DP Biology (last assessment 2024)

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Question 23M.3.SL.TZ2.13

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Date May 2023 Marks available [Maximum mark: 6] Reference code 23M.3.SL.TZ2.13
Level SL Paper 3 Time zone TZ2
Command term Describe, Distinguish, Explain Question number 13 Adapted from N/A
13.
[Maximum mark: 6]
23M.3.SL.TZ2.13

A study was done on the effect of heat stress on species of coral and their relationship with zooxanthellae algae.

(a)

Describe the relationship between these two types of organisms.

[3]

Markscheme
  1. symbiotic/mutually beneficial;
  2. the coral benefits from the algae/zoothanthellae by receiving nutrients/glucose/oxygen/products of photosynthesis;
  3. the algae benefit from the coral by receiving CO2
    OR
    the algae benefit from the coral by gaining a habitat;
Examiners report

Most candidates described the relationship between coral and zooxanthellae algae as mutualism and many described the advantages to the coral. Few described the advantages to the algae of the relationship.

The graphs show the effects of heat stress on the rates of photosynthesis in zooxanthellae and respiration of both organisms.

[Source: Reprinted from Marine Pollution Bulletin, Vol 152, Wooldridge, S.,
Excess seawater nutrients, enlarged algal symbiont densities and bleaching sensitive reef locations:
1. Identifying thresholds of concern for the Great Barrier Reef,
Copyright (2020), with permission from Elsevier.]

(b.i)

Distinguish between the temperatures at which heat stress begins to have an effect on the rates of the two processes.

[1]

Markscheme

(heat stress begins) in photosynthesis/oxygen production at a (slightly) lower temperature than in respiration/oxygen consumption;

Accept correct numerical distinction.

Examiners report

The term 'distinguish' was problematic. Many simply stated that enzyme activity began to decline.

(b.ii)

Global warming increases sea temperatures. Coral bleaching occurs at sea water temperatures above 33 °C. Using the data in the graphs, explain the reasons for coral bleaching.

[2]

Markscheme
  1. the photosynthesis (of zooxanthellae) has decreased at 33 °C
    OR
    less oxygen is provided (for the coral);
  2. the symbiotic relationship starts to break down as photosynthesis decreases/OWTTE;
  3. the coral expel the zooxanthellae;
Examiners report

Very few candidates described the effect of warm waters on the expulsion of algae by corals. Usually just one mark was given for a reduction in oxygen or photosynthesis. Most candidates did not understand that the zooxanthellae are inside the coral tissue. Many answers were given about increased CO2, ocean acidity and global warming.