Diffusion & Osmosis
Substances move in and out of cells through the cell membrane. 
All biological membranes such as the cell membrane, nuclear membrane and double membrane of the mitochondria, are similar in structure and in the way they operate. 
A permeable membrane (fully permeable) – allows substances to pass through it 
An impermeable membrane – does not allow substances to pass through it 
A selectively permeable membrane (semi-permeable membrane)– allows some but not all substances to pass through it. Biological membranes are selectively permeable, as are materials such as visking tubing and dialysis tubing. 
Substances can move across biological membranes in two ways – Diffusion and Active Transport
Diffusion 
Diffusion is the movement of usbstances from an area of high concentration to an area of low concentration 
Diffusion is a passive process which means that it does not need energy to occur 
Examples of diffusion are 
-	Carbon Dioxide diffusing into a leaf from the atmosphere an,d Oxygen diffusing out of a leaf into the atmosphere 
-	Carbon dioxide diffusing out of the cell into the blood, and Oxygen diffusion from the blood into a cell 

Active Transport 
Sometimes molecules are actively moved against the concentration gradient – they are forced from an area of low concentration to an area of high concentration. This process of called Active Transport. 
Active transport is an active process as energy is needed to achieve active transport. In order for cells to carry out active transport, lots of mitochondria are present to supply the energy released during respiration. 
Osmosis 
Osmosis is a special type of diffusion. 
It involves the movement of water from an area of high concentration to an area of low concentration across a selectively permeable membrane.
It is a passive process – no energy is required to achieve osmosis 

Osmosis and Animal Cells 
Condition 	Effects 	
If an animal cell is placed in a solution with the same concentration as their cytoplasm 	Water will move in and out of the cell via the cell membrane at the same rate 	
If an animal cell is placed in a solution less concentrated than their cytoplasm 	Excess water will move into the cell via osmosis causing it to enlarge and burst. 
	
If an animal cell is placed in a solution more concentrated than their cytoplasm	There will be an excessive amount of water lost from the cell via osmosis and the cell will shrivel into a condition called crenation 
	
Osmosis and Plant Cells 
Condition 	Effects 	
If a plant cell is placed in a solution with the same concentration as their cytoplasm 	Water will move in and out of the cell via the cell membrane at the same rate 	
If a plant cell is placed in a solution less concentrated than their cytoplasm 	Excess water will move into the cell via osmosis and into the vacuole which swells and pushes the cytoplasm against the cell wall. The cell is said to be turgid – it does not burst as plant cells have a cell wall 
 	
If a plant cell is placed in a solution more concentrated than their cytoplasm	Water passes out of the cell, turgor pressure is the lost, the cell becomes flaccid – this is called plasmolysis and it causes wilting in plants 	

Turgor Pressure – the outward pressure of the cytoplasm and vacuole against the cell wall of a plant cell
Application of Osmosis in Food Preservation
Bacteria and fungi cause food to rot. They need water to survive. Food is placed in a concentration solution (of sugar or salt), this draws the water from the cells of the food via osmosis. Micro-organisms cannot grow on the food and the food is preserved.