Osmosis
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of diffusion and the structure and properties of water. You can test your knowledge on these below.
Define simple diffusion.
Movement of small, non polar molecules from an area of higher concentration to an area of lower concentration (this will be across the phospholipid bilayer if referring to movement into or out of a cell)
Define facilitated diffusion.
Movement of larger, polar or charged particles from an area of higher concentration to an area of lower concentration through transport proteins in the membrane, such as carrier or channel proteins.
State the factors that affect the rate of facilitated diffusion.
Temperature, concentration gradient, surface area, diffusion distance, number of transport proteins.
Topic Explainer Videos
Check out this @LauraDoesBiology video that explains osmosis or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Osmosis Overview
Osmosis
- A passive process that involves the net movement of water molecules through a partially permeable membrane.
- Water moves down the water potential gradient (from higher Ψ to lower Ψ).
Water Potential (Ψ)
- Represented in kilopascals (kPa).
- Pure water has a water potential (Ψ) of 0 kPa. This is the highest possible water potential.
- Adding solute lowers water potential (making it more negative).
Factors Affecting The Rate of Osmosis
Water Potential Gradient
- The greater the difference in water potential, the faster osmosis occurs.
Surface Area
- A larger surface area of the membrane increases the rate of osmosis.
Thickness of Membrane
- A thinner membrane allows for a shorter diffusion pathway, increasing the rate of osmosis.
Temperature
- Higher temperatures increase the kinetic energy of water molecules, speeding up osmosis.
Number of channel proteins
- Water is polar but is also a very small molecule. Some water is able to pass through the phospholipid bilayer by osmosis, but this is slow. Aquaporins are channel proteins that allow the facilitated diffusion of water, this will be much faster.
Key Terms
- Hypotonic solution: A solution that has a lower concentration of solutes and therefore a higher water potential than inside the cell.
- Hypertonic solution: A solution that has a higher concentration of solutes and therefore a lower water potential than inside the cell.
- Isotonic solution: A solution that has the same concentration of solutes and therefore the same water potential as inside the cell.
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Effect of Osmosis on Cells
In Plant Cells
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When plant cells are placed in a hypotonic solution (a solution with a higher Ψ): Water enters the cell by osmosis, cell becomes turgid (the cell wall prevents bursting as it can withstand the turgor pressure).
- When plant cells are placed in an isotonic solution (a solution with the same Ψ): No net movement of water by osmosis, cell remains in normal state. The cell is described as flaccid.
- When plant cells are placed in a hypertonic solution (a solution with a low Ψ): Water leaves, the cell by osmosis. The cell is plasmolysed (cytoplasm shrinks, cell membrane pulls away from the cell wall).
In Animal Cells, such as red blood cells
- When animal cells are placed in a hypotonic solution (a solution with a higher Ψ): Water enters the cell by osmosis, the cell swells and bursts (lysis) due to lack of cell wall.
- When animal cells are placed in an isotonic solution (a solution with the same Ψ): No net movement of water by osmosis, normal cell function.
- When animal cells are placed in a hypertonic Solution (a solution with a low Ψ): Water leaves the cell by osmosis, the cell shrivels (crenation).
Key Terms
- Osmosis: Passive diffusion of water across a membrane.
- Turgid: Swollen plant cell due to water intake by osmosis.
- Plasmolysis: Shrinking of plant cytoplasm when water leaves.
- Crenation: Shriveling of an animal cell due to water loss by osmosis.
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Exam Tips
When describing osmosis, always mention the direction of water movement and refer to water potential differences rather than just solute concentration. e.g., water moves from a higher to a lower water potential.
Always say water moves by osmosis across a partially permeable membrane to ensure you get the marks.
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A student places a plant cell into a solution with a water potential of -0.8 kPa. The cell's internal water potential is -0.5 kPa. Explain what will happen to the plant cell. (3 marks)
- Water will move out of the plant cell by osmosis.
- This is because the external solution has a lower (more negative) water potential than the cell’s cytoplasm.
- The cytoplasm will shrink and the cell will become plasmolysed as the membrane pulls away from the cell wall.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!