King Edward VI Camp Hill School For Girls 1 1. Answers should be written in continuous prose. Credit will be given for biological accuracy, the organisation and presentation of the information and the way in which the answer is expressed. The following extract has been taken from a dictionary of biological terms. cell membrane: a membrane found either on the outside of a cell or within it. Cell membranes are extremely thin. They are only about 7 nm thick and so cannot be seen with a light microscope. A transmission electron microscope however shows a cell membrane consists of three lines forming a sandwich. The two outer lines are dark in colour while there is a lighter one in between. As it is impossible, even with an electron microscope, to see how the actual molecules are arranged in a cell membrane, it is necessary to produce a model to explain the membrane’s properties. The most accurate model of membrane structure that has been developed is the fluid mosaic model and this can be used to describe most of the properties of a cell membrane. Cell membranes play a very important part in the biology of cells and they are particularly important in regulating the movement of substances into and out of cells. Source: W.J.E. INDGE, The Complete A-Z Biology Handbook (Hodder & Stoughton) 1997 (a) (i) Describe the structure of a cell membrane. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... (5) (ii) Describe two ways in which the appearance of a plant cell wall would differ from a cell membrane when viewed with an electron microscope. ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... ........................................................................................................................... (2)
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King Edward VI Camp Hill School For Girls 1
1. Answers should be written in continuous prose. Credit will be given for biological accuracy, the organisation and presentation of the information and the way in which the answer is expressed.
The following extract has been taken from a dictionary of biological terms.
cell membrane: a membrane found either on the outside of a cell or within it. Cell membranes are extremely thin. They are only about 7 nm thick and so cannot be seen with a light microscope. A transmission electron microscope however shows a cell membrane consists of three lines forming a sandwich. The two outer lines are dark in colour while there is a lighter one in between. As it is impossible, even with an electron microscope, to see how the actual molecules are arranged in a cell membrane, it is necessary to produce a model to explain the membrane’s properties. The most accurate model of membrane structure that has been developed is the fluid mosaic model and this can be used to describe most of the properties of a cell membrane. Cell membranes play a very important part in the biology of cells and they are particularly important in regulating the movement of substances into and out of cells.
(ii) A human liver cell contains several hundred mitochondria. A cell from a plant root has only a small number. Suggest an explanation for this difference.
(b) Mitochondria may be separated from homogenised sells by differential centrifugation. During this process the cells must be kept in an isotonic solution. Explain why.
(c) Ribosomes in bacterial cells differ from those in the cytoplasm of eukaryotic cells. When centrifuged at high speed, the eukaryotic cell ribosomes sediment more rapidly than bacterial ribosomes. Explain what this tells you about the difference between bacterial and eukaryotic ribosomes
5. (a) Small samples of plant tissue were placed in a cold, isotonic solution and then treated to break open the cells to release the organelles. The different organelles were then separated. Describe a technique that could be used to
(b) One group of organelles was placed in a hypotonic solution. The diagram shows one of these organelles seen under an electron microscope before and after it was placed in the hypotonic solution.
6. Cell organelles can be separated by centrifuging a cell extract in a sucrose density gradient. The organelles settle at the level in the sucrose solution which has the same density as their own.
Some animal cells were broken open and the cell extract centrifuged in a sucrose density gradient. Three distinct fractions were obtained, A, B and C, as shown in the diagram.
A
B
C
Increasingdensity ofsucrosesolution
King Edward VI Camp Hill School For Girls 9
One fraction contained nuclei, one contained ribosomes and a third contained mitochondria.
Complete the table by identifying the organelle in each fraction and describing one function of each organelle.
Fraction Organelle Function
A
B
C
(Total 4 marks)
King Edward VI Camp Hill School For Girls 10
7. The diagram shows part of an animal cell as seen through an electron microscope.
X
A
B
Y
C
(a) Name organelles A, B and C.
A ....................................................................
B ....................................................................
C .................................................................... (3)
8. (a) The table shows some features of cells. Complete the table with ticks to show those features which are present in an epithelial cell from the small intestine and those features which may be present in a prokaryotic cell.
Feature Epithelial cell from small intestine
Prokaryotic cell
Golgi apparatus
Mitochondrion
Nuclear envelope
Plasmid
Ribosome
(2)
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(b) (i) Explain why it is possible to see the detailed structure of a prokaryotic cell with an electron microscope but not with a light microscope.
(ii) Care must be taken in interpreting electron micrographs. Some features visible in an electron micrograph may not be present in the living cell. Explain why.
(a) This drawing has been magnified 6000 times Calculate the actual length, in micrometres, of cell A. Show your working.
Answer ............................ µm (2)
(b) Each of these bacterial cells is surrounded by a capsule. The main chemical constituent of this capsule is a nitrogenous polysaccharide. List the elements present in this compound.
In a human, there are over 200 different types of cell clearly distinguishable from each other. What is more, many of these types include a number of different varieties. White blood cells, for example, include lymphocytes and granulocytes.
Although different animal cells have many features in common, each type has adaptations. 5 associated with its function in the organism. As an example, most cells contain the same
organelles, but the number may differ from one type of cell to another. Muscle cells contain many mitochondria, while enzyme-secreting cells from salivary glands have particularly large amounts of rough endoplasmic reticulum.
The number of a particular kind of organelle may change during the life of the cell. An 10 example of this change is provided by cells in the tail of a tadpole. As a tadpole matures into a
frog, its tail is gradually absorbed until it disappears completely. Absorption is associated with an increase in the number of lysosomes in the cells of the tail.
Use information from the passage and your own knowledge to answer the following questions.
(c) Starting with some lettuce leaves, describe how you would obtain a sample of undamaged chloroplasts. Use your knowledge of cell fractionation and ultracentrifugation to answer this question.
(b) Some cells similar to that shown in the diagram were grown in a culture. Radioactive amino acids were added to the solution in which they were being grown. The radioactivity acts as a label on the amino acid so that it can be detected wherever it is. This radioactive label allows amino acids to be followed through the cell. At various times, samples of the cells were taken and the amount of radioactivity in different organelles was measured. The results are shown in the table.
Time after radioactiveamino acids were
added to the solution/minutes
Golgi apparatus Rough endoplasmaticreticulum
Vesicles
Amount of radioactivity present/arbitrary units
1
20
40
60
90
120
21
42
86
76
50
38
120
68
39
28
27
26
6
6
8
15
28
56
(i) What happens to the amino acids in the rough endoplasmic reticulum?