What is the difference between a hypotonic and a hypertonic solution in the context of osmosis?

A hypotonic solution has a lower solute concentration (higher water potential) than the cell, causing water to enter. A hypertonic solution has a higher solute concentration (lower water potential), causing water to leave the cell.

Why is percentage change in mass used rather than the actual change in mass in grams?

Percentage change accounts for variations in the initial mass of the plant tissue cylinders. This allows for a valid comparison between samples that did not start at the exact same weight.

How does the use of a cork borer differ from using a knife to prepare the tissue samples?

A cork borer ensures that all cylinders have the same diameter, which standardizes the surface area. Using a knife alone makes it difficult to achieve uniform dimensions, which could affect the rate of osmosis.

What error occurs if the plant tissue cylinders are not blotted dry before the final weighing?

The final mass will be recorded as higher than it actually is because of excess surface liquid. This leads to an inaccurate calculation, potentially suggesting the tissue gained more water (or lost less) than it truly did.

Why might a student get a 'zero change' result for all concentrations in an osmosis experiment?

This usually happens if the tissue was not left in the solution long enough for a measurable amount of osmosis to occur. It could also indicate that the membrane was damaged (e.g., by high heat), making it fully permeable.

What is the consequence of leaving the skins on potato cylinders during the experiment?

The skin is relatively impermeable to water compared to the exposed tissue. This would restrict the surface area available for osmosis, leading to slower and inconsistent mass changes across samples.

Define the term 'partially permeable membrane' in the context of osmosis.

It is a barrier that allows the passage of small molecules, such as water, but prevents the passage of larger dissolved solutes. In this practical, the cell membrane of the plant tissue serves this function.

What is meant by the 'isotonic point' on an osmosis graph?

The isotonic point is the concentration where the line of best fit crosses the x-axis (0% mass change). It represents the concentration where the external solution's water potential equals the internal water potential of the tissue.

What is turgor pressure?

Turgor pressure is the pressure exerted by the cell contents against the cell wall when water enters the cell via osmosis. It provides structural support to the plant tissue.

Why does the mass of a potato cylinder decrease when placed in a high-concentration sugar solution?

The sugar solution has a lower water potential than the potato cells. Water moves out of the cells by osmosis, down the water potential gradient, resulting in a loss of mass.

Required Practical: Osmosis | AQA GCSE Science

Combined Science Trilogy / Biology

Cell Biology

Required Practical: Osmosis

Summary

This practical investigation determines the effect of varying solute concentrations on the mass of plant tissue to identify the internal concentration of the cells. By measuring the net movement of water across a partially permeable membrane, students can quantify the water potential of the tissue using the point of zero mass change.

1. Definition & Core Concepts

Osmosis is defined as the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution). This process occurs across a partially permeable membrane, which allows small water molecules to pass through while blocking larger solute molecules.
Water Potential ( $ψ$ ) is a measure of the tendency of water to move from one area to another. Pure water has the highest water potential, and adding solutes like sugar or salt lowers this potential, creating a gradient that drives osmotic flow.
In the context of this practical, plant cells (typically potato or carrot) act as the biological system where the cell membrane serves as the partially permeable barrier. The internal sap of the vacuole contains a specific concentration of solutes that the experiment aims to estimate.

Diagram showing a plant tissue cylinder submerged in an external solution within a beaker, with arrows indicating the direction of water movement via osmosis.

2. Underlying Principles

The movement of water is determined by the concentration gradient between the external solution and the cell sap. If the external solution is more dilute than the cell (hypotonic), water enters the cell, increasing its mass and making it turgid.
If the external solution is more concentrated than the cell (hypertonic), water leaves the cell via osmosis, causing the mass to decrease and the cell to become flaccid or plasmolysed. This loss of turgor pressure is a direct result of the water potential gradient.
The Isotonic Point occurs when the water potential of the external solution exactly matches the water potential inside the plant cells. At this specific concentration, there is no net movement of water, and the percentage change in mass is zero.

3. Methods & Techniques

4. Data Analysis & Calculations

5. Key Distinctions

6. Exam Strategy & Tips