Cambridge A-Level Biology: Infectious Disease – Malaria (Causes, Transmission & Control)
Master Cambridge A-Level Biology: Infectious Disease – Malaria (Causes, Transmission & Control)
Our advanced study modules align perfectly with aligned with Cambridge AS- A Level Biology (9700) appreciated at top-tier institutions like Brampton Manor Academy (London), Concord College (Shropshire) Westminster School (London), Cardiff Sixth Form College (Wales / London) Dubai College, Tanglin Trust, aur German Swiss International School—these resources are designed to simplify complex concepts and guarantee top grades in your board examinations.
Before diving into Cambridge A-Level Biology: Infectious Disease – Malaria (Causes, Transmission & Control) ensure you have gone through our previous guide : Infectious Diseases: HIV/AIDS – Causative Agent, Transmission, and Global Control Mechanics | Cambridge AS & A-Level Biology (9700)
Table of Contents
- Introduction to Malaria
- Disease Profile: Pathogen vs. Vector
- Modes of Transmission
- Biological Vector Transmission
- Alternative Transmission Routes
- The Life Cycle of Plasmodium inside the Human Host
- The Liver Stage (Hepatocytes)
- The RBC Stage (Erythrocytes & Symptoms)
- Prevention and Control Strategies
- Targeting the Vector (Anopheles)
- Disrupting Human-Vector Contact
- Prophylaxis and Anti-Malarial Treatment
- Cambridge Exam Focus: Why is Malaria Control So Difficult?
- Antigenic Variation Explained
- Eukaryotic Complexity & Resistance Issues
- AO1 Knowledge with Understanding (Direct & Recall Questions)
- AO2 Application of Knowledge (Diagram & Labeling Questions)
- AO3 Experimental Skills & Data Interpretation (Graph & Table Questions)
Introduction to Malaria
- Malaria is a significant global infectious disease primarily prevalent in tropical and subtropical regions.
- Depending on the geographical location and transmission rates, it is classified as an endemic disease in many parts of the world, posing a major global health and biological challenge.
- Malaria is caused by a unicellular eukaryotic parasite belonging to the kingdom Protoctist, specifically from the genus Plasmodium.
- The parasite is digenetic, meaning it requires two distinct hosts to complete its life cycle:
- Primary Host / Vector is Female Anopheles Mosquito Which acts as the biological vehicle that transmits the pathogen from one human to another.
- Secondary Host is human Where the parasite undergoes asexual reproduction.
๐ Cambridge A level Key Connection:
๐ Female Anopheles is regarded as primary host because sexual phase of Plasmodium life cycle occur in Mosquito .
Types of Malaria
- Different species of the Plasmodium genus cause varying forms of malaria, characterized by the frequency and severity of the recurrent fever:
Benign Tertian Malaria:
- It is Caused by Plasmodium vivax.
- It is widely distributed across tropical and subtropical regions, where fever recurs every 48 hours.
Quartan Malaria:
- It is Caused by Plasmodium malariae. It is characterized by a 72-hour fever cycle.
Tertian Malaria (Mild):
- It is Caused by Plasmodium ovale,
- It is predominantly found in West Africa and parts of South America.
Malignant Tertian Malaria:
- It is Caused by Plasmodium falciparum.
- This is the most severe, fatal, and clinically dangerous form of malaria, often referred to as pernicious malaria.
.
Disease Profile: Pathogen vs. Vector
- In the Cambridge examination, you must clearly distinguish between the causative agent and the vector.
- Name of the Disease: Malaria
- Causative Agent (Pathogen): Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, and Plasmodium ovale.
- Vector (Transmission Agent): Female Anopheles mosquito.
⚠️ Cambridge Exam Note:
๐Plasmodium is a protoctist (single-celled eukaryotic parasite). Examiners check whether you classify it as a Protoctist or bacterium/virus.
Mode of transmission:
- The transmission of the Plasmodium parasite relies heavily on its biological vector, but alternative pathways also exist where direct blood-to-blood contact occurs.
Biological Vector Transmission
- This is the primary and most common route of malaria transmission.
- It is an active biological process involving the vector and the human host.
- The female Anopheles mosquito acts as the biological vector.
๐ก Cambridge A-Level Note:
๐Male mosquitoes do not transmit malaria because they feed exclusively on plant nectar and lack the specialized piercing mouth parts required to break human skin.
The Inoculation Process:
- When an infected female Anopheles mosquito bites a human to take a blood meal, it punctures the skin and reaches a capillary.
- Before sucking blood, it injects its saliva directly into the human host's bloodstream.
Role of Saliva:
- The mosquito's saliva contains specialized anticoagulants that prevent the human blood from clotting inside its proboscis (feeding tube).
The Infective Stage:
- Along with the saliva, the mosquito inadvertently injects the infective, motile stage of the parasite, known as sporozoites, into the human circulatory system.
- From here, the sporozoites quickly migrate to the liver within 30 minutes.
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Alternative Transmission Routes
- While vector transmission is responsible for the vast majority of cases, Plasmodium can be transmitted whenever infected red blood cells (erythrocytes) from one individual enter the bloodstream of another.
- These routes bypass the mosquito stage entirely:
- Blood Transfusions: Receiving blood from an infected donor who carries the erythrocytic (RBC) stage of the parasite.
- Intravenous (IV) Needle Sharing: The use of unsterilized, shared hypodermic needles among intravenous drug users can mechanically transfer infected blood from person to person.
- Congenital (Vertical) Transmission: An infected pregnant mother can pass the parasite across the placental barrier directly to her unborn fetus (transplacental transmission), or the baby can become infected during childbirth through blood contact.
The Life Cycle of Plasmodium
- The life cycle of Plasmodium is alternating and complex, requiring two hosts: the human (where it reproduces asexually) and the female Anopheles mosquito (where it reproduces sexually).
 |
| Life cycle of Plasmodium |
The Asexual Phase (Inside the Human Host)
- To make asexual phase simple for my cambridge students , this phase has been splitted into various stages .
The Liver Stage (Pre-Erythrocytic Schizogony)
- When an infected female Anopheles mosquito bites a healthy human, it introduces the infective stage, called sporozoites, into the bloodstream.
- Each sporozoite is a motile, single-nucleated cell covered by a protective elastic Cuticle.
- Within 30 minutes, these sporozoites leave the bloodstream and enter the liver cells (hepatocytes).
- Inside the liver, the parasite undergoes an asexual reproduction process called schizogony.
- Initially, they form cryptozoites, which multiply to produce thousands of crypto-merozoites.
- These enter fresh liver cells to further develop into meta-crypto-merozoites and micro-crypto-merozoites.
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The RBC Stage (Erythrocytes & Symptoms)
- The micro-crypto-merozoites burst out of the liver cells, enter the bloodstream, and actively target the Red Blood Cells (erythrocytes).
Signet Ring Stage:
- Inside the RBC, the parasite assumes a disc-like shape with a large central vacuole, pushing the nucleus to the periphery.
- This gives it a classic signet ring appearance under a microscope.
Trophozoite (Amoeboid) Stage:
- The parasite begins to feed actively on the host's hemoglobin, growing into a flexible, moving trophozoite stage.
Rosette Stage :
- The mature trophozoite develops into a schizont. The schizont undergoes erythrocytic schizogony, losing its pseudopodia, and its nucleus divides to form a cluster of new merozoites (resembling a flower, known as the rosette stage).
๐ฅ Exam Keyword:
๐When the infected RBCs rupture synchronously to release new merozoites, they liberate a toxic crystalline byproduct called Hemozoin.
๐The release of hemozoin and cell debris into the blood plasma is directly responsible for the characteristic severe chills, shivering, and high recurring fever of malaria.
Gametocyte stage :
- After several cycles of infecting fresh RBCs, some merozoites stop dividing asexually and differentiate into sexual forms called gametocytes.
- Macrogametocyte are the large female gametocyte. whereas Microgametocytes are smaller, male gametocyte.
PHASE 1
Asexual Phase of Plasmodium
๐ Sporozoites enter via mosquito saliva.
⬇️
๐ Liver Cells (Hepatocytes): Multiply to form Crypto-merozoites.
⬇️
๐ Red Blood Cells (RBCs): Signet Ring → Trophozoite → Schizont.
⬇️
๐ Bursting: Releases Merozoites & Hemozoin (causes fever).
⬇️
๐ Differences: Turns into Male & Female Gametocytes.
The Sexual Phase (Inside the Mosquito Vector)
Ingestion: - When a female Anopheles mosquito bites an infected malaria patient, it sucks up the blood meal containing these macro and microgametocytes into its stomach (gut).
Syngamy (Fertilization):
- Inside the mosquito's stomach, the male and female gametocytes develop further and fuse (fertilization), forming a diploid zygote.
Ookinete & Encystment:
- After a period of rest, the zygote transforms into an elongated, motile stage called an ookinete.
- The ookinete penetrates the stomach wall of the mosquito and covers itself with a protective layer, turning into an oocyst (encysted stage).
Sporogony:
- Inside the oocyst, the parasite undergoes asexual multiplication called sporogony.
- The nucleus divides repeatedly, producing thousands of slender, infectious sporozoites.
Migration to Salivary Glands:
- Once the oocyst bursts, these sporozoites are liberated into the mosquito's body cavity (hemocoel) and travel directly to the salivary glands, where they are stored.
Next Cycle:
- When this mosquito bites another healthy person, the stored sporozoites are injected into the new host along with its saliva, ready to begin the next life cycle.
PHASE 2
Sexual Phase of Plasmodium
๐ Ingestion: Mosquito sucks blood with Gametocytes.
⬇️
๐ Mosquito Gut (Stomach): Gametes fuse to form a diploid Zygote.
⬇️
๐ Ookinete Stage: Zygote becomes motile, bores into stomach wall.
⬇️
๐ Oocyst & Sporogony: Divides asexually to create thousands of Sporozoites.
⬇️
๐ Salivary Glands: Oocyst bursts; sporozoites migrate to glands for next bite.
Prevention and Control Strategies for Malaria
- Malaria control programs operate on three main levels: eliminating the vector, preventing the vector from reaching humans, and using medical treatment to clear the parasite.
- The most effective way to stop malaria is to break the life cycle of the female Anopheles mosquito, particularly during its aquatic stages (Eggs, Larva, Pupa).
- It may be achieved by the following measures :
- Spraying oil or kerosene over standing water surfaces.
- The oil forms a thin film that clogs the breathing tubes (spiracles) of mosquito larvae and pupae, suffocating them.
- Introducing larvivorous fish, such as Gambusia affinis (Mosquito fish), into ponds, paddy fields, and drainage systems.
- These fish actively feed on mosquito larvae, reducing the adult population naturally without chemicals.
- Spraying residual insecticides (like pyrethroids or older compounds like DDT) on indoor walls where mosquitoes rest after feeding.
๐Extensive use of chemical insecticides has led to the natural selection of insecticide-resistant strains of Anopheles mosquitoes, making vector control increasingly difficult.
Disrupting Human-Vector Contact
Disrupting Human-Vector Contact
- If the vector cannot be eliminated, the next line of defense is to physically prevent the infected mosquito from biting a human host.
- Sleeping under bed nets treated with synthetic pyrethroid insecticides.
- This provides a dual layer of protection—a physical barrier and a chemical deterrent that kills the mosquito upon contact.
- Fitting windows and doors with fine wire mesh to prevent mosquitoes from entering residential houses, especially during dusk and dawn when Anopheles is most active.
- Applying formulations containing DEET (Diethyltoluamide) on exposed skin or wearing long-sleeved clothing to mask human scents (carbon dioxide and lactic acid) that attract mosquitoes.
Prophylaxis and Anti-Malarial Treatment
- When a person travels to an endemic region or is already infected, medical protocols are used to target the Plasmodium parasite inside the person.
Chemoprophylaxis:
- Taking preventative anti-malarial drugs before, during, and after traveling to a malaria-prone area.
- Drugs like chloroquine, doxycycline, or mefloquine are taken to kill any parasite that enters the bloodstream before it can establish a severe infection.
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Therapeutic Treatment (ACT):
- For confirmed malaria cases, Artemisinin-based Combination Therapy (ACT) is the global gold standard recommended by the WHO uses a combination of two different drugs to rapidly clear parasites from the blood.
⚠️ A-Level Critical Note:
๐Similar to mosquitoes, Plasmodium species (especially Plasmodium falciparum) have rapidly mutated and developed drug resistance to older anti-malarials like chloroquine, which is why combination therapies (ACT) are now mandatory to prevent further resistance.
Cambridge Exam Focus: Why is Malaria Control So Difficult?
- Despite decades of global effort, eradicating malaria remains one of the greatest challenges in modern medicine.
- The primary reasons lie within the genetic and structural complexity of the Plasmodium parasite itself.
Antigenic Variation Explained
- One of the main reasons our immune system fails to clear Plasmodium naturally and why making an effective vaccine is so difficult is Antigenic Variation.
- What are Antigens? The surface of the Plasmodium parasite is covered with specific proteins (antigens) that our immune system (B-lymphocytes) recognizes to produce specific antibodies.
- The Parasite's Strategy: Plasmodium possesses a large family of surface protein genes (such as the var genes in P. falciparum). It does not express all these genes at once. Instead, it constantly switches which surface protein it presents to the host's immune system.
- Immune Evasion: By the time the human body mounts a successful primary immune response and produces antibodies against one specific surface antigen, the parasite has already switched its genetic expression to display a completely different antigen.
- The Result: The existing antibodies become useless. The immune system has to start the recognition process all over again from scratch. This allows the parasite to multiply unchecked and establish a chronic, long-term infection.
Eukaryotic Complexity & Resistance Issues
- Unlike bacteria or viruses, Plasmodium is a highly evolved, multi-stage Eukaryotic organism, which creates severe clinical and biochemical challenges.
Large Genome and Structural Complexity:
- Being a eukaryote, Plasmodium has a much larger genome and a more complex cellular structure than bacteria.
- It has thousands of genes, giving it multiple metabolic pathways and redundant survival mechanisms.
Multi-Stage Life Cycle:
- The parasite constantly changes its morphology (form) and habitat within the same host.
- It moves from liver cells (intracellular) to the bloodstream (extracellular) and then hides inside Red Blood Cells (intracellular).
- A drug or vaccine that targets the liver stage (sporozoites) is completely ineffective against the blood stage (merozoites).
- Because Plasmodium is eukaryotic, its cellular machinery (ribosomes, enzymes, and metabolic pathways) is highly similar to human cells.
- Designing a drug that is toxic enough to kill the parasite but safe enough not to harm the human host is extremely difficult.
Rapid Drug Resistance:
- Due to its genetic flexibility, Plasmodium rapidly undergoes mutations under selection pressure. It has developed widespread resistance to traditional, cheap anti-malarial drugs like chloroquine and proguanil.
- Now, emerging resistance against the frontline drug Artemisinin poses a massive global threat.
Vector Resistance:
- To make matters worse, the eukaryotic complexity isn't limited to the parasite.
- The mosquito vector (Anopheles) also undergoes natural selection and has developed widespread resistance to the insecticides (like DDT and pyrethroids) used in bed nets and indoor spraying.
To understand the detail information about the Our Immune System: Cellular Component Framework & Self vs Non-Self Recognition | Cambridge A-Level Biology Core Notes read my next detailed guide
Q1. State the name of the causative agent of malignant tertian malaria and identify its primary and secondary hosts. [3 Marks]
Answer : Causative Agent: Plasmodium falciparum (Note: Species name must be underlined or italicized for full marks). [1 Mark]
Primary (Definitive) Host: Female Anopheles mosquito. [1 Mark]
Secondary (Intermediate) Host: Human (Homo sapiens). [1 Mark]
Q2. Explain the term Antigenic Variation with reference to the Plasmodium parasite. [3 Marks]
Answer : It is the ability of the parasite to alter/change the surface proteins (antigens) presented to the host immune system. [1 Mark]
The parasite possesses a large family of surface antigen genes (such as var genes) and switches their genetic expression over time. [1 Mark]
Consequence: This allows the parasite to evade the host's primary immune response, making existing antibodies ineffective. [1 Mark]
Q3. Describe the method by which the biological vector inoculates the Plasmodium parasite into a healthy human host. [3 Marks]
Answer :An infected female Anopheles mosquito bites a human to take a blood meal and punctures a blood capillary. [1 Mark]
It injects its saliva containing an anticoagulant to prevent human blood from clotting inside its mouthparts. [1 Mark]
Along with the saliva, the infective, motile stage of the parasite called sporozoites is injected directly into the host's bloodstream. [1 Mark]
Q4. Identify the toxic byproduct released during the erythrocytic stage of the Plasmodium life cycle and state its clinical effect on the human host. [2 Marks]
Answer: Toxic Byproduct: Hemozoin (malarial pigment/toxin). [1 Mark]
Clinical Effect: The synchronous bursting of RBCs releases this toxin into the plasma, triggering severe chills/shivering followed by recurring high fever. [1 Mark]
Q5. Suggest how introducing Gambusia fish into water bodies acts as a biological control method for malaria. [2 Marks]
Answer: Gambusia affinis is a larvivorous fish that acts as a natural predator to mosquitoes. [1 Mark]
It actively feeds on the aquatic larval stages of the Anopheles mosquito, breaking its life cycle and reducing the adult vector population without using chemical insecticides. [1 Mark]
๐AO2 Application of Knowledge (Diagram & Labeling Questions)
Context 1 : Consider the following diagram of Plasmodium life cycle and gives answer the following questions:
Q1. Identify the specific biological stages of the Plasmodium parasite labeled as A and B in the provided life cycle diagram. [2 Marks]
Answer: A: Sporozoite (or Sporozoites) [1 Mark]
B: Gametocyte (or Gametocytes / Macro & Microgametocytes) [1 Mark]
Q2. With reference to stage C shown in the diagram, describe its immediate target in the human host and explain the clinical consequence when these cells subsequently rupture. [3 Marks]
Answer : Target: Stage C represents merozoites, which actively target and invade the Red Blood Cells (Erythrocytes). [1 Mark]
Consequence of Rupture: When the infected RBCs rupture synchronously to release more merozoites, they liberate a malarial toxin/pigment called hemozoin into the bloodstream. [1 Mark]
Clinical Effect: This release triggers the characteristic clinical symptoms of malaria, which include severe shaking chills and high recurring fever. [1 Mark]
Q3. State how the transmission cycle would be affected if the development of stage B is successfully blocked inside the human host. [2 Marks]
Answer : Stage B represents gametocytes, which are the only stages capable of initiating sexual reproduction inside the mosquito vector. [1 Mark]
If stage B is blocked, a feeding female Anopheles mosquito will not ingest any viable gametocytes; hence, fertilization/zygote formation cannot occur, effectively halting the transmission of malaria to the next human host. [1 Mark]
Context 2 : Consider the following diagram of Plasmodium life cycle and gives answer the following questions.
Q1. Name the specific host cells represented by Site X and Site Y in the human body. [2 Marks]
Answer : Site X: Hepatocytes (or Liver cells). [1 Mark]
Site Y: Erythrocytes (or Red Blood Cells / RBCs). [1 Mark]
Q2. Name the type of cell division occurring at both Site X and Site Y, and explain why this rapid multiplication makes it difficult for the host's immune system to control the infection initially. [3 Marks]
Answer : Type of Cell Division: Mitosis / Asexual multiple fission / Schizogony. [1 Mark]
Immune Evasion Reason 1: The parasite multiplies intracellularly (inside the host's own liver and red blood cells), effectively "hiding" from circulating antibodies and white blood cells. [1 Mark]
Immune Evasion Reason 2: The sheer volume of merozoites released simultaneously after each cycle easily overwhelms the host’s initial immune response. [1 Mark]
Q3. Based on the diagram, outline the structural change that occurs to the parasite immediately after entering Site Y before it transitions into a mature Trophozoite. [2 Marks]
Answer : Upon entering the RBC (Site Y), the merozoite develops a large central vacuole that pushes the nucleus to the edge/periphery. [1 Mark]
This causes the parasite to take on a disc-like, ring-shaped appearance known as the Signet Ring Stage. [1 Mark]
๐ AO3 Experimental Skills & Data Interpretation
Question 1: A student wants to observe the Signet Ring Stage and mature trophozoites of Plasmodium falciparum inside infected human erythrocytes using a light microscope. The student is provided with a stained blood smear slide and a microscope fitted with an eyepiece graticule.
(a) Suggest a suitable staining reagent used to color the nucleic acid of the parasite distinct from the human red blood cell cytoplasm. [1 Mark]
๐ Reference Table for Microscope Calibration (High Power - 400 times)
Before measuring the parasite, the student calibrated the eyepiece graticule using a stage micrometer. The data obtained is recorded in the table below:
| Stage Micrometer Divisions (Each division = 10 ฮผm) | Eyepiece Graticule Units (epu) | Calculation for 1 epu | Actual Value of 1 epu (ฮผm) |
|---|---|---|---|
| 4 divisions (= 40 ฮผm) | 16 epu | 40 ฮผm / 16 epu | 2.5 ฮผm |
| 8 divisions (= 80 ฮผm) | 32 epu | 80 ฮผm / 32 epu | 2.5 ฮผm |
b) Describe the steps required to align and calibrate the eyepiece graticule against the stage micrometer to achieve the baseline values shown in the table above. [3 Marks]
(c) Using the final calibration factor
from the table (1 epu = 2.5 micrometer ) calculate the actual diameter of a Plasmodium trophozoite that measures exactly 3 eyepiece units under high power. Show your working and state appropriate units. [2 Marks]
Answers : (a) Staining Reagent:
Giemsa stain / Leishman’s stain / Wright's stain. [1 Mark]
(b) Calibration Steps:
Align the zero mark/scale of the eyepiece graticule with the zero mark/scale of the stage micrometer on the microscope stage under high power. [1 Mark]
Look across the scale to find a point where a line on the eyepiece graticule exactly coincides/lines up with a line on the stage micrometer scale (e.g., 16 epu}) lining up with 4\text{ divisions}). [1 Mark]
Divide the known distance of the stage micrometer (in micro meter ) by the number of coinciding eyepiece units to find the value of
1 epu = 40 micrometer \ 16 = 2.5 micrometer. [1 Mark ]
(C) Numerical calculation ;
Actual size = size in epu x calibration value
= 3 x 2.5 = 7.5 micro meter
Question 2: Scientists investigated the effectiveness of a new synthetic anti-malarial drug (Drug X) against a chloroquine-resistant strain of Plasmodium falciparum in vitro. Infected red blood cells were treated with different concentrations of Drug X nmol dm-3. The percentage of surviving parasites was measured after 48 hours.
The results are shown in the data table below:
| Concentration of Drug X (nmol dm⁻³) | Percentage of Surviving P. falciparum (%) |
|---|---|
| 0 (Control) | 100 % |
| 10 | 85 % |
| 20 | 50 % (IC₅₀ Value) |
| 40 | 12 % |
| 80 | 2 % |
a) With reference to the data, describe the relationship between the concentration of Drug X and the survival percentage of P. falciparum. [2 Marks]
(b) State the IC50 value (concentration required to inhibit 50% of parasite survival) for Drug X and explain why identifying this value is critical before moving to human clinical trials. [2 Marks]
Answer : (a).As the concentration of Drug X increases, the percentage of surviving P. falciparum parasites decreases / there is an inverse relationship. [1 Mark]
The decrease is non-linear, the sharpest drop occurs between 10 nmol dm3 (85%) and 40 nmol dm3 (12%), after which the curve flattens out. (Must include data points for full marks). [1 Mark]
(b) IC50 Value: 20 nmol dm3 [1 Mark]
Explanation: It determines the minimum effective baseline dose required to clear half the parasite load, ensuring the dosage is high enough to kill the eukaryotic parasite but low enough to avoid toxicity or side effects in human tissues. [1 Mark]
Question 3 : An investigation was carried out to study the development of Plasmodium schizonts over a period of 40 hours under three different experimental conditions (represented by the Blue, Green, and Red lines). The results are plotted in the graph below, with vertical error bars showing the standard deviation (SD) for each data point.
( Blue = Control Group, Green = Low Dose Drug, Red = High Dose Drug).
(a) Describe the trend shown by the Blue line regarding the change in schizont proportion over the course of the investigation. [2 Marks]
(b) State the time (in hours) at which the schizont proportion reaches its maximum value for both the Blue and Green lines. [2 Marks]
(c) With reference to the error bars plotted at 13 hours and 18 hours on the graph, explain what these bars indicate about the reliability of the data collected at these points. [2 Marks]
Answers : (a) Trend Description (Blue Line):
From 1 to 6 hours, the schizont proportion remains constant at zero. From 6 to 18 hours, there is a sharp/rapid increase, reaching a peak value of 50. [1 Mark]
After 18 hours, the proportion decreases significantly down to approximately 15 at 30 hours. (Data quote with correct units required for full marks). [1 Mark]
(b) Identifying Maximum Values:
Blue Line Peak: 18 hours (Value is approx 50). [1 Mark]
Green Line Peak: 13 hours (Value is approx 13). [1 Mark]
(c) Error Bar Evaluation:
The error bars show the variability of data around the mean (Standard Deviation). At 18 hours (Blue line), the error bar is relatively large, indicating higher data variability and lower reliability/precision at that point. [1 Mark]
At 13 hours (Green line), the error bar is very small, indicating that the data points are tightly clustered around the mean, representing high precision and reliability. [1 Mark]
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