Pathogens — Fungi and Protists

This lesson covers fungal and protist pathogens as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand how fungi and protists cause disease and describe the specific examples of rose black spot and malaria.

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Fungal Pathogens

What Are Fungi?

Fungi are eukaryotic organisms. They can be:

• Single-celled — e.g. yeast
• Multicellular — e.g. mushrooms, moulds

Multicellular fungi are made of thread-like structures called hyphae (singular: hypha). A network of hyphae is called a mycelium. Fungi reproduce using spores, which can be spread by wind, water or animals.

Not all fungi are pathogenic — many are decomposers that play a vital role in nutrient cycling. However, some fungi cause diseases in plants and animals.

How Fungi Cause Disease

Pathogenic fungi can cause disease by:

1. Invading tissues — fungal hyphae grow into plant or animal tissues, digesting and destroying cells.
2. Producing spores — spores can spread through the air or water to infect new hosts.
3. Some produce toxins — although this is less common than in bacteria.

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Example: Rose Black Spot

Rose black spot is a fungal disease that affects rose plants. It is caused by the fungus Diplocarpon rosae.

Feature	Detail
Type of pathogen	Fungus (Diplocarpon rosae)
Host	Rose plants
How it spreads	Spores are spread by wind and water (e.g. rain splash); can also spread via direct contact between infected and healthy leaves
How it causes disease	The fungus infects the leaves, causing purple or black spots to appear
Symptoms	Purple or black spots on leaves; leaves turn yellow and drop off prematurely
Impact	Fewer leaves means less photosynthesis, so the plant produces less glucose and grows more slowly
Treatment / control	Using fungicides (chemical sprays); removing and destroying infected leaves; some gardeners use resistant varieties

graph TD
    A["Fungal spores land<br/>on rose leaf"] --> B["Spores germinate<br/>and hyphae grow<br/>into leaf tissue"]
    B --> C["Purple/black spots<br/>appear on leaves"]
    C --> D["Leaves turn yellow<br/>and fall off"]
    D --> E["Reduced photosynthesis<br/>and growth"]
    C --> F["New spores produced<br/>on infected leaves"]
    F --> G["Spores spread by<br/>wind and water"]
    G --> A

Exam Tip: The key impact of rose black spot on the plant is reduced photosynthesis because the infected leaves drop off, meaning less light is absorbed. This is a common extended-response question point.

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Protist Pathogens

What Are Protists?

Protists are a diverse group of eukaryotic organisms. They are mostly single-celled (unicellular), although some form colonies. Protists are not bacteria, fungi, plants or animals — they form their own kingdom.

Some protists are parasites that cause serious diseases in humans and animals.

How Protist Pathogens Spread

Many protist pathogens are spread by vectors. A vector is an organism that carries a pathogen from one host to another without being affected by the disease itself.

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Example: Malaria

Malaria is a serious and potentially fatal disease caused by a protist parasite called Plasmodium. It is one of the most significant diseases globally, particularly in tropical and subtropical regions.

Feature	Detail
Type of pathogen	Protist (Plasmodium species)
Vector	Female Anopheles mosquito
How it spreads	The mosquito feeds on the blood of an infected person, taking in Plasmodium parasites. When the mosquito bites another person, it injects Plasmodium into their blood
Where it affects	Red blood cells and the liver
Symptoms	Recurring episodes of high fever, sweating, chills, headache, vomiting, muscle pain; can be fatal
Global impact	Hundreds of millions of cases per year; hundreds of thousands of deaths (mostly children under 5 in sub-Saharan Africa)

The Malaria Life Cycle

The life cycle of the malaria parasite involves both the mosquito and the human host:

graph TD
    A["Infected mosquito<br/>bites human"] --> B["Plasmodium enters<br/>human bloodstream"]
    B --> C["Parasites travel<br/>to the liver"]
    C --> D["Parasites multiply<br/>in liver cells"]
    D --> E["Parasites enter<br/>red blood cells"]
    E --> F["Parasites multiply<br/>inside red blood cells"]
    F --> G["Red blood cells burst<br/>releasing parasites<br/>causing fever"]
    G --> H["Uninfected mosquito<br/>bites infected human"]
    H --> I["Mosquito picks up<br/>Plasmodium parasites"]
    I --> J["Parasites develop<br/>inside mosquito"]
    J --> A

Preventing and Controlling Malaria

Because malaria is spread by mosquitoes, prevention focuses on reducing contact with the vector:

Prevention method	How it works
Mosquito nets (insecticide-treated)	Physical barrier preventing mosquitoes from biting during sleep; insecticide kills mosquitoes on contact
Insecticides	Spraying inside homes and on standing water to kill mosquitoes
Draining standing water	Mosquitoes breed in stagnant water; removing breeding sites reduces mosquito populations
Antimalarial drugs	Can be taken as prophylaxis (prevention) by travellers to high-risk areas; also used for treatment
Biological control	Introducing fish that eat mosquito larvae into ponds and water sources

Exam Tip: Malaria is caused by a protist (Plasmodium), NOT by the mosquito itself. The mosquito is the vector — it transmits the pathogen. Make sure you are precise about this in exam answers.

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Comparing Fungal and Protist Pathogens

Feature	Fungi	Protists
Cell type	Eukaryotic	Eukaryotic
Size	Varies (microscopic to large)	Mostly microscopic
Structure	Hyphae and spores (multicellular) or single-celled (yeast)	Mostly single-celled
Reproduction	Spores	Various (binary fission, spores)
Common spread method	Wind, water, contact	Often via a vector
Example disease	Rose black spot	Malaria
Example pathogen	Diplocarpon rosae	Plasmodium

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Summary of All Four Pathogen Types

Pathogen	Cell type	Example disease	How it causes illness
Bacteria	Prokaryotic	Salmonella (food poisoning)	Toxin production; cell damage
Virus	Not a cell	HIV, TMV	Hijacks host cells; cells burst
Fungus	Eukaryotic	Rose black spot	Invades and destroys plant tissue
Protist	Eukaryotic	Malaria	Destroys red blood cells

Exam Tip: Learn at least one example disease for each of the four pathogen types. A common 6-mark question asks you to compare how different types of pathogen cause disease.

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