Module 2. Cell structure - (set 2)

Question 1

Define Magnification

Answer 1

• The number of times greater image is than size of the real (actual) object
• Magnification = size of image / size of real object

Question 2

Define Resolution

Answer 2

minimum distance apart 2 objects can be to be distinguished as separate objects

Question 3

A scientist prepared a suspension of organelles in a solution that prevented damage to the organelles.
Describe three properties of this solution and explain how each property prevented damage to the organelles.
[3 marks]
[5.6 - JUN19] [AS]

Answer 3

1. (Ice) cold to prevent/reduce enzyme activity;
2. Buffered to prevent denaturing of enzyme/protein;
   Accept: prevent change of tertiary structure.
3. Same water potential/ Ψ to prevent lysis/bursting (of organelle);
   Accept: isotonic for same water potential

Question 4

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

Answer 4

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
   Ignore damage

Question 5

Describe and explain how cell fractionation and ultracentrifugation can be used to isolate mitochondria from a suspension of animal cells.
(5)

Answer 5

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;

Question 6

A biologist prepared a sample of organelles labelled C from liver.
Explain why the biologist used a blender and then filtered the mixture.
[2 marks]
[14d PMT]

Answer 6

1. Break open cells / homogenise / produce homogenate;
2. Remove unbroken cells / larger debris;

Question 7

Describe how a sample of chloroplasts could be isolated from leaves.
[4 marks]
[5.1 - JUN21]

Answer 7

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; Accept isotonic, buffered
3. Centrifuge/spin and remove nuclei/cell debris;
4. (Centrifuge/spin) at high(er) speed, chloroplasts settle out;

Question 8

Skin cells may be studied with a transmission electron microscope or an optical microscope. Explain the **advantages **and limitations of using a transmission electron microscope to study cells.
(6)

Answer 8

1. TEM uses (beam of) electrons;
2. These have short wavelength;
3. Allow high resolution/greater resolution/Allow more detail to be seen/greater useful magnification;
4. Electrons scattered (by molecules in air);
5. Vacuum established;
6. Cannot examine living cells;
7. Lots of preparation/procedures used in preparing specimens/ fixing/staining/sectioning;
8. May alter appearance/result in artefacts;

Question 9

Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of their use when studying cells.
(6)

Answer 9

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 10

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.
(4 marks)

Answer 10

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.