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Question 1

Topic: 16.3

(a) Fig. 1.1 is a diagram of an insect-pollinated flower.

(i) Using the information in Fig. 1.1, complete Table 1.1.

(ii) State the names of the two structures that form the stamen in a flowering plant.

(b) (i) Describe the stages in the reproduction of a flowering plant, from self-pollination to fertilisation.

(ii) Outline the advantages and disadvantages of self-pollination compared with cross-pollination.

▶️ Answer/Explanation

Solution

(a)(i)

Explanation: The stigma (A) is the sticky part that receives pollen. Petals (B) are colorful to attract pollinators. Sepals (F) are the protective outer covering of the flower bud.

(a)(ii) filament and anther

Explanation: The stamen, the male reproductive part of a flower, consists of the filament (stalk) that supports the anther (pollen-producing part).

(b)(i)

1. Self-pollination occurs when pollen is transferred from the anther to the stigma of the same flower or plant.
2. The pollen grain germinates and grows a pollen tube down through the style.
3. The pollen tube carries the male nucleus to the ovule through the micropyle.
4. Enzymes are released to help the pollen tube penetrate the ovule.
5. Fertilization occurs when the male nucleus fuses with the female nucleus in the ovule, forming a diploid zygote.

Explanation: Self-pollination begins with pollen transfer within the same plant, followed by pollen tube growth through the style to reach the ovule. The male nucleus travels down this tube to fuse with the female nucleus, completing fertilization inside the ovary.

(b)(ii)

Advantages of self-pollination:
1. Guarantees pollination even in isolated plants
2. Preserves successful genetic combinations
3. Requires less energy than attracting pollinators
4. Ensures reproduction when pollinators are scarce

Disadvantages of self-pollination:
1. Reduces genetic variation in offspring
2. Increases risk of harmful recessive traits appearing
3. Limits adaptability to environmental changes
4. May lead to weaker offspring over generations

Explanation: While self-pollination ensures reproduction, it limits genetic diversity, making populations more vulnerable to diseases and environmental changes. Cross-pollination promotes genetic variation but depends on external factors like pollinators.

Question 2

Topic: 6.2

(a) Fig. 2.1 is a diagram of a cross-section of part of a leaf.

Identify and explain how the structures labelled J and K are adapted for photosynthesis.

(b) In an experiment, thale cress plants (Arabidopsis thaliana) were grown in normal atmospheric and high carbon dioxide concentrations. The transpiration rate, the mean number of chloroplasts per cell and the concentrations of starch and magnesium ions were measured.

The results are shown in Table 2.1.

Complete the sentences about the data shown in Table 2.1.

Table 2.1 shows that increasing the carbon dioxide concentration caused more starch to be produced in the leaves. This shows that, at a normal carbon dioxide concentration, carbon dioxide is a …… for photosynthesis.

During photosynthesis, …… molecules of carbon dioxide are required to make one molecule of glucose.

The greater quantity of starch stored in the leaves grown in a high carbon dioxide concentration means, when needed, more sucrose can be produced for transport in the phloem, so the leaves act as a ……

The greater number of chloroplasts per cell in the leaves grown in the higher carbon dioxide concentration means that more energy can be absorbed from …… and transferred to …… energy.

The transpiration rate is lower when the carbon dioxide concentration is higher. This means reduced loss of …… from the leaves.

Magnesium ion concentration is lower in these leaves because they have used the magnesium ions to make ……

▶️ Answer/Explanation

Solution

(a)

J – (Upper) epidermis cell:

The upper epidermis is transparent (translucent) and thin, which allows maximum light penetration to reach the palisade mesophyll cells underneath where most photosynthesis occurs. The lack of chloroplasts in these cells ensures no light is absorbed before reaching the photosynthetic cells.

K – Xylem vessel:

The xylem is adapted with its hollow, continuous tube structure (no end walls) and lignin-strengthened walls to efficiently transport water and mineral ions to the leaf. This constant water supply is crucial for photosynthesis. The structural support from lignin also helps maintain leaf shape for optimal light absorption.

(b)

During photosynthesis, 6 molecules of carbon dioxide are required to make one molecule of glucose.

The greater number of chloroplasts per cell in the leaves grown in the higher carbon dioxide concentration means that more energy can be absorbed from sunlight and transferred to chemical energy.

The transpiration rate is lower when the carbon dioxide concentration is higher. This means reduced loss of water vapor from the leaves.

Magnesium ion concentration is lower in these leaves because they have used the magnesium ions to make chlorophyll.

Detailed Explanation:

The experiment demonstrates several key concepts about photosynthesis and plant responses to CO₂ concentration:

The increased starch production shows CO₂ is normally limiting – when more is available, photosynthesis increases.
The chemical equation for photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂) explains why 6 CO₂ molecules are needed per glucose.
Leaves become sources when they produce excess carbohydrates that can be transported to other plant parts.
More chloroplasts develop to handle the increased photosynthetic activity, absorbing more light energy.
Stomata don’t need to open as wide when CO₂ is plentiful, reducing water loss through transpiration.
Magnesium is a key component of chlorophyll molecules, so more chlorophyll production uses up available magnesium ions.

Question 3

Topic: 11.1

(a) (i) Complete Table 3.1 by writing in the percentages of carbon dioxide and oxygen in inspired air and in expired air.

(ii) A scientist measured the number of dust particles in inspired air and in expired air. They found fewer dust particles in expired air. Suggest a reason for their observation.

(b) Fig. 3.1 is a diagram of alveoli and associated blood vessels.

(i) Explain how the structure of a capillary is related to its function.

(ii) State the name of the component of blood that transports oxygen.

(iii) State the name of the blood vessel that transports blood from the heart towards the capillaries in the lungs.

(iv) State the location and function of cartilage in the breathing system.

(c) A student measured the rate and depth of breathing of an athlete for 30 seconds at rest. The data are shown in Fig. 3.2.

(i) Using the information in Fig. 3.2, calculate the rate of breathing at rest.

The measurements were repeated while the athlete was running on a treadmill. The data are shown in Fig. 3.3.

(ii) Using the information in Fig. 3.3, calculate the volume of air inspired in one breath from 25 seconds.

(iii) Explain the effect of exercise on the rate and depth of breathing shown in Fig. 3.2 and Fig. 3.3.

▶️ Answer/Explanation

Solution

(a)(i)

Explanation: Inspired air contains approximately 21% oxygen and only 0.04% carbon dioxide, which is the normal atmospheric concentration. During respiration, oxygen is used up and carbon dioxide is produced, so expired air contains less oxygen (about 16%) and more carbon dioxide (about 4%).

(a)(ii) Dust particles are trapped in nose hairs or mucus in the respiratory tract.

Explanation: The respiratory system has several defense mechanisms to prevent dust and other particles from reaching the lungs. Nose hairs act as a physical barrier, while mucus produced by the lining of the respiratory tract traps particles. Cilia then move this mucus upwards to be swallowed or coughed out, resulting in fewer particles being exhaled.

(b)(i) Capillaries have thin walls (one cell thick) for efficient diffusion of gases; small lumen to slow blood flow allowing more time for gas exchange; and are numerous creating a large surface area for exchange.

Explanation: The structure of capillaries is perfectly adapted for their function of gas exchange. Their extremely thin walls (just one cell thick) minimize the diffusion distance for oxygen and carbon dioxide. The narrow lumen slows down blood flow, maximizing the time available for gas exchange. Their extensive network throughout the body ensures a large surface area for this vital process.

(b)(ii) Red blood cells.

Explanation: Red blood cells contain hemoglobin, a protein that binds oxygen in the lungs and releases it to tissues throughout the body. Each red blood cell can carry about a billion oxygen molecules.

(b)(iii) Pulmonary artery.

Explanation: The pulmonary artery carries deoxygenated blood from the right ventricle of the heart to the lungs for oxygenation. It’s the only artery in the body that carries deoxygenated blood.

(b)(iv) Location: trachea; Function: prevents airway collapse and keeps airway open.

Explanation: Cartilage rings are found in the trachea and bronchi. These C-shaped rings provide structural support to prevent the airways from collapsing during breathing, while still allowing some flexibility for movement. The open part of the C-shape faces the esophagus, allowing food to pass easily during swallowing.

(c)(i) 12 breaths per minute.

Explanation: From the graph, we can count that the athlete takes about 6 breaths in 30 seconds. To calculate breaths per minute, we multiply by 2 (since there are two 30-second periods in a minute).

(c)(ii) 2.5 dm³.

Explanation: At 25 seconds on the graph during exercise, the volume change between inhalation and exhalation is about 2.5 dm³, which represents the volume of air inspired in one breath.

(c)(iii) Exercise increases both the rate and depth of breathing. More oxygen is needed for aerobic respiration in muscles, and more carbon dioxide needs to be removed. The brain detects increased CO₂ levels and sends impulses to respiratory muscles to contract more frequently and forcefully.

Explanation: During exercise, muscles require more energy, which is produced through increased aerobic respiration. This requires more oxygen and produces more carbon dioxide. The brain’s respiratory center detects the rising CO₂ levels (or the accompanying drop in pH) and responds by increasing both the rate (number of breaths per minute) and depth (tidal volume) of breathing. This ensures that more oxygen is delivered to the muscles and more CO₂ is removed from the body. Additionally, adrenaline released during exercise stimulates faster breathing.

Question 4

Topic: 14.4

(a) Complete the sentences about the control of body temperature.

The human body maintains a constant internal temperature. This is an example of …… . When the temperature moves away from the set point, the mechanism of …… returns the temperature to the set point.

(b) Fig. 4.1 is a diagram of a section of human skin.

(i) State the names of the structures labelled L, O and J in Fig. 4.1.

(ii) Describe how humans maintain a constant body temperature when the external temperature decreases.

Use the structures labelled in Fig. 4.1 in your answer.

(i) State the organ that secretes glucagon.

(ii) Describe the effect of glucagon on the body.

▶️ Answer/Explanation

Solution

(a) homeostasis; negative feedback

Explanation: The human body maintains a constant internal temperature through homeostasis, which is the process of maintaining stable internal conditions despite changes in the external environment. When temperature deviates from the set point, negative feedback mechanisms work to return it to normal. For example, if body temperature rises, mechanisms like sweating are triggered to cool the body down, while if temperature drops, shivering generates heat to warm the body up.

(b)(i)

L – sweat gland

O – receptor(s)

J – (hair) erector muscle

Explanation: These structures are key components of the skin’s thermoregulatory system. Sweat glands produce sweat for cooling, receptors detect temperature changes, and hair erector muscles control hair position to trap insulating air when contracted.

(b)(ii)

When external temperature decreases:

Temperature receptors in the skin detect the cold stimulus
Nerve impulses are sent to the hypothalamus in the brain
Hair erector muscles contract, making hairs stand up to trap insulating air
Blood vessels near the skin surface constrict (vasoconstriction) to reduce heat loss
Sweat glands reduce or stop sweat production to conserve heat
Shivering may occur as muscles contract rapidly to generate heat
Fatty tissue under the skin acts as insulation

Explanation: This coordinated response helps maintain core body temperature by reducing heat loss and increasing heat production when exposed to cold environments. The hypothalamus acts as the body’s thermostat, coordinating these responses through the nervous and endocrine systems.

(c)(i) pancreas

Explanation: Glucagon is secreted by the alpha cells in the islets of Langerhans within the pancreas, which is both an exocrine and endocrine gland located behind the stomach.

(c)(ii) Glucagon stimulates the breakdown of glycogen to glucose in the liver, increasing blood glucose concentration.

Explanation: When blood glucose levels drop too low, glucagon is released. It acts primarily on liver cells, triggering the conversion of stored glycogen into glucose through glycogenolysis. This glucose is then released into the bloodstream, raising blood sugar levels back to normal. Glucagon works in opposition to insulin to maintain glucose homeostasis.

Question 5

Topic: 21.2

(a) State two cell structures found in both animal and bacterial cells.

(b) State why bacteria are useful in biotechnology.

(c) Genetically modified bacteria were grown in a fermenter. The number of bacteria was measured, and the data are shown in Fig. 5.1.

(i) On Fig. 5.1, draw an X to identify the lag phase.

(ii) On Fig. 5.1, draw a Y to show where the birth rate is equal to the death rate.

(iii) Calculate how long it takes for the number of bacteria to reduce by half after the bacteria have been in the fermenter for 24 hours.

(iv) Describe and explain the change in bacterial population size from 24 hours to 50 hours shown in Fig. 5.1.

(v) The fermenter is kept at the optimum temperature for the bacteria. Explain why this is important for enzyme function.

(vi) State why the bacteria are grown in a liquid that contains amino acids.

▶️ Answer/Explanation

Solution

(a) Cell membrane and cytoplasm (or ribosomes or DNA).

Explanation: Both animal and bacterial cells share these fundamental structures. The cell membrane controls what enters and exits the cell, while the cytoplasm is the jelly-like substance where metabolic reactions occur. Ribosomes are present in both for protein synthesis, and DNA carries genetic information in both cell types.

(b) Any three from: rapid reproduction rate, reproduce asexually, small size, simple growth requirements, ability to make complex molecules, few ethical concerns, presence of plasmids, same genetic code as other organisms.

Explanation: Bacteria are extremely useful in biotechnology due to several key characteristics. Their rapid reproduction allows quick production of desired products. Asexual reproduction ensures genetic consistency. Their small size means they can be grown in large quantities in small spaces. They have simple nutritional needs, often just requiring basic nutrients. Many bacteria naturally produce complex molecules like enzymes. There are fewer ethical concerns compared to using animal or human cells. Plasmids make genetic modification easier, and their universal genetic code means genes from other organisms can be expressed in bacteria.

(c)(i) X placed between 0-8 hours.

Explanation: The lag phase is the initial period where bacteria are adjusting to their environment, synthesizing enzymes and preparing for growth, before population numbers begin to increase significantly.

(c)(ii) Y placed between 18-24 hours.

Explanation: The stationary phase occurs when birth rate equals death rate, resulting in no net population growth. This typically happens when resources become limited.

(c)(iii) 12 hours.

Explanation: At 24 hours the population is at its peak (about 10 million cells/cm³). By 36 hours it has halved to about 5 million cells/cm³, so the halving time is 12 hours.

(c)(iv) The population enters death phase where death rate exceeds birth rate, decreasing due to limited resources, increased competition, waste buildup, or other unfavorable conditions.

Explanation: After 24 hours, the bacterial population begins to decline. This death phase occurs because essential nutrients become depleted, waste products accumulate to toxic levels, and competition for remaining resources intensifies. The environment becomes increasingly hostile, causing more cells to die than are being produced through reproduction.

(c)(v) To maximize enzyme activity while preventing denaturation.

Explanation: Enzymes function best at their optimum temperature. At this temperature, they have sufficient kinetic energy for frequent collisions with substrates, forming many enzyme-substrate complexes. The active site maintains its correct shape for substrate binding. Temperatures above the optimum would cause denaturation (permanent shape change), while lower temperatures would reduce reaction rates.

(c)(vi) To make proteins/enzymes/nucleic acids.

Explanation: Amino acids are the building blocks of proteins, which bacteria need for growth, enzyme production, and cellular structures. Providing amino acids in the growth medium allows for efficient protein synthesis without requiring the bacteria to produce all amino acids from scratch, thus supporting faster growth and higher yields of desired products.

Question 6

Topic: 18.2

(a) Xerophytes are plants that are adapted for an environment which has very little available water. Describe the meaning of adaptation.

(b) Fig. 6.1 is a photograph of a saguaro cactus, Carnegiea gigantea, which lives in a desert. The climate in a desert has very low rainfall and very high daytime temperatures.

Describe two visible adaptive features shown in Fig. 6.1 and explain how each feature is beneficial for living in a desert.

(c) Table 6.1 shows some data about stomatal density in the leaves of one plant that is not a xerophyte and three xerophyte plants.

(i) Using the information in Table 6.1, estimate the total number of stomata in an ice plant leaf with a lower leaf surface area of 8 cm².

(ii) Explain the data shown in Table 6.1.

(d) There are xerophytic forests which are threatened by human overexploitation. Suggest reasons why it is important to conserve xerophytic ecosystems.

▶️ Answer/Explanation

Solution

(a) Adaptation refers to the process resulting from natural selection where populations become better suited to their environment over many generations.

Explanation: Adaptations are characteristics that help organisms survive and reproduce in their specific environment. These traits develop over long periods through natural selection, where individuals with beneficial traits are more likely to survive and pass on their genes. In the case of xerophytes, their adaptations help them conserve water in arid environments.

(b)

Feature 1: Spines/needles/thorns

Explanation: These structures reduce the surface area of the plant, minimizing water loss through transpiration. They also serve as a defense mechanism against herbivores that might otherwise consume the plant’s precious water reserves.

Feature 2: Fleshy/thick/swollen stem

Explanation: The enlarged stem acts as a water storage organ, allowing the cactus to store large amounts of water during rare rainfall events and slowly utilize it during prolonged dry periods.

(c)(i) 33,600 stomata

Explanation: Calculation: 8 cm² = 800 mm² (since 1 cm² = 100 mm²). The ice plant has 42 stomata per mm² on its lower surface, so total stomata = 800 × 42 = 33,600.

(c)(ii) Xerophytes generally have fewer stomata than non-xerophytes to reduce water loss through transpiration. The stomata are often concentrated on the lower leaf surface which is cooler and more shaded, further minimizing water loss. The data shows that xerophytes like the tongue leaf plant and ice plant may have no stomata on their upper surfaces at all.

Detailed Explanation: The table demonstrates how xerophytes have evolved to minimize water loss while still allowing for gas exchange. The oak tree (non-xerophyte) has many more stomata (503 per mm² on lower surface) compared to the xerophytes (15-42 per mm²). Some xerophytes like the tongue leaf plant have no stomata on their upper surface at all (0 per mm²). This adaptation reduces transpiration while still allowing some gas exchange through the fewer stomata on the cooler, shaded lower surface.

(d) Reasons to conserve xerophytic ecosystems include:

Maintaining biodiversity and genetic diversity
Preventing extinction of unique species
Ensuring stability of food chains and nutrient cycling
Protecting vulnerable ecosystems that may have important ecological functions

Detailed Explanation: Xerophytic ecosystems, while appearing harsh, support unique biodiversity that may have undiscovered benefits for medicine, science, or agriculture. They often contain species found nowhere else. Conservation helps maintain ecological balance, protects potential resources, and preserves these unique environments for future generations. Additionally, many xerophytic plants are important carbon sinks and help stabilize soils in fragile environments.

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