ZIMSEC O Level Combined Science Notes:The movement of molecules
- 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.
Diffusion 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.
- 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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