temperature-dependent (colder = less fluid; warmer = more fluid) and supported by cholesterol.

• Phospholipids exhibit rapid lateral movement ($\sim 10^7$ times per second) and rare flip-flop ($\sim$ once per month).

Fluid Mosaic Model

• Current understanding: membrane is fluid with various embedded and associated molecules.

• Small, non-polar molecules ($O<em>2$, $CO</em>2$) pass through easily.

• Water ($H_2O$), though slightly polar, can pass at a low rate.

Cell Membrane Proteins

• Carrier proteins (integral): Facilitate movement of neutral, water-soluble molecules (e.g., glucose).

• Channel proteins (integral): Allow ions ($H^+$, $Ca^{2+}$) to pass through.

• Aquaporins (integral): Specialized channels for water ($H_2O$) passage.

Cell Membrane Carbohydrates

• Attached to lipids (glycolipids) or proteins (glycoproteins).

• Serve as recognition sites, markers for cytoskeleton attachment, and aid in cell-cell adhesion.

Cell Transport

• Cell membranes are selectively permeable, maintaining homeostasis.

• Solution: Solute dissolved in a solvent ($H_2O$ in cells).

• Solutes: Sugars, salts, ions ($Na^+$, $Cl^-$, $K^+$, ATP).

• Concentration gradients: Cells actively maintain unequal solute distribution across the membrane.

Factors Affecting Solute Movement

• Size of molecule: Smaller molecules move more easily.

• Charge and polarity: Neutral and nonpolar molecules pass the bilayer more easily.

• Concentration gradient: Easier movement from high to low concentration.

• Distance: Surface area to volume ratio affects movement in larger cells.

Three Main Transport Methods

1. Passive Transport: No energy required.

2. Active Transport: Requires energy (ATP).

3. Bulk Membrane Transport: Requires energy (ATP).

1. Passive Transport

• A. (Simple) Diffusion: Movement of molecules from high to low concentration until equilibrium.

• B. Osmosis: Diffusion of water ($H<em>2O$) from high water concentration ($[H</em>2O]$) to low water concentration across a semi-permeable membrane.

  • Hypertonic solution: Higher external solute concentration; cell loses $H_2O$ (animal cell crenates, plant cell plasmolysis).

  • Hypotonic solution: Lower external solute concentration; cell gains $H_2O$ (animal cell lyses, plant cell becomes turgid).

  • Isotonic solution: Equal solute concentration; no net water movement.

• C. Facilitated Diffusion: Movement from high to low concentration using specific proteins (carrier or channel proteins); no energy required.

  • Carrier proteins: Aid larger molecules (e.g., glucose).

  • Channel proteins: Aid ions (e.g., $H^+$, $Ca^{2+}$).

2. Active Transport

• Moves molecules against their concentration gradient (low to high concentration).

• Requires energy, typically from ATP hydrolysis (Primary Active Transport) or electrical gradients (Secondary Active Transport).

• Primary Active Transport: Direct use of ATP.

  • Example: Sodium-Potassium pump (moves $3 Na^+$ out and $2 K^+$ in).

  • Other examples: Proton pump ($H^+$), Calcium pump ($Ca^{2+}$).

  • Establishes electrochemical gradients.

• Secondary Active Transport: Uses the concentration gradient of a driving ion (established by primary transport) as an energy source.

  • Symport: Driving ion and transported substance move in the same direction (e.g., $Na^+$ and glucose).

  • Antiport: Driving ion and transported substance move in opposite directions.

3. Bulk Membrane Transport

• Requires energy (ATP) for moving large quantities of material.

• A. Endocytosis: Cell membrane folds inward, forming a vesicle to internalize extracellular material.

  • Pinocytosis ("cell drinking"): Ingestion of fluids and small dissolved particles via small vesicles.

  • Phagocytosis ("cell eating"): Engulfment of large particles (e.g., bacteria) via larger vesicles; can be receptor-mediated.

• B. Exocytosis: Vesicles containing intracellular material fuse with the cell membrane, expelling contents into the extracellular fluid (e.g., hormone secretion like insulin).