2.1 Cell structure

Question 1

Describe the structure of the nucleus.

Answer 1

1. Nuclear envelope (double membrane) with pores
2. Chromatin (chromosomes consisting of protein-bound, linear DNA, either more or less condensed)
3. Nucleolus

Question 2

Describe the function of the nucleus.

Answer 2

1. Stores genetic information needed to code for polypeptide production.
2. Site of DNA replication.
3. Site of transcription / production of mRNA.
4. Site of tRNA production (but NOT the joining of specific amino acids to tRNA - this happens in the cytoplasm).
5. Production of rRNA/ribosomes.

Question 3

Describe the function of the Golgi apparatus.

Answer 3

1. Modify and package proteins (eg by adding carbohydrate to make glycoprotein)
2. Modify and package lipids (eg by adding carbohydrate to make glycolipids)
3. Forming and releasing Golgi vesicles
4. Forming and releasing lysosomes

Question 4

Describe the function of lysosomes.

Answer 4

Digest cell parts / toxins / cells in phagosomes, using hydrolytic enzymes

Question 5

Give the two types of molecule from which a ribosome is made.

Answer 5

1. rRNA
2. Protein

Question 6

How do prokaryotic cells differ from eukaryotic cells?

Answer 6

1. Much smaller
2. Cytoplasm that lacks membrane-bound organelles
3. Smaller ribosomes (70S, as opposed to 80S)
4. One circular DNA molecule that is free in the cytoplasm and is not associated with proteins
5. A cell wall that contains murein, a glycoprotein
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6. Maybe, but not always, plasmids
7. Maybe, but not always, a capsule surrounding the cell
8. Maybe, but not always, one or more flagella. Eukaryotes can ALSO have flagella, but they have a different internal structure.

Question 7

Describe the structure of a virus.

Answer 7

1. genetic material, either DNA or RNA
2. a protein capsid
3. attachment proteins, for binding to receptors on host cells.

Question 8

Why are viruses described as acellular and non-living?

Answer 8

Acellular: not made of cells
Non-living: have no metabolism (eg cannot independently respire or replicate)

Question 9

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

Answer 9

1. Electrons pass through a very thin specimen;
2. Denser parts / parts stained with metal ions absorb more electrons so appear darker;
3. Electron beams have short wavelength so give high resolution;

Question 10

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

Answer 10

1. Cannot look at living material as must be in a vacuum;
2. Specimen must be very thin;
3. Artefacts present;
4. Complex staining method / complex / long preparation time;
5. Image black and white and not in 3D / only black and white 2D images produced.

Question 11

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

Answer 11

1. TEM use electrons and optical use light;
2. TEM allows a greater resolution;
3. (So with TEM) smaller organelles can be observed / organelles can be observed in greater detail
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 12

Describe and explain how cell fractionation and ultracentrifugation can be used to isolate organelles from a suspension of cells.

Answer 12

Cell fractionation:

1. Cell homogenisation to break open cells (eg blender / grind with sand with a pestle and mortar)
2. Filter to remove larger debris / whole cells;
3. Use isotonic solution to prevent damage to organelles;
4. Keep cold to prevent / reduce damage by enzymes;
5. Use pH buffer to prevent protein / enzyme denaturation;

Ultracentrifugation:

6. Centrifuge at lower speed to separate heavier organelles (in pellet);
7. Re-spin supernatant (after pellet removed) at higher speed to get less dense organelles in pellet.

Question 13

Scanning electron microscopes

Answer 13

SEMs scan a beam of electrons across the specimen.
This beam reflects off the surface of the specimen and the electrons are detected, forming a three-dimensional image that shows the surface of the specimen.
SEMs can be used on thick or 3-D specimens, but they must be dead (as they will be placed in a vacuum).
SEMs give lower resolution images than TEMs but higher resolution images than optical microscopes. False colour is often added afterwards.