Passive Transport and Osmosis: Lab Pre-Study Guide

Passive Transport Lab (Prelab)

Lab Objectives

This section introduces the main goals of the lab, focusing on the movement of water across cell membranes and the concept of tonicity.

• Osmosis: The process by which water moves across a semi-permeable membrane from an area of lower solute concentration to an area of higher solute concentration.

• Tonicity: The effect of extracellular solution concentration on cell shape and volume.

• Toxicity: The impact of various extracellular environments on cell health.

Background: Osmosis and Tonicity

Water movement across cell membranes is essential for maintaining cellular homeostasis. The direction and rate of osmosis depend on the relative concentrations of solutes inside and outside the cell.

• Semi-permeable membrane: A barrier that allows water to pass but restricts the movement of many solutes.

• Extracellular solutions: Solutions outside the cell that can affect cell volume and shape depending on their concentration relative to the cell's interior.

Types of Solutions and Their Effects on Cells

Solutions are classified based on their solute concentration compared to the cell:

• Hypertonic: Higher solute concentration than the cell. Water will diffuse out of the cell, causing the cell to shrink (crenation).

• Hypotonic: Lower solute concentration than the cell. Water will diffuse into the cell, causing the cell to swell and possibly burst (lysis).

• Isotonic: Same solute concentration as the cell. There will be no net movement of water, and the cell will neither shrink nor swell.

Example: If a cell is placed in a 10% solute solution and its cytoplasm is 5% solute, the solution is hypertonic to the cell.

Key Definitions

• Osmosis: Movement of water across a semi-permeable membrane from low to high solute concentration.

• Tonicity: The ability of a solution to cause a cell to gain or lose water.

• Non-penetrating solutes: Solutes that cannot cross the cell membrane, such as sodium ions (Na+) and chloride ions (Cl-).

Formulas and Equations

• Osmotic Pressure Equation:

$\Pi = iMRT$

• Where $\Pi$ is osmotic pressure, $i$ is the van 't Hoff factor, $M$ is molarity, $R$ is the gas constant, and $T$ is temperature in Kelvin.

Comparing Solution Types

The following table summarizes the effects of different solution types on cells:

Solution Type	Relative Solute Concentration	Water Movement	Effect on Cell
Hypertonic	Higher than cell	Out of cell	Cell shrinks (crenation)
Hypotonic	Lower than cell	Into cell	Cell swells (lysis)
Isotonic	Equal to cell	No net movement	Cell remains unchanged

Pre-Lab Assignment Questions

These questions are designed to reinforce understanding of osmosis and tonicity:

1. Define osmosis.

2. Define tonicity.

3. Describe two solutions in terms of cell tonicity.

4. Explain how tonicity affects cell shape and volume.

5. Identify two non-penetrating solutes present across the cell membrane.

6. Define hypotonic, hypertonic, and isotonic relative to intracellular fluid.

Additional info: Non-penetrating solutes are important in determining tonicity because they cannot cross the membrane and thus influence water movement. Common examples include sodium chloride and glucose.