# Mass Transport: Edexcel Unit 1 (High band Biology)

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. ‘ 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 Page…1

ficks law has never come in the past papers before

MM