e-Consult | Revision Qns

1.

Explain the movement of water between cells and solutions in terms of water potential. In your answer, define water potential and describe how differences in water potential drive the movement of water. Consider the direction of water movement in different scenarios.

Water potential is a measure of the tendency of water to move from one area to another. It's defined as the difference in water potential between pure water and a solution. Pure water has a water potential of 0. Solutions with solutes have a negative water potential. The more solutes present, the more negative the water potential.

Water moves from areas of high water potential (less negative) to areas of low water potential (more negative). This movement is driven by the tendency of water to equalize the water potential across a membrane or between different solutions. Water moves via osmosis, which is the net movement of water across a semi-permeable membrane from a region of higher water potential to a region of lower water potential.

Consider these scenarios:

A pure water solution in contact with a solution containing solutes: Water will move from the pure water into the solution with solutes, attempting to equalize the water potential.

A plant cell with a high solute concentration in contact with a cell with a lower solute concentration: Water will move from the cell with lower solute concentration to the plant cell, driven by the higher water potential of the lower solute concentration solution.

A soil with a high water potential in contact with a plant root cell with a low water potential: Water will move from the soil into the root cell, driven by the higher water potential of the soil.

2.

Using agar plates, design an experiment to investigate the effect of different concentrations of sucrose solution on the rate of osmosis in potato tissue. Include a clear description of the experimental setup, the variables you would control and investigate, and how you would analyse your results. What would be the expected trend in your results and why?

Experimental Setup:

Prepare agar plates with a known concentration of sucrose solution. The sucrose solution will be the hypotonic solution.

Cut uniform discs from potato using a cork borer.

Place the potato discs into separate wells on the agar plates.

Use different concentrations of sucrose solution (e.g., 0%, 10%, 20%, 30%, 40%) as the external solution.

Allow the discs to equilibrate in the sucrose solution for a set time (e.g., 30 minutes).

Measure the mass of each potato disc before and after the equilibration period.

Variables:

Independent Variable: Concentration of sucrose solution (e.g., 0%, 10%, 20%, 30%, 40%).

Dependent Variable: Change in mass of the potato disc (mass after - mass before).

Controlled Variables:
- Size and type of potato used.
- Temperature of the sucrose solution.
- Equilibration time.
- Volume of sucrose solution.

Analysis: The change in mass of the potato disc will be plotted against the concentration of sucrose solution. The expected trend is that the mass of the potato disc will increase as the concentration of sucrose solution increases. This is because water will move into the potato cells by osmosis, increasing their mass. At very high concentrations of sucrose, the rate of osmosis will slow down as the water potential gradient between the inside of the potato cell and the external solution becomes smaller. The data can be presented in a graph, with sucrose concentration on the x-axis and change in mass on the y-axis. A scatter graph would be appropriate.

3.

Describe and explain the processes of simple diffusion, facilitated diffusion, and osmosis. In your answer, clearly outline the factors that influence the rate of each process.

Simple Diffusion: This is the movement of a substance across a semi-permeable membrane from an area of high concentration to an area of low concentration. It occurs without the need for any cellular energy (ATP). The rate of simple diffusion is influenced by several factors:

Concentration Gradient: A steeper concentration gradient results in a faster rate of diffusion.

Temperature: Higher temperatures increase the kinetic energy of molecules, leading to faster diffusion.

Surface Area: A larger surface area allows for more molecules to diffuse across the membrane, increasing the rate.

Thickness of the Membrane: A thinner membrane reduces the diffusion distance, increasing the rate.

Molecular Size: Smaller molecules diffuse faster than larger molecules.

Facilitated Diffusion: This is also passive transport, but it requires the assistance of membrane proteins (channel proteins or carrier proteins). These proteins bind to the substance being transported and help it cross the membrane down its concentration gradient. Unlike simple diffusion, facilitated diffusion has a limited capacity and can become saturated. The rate is influenced by the same factors as simple diffusion, but also by:

Number of Transport Proteins: More transport proteins mean a higher rate of facilitated diffusion.

Specificity of Transport Proteins: The type of protein determines which substance it can transport.

Osmosis: This is the specific case of diffusion involving water. It's the movement of water across a semi-permeable membrane from an area of high water potential (low solute concentration) to an area of low water potential (high solute concentration). Water potential is influenced by solute concentration, pressure, and temperature. The rate of osmosis is influenced by:

Solute Concentration: Higher solute concentration lowers water potential, driving water movement.

Temperature: Higher temperatures increase the rate of osmosis.

Surface Area: Larger surface area increases the rate.

Thickness of the Membrane: A thinner membrane allows for faster osmosis.

Movement into and out of cells (3)