A Level Biology: Edexcel Unit 1 – Mass Transport
Hi!
Welcome to today’s video.
Today, we are going to be looking at Edexcel AS Level Biology Unit 1 and we are going to
start off with transport around the body and today’s objectives, we are going to look
at why many animals have a heart and a mass transport system.
So it’s the circulatory system.
We are going to look at the limits of diffusion.
To start us off, we need to remember that when in metabolism, most of the chemical reactions
actually occur in the interior of the cell.
Therefore, the actual size of the cell is going to affect the rate of which these reactions
take place.
A thing to remember in terms of the exchange between the cell and its environment is that
the surface of the cell or the cell membrane actually controls what enters and leaves the
cell.
Also, those cells with the largest surface area can obviously exchange more materials.
What we are going to try and do now is work out the surface area to form a ratio of these
three following cubes.
They should be able to work out the surface area and the also the volume.
While we have a look at it, the surface area of this small cube is 6 mm2.
The medium one is 24 mm2 and obviously, the largest square is 96 mm2.
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If you don’t know how to calculate the surface area of a cube, you work out the area of one
side and multiply it by 6.
The volume is three-dimensional so you are going to times these three numbers together
and so the volume for this small cube is 1 mm3, the middle one is 8 mm3 and for the large
cube, it is 64 mm3.
Because it is a ratio, you are just basically going to divide the surface area by the volume
and that gives you a ratio of 6:1.
The medium one is 3:1 and the largest one is 1.5:1.
So obviously showing is that the volume is actually increasing faster than the surface
area.
But wait, why is that a bad thing?
Cells which are smaller have a much larger surface area to volume ratio.
Just like this image of an amoeba here, it can move things into and out of its surface
and into the cell simply by diffusion.
Much larger cells and larger organisms have obviously gotten much smaller surface area
to volume ratio, therefore, diffusion is too slow.
Therefore, the cells are limited by what they can actually do.
They need some form of mass transport system in order to be able to get the nutrients from
its environment into the cell, so that they can carry out both basic metabolic reactions.
To do that, as I said before, it uses mass transport system and also specialist organs
as well.
To summarize the first part, we can see that as these organisms get larger, the surface
area to volume ration becomes smaller.
Therefore, the exchanges with the environment become too slow.
We use Fick’s Law to actually calculate the rate to which diffusion occurs.
Now, we basically took out where we looked at those three cubes.
Basically, Fick’s Law revolves around three factors: effect of diffusion and concentration
gradients, the surface area and also the thickness of the exchange surface.
If we apply this particular calculation to this cube, you can see that the rate of which
O2 or oxygen reaches the center of the cube is going to be a product of a ratio of the
surface area compared to the volume.
Therefore, there is a need for a mass transport system to carry substances around the organism.
Some features of some common mass transport systems:
- You need a system of vessels that carry substances around the organism;
- You need a way of making sure about those substances are moved in the right direction;
and - You need a means of movement fast enough
and you also need a suitable transport medium.
In humans, the mass transport system is the circulatory system.
The specialized exchange organs include things like the lungs and the digestive system.
This concludes the short presentation.
I hoped you enjoyed it.
Don’t forget to keep a lookout for some more videos coming out soon.
[end of audio – 05:08] A Level Biology: Edexcel Unit 1 – Mass Transport
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