Why does a single-celled organism not need a gas exchange system?

It has a large SA:V and a short diffusion distance.

Openings on the surface of an insect that lead into the tracheal system and can open and close.

Larger air-filled tubes that carry air from the spiracles into the insect's body.

Very fine, thin-walled branching tubes that end next to or inside respiring cells, where gas exchange occurs.

How does oxygen reach the cells of an insect?

It diffuses along the air-filled tracheae and tracheoles directly to the cells down a concentration gradient.

How do terrestrial insects limit water loss?

Spiracles are closed as much as possible; waterproof cuticle; hairs around the spiracles trap moist air.

Why is the insect system a compromise?

Open spiracles allow gas exchange but also lose water vapour.

What are gill filaments and lamellae?

Filaments are on the gill arch; lamellae are thin plates on the filaments where gas exchange occurs.

Why do gill lamellae have thin epithelium and many capillaries?

Thin epithelium gives a short diffusion distance and capillaries maintain a steep concentration gradient.

What is the counter-current principle?

Water and blood flow in opposite directions across the lamellae.

Why is counter-current flow more efficient than parallel flow?

A concentration gradient is maintained along the whole lamella, so equilibrium is not reached and more oxygen is absorbed.

Gas exchange in single-celled organisms, insects and fishAQA A-Level Biology: Flashcards

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How does gas exchange occur in a single-celled organism?

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How does gas exchange occur in a single-celled organism?: By diffusion across the cell surface membrane.
Why does a single-celled organism not need a gas exchange system?: It has a large SA:V and a short diffusion distance.
What are spiracles?: Openings on the surface of an insect that lead into the tracheal system and can open and close.
What are tracheae?: Larger air-filled tubes that carry air from the spiracles into the insect's body.
What are tracheoles?: Very fine, thin-walled branching tubes that end next to or inside respiring cells, where gas exchange occurs.
How does oxygen reach the cells of an insect?: It diffuses along the air-filled tracheae and tracheoles directly to the cells down a concentration gradient.
How do terrestrial insects limit water loss?: Spiracles are closed as much as possible; waterproof cuticle; hairs around the spiracles trap moist air.
Why is the insect system a compromise?: Open spiracles allow gas exchange but also lose water vapour.
What are gill filaments and lamellae?: Filaments are on the gill arch; lamellae are thin plates on the filaments where gas exchange occurs.
Why do gill lamellae have thin epithelium and many capillaries?: Thin epithelium gives a short diffusion distance and capillaries maintain a steep concentration gradient.
What is the counter-current principle?: Water and blood flow in opposite directions across the lamellae.
Why is counter-current flow more efficient than parallel flow?: A concentration gradient is maintained along the whole lamella, so equilibrium is not reached and more oxygen is absorbed.

Exam questions on Gas exchange in single-celled organisms, insects and fish

Amoeba is a single-celled organism that lives in freshwater ponds. It takes in oxygen and loses carbon dioxide across its cell surface membrane and has no gas exchange organs.
Explain how a concentration gradient for oxygen is maintained across the cell surface membrane of Amoeba.2 marks
A locust is a terrestrial insect. Air enters through openings along the surface of its thorax and abdomen and passes into branching tubes that end next to the muscle cells. The outer surface of the locust is covered by a waterproof layer.
Explain how the locust's gas exchange system is a compromise between gas exchange and limiting water loss.2 marks
In a bony fish, water passes over the gills and flows across the gill lamellae in the opposite direction to the flow of blood in the capillaries inside each lamella. Water entering the gills is 90% saturated with oxygen and leaves 20% saturated. Blood entering the lamellae is 5% saturated with oxygen and leaves 85% saturated.
Explain how counter-current flow allows the fish to absorb a high percentage of the oxygen from the water.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).