Movement of Substances
Three Transport Processes
| Process | Substance moved | Direction | Energy required? |
|---|---|---|---|
| Diffusion | Any small molecule | High → low concentration | ✗ (passive) |
| Osmosis | Water only | High → low water potential (dilute → concentrated solution) | ✗ (passive) |
| Active transport | Ions and molecules | Low → high concentration (against gradient) | ✓ (uses ATP) |
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down the concentration gradient.
- Passive process (no energy required)
- Occurs in gases and liquids
- Continues until concentrations are equal (equilibrium)
Examples in biology:
- Oxygen diffuses from alveoli into blood (high O₂ in alveoli → low O₂ in blood)
- CO₂ diffuses from cells into blood during respiration
- Glucose diffuses into cells from blood
Osmosis
Osmosis is the movement of water molecules through a partially (selectively) permeable membrane from a region of higher water potential (more dilute solution) to a region of lower water potential (more concentrated solution).
- Only water moves by osmosis (not solutes)
- Partially permeable membrane allows water through but not large solute molecules
Osmosis in Cells
Animal cells:
- In dilute solution (high water potential): water enters by osmosis → cell swells and may burst (lysis)
- In concentrated solution (low water potential): water leaves → cell shrinks (crenation)
Plant cells:
- In dilute solution: water enters → vacuole expands → cell becomes turgid (firm, supports the plant)
- In concentrated solution: water leaves → vacuole shrinks → cell becomes flaccid → severe: plasmolysis (membrane pulls away from cell wall)
Turgidity in Plants
Turgor pressure (turgid cells pushing against their cell walls) is what keeps non-woody plants upright. Wilting occurs when cells lose water and become flaccid.
Active Transport
Active transport moves substances against the concentration gradient (from low to high concentration).
- Requires energy from ATP (produced by respiration)
- Uses carrier proteins in the cell membrane
- Essential when cells need to absorb substances that are already at a higher concentration inside
Examples:
- Absorption of glucose from the small intestine into blood (when glucose concentration in blood is already higher than in the gut)
- Absorption of mineral ions (e.g. nitrates) by root hair cells from soil water
Factors Affecting Rate of Movement
| Factor | Effect on diffusion/osmosis | Reason |
|---|---|---|
| Concentration gradient | Larger gradient → faster rate | Greater difference drives faster net movement |
| Temperature | Higher temp → faster rate | Particles have more kinetic energy; move faster |
| Surface area | Larger area → faster rate | More space for particles to cross simultaneously |
| Distance | Shorter distance → faster rate | Particles reach the other side more quickly |
| Surface area:volume ratio | Higher ratio → faster exchange | Smaller cells have relatively more surface for exchange |
Practical: Investigate Diffusion and Osmosis (Spec 2.17)
Osmosis in potato cylinders:
- Cut potato cylinders to equal length/mass.
- Place each in a different concentration of sugar solution (0%, 0.25%, 0.5%, 1 mol/dm³, etc.).
- Leave for 30–60 minutes.
- Remove, dry and re-measure mass/length.
- Plot % change in mass vs. concentration.
- The concentration at which there is no change in mass = the solute concentration of potato cells.
Exam Questions
Q: Define osmosis.
Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water potential (more dilute solution) to a region of lower water potential (more concentrated solution).
Q: Explain why a plant cell becomes turgid when placed in distilled water.
Distilled water has a higher water potential than the cell contents. Water moves by osmosis into the cell, entering the vacuole. The vacuole expands, pushing the cell membrane against the rigid cell wall. The cell wall resists further expansion, making the cell firm (turgid).
Q: Explain why active transport requires energy but diffusion does not.
Diffusion moves substances down a concentration gradient (from high to low): this happens spontaneously and requires no energy. Active transport moves substances against the concentration gradient (from low to high), which is not spontaneous and requires ATP to power carrier proteins in the membrane.
Q: State two ways in which active transport is different from diffusion.
- Active transport moves substances against the concentration gradient; diffusion moves substances down it. 2. Active transport requires energy (ATP); diffusion does not.
Exam Tip
Osmosis is a special case of diffusion: water only, through a partially permeable membrane. Don’t say “water diffuses”; say “water moves by osmosis.” Also: osmosis is driven by water potential, not just concentration: higher water potential = more dilute.