Diffusion (Biology)
Diffusion is the passive net movement of molecules or ions from a region of higher concentration to a region of lower concentration, driven by the concentration gradient until equilibrium is reached. In biology, simple diffusion across cell membranes occurs for small, nonpolar molecules such as oxygen, carbon dioxide, and ethanol, which can pass directly through the phospholipid bilayer. Facilitated diffusion involves channel or carrier proteins to transport polar or charged molecules down their concentration gradient without requiring energy.
Key Formula
J = -D × (dC/dx) [Fick's first law]
LaTeX: J = -D \frac{dC}{dx}
| Symbol | Meaning | Unit |
|---|---|---|
| J | Diffusion flux (amount of substance per unit area per unit time) | mol/(m²·s) |
| D | Diffusion coefficient | m²/s |
| dC/dx | Concentration gradient | mol/m⁴ |
Worked Example
Problem
Oxygen diffuses across a cell membrane 8 nm thick. The concentration on the outside is 0.25 mol/m³ and inside is 0.05 mol/m³. The diffusion coefficient for O₂ in the membrane is 1.0 × 10⁻⁹ m²/s. Calculate the diffusion flux.
Solution
Step 1: Identify values — D = 1.0 × 10⁻⁹ m²/s, ΔC = 0.25 − 0.05 = 0.20 mol/m³, Δx = 8 × 10⁻⁹ m Step 2: Apply Fick's first law: J = D × (ΔC / Δx) Step 3: J = 1.0 × 10⁻⁹ × (0.20 / 8 × 10⁻⁹) Step 4: J = 1.0 × 10⁻⁹ × 2.5 × 10⁷ Step 5: J = 0.025 mol/(m²·s)
Answer
The diffusion flux is 0.025 mol/(m²·s) or 2.5 × 10⁻² mol/(m²·s)
Comparison of Types of Diffusion Across Cell Membranes
| Type | Energy Required | Molecules | Proteins Used | Example |
|---|---|---|---|---|
| Simple diffusion | No | Small, nonpolar | None | O₂, CO₂ across membrane |
| Facilitated diffusion | No | Polar, charged, large | Channel or carrier proteins | Glucose (GLUT1), ions (K⁺) |
| Active transport | Yes (ATP) | Any | Pump proteins | Na⁺/K⁺ ATPase pump |
| Osmosis | No | Water only | Aquaporins (facilitated) | Water into plant root cells |
Interactive Tools
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Related Terms
Osmosis (Biology)
Osmosis is the passive movement of water molecules across a selectively permeable membrane from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration). This net movement continues until equilibrium is reached or until an opposing pressure prevents further flow. Osmosis is fundamental to cell function, driving turgor pressure in plant cells, reabsorption of water in kidney tubules, and the movement of water into roots from soil.
Active Transport
Active transport is the movement of molecules or ions across a cell membrane against their concentration gradient (from low to high concentration), requiring the expenditure of cellular energy in the form of ATP. Primary active transport uses ATP directly to power transport proteins called pumps, while secondary active transport uses the electrochemical gradient established by primary active transport to drive the movement of another solute. Active transport is essential for maintaining cellular ion balances, nutrient uptake, and nerve impulse transmission.
Cell Wall
The cell wall is a rigid or semi-rigid layer located outside the plasma membrane in plant cells, fungi, bacteria, and some algae, providing structural support and protection against mechanical stress and osmotic lysis. In plants, the primary cell wall is composed mainly of cellulose microfibrils embedded in a matrix of hemicellulose and pectin, while the secondary cell wall may also contain lignin for added rigidity. The cell wall is absent in animal cells, which instead rely on the extracellular matrix for structural support.
From Latin "diffusio" (a spreading out), from "diffundere" (to pour out or spread). The mathematical description of diffusion was formalized by Adolf Fick in 1855 with his publication of Fick's laws of diffusion.