Osmosis

1. Overview

Osmosis is a specific type of diffusion focusing exclusively on the movement of water. It is a fundamental process in biology that determines how cells maintain their shape, how plants stay upright, and how nutrients and waste products are transported in solution throughout an organism.

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Key Definitions

• Osmosis: The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
• Partially Permeable Membrane: A barrier (like the cell membrane) that allows small molecules like water to pass through but prevents larger solute molecules (like sugar or salt) from crossing.
• Solvent: A substance (usually a liquid) in which other materials dissolve to form a solution. Water is the "universal solvent" in biological systems.
• Water Potential: A measure of the "free" water molecules in a solution; pure water has the highest water potential.
• Turgid: A term used to describe a plant cell that is swollen and firm due to high internal water pressure.
• Plasmolysis: The process where the cell membrane pulls away from the cell wall because the cell has lost too much water.

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Core Content

The Role of Water as a Solvent

Water is essential for life because it acts as a solvent in:

• Digestion: Food molecules must be dissolved in water to be broken down by enzymes and absorbed into the blood.
• Excretion: Waste products like urea and excess salts are dissolved in water to form urine, allowing them to be removed from the body.
• Transport: In animals, blood plasma (mostly water) transports glucose and CO2. In plants, water in the xylem and phloem transports minerals and sucrose.

Mechanism of Osmosis

• Water molecules move randomly due to kinetic energy.
• When a membrane separates two solutions of different concentrations, more water molecules move from the dilute side to the concentrated side than vice-versa.
• This results in a net movement of water.
• 📊Two compartments separated by a dashed line (membrane). Left side has many blue dots (water) and few red circles (sugar). Right side has few blue dots and many red circles. An arrow shows the net flow of blue dots from left to right.

Investigating Osmosis: Dialysis (Visking) Tubing

Dialysis tubing is an artificial partially permeable membrane used to model the cell membrane.

1. Fill tubing with a concentrated sugar solution.
2. Place the tubing in a beaker of pure water.
3. Observation: The tubing becomes firm and increases in volume/mass as water moves in by osmosis.

Osmosis in Plant Tissues

When plant tissues (like potato cylinders) are immersed in solutions:

• In Pure Water: Water enters the cells. The cells increase in mass and length.
• In Concentrated Sugar/Salt Solution: Water leaves the cells. The cells decrease in mass and length; the tissue becomes "soft" or "floppy."

Support in Plants

Plants do not have a skeleton. They rely on turgor pressure.

• When water enters a plant cell, it pushes the cytoplasm against the rigid cell wall.
• This internal pressure keeps the cells firm (turgid).
• These turgid cells press against each other, providing structural support for the stem and leaves.

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Extended Content (Extended Only)

Water Potential and Movement

Instead of "concentration," use the term Water Potential ($\Psi$).

• High Water Potential: A dilute solution (lots of water, little solute).
• Low Water Potential: A concentrated solution (little water, lots of solute).
• Water always moves down a water potential gradient.

Effects on Plant Cells: Step-by-Step

1. Turgid (Hypotonic solution): The external solution has a higher water potential than the cell sap. Water enters by osmosis. The vacuole increases in size, pushing the cell membrane against the cell wall.
2. Flaccid (Isotonic solution): The water potential is equal inside and outside. There is no net movement of water. The cell is not firm.
3. Plasmolysed (Hypertonic solution): The external solution has a lower water potential than the cell sap. Water leaves the cell. The vacuole shrinks, and the cell membrane pulls away from the cell wall (plasmolysis).

• 📊Three plant cells. 1. Turgid: Rectangular, large vacuole, membrane tight against wall. 2. Flaccid: Slightly rounded corners, vacuole smaller. 3. Plasmolysed: Clear gaps between the cell wall and the shrunken cell membrane/contents.

Importance of Water Potential in Organisms

• Root Hair Cells: These cells maintain a lower water potential than the soil water (by pumping in ions), ensuring water enters the plant via osmosis.
• Animal Cells: Unlike plants, animal cells lack a cell wall. If placed in pure water (high water potential), they will take in so much water that the cell membrane bursts (lysis). This is why blood plasma concentration must be strictly controlled.

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Key Equations

Percentage Change in Mass This is the most common calculation in Osmosis exam questions.

$$\text{Percentage Change} = \frac{\text{Change in Mass}}{\text{Initial Mass}} \times 100$$

• Change in Mass = Final Mass - Initial Mass
• Units: Percent (%)
• Note: If the final mass is lower than the initial mass, the percentage change will be a negative number (indicating mass loss).

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Common Mistakes to Avoid

• ❌ Wrong: Water moves from a concentrated solution to a dilute solution.
• ✅ Right: Water moves from a dilute solution (high water potential) to a concentrated solution (low water potential).
• ❌ Wrong: Saying the plant cell "bursts" when placed in water.
• ✅ Right: Plant cells become turgid; the cell wall prevents them from bursting. (Only animal cells burst).
• ❌ Wrong: Using "amount" of water.
• ✅ Right: Use "water potential" or "concentration of water molecules."

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Exam Tips

• Command Word - "Explain": If asked to explain why a potato lost mass, you must mention: 1. Water potential gradient (higher inside, lower outside), 2. Direction of movement (out of cell), 3. The process (osmosis), and 4. The membrane (partially permeable).
• Typical Numerical Values: In experiments, concentrations are often given in mol/dm³. 0.0 mol/dm³ is pure water; values like 0.5 to 1.5 mol/dm³ are common for sugar/salt solutions.
• The X-Intercept: In graphs showing percentage change in mass, the point where the line crosses the X-axis (0% change) indicates the concentration of the cell sap inside the tissue.
• Contexts: Expect questions involving Visking tubing, potato cylinders, or red blood cells. Always identify where the water potential is highest first.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

A student investigates the effect of different sugar concentrations on potato tissue. They cut potato cylinders of equal length and mass and place them in five different sugar solutions for 24 hours.

(a) State the name of the process by which water moves into or out of the potato cells. [1]

(b) Describe how the student could ensure that the experiment is a fair test. [2]

(c) Explain why the potato cylinders change mass when placed in the sugar solutions. [2]

Worked Solution:

(a)

1. Osmosis [Name of the process]

How to earn full marks:

• Correctly state "Osmosis"

(b)

1. Use potato cylinders of the same initial length/mass/variety. [Ensures a consistent starting point]
2. Keep the volume of sugar solution the same for each cylinder and at the same temperature. [Ensures the concentration gradient has the same effect]

How to earn full marks:

• Mention same initial length/mass/variety.
• Mention same volume of solution and same temperature.

(c)

1. Water moves by osmosis from a region of high water potential (dilute solution) to a region of low water potential (concentrated solution) through the partially permeable membrane of the potato cells. [Explains the direction of water movement based on water potential gradient]
2. If the sugar solution is more concentrated than the potato cells, water will move out of the cells, causing them to lose mass. If the sugar solution is less concentrated, water will move into the cells, causing them to gain mass. [Explains the relationship between solution concentration and mass change]

How to earn full marks:

• Mention high to low water potential through the cell membrane by osmosis.
• Relate the direction of water movement to the relative concentrations of the sugar solution and potato cells.

Common Pitfall: Many students forget that osmosis is specifically the movement of water molecules. Also, be sure to state that the water moves through the partially permeable membrane.

Exam-Style Question 2 — Extended Response [8 marks]

Question:

A student uses dialysis tubing to model osmosis. They fill a section of dialysis tubing with a 20% sucrose solution and seal the ends. They then place the tubing in a beaker of distilled water. The mass of the tubing is recorded every 10 minutes for 60 minutes. The results are shown in Table 2.1.

Table 2.1

Time (minutes)	Mass of tubing (g)
0	10.0
10	10.8
20	11.4
30	11.8
40	12.0
50	12.1
60	12.1

(a) Describe the trend shown in the results in Table 2.1. [2]

(b) Explain why the mass of the dialysis tubing changes during the experiment. [4]

(c) Predict what would happen to the mass of the dialysis tubing if the experiment was continued for another 60 minutes. Explain your prediction. [2]

Worked Solution:

(a)

1. The mass of the tubing increases over time. [Describes the overall change]
2. The rate of increase decreases over time, eventually levelling off. [Describes the change in the rate of change]

How to earn full marks:

• Mention that the mass increases.
• Mention that the rate of increase decreases, eventually levelling off.

(b)

1. The dialysis tubing is partially permeable, meaning that water molecules can pass through, but sucrose molecules cannot. [States the permeability of the tubing]
2. The water potential inside the dialysis tubing is lower than the water potential outside in the distilled water. [Establishes the water potential gradient]
3. Water moves by osmosis from the region of higher water potential (distilled water) to the region of lower water potential (sucrose solution) through the dialysis tubing. [Explains the direction of water movement based on water potential gradient]
4. This causes the volume of the sucrose solution inside the tubing to increase, hence the mass increases until equilibrium is reached. [Links water movement to mass change]

How to earn full marks:

• Mention partially permeable and that water moves but sucrose does not.
• Correctly identify the relative water potentials inside and outside the tubing.
• Mention through the tubing.
• Link the increase in volume to the increase in mass until equilibrium.

(c)

1. The mass will remain constant. [States the predicted outcome]
2. Because the water potential inside and outside the dialysis tubing will eventually reach equilibrium/become equal, so there will be no further net movement of water. [Explains the prediction based on water potential equilibrium]

How to earn full marks:

• State that the mass will remain constant (or similar).
• Explain that equilibrium is reached, so no net movement of water.

Common Pitfall: Remember that only water moves across the dialysis tubing in this setup. Sucrose molecules are too large to pass through the membrane. Also, the rate of mass increase will slow down as the water potential difference decreases.

Exam-Style Question 3 — Short Answer [6 marks]

Question:

Plant cells rely on osmosis for support.

(a) State the name of the structure in a plant cell that provides support when the cell is turgid. [1]

(b) Define the term turgor pressure. [2]

(c) Describe what happens to a plant cell when it is placed in a concentrated salt solution. Use the term plasmolysis in your answer. [3]

Worked Solution:

(a)

1. Cell wall [Name of the supporting structure]

How to earn full marks:

• Correctly state "Cell wall".

(b)

1. Turgor pressure is the pressure exerted by the water inside the vacuole/cytoplasm [Identifies the source of the pressure]
2. Against the cell wall. [States where the pressure is exerted]

How to earn full marks:

• Mention pressure exerted by the water inside the vacuole/cytoplasm.
• Mention that it is exerted against the cell wall.

(c)

1. The plant cell will lose water by osmosis to the more concentrated salt solution. [Describes water movement]
2. The cytoplasm shrinks and pulls away from the cell wall. [Describes the change in cytoplasm]
3. This process is called plasmolysis. [States the name of the process]

How to earn full marks:

• Mention water loss by osmosis to the salt solution.
• Describe the cytoplasm shrinking and pulling away from the cell wall.
• Use the term plasmolysis.

Common Pitfall: Be precise when defining turgor pressure. It's the pressure exerted by the water inside the vacuole or cytoplasm. When describing plasmolysis, make sure to mention that the cytoplasm pulls away from the cell wall, not just that it shrinks.

Exam-Style Question 4 — Extended Response [10 marks]

Question:

A farmer notices that some of their crops are wilting, even though the soil appears to be moist. They suspect that the soil may have a high salt concentration. They conduct an experiment to investigate the effect of different salt concentrations on plant cells. They take samples of plant tissue and immerse them in solutions with different salt concentrations (0%, 1%, 2%, 3%, and 4%). After 2 hours, they observe the cells under a microscope and classify them as turgid, flaccid, or plasmolysed.

(a) State what is meant by the terms flaccid and turgid when describing plant cells. [2]

(b) Explain why plant cells become plasmolysed in high salt concentrations. [4]

(c) The farmer also wants to investigate the effect of salt concentration on root hair cells. Suggest how the farmer could modify their experiment to observe the changes in root hair cells specifically. [2]

(d) Predict the results the farmer would obtain for the plant tissue at each salt concentration (0%, 1%, 2%, 3%, and 4%). Explain the pattern you predict. [2]

Worked Solution:

(a)

1. Flaccid: The cell is limp and not swollen, with reduced turgor pressure; the cell membrane is not pressed tightly against the cell wall. [Describes a flaccid cell]
2. Turgid: The cell is swollen and firm due to the pressure of the cell contents against the cell wall. [Describes a turgid cell]

How to earn full marks:

• For flaccid, mention it is limp and has reduced turgor pressure; also mention the cell membrane not pressed tightly against the cell wall.
• For turgid, mention it is swollen and firm due to pressure against the cell wall.

(b)

1. High salt concentrations in the surrounding solution create a lower water potential outside the plant cell than inside. [Establishes the water potential gradient]
2. Water moves by osmosis from the region of higher water potential (inside the cell) to the region of lower water potential (outside the cell) through the cell membrane. [Explains the direction of water movement]
3. This causes the cytoplasm to shrink and pull away from the cell wall. [Describes the change in the cytoplasm]
4. Resulting in plasmolysis, where the cell membrane pulls away from the cell wall. [States the outcome]

How to earn full marks:

• Correctly identify the relative water potentials inside and outside the cell.
• Mention water movement by osmosis through the cell membrane.
• Describe the cytoplasm shrinking and pulling away from the cell wall.
• Mention plasmolysis, where the cell membrane pulls away from the cell wall.

(c)

1. Gently scrape the roots to collect root hair cells. [Isolate the cells of interest]
2. Observe the root hair cells specifically under the microscope after immersion in each salt solution, rather than the bulk plant tissue. [Focus on root hair cells during observation]

How to earn full marks:

• Mention scraping the roots to collect the root hair cells.
• Mention observing only the root hair cells under the microscope.

(d)

1. Prediction: 0% - Turgid, 1% - Turgid/Flaccid, 2% - Flaccid, 3% - Plasmolysed, 4% - Plasmolysed. [Predicts the results at each concentration]
2. Explanation: As the salt concentration increases, the water potential outside the cells decreases, causing more water to move out of the cells by osmosis, leading to the cells becoming flaccid and eventually plasmolysed. [Explains the predicted pattern based on osmosis and water potential]

How to earn full marks:

• Correctly predict the general trend (more turgid at low concentrations, more plasmolysed at high concentrations).
• Explain that increasing the salt concentration decreases the water potential outside the cells, causing more water to move out by osmosis.

Common Pitfall: When describing flaccid cells, remember to mention the reduced turgor pressure and that the cell membrane isn't pressed tightly against the cell wall. Also, be sure to link the water potential gradient to the movement of water out of the cell in high salt concentrations.