Chapter 3: Movement of Substances Across a Plasma Membrane

3.1 Structure of Plasma Membrane

Necessity of Movement Across the Plasma Membrane

• Essential for Cell Function:
  • Cells need external substances for metabolic processes.
  • Waste products from metabolism must be disposed of.
  • Movement of substances is regulated by the plasma membrane.

Structure of the Plasma Membrane

• Composition:

  • Phospholipid Bilayer: Composed of polar (hydrophilic) heads and nonpolar (hydrophobic) tails.
  • Protein Molecules: Embedded within the bilayer, forming a dynamic mosaic pattern.
  • Glycoproteins & Glycolipids: Carbohydrate chains attached to proteins and lipids, acting as hormone receptors, membrane stabilizers, and cell identifiers.
  • Cholesterol: Provides strength, flexibility, and decreases permeability to water-soluble substances.

• Dynamic Nature:

  • The arrangement of phospholipids, proteins, and cholesterol results in a fluid and flexible membrane structure.

Permeability

• Types of Permeability:
  • Permeable: Allows all substances to pass freely.
  • Impermeable: Does not allow substances to pass.
  • Semi-permeable/Selectively Permeable: Allows certain molecules to pass based on criteria.
• Plasma Membrane: Selectively permeable due to phospholipid and protein composition.

3.2 Concept of Movement of Substances Across a Plasma Membrane

Characteristics of Movement

• Types of Molecules:
  • Large Molecules: e.g., glucose, amino acids
  • Nonpolar Molecules: e.g., fatty acids, oxygen, carbon dioxide
  • Polar Molecules: e.g., water
  • Ions: e.g., K+, Na+, Ca2+, Mg2+

Modes of Transport

• Passive Transport:

  • Simple Diffusion: Movement from high to low concentration without energy.
  • Osmosis: Movement of water through a selectively permeable membrane.
  • Facilitated Diffusion: Movement with the help of transport proteins (carrier or channel proteins).

• Active Transport:

  • Requires energy (ATP) and moves substances against the concentration gradient.
  • Involves specific carrier proteins known as pumps (e.g., sodium-potassium pump, proton pump).

Differences Between Passive and Active Transport

• Passive Transport: No energy, follows concentration gradient, achieves equilibrium.
• Active Transport: Requires energy, goes against gradient, involves accumulation or disposal.

3.3 Movement in Living Organisms

Passive Transport

• Examples:
  • Gaseous exchange in alveoli.
  • Water absorption by root hairs.
  • Fructose absorption in villi.

Active Transport

• Examples:
  • Glucose and amino acids absorption in kidneys.
  • Mineral ions absorption by root hairs.

Effects of Solutions on Cells

• Isotonic: No net water movement; cells maintain normal shape.
• Hypotonic: Water enters; animal cells may burst (haemolysis), plant cells become turgid.
• Hypertonic: Water exits; cells shrink (crenation in animal cells, plasmolysis in plant cells).

3.4 Applications in Daily Life

Phenomenon of Plant Wilting

• Cause: Excessive fertilizer makes soil hypertonic, causing water to leave plant cells, leading to wilting.

Practical Applications

• Rehydration Drinks: Help recover water/electrolytes lost due to diarrhea or perspiration.
• Saline Solutions: Used in medicine, isotonic to blood plasma.
• Liposomes: Protect drugs from gastric juices.
• Reverse Osmosis: Desalination technique to produce fresh water from seawater.

Prepared by Cikgu Husrita MRSM Transkrian.