Osmosis and plant transport
Diffusion
Diffusion is the net movement of particles from a region of high concentration to a region of low concentration, down a concentration gradient. It is a passive process (no energy required).
Factors that increase the rate of diffusion:
- Steeper concentration gradient
- Shorter diffusion distance
- Larger surface area
- Higher temperature (particles have more kinetic energy)
Osmosis
Osmosis is the net movement of water molecules from a region of higher water potential (more dilute solution) to a region of lower water potential (more concentrated solution) through a partially permeable membrane.
Water potential is essentially a measure of how "free" water molecules are. Pure water has the highest water potential; adding solutes lowers it.
Effects on cells:
| Solution | Effect on animal cell | Effect on plant cell |
|---|---|---|
| Hypotonic (dilute) | Swells → lysis (bursts) | Becomes turgid (swells; cell wall prevents burst) |
| Isotonic (same concentration) | No net change | No net change |
| Hypertonic (concentrated) | Crenation (shrinks) | Becomes plasmolysed (cytoplasm pulls away from wall) |
Plasmolysis: plant cell in concentrated solution — water leaves by osmosis, cytoplasm shrinks, cell membrane pulls away from cell wall.
Turgor pressure: water-filled vacuole pushes against cell wall, keeping plant rigid. Loss of turgor = wilting.
Root structure and water uptake
Water enters the root hair cells by osmosis — soil solution is more dilute than cell contents initially.
Root hair cells are adapted:
- Large surface area (the root hair extends into the soil)
- Thin walls and cell membrane — short diffusion distance
Water travels through the root to the xylem by:
- Apoplast pathway: through cell walls (doesn't cross membranes).
- Symplast pathway: through cytoplasm via plasmodesmata.
Xylem and phloem
| Tissue | Substance transported | Direction | Cells | Energy? |
|---|---|---|---|---|
| Xylem | Water and mineral ions | Upwards only (roots → leaves) | Dead, hollow, lignified | Passive (transpiration pull) |
| Phloem | Sugars (sucrose) | Both directions (source → sink) | Living (sieve tube elements + companion cells) | Active (uses ATP) |
Transpiration: loss of water vapour from leaves (mainly through stomata). The transpiration stream pulls water up the xylem.
Stomata: pores in the leaf (mainly underside) surrounded by guard cells. Guard cells open stomata when turgid (daytime — photosynthesis); close when flaccid (drought, night).
Factors affecting transpiration rate:
- Temperature (higher → faster)
- Humidity (lower → faster; steeper gradient)
- Wind speed (higher → faster; removes water vapour)
- Light intensity (more light → stomata open wider)
⚠Common mistakes
- Defining osmosis as movement of "particles" — it is specifically water molecules through a partially permeable membrane.
- Forgetting "net" — water moves in both directions; osmosis describes the net movement.
- Confusing xylem and phloem — xylem carries water (dead cells); phloem carries sugars (living cells).
- Stating transpiration is bad for plants — it also drives mineral uptake and cools the leaf.
AI-generated · claude-opus-4-7 · v3-ccea-biology