0610_m25_qp

Question 1

The relationships of four species of bacteria, $P, Q, R$ and $S$, were compared.

Species $P$ and $Q$ have a more recent common ancestor than either species has with species $R$.
Species $S$ developed recently from species $P$.

Which species has the least similar base sequences in their DNA compared to the other three species of bacteria?

(A) $P$
(B) $Q$
(C) $R$
(D) $S$

▶️ Answer/Explanation

The degree of similarity in DNA base sequences is determined by how recently species shared a common ancestor. According to the data, $P$ and $Q$ are closely related, and $S$ is very closely related to $P$ because it branched off recently. This places $P, Q,$ and $S$ in a relatively tight evolutionary cluster. In contrast, species $R$ shared a common ancestor with the others much further back in time. Because more time has passed since $R$ diverged, more mutations have accumulated, resulting in the greatest number of differences in its DNA sequence compared to the other three. Therefore, $R$ is the most genetically distinct.
✅ Answer: (C)

Question 2

The diagram shows an arthropod.

To which group does it belong?

(A) arachnids
(B) crustaceans
(C) insects
(D) myriapods

▶️ Answer/Explanation

The organism shown is a scorpion, which belongs to the class Arachnida. Key identifying features include having exactly $4$ pairs of jointed walking legs (totaling $8$ legs), which distinguishes them from insects ($3$ pairs). Additionally, they possess specialized appendages called pedipalps (the pincers) and lack antennae, another hallmark of the arachnid group. Unlike crustaceans, they are primarily terrestrial and lack a heavy calcified exoskeleton with numerous pairs of legs. Their body is typically divided into two main segments: the cephalothorax and the abdomen.
✅ Answer: (A)

Question 3

Which structures are found in both plant cells and bacterial cells?

▶️ Answer/Explanation

Both plant cells (eukaryotic) and bacterial cells (prokaryotic) share basic structural features like the cell membrane and ribosomes for protein synthesis. While both possess a cell wall, the composition differs (cellulose in plants vs. peptidoglycan in bacteria). However, plasmids—small, circular DNA loops—are characteristic of bacteria and are not typically found in plant cells. Therefore, the combination of “yes” for membrane, wall, and ribosomes, but “no” for plasmid, is correct.
✅ Answer: (D)

Question 4

A cell is $65 \text{ \mu m}$ across.

What is this measurement in $\text{mm}$?

(A) $0.00065 \text{ mm}$ (B) $0.0065 \text{ mm}$ (C) $0.065 \text{ mm}$ (D) $0.65 \text{ mm}$

▶️ Answer/Explanation

To convert from micrometers ($\text{\mu m}$) to millimeters ($\text{mm}$), you need to understand the relationship between the two units. There are $1,000$ micrometers in $1$ millimeter ($1 \text{ mm} = 1,000 \text{ \mu m}$). Therefore, to convert a value from $\text{\mu m}$ to $\text{mm}$, you must divide the value by $1,000$.

Calculation: $\frac{65}{1,000} = 0.065$.

This shift moves the decimal point three places to the left. Starting from $65.0$, one jump makes it $6.5$, two jumps make it $0.65$, and the third jump results in $0.065$. This matches option C.
✅ Answer: (C)

Question 5

The diagram shows four mesophyll cells, $W$, $X$, $Y$ and $Z$. Water moves through these cells.

Which arrow shows the direction of movement of water through these cells?

▶️ Answer/Explanation

Water always moves from a region of higher water potential (less negative value) to a region of lower water potential (more negative value) down a water potential gradient. In option B, the water potentials are $-350 \rightarrow -400 \rightarrow -450 \rightarrow -600$. Since $-350$ is the highest value and $-600$ is the lowest, the water will flow from left to right ($W$ to $Z$). This matches the direction arrow provided in the row. In other options, the values either increase in the direction of the arrow or the arrow contradicts the gradient.
✅ Answer: (B)

Question 6

Three processes are listed.

$1$ Molecules or ions are moved across membranes.
$2$ Molecules or ions are moved by protein carriers.
$3$ Ions are taken up by root hairs.

Which processes can involve active transport?

(A) $1$, $2$ and $3$
(B) $1$ and $2$ only
(C) $1$ and $3$ only
(D) $2$ and $3$ only

▶️ Answer/Explanation

Active transport is the movement of substances against a concentration gradient using energy from respiration. Statement $1$ is correct because active transport specifically moves solutes across biological membranes. Statement $2$ is correct because this process requires specific protein carriers (pumps) to transport molecules. Statement $3$ is a classic biological example, as root hair cells use active transport to absorb mineral ions from the soil where they are in low concentration. Since all three statements accurately describe or exemplify active transport, option (A) is the correct choice.
✅ Answer: (A)

Question 7

A section of a chromosome contains $4500$ bases. There are $1450$ $G$ bases.

How many $A$ bases are there?

(A) $800$
(B) $1450$
(C) $1600$
(D) $2900$

▶️ Answer/Explanation

According to Chargaff’s rules, in a double-stranded DNA molecule, the amount of Guanine ($G$) equals Cytosine ($C$), and Adenine ($A$) equals Thymine ($T$). Given there are $1450$ $G$ bases, there must also be $1450$ $C$ bases. Together, $G + C = 1450 + 1450 = 2900$. Subtracting this from the total gives $4500 – 2900 = 1600$ bases for the $A + T$ pairs. Since $A$ and $T$ are present in equal amounts, we divide by $2$: $1600 \div 2 = 800$. Therefore, there are $800$ Adenine bases.

✅ Answer: (A)

Question 8

Which chemical element is found in proteins but not in carbohydrates?

(A) carbon
(B) hydrogen
(C) nitrogen
(D) oxygen

▶️ Answer/Explanation

Carbohydrates are organic compounds consisting primarily of carbon, hydrogen, and oxygen, typically in a $2:1$ ratio of hydrogen to oxygen. Proteins, however, are made of amino acids which contain an amino group ($-NH_{2}$). This means all proteins must contain nitrogen in addition to carbon, hydrogen, and oxygen. While some proteins also contain sulfur, nitrogen is the defining element that distinguishes them from carbohydrates in this list. Carbon, hydrogen, and oxygen are common to both macromolecules, making nitrogen the correct distinguishing factor.

✅ Answer: (C)

Question 9

Which statement describes enzymes?

(A) Enzymes are carbohydrates.
(B) Enzymes are involved in metabolic reactions.
(C) Enzymes decrease the rate of reaction.
(D) Enzymes have the same shape of active site as their substrate.

▶️ Answer/Explanation

Enzymes are biological catalysts, primarily made of proteins, that speed up chemical reactions in living organisms by lowering the activation energy. Statement (B) is correct because metabolism encompasses all chemical reactions in a cell, almost all of which require enzymes to occur at a life-sustaining rate. Statement (A) is incorrect as they are proteins, not carbohydrates. Statement (C) is the opposite of their function, as they increase the rate of reaction. Finally, statement (D) is a common misconception; the active site is complementary in shape to the substrate, not the “same” shape, allowing them to fit together like a lock and key.
✅ Answer: (B)

Question 10

The graphs show how the rate of photosynthesis and the rate of respiration in a plant changed over a $24$-hour period.

Which graph shows the correct rate of photosynthesis and rate of respiration?

▶️ Answer/Explanation

Photosynthesis is a light-dependent process; therefore, its rate is zero during the night and peaks at midday ($12$ hours) when light intensity is highest. Respiration, however, is a continuous process that occurs in all living cells $24$ hours a day to provide energy for survival. While the rate of respiration may fluctuate slightly with temperature changes, it remains relatively constant compared to the dramatic spikes in photosynthesis. Graph A correctly depicts photosynthesis as a bell curve peaking at noon and respiration as a steady, ongoing line throughout the entire $24$-hour cycle. The other graphs incorrectly suggest that respiration stops in the dark or follows light intensity identically.
✅ Answer: (A)

Question 11

The photomicrograph shows part of the lower surface of a leaf.

What is the structure labelled $X$?

(A) guard cell
(B) palisade mesophyll cell
(C) spongy mesophyll cell
(D) xylem vessel

▶️ Answer/Explanation

The image displays the lower epidermis of a leaf, characterized by specialized structures for gas exchange. The label $X$ points specifically to one of the bean-shaped cells that flank a small opening or pore. These are guard cells, which control the opening and closing of the stoma (the pore itself). Unlike regular epidermal cells, guard cells contain chloroplasts, which are visible as small grains in the photomicrograph. Options B and C are located in the internal layers of the leaf, and option D is a transport tissue found within vascular bundles, not on the surface.

✅ Answer: (A)

Question 12

A student is told that their diet is not balanced and they may be at risk of developing scurvy.

What should the student do to reduce their risk of scurvy?

(A) The student should drink more milk which will provide more calcium.
(B) The student should drink more milk which will provide more vitamin C.
(C) The student should eat more citrus fruits which will provide more calcium.
(D) The student should eat more citrus fruits which will provide more vitamin C.

▶️ Answer/Explanation

Scurvy is a deficiency disease caused by a lack of vitamin C (ascorbic acid) in the diet. This nutrient is essential for the synthesis of collagen, which helps maintain the health of skin, blood vessels, and gums. Citrus fruits, such as oranges, lemons, and limes, are well-known primary sources of vitamin C. While milk is a good source of calcium, it contains very little vitamin C and would not prevent scurvy. Therefore, increasing the intake of citrus fruits is the most effective way to address this specific deficiency.

✅ Answer: (D)

Question 13

The diagram shows the four types of human tooth.

Which teeth are used for grinding food?

(A) $1$ and $2$
(B) $1$ and $4$
(C) $2$ and $3$
(D) $3$ and $4$

▶️ Answer/Explanation

In the human mouth, different teeth are specialized for specific mechanical digestion tasks. Incisors (1) are chisel-shaped for biting and cutting, while canines (2) are pointed for tearing food. In contrast, premolar (3) and molar (4) teeth have broad, flat surfaces with ridges called cusps. These wide surfaces are specifically designed to crush and grind food into smaller pieces, increasing the surface area for enzymes to work. Therefore, the teeth responsible for grinding are the posterior teeth.
✅ Answer: (D) $3$ and $4$

Question 14

Which substance causes the change in $pH$ when food enters the small intestine?

(A) amylase
(B) bile
(C) gastric juice
(D) trypsin

▶️ Answer/Explanation

When food (chyme) moves from the stomach into the small intestine, it is highly acidic due to hydrochloric acid in the gastric juice. To protect the intestinal lining and allow enzymes like trypsin to function, this acid must be neutralized. Bile, produced by the liver and stored in the gallbladder, is an alkaline fluid that helps neutralize the stomach acid, raising the $pH$ to a more basic level. Additionally, bile plays a crucial role in emulsifying fats to aid digestion. While enzymes like amylase and trypsin work in this $pH$, they do not cause the change itself.
✅ Answer: (B)

Question 15

Which conditions cause the highest rate of transpiration?

(A) high temperature, high wind speed and high humidity
(B) high temperature, high wind speed and low humidity
(C) low temperature, low wind speed and low humidity
(D) low temperature, low wind speed and high humidity

▶️ Answer/Explanation

Transpiration is the evaporation of water from leaf surfaces, which is driven by a concentration gradient. High temperatures increase the kinetic energy of water molecules, speeding up evaporation. High wind speeds prevent water vapor from accumulating near the leaf, maintaining a steep diffusion gradient between the inside and outside of the leaf. Conversely, low humidity ensures the surrounding air is dry, further maximizing the rate at which water diffuses out of the stomata. Therefore, the combination of hot, windy, and dry conditions results in the maximum water loss.

✅ Answer: (B)

Question 16

The diagram shows a heart defect that some babies are born with. The arrows indicate direction of blood flow.

Which statement explains why this heart defect might cause the baby to have poor growth?

(A) Blood flows to the lungs at high pressure increasing rate of gas exchange.
(B) Blood flows to the lungs at low pressure decreasing rate of gas exchange.
(C) Mixing of blood results in less oxygen available for respiration.
(D) Mixing of blood results in more glucose available for respiration.

▶️ Answer/Explanation

The diagram illustrates a ventricular septal defect (VSD), where a hole in the septum allows oxygenated blood from the left ventricle to mix with deoxygenated blood in the right ventricle. This mixing means that the blood pumped out to the body via the aorta contains a lower concentration of oxygen than normal. Since oxygen is a vital reactant for aerobic respiration, a reduced supply limits the amount of energy released in the baby’s cells. Without sufficient energy, cellular processes such as protein synthesis and cell division are hindered, leading to stunted or poor growth. Option (D) is incorrect because the defect affects gas transport, not glucose levels.
✅ Answer: (C)

Question 17

The diagram shows the effect of vaccination on the human body. Three components in the diagram are represented by numbers.

Which components are memory cells?

(A) $1$ only
(B) $2$ and $3$
(C) $2$ only
(D) $3$ only

▶️ Answer/Explanation

In this process, component $1$ represents the antigen (or weakened pathogen) introduced by the vaccine. This triggers an immune response where the body produces both antibodies (component $2$) and memory cells (component $3$). Antibodies provide immediate defense and can be transferred to others (e.g., via breast milk) to provide passive immunity. Memory cells, however, remain in the body for a long duration to recognize the pathogen if it enters again, thus providing long-term immunity. Since component $3$ is specifically linked to long-term protection, it represents the memory cells.
✅ Answer: (D) $3$ only

Question 18

What helps to keep the trachea open during breathing?

(A) cartilage
(B) cilia
(C) external intercostal muscles
(D) larynx

▶️ Answer/Explanation

The trachea, or windpipe, is supported by a series of $C$-shaped rings made of cartilage. This stiff yet flexible tissue provides structural support that prevents the airway from collapsing under the negative pressure created during inhalation. While the other options are part of the respiratory system, they serve different functions:

Cilia are microscopic hairs that sweep mucus and debris out of the airways.
External intercostal muscles assist in expanding the ribcage during breathing.
The larynx is the voice box located at the top of the trachea.

Therefore, cartilage is specifically responsible for maintaining the patency of the tracheal tube.

✅ Answer: (A)

Question 19

Yeast can respire aerobically and anaerobically.

For each molecule of glucose used in yeast, how many more molecules of carbon dioxide gas are produced by aerobic respiration compared with anaerobic respiration?

(A) $1$ more
(B) $2$ more
(C) $4$ more
(D) $6$ more

▶️ Answer/Explanation

To solve this, we compare the balanced equations for both types of respiration in yeast. In aerobic respiration, one molecule of glucose reacts with oxygen to produce $6$ molecules of $CO_{2}$ ($C_{6}H_{12}O_{6} + 6O_{2} \rightarrow 6CO_{2} + 6H_{2}O$). In anaerobic respiration (fermentation), yeast breaks down one molecule of glucose into ethanol and only $2$ molecules of $CO_{2}$ ($C_{6}H_{12}O_{6} \rightarrow 2C_{2}H_{5}OH + 2CO_{2}$). By subtracting the anaerobic yield from the aerobic yield ($6 – 2 = 4$), we find that aerobic respiration produces $4$ additional molecules of carbon dioxide per glucose molecule.

✅ Answer: (C)

Question 20

In a healthy human, in which blood vessel would you find the lowest concentration of urea?

(A) hepatic artery
(B) hepatic vein
(C) renal artery
(D) renal vein

▶️ Answer/Explanation

To solve this, consider the organs where urea is produced and removed. Urea is synthesized in the liver, meaning the hepatic vein carries the highest concentration as it leaves the production site. Conversely, the kidneys are responsible for filtering urea out of the blood to be excreted as urine. The renal artery brings urea-rich blood into the kidneys, while the renal vein carries blood away after it has been filtered. Consequently, the blood in the renal vein has the lowest urea content in the entire body.

✅ Answer: (D)

Question 21

A specific eye condition can cause permanent contraction of the ciliary muscles.

Which row is correct for an eye with this condition compared to an eye without this condition?

▶️ Answer/Explanation

When the ciliary muscles contract, the ring of muscle becomes smaller in diameter. This reduces the tension on the suspensory ligaments, making them slack. As a result, the elastic lens is no longer pulled thin and becomes more convex (rounder). A rounder lens has a higher refractive power, meaning there is more refraction of light rays as they pass through the lens to focus on the retina. This state is normally used for viewing near objects.
✅ Answer: (B)

Question 22

The liver and the pancreas work together to control the concentration of glucose in the blood.

Which statement is correct?

(A) The liver converts the small molecule glucose into the large molecule glucagon.
(B) The liver releases the hormone insulin when blood glucose concentration is too high.
(C) The pancreas does not respond to an increase in blood glucose concentration.
(D) The pancreas responds to a fall in the blood glucose concentration by increasing the release of the hormone glucagon.

▶️ Answer/Explanation

Blood glucose levels are regulated via negative feedback. When glucose levels fall, the pancreas detects this change and secretes the hormone glucagon. Glucagon then travels to the liver, triggering the breakdown of stored glycogen back into glucose to raise blood sugar levels. Option (A) is incorrect because glucose is converted into glycogen for storage, not glucagon (which is a hormone). Option (B) is incorrect because the pancreas, not the liver, releases insulin. Option (C) is false as the pancreas actively responds to high glucose by secreting insulin.
✅ Answer: (D)

Question 23

In the dark, plant shoots grow upwards and roots grow downwards.

What causes these responses?

▶️ Answer/Explanation

The question specifies that the plant is growing “in the dark,” which immediately eliminates phototropism (response to light) as a cause for the current movement. Gravitropism is the growth response of a plant to gravity. Shoots exhibit negative gravitropism by growing away from the gravitational pull, while roots exhibit positive gravitropism by growing toward it. Even without light, gravity acts as a constant stimulus that guides the vertical orientation of the plant. Therefore, the observed growth is caused by gravitropism, but not phototropism.
✅ Answer: (C)

Question 24

What is the reason why antibiotics should be used only when essential?

(A) It increases the artificial selection of antibiotic resistant bacteria.
(B) It increases the natural selection of antibiotic resistant bacteria.
(C) It reduces the artificial selection of antibiotic resistant bacteria.
(D) It reduces the natural selection of antibiotic resistant bacteria.

▶️ Answer/Explanation

Antibiotics act as a selective pressure in a population of bacteria. When antibiotics are used, they kill susceptible bacteria, while those with rare mutations for resistance survive and reproduce. Overusing antibiotics provides more opportunities for this natural selection process to occur, leading to a rise in resistant strains. By using antibiotics only when essential, we limit the exposure of bacteria to the drug, thereby reducing the rate at which natural selection for resistance occurs. This helps preserve the effectiveness of treatments for future infections.

✅ Answer: (D)

Question 25

Which row describes self-pollination?

▶️ Answer/Explanation

Self-pollination occurs when pollen from the anther is deposited on the stigma of the same flower or another flower on the same plant. Because the genetic material comes from the same individual, it is considered “selfing.” In contrast, cross-pollination involves the transfer of pollen to a flower on a different plant of the same species. Therefore, a row describing self-pollination must have a “yes” (✓) for transfers within the same flower and between different flowers on the same plant, but a “no” (✗) for transfers to different plants.

✅ Answer: (B)

Question 26

The list shows terms used when describing sexual reproduction.

$1$ male gamete
$2$ female gamete
$3$ ovule
$4$ ovary
$5$ filament
$6$ fertilisation
$7$ haploid

Which terms are used only when describing sexual reproduction in plants?

(A) $1$ and $7$(B) $2$ and $4$(C) $2$ and $6$(D) $3$ and $5$

▶️ Answer/Explanation

To find the terms used only for plants, we must exclude terms common to both plants and animals. Male/female gametes, fertilisation, and the term haploid are universal to almost all sexually reproducing organisms. While animals have ovaries, they do not have ovules (which become seeds) or filaments (the stalk supporting the anther). In botany, the filament is a specific part of the stamen, whereas in animals, “ovary” is a shared term but the internal structures differ significantly. Therefore, the combination of ovule ($3$) and filament ($5$) is specific to plant biology in this context.
✅ Answer: (D)

Question 27

Which statement correctly describes the effect of a hormone in a human menstrual cycle that lasts for $28$ days?

(A) FSH causes maturation of an egg up to day $21$.
(B) LH stimulates the release of an egg on day $14$.
(C) Oestrogen maintains the uterus lining between days $14$ and $28$.
(D) Progesterone causes the repair and thickening of the uterus lining between days $4$ to $14$.

▶️ Answer/Explanation

In a typical $28$-day menstrual cycle, Luteinizing Hormone (LH) reaches its peak concentration just before the midpoint of the cycle. This “LH surge” triggers ovulation, which is the release of a mature egg from the ovary, typically occurring on day $14$.

The other options are incorrect because: * FSH (Follicle Stimulating Hormone) acts during the follicular phase (days $1$-$14$) to mature the egg, not up to day $21$. * Oestrogen primarily repairs the lining in the first half of the cycle, while Progesterone (produced by the corpus luteum) is responsible for maintaining the lining from day $14$ to $28$. * The repair and thickening between days $4$ and $14$ is stimulated by Oestrogen, not Progesterone.
✅ Answer: (B)

Question 28

What is the main hormone responsible for the development of male secondary sexual characteristics?

(A) adrenaline
(B) insulin
(C) oestrogen
(D) testosterone

▶️ Answer/Explanation

Testosterone is the primary androgen produced by the testes in males. During puberty, its levels increase significantly, triggering the development of secondary sexual characteristics such as a deeper voice, facial and body hair growth, and increased muscle mass. While adrenaline regulates the “fight or flight” response and insulin manages blood glucose, oestrogen is the primary hormone for female sexual development. Therefore, testosterone is the hormone specifically responsible for masculine physical traits.
✅ Answer: (D)

Question 29

Which term is defined as a length of DNA that codes for a protein?

(A) amino acid
(B) chromosome
(C) gene
(D) mutation

▶️ Answer/Explanation

A gene is the fundamental physical and functional unit of heredity, consisting of a specific sequence of nucleotides within a DNA molecule. These sequences provide the instructions required to synthesize a specific protein or RNA molecule. While a chromosome contains hundreds to thousands of genes, it is the individual gene that serves as the “coding unit.” Amino acids are the building blocks that make up proteins, rather than segments of DNA themselves, and a mutation is simply a change in the DNA sequence. Therefore, the specific segment of DNA responsible for protein production is a gene.
✅ Answer: (C)

Question 30

Which statement about the sex chromosomes is correct?

(A) Men and women can inherit characteristics controlled by genes on the $X$ chromosome.
(B) Men and women can inherit characteristics controlled by genes on the $Y$ chromosome.
(C) Only men can inherit characteristics controlled by genes on the $X$ chromosome.
(D) Only women can inherit characteristics controlled by genes on the $Y$ chromosome.

▶️ Answer/Explanation

In humans, biological sex is determined by the $XY$ system. Females possess two $X$ chromosomes ($XX$), while males possess one $X$ and one $Y$ chromosome ($XY$). Because both males and females have at least one $X$ chromosome, both sexes can inherit and express traits located on it. In contrast, the $Y$ chromosome is typically only present in males, meaning $Y$-linked traits are not inherited by females. Therefore, option (A) is the only scientifically accurate statement.

✅ Answer: (A)

Question 31

The graph shows how the mass of DNA changes during a mitotic cell division.

Where on the graph are two cells formed?

▶️ Answer/Explanation

During the cell cycle, DNA replication occurs during the S phase of interphase, which is represented by the slope between points $A$ and $C$. At point $C$, the cell has double the original amount of DNA ($2 \times$ mass). This doubled mass remains constant through the $G_{2}$ phase and the early stages of mitosis (prophase, metaphase, and anaphase).

The sudden drop in DNA mass per cell at point D represents cytokinesis, the physical division of the cytoplasm. At this moment, the single parent cell splits into two identical daughter cells. Consequently, the mass of DNA measured per cell returns to the original baseline level because the total genetic material has been partitioned into two separate nuclei.

✅ Answer: (D)

Question 32

A man with blood group $A$ and genotype $I^AI^o$ has a child with a woman who has blood group $B$ and genotype $I^BI^o$.

What is the probability of the child having blood group $O$?

A $0.00$ B $0.25$ C $0.50$ D $1.00$

▶️ Answer/Explanation

To find the probability, we use a Punnett square to cross the parents’ genotypes: $I^AI^o \times I^BI^o$. The possible combinations for the offspring are $I^AI^B$ (Type $AB$), $I^AI^o$ (Type $A$), $I^BI^o$ (Type $B$), and $I^oI^o$ (Type $O$). Each of these four combinations has an equal chance of occurring. Since only one out of the four possibilities results in blood group $O$, the probability is $1/4$. This converts to a decimal value of $0.25$.

✅ Answer: (B)

Question 33

What are sources of genetic variation in populations?

(A) meiosis, mutation, random mating
(B) meiosis, random mating, asexual reproduction
(C) mitosis, mutation, random fertilisation
(D) mitosis, random fertilisation, asexual reproduction

▶️ Answer/Explanation

Genetic variation arises from processes that shuffle or change DNA. Meiosis contributes through crossing over and independent assortment, while mutations create entirely new alleles. Random mating ensures diverse combinations of these alleles within a population. In contrast, mitosis and asexual reproduction produce genetically identical clones, which limits rather than increases variation. Therefore, option (A) is the only set containing three genuine sources of diversity.

✅ Answer: (A)

Question 34

The food chain shows how energy is transferred to a human.

lettuce $\rightarrow$ snails $\rightarrow$ duck $\rightarrow$ human

Where did the energy in the lettuce come from?

(A) snails
(B) soil
(C) the Sun
(D) water

▶️ Answer/Explanation

The primary source of energy for almost all life on Earth is the Sun. In this food chain, lettuce acts as a producer. Producers use a process called photosynthesis to capture light energy and convert it into chemical energy stored in glucose. While plants like lettuce absorb nutrients from the soil and water for growth, these are not the source of the energy itself. The snails, ducks, and humans then obtain this energy by consuming the level below them. Therefore, the original energy entering this specific biological system started with solar radiation.

✅ Answer: (C)

Question 35

The processes listed affect the concentration of carbon dioxide in the atmosphere.

$1$ increased deforestation
$2$ increased tree planting
$3$ decreased fossil fuel combustion
$4$ increased fossil fuel combustion

Which processes increase the concentration of carbon dioxide in the atmosphere?

(A) $1$ and $3$
(B) $1$ and $4$
(C) $2$ and $3$
(D) $2$ and $4$

▶️ Answer/Explanation

To determine which processes increase atmospheric $CO_{2}$, we analyze the carbon cycle. Increased deforestation ($1$) leads to higher $CO_{2}$ levels because there are fewer trees to perform photosynthesis and remove the gas, and the clearing process often involves burning or decay. Increased fossil fuel combustion ($4$) directly releases stored carbon into the atmosphere as $CO_{2}$ through chemical oxidation. Conversely, tree planting removes $CO_{2}$, and decreasing combustion reduces the rate of addition. Therefore, only processes $1$ and $4$ contribute to an increase.
✅ Answer: (B)

Question 36

Which organisms can be sources of energy for decomposers?

▶️ Answer/Explanation

Decomposers, such as bacteria and fungi, obtain their energy by breaking down organic matter from dead organisms and waste products. Since all living things eventually die, every level of a food web—including producers (plants), herbivores (primary consumers), and carnivores (secondary/tertiary consumers)—serves as a source of energy for decomposers. This process is essential for recycling nutrients back into the ecosystem. Therefore, the correct row must show a “yes” (✓) for all three categories.
✅ Answer: (A)

Question 37

A coral reef contains species such as stony corals, sponges, seahorses and whale sharks.

These species interact with each other and are affected by the temperature, salt concentration and oxygen concentration of the water.

Using all the information given, what is being described?

(A) community
(B) ecosystem
(C) food web
(D) population

▶️ Answer/Explanation

An ecosystem is defined as a biological system consisting of all the living organisms (biotic factors) in a particular area interacting with their physical, non-living environment (abiotic factors). In this question, the text identifies diverse species like corals and sharks, which represent the biotic component. Crucially, it also mentions temperature, salt, and oxygen levels, which are abiotic factors. While a “community” only refers to the interacting living species, the inclusion of the physical water conditions makes “ecosystem” the most complete and accurate description. A population would only involve one species, and a food web focuses strictly on feeding relationships.
✅ Answer: (B)

Question 38

What can cause an increase in the size of a rabbit population?

(A) an increase in a disease that affects rabbits
(B) an increase in available food for the rabbits
(C) an increase in pollution in the habitat of the rabbits
(D) an increase in predation of rabbits

▶️ Answer/Explanation

The size of a population increases when the birth rate and immigration exceed the death rate and emigration. Option (B) provides more resources, which reduces competition and allows for higher survival and reproductive success. In contrast, disease (A), pollution (C), and predation (D) are all limiting factors that increase the mortality rate, leading to a decrease in population size. Since food is a primary requirement for growth and energy, its abundance directly supports a larger carrying capacity in the ecosystem.
✅ Answer: (B)

Question 39

How does eutrophication lead to the death of fish?

(A) A sudden increase in the number of bacteria increases the spread of disease.
(B) Algae respire instead of photosynthesise.
(C) Decomposer bacteria lower the oxygen concentration in the water.
(D) Fertilisers reduce the availability of nitrates in the water.

▶️ Answer/Explanation

Eutrophication begins when excess nutrients, like nitrates from fertilizers, cause an algal bloom. This thick layer of algae blocks sunlight, causing submerged plants to die. When the algae and plants die, decomposer bacteria break down the organic matter. These bacteria reproduce rapidly and use up the dissolved oxygen in the water through aerobic respiration. As the oxygen concentration drops significantly, fish and other aquatic organisms cannot breathe and eventually die from suffocation. This process creates “dead zones” where life cannot be sustained.
✅ Answer: (C)

Question 40

The diagram shows a bacterial cell.

Which cell structure makes bacteria useful in genetic modification?

▶️ Answer/Explanation

The correct structure is B, which represents a plasmid. Plasmids are small, circular loops of extra-chromosomal DNA found in bacteria that are separate from the main nucleoid (D). In genetic engineering, plasmids are highly useful because they can be easily isolated, cut using restriction enzymes, and joined with foreign genes to act as vectors. Once the recombinant plasmid is reinserted into a bacterium, the cell will express the new genetic information. This mechanism is widely used to mass-produce substances like human insulin.
✅ Answer: (B)