IB 103: Lec 20 — Symbiosis

Symbiosis

A close, long-term interaction between two different species.

Commensalism

A relationship where one species benefits & the other is unaffected.

Mutualism

A relationship where both species benefit.

Parasitism

A relationship where one organism benefits & the other is harmed.

Rhizobia

Nitrogen-fixing bacteria that live in plant roots.

Nodules

Structures on plant roots where rhizobia live & fix nitrogen.

Mycorrhizae

Symbiotic associations between plant roots & fungi.

Haustoria

Specialized structures that parasitic plants use to extract nutrients from host plants.

Give 2 examples of plant-animal mutualism.

• Pollination (bees & flowers)

• Seed dispersal (animals & fruits)

Give 2 examples of plant-animal commensalism.

• Birds nesting in trees

• Animals using plants for shelter

Why does nitrogen need to be fixed?

Atmospheric nitrogen (N2) cannot be used by plants; it must be converted into usable forms like ammonia.

Why is plant-rhizobia interaction mutualistic?

Plants get usable nitrogen; bacteria get carbohydrates & shelter.

Plant-bacteria mutualism is mostly found in which plants?

Legumes (e.g., beans, peas).

Why do farmers rotate legumes with other crops?

Legumes enrich soil nitrogen, improving soil fertility for future crops.

Plant-fungi mutualism is mostly found in which plants?

Most land plants.

Why is plant-mycorrhizal association mutualistic?

Fungi help plants absorb water & nutrients; plants provide sugars to fungi.

What is a facultative parasite?

A parasite that can survive without a host.

What is an obligate parasite?

A parasite that must have a host to survive.

What is a hemiparasite?

A parasitic plant that can photosynthesize but still depends on a host.

What is a holoparasite?

A parasitic plant that cannot photosynthesize & fully depends on a host.

Are all obligate parasites holoparasites?

No — some obligate parasites are hemiparasites.

What are 2 major crop parasites discussed in class?

• Witchweed (Striga)

• Dodder, strangleweed (Cuscuta)

Why are these parasites harmful?

They drain nutrients from crops, reducing yield & causing economic loss.

Are all parasitic plants “bad” (for humans)?

No — some play ecological roles or are used in research/medicine.