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Diffusion, Osmosis and Active transport
What you need to know
Reflections and Exam tips
Why do substances need to move in and out of cells?
How do substances move?
The way a substance enters or leaves a cell depends on its properties and the properties of membranes:
Factors affecting movement
Cell walls are freely permeable which means that they do not block the movement of substances.
Cell membranes are partially permeable ( some people say semi-permeable)- this means that they only allow small substances through and prevent the passage of larger ones.
This is the movement of particles of a substance from high concentration (where they are crowded) to low concentration.
Diffusion is a passive process, which means that it doesn't need energy on the part of the cell. It relies only on the inbuilt energy of the particles (Brownian motion).
In most cases, only small molecules move by diffusion.
The concentration difference can also be called a gradient (slope).
So we can redefine diffusion as:
The movement of particles of a substance down a concentation gradient. (simply means down the slope)
Factors affecting rate (speed) of diffusion
You can easily remember these using SAD TOES. Yes Sad toes.
Rate of diffusion = SAD/TOES i.e. Surface Area and Diffusion gradient (SAD) divided by the Thickness Of the Exchange Area.
So in general, the bigger SAD is and the smaller TOES is the faster diffusion will take place.
So what does it mean in terms of diffusion surfaces?
They have to be:
Is the movement of water molecules from a weak solution (dilute solution) to a concentrated (strong) solution through a partially permeable membrane.
This is because the membrane only allows small (water) molecules through while stoping the large (salt/sugar) molecules which can't fit through the pores.
This is the movement of substance molecules from low concentration to high concentration using energy from respiration.
You can see why it needs energy from the cell; the particles are moving up the slope.
Active transport is the movement of molecules up a concentration gradient using energy (ATP) from respiration.
For this reason, active transport can only happen in living cells which have many mitochondria for respiration.