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Osmosis Is the Movement of Water Across a Membrane

Definition

Osmosis

The net movement of water molecules across a partially permeable membrane from a dilute solution to a more concentrated solution.

  1. Osmosis is the net movement of water molecules across a partially permeable membrane.
  2. Water moves from a dilute solution to a more concentrated solution.
  3. A dilute solution has a high proportion of water and little solute, while a concentrated solution has more solute and less water.
  4. Only water crosses the membrane, because the dissolved solute is usually too large to pass through.
  5. Water keeps moving until the concentrations on the two sides are more equal.
Key Idea

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

Osmosis Is a Special Case of Diffusion

  1. Osmosis is a special type of diffusion in which the moving particles are water molecules.
  2. A partially permeable membrane has tiny gaps that let small water molecules through.
  3. The larger solute molecules cannot fit through the membrane.

A diagram of a partially permeable membrane. It shows that small water molecules can pass through the phospholipid bilayer, while larger solute molecules like sucrose are unable to pass.

  1. Like all diffusion, osmosis is passive and driven by the random movement of the water molecules.
Analogy

The membrane works like a sieve with holes big enough for water but too small for the larger solute particles.

Water Potential (Higher Tier)

Definition

Water potential

A measure of the tendency of water to move; water moves by osmosis from a higher water potential (dilute) to a lower water potential (concentrated).

  1. At Higher tier, osmosis can be described using water potential.
  2. A dilute solution has a higher water potential, and a concentrated solution has a lower water potential.

A diagram showing how adding solute to water decreases its water potential. Pure water has a water potential of 0 kPa, while solutions with more solute have increasingly negative water potential values, such as -100 kPa and -200 kPa.

  1. Water moves by osmosis from a region of higher water potential to a region of lower water potential.

An illustration of water potential in action, showing water moving from a region of higher water potential in the soil to a region of lower water potential in a root hair cell.

Note

Moving from a high to a low water potential is the same as moving from a dilute to a concentrated solution.

Osmosis Into and Out of Cells

Definition

Cell membrane

The partially permeable layer around a cell that controls which substances move into and out of it.

  1. Water moves into and out of cells by osmosis across the partially permeable cell membrane.
  2. If the solution outside is more dilute than the cell contents, water moves into the cell.
  3. If the solution outside is more concentrated than the cell contents, water moves out of the cell.
  4. If the two are equal, there is no net movement of water.

A diagram illustrating osmosis in plant cells under three conditions: hypertonic (plasmolyzed), isotonic (flaccid), and hypotonic (turgid). It shows the movement of water (H2O) and the resulting state of the vacuole and cell membrane relative to the cell wall.

  1. Body cells must keep their water and solute balance, or they cannot work properly.

A table comparing the effects of osmosis on animal and plant cells in hypertonic, isotonic, and hypotonic solutions. It shows animal cells shriveling or lysing, and plant cells becoming plasmolysed, flaccid, or turgid.

Common Mistake

Water does not always enter a cell; it leaves if the surrounding solution is more concentrated than the cell.

Note

Required Practical: Investigating Osmosis

  • Aim: to investigate how the concentration of sugar or salt solution affects the mass of plant tissue.
  • Cut cylinders of potato of equal size with a cork borer, and measure the mass of each.
  • Place each cylinder in a different concentration of sugar or salt solution, using at least five concentrations plus distilled water.
  • Leave them for a set time at the same temperature, then remove, blot dry and measure the mass again.
  • The independent variable is the concentration of the solution and the dependent variable is the change in mass.
  • Control the type and volume of solution, the temperature and the time.
Self review
  • Define osmosis.
  • Which way does water move, from a dilute or a concentrated solution?
  • What moves in osmosis, the water or the solute?
  • In the required practical, why is percentage change in mass used rather than change in mass?
  • If a potato cylinder gains mass, what does that tell you about the solution it was in?
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1.3.2 Osmosis Revision Guide

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