Cell Membrane Transport: Diffusion, Osmosis & Active Transport
Unlock the secrets of molecular transport across cell membranes with our comprehensive interactive guide
Understanding Cell Membrane Transport
Cell membrane transport controls how substances move into and out of cells. This fundamental biological process ensures cells maintain proper concentrations of nutrients, waste products, and signaling molecules.
🌊 Diffusion: Passive Movement
What is Diffusion?
Diffusion occurs when molecules spread out from areas of high concentration to low concentration. This passive transport requires no energy input from the cell.
Key Characteristics:
- Moves down concentration gradient
- No energy required (passive)
- Continues until equilibrium
- Rate depends on temperature and molecular size
🎯 Interactive Diffusion Demo
📊 Problem 1: Diffusion Rate Calculation
A glucose molecule (molecular weight: 180 g/mol) diffuses across a membrane. If the concentration difference is 5 mM and the membrane area is 100 μm², calculate the diffusion rate using Fick’s Law.
Given: Diffusion coefficient (D) = 6.7 × 10⁻⁶ cm²/s
Solution:
Using Fick’s Law: J = -D × (ΔC/Δx)
J = 6.7 × 10⁻⁶ × (5 × 10⁻³) / (membrane thickness)
Rate = 3.35 × 10⁻⁸ mol/cm²·s
💧 Osmosis: Water’s Special Journey
Understanding Osmosis
Osmosis is the diffusion of water across a selectively permeable membrane. Water moves from areas of low solute concentration to high solute concentration.
Tonicity Effects:
- Hypotonic: Cell swells as water enters
- Isotonic: No net water movement
- Hypertonic: Cell shrinks as water exits
📊 Problem 2: Plant Cell in Hypotonic Solution
What happens to a plant cell placed in a hypotonic solution with 0.1 M sucrose when the cell’s internal concentration is 0.3 M?
Solution:
Water enters the cell due to lower external solute concentration. The cell becomes turgid (swollen) but doesn’t burst due to the rigid cell wall. This creates turgor pressure, essential for plant structure.
🌱 How Plants Take in Water
Plants absorb water through root hairs via osmosis. The cohesion-tension theory explains how water travels from roots to leaves through xylem vessels.
⚡ Active Transport: Energy-Powered Movement
Active Transport Mechanisms
Active transport moves substances against their concentration gradient using cellular energy (ATP). This process is essential for maintaining cellular homeostasis.
Primary Active Transport
- Direct ATP usage
- Sodium-potassium pump
- Calcium pumps
- Proton pumps
Secondary Active Transport
- Uses electrochemical gradients
- Glucose transport in intestines
- Amino acid uptake
- Ion exchangers
📊 Problem 3: Glucose Transport Protein
Does glucose need a transport protein to cross cell membranes? Calculate the energy required to transport glucose against a 10:1 concentration gradient at 37°C.
Solution:
Yes, glucose requires transport proteins (GLUT transporters) due to its polar nature.
Energy calculation: ΔG = RT ln(C₂/C₁)
ΔG = (8.314 J/mol·K)(310 K) ln(10) = 5.9 kJ/mol
This positive ΔG indicates energy input is required.
🔍 Cell Transport Examples
Real-World Applications
Nutrient Transport from Lumen to Blood
In the small intestine, transport of nutrients from lumen to blood occurs through:
- Glucose: SGLT1 (secondary active transport)
- Amino acids: Various specific transporters
- Fatty acids: Passive diffusion after processing
- Vitamins: Specific carrier proteins
Medical Applications
Understanding transport mechanisms helps in:
- Drug delivery design
- Treatment of transport disorders
- Dialysis procedures
- IV fluid composition
⚖️ Active vs Passive Transport
Key Differences
| Aspect | Passive Transport | Active Transport |
|---|---|---|
| Energy | No ATP required | Requires ATP |
| Direction | Down gradient | Against gradient |
| Examples | Diffusion, Osmosis | Na⁺/K⁺ pump |
❓ Frequently Asked Questions
Membrane permeability refers to the ability of a biological membrane to allow substances to pass through it. It depends on factors like molecular size, charge, and lipid solubility. Selective permeability allows cells to control what enters and exits.
When molecules spread out from areas of high concentration to low concentration, it is called diffusion. This natural process continues until equilibrium is reached and occurs in gases, liquids, and across membranes.
The cohesion-tension theory explains water transport in plants. Water molecules stick together (cohesion) and are pulled up through xylem vessels by transpiration-created tension. This creates a continuous water column from roots to leaves.
In hypotonic solutions, water enters plant cells through osmosis, making them turgid (swollen). The rigid cell wall prevents bursting and creates turgor pressure, which is essential for plant structure and growth.