K224 / K225 / 2027

Lesson 2 of 5 / Movement of substances

Osmosis

Which way will water move across a cell membrane?

I can predict the net movement of water using water potential and membrane permeability.

  • I can define osmosis and investigate and describe its effects on plant and animal tissues.Syllabus

    Syllabus K224 / K225, Topic 2(b). Define osmosis and investigate and describe its effects on plant and animal tissues.

About 4 min

Make a guess. You are not marked.

A membrane separates pure water from a strong sugar solution. Only water can cross. Which side has the higher water potential?

  1. The pure water side.
  2. The sugar solution side, because it is more concentrated.
  3. Neither, because water potential only applies inside cells.
Show the answer

The pure water side.

Dissolving sugar lowers water potential. Pure water has the highest water potential, so water moves from it into the sugar solution.

The key idea

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

Water potential describes the tendency of water to move from one place to another. At the same pressure, a more dilute solution has a higher water potential than a more concentrated solution.

Explore the idea

Which way will water move overall?

Outside water potential, compared with inside
Outside
Higher water potentialInside
Lower water potential
Net movement into the cellThe outside solution has a higher water potential. Water crosses the partly permeable membrane in both directions, with a net movement into the cell. A fixed partly permeable membrane separates the solutions. Blue water molecules can cross it. Orange solute particles cannot cross this membrane. Arrows show water moving both ways.

The membrane lets water through, but not these solute particles.

Net movement into the cell.The outside solution has a higher water potential. Water crosses the partly permeable membrane in both directions, with a net movement into the cell.

  • Water molecule: can cross
  • Solute particle: cannot cross here
  • Partly permeable membrane

The examples compare the same solute at equal pressure. More dissolved solute lowers water potential under these conditions. The picture shows starting conditions; it does not model changes in pressure or volume. Particle numbers and arrow widths are illustrative.

Explanation

A partially permeable membrane lets some substances through more easily than others. In a simple model of osmosis, water can cross the membrane but the dissolved substance, the solute, cannot. The membrane separates two solutions.

Dissolving more solute in water lowers its water potential. So a dilute solution has a higher water potential than a concentrated one. Pure water has the highest water potential of all. Always compare the two solutions before deciding which way water moves.

Water molecules move across the membrane in both directions. When the water potentials are different, more water moves from the higher water potential to the lower water potential. This net movement of water is osmosis. Osmosis does not need energy from respiration.

If the water potentials are equal, water still moves both ways, but there is no net movement. Remember: osmosis is about water and needs a partially permeable membrane. Without the membrane, the movement is not osmosis.

Step by step
  1. 1

    Identify the membrane

    The question must involve a partially permeable membrane separating the solutions.

  2. 2

    Compare water potential

    For the simple solutions here, at the same pressure, the more dilute solution has the higher water potential.

  3. 3

    Explain the net movement of water

    Water moves by osmosis from higher to lower water potential through the membrane.

Worked example

A membrane bag in sugar solution

A bag contains a dilute sugar solution. It is placed in a more concentrated sugar solution. The membrane allows water, but not sugar, to pass. Predict what happens to the bag.

One way to explain it

The bag loses water and its mass decreases. Water moves by osmosis from the higher water potential inside the bag to the lower water potential outside, through the partially permeable membrane.

Why this answer works
  • Compare the two water potentials.
  • Name water as the substance moving.
  • Connect water loss to the decrease in mass.

Watch out for this

A student says: "The more concentrated sugar solution has the higher water potential." What is wrong with this?

Show the answer

For these solutions at the same pressure, more dissolved sugar means lower water potential. Be precise: solute concentration and water potential change in opposite directions.

Check your understanding

A membrane lets water pass but not sucrose. Water potential is higher on the left than on the right. What is the net movement?

  1. Water moves from left to right by osmosis.
  2. Sucrose moves from right to left by osmosis.
  3. Water moves from right to left using energy from respiration.
Show the answer

Water moves from left to right by osmosis.

Water moves from higher to lower water potential through the partially permeable membrane.

Quick recall

Card 1 of 3. Answer in your head, then check.

Which way will water move across a cell membrane?

You can return to this lesson any time.