1. Molecules in Motion
Year 11 biology

2. Molecules in motion notes
In sciences, a molecule is a group of atoms in a definite arrangement held together by chemical bonds.
Molecules move into cells in different ways depending on the size of the molecules, the permeability of a membrane and the concentration of those molecules in the intracellular and extra cellular environment.

3. diffusion Oxygen moving into cells and carbon dioxide moving out
In a gas or liquid molecules move freely, bumping and bouncing into one another
This is random movement
Which results in an even spread of molecules throughout the space they occupy 
In a liquid the molecules move from a region of high concentration to a region of low concentration, in an attempt to balance the levels or reach-equilibrium.
The difference between these two regions is referred to as a the concentration gradient or diffusion gradient
E.g.
Oxygen moving into cells and carbon dioxide moving out
Needs the molecules to be solution
Membrane to be moist and permeable to them

4. Contd: Diffusion is slow – It can be sped up in the following ways:
Steeper gradient
Stirring the molecules
Heating
Size is important
Small ones (oxygen) diffuse quicker than large ones (glucose)
Diffusion is related to the surface area to volume ratio
Quicker the greater the surface area is to volume
Key Questions page 67

5. OSMOSIS
Movement of water across a selectively permeable membrane, to balance the levels of solutes.
Water actually moves from a region of low concentration of solutes to a region of high concentration of solutes
It is still diffusion but only of water
This happens when the membrane is permeable to water but not to larger molecules like sugar
Refer to diagram on page 65 fig. 4.13
Copy if needed

6. Osmosis contd:
Note: Osmosis in animal cells, especially blood cells could be fatal, as the water would cause the cell to burst
Water entering a red blood cell is endo-osmosis.
If red blood cells are put in salt water the water leaves the cell to balance the solutes and the cell shrivels and dies – exo-osmosis
This means that the fluid part of our body blood cells, the plasma, must be in balance to prevent osmosis and disaster.

7. Osmosis in plant cells Plants have cellulose in their cells walls
They also have a vacuole inside which contains salts in solution
Membrane of the vacuole is semi permeable
Cell wall is fully permeable to salts and water
If a plant cell is in water it swells, but not bursts because the cell wall is tough:
Eventually it gets full and firm and is said to be turgid which keeps the plant firm and once here, the osmosis stops
If a plant cell is placed into a high solute solution, water is taken out of the vacuole and the cell goes flaccid eventually, cytoplasm pulls away from the cellulose – plasmolysis

8. Active transport
This when the cells need molecules of substance which are not in a higher concentration outside the cell, but the cell needs more of them anyway.
Therefore it moves molecules from a region of low concentration to one of high concentration – against the concentration gradient.
It needs energy for this to happen supplied by the process of cellular respiration, which occurs in the mitochondria
 PAGE 66 NOTE Endocytosis and Exocytosis