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.

Calculating the Rate of Water Uptake

The average rate of water uptake tells us how much water is taken up per unit of time. To calculate it, divide the quantity of water taken up by the time taken.

\text{average rate of water uptake} = \frac{\text{water uptake}}{\text{time taken}}

Water uptake can be measured by mass or by volume. The rate has a compound unit combining the uptake unit with the time unit. For example, uptake in grams over minutes gives a rate in grams per minute ( $\text{g/min}$ ). Uptake in cubic centimetres over minutes gives $\text{cm}^3/\text{min}$ .

Use consistent units before calculating. For example, if the rate is required per minute, convert the whole time to minutes first.

Calculating water uptake per minute

Suppose $3.0\ \text{cm}^3$ of water is taken up in 2 minutes 30 seconds. The time is $$$2.5$$$ minutes, so:

\text{average rate} = \frac{3.0\ \text{cm}^3}{2.5\ \text{min}} = 1.2\ \text{cm}^3/\text{min}

This means that, averaged over the measurement period, $1.2\ \text{cm}^3$ of water was taken up each minute.

Percentage Change in Mass

A percentage change in mass expresses a gain or loss as a percentage of the initial mass. This makes the size of the change relative to the mass at the start.

Subtract the initial mass from the final mass, then divide by the initial mass:

\text{Percentage change in mass} = \frac{\text{final mass} - \text{initial mass}}{\text{initial mass}} \times 100\%

The initial and final masses must be expressed in the same unit before calculating. A positive result indicates a gain in mass; a negative result indicates a loss.

For example, if a sample’s mass falls from $5.0\ \text{g}$ to $4.0\ \text{g}$ :

\frac{4.0 - 5.0}{5.0} \times 100\% = -20\%

The negative sign shows that the sample lost mass. The result can also be reported as a 20% loss: the word “loss” gives the direction, so the percentage is written as a positive value.

Plotting and Interpreting Graphs

A graph shows the relationship between quantities that have been measured or calculated. It makes it easier to see how one quantity changes as another changes.

Choosing and labelling a graph

Choose a graph type that suits the data. A line graph is useful for showing how a quantity changes with a continuous variable, such as time. A bar chart is useful for comparing separate categories, such as different plant species.

When plotting one variable against another, the independent variable is normally placed on the horizontal axis. The dependent variable, which is measured or calculated in response, is placed on the vertical axis. For example, when measuring plant height over time, time goes on the horizontal axis and plant height on the vertical axis.

Label each axis with the quantity and its unit, such as Time (days) and Plant height (cm). Choose scales that cover the data, use enough of the graph to show the values clearly, and have equal intervals for equal changes in value.

Plotting values

Each point on a graph represents a pair of values. To plot a plant height of 6 cm at 2 days, find 2 days on the horizontal axis and 6 cm on the vertical axis. Place a point where those positions meet. Repeat for each pair of values.

Where appropriate, draw a line or smooth curve to show the relationship between the points. Depending on the data and the purpose of the graph, this may be a line of best fit rather than a line joining every point. Separate categories on a bar chart are not joined by a line.

Reading and interpreting a graph

To read a value from a line graph, find the given value on one axis, move to the line or curve, then read the corresponding value from the other axis. Check the scale and units before stating the answer.

Describe the pattern using both variables. For example, if the line rises, plant height increases as time increases; a flat section shows that the height stays the same over that interval. A falling line shows that the dependent variable decreases as the independent variable increases. Values read between plotted points are estimates.