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Not all relationships between species involve eating each other. Some organisms live in close association with other species in ways that can be beneficial, harmful or neutral. This lesson focuses on two important types of relationship: parasitism and mutualism.
| Relationship | Organism A | Organism B |
|---|---|---|
| Parasitism | Benefits (parasite) | Harmed (host) |
| Mutualism | Benefits | Benefits |
| Commensalism | Benefits | Unaffected |
| Predation | Benefits (predator) | Killed (prey) |
For Edexcel GCSE, you need to understand parasitism and mutualism in detail.
Parasitism is a relationship in which one organism (the parasite) lives on or inside another organism (the host) and benefits at the host's expense. The host is harmed but usually not killed immediately — it is in the parasite's interest to keep the host alive.
| Parasite | Host | How it harms the host |
|---|---|---|
| Fleas | Dogs, cats, humans | Feed on blood; cause itching, irritation and can transmit diseases |
| Tapeworms | Humans, pigs, cattle | Live in the intestines; absorb digested food before the host can use it, causing malnutrition |
| Head lice | Humans | Feed on blood from the scalp; cause itching |
| Mosquitoes | Humans, animals | Feed on blood; transmit diseases like malaria (mosquitoes are vectors — they carry the Plasmodium parasite) |
| Mistletoe | Trees (e.g. apple, oak) | A parasitic plant that grows on branches; absorbs water and minerals from the host tree through modified roots (haustoria) |
| Plasmodium | Humans (via mosquito vector) | Causes malaria; reproduces inside red blood cells, destroying them |
Parasites have evolved specific adaptations to survive on or in their hosts:
| Adaptation | Example | Purpose |
|---|---|---|
| Attachment mechanisms | Tapeworm has hooks and suckers on its head (scolex) | Grips onto the intestine wall to avoid being dislodged |
| Flattened body | Tapeworm is ribbon-like and flat | Large surface area for absorbing nutrients; fits inside the intestine |
| Thick outer layer | Tapeworm has a tegument resistant to digestive enzymes | Protects from the host's digestive system |
| Rapid reproduction | Fleas and lice lay many eggs | Ensures survival of offspring even if host grooms some away |
| Ability to evade immune system | Plasmodium lives inside red blood cells | Hidden from immune detection; can change surface proteins |
| No digestive system | Tapeworm absorbs pre-digested food | Does not need its own gut; takes nutrients directly from host |
Many parasites are pathogens (they cause disease) or act as vectors (they carry pathogens between hosts). Mosquitoes, for example, are not themselves the cause of malaria — they carry the Plasmodium parasite, which causes the disease.
Exam tip: When describing parasitism, always clearly state which organism benefits and which is harmed. For full marks, give a specific example and explain HOW the host is harmed (e.g. "the tapeworm absorbs digested food in the intestine, reducing the nutrients available for the host, which can lead to malnutrition and weight loss").
Mutualism is a relationship in which both organisms benefit. It is a win-win interaction.
| Organism | Benefit |
|---|---|
| Oxpecker bird | Gets a reliable food source (ticks, fleas and other parasites on the mammal's skin) |
| Buffalo/mammal | Gets parasite removal — fewer parasites means less irritation and disease |
The oxpeckers also act as an "alarm system" — they fly up and make noise when predators approach, alerting the mammals.
| Organism | Benefit |
|---|---|
| Cleaner fish (e.g. cleaner wrasse) | Gets food — eats parasites and dead skin from the larger fish |
| Larger fish (e.g. shark) | Gets cleaned — parasites removed, reducing risk of infection |
Larger fish visit "cleaning stations" on coral reefs and allow the tiny cleaner fish to enter their mouths and gills without eating them.
| Organism | Benefit |
|---|---|
| Nitrogen-fixing bacteria (e.g. Rhizobium) | Gets sugars (glucose) from the plant as an energy source |
| Leguminous plant (e.g. peas, beans, clover) | Gets usable nitrogen compounds (nitrates/ammonium) — bacteria convert atmospheric nitrogen (N₂) into a form the plant can absorb |
The bacteria live in special root nodules — pink swellings on the roots of legumes. This is a very important relationship because most plants cannot use nitrogen gas directly from the air.
| Organism | Benefit |
|---|---|
| Mycorrhizal fungi | Gets sugars (from photosynthesis) from the plant |
| Plant | Gets increased mineral ion absorption — the fungal hyphae (threads) extend far into the soil, dramatically increasing the effective surface area of the root system |
Approximately 90% of all plant species have mycorrhizal associations. Plants with mycorrhizae grow significantly better, especially in nutrient-poor soils.
| Organism | Benefit |
|---|---|
| Bee/butterfly | Gets nectar (a sugary liquid) and pollen (protein-rich food) |
| Flowering plant | Gets pollinated — pollen is transferred from one flower to another, enabling fertilisation and seed production |
Plants have evolved colourful petals, scent and nectar guides to attract pollinators. Pollinators have evolved long tongues and specific body shapes to access nectar.
Exam tip: For mutualism questions, always clearly state the benefit for EACH organism. Writing "both benefit" is not enough — you must say specifically what each organism gains. For example: "The bee benefits by obtaining nectar as a food source, while the plant benefits because the bee transfers pollen to other flowers, enabling cross-pollination."
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