Mini-Lesson

Form & Function: Organisms & Ecosystems (HL)

Whole-organism systems are exquisitely adapted. This HL lesson covers B3.1 Gas exchange, B3.2 Transport, B3.3 Muscle & motility (HL), B4.1 Adaptation and B4.2 Ecological niches.

You will link exchange surfaces to Fick's law, follow circulation and the oxygen dissociation curve, and study the sliding filament model.

Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you are ready.

B3.1 Gas exchange

Gas exchange surfaces

Efficient exchange surfaces are large, thin, moist and well ventilated with a good blood supply to keep steep gradients.

rate ∝ (area × concentration difference) ÷ thicknessFick's law

Alveoli, fish lamellae (with countercurrent flow) and leaf spongy mesophyll all maximise these features.

B3.2 Transport

Circulation and haemoglobin (HL)

Mammals have a double circulation: blood passes through the heart twice per body circuit, giving high pressure to the body.

Arteries: thick, elastic, high pressure; veins: valves, low pressure; capillaries: thin, exchange.
The oxygen dissociation curve is sigmoid because oxygen binding to haemoglobin is cooperative.
The Bohr shift: high CO2 (and low pH) makes haemoglobin release more oxygen to active tissues.

Calculate

Your turn — calculate

1A person has a tidal volume of 0.5 L and breathes 15 times per minute. Calculate the ventilation rate.

L/min

Hint: ventilation rate = tidal volume × breathing rate = 0.5 × 15.

Calculate

Your turn — calculate

2The heart pumps a stroke volume of 70 mL at a heart rate of 75 beats per minute. Calculate the cardiac output.

mL/min

Hint: cardiac output = stroke volume × heart rate = 70 × 75.

B3.3 Muscle & motility

Sliding filament model (HL)

Skeletal muscle contracts by the sliding filament mechanism. Within each sarcomere, myosin heads form cross-bridges and pull actin filaments inward, shortening the sarcomere.

ATP is needed to detach each myosin head from actin and to re-cock it for the next cycle. Muscles work in antagonistic pairs; motility in unicells uses cilia, flagella or pseudopodia.

Sort it

Sort each item into its group

Sort each feature by the blood vessel it describes.

Artery

Vein

Capillary

B4.1-B4.2 Adaptation & niches

Adaptation and ecological niches

Organisms are adapted to abiotic factors; xerophytes reduce water loss, for example.

A species' niche is its role and resource use. Its fundamental niche (all it could occupy) is usually reduced to a smaller realised niche by competition. Two species cannot coexist on an identical niche — competitive exclusion.

Quick check

Question

?During muscle contraction, ATP is required to...

Quick check

Question

?The oxygen dissociation curve of haemoglobin is S-shaped (sigmoid) because oxygen binding is...

Quick check

Question

?Alveoli are efficient gas-exchange surfaces because they provide...

Match it

Match each item to its partner

Match each concept to its correct description.

Item

Partner

Quick check

Question

?A species' realised niche is usually smaller than its fundamental niche because of...

Recap

The big ideas to take away

B3.1: exchange surfaces obey Fick's law (large, thin, moist, ventilated)

B3.2 (HL): double circulation; sigmoid O2 curve; Bohr shift

B3.3 (HL): sliding filament model; ATP detaches and re-cocks myosin heads

B4.1-B4.2: adaptation to abiotic factors; realised niche and competitive exclusion

Press Finish to see your score.

🏆

Mini-lesson complete!

⭐⭐⭐

You have worked through Theme B: Form & Function — Organisms & Ecosystems for IB Diploma Biology. 🎉

Your stars: 0 / 0

Next: test yourself in the Evaluate stage Confidence Quiz, then lock it in with Verify.

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