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Movement Across Membranes

Diffusion, osmosis, active transport

Diffusion

Movement of particles from HIGH to LOW concentration

Key point: Particles move randomly. NET movement is down the concentration gradient until equilibrium.

No energy (ATP) required. Passive transport.

Examples: Oxygen diffusing into cells, glucose diffusing into an amoeba, ammonia diffusing from soil into root

Factors Affecting Diffusion Rate

1. Concentration gradient: Larger difference = faster diffusion

2. Temperature: Higher temp = faster particle movement = faster diffusion

3. Surface area: Larger area = more diffusion

4. Membrane properties: Must be permeable to the substance

Osmosis

Movement of water across a partially permeable membrane from dilute to concentrated solution

Water molecules move randomly. Net movement = towards solution with more dissolved solute (lower water potential)

No energy required. Water molecules are not carried by channels — they move by diffusion.

Partially permeable membrane: allows water and small ions through, but NOT large molecules like proteins/starch

Osmosis in Plant Cells — Turgor & Plasmolysis

Turgid cell: Plant cell in dilute solution. Water enters by osmosis. Cell swells. Cell wall resists — creates turgor pressure. Cell stays rigid & firm.

Plasmolysis: Plant cell in concentrated solution. Water leaves by osmosis. Cytoplasm shrinks away from cell wall. Causes wilting.

Flaccid cell: Plant cell with reduced turgor. No longer rigid. (Different from plasmolysis — membrane still touches wall)

Osmosis in Animal Cells

Hypotonic solution (dilute): Water enters by osmosis. Cell swells & may burst (lysis). No cell wall to resist.

Hypertonic solution (concentrated): Water leaves by osmosis. Cell shrives (crenation). Affects transport of O₂.

Isotonic solution: No net water movement. Cell volume stays same. Water enters = water leaves.

This is why blood transfusions must use isotonic solutions (0.9% NaCl)

Active Transport

Movement of particles AGAINST concentration gradient (low → high)

Requires energy from ATP (respiration). Active process.

Carrier proteins in membrane use energy to pump substances in/out

Examples:
• Glucose uptake in small intestine (against gradient)
• Mineral uptake by root cells
• Sodium-potassium pump in nerve cells

Comparison: Three Transport Types

Type	Direction	Energy	Example
Diffusion	High → Low	No	O₂ into cell
Osmosis	Dilute → Concentrated	No	Water into turgid cell
Active Transport	Low → High	Yes (ATP)	Glucose up intestine

Exam Tips

✓ Osmosis = water movement only (not solutes)

✓ Active transport requires ATP (from mitochondria)

✓ Don't confuse turgor with lysis — turgor = plant cell, lysis = animal cell

✓ Plasmolysis = membrane detaches from wall (reversible)

Key Takeaways

✅ Diffusion: High → Low (passive, no energy)

✅ Osmosis: Water across partially permeable membrane (passive)

✅ Active transport: Low → High (active, needs ATP)

✅ Turgor = plant cell rigid; lysis = animal cell bursts

✅ Plasmolysis = cytoplasm shrinks (reversible)

Passive vs active — it's all about energy! ⚡💧