Cell structure and microscopy Qp

Question 1

All prokaryotic cells contain a circular DNA molecule and some prokaryotic cells
contain plasmids.
(b) 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 1

Mark in pairs: 1 and 2 OR 3 and 4
1. (Amino acids used in) protein synthesis;
Accept for ‘protein synthesis’, translation
2. (So) more enzymes (for DNA/plasmid replication)
OR
(So) more DNA polymerase;
3. (Amino acids used in) respiration;
4. (So) more energy/ATP (for DNA/plasmid replication);

Question 2

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

Answer 2

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

Question 3

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 3

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

Question 4

Suggest why a nucleus is not visible in above image.

Answer 4

A section/slice (so nucleus in another part of cell)
OR
(Nucleus) not stained;

Question 5

Figure 2 shows a photograph of part of a mitochondrion from a mouse liver
cell taken using a transmission electron microscope at × 62 800
magnification.
Produce a scientific drawing of the mitochondrion in Figure 2 in the box
below.
Label the following parts of the mitochondrion on your drawing.
* Matrix
* Crista

Answer 5

1. No sketched / hanging / crossing lines / shading;
   Ignore stippling
2. Must look similar;
3. Matrix and crista correctly labelled;
   Ignore any other labels
4. Correct scale stated (x 62 800);
   Accept other suitable scale gi

Question 6

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

Answer 6

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 7

Explain why the solution the biologist used was ice-cold, buffered and the
same water potential as the liver tissue

Answer 7

1. Ice-cold – Slows / stops enzyme activity to prevent digestion of
   organelles / mitochondria;
2. Buffered – Maintains pH so that enzymes / proteins are not
   denatured;
   Reject reference to cells
3. Same water potential – Prevents osmosis so no lysis / shrinkage of
   organelles / mitochondria /

Question 8

The second centrifuge tube was spun at a higher speed to obtain the
sample of organelles labelled C in the diagram (step 5).
Suggest why.

Answer 8

Mitochondria / organelle C less dense than nucleus / organelle in first pellet;
Accept ‘lighter’ for less dense

Question 9

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

Answer 9

Electron microscope has higher resolution (than optical microscope)

Question 10

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

Answer 10

Cytoplasm of red blood cell filled with haemoglobin

Question 11

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 11

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

Question 12

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

Answer 12

Mitochondrion / ribosome / endoplasmic reticulum / lysosome / cellsurface membrane.

Question 13

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 13

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. Allow any appropriate method that avoids trapping air bubbles
5. Lower cover slip using mounted needle