IB Diploma Biology SL · Theme B: Form & Function — Organisms & Ecosystems
Mini-Lesson
Form & Function: Organisms & Ecosystems
Whole organisms show adaptations that fit their environment. This SL lesson covers B3.1 Gas exchange, B3.2 Transport, B4.1 Adaptation to environment and B4.2 Ecological niches.
You will link exchange-surface features to Fick's law, compare transport systems, and use ventilation and uptake calculations.
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 gas-exchange surfaces share features: large surface area, thin (short diffusion distance), moist, and well ventilated / good blood supply to keep steep concentration gradients.
rate ∝ (surface area × concentration difference) ÷ thicknessFick's law of diffusion
Examples: alveoli (lungs), lamellae (fish gills, with countercurrent flow), spongy mesophyll and stomata (leaves).
Quick check
Question
?According to Fick's law, the rate of diffusion across a gas-exchange surface increases when...
Calculate
Your turn — calculate
1At rest a person has a tidal volume of 0.45 L and breathes 14 times per minute. Calculate the ventilation rate (tidal volume × breathing rate).
L/min
Hint: ventilation rate = 0.45 × 14.
B3.2 Transport
Transport systems
Large organisms need mass-transport systems.
Animals: a circulatory system with the heart pumping blood through arteries, capillaries and veins.
Plants: xylem carries water and minerals up from roots (pulled by transpiration); phloem carries sugars from source to sink (translocation).
Transpiration is the evaporation of water from leaves that pulls the xylem water column upward — a direct use of water's cohesion.
Calculate
Your turn — calculate
2A leafy shoot in a potometer took up 1.2 cm³ of water over 30 minutes. Calculate the mean rate of water uptake in cm³ per minute.
cm³/min
Hint: rate = volume ÷ time = 1.2 ÷ 30.
Sort it
Sort each item into its group
Sort each statement into xylem, phloem, or both.
Xylem
Phloem
Both
B4.1 Adaptation
Adaptation to environment
Organisms are adapted to the abiotic factors of their habitat (temperature, water, light, salinity).
Xerophytes (e.g. cacti) reduce water loss: thick waxy cuticle, few sunken stomata, spines, water storage.
Hydrophytes and others show different adaptations suited to their conditions.
Adaptations arise by natural selection because they raise survival and reproduction.
B4.2 Ecological niches
Ecological niches
A species' niche is its role and the resources it uses in a community — how it feeds, where it lives, and its interactions.
Modes of nutrition include autotrophs (make their own food), heterotrophs (consume others), and specialists such as saprotrophs.
Two species cannot occupy exactly the same niche indefinitely — competition tends to exclude one (competitive exclusion).
Quick check
Question
?An ecological niche describes...
Quick check
Question
?In flowering plants, water is transported from roots to leaves through...
Match it
Match each item to its partner
Match each gas-exchange feature to its role.
Item
Partner
Quick check
Question
?A xerophyte such as a cactus reduces water loss by having...
Recap
The big ideas to take away
B3.1: exchange surfaces are large, thin, moist and ventilated (Fick's law)
B3.2: animal circulation; xylem (water up) and phloem (sugars) in plants
B4.1: adaptations such as xerophyte features fit abiotic factors
B4.2: the niche is a species' role; competitive exclusion
Press Finish to see your score.
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You have worked through Theme B: Form & Function — Organisms & Ecosystems for IB Diploma Biology. 🎉
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