Cells

Question 1

Describe and explain how centrifuging the culture allowed the scientists to obtain a cell-free liquid (3 marks)

Answer 1

1. cold, buffered, isotonic solution
2. homogenise sample to break up cells and filter to remove debris
3. centrifuge, heaviest cells/organelles form pellet
4. remove supernatant

Question 2

H. pylori cells produce an enzyme that neutralises acid.

Suggest one advantage to the H. pylori of producing this enzyme. (2 marks)

Answer 2

1. to stop the bacterium being killed

2. by stomach acid

Question 3

The scientists measured cell damage by measuring the activity of lysosomes. Give one function of lysosomes. (1 mark)

Answer 3

Break down cells/organelles/toxins

Question 4

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. (3 marks)

Answer 4

1. toxin produced by bacterium is a protein
2. so no toxin is produced
3. enzyme is a protein so broken down/hydorlysed

Question 5

Describe how you could use cell fractionation to isolate chloroplasts from leaf tissue. (4 marks)

Answer 5

1. cold, buffered (pH controlled), isotonic (same water potential) solution
2. homogenise to break up cells and filter to remove debris
3. remove pellet and ultracentrifuge supernatant at higher speed
4. chloroplast second pellet

Question 6

Name two structures in a eukaryotic cell that cannot be identified using an optical microscope. (2 marks)

Answer 6

Ribosomes, mitochondria, endoplasmic reticulum

Question 7

Explain the difference in the structure of the starch molecule and the cellulose molecule (2 marks)

Answer 7

1. starch is made up of alpha glucose but cellulose is made up of beta glucose
2. the hydrogen and hydroxyl group on carbon 1 is inverted

Question 8

Starch molecules and cellulose molecules have different functions in plant cells. Each molecule is adapted for its function. Explain one way in which starch molecules are adapted for their function in plant
cells. (2 marks)

Answer 8

1. Insoluble;
2. Don’t affect water potential;
   OR
3. Helical;
   Accept form spirals
4. Compact;
   OR
5. Large molecule;
6. Cannot leave cell.

Question 9

Explain how cellulose molecules are adapted for their function in plant cells. (3 marks)

Answer 9

1. long, straight chains
2. hydrogen bonds between the chains form microfibrils
3. gives strength to cell wall

Question 10

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

Answer 10

1. gene gets into all / most of cells of silkworm;

2. So gets into cells that make silk.

Question 11

Suggest why the scientists used a marker gene and why they used the EGFP gene. (2 marks)

Answer 11

1. Not all eggs will successfully take up the plasmid;

2. Silkworms that have taken up gene will glow.

Question 12

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? (1 mark0

Answer 12

promoter region

Question 13

Suggest two reasons why it was important that the spider gene was expressed only in the silk glands of the silkworms. (2 marks)

Answer 13

1. So that protein can be harvested;

2. Fibres in other cells might cause harm

Question 14

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 14

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.

Question 15

Describe how phospholipids are arranged in a plasma membrane. (2 marks)

Answer 15

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

Question 16

Cells that secrete enzymes contain a lot of rough endoplasmic reticulum (RER) and a large Golgi apparatus.
Describe how the RER is involved in the production of enzymes (2 marks)

Answer 16

1. ribosomes;

2. To make protein ;

Question 17

Describe how the Golgi apparatus is involved in the secretion of enzymes. (2 marks)

Answer 17

(Golgi apparatus) modifies (protein)
OR
packages / put into (Golgi) vesicles
OR
transport to cell surface / vacuole;

Question 18

Suggest why during fractionation the sample should be in a cold, buffered, isotonic solution. (3 marks)

Answer 18

1. cold: slows down metabolic reactions
2. isotonic: so there is no osmosis (cells don’t burst/shrivel)
3. buffered: so the pH doesn’t change + denature

Question 19

Name 2 structures that are present in eukaryotic cells but not prokaryotic (2 marks)

Answer 19

1. nucleus

2. rough endoplasmic reticulum

Question 20

Suggest 1 piece of evidence that would suggest that the mitochondria could still function after isolation (1 mark)

Answer 20

Test for CO2 bubbles. Put supernatant in lime water

Question 21

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

Answer 21

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 to separate nuclei
6. Re-spin (supernatant / after nuclei / pellet removed) at higher speed toget mitochondria in pellet / at bottom.

Question 22

Describe the principles and the limitations of using a transmission electron microscope to investigate cell structure. (5 marks)

Answer 22

Principles:

1. Electrons pass through / enter (thin) specimen;
2. Denser parts absorb more electrons;
3. (So) denser parts appear darker;
4. Electrons have short wavelength so give high resolution;

Limitations:

5. Cannot look at living material / Must be in a vacuum;
6. Specimen must be (very) thin;
7. Artefacts present;
8. Complex staining method / complex / long preparation time;
9. Image not in 3D / only 2D images produced.

Question 23

The events that take place during interphase and mitosis lead to the production of two genetically identical cells. Explain how. (4 marks)

Answer 23

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

Question 24

The student cut thin sections of tissue to view with an optical microscope. Explain why it was important that the sections were thin. (2 marks)

Answer 24

1. To allow (more) light through;
   Accept: transparent
2. A single / few layer(s) of cells to be viewed.
   Accept: (thin) for better / easier stain penetration

Question 25

The student prepared a temporary mount of the onion tissue on a glass slide. She covered the tissue with a cover slip. She was then given the following instruction.“Push down hard on the cover slip, but do not push the cover slip sideways.”
Explain why she was given this instruction. (2 marks)

Answer 25

1. Push hard – spread / squash tissue;
2. Not push sideways – avoid rolling cells together / breaking
   chromosomes.

Question 26

The doctors compared median survival times for patients in each group. How would you find the median survival time for a group of patients? (2 marks)

Answer 26

1. Rank all mean survival time in ascending order;

2. Find value with same number (of people) above and below

Question 27

MM is caused by a faulty receptor protein in cell-surface membranes. Cells in MM tumours can be destroyed by the immune system. Suggest why they can be destroyed by the immune system. (3 marks)

Answer 27

1. Faulty protein recognised as an antigen / as a ‘foreign’ protein;
2. T cells will bind to faulty protein / to (this) ‘foreign’ protein;
3. T cells will stimulate clonal selection of B cells;
4. release of antibodies against faulty protein.

Question 28

Watering treatment was the independent variable in this investigation. Explain what is meant by the independent variable. (1 mark)

Answer 28

Variable that is changed;

Question 29

Describe the role of the spindle fibres and the behaviour of the chromosomes during each of these phases (5 marks)

Answer 29

1. Prophase chromatin condenses to form chromosomes
2. spindle fibres attach to centromere
3. Metaphase: chromosomes line up along the equator of the cell
4. Anaphase: centromeres divide
5. Chromatids to opposite ends of oles of the cell