Tip:
Highlight text to annotate it
X
A Level Biology: Homeostasis 6 – Osmosis
Hi! Welcome to today’s video. Today, we are going to be looking at Osmosis.
Osmosis is just an example of diffusion and diffusion is the movement of molecules down
a concentration gradient. That’s from an area of high concentration to an area of low
concentration. A simple diffusion is just an example of passive transport and that it
requires no energy inputs from the cell.
What does osmosis actually mean?
It is defined as the movement of water molecules across a partially permeable membrane from
an area of high concentration to an area of low concentration. Partially permeable means
it’s a barrier and only allows certain types of molecules to pass through. With this example,
we’ve got water on one side over here and we got a sugar solution on the other side.
What actually happens is the water is able to cross over this barrier because it is small
enough. However, the sugar molecules, as you can see, are too large to pass through into
the opposite direction. Therefore, we have a net flow of water moving from left to right.
So you can actually demonstrate how osmosis works by using a very simple and practical
– all you need is an egg, some vinegar and a couple of days’ patience. What you are
eventually left with is an egg without a shell. Now, the rubber egg, as a few people like
to call it, is a good demonstration of a very, very basic cell. So you got the nucleus which
is obviously the yolk. You got the albumen and then you got the membrane itself. The
reason why it grows in size when you place it within the vinegar is because the albumen
is about 90% water. What happens is that the water from the acid enters the cell and that’s
why the egg cell increases. Once it reaches equilibrium, then we say that it has reached
its isotonic state. That’s basically where the net flow of water is equal in both directions.
If we were to place this egg cell into, for example, a pure water solution, something
like this filled with water, then, we’d see a net flow of water going into the cell.
Therefore, the egg would gain in weight and size. If we were to place the egg into a sugar
solution, what we would expect to see is the net flow of water move out of the cell. So
water actually leaves the cell and therefore, the cell would start to shrink.
Finally, the movement of water needs to be closely monitored and controlled by both plants
and animals. This is done through a process known as osmoregulation. If for example, here
we got a red blood cell and as you can see in an isotonic solution, it keeps its shape.
However, with a hypertonic solution, it loses water and shrivels up while with a hypotonic
solution, it takes on water until it bursts.
Osmoregulation is just one of many homeostatic systems which are designed to keep a stable
internal environment. It is carried out in animals through the skin and the kidneys.
In plants, it is carried out by the stomata.
[end of audio – 04:06] A Level Biology: Homeostasis 6 - Osmosis
Page…1