BIOLOGY REVISION [ GCSE ]

Cell Biology - Osmosis, Diffusion and Active Transport

OSMOSIS

Here's your very important definition of osmosis that you really can't forget.  If you do, I'll sue you.

OSMOSIS IS THE MOVEMENT OF WATER MOLECULES THROUGH A PARTIALLY PERMEABLE MEMBRANE FROM A REGION OF A HIGHER WATER CONCENTRATION TO A REGION OF A LOWER CONCENTRATION.

Easy enough, right?  So, here's your explanation as to what that little important sentence means.

A partially permeable membrane is basically just like a wall with lots of tiny holes in it - by tiny, I mean absolutely tiny.  So small that H2O molecules can fit through it but sucrose molecules cannot.

The water molecules can actually pass both ways through the membrane during osmosis because y'know, water molecules gonna do what water molecules gonna do.  Ain't no one gonna direct them.  (They move about randomly all the time).

But, because there are more water molecules than there are water molecules on the side with higher water concentration than lower, there is still a steady net flow of water into the region with fewer molecules i.e into the stronger sugar solution.

This means the strong sugar sugar solution gets more dilute.  The water acts like it's trying to "even up" (meet the equilibrium for) the concentration either side of the membrane.

DIFFUSION

Diffusion is really impotent in living organisms.  It's how a lot of substances get in and out of cells.

- it is the gradual movement of particles from places where there are lots of them to places where there are fewer of them.  And here's the version you need to know because school hates us:

DIFFUSION IS THE SPREADING OUT OF PARTICLES FROM AN AREA OF HIGHER CONCENTRATION TO AN AREA OF LOWER CONCENTRATION.

So, like, when that annoying kid who thinks he's so smart sprays his can of lynx anywhere and you're glad you're at the other end of the classroom but it still gets to you and you question how hard it would be to shove that can of lynx up his a— moving on...

ACTIVE TRANSPORT

Sometimes substances need to be absorbed against a concentration gradient, i.e. from a lower to a higher concentration.  This process is referred to as 'active transport'.  Another way of remembering it is that it's the one that uses the energy and stuff.

Oh, and get this, there's no fancy definition to memorise this time.  Kinda.

Basically, active transport is used by things that need things to actually be things.  So like our little seagull (aka root hair cell) who needs water and mineral ions which are in whatever it is in.  So they suck 'em up, I guess.  But to do this they need a bit of energy too.

Active transport also happens in people — it's used when there is a lower concentration of nutrients in the gut but a higher concentration of nutrients in the blood.  So, technically the gut should be taking nutrients out of the blood, but that isn't a good idea so a little bit of energy is used to shift the active transport so it's still taking nutrients from the gut and drawing it into the blood. 

But like, seriously mate, eat some food.  Active transport is comiNG foR yOu—