Passive Transport across Membranes

Osmosis

Osmosis

Learning Objectives• - Explain what is meant by

osmosis, in terms of water potential.

• (No calculations of water potential will be required);

• - Recognise and explain the effects that solutions of different water potentials can have upon plant and animal cells.

Success Criteria• Identify the direction of

osmosis using water potential values

• Complete a quantitative practical task (practice)

What is osmosis?Osmosis is the diffusion of water. It is the net movement of water molecules from a region of high water concentration to a region of low water concentration, through a partially-permeable membrane.

net movement of water molecules

Osmosis is the process by which cells exchange water with their environment, such as in the mammalian kidney.

What is water potential?

Pure water has the highest water potential, and has a value of 0 kPa. Solutions have a lower water potential than pure water, and have a negative water potential.

The net movement of water by osmosis is determined by differences in water potential between two solutions connected by a partially-permeable membrane.

Water potential is the tendency of water molecules in a system to move. It is denoted by the symbol Ψ and is measured in kiloPascals (kPa).

Water molecules always move from a region of high water potential to a region of low (more negative) water potential.

Water movement during osmosis

free water molecule

solute molecule

What is solute potential?The water potential of a solution is affected by the amount of solute it contains. The greater the amount of solute, the lower the water potential.

This is because water molecules bind to the solute molecules, reducing the number of water molecules that are free to diffuse.

The contribution that solutes make to the water potential of a solution is the solute potential (ΨS), and is a negative value.

What is pressure potential?

In plant cells, the pressure potential is a result of the cell wall exerting pressure on the cytoplasm.

Water potential is calculated using the following equation:

The water potential of a solution is also affected by the pressure applied to it. The greater the pressure, the higher the water potential.

This is called the pressure potential (ΨP) and is always a positive value.

water potential solute potential pressure potential+=

+= ΨPΨSΨ

Calculating water potential

Osmosis in plant cells

Osmosis in animal cells

Water potential of potatoes

Osmosis: true or false?

Direction of water movement