Osmosis diagrams, graphs and practicals
Cell biology • Transport in cells
Flashcards
Test your knowledge with interactive flashcards
Key concepts
What you'll likely be quizzed about
Definition and mechanism of osmosis
Osmosis is the net diffusion of water from a region of higher water concentration to a region of lower water concentration across a partially permeable membrane. Water movement is passive and occurs down a water concentration gradient, producing a net flow even though individual molecules move randomly . Cause → effect: when the solution outside a cell has fewer solute particles (higher water concentration), water moves into the cell; when the outside solution contains more solute particles (lower water concentration), water moves out of the cell. The same-volume exchange in both directions occurs only when solutions are at equal water concentration (no net movement).
Diffusion versus osmosis
Diffusion is the net movement of any particles from an area of high concentration to an area of lower concentration until equilibrium. Osmosis is a special case of diffusion that applies to water across a partially permeable membrane . Cause → effect: particles (gases, solutes) spread down their concentration gradients by diffusion; water spreads down its water concentration gradient by osmosis. Diffusion may allow many particle types through a membrane if the membrane is permeable to them; osmosis requires a membrane that is partially permeable to water but not to the dissolved solute.
Partially permeable membranes, isotonic, hypotonic and hypertonic
A partially permeable membrane allows some molecules (commonly water) to pass while blocking larger solute particles. Comparison of water concentration determines direction of osmosis: isotonic solutions have equal water concentration and show no net water movement; hypotonic solutions have higher water concentration and tend to cause water to move into cells; hypertonic solutions have lower water concentration and draw water out of cells . Cause → effect: placing a plant cell in a hypotonic medium causes the vacuole to fill and the cell to become turgid; placing it in a hypertonic medium causes the plasma membrane to pull away from the cell wall (plasmolysis). Diagrams should label water movement arrows and solute concentration in each compartment.
Interpreting and drawing model diagrams
Model diagrams show relative solute concentrations inside and outside the cell, a partially permeable membrane, and arrows showing net water movement. Plant cell diagrams require cell wall, cell membrane, vacuole and cytoplasm; animal cell diagrams require only membrane and cytoplasm. Diagrams must indicate whether the cell is turgid, flaccid or plasmolysed and show direction of net water movement . Limiting factors: scale is not required, but relative sizes and clear labelling are essential. Net movement should be shown by single thicker arrows or by correctly indicating positive/negative mass change when modelling an experiment.
Plotting and interpreting osmosis graphs
Experimental results from varying external solute concentrations are plotted with solute concentration on the x-axis and percentage change in mass on the y-axis. Axes must allow both positive and negative y-values to show gain or loss of mass. Data points are plotted and a suitable line of best fit (straight line or curve) is drawn to show the trend; the concentration at which percentage change is zero indicates isotonic conditions for the tissue sample . Cause → effect: as external solute concentration increases, water potential outside the tissue falls and the percentage change in mass falls; the graph crosses the x-axis at the concentration where tissue water potential equals solution water potential. Use the graph to estimate the concentration giving no net mass change.
Required practical: mass change in plant tissue across concentrations
Standard practical procedure uses a range of concentrations made from a stock solution and distilled water, one matched tissue sample per concentration, and fixed incubation time (example uses 0.0–1.0 M at 0.2 M intervals and 30 minutes incubation) . Equal-sized pieces of plant tissue (for example 1 cm lengths of potato core) receive starting and end mass measurements; samples are blotted dry before massing to remove surface solution that would distort results . Variables and limiting factors: independent variable is solute concentration; dependent variable is percentage change in mass; control variables include tissue size, incubation time, temperature and blotting procedure. Common errors include inconsistent blotting, unequal sample size, evaporation during incubation and measurement precision.
Calculations: percentage change and simple rate measures
Percentage change in mass is calculated as % change = (change in mass ÷ starting mass) × 100, where change in mass = end mass − starting mass. Positive values denote mass gain (water uptake); negative values denote mass loss (water loss) . Simple compound measures of rate of water uptake use change per unit time. Examples: rate in g min−1 = (change in mass in g) ÷ (time in min); rate in % min−1 = (percentage change in mass) ÷ (time in min). These measures allow comparison between experiments conducted for different durations.
Key notes
Important points to keep in mind