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CIE iGCSE Co-Ordinated Science B17.1 Variation Exam Style Questions Paper 4

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Question

(a) Variation occurs in all organisms.
There are two types of variation, continuous variation and discontinuous variation.
Complete Table 4.1 by placing a tick (✓) in each row to show if the description is about continuous or discontinuous variation.
(b) Antibiotics are used to treat people with bacterial infections.
Some strains of bacteria have become resistant to antibiotics.
Describe how strains of antibiotic-resistant bacteria develop by natural selection.
(c) (i) Many bacteria are decomposers in food chains.
Describe what is meant by a decomposer.
(ii) Energy in food chains is lost through processes such as excretion of urea.
State two other ways that energy is lost between one trophic level and the next.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic B17.1 — Variation (Part (a))
• Topic B17.2 — Selection / Natural selection (Part (b))
• Topic B18.2 — Food chains and food webs / Decomposers (Part (c)(i))
• Topic B18.2 — Energy transfer between trophic levels (Part (c)(ii))

▶️ Answer/Explanation

(a) Table 4.1 completed:

Discontinuous variation has a limited number of phenotypes with no intermediates (like blood groups); continuous variation shows a range between extremes (like height).

(b) Some bacteria mutate and become resistant to antibiotics. These resistant bacteria survive and reproduce, passing on their alleles for resistance to offspring, increasing the frequency of resistance in the population.
Antibiotics act as a selective pressure, killing non-resistant bacteria. The resistant ones survive and multiply, so the resistant trait becomes more common over generations.

(c)(i) A decomposer is an organism that gets its energy from dead or waste organic material.
Decomposers like bacteria and fungi break down dead organisms and waste, recycling nutrients back into the ecosystem.

(c)(ii) 1. Egestion (faeces / indigestible material) ; 2. Respiration (heat loss).
Energy is lost as heat during respiration, and in uneaten or indigestible parts like bones and teeth. Other losses include movement and excretion.

Question

(a) The circulatory system in horses is similar to in humans.
Fig. 1.1 is a simplified diagram of the circulatory system in horses.
The arrows represent the direction of blood flow.
(i) Identify from Fig. 1.1: blood vessel \(X\); organ \(Y\); blood vessel \(Z\).
(ii) Describe evidence from Fig. 1.1 that horses have a double circulatory system.
(b) The circulatory system transports blood around the body.
(i) State two blood groups in humans.
(ii) White blood cells are one component of blood.
Tick (✓) all the boxes that show the correct functions of white blood cells.
(c) Table 1.1 shows the average thickness of wall for an artery and a capillary.
(i) Calculate how many times thicker an artery wall is than a capillary wall.
(ii) Explain the reasons for this difference in thickness shown in Table 1.1.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654, 2025–2027 syllabus):

• Topic B9.1/B9.3 — Circulatory systems & blood vessels (Part (a))
• Topic B17.1 — Variation (ABO blood groups) (Part (b)(i))
• Topic B9.4 — Blood (Part (b)(ii))
• Topic B9.3 — Blood vessels (Part (c))

▶️ Answer/Explanation

(a)(i) X = pulmonary artery; Y = kidney; Z = vena cava

\(X\) carries blood from the heart to the lungs, so it is the pulmonary artery.
\(Y\) receives blood from the renal artery, so it must be the kidney.
\(Z\) returns blood from the head to the heart, so it is the vena cava.

(a)(ii) Two separate loops through the heart

The diagram shows blood passing through the heart twice in one full circuit.
One loop goes heart → lungs → heart (oxygenation).
The second loop goes heart → rest of body/head/kidney → heart (delivery to tissues), confirming a double circulation.

(b)(i) Any two of: A, B, AB, O

Human blood is classified into the ABO blood group system.
This is an example of discontinuous variation with no intermediate phenotypes.
Any two of the four groups (A, B, AB, O) are acceptable.

(b)(ii) 

White blood cells defend the body against pathogens.
Lymphocytes produce antibodies, while phagocytes engulf and digest pathogens.
Blood clotting, hormone transport, and oxygen transport are not functions of white blood cells (these belong to platelets, plasma, and red blood cells respectively).

(c)(i) 2000 times

\( \dfrac{1.0000}{0.0005} = 2000 \)
The artery wall is 2000 times thicker than the capillary wall.

(c)(ii) Different functions require different wall thickness

The artery wall is thick (and muscular/elastic) to withstand and maintain the high pressure of blood leaving the heart.
The capillary wall is only one cell thick to provide the shortest possible diffusion distance for the exchange of substances such as oxygen, carbon dioxide, and nutrients between blood and tissues.

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