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Movement in and out of Cells - Osmosis

Grade 11IGCSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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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), through a partially permeable membrane.

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Water potential is a measure of the tendency of water to move from one area to another. Pure water has the highest water potential, often denoted as 00 ψ\psi.

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A partially permeable membrane (like the cell membrane) allows small molecules like H2OH_2O to pass through but prevents larger solute molecules from moving.

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In plant cells, water entering the cell via osmosis builds up internal pressure against the cell wall, known as turgor pressure. This makes the cell 'turgid'.

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When a plant cell loses water, the cell membrane pulls away from the cell wall, a process known as plasmolysis, making the cell 'flaccid'.

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Animal cells lack a cell wall; therefore, placing them in a solution with high water potential leads to the cell bursting (lysis), while placing them in a solution with low water potential leads to shrinking (crenation).

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Plants absorb water from the soil through root hair cells via osmosis because the water potential in the soil is higher than the water potential inside the cell cytoplasm.

📐Formulae

Percentage Change in Mass=Final Mass−Initial MassInitial Mass×100\text{Percentage Change in Mass} = \frac{\text{Final Mass} - \text{Initial Mass}}{\text{Initial Mass}} \times 100

Net Movement=Movement In−Movement Out\text{Net Movement} = \text{Movement In} - \text{Movement Out}

💡Examples

Problem 1:

A potato cylinder has an initial mass of 2.502.50 g. After being placed in a 0.80.8 mol/dm3mol/dm^3 sucrose solution for 22 hours, its final mass is 2.152.15 g. Calculate the percentage change in mass.

Solution:

2.15 g−2.50 g2.50 g×100=−14.0%\frac{2.15\text{ g} - 2.50\text{ g}}{2.50\text{ g}} \times 100 = -14.0\%

Explanation:

The negative result indicates a loss in mass. This occurred because the sucrose solution had a lower water potential than the potato cells, causing H2OH_2O to move out of the potato by osmosis.

Problem 2:

Explain what happens to a Red Blood Cell (RBC) when placed in distilled water (100%100\% H2OH_2O).

Solution:

The RBC will undergo haemolysis (burst).

Explanation:

Distilled water has a higher water potential than the cytoplasm of the RBC. Water moves into the cell via osmosis. Since animal cells do not have a rigid cell wall to withstand the internal pressure, the cell membrane eventually ruptures.

Osmosis - Revision Notes & Key Diagrams | IGCSE Grade 11 Biology