1.7 Osmosis and Osmosis in Plant Cells

A free GCSE Biology lesson for AQA (8461) from OBA School: what osmosis is, and how it affects animal and plant cells in solutions of different concentrations. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Define osmosis using the idea of a partially permeable membrane
• Use the terms isotonic, hypertonic and hypotonic
• Describe and explain what happens to animal cells and plant cells in different solutions
• Calculate the percentage change in mass of a tissue sample

Key words

• Osmosis: The diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane.
• Partially permeable membrane: A membrane that lets water molecules through but not larger solute molecules such as sugar.
• Isotonic: A solution with the same concentration of solutes as the cell contents. No net movement of water.
• Hypertonic: A solution more concentrated than the cell contents. The cell loses water by osmosis.
• Hypotonic: A solution more dilute than the cell contents. The cell gains water by osmosis.
• Turgor: The pressure of the cell contents pushing against the cell wall. It keeps plant cells firm (turgid).

What is osmosis?

Osmosis is a special case of diffusion. It is about the movement of water only. A dilute solution has lots of water molecules (a high water concentration). A concentrated solution has fewer. Water moves by osmosis from the dilute solution to the concentrated solution, across a partially permeable membrane such as a cell membrane.

Key fact: Always talk about the concentration of the solution (or of the solutes). Water moves from a dilute solution to a more concentrated solution. Like diffusion, osmosis is passive and needs no energy.

Osmosis in animal cells

Animal cells have no cell wall, so they are easily damaged by osmosis. The body keeps the concentration of the blood plasma and tissue fluid steady so that cells stay in an isotonic solution.

• Hypotonic (e.g. pure water); Movement of water into the cell; What happens to a red blood cell swells and may burst (lysis)
• Isotonic; Movement of water no net movement; What happens to a red blood cell stays the same, works normally
• Hypertonic (e.g. strong salt solution); Movement of water out of the cell; What happens to a red blood cell shrinks and becomes wrinkled (crenated)

Osmosis in plant cells

Plant cells have a strong cellulose cell wall. When water moves in, the vacuole swells and the cytoplasm pushes against the wall, but the wall stops the cell bursting. This firmness is called turgor. Turgid cells hold stems upright and keep leaves spread out to the light.

• If a plant loses more water than it takes in, its cells become flaccid (soft) and the plant wilts.
• In a very concentrated solution, so much water leaves that the cell membrane pulls away from the cell wall. The cell is plasmolysed. This usually kills the cell.

Measuring osmosis: percentage change in mass

Pieces of plant tissue (such as potato) gain mass in a hypotonic solution and lose mass in a hypertonic one. Pieces start at different masses, so we compare the percentage change.

percentage change in mass = (final mass − starting mass) ÷ starting mass × 100

A positive answer is a gain in mass; a negative answer is a loss.

Worked example: percentage change

A potato cylinder has a mass of 3.20 g. After 30 minutes in a strong sugar solution its mass is 2.72 g.
Change = 2.72 − 3.20 = −0.48 g
Percentage change = −0.48 ÷ 3.20 × 100 = −15%

Answer: −15% (a loss in mass, so water left the cells by osmosis). You will do this as a required practical in section 1.11.

Common mistakes

• Saying water moves from a “high concentration to a low concentration” without saying of what. Say “from a dilute solution to a more concentrated solution”.
• Saying sugar or salt moves by osmosis. Osmosis is the movement of water only.
• Saying plant cells burst in pure water. The cell wall stops them bursting; they become turgid.
• Forgetting the minus sign for a loss in mass, or dividing by the final mass instead of the starting mass.

Revision questions

Try these, then check your answers below each one.

Question 1. Which is the correct definition of osmosis? (Choose from: Movement of any particles from high to low concentration, Diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane, Movement of water from a concentrated solution to a dilute solution, Movement of sugar across a membrane using energy.)

Answer: Diffusion of water from a dilute solution to a more concentrated solution through a partially permeable membrane

Working: Osmosis is the diffusion of water from a dilute solution to a more concentrated one, through a partially permeable membrane.

Question 2. True or false? In osmosis, water moves from a concentrated solution to a dilute solution.

Answer: False

Working: False. Water moves from a dilute solution (more water) to a more concentrated solution (less water).

Question 3. A potato cylinder has a mass of 2.50 g. After an hour in pure water its mass is 2.80 g. Calculate the percentage change in mass.

Answer: 12 %

Working: (2.80 − 2.50) ÷ 2.50 × 100 = 0.30 ÷ 2.50 × 100 = +12%.

Related lessons

• Previous lesson: 1.6 Diffusion
• Next lesson: 1.9 Active Transport
• In this chapter: 1.1 The World of the Microscope
• In this chapter: 1.2 Animal and Plant Cells
• In this chapter: 1.3 Eukaryotic and Prokaryotic Cells

Continue in the full course

This lesson is section 1.7 (AGCBH0108) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.