Bio 100 LAB 2 Chapter 2

Properties of Biological Membranes: Osmosis and Diffusion

Membranes: Structure and Function

Biological membranes are essential structures that define cell boundaries and regulate interactions with the environment. They are composed primarily of lipids, proteins, and carbohydrates, each contributing to membrane function.

• Phospholipid Bilayer: The main structural component, forming a semi-permeable barrier that restricts the passage of most substances except water, glycerol, and small lipid-soluble molecules.

• Membrane Proteins: Embedded within the bilayer, these proteins facilitate transport, communication, and enzymatic activity.

• Carbohydrates: Present on the external surface, they play roles in cell recognition and adhesion.

• Organelles: Eukaryotic cells contain membrane-bound organelles (e.g., mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus) that compartmentalize cellular functions.

Main Functions of Biological Membranes

1. Barrier: Separates the cell from its environment, selectively allowing substances to cross.

2. Transport: Regulates uptake of nutrients and elimination of wastes. Transport can be passive (no energy required) or active (energy required).

3. Organization: Provides a framework for organizing enzymes and electron carriers, facilitating complex processes like respiration and photosynthesis.

4. Communication: Contains receptors for hormones and signaling molecules, enabling cells to respond to external signals.

5. Recognition: Surface molecules allow cells to identify each other and distinguish self from non-self, crucial for immune responses.

Diffusion

Diffusion is the random movement of molecules from regions of higher concentration to regions of lower concentration. It is a fundamental process for the movement of substances across biological membranes.

• Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) directly through the lipid bilayer without energy input.

• Facilitated Diffusion: Movement of ions and polar molecules across membranes via specific protein channels or carriers, still down their concentration gradient and without energy input.

Key Principle: The net movement is always from high to low concentration until equilibrium is reached.

Osmosis

Osmosis is the diffusion of water across a semipermeable membrane in response to differences in solute concentration. Water moves from regions of higher water concentration (lower solute concentration) to regions of lower water concentration (higher solute concentration).

• Semipermeable Membrane: Allows passage of water but not certain solutes.

• Direction of Water Flow: Water moves toward the side with higher solute concentration to balance solute levels.

Example: If a membrane separates pure water from a salt solution, water will move into the salt solution compartment.

Concentration Units

• Mole (mol): Standard unit for amount of substance; 1 mole = molecules.

• Molarity (M): Moles of solute per liter of solution.

Example: 0.1 M glucose solution is equivalent in osmotic effect to 0.1 M sucrose, but only to 0.05 M NaCl, since NaCl dissociates into two ions.

Experimental Study: Dialysis Tubing as a Model Membrane

Dialysis tubing is a semipermeable membrane used to simulate cell membranes in laboratory experiments. It allows passage of small molecules (e.g., glucose, water) but not larger molecules (e.g., starch, proteins).

• Molecular Weight Cutoff: Approximately 10,000 daltons; molecules smaller than this can pass through.

• Dialysis: The process by which small molecules diffuse across the membrane, used in artificial kidney machines.

Osmosis Procedure (Summary)

1. Prepare dialysis tubing and fill with a solution containing glucose, ovalbumin (protein), and starch.

2. Seal and rinse the tubing, then immerse in deionized water.

3. After incubation, test both inside and outside solutions for presence of glucose, starch, and protein using specific reagents.

Table: Experimental Tests for Solutes

Osmosis in Plant and Animal Cells

Plant and animal cells respond differently to osmotic conditions due to structural differences (e.g., presence of cell wall in plants).

• Hypotonic Solution: Lower solute concentration outside the cell; water enters the cell.

• Isotonic Solution: Equal solute concentration inside and outside; no net water movement.

• Hypertonic Solution: Higher solute concentration outside; water leaves the cell.

Effects on Animal Cells

• In hypotonic solutions, animal cells swell and may burst (lyse).

• In hypertonic solutions, animal cells shrink (crenate).

• In isotonic solutions, cell volume remains stable.

Effects on Plant Cells

• In hypotonic solutions, plant cells swell but do not burst due to the rigid cell wall; turgor pressure increases.

• In hypertonic solutions, the plasma membrane pulls away from the cell wall (plasmolysis).

• In isotonic solutions, no net change in cell volume.

Table: Osmotic Properties of Plant and Animal Cells

Key Definitions and Concepts

• Passive Transport: Movement of substances across membranes without energy input (includes simple and facilitated diffusion).

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP).

• Facilitated Diffusion: Passive movement of molecules via membrane proteins.

• Semipermeable Membrane: Allows certain molecules to pass while restricting others.

• Isotonic, Hypotonic, Hypertonic: Terms describing relative solute concentrations and their effects on cells.

Sample Calculations and Equivalencies

• Mole: molecules

• Molarity:

• Osmotic Equivalence: 300 mM glucose is isotonic to 150 mM NaCl because NaCl dissociates into two particles (Na+ and Cl-).

Laboratory Applications

• Dialysis tubing experiments model selective permeability of membranes.

• Benedict's, Lugol's, and Biuret tests are used to detect glucose, starch, and protein, respectively.

• Microscopy of plant and animal cells in different solutions demonstrates osmotic effects visually.

Summary Table: Comparison of Simple Diffusion, Facilitated Diffusion, and Active Transport

Additional info:

• Plant cells maintain turgor pressure in hypotonic environments, which is essential for structural support.

• Animal cells lack a cell wall and are more susceptible to osmotic lysis in hypotonic solutions.

• Dialysis tubing is a useful model but does not replicate all selective properties of biological membranes, especially for polar molecules.