Osmosis
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
Water as a Solvent in Organisms
Water is the most important solvent in living organisms. Many vital processes depend on substances being dissolved in water:
- Digestion: digestive enzymes and the products of digestion (glucose, amino acids) are dissolved in water in the gut
- Transport: blood plasma (mostly water) dissolves and carries glucose, amino acids, hormones, urea, carbon dioxide and other substances around the body
- Excretion: urea and other waste products are dissolved in water to form urine, which is removed by the kidneys
What is Osmosis?
Osmosis is a special case of diffusion - it concerns only the movement of water molecules across a partially permeable membrane.
- Water diffuses through partially permeable membranes by osmosis
- Water moves into and out of cells by osmosis through the cell membrane
- A partially permeable membrane allows small water molecules to pass through but not large solute molecules
- Like diffusion, osmosis requires no energy from the cell - it is driven by the concentration gradient of water molecules
Water moves by osmosis from a dilute solution to a concentrated solution through a partially permeable membrane
The Formal Definition of Osmosis (Supplement)
- Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane
- Water potential is a measure of the tendency of water to move. Pure water has the highest water potential; adding solute lowers water potential
- A dilute solution has a higher water potential than a concentrated solution - water always moves from higher to lower water potential
Effects on Plant Cells (Supplement)
When plant cells are placed in solutions of different concentrations, osmosis causes them to change state. You must know and use four key terms: turgid, turgor pressure, plasmolysis and flaccid.
- Cell placed in a dilute solution (higher water potential than inside the cell):
- Water enters the cell by osmosis; the vacuole expands and the cell swells, pushing the cell wall outward
- The cell becomes turgid, and turgor pressure acts on the cell wall
- The rigid cell wall prevents the cell from bursting
- Cell placed in a concentrated solution (lower water potential than inside the cell):
- Water leaves the cell by osmosis; the vacuole shrinks and the cytoplasm and membrane pull away from the cell wall
- The cell becomes flaccid, and once the membrane has pulled away from the wall, the cell is said to be plasmolysed (plasmolysis)
- Cell placed in a solution of the same concentration as the cell contents:
- There is no net movement of water, since there is no water potential gradient - the cell neither gains nor loses water
A plant cell becomes turgid in a dilute solution, flaccid in a slightly concentrated solution, and plasmolysed in a strongly concentrated solution
Common Mistakes
- ❌ Thinking animal cells respond to osmosis in the same way as plant cells
- ✓ plant cells cannot burst in a dilute solution because the rigid cell wall resists expansion and creates turgor pressure. Animal cells (e.g. red blood cells) have no cell wall - they will swell and burst (lyse) in very dilute solutions, or shrivel (crenate) in concentrated solutions
Support in Plants
Plants are supported by the pressure of water inside the cells pressing outwards on the cell wall. When cells are turgid, turgor pressure from each cell presses against neighbouring cells, keeping stems and leaves firm. If a plant loses too much water, its cells become flaccid and the plant wilts and droops.
Investigating Osmosis (Practical)
Osmosis can be investigated using dialysis tubing or plant tissue (e.g. potato, yam, cassava):
- Dialysis tubing method: fill tubing with a concentrated sugar solution, seal it, weigh it, and place it in water. The tubing gains mass as water enters by osmosis. Compare with tubing placed in a concentrated solution, which loses mass
- Plant tissue method: cut equal-sized cylinders of potato, weigh them, place them in solutions of different concentrations for a set time, then re-weigh. Plot percentage change in mass against concentration - the concentration at which mass does not change is the water potential of the cells
Worked Example: Calculating Percentage Change in Mass
- Question: a potato cylinder has an initial mass of 4.0 g. After immersion in a solution, it has a final mass of 4.6 g. Calculate the percentage change in mass.
- Step 1: change in mass = 4.6 − 4.0 = +0.6 g
- Step 2: percentage change = (0.6/4.0) × 100 = +15\%
- The potato gained mass, so water entered the cylinder by osmosis - the external solution was more dilute than the cell contents
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