Lab 4 Membrane and Cell Permeability

Brownian Motion

Definition: Brownian motion refers to the random, erratic movement of particles suspended in a fluid (liquid or gas) due to collisions with fast-moving molecules of the fluid. It’s the physical phenomenon that explains how molecules move in any medium, not just gases or liquids.

Functions of a Cell Membrane

• Protection: Acts as a barrier to protect the cell’s internal environment from external surroundings.

• Regulation of Transport: Controls the entry and exit of molecules and ions.

• Communication: Contains receptors for cell signaling and communication with other cells.

• Structural Support: Provides structural support to maintain the shape of the cell.

• Selective Permeability: Determines which substances can pass through the membrane based on size, charge, and other properties.

Methods of Substance Transport Across Cell Membranes

• Passive Transport (No energy required):

  • Diffusion: Movement of molecules from high to low concentration.

  • Facilitated Diffusion: Movement via a transport protein for larger or charged molecules.

  • Osmosis: A special type of diffusion, specifically for water molecules across a semi-permeable membrane.

• Active Transport (Requires energy):

  • Active Transport Pumps: Movement of molecules against their concentration gradient (low to high concentration) using energy (ATP).

  • Endocytosis/Exocytosis: Bulk transport of large molecules or particles into or out of the cell by engulfing them in vesicles.

Diffusion vs. Osmosis

• Diffusion: Movement of molecules from an area of high concentration to an area of low concentration.

• Osmosis: A type of diffusion specifically for water molecules through a selectively permeable membrane. Water moves from an area of lower solute concentration to higher solute concentration.

Effects of Osmotic Solutions on Cells

• Hypotonic Solution (Lower solute concentration outside the cell):

  • Animal Cells: Water enters the cell, causing it to swell and potentially burst (lysis).

  • Plant Cells: Water enters, causing the cell to swell, but the cell wall prevents bursting (turgidity).

• Hypertonic Solution (Higher solute concentration outside the cell):

  • Animal Cells: Water leaves the cell, causing it to shrink (crenation).

  • Plant Cells: Water leaves, causing the cell to shrink and the membrane to pull away from the wall (plasmolysis).

• Isotonic Solution (Equal solute concentration inside and outside the cell):

  • Animal Cells: No net movement of water; the cell maintains its shape.

  • Plant Cells: Water moves in and out at an equal rate, but the cell is less turgid than in a hypotonic solution.

Plasma Membrane Structure and Permeability

• Structure: The plasma membrane is primarily made of a phospholipid bilayer, with embedded proteins, cholesterol, and carbohydrates.

  • Phospholipids: Hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails create a selective barrier.

  • Proteins: Integral (spanning the membrane) and peripheral (associated with the surface) proteins serve various functions like transport and signaling.

  • Cholesterol: Stabilizes the membrane and maintains its fluidity.

• Permeability: The membrane is selectively permeable, allowing small, nonpolar molecules (like oxygen and carbon dioxide) to pass easily, while larger or polar molecules (like glucose) need transport proteins.

Factors Affecting Diffusion Across the Membrane

• Concentration Gradient: A larger difference in concentration leads to faster diffusion.

• Temperature: Higher temperatures increase molecular motion, speeding up diffusion.

• Surface Area: A larger surface area allows for more molecules to diffuse at once.

• Size of Molecules: Smaller molecules diffuse faster than larger molecules.

• Membrane Permeability: If the membrane is more permeable to a substance, it will diffuse faster.

Why Not All Molecules Cross the Membrane

• Size and Charge: Large or charged molecules cannot pass freely through the lipid bilayer.

• Membrane Proteins: Only molecules that fit specific transport proteins can cross the membrane via facilitated diffusion or active transport.

• Non-Permeable Molecules: Some substances are completely non-permeable, requiring specialized mechanisms to cross.

Release of Non-Permeable Substances

• Non-permeable substances might be released from the cell in cases of membrane damage (e.g., by a toxin or physical disruption) or temperature changes (which can make the membrane more fluid or disrupt its structure).

Lab Concepts

• “De-shelled” Experiment: Likely refers to studying osmosis in egg cells by removing the shell, leaving the semi-permeable membrane intact to observe osmotic behavior.

• “Dialysis Bag” Experiment: Simulates a semi-permeable membrane, demonstrating osmosis and diffusion. It shows how substances move from areas of high to low concentration across a membrane.

• “Elodea Leaf” Experiment: Observes the effect of osmosis in plant cells, often showing how vacuole size changes in different osmotic solutions.

• “Beet Root” Experiment: Studies the effect of osmotic pressure and temperature on the permeability of the plasma membrane, typically by observing the leakage of betalains (pigments) from beet cells.

Semi-Permeability of Plasma Membrane

The plasma membrane is semi-permeable because it allows some molecules to pass freely (small, nonpolar molecules) while restricting others (large, polar molecules or ions). This is due to the unique arrangement of phospholipids and proteins that create a selective barrier.