SL Paper 3
When feed for animals is stored, moisture levels need to be kept low to prevent the growth of fungi. Feed was exposed to 15 % moisture and placed into a respirometer. O2 consumption was monitored using the respirometer over a period of six days.
[Source: © International Baccalaureate Organization 2019]
Outline how O2 consumption is measured using a respirometer.
Suggest one conclusion that can be drawn from the results shown in the graph.
Other than humidity, suggest a variable that would need to be controlled in this experiment.
Outline the role of fungi in nutrient cycling.
Markscheme
a. a CO2 absorber/KOH is added ✔
b. a bubble/starting water level is measured ✔
c. «the rate of» movement of the bubble/pressure reduction is a measure of O2 consumption ✔
a. as fungus grows, the rate of oxygen consumption increases ✔
b. around day 4/5 fungus begins to grow/no growth in days 1–3 ✔
c. the rate of fungus growth/rate of respiration is highest after 5 days ✔
temperature/amount or type of feed «in the respirometer»/amount of CO2 absorber
OR
other reasonable suggestion ✔
Do not accept light
a. act as saprotrophs/decomposers ✔
b. rot/feed on/break down organic matter/food/organisms
OR
return nutrients trapped in organic matter to the cycle/soil ✔
Do not accept detritivores
Examiners report
Very few candidates achieved a mark. Some confused a respirometer with a spirometer; others mentioned data logging probes. Credit was given for mention of a water column. Only the highest scoring candidates understood that a carbon dioxide absorber is required to allow pressure changes to be measured.
Many candidates described what was shown by the graph rather than making a conclusion related to the growth of fungi. Thus “oxygen consumption increased after day 4” or references to the feed respiring were common answers.
Often more than one answer was given but only the first was marked. Temperature and amount of feed were the most common answers.
A study was carried out on the activity of catalase in bacteria living in extreme environments. Catalase decomposes hydrogen peroxide (H2O2). The data were obtained for a strain of bacteria called Vibrio rumoiensis which exhibits high catalase activity.
Describe the relationship between increasing H2O2 concentration and catalase activity.
Identify two factors that would need to be controlled in generating these data.
Evaluate the claim that, based on the data, the optimum H2O2 concentration for catalase is 70 mM.
Markscheme
a. as H2O2 concentration increases catalase activity increases / positive correlation
(up to 70 mM)
b. peak activity at approximately 70 mM
c. activity decreases after the peak
temperature / pH / enzyme concentration / enzyme volume / quantity of bacteria
2 answers for 1 mark.
a. maximum peak / optimum around 70 mM
b. but overlapping error bars around 70 mM
c. no clear optimum / range between 60 − 90 mM
d. may only be true for this strain Vibrio rumoiensis / other variables not reported (e.g. another form of catalase)
Examiners report
Students exposed the water plant Cabomba caroliniana to different light intensities. The bubbles of oxygen gas released by the plant were counted each minute.
Compare and contrast the experimental results for the effect of light intensity on the rate of photosynthesis of a green plant with the expected trend line.
C. caroliniana can grow well in water at 27°C. This experiment was carried out at 25°C.
Describe the effect of carrying out the same experiment at 15°C.
This experiment measured the quantity of oxygen gas released by the water plant. State one other way of measuring the rate of photosynthesis.
Markscheme
Compare:
a. both the theoretical data and the experimental data show an increase in photosynthetic rate/oxygen production with increased light intensity
OR
the rate of photosynthesis in the experimental data starts to level off at high light intensity as expected in theoretical data
Contrast:
b. the experimental data do not reach a plateau whereas theoretical data do
OR
data show photosynthesis is occurring at zero lux/light intensity whereas theoretical data would not
Accept answers using an annotated graph.
Accept answers as annotations of the graph or a new graph drawn:
«the rate of photosynthesis» would be slower/fewer bubbles
OR
rise to lower level/would plateau at a lower level/would be half the height
a. measure the absorption/decrease of CO2
b. measure the increase in biomass
c. measure rise in pH
Examiners report
Thin-layer chromatography was carried out on red and brown algae to discover what photosynthetic pigments they contained. The results were compared with the known pigments found in spinach leaves.
Identify pigment labelled P.
State a suitable solvent for extracting photosynthetic pigments from plant tissue.
Explain how the pigments in the chromatogram of spinach are identified.
Markscheme
xanthophyll
acetone
OR
alcohol
OR
ether
Accept other named organic solvent
If there is more than one answer accept only the first one. (Note: “Water mixed with alcohol” would be correct as would “alcohol, water” but “water, alcohol” would be incorrect)
a. they can be identified by their colour/analysis with spectrometer
b. measure the distance travelled by the solvent front
c. measure the distance travelled by the pigment
d. calculate the Rf value
e. they can be identified by comparing Rf values to known values
[Max 3 Marks]