3.1 Structure of the Plasma Membrane
The structure of the plasma membrane is best described by the Fluid Mosaic Model proposed by S. J. Singer and G. L. Nicolson.
Fluid Mosaic Model Components
- Phospholipid Bilayer: Arranged in two layers. Each phospholipid molecule consists of:
- A hydrophilic head (water-loving) facing outwards towards the aqueous extracellular and intracellular environments.
- A hydrophobic tail (water-fearing) pointing inwards, away from water.
- Proteins: Embedded within or attached to the membrane.
- Carrier Proteins: Bind specific molecules (e.g., glucose, amino acids) and undergo conformational changes to transport them across the membrane.
- Pore / Channel Proteins: Form water-filled channels allowing specific ions or water-soluble molecules to diffuse freely.
- Glycoproteins and Glycolipids: Carbohydrate chains attached to proteins and lipids on the outer surface; act as receptors for signaling and cell recognition.
- Cholesterol: Interspersed between phospholipid molecules to stabilize membrane structure, regulate fluidity, and reduce permeability to water-soluble substances.
3.2 Concept of Movement of Substances Across the Plasma Membrane
The plasma membrane is a selectively permeable membrane that allows only certain substances to pass through freely based on molecular size, polarity, and charge.
Properties of Transported Substances
- Lipid-soluble / Non-polar substances: (e.g., fatty acids, glycerol, steroid hormones, fat-soluble vitamins A, D, E, K, oxygen, carbon dioxide) pass directly through the phospholipid bilayer by simple diffusion.
- Water-soluble / Polar / Charged substances: (e.g., glucose, amino acids, ions like $Na^+$, $K^+$, $Ca^{2+}$) cannot pass through the hydrophobic lipid core; they require transport proteins (carrier or channel proteins).
Passive Transport vs Active Transport
- Passive Transport: Movement of substances down a concentration gradient (from higher to lower concentration) without consuming cellular energy (ATP).
- Simple Diffusion: Net movement of small, lipid-soluble molecules or uncharged gases directly through the phospholipid bilayer.
- Osmosis: Net movement of water molecules from a region of higher water potential (hypotonic) to a region of lower water potential (hypertonic) across a selectively permeable membrane.
- Facilitated Diffusion: Movement of large or polar water-soluble molecules down a concentration gradient aided by channel or carrier proteins.
- Active Transport: Movement of substances against a concentration gradient (from lower to higher concentration) requiring carrier proteins and energy derived from ATP hydrolysis (e.g., Sodium-Potassium Pump).
3.3 Movement of Substances Across the Plasma Membrane in Living Organisms
Effects of Hypotonic, Hypertonic, and Isotonic Solutions
The behavior of cells depends on the solute concentration of the surrounding environment relative to the cell cytoplasm:
1. Animal Cells (e.g., Red Blood Cells / Erythrocytes)
- Hypotonic Solution: Water enters the cell by osmosis. The cell swells and eventually bursts (hemolysis) due to lack of a rigid cell wall.
- Hypertonic Solution: Water leaves the cell by osmosis. The cell shrinks and crenates, forming spikey edges (crenation).
- Isotonic Solution: Water potential is equal inside and outside. No net movement of water; the cell maintains its normal biconcave disc shape.
2. Plant Cells
- Hypotonic Solution: Water enters the vacuole by osmosis. The vacuole expands, pushing cytoplasm and plasma membrane against the rigid cell wall, creating high turgor pressure (the cell becomes turgid).
- Hypertonic Solution: Water leaves the vacuole by osmosis. The central vacuole shrinks and the plasma membrane pulls away from the cell wall (plasmolysis). When returned to a hypotonic solution, the cell absorbs water and regains turgidity (deplasmolysis).
- Isotonic Solution: No net movement of water. Turgor pressure drops to zero; the plant cell becomes flaccid.
3.4 Applications of the Concept of Movement of Substances in Everyday Life
- Rehydration Drinks & Oral Rehydration Salts (ORS): Formulated with glucose and electrolytes in isotonic proportions to treat dehydration caused by diarrhea or intense exercise.
- Food Preservation: High concentrations of sugar (jams/jellies) or salt (pickled vegetables, salted fish) create a hypertonic environment that dehydrates food spoilage bacteria and fungi via plasmolysis/crenation, inhibiting microbial growth.
- Saline Injections / IV Drips: Intravenous fluids ($0.9\%\text{ NaCl}$) are isotonic to human blood plasma to prevent red blood cells from undergoing hemolysis or crenation during medical treatment.
- Fertilizer Overuse in Agriculture: Excess synthetic fertilizers make soil water hypertonic relative to root hair cell sap, causing water to leave root cells via osmosis, resulting in wilting and plant death.