Membranes [5]: Exam Questions (A Level Biology)

Uploaded by freeeschool on 29.09.2012

A Level Biology: Membranes 5 – Exam Questions
Hi! Welcome to my fifth video on the series about Cell Membranes. Today, we are going
to be looking at answering exam questions.
The questions that we are going to be looking at today are from Excel Qualification of IS
Biology and this is taken from the Unit 1 paper.
The question states: The diagram below represents the structure of the cell surface membrane.
(a) Explain why the phospholipid molecules form a bilayer.
The question is asking why the phospholipid molecules form a bilayer. So looking at the
diagram, you got a number of different things labeled here. You’ve got intrinsic proteins,
you have glycoproteins or glucocalyx, and you also have cholesterol molecules. What
we are interested in is the phospholipid molecules.
The phospholipid is made up of two parts: they have a head part and so the first mark
for this question will be the fact that the fatty acids or the tails are hydrophobic,
which means they don’t like water. The fact that they are hydrophobic means they orientate
themselves away from the water or the polar environment in this case. Obviously, the tails
are hydrophobic and move away from water; the heads are hydrophilic and therefore, that’s
another mark.
From there, you can basically say that they interact or orientate themselves toward the
water or the polar environment, the same where they position themselves. Therefore, if you
reference the fact that the cytoplasm, the tissue fluid and that they are found in a
polar environment or the fact that there is a large amount of water there present, you
therefore get the fifth mark and this question is worth three marks, so it is any three from
those that are available.
The next part of the question goes on to a section about beet roots and about the permeability
of beet root in the presence of alcohol. Essentially, what the experiment is about is they put pieces
of beet root in with different concentrations of ethanol in this case and they see the intensity
of the red coloration of the liquid that is produced.
The question states: Suggest two variables over those stated above which could be kept
constant during the experiment. If you actually look at the text first, we see some of the
things that are actually kept constant. Some of the things like one piece of beet root
was placed into a tube containing 15 cm2 of water and left for 15 minutes. Those are all
controlled. The procedure was repeated seven times and during repeats, each piece of beet
root was removed from the tubes and a sample of fluid removed and placed in a colorimeter,
essentially a piece of equipment to determine the intensity of the red coloration.
Suggest two of the variables other than those stated above which could be kept constant
during the experiment there and a number of things that haven’t been mentioned in the
method. Anything from the temperature of the ethanol, the surface area or the volume of
the beet root and the part of the beet root that was used, potentially the age, the variety,
how it was stored and the source we got it from and the volume of ethanol, although the
concentration is the variable that you alter. The volume of ethanol in each case must be
the same and they must be measured in the same wavelength of light in the colorimeter.
Any of those two are valid.
The part two for this question states: There was a red coloration in the tube containing
only water. Suggest an explanation for this.
What this is saying is if we believe that it is the ethanol causing the breakdown of
the cell membrane and causing the leaching of the pigment that gives the beet root its
red color, then how come we got some red coloration from only water? Clearly, when the beet root
was cut, some damage may have been done to the cells and so therefore, the cell membranes
could have been damaged and hence, some of the pigment could have leaked out of the cells.
Saying first of all that damage was caused physically when the beet root was cut and
therefore, causing the leakage of some of the pigment gets you the second mark.
Part 3. Describe what the student could have done to reduce the red coloration in the tube
containing only the water.
So two possibilities here: One is rinse it off with water before you start. Or two, you
potentially dab it to remove any of the excess pigment on the outside.
Part c. The graph shows on page 10 that the ethanol has an effect on the permeability
on the beet root. State the effect that the ethanol concentration on the intensity of
the red coloration. If we go back to the graph, we can see here, generally speaking, that
as the concentration of ethanol increases, the intensity of the red coloration also increases.
You can tell that it is a reasonably simple answer because there is only one mark available.
If there were more marks available, you may be expected to quote numbers from the graph
and to suggest an explanation for this is part two and this is worth two marks and you
can get it from the following, so referenced to the disruption of the membrane, saying
that the ethanol has some effect on the membrane. This is a single mark and from there, you
can say that ethanol is a solvent which means that it is something that could dissolve like
a bit how water can dissolve sugar. Water in this case is a solvent and the ethanol
is a solvent in this case and the idea that some of the lipids can dissolve within the
ethanol; hence, it is a solvent.
You can also get marks here for saying the following, so the idea that the increase in
the ethanol causes the solution to be less polar. Essentially, the parts on the outside
of the beet root cells have less water in them and so they become less polar. That therefore
changes the orientation of the phospholipids and if you remember back and you look at the
diagram, you can see that you got phospholipids and have the tails on the inside and the head
on the outer. That’s only dependent on the fact that the heads are hydrophilic and the
toes hydrophobic. If less water is available surrounding the membrane, then you can expect
the phospholipids not to be in a bilayer.
[end of audio – 06:34] A Level Biology: Membranes 5 – Exam Questions