▶️ Answer/Explanation
(a)
- The SBP + PCHS combination is the most efficient treatment.
- It results in the lowest percentage of bacteria remaining after both 1 day and 3 days.
- Also, it shows a lasting effect, maintaining nearly 0% bacteria even after 3 days.
(b)
- Bacteriophages infect and destroy bacteria directly.
- They can penetrate the biofilm and kill bacteria throughout, not just at the surface.
- Biofilms have a protective matrix (EPS) that often blocks chemicals but not phages.
- Phages are highly specific, targeting only the pathogen (e.g., S. aureus or P. aeruginosa).
Chemicals can cause environmental contamination or lead to chemical resistance in bacteria.
(c)
- This is a form of bioremediation.
- Pseudomonas putida can convert toxic methyl mercury into less toxic forms.
- Methyl mercury is broken down into methane and mercury ions (Hg²⁺).
- Mercury ions can be further converted to elemental mercury (Hg⁰) by other microbes.
- Elemental mercury is insoluble and easier to remove from wastewater.
Markscheme
(a)
SBP+PCHS as more bacteria removed/killed
OR
SBP+PCHS as the effect lasts longer/more than 3 days;
(b)
a. bacteriophages (are viruses that) infect bacteria;
b. they can spread through entire biofilm to kill bacteria;
OR
chemicals only kill bacteria near the surface of biofilm
c. polysaccharide matrix/EPS/biopolymers act as a (physical) barrier to chemicals;
d. bacteriophages are specific/can be added to destroy a specific pathogen;
e. chemicals can contaminate water
OR
can lead to bacteria resistant to the chemical;
(c)
a. bioremediation;
b. Pseudomonas putida used;
c. methyl mercury converted/decomposed to (methane and) mercury ions;
d. mercury ions turned into mercury (by other bacteria);
e. insoluble mercury can be separated from waste water;
▶️ Answer/Explanation
b.
An indicator species is an organism that is present or absent when specific environmental conditions exist.
c.
- Example: DDT (a pesticide).
- DDT is not excreted easily and accumulates in fat tissue of organisms.
- When contaminated organisms are eaten, predators consume large amounts of DDT.
- This leads to increased concentration of DDT at each trophic level (biomagnification).
- Top predators like bald eagles or ospreys are severely affected — for example, thinner eggshells that lead to population decline.
Biomagnification affects some organisms more due to their position in the food chain and fat content.
e. Islands can be considered both:
- Closed ecosystems because they have limited exchange of matter (like nutrients) with their surroundings.
- Open ecosystems because energy (like sunlight) and some matter (like seeds, birds, or ocean debris) can still enter or leave.
Markscheme
b.
Organism that is present/absent when specific environmental conditions exist
OR
Organism used to assess a specific environmental condition
c.
a. Example, e.g., DDT/mercury/cadmium
b. Substance accumulates in fat tissue/not excreted when consumed
c. Contaminated organisms consumed in large quantities by higher-level consumers
d. Pollutant becomes more concentrated at each higher trophic level/through the food chain
e. Some pollutants are more likely to be biomagnified (accumulate in fat tissue)
OR
Some organisms are more likely to be affected by biomagnification than others
OR
Biomagnification not the same at each trophic level
Only [2] if verified example not given.
e.
Closed because islands do not exchange matter/nutrients with surroundings
OR
Open because islands do exchange matter/nutrients with surroundings
▶️ Answer/Explanation
a.
- The fundamental niche is the potential range of environmental conditions and resources an organism can occupy and use, without competition or limiting factors.
- The realized niche is the actual niche the organism occupies, considering biotic interactions like competition or predation.
b.
- Competition limits the niche an organism can occupy.
- It reduces available resources such as food, space, or mates.
- If competition is removed, the organism’s niche may expand toward its fundamental niche.
- Strong competition can lead to competitive exclusion, where one species is driven out of a niche.
c.
- A keystone species has a disproportionate effect on the structure and function of an ecosystem relative to its abundance.
- The removal of a keystone species would drastically alter the ecosystem, often resulting in the collapse or severe change in the food web.
Markscheme
a.
Fundamental niche is the potential niche and realized niche is the actual niche
OWTTE (or words to that effect).
b.
a. Competition can limit the niche
b. Competition can limit resources
c. When competition is removed, the niche can expand
d. Can result in exclusion or removal of a species from a niche / only one species can occupy a niche
c.
a. Keystone species has a strong/disproportionate effect on a community/food web/ecosystem
b. Absence of keystone species would completely alter the ecosystem
▶️ Answer/Explanation
a.
- A: Decomposition– the process by which litter is broken down and nutrients are transferred to the soil.
- B: Leaching– the process by which nutrients are lost from the soil, often due to water movement.
b.
- Ecosystem I is the tropical rainforest.
- This is because it shows:
- Low litter levels due to rapid decomposition (thicker arrow from litter to soil).
- High biomass, consistent with high productivity typical of rainforests.
- High leaching from soil (thick arrow B), which is due to heavy rainfall in tropical regions.
Markscheme
a.
a. Process A: decomposition/decay
b. Process B: leaching/erosion/run-off/weathering
b.
a. Ecosystem I
b. Low levels of litter due to high rates of decomposition
OR
High amounts of biomass related to high rates of productivity
OR
Weathering/leaching due to high rates of precipitation