OSMOSIS

Photos taken from:

Standard Grade Biology(third edition), James Torrance, Hodder & Stoughton, 2001.

Intermediate 2 Biology, James Torrance et.al., Hodder & Stoughton, 1999.

PRACTICAL Osmosis in potato

• Use a cork borer to get 10 cores of potato.

• Place them on a tile and, using a sharp knife, trim them all to the same length.

• Make sure they are cut at right angles to make them easy to measure.

• The longer the cores, the better your result will be.

• Pour different concentrations of sugar solutions into five Petri dishes and label the dishes 1–5, as shown in your results table .

• Measure two cores, record their lengths and put them in one dish.

• Repeat this with the other four dishes.

• Put covers over the dishes and leave for at least 1 hour.

• Measure the lengths of the cores again and record the results.

You will need:

•a large potato

•cork borer

•sharp knife

•tile

•five Petri dishes with labels

•five different concentrations of sugar solution

•ruler

Health and safety! Take care with the sharp knife.

OBJECTIVES

Key Objective

•Describe the process of osmosis

•State that osmosis is a type of diffusion (C/D)

•Describe osmosis as the movement of water across a partially-permeable membrane from an area of high water concentration (i.e. dilute solution) to an area of low water concentration (i.e. concentrated solution) (C/D)

•Explain that osmosis is the net movement of water molecules from an area of high water concentration to an area of low water concentration across a partially-permeable membrane and is a consequence of the random movement of individual particles (B-A*)

When smelling body spray where is the smell strongest and where is the smell weakest?

Smell is strongest at source.

Diffusion means the smell spreads out and gets weaker further away from the source.

How does the concentration of smell molecules change during diffusion?

Smell molecules move from an area of high concentration to an area of lower concentration. This is called a concentration gradient.

Osmosis is a special type of diffusion.

Diffusion involves gas or dissolved molecules, but osmosis only involves the movement of water molecules.

What is osmosis?

Osmosis occurs across a semipermeable membrane which has tiny holes in it.

These holes are small enough for water molecules to pass

watermolecule

through but larger molecules cannot pass through.

Osmosis:

The net movement of water molecules along a concentration gradient from an area of high water concentration (HWC) to an area of low water concentration (LWC) through a selectively permeable membrane.

A bag made from a semipermeable membrane is tied to a glass tube and filled with a strong sugar solution.

What happens to the water molecules in the weak solution?

What happens to the liquid inside the semipermeable bag?

This bag is placed in a weak sugar solution.

strong sugar solution

weak sugar solution

strong sugar solution

weak sugar solution

Water molecules diffuse across the membrane from the weak sugar solution into the strong sugar solution. This continues until the concentration is the same on both

The volume of liquid in the semipermeable bag increases. The liquid rises up the glass tube and then stops. Why does this happen?

sides of the membrane.

Osmosis animation 1

Osmosis animation 2

Osmosis:

The net movement of water molecules along a concentration gradient from an area of high water concentration (HWC) to an area of low water concentration (LWC) through a selectively permeable membrane.

Water Movement by Osmosis

Osmosis in Eggs

The Effect of Two Sugar Solutions on Beetroot Cells

Molecular Model of Osmosis

• Hypotonic (HWC) - a region of higher water concentration.

• Hypertonic (LWC) - a region of lower water concentration.

• Isotonic - a region where there are equal water concentrations on either side of a membrane.

Some Important Terms

Osmosis in Red Blood Cells

Shrunken RBC

Normal RBC

Osmosis in Plant Cells

Normal Red Onion Cells

Plasmolysed Red Onion Cells

Important Terms

• Turgid - the term used to describe a plant cell whose contents have shrunk due to loss of water by osmosis.

• Plasmolysed - the term used to describe a plant cell whose contents have shrunk due to loss of water by osmosis.