Q1.
The diagram shows part of a plasma membrane. The arrows show the path taken by
sodium ions and by substance X when they diffuse through the membrane into a cell.
(a)
An optical microscope cannot be used to see a plasma membrane. Explain why.
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(2)
(b)
Give one property of the molecules of substance X which allows them to diffuse
through the membrane at the position shown.
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(1)
(c)
The effect of the concentration of sodium ions in the surrounding solution on their
rate of diffusion across the membrane was investigated. The graph shows the
results.
Page 1
(i)
What limits the diffusion of sodium ions across the membrane between
A and B on the graph? Give the evidence for your answer.
Limiting factor .....................................................................................
Evidence .............................................................................................
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(2)
(ii)
Explain the shape of the curve between C and D.
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(2)
(Total 7 marks)
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Q2.
The diagram shows part of a plasma membrane.
(a)
Describe two functions of the structure made from the parts labelled X.
1 ...................................................................................................................
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2 ...................................................................................................................
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(2)
(b)
Give one function of the molecule labelled Y.
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(1)
(c)
The part labelled Z is involved in facilitated diffusion of substances across the
membrane.
(i)
Give one similarity in the way in which active transport and facilitated diffusion
transport substances across the membrane.
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(ii)
Give one way in which active transport differs from facilitated diffusion.
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(2)
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(iii)
The graph shows the relationship between the concentration of a substance
outside a cell and the rate of entry of this substance into the cell.
Explain the evidence from the graph that this substance is entering the cell by
facilitated diffusion and not by simple diffusion.
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(2)
(Total 7 marks)
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M1.
(a)
Does not have the resolution / cannot distinguish between points this close
together;
As light has longer wavelength;
The key ideas in marking this part of the question are resolution
and wavelength.
2
(b)
Lipid soluble / small / non-polar / not charged;
1
(c)
(i)
Concentration of sodium ions (outside cell);
As concentration/independent variable increases so does
the rate of diffusion;
2
(ii)
Sodium ions are passing through the channels/pores;
At their maximum rate;
Rate is limited by the number of sodium channels / another limiting
factor;
max 2
[7]
M2.
(a)
two of the following:
form(water) impermeable barrier to water-soluble substances / selectively
permeable / allows non-polar molecules to pass through;
allows cell to maintain different concentrations either side;
makes membranes self-sealing/able to fuse with other
membranes/able to form
vesicles / gives flexibility/fluidity;
2 max
(b)
(surface/extrinsic protein) for cell recognition / binding to
hormones/identification
1
(c)
(i)
involves carrier/transmembrane/transport proteins;
(reject channel proteins)
1
Page 5
(ii)
requires energy/requires use of ATP / moves
substances/ions/molecules against a concentration gradient;
1
(iii)
the curve levels off above a certain external concentration of
substance; as channel proteins are saturated with molecules
(and no more can be carried);
2
[7]
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E1.
E2.
(a)
Candidates are increasingly aware that observation of fine detail is a
consequence of greater resolution, and there were few answers based purely on the
idea of limited magnification. Most of the problems that arose stemmed from
imprecise use of pronouns. The statement ‘it has a longer wavelength’ needed to be
linked to light. In the context of the question, ‘it’ could only refer to the microscope
itself.
(b)
The most frequent correct answers related to small size and lipid solubility and
many candidates correctly identified one of these properties. There were some
problems with terminology, however, particularly where candidates contradicted
themselves in attempting to explain what they meant by nonpolar or un-ionised.
(c)
Two skills were being tested in part (i); interpretation of the graph, and the ability to
translate graphical information into words. Many candidates experienced difficulties
with both of these skills. The limiting factor was often incorrectly identified but more
often than not described imprecisely as ‘concentration’ or ‘sodium’ while the
dependent variable became ‘rate of reaction’. Candidates also reversed
independent and dependent variables and wrote that ‘as the rate of diffusion
increased, the concentration of ions increased’. Others insisted on describing the
complete curve rather than the part between A and B. Many of the difficulties that
arose in part (ii) stemmed from a failure to understand that the rate of diffusion was
constant after point C. Candidates frequently equated constant with zero. Better
candidates appreciated that something other than the concentration of sodium ions
was now limiting, but then lost sight of the fact that this curve related to facilitated
diffusion not to active transport or enzyme-controlled reactions.
This question was generally well answered and the majority of candidates gained at
least half marks.
(a)
Most candidates gained one mark for selectively permeability, or a description of
this concept. Only the more able candidates gained a second mark, usually for
describing membranes as being fluid. Virtually no one gained the alternative mark
for stating that membranes maintain different concentrations either side of the
membrane.
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(b)
There were mixed responses to this part and they tended to be centre-specific. The
most common mistakes related to transferring materials across the membrane.
(c)
(i)
There were mixed responses to this part. Some candidates gave the correct
answer of ‘carrier protein’, but then disqualified their answer by stating that it
was also a ‘channel protein’. Other candidates simply said ‘have proteins’.
(ii)
This was answered well by the majority of candidates, even those getting the
previous part wrong.
(iii)
Many candidates gained the first mark for the ‘graph levelling off’, but there
were some very poor descriptions relating to the carrier proteins being limiting.
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