Osmosis

To explain the importance of osmosis in plant and animal cells and describe the effects of water gain or loss.
To describe the method for investigating the effect of different solution concentrations on the mass of plant tissue using osmosis.
To calculate percentage change in mass and interpret a graph to estimate the concentration of dissolved substances in plant tissue.

Osmosis is a special type of diffusion that involves the movement of water molecules across a partially permeable membrane.
During osmosis:
- Water moves from an area of dilute solution (where there is a higher concentration of water molecules) to an area of concentrated solution (where there is a lower concentration of water molecules).
- This process continues until the concentration on both sides of the membrane is balanced.

Osmosis.

Plant Cells:
- Plants rely on osmosis to absorb water from the soil and to maintain cell turgidity.
- Turgid cells are swollen with water, which helps the plant stay upright and supported.
- If plant cells lose water, they become flaccid (soft) and the plant may wilt.
Animal Cells:
- Animal cells also use osmosis to balance water levels.
- Unlike plant cells, animal cells lack a cell wall. This means that if an animal cell absorbs too much water, it can swell and eventually burst (lysis) due to the pressure inside the cell.
- If too much water leaves the cell, it will shrink and may become dehydrated.

In cells, different organisms, organs, and tissues have varying concentrations of dissolved substances in their cytoplasm.
We can investigate the concentration of dissolved substances in potato tissue by using osmosis to observe how the potato’s mass changes when placed in solutions of different salt concentrations.

Experimental set-up.

Expected Results:
- Potato pieces in dilute (low concentration) salt solutions will gain mass as water moves into the cells by osmosis.
- Potato pieces in concentrated (high concentration) salt solutions will lose mass as water moves out of the cells by osmosis.
Plotting Results:
- Calculate the percentage change in mass for each potato cube using the formula:

Percentage Change = (End mass - start mass) / start mass x 100

- Plot these results on a graph with percentage change in mass on the y-axis and salt solution concentration on the x-axis. By drawing a line of best fit, you can estimate the concentration of dissolved substances within the potato cells. The point at which the line crosses the x-axis (0% change in mass) indicates the concentration at which there is no net movement of water, as the inside and outside concentrations are equal.

Expected results

Osmosis: The diffusion of water molecules across a partially permeable membrane from a dilute solution to a concentrated solution.
Partially permeable membrane: A membrane that allows some molecules (like water) to pass through but not others (like larger solute molecules).
Dilute solution: A solution with a high concentration of water molecules (low solute concentration).
Concentrated solution: A solution with a low concentration of water molecules (high solute concentration).
Turgid: A plant cell swollen with water due to osmosis, providing support to the plant.
Flaccid: A plant cell that has lost water by osmosis, causing it to become soft and the plant to wilt.
Lysis: The bursting of an animal cell due to excessive water uptake by osmosis.

Osmosis in Action (Egg Experiment): Carefully remove the shell from an egg by soaking it in vinegar, leaving the membrane intact. Place the de-shelled egg in different solutions (e.g., pure water, sugar solution) and observe the changes in size over time due to osmosis.
Graph Analysis Practice: Find example graphs online showing the percentage change in mass of plant tissue placed in solutions of varying concentrations. Practice describing the trends shown and explaining them in terms of osmosis.