Cell: The Building Block of Life · Lesson 2 of 9
Structure of a Cell
“A cell is a tiny organised workplace where every structure has a job and none gets a lunch break.”
• Explain why a cell needs a boundary. • Describe the selective permeability of the cell membrane. • Distinguish diffusion from osmosis. • Predict the effect of isotonic, hypotonic and hypertonic solutions on a cell. • Interpret the potato experiment using water movement. • Describe the fluid-mosaic model of the cell membrane.
Introduction
Place two equal pieces of potato in different liquids. One rests in plain water; the other rests in a concentrated salt solution. After some time, the first becomes swollen while the second becomes smaller and softer. Nothing pushed or pulled the pieces from outside. The change began at the boundaries of their cells, where water moved between the cells and their surroundings.
Cells must take in useful substances, release wastes and respond to changes outside them. These exchanges occur at the cell boundary. Even a single-celled organism depends on this boundary to remain separate from its surroundings while still exchanging material with them.
Cell membrane — The Universal Feature of a Cell
A very thin boundary surrounding the cell. It protects the cell contents, defines the cell’s individuality and controls the movement of substances between the cell and its surroundings. It is also called the plasma membrane.
Allowing certain substances to pass while restricting others.
Activity: Let us experiment
| Step | What is done | Purpose |
|---|---|---|
| 1 | Cut two potato pieces of roughly equal size. | Keeps their starting conditions comparable. |
| 2 | Measure their initial masses. | Provides a baseline. |
| 3 | Place one in plain water. | Creates a surrounding with more water and less dissolved solute. |
| 4 | Place the other in 20% salt or sugar solution. | Creates a surrounding with less water and more solute. |
| 5 | Wait and measure the final masses. | Shows the effect of water movement. |
| Beaker | Visible observation | Expected mass change | Explanation |
|---|---|---|---|
| Plain water | Potato swells | Mass increases | More water enters the cells. |
| Concentrated solution | Potato shrinks | Mass decreases | More water leaves the cells. |
The potato pieces should be similar in size, measured before treatment and left for the same amount of time. Otherwise, a difference might be caused by unequal starting conditions rather than the solutions.
A difference in the concentration of particles between two regions.
The net movement of particles from a region of higher concentration to a region of lower concentration.
The diffusion of water through a selectively permeable membrane from a region with more water and less solute towards a region with less water and more solute.
| Feature | Diffusion | Osmosis |
|---|---|---|
| What moves? | Particles in general | Water |
| Direction | Higher particle concentration to lower particle concentration | More water and less solute to less water and more solute |
| Membrane required? | Not necessarily | A selectively permeable membrane is required |
| Example | A dissolved dye spreading through water | Water entering plant-root cells |
Problem
Decide which process explains fragrance spreading through air and which explains a soaked seed changing size.
- 1.Fragrance particles spread from a region where they are more concentrated to regions where they are less concentrated.
- 2.This is diffusion because particles spread without a selectively permeable membrane being essential.
- 3.A soaked seed contains living cells surrounded by cell membranes.
- 4.Water crosses those selectively permeable membranes and enters the cells.
- 5.This water movement is osmosis.
What if ...
The effect of a solution depends on the concentration outside the cell compared with the concentration inside it. The words isotonic, hypotonic and hypertonic describe this comparison; they are not permanent labels for a liquid when no cell or second solution is specified.
| Solution around the cell | Outside solute concentration | Net water movement | Likely result |
|---|---|---|---|
| Isotonic | Equal to the inside | No net movement | Cell size remains broadly unchanged |
| Hypotonic | Lower than the inside | Water moves into the cell | Cell swells |
| Hypertonic | Higher than the inside | Water moves out of the cell | Cell shrinks |
Problem
A cell contains more dissolved solute than the surrounding liquid. Predict what happens.
- 1.Compare solute concentration: the outside has less solute than the inside.
- 2.Therefore, the outside solution is hypotonic relative to the cell.
- 3.The outside has a greater proportion of water.
- 4.Water moves into the cell through its selectively permeable membrane.
- 5.The cell swells. The final effect also depends on whether a rigid cell wall is present.
Problem
Why does a potato piece lose mass in concentrated salt solution even though salt surrounds it?
- 1.The salt solution has a greater solute concentration and a smaller proportion of water than the potato cells.
- 2.The cell membrane allows water to cross more readily than the dissolved salt in this situation.
- 3.Water therefore moves out of the potato cells by osmosis.
- 4.The potato piece loses water, so its mass decreases and it shrinks.
The cell membrane is approximately 7–10 nanometres thick and is mainly composed of lipids and proteins. Its lipids form two layers. Water-attracting heads face the watery environments inside and outside the cell, while water-repelling tails face inward, away from water. Proteins are embedded within this lipid bilayer and can assist selected substances in crossing.
| Feature | Meaning |
|---|---|
| Lipid bilayer | Two layers of lipid molecules form the basic boundary. |
| Fluid | Membrane components can move sideways, flip and rotate. |
| Mosaic | Different proteins are arranged among the lipids like pieces in a pattern. |
| Gatekeeper proteins | Membrane proteins help selected substances pass through. |
Osmosis is described through water movement. Water moves towards the side with more dissolved solute and therefore a lower proportion of water, until the concentrations become balanced.
Quiz
Which description best matches Cell membrane?
Which description best matches Selectively permeable?
Which term matches this description: A very thin boundary surrounding the cell.
Which term matches this description: Allowing certain substances to pass while restricting others.
Which statement is a key takeaway from this lesson?
Practice Problems
- Explain why the cell membrane is described as selectively permeable.
- Distinguish diffusion from osmosis using the role of a membrane.
- Predict what happens to a cell placed in a hypertonic solution and explain each step.
- Why should the potato pieces in Activity 2.2 begin with roughly equal sizes?
- Explain why the membrane model is described as both fluid and mosaic.
Key Takeaways
• The cell membrane separates the cell from its surroundings while allowing controlled exchange. • Diffusion is net particle movement down a concentration gradient. • Osmosis is the diffusion of water through a selectively permeable membrane. • Hypotonic surroundings cause net water entry, while hypertonic surroundings cause net water loss. • The cell membrane contains a fluid lipid bilayer with embedded proteins.