Membranes [6]: Plasma Membranes - Exam Questions (A Level Biology)


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Transcript:
A Level Biology: Membranes 6 – Plasma Membranes
Hi! Welcome to my next video on the series about Plasma Membranes. Today, we are going
to be looking at answering more exam questions.
These questions from the Excel Biology Course and these are taken from the Unit 1 paper.
The size and the solubility of molecules have an effect on their ability to be taken up
by cells. Describe the experiment you carried out to investigate the permeability of cell
membranes.
The key term is “describe” and so all you have to do is list down the method that
you have done to test the permeability and the experiment hopefully that you have done
is the beet root experiment. This question is five marks and so you need to be saying
at least five things.
The first thing that gets a mark is by mentioning the fact that you used beet root to do this.
When you have actually done this, the first thing you have done is wash the beet root
to remove excess pigment. From there, you probably could have done one or two experiments,
either changing the temperature or changing the level of ethanol.
If we are looking at temperature, you could use a water bath to control the temperature,
so this could be using the five or six different temperatures you are looking at. Of if with
the ethanol experiment, it could simply be keeping it at a fixed temperature.
You then need to mention at least five different temperatures: 10, 20, 30, 40, 50, 60 and then
concentration of ethanol is exactly the same, you need to mention at least five. It is usually
done as a percentage.
You then need to talk about your control variables. So these will be things like how long it is
left on the solution, then the size of the beet root, the condition of the beet root
that you have used, how it was stored and basically, all the things that you do to keep
it a fair test and you probably need to list down at least two or three.
The dependent variable, you need tell what it is you are measuring and so this will be
using a piece of equipment like a colorimeter and this measure things like absorbance, transmission
or maybe the intensity of colouration. But basically, this is where you state what the
dependent variable will be. Mention the fact that use a colorimeter as a piece of equipment
to determine the intensity of the color of the liquid produced gets you another mark
and the final mark is to do with repeats and reliability. So mention that you repeat each
concentration or each temperature a number of times. It is any five from those possible
eight.
An investigation was carried out into the permeability of the cell membrane to a number
of different, non-polar organic molecules. The molecules differed in their size and their
solubility in oil compared with their solubility in water. The higher the solubility, the more
soluble the molecule is in oil compared with water. The graph below shows results of the
investigation.
The size of the circle drawn on the graph indicates the size of the molecule. The larger
the circle, the larger the molecule. The first question is: Describe what relationship, if
any, there is between the permeability of the cell membrane and the size of the molecules.
You essentially have three things on this graph. You not only have solubility in oil
compared with water, you also have its permeability and you also have the size of the molecule.
In this first part, it is asking you the relationship between the permeability of the cell membrane
and the size. The membrane permeability is this one and the size of the molecule indicates
by how big the circle is. There is no pattern here. You have big ones down here. You have
large ones here; small ones are dotted around in different locations. There seems to be
no relationship so there is no correlation.
(2) Describe what relationship, if any, there is between the permeability of the cell membrane
and the solubility of the molecule in oil compared with water. This is asking for a
comparison of the permeability and solubility. Clearly, the most soluble it is in oil compared
with water, the greater the level of permeability there is. That is reasonably straightforward.
(3) This is a little bit more difficult. Water is able to diffuse across the cell membrane
even though it is a polar molecule. On the graph or circle, put W to indicate permeability
of the cell membrane to water, the size the circle should represent the size of the water
molecule.
It says in the first sentence, water is able to diffuse through the cell membrane. That
must mean it has a very high permeability, therefore, it must be somewhere at the top
portion of the graph. If it is high permeability, it must be near the top portion somewhere.
In the event you look at the solubility in oil compared with water, or clearly, water
is not very soluble in oil, so therefore, its solubility compared with oil must be quite
low.
So you draw a circle anywhere on the top left. It is some way like that. The important thing
here is the size of the circle should represent the size of the water molecule. Now, this
is slightly too large the circle that I have drawn. It should be less than one square because
water is a very, very small molecule and hence, water should be exceptionally small. What
they give the mark for on the test paper is it must be smaller than the single square.
The following question: Use you knowledge of the structure and the properties of the
cell membrane and to explain the results of the investigation.
Clearly, what it is showing is the high level of solubility in oil compared with water;
the high level of permeability. So what you got to do is you got to think about what you
know about the structure and the property of cell membrane. This explains the investigation.
The structure, you simply get a mark for it describing the membrane as a phospholipid
bilayer. That’s a single mark for that. The property of that, the actual phospholipid
is made of two parts, the phosphate head and the fatty acids. By mentioning the fact that
the tail is hydrophobic and doesn’t like water, you therefore gain an extra mark for
that.
Describing the results of the experiments is saying the most soluble it is in oil compared
with water means it has a great permeability, gets you another mark. Essentially sayable,
the results have shown.
The following two marks: the idea that the permeability is linked to the readiness to
dissolve and so the higher the solubility, the more likely is it to dissolve in the phospholipid
bilayer. Finally, if you referenced to something of the fluidity of the movement of the phospholipid,
that also gains a mark.
But the top three are the easiest ones, really, to get. Mentioning the phospholipid bilayer,
that’s simply the structure referenced to the hydrophobic nature that referenced the
properties and then saying what happened in the practical, so saying the most soluble
it is oil, the more permeable it is. Simply, you will get the three marks now.
[end of audio – 06:53] A Level Biology: Membranes 6 – Plasma Membranes
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