Cell Structure - Mark Scheme Answers

Question 1

Describe how you could make a temporary mount of a piece of plant tissue to observe the
position of starch grains in the cells when using an optical (light) microscope.

Answer 1

1. Add drop of water to glass slide
2. Obtain thin section of plant tissue and place on slide/ float on drop of water
3. Stain with iodine.
4. Lower cover slip using mounted needle.

Question 2

A transmission electron microscope was used to produce the image in the figure
above.
Explain why.

Answer 2

1. High resolution;
2. Can see internal structure of organelles.

Question 3

Suggest why the plasmids were injected into the eggs of silkworms, rather than into
the silkworms.

Answer 3

(If injected into egg), gene gets into all / most of cells of silkworm; 2.

So gets into cells that make silk.

Question 4

Suggest why the scientists used a marker gene and why they used the EGFP gene.

Answer 4

1. Not all eggs will successfully take up the plasmid;
2. Silkworms that have taken up gene will glow.

Question 5

What would the scientists have inserted into the plasmid along with the spider gene
to ensure that the spider gene was only expressed in the silk glands of the
silkworms?

Answer 5

Promoter (region / gene).

Question 6

Suggest two reasons why it was important that the spider gene was expressed only
in the silk glands of the silkworms.

Answer 6

1. So that protein can be harvested;
2. Fibres in other cells might cause harm.

Question 7

Describe how phospholipids are arranged in a plasma membrane.

Answer 7

1. Bilayer;
2. Hydrophobic / fatty acid / lipid (tails) to inside;
3. Polar / phosphate group / hydrophilic (head) to outside;

Question 8

Cells that secrete enzymes contain a lot of rough endoplasmic reticulum (RER) and
a large Golgi apparatus.

(i) Describe how the RER is involved in the production of enzymes.

Answer 8

1. (Rough endoplasmic reticulum has) ribosomes;
2. To make protein (which an enzyme is);

Question 9

Describe how the Golgi apparatus is involved in the secretion of enzymes.

Answer 9

(Golgi apparatus) modifies (protein)

Or

packages / put into (Golgi) vesicles

Or

transport to cell surface / vacuole;

Question 10

Describe how you could use cell fractionation to isolate chloroplasts from leaf tissue.

Answer 10

1. How to break open cells and remove debris;
   2.Solution is cold / isotonic / buffered;
2. Second pellet is chloroplast.

Question 11

Name two structures in a eukaryotic cell that cannot be identified using an optical
microscope.

Answer 11

• mitochondria
• ribosome

Question 12

Describe and explain how centrifuging the culture allowed the scientists to obtain a
cell-free liquid.

Answer 12

1. Large / dense / heavy cells;
2. Form pellet / move to bottom of tube (when centrifuged);
3. Liquid / supernatant can be removed.

Question 13

The scientists measured cell damage by measuring the activity of lysosomes.
Give one function of lysosomes.

Answer 13

Break down cells / cell parts / toxins.

Question 14

H. pylori cells produce an enzyme that neutralises acid.
Suggest one advantage to the H. pylori of producing this enzyme.

Answer 14

1. To stop / reduce them being damaged / destroyed / killed;
2. By stomach acid.

Question 15

What do these data suggest about the damage caused to human cells by the toxin
and by the enzyme that neutralises acid?
Explain your answer.

Answer 15

1. More cell damage when both present / A;
2. Some cell damage when either there on their own / some cell damage in B and C;
3. Standard deviation does not overlap for A with B and C so difference is real;
4. Standard deviations do overlap between B and C so no real difference.

Question 16

The scientists carried out a further investigation. They treated the liquid from strain
A with a protein-digesting enzyme before adding it to a culture of human cells. No
cell damage was recorded.
Suggest why there was no damage to the cells.

Answer 16

1. Enzyme (a protein) is broken down (so no enzyme activity);
2. No toxin (as a result of protein-digesting enzyme activity);
3. (So) toxin is protein.

Question 17

Explain why it is not possible to determine the identity of the structures
labelled X using an optical microscope.

Answer 17

1. Resolution (too) low;
2. Because wavelength of light is (too) long;

Question 18

U. marinum cells ingest bacteria and digest them in the cytoplasm.
Describe the role of one named organelle in digesting these bacteria.

Answer 18

1. Lysosomes;
2. Fuse with vesicle;
3. (Releases) hydrolytic enzymes;

Question 19

Give two structures found in all prokaryotic cells and in all eukaryotic cells.

Answer 19

1. Cell(-surface) membrane;
2. Ribosomes;

Question 20

Scientists have found that the rate of plasmid replication is faster in cells
growing in a culture with a high concentration of amino acids than in a
culture with a lower concentration of amino acids.

Suggest one explanation for the faster rate of plasmid replication in cells
growing in a culture with a high amino acid concentration.

Answer 20

1. (Amino acids used in) protein synthesis;
2. (So) more enzymes (for DNA/plasmid replication)

Question 21

(c) What can you conclude from the figure above about a structural difference
between the plasmids and the circular DNA? Explain your answer.

Answer 21

1. Circular DNA is bigger/heavier/denser;
2. (Because band) moved further/is lower (in tube)/closer to bottom (of
   tube);

Question 22

Describe how a sample of chloroplasts could be isolated from leaves.

Answer 22

1. Break open cells/tissue and filter OR
   Grind/blend cells/tissue/leaves and filter;
2. In cold, same water potential/concentration, pH controlled solution;
3. Centrifuge/spin and remove nuclei/cell debris;
4. (Centrifuge/spin) at high(er) speed, chloroplasts settle out;

Question 23

Contrast the structure of the two cells visible in the electron micrographs
shown in the figure above.

Answer 23

1. Magnification (figures) show A is bigger than B;
2. A has a nucleus whereas B has free DNA;
3. A has mitochondria whereas B does not;
4. A has Golgi body/endoplasmic reticulum whereas B does not;
5. A has no cell wall whereas B has a murein/glycoprotein cell wall;
6. A has no capsule whereas B has a capsule;
7. A has DNA is bound to histones/proteins whereas B has
   DNA not associated with histones/proteins
   OR
   A has linear DNA whereas B has circular DNA;
8. A has larger ribosomes;

Question 24

Eukaryotic cells produce and release proteins.

Outline the role of organelles in the production, transport and release of
proteins from eukaryotic cells.

Do not include details of transcription and translation in your answer.

Answer 24

1. DNA in nucleus is code (for protein);
2. Ribosomes/rough endoplasmic reticulum produce (protein);
3. Mitochondria produce ATP (for protein synthesis);
4. Golgi apparatus package/modify;
   OR
   Carbohydrate added/glycoprotein produced by Golgi apparatus;
5. Vesicles transport
6. (Vesicles) fuse with cell(-surface) membrane;

Question 25

Give one advantage of viewing a biological specimen using a transmission electron microscope compared with using a scanning electron microscope.

Answer 25

Higher resolution View internal structures;

Question 26

Describe three properties of this solution and explain how each property prevented damage to the organelles.

Answer 26

1. (Ice) cold to prevent/reduce enzyme activity; 2. Buffered to prevent denaturing of enzyme/protein; 3. Same water potential/ Ψ to prevent lysis/bursting (of organelle);

Question 27

Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of their use when studying cells.

Answer 27

1. TEM use electrons and optical use light; 2. TEM allows a greater resolution; 3. (So with TEM) smaller organelles / named cell structure can be observed OR greater detail in organelles / named cell structure can be observed; 4. TEM view only dead / dehydrated specimens and optical (can)view live specimens; 5. TEM does not show colour and optical (can); 6. TEM requires thinner specimens; 7. TEM requires a more complex/time consuming preparation; 8. TEM focuses using magnets and optical uses (glass) lenses;

Question 28

Name two structures present in plant cells that are not present in animal cells.

Answer 28

1. Chloroplasts / plastids 2. Cell wall

Question 29

The cell-surface membrane can be seen with a transmission electron microscope but not with an optical microscope. Explain why.

Answer 29

Electron microscope has higher resolution (than optical microscope).

Question 30

No organelles are visible in the cytoplasm of this red blood cell.

Answer 30

Cytoplasm of red blood cell filled with haemoglobin.

Question 31

Before the cell was examined using the electron microscope, it was stained. This stain caused parts of the structure of the cell-surface membrane to appear as two dark lines. Suggest an explanation for the appearance of the cell-surface membrane as two dark lines.

Answer 31

1. Membrane has phospholipid bilayer; 2. Stain binds to phosphate / glycerol; 3. On inside and outside of membrane.

Question 32

Describe how you could make a temporary mount of a piece of plant tissue to observe the position of starch grains in the cells when using an optical (light) microscope.

Answer 32

1. Add drop of water to (glass) slide; 2. Obtain thin section (of plant tissue) and place on slide / float on drop of water; 3. Stain with / add iodine in potassium iodide. 4. Lower cover slip using mounted needle.

Question 33

Describe how you could use cell fractionation to isolate chloroplasts from leaf tissue.

Answer 33

1. How to break open cells and remove debris; 2. Solution is cold / isotonic / buffered; 3. Second pellet is chloroplast.

Question 34

Describe and explain how centrifuging the culture allowed the scientists to obtain a cell-free liquid.

Answer 34

1. Large / dense / heavy cells; 2. Form pellet / move to bottom of tube (when centrifuged); 3. Liquid / supernatant can be removed.

Question 35

The events that take place during interphase and mitosis lead to the production of two genetically identical cells. Explain how.

Answer 35

1. DNA replicated; 2. (Involving) specific / accurate / complementary base-pairing; 3. (Ref to) two identical / sister chromatids; 4. Each chromatid / moves / is separated to (opposite) poles / ends of cell.

Question 36

Describe the principles and the limitations of using a transmission electron microscope to investigate cell structure.

Answer 36

1. Cell homogenisation to break open cells; 2. Filter to remove (large) debris / whole cells; 3. Use isotonic solution to prevent damage to mitochondria / organelles; 4. Keep cold to prevent / reduce damage by enzymes / use buffer to prevent protein / enzyme denaturation; 5. Centrifuge (at lower speed / 1000 g) to separate nuclei / cell fragments / heavy organelles; 6. Re-spin (supernatant / after nuclei / pellet removed) at higher speed to get mitochondria in pellet / at bottom.