Osmosis

What you'll learn

Osmosis is a specific type of diffusion involving water molecules moving across partially permeable membranes. This topic is fundamental to understanding how cells regulate water balance and maintain their structure. You'll learn to explain osmosis, predict water movement in different solutions, and apply your knowledge to practical investigations that frequently appear in AQA GCSE Biology exams.

Key terms and definitions

Osmosis — the diffusion of water molecules from a dilute solution (high water concentration) to a concentrated solution (low water concentration) through a partially permeable membrane

Partially permeable membrane — a membrane that allows only certain molecules to pass through it, such as water molecules, but not larger molecules like sugar or protein

Dilute solution — a solution with a low concentration of solute and a high concentration of water molecules

Concentrated solution — a solution with a high concentration of solute and a low concentration of water molecules

Isotonic — when two solutions have the same concentration of solutes, resulting in no net water movement by osmosis

Hypertonic — a solution that has a higher concentration of solutes (lower water concentration) compared to another solution

Hypotonic — a solution that has a lower concentration of solutes (higher water concentration) compared to another solution

Turgor pressure — the pressure exerted by water inside plant cells pushing the cell membrane against the cell wall, keeping the plant firm and upright

Core concepts

Understanding osmosis as a special type of diffusion

Osmosis is a passive process, meaning it does not require energy from respiration. Water molecules move randomly due to their kinetic energy, but overall movement occurs from areas of higher water concentration to areas of lower water concentration.

The key difference between osmosis and diffusion is specificity:

• Diffusion involves any particles moving down their concentration gradient
• Osmosis specifically involves water molecules moving across a partially permeable membrane

The partially permeable membrane is critical. Cell membranes act as partially permeable barriers, allowing water to pass through but preventing larger solute molecules from moving freely. This selective permeability enables cells to control their internal environment.

Water molecules are small enough to pass through gaps in the phospholipid bilayer or through specialized protein channels called aquaporins. Larger molecules like glucose, salt ions, and proteins cannot pass through as easily, creating concentration gradients that drive osmosis.

Osmosis in animal cells

Animal cells lack cell walls, making them vulnerable to changes in water content. The cell membrane alone provides limited structural support.

When an animal cell is placed in different solutions:

In a hypotonic solution (dilute):

• Water concentration outside the cell is higher than inside
• Water moves into the cell by osmosis
• The cell swells and may burst (lysis)
• Red blood cells are particularly susceptible to bursting

In an isotonic solution:

• Water concentration is equal inside and outside the cell
• Water moves in and out at equal rates
• No net water movement occurs
• The cell remains the same size
• This is the ideal state for animal cells

In a hypertonic solution (concentrated):

• Water concentration outside the cell is lower than inside
• Water moves out of the cell by osmosis
• The cell shrinks and becomes shrivelled (crenation)
• Cell function is impaired

This is why maintaining isotonic conditions is crucial for animal cells. Blood plasma, for example, must be carefully regulated to prevent red blood cells from bursting or shrivelling.

Osmosis in plant cells

Plant cells behave differently from animal cells due to their rigid cell walls, which provide structural support and prevent bursting.

In a hypotonic solution (dilute):

• Water moves into the cell by osmosis
• The vacuole expands, pushing the cytoplasm against the cell wall
• The cell becomes turgid (firm and swollen)
• Turgor pressure builds up as the cell wall resists further expansion
• Eventually, the cell wall prevents more water entering
• Turgid cells are essential for plant support — they keep stems upright and leaves spread out

In an isotonic solution:

• Water moves in and out at equal rates
• The cell is between turgid and flaccid
• Some turgor pressure exists but the cell is not at maximum firmness

In a hypertonic solution (concentrated):

• Water moves out of the cell by osmosis
• The vacuole shrinks
• The cytoplasm pulls away from the cell wall
• This process is called plasmolysis
• The cell becomes flaccid (limp)
• Plants wilt when too many cells become plasmolysed

The cell wall prevents plant cells from bursting in dilute solutions, which is why plants can absorb water from soil without their cells rupturing.

Water potential and concentration gradients

Though water potential is more commonly taught at A-level, GCSE students should understand that:

• Water moves from high water concentration to low water concentration
• Adding solute to water decreases the concentration of water molecules
• Pure water has the highest possible water concentration
• The greater the concentration difference, the faster osmosis occurs

In practical terms:

• A dilute sugar solution has more water molecules per unit volume than a concentrated sugar solution
• Water will move from the dilute solution toward the concentrated solution
• This continues until concentrations equalize (equilibrium is reached)

Osmosis investigations — required practical

AQA GCSE Biology includes a required practical investigating osmosis using plant tissue. You must be able to describe, explain, and evaluate this investigation.

Standard method:

1. Cut equal-sized chips from a potato or other plant tissue
2. Measure and record the initial mass (or length) of each chip
3. Place each chip in a different concentration of sugar or salt solution
4. Leave for a set time (typically 30 minutes to 24 hours)
5. Remove chips, blot dry to remove surface water
6. Measure and record the final mass (or length)
7. Calculate percentage change in mass
8. Plot a graph of percentage change against solution concentration

Calculating percentage change:

Percentage change = ((final mass - initial mass) ÷ initial mass) × 100

Key variables:

• Independent variable: concentration of solution
• Dependent variable: change in mass or length
• Control variables: volume of solution, time, temperature, size of chips, type of plant tissue

What the results show:

• In dilute solutions: chips gain mass (water enters by osmosis)
• In concentrated solutions: chips lose mass (water leaves by osmosis)
• At a specific concentration: no change in mass (isotonic point)
• The isotonic point indicates the concentration of the cell sap inside the plant cells

Sources of error and improvements:

• Chips not fully dried — leads to falsely high final mass
• Uneven chip sizes — affects surface area to volume ratio and rate of osmosis
• Temperature changes — affects rate of osmosis (higher temperature increases kinetic energy)
• Evaporation from solutions — changes concentration over time
• Damaged cells on cut surfaces — may leak cell contents

Osmosis in biological systems

Understanding osmosis helps explain many biological phenomena:

In the digestive system:

• Water is absorbed from the small intestine into the bloodstream by osmosis
• The intestinal lining maintains concentration gradients to facilitate water absorption
• After nutrients are absorbed, water follows by osmosis

In plant roots:

• Root hair cells absorb water from soil by osmosis
• Soil water is usually more dilute than the cell sap inside root cells
• Water moves through root cells toward xylem vessels
• This process requires the soil to be more dilute than the plant cells

In kidneys:

• The kidney regulates blood water content through osmosis
• Water is reabsorbed from kidney tubules back into the blood
• Antidiuretic hormone (ADH) controls the permeability of kidney tubules to water

Preserving food:

• Adding salt or sugar to food removes water from bacterial and fungal cells by osmosis
• Microorganisms become plasmolysed and cannot function
• This is why jam (high sugar), salted fish, and pickles last longer

Worked examples

Example 1: Describing osmosis (3 marks)

Question: A student places a piece of Visking tubing (a partially permeable membrane) containing sugar solution into a beaker of pure water. Describe and explain what will happen to the mass of the Visking tubing.

Answer: The mass will increase (1 mark) because water molecules move from the beaker into the Visking tubing (1 mark) by osmosis, from a region of higher water concentration to lower water concentration through the partially permeable membrane (1 mark).

Mark scheme notes:

• One mark for stating mass increases
• One mark for identifying direction of water movement
• One mark for explaining osmosis in terms of concentration gradient and partially permeable membrane

Example 2: Calculating percentage change (4 marks)

Question: A student investigates osmosis in potato chips. A potato chip with an initial mass of 2.5 g is placed in salt solution for 30 minutes. The final mass is 2.1 g. Calculate the percentage change in mass. Show your working.

Answer: Change in mass = 2.1 - 2.5 = -0.4 g (1 mark)

Percentage change = (-0.4 ÷ 2.5) × 100 (1 mark)

= -16% (1 mark)

The negative value indicates the potato chip lost mass / water moved out by osmosis (1 mark).

Mark scheme notes:

• Correct calculation of change in mass
• Correct formula applied
• Correct numerical answer with unit (%)
• Interpretation of negative value

Example 3: Evaluating an osmosis investigation (6 marks)

Question: A student investigated osmosis by placing potato chips in different concentrations of sugar solution. The student measured only length, not mass. Evaluate this method and suggest improvements.

Answer: Measuring length is less accurate than measuring mass (1 mark) because length only measures one dimension whereas mass reflects the total water content (1 mark). The potato chip may swell unevenly, making length measurements inconsistent (1 mark).

The student could improve the method by measuring mass instead of length (1 mark) and by ensuring all surface water is removed by blotting with paper towels before measuring (1 mark). Using multiple potato chips at each concentration and calculating a mean would improve reliability (1 mark).

Mark scheme notes:

• Identification of limitation
• Explanation of why it's a limitation
• Suggestion for improvement with justification
• Additional improvements for reliability

Common mistakes and how to avoid them

• Confusing osmosis with diffusion — Always specify that osmosis involves water molecules moving through a partially permeable membrane. Diffusion is the general term for any particle movement down a concentration gradient.

• Getting the direction of water movement wrong — Water moves from dilute to concentrated, or from high water concentration to low water concentration. Remember: water moves toward the solute. Don't say "water moves from low to high concentration" without specifying it's solute concentration that's high.

• Forgetting the partially permeable membrane — Osmosis only occurs across a partially permeable membrane. Without mentioning this, your answer is describing diffusion, not osmosis.

• Misunderstanding turgid and flaccid — Turgid cells are swollen and firm (in dilute solutions), flaccid cells are limp (in concentrated solutions). Don't confuse turgid with bursting — plant cells become turgid but don't burst due to the cell wall.

• Not removing surface water in practicals — When measuring mass after osmosis experiments, always blot chips dry. Surface water is not water that entered by osmosis and will falsely increase the measured mass.

• Incorrect percentage change calculations — Always use the initial mass as the denominator. The formula is: ((final - initial) ÷ initial) × 100. A negative answer means mass decreased; positive means mass increased.

Exam technique for "Osmosis"

• "Describe" questions require you to state what happens but not why. For example: "Water moves into the cell and it swells." You don't need to explain the concentration gradient unless asked to explain.

• "Explain" questions require mechanisms and reasons. Always mention: water molecules, partially permeable membrane, concentration gradient (high to low water concentration), and the direction of movement. A full explanation typically requires all these components for full marks.

• Command words matter — "Define osmosis" needs a precise definition including water, concentration gradient, and partially permeable membrane (usually 2-3 marks). "Describe what happens" needs observations, not explanations.

• Practical questions often ask about variables, improvements, and calculations. Know how to calculate percentage change, identify control variables, and suggest realistic improvements like using more replicates or controlling temperature. Each practical skill is typically worth 1 mark, so give multiple improvements if asked.

Quick revision summary

Osmosis is the movement of water molecules from high to low water concentration through a partially permeable membrane. Water moves from dilute solutions to concentrated solutions. In animal cells, osmosis can cause bursting in dilute solutions or shrinking in concentrated solutions. Plant cells become turgid in dilute solutions and plasmolysed in concentrated solutions; the cell wall prevents bursting. In required practicals, calculate percentage change in mass using: ((final - initial) ÷ initial) × 100. Always identify the isotonic point where no net water movement occurs.