T2 Cells

Question 1

Name the process by which prokaryotic cells divide. 1 mark

Answer 1

Binary fission

Reject fusion

Question 2

Name two structures present in plant cells that are not present in animal cells. 1 mark

Answer 2

1. Chloroplasts / plastids
2. Cell wall
3. Cell vacuole
4. Starch grains / amyloplasts;
   Any 2 for 1 mark

Question 2

Give one advantage of viewing a biological specimen using a transmission electron
microscope compared with using a scanning electron microscope. 1 mark

Answer 2

Higher resolution
or
View internal structure

Question 2

A biologist prepared a sample of organelles labelled C from liver. He used the following method.
1. Added to the liver tissues an ice-cold, buffered solution with the same water potential as the liver tissue.
2. Mixed the liver and solution in a blender.
3. Filtered the mixture from the blender.
4. Spun the filtered liquid in a centrifuge at a low speed. A pellet appeared in the bottom
of the centrifuge tube.
5. Poured off the liquid above the pellet into a second centrifuge tube and spun this at a
higher speed to obtain the sample of organelles labelled C.
(c) Explain why the solution the biologist used was ice-cold, buffered and the same water
potential as the liver tissue (step 1) 3 marks

Answer 2

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 / C;
   Ignore damage
   For each mark must link reason to relevant property

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. 4 marks

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 3

Starting with some lettuce leaves, describe how you would obtain a sample of undamagedchloroplasts. Use your knowledge of cell fractionation and ultracentrifugation to answer this question. 6 marks

Answer 3

1. Chop up (accept any reference to crude breaking up);
2. Cold;
3. Buffer solution;
4. Isotonic / same water potential;
5. Filter and centrifuge filtrate;
6. Centrifuge supernatant;
7. At higher speed;
8. Chloroplasts in (second) pellet;

Question 3

  Name the fixed position occupied by a gene on a DNA molecule 1 mark

Answer 3

Locus/Loci

Question 3

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 3

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 3

The student produced the sucrose solutions with different concentrations from a
concentrated sucrose solution.
Name the method she would have used to produce these sucrose solutions. 1 mark

Answer 3

Dilution series
Accept serial dilution

Question 4

Describe how a gene is a code for the production of a polypeptide. Do not include
information about transcription or translation in your answer. 3 marks

Answer 4

1. (Because) base/nucleotide sequence;
2. (In) triplet(s);
3. (Determines) order/sequence of amino acid sequence/primary structure (in
   polypeptide);

Question 5

Trexall is a drug that can be used to slow the development of various forms of cancer.
Trexall slows cell division by interacting with an enzyme called dihydrofolate reductase (DR).
DR is involved in making nucleotides; the substrate for DR is folic acid.
Use the information provided to suggest how Trexall slows cell division. 3 marks

Answer 5

1. (Trexall acts as a) competitive inhibitor
   OR
   (Trexall) competes (with folic acid/substrate) for/is able to fit into/binds at active site (on dihydrofolate reductase / enzyme);
   Reject Trexall and folic acid have the same shape
2. Less folic acid/substrate attaches
   OR
   Fewer enzyme-substrate complexes;
   Accept folic acid/substrate is prevented from binding
   3.Fewer/not enough nucleotides available for DNA replication;
   Accept fewer/not enough nucleotides available during interphase/for semi-conservative replication/to add to (all) template strands/for
   transcription

Question 6

Mitosis is important in the life of an organism. Give two reasons why 2 marks

Answer 6

1. Growth / increase in cell number;
   Ignore growth of cells
2. Replace cells / repair tissue / organs / body;
   Ignore repair cells
   Reject bacteria
3. Genetically identical cells;
   ‘Produces 2 genetically identical cells’ does not reach MP1 as well
   as MP3
4. Asexual reproduction / cloning;
   Allow example or description

Question 7

In which phase of the cell cycle does DNA replication take place?
1 mark

Answer 7

Interphase

Question 8

Describe the role of the spindle during mitosis 2 marks

Answer 8

Attachment of centromeres / chromosomes / chromatids; Separation of centromeres /
chromatids / chromosomes;

Question 9

Meiosis also occurs during the life cycle of organisms. What is the importance of meiosis?
2 marks

Answer 9

Halves chromosome number / haploid;
Diploid / full number restored at fertilisation;
Allow correct reference to variation

Question 10

A student prepared a stained squash of cells from the root tips of garlic to calculate a mitotic
index. He:
1. cut the end 5 mm from 10 garlic roots
2. placed the root tips into a Petri dish containing 5 cm3
of hydrochloric acid for 12 minutes
3. rinsed the root tips in distilled water
4. placed one of the root tips on a microscope slide and added toluidine blue stain
5. placed a coverslip onto the microscope slide, and gently pressed the coverslip downwards on the root tip
6. observed the root tip using an optical microscope.
7.
(a)  Suggest why the student soaked the root tips in hydrochloric acid in step 2. 2 marks

Answer 10

1. To break down links between cells/cell walls
   OR
   To separate cells/cell walls
   OR
   To break down/hydrolyse cellulose/cell wall;
2. Allowing the stain to pass/diffuse into the
   cells
   OR
   Allowing the cells to be (more easily)
   squashed;
3. To stop mitosis;

Question 11

Describe how a phagocyte destroys a pathogen present in the blood. 3 marks

Answer 11

1. Engulfs;
   Accept endocytosis
   OR
   Description
   Ignore ‘taken in’
2. Forming vesicle/phagosome and fuses with lysosome;
3. Enzymes digest/hydrolyse;
   Accept lysozymes for ‘enzymes’

Question 12

Give two types of cell, other than pathogens, that can stimulate an immune response. 2 marks

Answer 12

1. (Cells from) other organisms/transplants;
2. Abnormal/cancer/tumour (cells);
3. (Cells) infected by virus;
   Accept ‘own cells’ if autoimmune response suggested
   Accept APCs
   Accept non-self

Question 13

What is the role of the disulfide bridge in forming the quaternary structure of an antibody? 1 mark

Answer 13

Joins two (different) polypeptides;
Accept holds/attaches
Accept ‘prevents polypeptide chains separating’

Question 14

In Europe, viruses have infected a large number of frogs of different species. The viruses are
closely related and all belong to the Ranavirus group.
Previously, the viruses infected only one species of frog.
3.
(a) Suggest and explain how the viruses became able to infect other species of frog.
2 marks

c. Describe how the B lymphocytes of a frog would respond to vaccination against Ranavirus.
You can assume that the B lymphocytes of a frog respond in the same way as B
lymphocytes of a human.
Do not include details of the cellular response in your answer. 3 marks

Answer 14

  1. Mutation in the viral DNA/RNA/genome/genetic material;
Accept named examples mutations
2. Altered (tertiary structure of the) viral attachment protein;
Accept ‘antigen’ for ‘attachment protein’
Accept causes antigenic variability
3. Allows it/attachment protein/virus to bind (to receptors of other species);
Accept descriptions of binding eg is complementary

1. B cell (antibody) binds to (viral) specific/complementary receptor/antigen;
   Accept B cell forms antigen-antibody complex
2. B cell clones
   OR
   B cell divides by mitosis;
3. Plasma cells release/produce (monoclonal) antibodies (against the virus);
4. (B/plasma cells produce/develop) memory cells;
   Accept B cell undergoes clonal selection/expansion

Question 15

Name two techniques the scientists may have used when analysing viral DNA to determine
that the viruses were closely related 1 mark

Answer 15

For one mark, accept any two of the following:
*   The polymerase chain reaction
*   Genetic/DNA fingerprinting
*   (Gel) electrophoresis
*   DNA/genome sequencing;
Accept PCR for polymerase chain reaction
Accept autoradiography
Accept DNA hybridisation
Accept compare DNA/base sequence for ‘DNA sequencing’
Ignore compare mRNA base sequence
Ignore compare amino acid sequence
Ignore DNA probes

Question 16

Determining the genome of the viruses could allow scientists to develop a vaccine.
Explain how. 2 marks

Answer 16

1. (The scientists) could identify proteins (that derive from the genetic code)
   OR
   (The scientists) could identify the proteome;
2. (They) could (then) identify potential antigens (to use in the vaccine);
   Reject if answer suggests vaccine contains antibodies

Question 17

What is a monoclonal antibody?
1 mark

Answer 17

(Antibodies with the) same tertiary structure
OR
(Antibody produced from) identical/cloned plasma cells/B cells/B lymphocytes;
Accept in context of single plasma/B cell/B lymphocyte
Reject: genetically identical antibody

Question 18

After a disease is diagnosed, monoclonal antibodies are used in some medical treatments.
Give one example of using monoclonal antibodies in a medical treatment. 1 mark

Answer 18

Accept any one suitable use, eg
Targets/binds/carries drug/medicine to specific cells/antigens/receptors
OR
Block antigens/receptors on cells;
Accept cancer/diseased cells (as a specific cell).
Ignore medical diagnosis/pregnancy/ PSA/ELISA test

Question 19

Describe the role of antibodies in producing a positive result in an ELISA test 4 marks

Answer 19

1.   (First) antibody binds/attaches /complementary (in shape) to antigen;
2.   (Second) antibody with enzyme attached is added;
3.   (Second) antibody attaches to antigen;
Accept (second) antibody attaches to (first) antibody (indirect ELISA test).
4.   (Substrate/solution added) and colour changes;
Only award if enzyme mentioned.

Question 20

When a person is bitten by a venomous snake, the snake injects a toxin into the person.
Antivenom is injected as treatment. Antivenom contains antibodies against the snake toxin.
This treatment is an example of passive immunity.
Explain how the treatment with antivenom works and why it is essential to use passive
immunity, rather than active immunity. 2 marks

Answer 20

1. (Antivenom/Passive immunity) antibodies bind to the toxin/venom/antigen and
   (causes) its destruction;
   For ‘bind’ accept ‘attach’, ignore ‘attack’.
   For ‘destruction of toxin’ accept agglutination or phagocytosis.
   Ignore reference to antibodies ‘neutralising toxin/stopping damage’
   Reject reference to ‘killing’ toxin/venom.
2. Active immunity would be too slow/slower;
   Accept ‘passive immunity is fast er’, not simply ‘passive immunity is
   fast’

Question 21

During vaccination, each animal is initially injected with a small volume of venom. Two
weeks later, it is injected with a larger volume of venom.
Use your knowledge of the humoral immune response to explain this vaccination
programme. 3 marks

Answer 21

1. B cells specific to the venom reproduce by mitosis;
   Accept in context of primary or secondary immune response.
   Credit idea of specificity if given once in relation to T or B cell.
   Accept a description for specificity.
   Accept ‘clone’ for ‘reproduce by mitosis’.
   ‘Clonal selection of B cells’ = MP1.
2. (B cells produce) plasma cells and memory cells;
3. The second dose produces antibodies (in secondary immune response) in higher
   concentration and quickly
   OR
   The first dose must be small so the animal is not killed;
   Accept ‘a lot of antibody’ for ‘higher concentration of antibody’.

Question 22

A high mutation rate of a virus makes it difficult to develop a vaccine. Explain why. 3 marks

Answer 22

1. (High mutation rate leads to) antigens change/antigenic
   variability;
   Accept (high mutation rate leads to) changes in base sequence
   coding for antigen;
2. Vaccine contains specific antigen;
3. Antibodies not complementary to (changed) antigen / won’t bind to
   (changed) antigens;

Question 23

Describe how HIV is replicated 4 marks

Answer 23

1. Attachment proteins attach to receptors on helper T cell/lymphocyte;
2. Nucleic acid/RNA enters cell;
3. Reverse transcriptase converts RNA to DNA;
4. Viral protein/capsid/enzymes produced;
5. Virus (particles) assembled and released (from cell);

Question 24

Describe how the human immunodeficiency virus (HIV) is replicated once inside helper T
cells (TH cells). 4 marks

Answer 24

1. RNA converted into DNA using reverse transcriptase;
   Reject ‘messenger’ or ‘m’ before RNA
2. DNA incorporated/inserted into (helper T cell) DNA/chromosome/genome/nucleus;
3. DNA transcribed into (HIV m)RNA;
   Accept descriptions of transcription
4. (HIV mRNA) translated into (new) HIV/viral proteins (for assembly into
   viral particles);
   Accept descriptions of translation
   Accept named viral protein, eg capsid
   Reject viral cells

Question 25

Describe the structure of the human immunodeficiency virus (HIV).
4 marks

Answer 25

1.   RNA (as genetic material);
Reject nucleus/DNA/plasmids.
2.   Reverse transcriptase;
3.   (Protein) capsomeres/capsid;
Reject capsule.
4.   (Phospho)lipid (viral) envelope
OR
Envelope made of membrane;
Reject if HIV has a cell membrane or a cell wall.
5.   Attachment proteins;
Accept gp41 and/or gp 120.
Accept glycoprotein.
Accept description of attachment protein.
Ignore ‘receptor protein’.
Ignore cytoplasm.