Chapter 3: Movement of Substances Across a Plasma Membrane

Learning Objectives

By the end of this chapter, you should be able to:

• Describe the fluid mosaic model of plasma membrane structure
• Explain the difference between passive and active transport mechanisms
• Understand osmosis and its effects on animal and plant cells
• Apply knowledge of membrane transport to practical situations

Overview

The plasma membrane is a selectively permeable barrier that controls the movement of substances into and out of cells. The fluid mosaic model describes its structure as a dynamic bilayer with embedded proteins. Substances move across membranes through various mechanisms, including passive transport (no energy required) and active transport (energy required).

Structure of the Plasma Membrane

The Fluid Mosaic Model

The plasma membrane is described as a fluid mosaic because:

• Fluid: Phospholipids can move laterally
• Mosaic: Proteins are embedded like tiles in a mosaic

Membrane Structure Diagram

Phospholipid Bilayer Structure

Membrane Components and Functions

Membrane Permeability Analysis

Membrane Permeability:

• Impermeable to large molecules and ions
• Freely permeable to small, nonpolar molecules ($O_2$, $CO_2$)
• Selectively permeable to water and some ions

Did You Know? The plasma membrane is only about 7-10 nanometers thick, yet it's incredibly complex and controls everything that enters and leaves the cell!

Mechanisms of Movement Across Membranes

Transport Classification

Passive Transport

Passive transport moves substances down their concentration gradient (from high to low concentration) and does not require energy.

Simple Diffusion

• Definition: Movement of small, nonpolar molecules directly through the phospholipid bilayer
• Substances: Oxygen ($O_2$), carbon dioxide ($CO_2$), lipids
• Direction: High concentration → Low concentration
• Factors affecting rate: Concentration gradient, temperature, membrane permeability
• Energy Equation: $\Delta G = RT \ln \frac{[C_2]}{[C_1]}$ (negative for spontaneous diffusion)

Example: Oxygen diffusing from alveoli into blood cells

Facilitated Diffusion

• Definition: Movement of ions and large molecules through channel or carrier proteins
• Substances: Glucose, amino acids, ions ($Na^+$, $K^+$, $Cl^-$)
• Direction: High concentration → Low concentration
• Protein types:
  • Channel proteins: Form pores for specific ions
  • Carrier proteins: Bind to specific molecules and change shape

Example: Glucose entering cells via carrier proteins

Osmosis

• Definition: Special case of diffusion involving water movement across a selectively permeable membrane
• Direction: Water moves from area of higher water potential to area of lower water potential
• Types of solutions:
  • Hypotonic: Lower solute concentration, higher water potential
  • Hypertonic: Higher solute concentration, lower water potential
  • Isotonic: Equal solute concentrations, equal water potential
• Water Potential Formula: $\Psi = \Psi_s + \Psi_p$ where $\Psi_s$ = solute potential, $\Psi_p$ = pressure potential

Active Transport

Active transport moves substances against their concentration gradient (from low to high concentration) and requires energy (ATP).

Primary Active Transport

• Direct use of ATP to pump substances
• Example: Sodium-potassium pump ($Na^+/K^+$ pump)
  • 3 Na⁺ pumped out of cell
  • 2 K⁺ pumped into cell
  • Requires ATP hydrolysis
  • Energy Equation: $ATP + H_2O \rightarrow ADP + P_i + \text{energy}$

Secondary Active Transport

• Uses energy stored in ion gradients (established by primary active transport)
• Example: Glucose transport in intestines
  • Na⁺ gradient drives glucose uptake

Other Transport Mechanisms

Endocytosis

• Definition: Movement of substances into cells via vesicle formation
• Types:
  • Phagocytosis: "Cell eating" of large particles
  • Pinocytosis: "Cell drinking" of fluids
  • Receptor-mediated endocytosis: Specific molecule uptake

Exocytosis

• Definition: Movement of substances out of cells via vesicle fusion
• Function: Releases neurotransmitters, hormones, and waste products
• Energy Requirement: ATP-dependent process

Effects of Solutions on Animal and Plant Cells

Osmosis Effects Comparison

Animal Cells (e.g., Red Blood Cells)

Plant Cells

Plasmolysis Process

Plasmolysis Process:

1. Hypertonic solution causes water to leave the cell
2. Cytoplasm shrinks and pulls away from the cell wall
3. Cell becomes flaccid and may wilt
4. Reversibility: Plasmolysis can be reversed by returning to isotonic/hypotonic solution (deplasmolysis)

Practical Applications

Food Preservation Methods

• Principle: Create hypertonic environment to remove water from microorganisms
• Methods: Salting (fish, meats), sugaring (jams, preserves), pickling
• Chemical Basis: Reduces water potential ($\Psi$) to levels below microbial growth requirements

Plant Water Relations

• Root Water Absorption: Osmosis occurs as root cells take up water from soil
• Wilting: Occurs when plant cells lose water and become flaccid
• Turgor Pressure: Essential for maintaining plant structure and growth
• Water Potential Formula: $\Psi_{\text{soil}} > \Psi_{\text{root}}$ drives uptake

Medical Applications

• Intravenous Fluids: Must be isotonic to prevent cell damage
• Dialysis: Uses osmosis to remove waste from blood
• Drug Delivery: Controlled release mechanisms often rely on membrane transport
• Clinical Relevance: Understanding membrane transport is crucial for therapeutic interventions

SPM Exam Tip: When explaining osmosis effects, always mention the presence or absence of cell walls as the key difference between animal and plant cell responses. This is a frequently tested concept!

Experimental Investigation of Membrane Transport

Red Onion Cell Plasmolysis Experiment

1. Place red onion epidermis in water (hypotonic solution)
2. Observe cells become turgid
3. Transfer to salt solution (hypertonic solution)
4. Observe plasmolysis as membrane pulls away from cell wall
5. Return to water to observe deplasmolysis

Potato Osmosis Experiment

1. Cut potato cylinders and weigh
2. Place in different sucrose solutions
3. After time, reweigh and measure length changes
4. Calculate percentage change to determine water movement

Practice Tips for SPM Students

Key Concepts to Master

1. Differentiate between transport types:

   • Passive vs. Active
   • Simple vs. Facilitated diffusion
   • Endocytosis vs. Exocytosis

2. Membrane structure relationships:

   • Hydrophobic/hydrophilic properties
   • Protein functions and types
   • Fluidity factors

3. Osmosis calculations:

   • Water potential concepts
   • Solution concentration effects
   • Cell response predictions

Experimental Skills

1. Design experiments to test membrane transport
2. Interpret results from osmosis experiments
3. Calculate concentration changes and water movement

Environmental and Health Connections

Water Balance in Organisms

• Osmoregulation: Maintains water balance in cells
• Kidney function: Filters blood and regulates water content
• Plant adaptation: Root structures for water absorption

Disease Connections

• Chronic kidney disease: Impaired osmoregulation
• Dehydration: Water loss affecting cellular function
• Edema: Fluid accumulation due to membrane transport issues

Summary

• The plasma membrane follows the fluid mosaic model with phospholipids and proteins
• Passive transport (diffusion, facilitated diffusion, osmosis) doesn't require energy
• Active transport requires energy and moves substances against concentration gradients
• Osmosis affects animal and plant cells differently due to cell wall presence
• Understanding membrane transport is crucial for understanding many biological processes

Related Topics

• Chapter 1: Introduction to Biology
• Chapter 2: Cell Biology and Cell Organization
• Chapter 4: Chemical Composition in Cells