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Date May 2022 Marks available 5 Reference code 22M.2.SL.TZ1.6
Level Standard level Paper Paper 2 Time zone Time zone 1
Command term Describe Question number 6 Adapted from N/A

Question

Outline reasons for the therapeutic use of stem cells.

[3]
a.

Describe how leaf cells make use of light energy.

[5]
b.

Explain how cells and cell components in the blood defend the body against infectious disease.

[7]
c.

Markscheme

  1. unspecialized/undifferentiated stem cells can divide / differentiate along different pathways;
  2. (stem cells are accessible as they) come from embryos/bone marrow/umbilical cord blood/adult tissue;
  3. (stem cells) can regenerate/repair diseased/damaged tissues in people;
  4. valid specific example;
  5. drugs can be tested on stem cells (in laboratories to see if they are harmful);
a.
  1. leaf cells contain chloroplasts;
  2. light is absorbed by chlorophyll (in chloroplasts);
  3. other pigments absorb different wavelengths;
  4. light energy is used in photosynthesis;
  5. (light is needed) to combine water and carbon dioxide/fix carbon dioxide;
  6. carbon compounds/organic compounds/glucose/starch/carbohydrate are produced;
  7. blue and red light is absorbed;
  8. perform photolysis
    OR
    split water molecules;

Wavelengths accepted for mpg.

b.

Platelets: [3 max]

  1. damage/cuts to blood vessels causes platelets to be activated;
  2. the platelets release clotting factors;
  3. initiates cascade of reactions
    OR
    fibrinogen is converted to fibrin;
  4. forms a mesh over the damaged area;
  5. prevents pathogens from entering the body;

    Phagocytes: [3 max]
  6. phagocytes/phagocytic white blood cells in the blood travel to the site of infection;
  7. (phagocytes) squeeze between the capillary cells;
  8. (phagocytes) engulf/ingest/take in pathogens;
  9. the pathogen is digested/broken down by/within the phagocyte;

    Lymphocytes: [3 max]
  10. lymphocytes recognize a particular fragment/antigen of a pathogen;
  11. (lymphocytes) release antibodies;
  12. (antibodies) provide specific immunity;
  13. memory cells provide rapid response giving long-term immunity (to pathogens previously recognized);
  14. antibodies destroy pathogens;

ECF may be applied when candidates use white blood cells in place of specific terms.

c.

Examiners report

Some excellent answers; stem cells was usually well answered, leaf cells was often more complex than necessary but marks were usually able to be gained, though many answers were missing the obvious, for example leaf cells contain chloroplasts. Defence against disease; far too often over-complicated and confused answers. It is important that students are aware of the level they need to know. There is no need to over-complicate and risk loss of understanding, which was evident across many answers and which was so confused across the roles of macrophages, B cells and T cells. Equally, for cell components, more often than not this was ignored or mistakenly discussed as skin and mucus over platelets.

Mostly well done. Appropriate examples provided for stem cell therapies. It seems that many candidates think that all stem cells can differentiate into "any" type of cells, which is not correct. Other candidates showed understanding of the discrimination between the source and the potential of the cells. Occasionally candidate showed confusion about the term "stem" in that "stem cells show the way plants grow"; sometimes therapeutic use of stem cells was confused with gene editing.

a.

Generally speaking, students know how leaf cells make use of light energy; at least three marks were usually gained. Some candidates made the answer too complex and involved, when a simple straight-forward answer would have been better.

b.

Among all candidates, there was knowledge of how each of the blood components (platelets, phagocytes, lymphocytes) can defend against infectious disease; each of the marking points in the mark scheme was eventually awarded; popular ideas were that fibrin is formed, phagocytes engulf pathogens and antibodies provide specific immunity.

Correct terminology was confused or was not employed in too many answers.

c.

Syllabus sections

Core » Topic 2: Molecular biology » 2.9 Photosynthesis
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