Osmosis Revision Notes

Water molecules are polar, meaning they have positive and negative ends. This polarity allows water to surround and dissolve many different substances, including salts, sugars, gases, and waste products.

Because of this, water is often called a universal solvent in living organisms, enabling essential biological processes to occur efficiently.

• Water's polarity allows it to dissolve a wide range of substances.
• It forms hydrogen bonds with other polar molecules, aiding dissolution.

During digestion, water dissolves food molecules, breaking them into smaller particles that enzymes can act upon more effectively.

Dissolving food increases the surface area exposed to enzymes, speeding up chemical breakdown and nutrient absorption.

• Water dissolves food molecules to facilitate enzyme action.
• Enzymes only work on dissolved substances.

Water dissolves waste products such as urea, allowing them to be transported in urine and removed from the body.

In animals, water carries dissolved nutrients, oxygen, and carbon dioxide in the blood to and from cells.

In plants, water transports dissolved minerals and sugars through the sap in xylem and phloem vessels.

• Water dissolves wastes for excretion.
• Water transports dissolved substances in blood and plant sap.
• This transport supports cellular metabolism and homeostasis.

Osmosis is the movement of water molecules through a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.

A partially permeable membrane allows water molecules to pass but blocks many solutes, enabling selective movement of water.

Osmosis occurs to balance water concentrations on both sides of the membrane.

• Water moves only, solutes do not cross during osmosis.
• Membrane is selectively permeable to water.
• Movement is passive, requiring no energy.

Water moves down its concentration gradient, from where water molecules are more abundant to where they are less abundant.

This process helps cells maintain their water balance, preventing them from swelling or shrinking excessively.

Osmosis is essential for many biological functions, including nutrient absorption and waste removal.

• Water moves from high to low water concentration.
• Osmosis maintains cell volume and function.
• It is a passive process driven by concentration differences.

Partially permeable membranes allow water molecules to pass freely but restrict larger molecules or ions.

This selective permeability is crucial for controlling water movement and maintaining cellular homeostasis.

• Membrane blocks many solutes but allows water.
• Selective permeability enables osmosis.
• Membrane controls internal environment of cells.

The cell membrane is selectively permeable, allowing water molecules to pass but restricting many solutes.

Water moves through the membrane by osmosis, from areas of higher water concentration (less solute) to lower water concentration (more solute).

This movement is passive and does not require energy.

• Cell membrane controls water movement.
• Water moves down its concentration gradient by osmosis.
• Osmosis is passive and energy-free.

If the fluid outside the cell has a higher water concentration than inside, water enters the cell, causing it to swell.

If the outside fluid has a lower water concentration, water leaves the cell, causing it to shrink.

Maintaining proper water balance is vital for cell survival and function.

• Water entering cells causes swelling.
• Water leaving cells causes shrinking.
• Osmosis helps maintain cell volume and health.

Osmosis regulates water content, preventing cells from bursting or becoming dehydrated.

It supports nutrient uptake and waste removal by controlling water flow.

Understanding osmosis explains how cells interact with their environment.

• Osmosis maintains cell water balance.
• It supports essential cellular processes.
• Cell membrane selectively regulates water movement.

Dialysis tubing acts as an artificial selectively permeable membrane, allowing water molecules to pass through while blocking larger solute molecules such as sugar.

When the tubing is filled with a sugar solution and placed in distilled water, water moves by osmosis from the area of lower solute concentration (outside) to higher solute concentration (inside).

This movement causes the tubing to swell and increases its mass or volume, demonstrating osmosis in action.

• Dialysis tubing mimics a cell membrane's selective permeability.
• Water moves into tubing if internal sugar concentration is higher.
• Osmosis causes swelling and mass increase of the tubing.

The independent variable is the sugar concentration inside the dialysis tubing, which is deliberately changed to observe its effect.

The dependent variable is the change in mass or volume of the dialysis tubing after immersion in distilled water, reflecting water movement.

Control variables such as temperature, duration of the experiment, and volume of solutions must be kept constant to ensure a fair test.

Accurate measurement of mass before and after the experiment using a balance is essential to quantify osmosis.

Repeating the experiment multiple times improves reliability by reducing random errors.

• Independent variable: sugar concentration inside tubing.
• Dependent variable: change in mass or volume of tubing.
• Control variables: temperature, time, solution volume.
• Use precise balances for measurement.
• Repeat trials to confirm results.

Handle glassware and solutions carefully to avoid spills and injury during the experiment.

Ensure the dialysis tubing ends are securely tied to prevent leaks of the sugar solution.

Blot the tubing dry before weighing to avoid excess water affecting mass measurements.

Conduct the experiment at room temperature to maintain consistent conditions and control variables.

Record all measurements carefully and maintain consistent timing for each trial.

• Handle equipment carefully to prevent accidents.
• Secure tubing ends to avoid leaks.
• Dry tubing before weighing for accuracy.
• Maintain consistent temperature and timing.

When plant tissues like potato cylinders are placed in solutions with different solute concentrations, water moves across their cell membranes by osmosis.

If the external solution is hypertonic (more concentrated than cell sap), water leaves the cells, causing the tissue to lose mass and become flaccid.

If the solution is hypotonic (less concentrated), water enters the cells, increasing mass and causing swelling.

In isotonic solutions, solute concentrations are equal inside and outside, so there is no net water movement and mass remains stable.

• Water moves by osmosis depending on solution concentration.
• Hypertonic solution causes water loss and mass decrease.
• Hypotonic solution causes water gain and mass increase.
• Isotonic solution causes no net water movement.

The independent variable is the concentration of the solution in which the plant tissue is immersed.

The dependent variable is the change in mass of the plant tissue after immersion.

Control variables include size and type of tissue, temperature, and immersion time to ensure fair testing.

Percentage change in mass is calculated using: ((Final mass - Initial mass) ÷ Initial mass) × 100.

Repeating the experiment improves reliability and identifies anomalies.

• Independent variable: solution concentration.
• Dependent variable: tissue mass change.
• Control variables: tissue size, temperature, time.
• Calculate percentage mass change.
• Repeat for reliable results.

Use a sharp knife carefully, cutting away from yourself to avoid injury.

Handle solutions and equipment carefully to prevent spills and accidents.

Blot plant tissues dry before weighing to avoid excess water affecting mass measurements.

Maintain consistent temperature and timing during the experiment.

• Handle sharp tools with care.
• Avoid spills and contamination.
• Dry tissues before weighing.
• Keep conditions consistent.

Plant cells contain water within their central vacuole and cytoplasm. This water creates an internal pressure called turgor pressure, which pushes the cell membrane against the rigid cell wall.

The cell wall is strong and does not stretch easily, so the pressure from inside keeps the cell firm and helps the plant maintain its shape.

• Water inside cells creates turgor pressure.
• Turgor pressure presses the cell membrane against the cell wall.
• This pressure supports the plant's structure and keeps it upright.

When a plant loses water, the amount of water inside its cells decreases, reducing turgor pressure.

Lower turgor pressure means the cells become less firm, causing the plant to wilt and lose its upright shape.

• Loss of water reduces turgor pressure.
• Reduced pressure causes cells to become flaccid.
• Plants wilt when turgor pressure drops.

Maintaining adequate water levels inside cells is essential for plants to stay firm and healthy.

Watering plants helps maintain turgor pressure, preventing wilting and supporting growth.

• Water is essential to maintain turgor pressure.
• Turgor pressure keeps plants firm and upright.
• Proper watering prevents wilting.

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

Each part of the definition can earn credit, so include the particles, direction and membrane.

• Particle: water molecules.
• Overall direction: higher water potential to lower water potential.
• Barrier: a partially permeable membrane.
• Movement: net movement.

Water molecules move randomly in both directions through the membrane. Net movement describes the overall result: more water molecules move in one direction than in the other.

The overall direction is towards the region with the lower water potential.

A partially permeable membrane allows some small molecules, including water, to pass through but prevents some other particles from crossing.

Without the membrane condition, the process described is not a complete definition of osmosis.

When two regions are labelled, compare their water potentials before writing the direction.

• Step 1: identify the region with the higher water potential.
• Step 2: identify the region with the lower water potential.
• Step 3: draw or state the direction from higher to lower water potential.
• Step 4: mention that movement is through a partially permeable membrane.

When a plant cell is placed in a dilute (hypotonic) solution, water enters the cell by osmosis because the solution has a higher water potential than the cell sap.

Water entering the cell fills the central vacuole, increasing turgor pressure, which pushes the cytoplasm against the cell wall, making the cell turgid and firm.

• Water moves into the cell by osmosis in dilute solutions.
• Turgor pressure increases, making cells firm (turgid).
• Turgid cells support the plant structure.

In a concentrated (hypertonic) solution, water moves out of the plant cell by osmosis because the external solution has lower water potential.

Loss of water causes the vacuole and cytoplasm to shrink, and the cell membrane pulls away from the cell wall, a process called plasmolysis.

The cell becomes flaccid and limp, reducing support and causing the plant to wilt.

• Water leaves the cell in concentrated solutions.
• Plasmolysis is the shrinking of the cell membrane away from the cell wall.
• Cells become flaccid and plants wilt.

In an isotonic solution, the concentration of solutes inside and outside the cell is equal.

There is no net movement of water, so the cell remains in its normal state, neither turgid nor plasmolysed.

• No net water movement in isotonic solutions.
• Cells maintain normal turgor pressure.
• Plant cells remain stable in isotonic conditions.

Water potential measures the potential energy of water in a system and determines the direction water moves.

Water moves from areas of higher water potential to areas of lower water potential.

This movement balances solute concentrations and water distribution.

• Water potential indicates water's tendency to move.
• Water flows from higher to lower water potential.
• Water potential differences drive osmosis.

Osmosis is the diffusion of water across a partially permeable membrane from a dilute solution (higher water potential) to a concentrated solution (lower water potential).

Partially permeable membranes allow water to pass but block many solutes.

Osmosis helps cells regulate their water content and maintain turgidity.

• Osmosis moves water across membranes.
• Water moves from dilute to concentrated solutions.
• Membranes are selectively permeable.

Cells absorb water when the external environment has higher water potential, keeping cells turgid and functional.

Cells lose water when the environment has lower water potential, which can cause plasmolysis or shrinkage.

Maintaining water balance through osmosis is vital for nutrient transport, cell shape, and survival.

• Osmosis controls water absorption and loss.
• Water balance is essential for cell health.
• Organisms adapt to regulate water potential and osmosis.