Osmosis

Water can move across a membrane that allows water molecules through but prevents some dissolved substances from crossing. Such a membrane is called partially permeable. The diffusion of water through it is called osmosis.

When the solutions on either side have different concentrations, there is a net movement of water from the more dilute solution to the more concentrated solution. Water molecules move in both directions, but more move towards the concentrated solution than in the opposite direction.

A cell membrane is partially permeable, so water can move into or out of a cell by osmosis. If the solution outside is more dilute than the solution inside, there is a net movement of water into the cell. If the solution outside is more concentrated, there is a net movement of water out of the cell.

Percentage Change in Plant Tissue Mass

A plant tissue sample may gain or lose mass. To express the change relative to its starting mass, calculate the percentage change in mass.

First subtract the initial mass from the final mass. Then divide the change by the initial mass and multiply by 100%:

A positive result means the tissue gained mass; a negative result means it lost mass. A result of zero means its mass did not change.

Comparing changes in tissue mass

For example, if a sample starts at $$5,\text{g}$$ and ends at $$4,\text{g}$$:

The sample has lost 20% of its initial mass. If another sample starts at $$10,\text{g}$$ and also loses $$1,\text{g}$$, its percentage change is $$-10%$$. Both samples lose the same mass, but the loss is a smaller proportion of the second sample’s starting mass.

Using the initial mass as the reference therefore allows changes in tissues with different starting masses to be compared.

Plotting and Interpreting Experimental Graphs

A graph shows how one measured quantity varies with another. It makes patterns in the data easier to see than a list of measurements alone.

The independent variable is the quantity changed or selected in the investigation. The dependent variable is the quantity measured in response. Plot the independent variable on the horizontal axis and the dependent variable on the vertical axis.

Choosing and plotting a graph

Choose a graph type that suits the data. A line graph is suitable when the independent variable is continuous, such as time, and the aim is to show how the dependent variable changes across its range. A bar chart is suitable for comparing separate categories, such as different types of treatment; leave gaps between the bars to show that the categories are distinct.

Label each axis with the quantity and its unit, for example Time (minutes) or Length (centimetres). Choose a regular scale that covers the data, uses the available graph space clearly and allows values to be plotted accurately.

Plot each pair of values at the correct position. For a line graph, draw a suitable line or curve to show the pattern. A line of best fit need not pass through every point or through the origin. For a bar chart, draw each bar to the measured value for its category.

Reading patterns and gradients

To read a value from a line graph, find the relevant position on one axis, move to the line or curve, then read across to the other axis. When describing a pattern, identify what happens to the dependent variable as the independent variable increases: it may increase, decrease or stay approximately constant. State the values and units where they help to describe the change.

The gradient of a straight line describes how much the vertical quantity changes for a change in the horizontal quantity:

For example, if length increases by $$2,\mathrm{cm}$$ over $$4,\mathrm{min}$$, the gradient is $$0.5,\mathrm{cm/min}$$. The meaning and units of a gradient depend on the quantities shown on the axes.

An upward-sloping line has a positive gradient; a downward-sloping line has a negative gradient. A horizontal line has zero gradient, meaning the vertical quantity is not changing. On a curve, the gradient can change: a steeper section shows a larger change in the vertical quantity for the same change in the horizontal quantity.