Osmosis Revision Notes

A partially permeable membrane is a barrier that allows certain small molecules, such as water, to pass through while blocking larger molecules or solutes. In biological systems, the cell membrane acts as a partially permeable membrane, controlling the substances entering and leaving the cell.

This selective permeability is crucial because it enables the cell to regulate its internal environment, maintaining conditions necessary for life. For example, water molecules can move freely, but many dissolved substances cannot cross without specific transport mechanisms.

• Allows passage of water molecules.
• Restricts movement of many solutes.
• Enables selective exchange between cell and environment.

Osmosis is the movement of water molecules through a partially permeable membrane from a region where water concentration is higher (or solute concentration is lower) to a region where water concentration is lower (or solute concentration is higher).

This movement occurs because water tends to move towards the side with more solute particles to balance concentrations on both sides of the membrane. Osmosis continues until equilibrium is reached or until physical pressure prevents further movement.

For example, if a cell is placed in pure water, water will move into the cell where solute concentration is higher. Conversely, if placed in a concentrated salt solution, water will move out of the cell.

• Water moves from low solute concentration to high solute concentration.
• Movement aims to equalise solute concentrations.
• Occurs until equilibrium or opposing pressure is reached.

Osmosis does not require energy input from the cell; it is a passive process driven by the concentration gradient of water molecules. The random movement of water molecules results in net movement towards the area with fewer free water molecules (higher solute concentration).

This distinguishes osmosis from active transport, which requires cellular energy to move substances against their concentration gradients.

• No energy required (passive process).
• Driven by concentration gradient of water.
• Results from random molecular motion.

Osmosis is vital for maintaining cell turgidity and volume. In plant cells, water entering by osmosis fills the central vacuole, creating turgor pressure that supports the plant structure.

In animal cells, osmosis regulates fluid balance. Cells can shrink or swell depending on the surrounding solution's concentration, affecting cell function.

For example, red blood cells placed in distilled water swell and may burst due to water intake by osmosis, whereas in concentrated salt solutions they shrink as water leaves the cell.

• Maintains plant cell turgor pressure.
• Regulates animal cell volume and fluid balance.
• Essential for nutrient and waste transport.

Consider a U-shaped tube divided by a partially permeable membrane. One side contains pure water, and the other contains a sugar solution. Over time, water moves from the pure water side to the sugar solution side, causing the liquid level to rise on the sugar side.

This demonstrates water moving from low solute concentration to high solute concentration through the membrane, illustrating osmosis in action.

• Water moves towards sugar solution side.
• Liquid level rises on concentrated side.
• Shows osmosis driven by solute concentration difference.

Cells are enclosed by a cell membrane, which acts as a selective barrier controlling what substances enter and leave the cell. This membrane is partially permeable, meaning it allows certain molecules, such as water, to pass through while blocking others like large solutes or ions.

Water molecules can move freely through the membrane, but many dissolved substances cannot. This selective permeability is essential for maintaining the internal environment of the cell, known as homeostasis.

• Cell membrane is partially permeable.
• Allows water molecules to pass but restricts many solutes.
• Maintains internal cell conditions.

Osmosis is the movement of water molecules from an area where water is more concentrated (or where solute concentration is lower) to an area where water is less concentrated (or solute concentration is higher) through a partially permeable membrane.

In cells, osmosis occurs when there is a difference in water concentration between the inside of the cell and the surrounding fluid. Water moves to balance these concentrations, moving either into or out of the cell depending on the relative concentrations.

For example, if a cell is placed in pure water (high water concentration outside), water will move into the cell. Conversely, if the cell is in a concentrated salt solution (low water concentration outside), water will move out of the cell.

• Water moves from high to low water concentration.
• Occurs through the partially permeable cell membrane.
• Direction depends on relative water concentrations inside and outside the cell.

Water concentration can be thought of in terms of water potential, which is higher in dilute solutions and lower in concentrated solutions. Osmosis moves water from regions of higher water potential to lower water potential.

This movement continues until equilibrium is reached or until physical constraints prevent further movement, such as the cell wall in plant cells.

Understanding water potential helps predict the direction of water movement in different environments.

• Water moves from higher to lower water potential.
• Water potential relates inversely to solute concentration.
• Equilibrium or physical limits stop osmosis.

Osmosis is vital for maintaining cell turgor, which is the pressure of the cell contents against the cell wall in plant cells. This pressure keeps plant cells firm and supports the plant structure.

In animal cells, osmosis helps regulate the balance of fluids, ensuring cells neither swell excessively nor shrink.

Disruption of normal osmosis can cause cells to become flaccid or plasmolysed in plants, or to burst or shrivel in animals, affecting their function and survival.

• Maintains turgor pressure in plant cells.
• Regulates fluid balance in animal cells.
• Essential for cell survival and function.

Consider a raisin placed in water. The raisin’s skin acts like a partially permeable membrane. Water moves into the raisin by osmosis because the inside of the raisin has a higher solute concentration than the surrounding water.

As water enters, the raisin swells and becomes plumper. This demonstrates water moving through a membrane from a dilute solution to a concentrated one.

This example illustrates osmosis in a familiar context, showing how water moves to balance concentrations.

• Raisin skin acts as a partially permeable membrane.
• Water moves into raisin due to higher solute concentration inside.
• Raisin swells as a result of osmosis.

Osmosis is the passive movement of water molecules through a partially permeable membrane from a region of higher water potential (more dilute solution) to a region of lower water potential (more concentrated solution). In plant cells, the cell membrane acts as the partially permeable membrane, allowing water to move in and out depending on the external solution concentration.

When plant tissues are placed in solutions with different concentrations, water moves to balance the concentration gradient. This movement affects the volume and pressure inside the cells, leading to observable physical changes.

For example, if a plant tissue is placed in a hypotonic solution (lower solute concentration than the cell sap), water enters the cells, causing them to swell and become turgid. Conversely, in a hypertonic solution (higher solute concentration than the cell sap), water leaves the cells, causing them to shrink and the tissue to become flaccid or plasmolysed.

• Osmosis moves water from dilute to concentrated solutions.
• Plant cell membranes are partially permeable.
• Water movement changes cell volume and pressure.

To investigate how different solution concentrations affect plant tissues, a common approach is to use potato tuber cylinders or strips. These tissues have uniform cells and are easy to handle.

Apparatus needed includes: potato samples, distilled water, sucrose solutions of varying concentrations (e.g., 0.0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M), a balance for measuring mass, a ruler for measuring length, beakers, and a timer.

The independent variable is the concentration of the sucrose solution. The dependent variable is the change in mass or length of the potato tissue. Control variables include the size and shape of the potato pieces, temperature, and duration of the experiment.

• Use potato cylinders for uniformity.
• Prepare sucrose solutions of known concentrations.
• Measure initial and final mass or length of samples.
• Control variables to ensure fair testing.

1. Cut potato cylinders of equal size and measure their initial mass using a balance.

2. Prepare sucrose solutions of different concentrations in separate beakers.

3. Place each potato cylinder into a different sucrose solution and leave them for a fixed time, typically 30 minutes, at room temperature to allow osmosis to occur without other metabolic changes interfering significantly.

• Measure initial mass of potato pieces.
• Immerse in solutions of varying concentration.
• Leave for a fixed time period.
• Remove, blot dry, and measure final mass.

After the incubation period, remove the potato samples, gently blot them to remove surface solution, and measure their final mass.

Calculate the percentage change in mass using the formula:

Percentage change in mass = ((Final mass - Initial mass) / Initial mass) × 100% For example, if the initial mass is 5.00 g and the final mass is 5.50 g, then: Percentage change = ((5.50 - 5.00) / 5.00) × 100% = 10% increase in mass, indicating water uptake due to osmosis in a hypotonic solution.

• Calculate percentage mass change for each concentration.
• Plot a graph of percentage mass change against solution concentration.
• Identify the concentration where there is no change in mass (isotonic point).

In hypotonic solutions (low solute concentration), water enters the cells, increasing mass and causing cells to become turgid. Turgor pressure supports plant structure.

In isotonic solutions, there is no net water movement; mass remains constant.

In hypertonic solutions (high solute concentration), water leaves the cells, decreasing mass and causing plasmolysis where the cell membrane pulls away from the cell wall, making the tissue flaccid and limp, which can damage the plant if prolonged.

• Turgid cells have high turgor pressure, supporting plants.
• Plasmolysis occurs in hypertonic solutions, damaging cells.
• Isotonic solutions cause no net water movement.

Water potential is a measure of the tendency of water molecules to move from one area to another. Pure water has the highest water potential, and adding solutes lowers the water potential because solute molecules bind water molecules, reducing their free movement.

In biological systems, solutions with fewer solutes are described as dilute and have higher water potential, while solutions with more solutes are concentrated and have lower water potential.

Water moves from regions of higher water potential (dilute solutions) to regions of lower water potential (concentrated solutions) to balance solute concentrations across membranes.

• Water potential decreases as solute concentration increases.
• Water moves down the water potential gradient during osmosis.

A partially permeable membrane allows only certain molecules, such as water, to pass through while blocking larger solute molecules.

This selective permeability is crucial because it enables water to move freely while preventing solutes from moving, creating a difference in water potential across the membrane.

Examples include the cell membrane in plant and animal cells, which controls the internal environment by regulating water movement.

• Membranes allow water but not most solutes to pass.
• Selective permeability creates conditions for osmosis.

Osmosis occurs when water molecules move from the side with higher water potential (more free water molecules) to the side with lower water potential (fewer free water molecules) through a partially permeable membrane.

This movement continues until equilibrium is reached, meaning the water potential is equal on both sides or until physical constraints prevent further movement.

For example, if a plant cell is placed in a dilute sugar solution, water will enter the cell because the cell sap is more concentrated, lowering its water potential compared to the surrounding solution.

• Water moves to balance solute concentrations.
• Osmosis is a passive process requiring no energy.
• Equilibrium stops net water movement.

Osmosis maintains cell turgor pressure, which keeps plant cells firm and supports the plant structure.

In animal cells, osmosis regulates the balance of fluids inside and outside cells, affecting cell volume and function.

Disruptions in osmosis can cause cells to become turgid (swollen) or plasmolysed (shrunken), affecting their survival and activity.

• Osmosis supports plant rigidity through turgor pressure.
• It controls fluid balance in animal cells.
• Proper osmosis is essential for cell health and function.

If potato cylinders are placed in solutions of different concentrations, water will move by osmosis either into or out of the potato cells depending on the water potential gradient.

In pure water (high water potential), water enters the potato cells, making them swell and increase in mass.

In concentrated sugar solutions (low water potential), water leaves the potato cells, causing them to shrink and lose mass.

• Mass change in potato indicates direction of osmosis.
• Higher water potential outside causes water uptake.
• Lower water potential outside causes water loss.

Osmosis is the movement of water molecules across a partially permeable membrane from a region of higher water potential (more dilute solution) to a region of lower water potential (more concentrated solution). In plant cells, the cell membrane acts as the partially permeable membrane, allowing water to move in or out depending on the surrounding solution concentration. This movement affects the cell's volume and internal pressure.

When a plant cell is placed in a hypotonic solution, the external solution has a lower concentration of solutes than the cell sap inside the vacuole. Water moves into the cell by osmosis, increasing the volume of the cytoplasm and vacuole. As the cell swells, the rigid cell wall resists further expansion, creating turgor pressure — the pressure of the cell contents against the cell wall.

This pressure supports the plant structurally and keeps cells firm. A cell in this state is described as turgid. For example, when a plant root cell is in pure water, it becomes turgid, helping the plant maintain upright posture.

• Water moves into the cell due to higher water potential outside.
• Cell swells and presses against the cell wall.
• Turgor pressure builds up, preventing bursting.
• Cell is described as turgid.

In an isotonic solution, the concentration of solutes outside the cell is equal to that inside the cell sap. There is no net movement of water into or out of the cell because the water potential is balanced.

Without water entering, the cell does not swell and remains in a relaxed state. The cell membrane is not pressing tightly against the cell wall, so the cell is described as flaccid. This state can cause the plant to wilt if many cells become flaccid.

• Equal solute concentration inside and outside the cell.
• No net water movement by osmosis.
• Cell volume remains constant.
• Cell is flaccid, not firm.

When a plant cell is placed in a hypertonic solution, the external solution has a higher concentration of solutes than the cell sap. Water moves out of the cell by osmosis, causing the cytoplasm and vacuole to shrink.

As water leaves, the cell membrane pulls away from the cell wall in a process called plasmolysis. This can damage the cell and is often irreversible if prolonged. For example, placing a plant cell in a concentrated salt solution causes plasmolysis, leading to wilting and cell damage.

• Water moves out of the cell due to lower water potential outside.
• Cell contents shrink and membrane detaches from cell wall.
• Process is called plasmolysis.
• Cell becomes plasmolysed and loses firmness.

The response of plant cells to different solution concentrations depends on the direction of water movement by osmosis. Hypotonic solutions cause cells to become turgid with high turgor pressure, isotonic solutions result in flaccid cells with no net water movement, and hypertonic solutions cause plasmolysis due to water loss.

These changes affect plant rigidity and health, making understanding osmosis essential in biology and agriculture.

• Turgid cells support plant structure.
• Flaccid cells may cause wilting.
• Plasmolysis can damage or kill cells.