The movement of molecules in plants: Diffusion

• Both plants and animals contain a large amount of water in their tissue.
• Plants like water melons and cucumbers have a lot of water which sometimes constitute 95% of their weight.
• Most animal tissue is at least 70% water.
• Water must be able to enter and leave both plant and animal cells.
• The process by which water enters plants is called diffusion.
• All matter is made up of molecules.
• The kinetic theory states that all molecules and particles are always in constant movement.
• In gasses and liquids particular these molecules are able to move freely from one place to another.
• If there is a higher concentration of certain molecules in one place than in another, they ( molecules)will move from where there is a higher concentration to regions where the concentration is lower.
• This movement is referred to as diffusion.
• Water vapor molecules in the air and inside leaves move by diffusion.
• These molecules will continue to move as long as there is a difference in concentration between the two places involved.
• The difference in the concentration of a certain molecule in one place from another creates what is known as a a concentration gradient.

Molecules move (diffuse) from higher concentration to lower concentration areas.

Diffusion of water

Demonstrating diffusion using a semi-permeable membrane and equal concentrations of water.

• Water molecules are always moving because they have enrgey.
• If water is separated into two sections by a thin membrane which is permeable, as shown in the diagram above,
• molecules will pass through and move in both directions.
• There is equal movement of the water molecules in each direction.

When sugar is added to the left side of the partition more water molecules move there.

• If salt/sugar is added to the water on the left side of the partition as shown in the diagram above,
• There will be relatively more water molecules on the right side of the membrane.
• The membrane is semi-permeable as it only allow water but not salt/sugar molecules to pass.
• More water molecules will move from the right to the left than will move to the left from the right.
• The diffusion of water across a semi-permeable membrane from higher concentration to lower concentration places is known as osmosis.
• Osmosis-the diffusion of water molecules from regions of higher concentration to regions of lower concentration via a semi-permeable membrane.

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