By the end of this lesson
- Define diffusion, osmosis and active transport precisely.
- Predict net movement from concentration or water-potential information.
- Explain cellular examples using membranes, gradients and energy.
BIG QUESTION
How can the same cell move one substance downhill, water towards lower water potential and another substance uphill?
Direction depends on the transported substance and gradient; membranes and proteins determine which pathway is available.
Diffusion is net movement down a gradient
diffusionThe net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. Individual particles move randomly in both directions; “net” describes the overall imbalance.
Diffusion is passive and does not require energy released by respiration. Oxygen can diffuse into respiring cells while carbon dioxide can diffuse out when suitable gradients exist.
Osmosis refers only to water
osmosisThe net movement of water molecules, through a partially permeable membrane, from a region of higher water potential to a region of lower water potential; a dilute solution has a higher water potential than a concentrated one. is the net movement of water molecules from higher water potential to lower water potential through a partially permeable membrane. A dilute solution has a higher water potential than a more concentrated solution under comparable conditions.
At equilibrium, water molecules still move in both directions, but at equal rates, so there is no net movement. Solute does not “pull” individual water molecules with intention; random movement plus membrane selectivity produces the net effect.
Cells respond differently to water gain and loss
An animal cell in a solution of higher water potential gains water and may lyse because it lacks a cell wall. In lower external water potential it loses water and shrivels.
A plant cell gaining water becomes turgidDescribes a plant cell that has taken in water by osmosis so its vacuole presses the cytoplasm against the cell wall, making the cell firm (turgor). as its contents press against the cellulose wall. A cell losing some water becomes flaccidDescribes a plant cell that has lost water by osmosis, so its vacuole has shrunk and the cell is soft rather than firm.; with sufficient loss, the membrane pulls away from the wall and the cell is plasmolysedDescribes a plant cell that has lost so much water by osmosis that the cell membrane and cytoplasm pull away from the cell wall.. Flaccid and plasmolysed are not synonyms.
Active transport moves against a gradient
active transportMovement of substances across a membrane against a concentration gradient, from lower to higher concentration, using carrier proteins and energy released by respiration. uses carrier proteins and energy released by respiration to move substances against their concentration gradient, from lower to higher concentration.
Root hair cells can absorb mineral ions from dilute soil, and cells of the small intestine can continue absorbing nutrients when their concentration is lower in the gut than in cells or blood. Blocking respiration reduces the energy available for this transport.
Three transport processes compared
Direction alone is not enough: identify the substance, membrane and energy requirement.
| Process | Direction and requirement | Example |
|---|---|---|
| Diffusion | Particles move net high → low concentration; passive; membrane not always required. | Oxygen moving from an alveolus into blood. |
| Osmosis | Water moves net high → low water potential through a partially permeable membrane; passive. | Water entering a root hair cell. |
| Active transport | Solute moves low → high concentration through carrier proteins; requires energy released by respiration. | Mineral ions entering a root hair cell against the gradient. |
Language that earns precision
- concentration gradient
- A difference in concentration between two regions.
- water potential
- A measure of the tendency of water to move; pure water has the highest value under standard conditions.
- partially permeable
- Allowing some particles to cross more readily than others.
- net movement
- The overall movement after movement in both directions is considered.
- active transport
- Energy-requiring movement across a membrane against a concentration gradient.
Common misconceptions
Claim: Particles stop moving at equilibrium.
Correction: Random movement continues, but equal movement in both directions gives no net change.
Claim: Osmosis is the movement of any molecule through a membrane.
Correction: Osmosis describes water molecules only.
Claim: A flaccid plant cell is necessarily plasmolysed.
Correction: Plasmolysis is a more extreme state in which the membrane pulls from the wall.
Claim: Active transport moves substances from high to low concentration.
Correction: It moves against the gradient, from lower to higher concentration.
Match the answer to the command word
Include net movement, the correct gradient and the membrane requirement for osmosis.
Compare both sides first, then state the substance and direction of net movement.
Link respiration to energy release and carrier proteins for active transport.
Recall, apply, then justify
Recall, apply, then justify (continued)
Define osmosis.
Hint
Include substance, direction and membrane.
Suggested marking points
- Net movement of water molecules.
- From higher to lower water potential.
- Through a partially permeable membrane.
Explain why a plant cell becomes turgid rather than bursting in distilled water.
Hint
Follow osmosis, pressure and the wall.
Suggested marking points
- Water enters by osmosis from higher external water potential.
- The cell contents press against the wall.
- The cellulose wall resists expansion and prevents bursting under ordinary conditions.
Explain how a root hair cell absorbs mineral ions when their concentration is lower in the soil than in the cell.
Hint
Name direction, mechanism and energy source.
Suggested marking points
- Ions move against their concentration gradient.
- Carrier proteins in the membrane transport them.
- Active transport uses energy released by respiration.
At equilibrium, explain why there is no net diffusion even though particles continue moving.
Hint
Compare rates in opposite directions.
Suggested marking points
- Particles continue random movement in both directions.
- Equal numbers move each way per unit time, so net movement is zero.
Predict how a respiratory inhibitor would affect glucose uptake when glucose is more concentrated inside an intestinal cell than outside.
Planning hint
Identify the required transport direction and its energy dependence.
What a strong design includes
- Uptake is against the glucose concentration gradient.
- It therefore requires active transport or energy-dependent co-transport.
- The inhibitor reduces ATP supply or energy released by respiration.
- Carrier cycling and glucose uptake decrease.
- Passive diffusion alone cannot maintain uphill accumulation.