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Date	May 2019	Marks available	4	Reference code	19M.2.SL.TZ2.6
Level	Standard level	Paper	Paper 2	Time zone	Time zone 2
Command term	Outline	Question number	6	Adapted from	N/A

Question

Describe briefly the endosymbiotic theory.

[3]

Outline how photosynthesis produces glucose.

[4]

Discuss the control of blood glucose levels and the consequences if they are not maintained.

[8]

Markscheme

a. eukaryotes evolved from prokaryotes ✔

b. prokaryotes engulfed other prokaryotes without digesting them ✔

c. engulfed aerobic cell/prokaryote became mitochondria ✔

d. engulfed photosynthetic cell/ prokaryotes became chloroplasts ✔

e. these organelles have a double membrane «due to the engulfing process» ✔

f. mitochondria/chloroplasts contain DNA/small ribosomes/70S ribosomes ✔

a. solar/light energy is converted to chemical energy ✔

b. energy needed to produce glucose ✔

c. only specific wavelengths are absorbed by chlorophyll
OR
red and blue absorbed most strongly.
OR
chlorophyll is the pigment that absorbs light energy ✔

d. H⁽⁺⁾/electrons from water are used to reduce compounds ✔

e. CO₂ is absorbed/used/reduced to produce carbohydrates ✔

f. correct word/balanced symbol equation of photosynthesis ✔

Accept correct reference to NADPH/ATP from AHL.

control: [6 max]

a. homeostasis is the maintenance of a constant internal environment ✔

b. the pancreas produces hormones that control the levels of glucose ✔

c. if glucose levels in blood are high, beta-cells «of the pancreas» produce insulin ✔

d. «insulin» causes the cells to take up /absorb glucose ✔

e. liver stores excess glucose as glycogen ✔

f. if glucose levels in blood are low, alpha-cells «of the pancreas» produce glucagon ✔

g. «glucagon» causes the liver to break down glycogen into glucose ✔

h. «glucagon» increase levels of glucose in the blood ✔

i. negative feedback controls the glucose levels ✔ OWTTE

consequences:

j. if the pancreas produces little/no insulin a person can develop type I diabetes ✔

k. a person with type I diabetes «usually» needs/is dependent on injections of insulin ✔

l. type II diabetes occurs when the body becomes resistant to insulin/cells do not respond to insulin ✔

m. type II diabetes can «sometimes» be controlled by diet and exercise ✔

n. named consequence of having diabetes «eg: eye damage» ✔

Award [6 max] if no consequences are given.

Examiners report

Endosymbiotic theory. Those who had been taught it gained at least one mark. Few mentioned that the cell that became the mitochondrion was aerobic, and that the one that became the chloroplast was photosynthetic.

Glucose and photosynthesis. Again, this was an example of leaping into an answer without planning. Despite G2 protestations, HL material was not expected. Better candidates were able to gain full marks by stating that the light was trapped by chloroplasts, processes converted this into chemical energy and energy was needed to make glucose and included a correct equation ( if a symbol equation, it needed to be balanced).

Control of blood glucose levels. Weaker candidates tied themselves in knots trying to explain. Weak candidates also described insulin and glucagon as enzymes, not hormones. Other common mistakes were Alpha cells and Beta cells do not move, they just produce the hormones and glucagon and insulin do not have any direct effect on the glycogen molecules. The causes and treatment of type 2 diabetes were quite well known. A large number of students used the word ‘digest’ incorrectly, stating that glucose was ‘digested’ to glycogen.

Syllabus sections

Core » Topic 2: Molecular biology » 2.9 Photosynthesis

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