Muscle Contraction, Adaptation & Ecological Niches

Complete Study Guide

Topics Covered

1. Muscle Contraction (B3.1) HL
2. Adaptation to Environment (B4.1)
3. Ecological Niches (B4.2)
4. Mixed Practice — Exam Style
5. May 2026 Prediction Questions

Aligned to IB Biology HL 2025 syllabus — B3.1, B4.1, B4.2

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Section 1: Muscle Contraction (B3.1) HL

Muscle contraction is one of the most commonly examined HL topics. You must be able to describe the structure of skeletal muscle from the whole-muscle level down to the molecular arrangement of filaments within a sarcomere, and explain every step of the sliding filament mechanism.

1.1 Structure of Skeletal Muscle

Skeletal muscle is composed of bundles of elongated cells called muscle fibres (also known as muscle cells). Each muscle fibre is multinucleated — it forms from the fusion of many embryonic cells and therefore contains multiple nuclei and a shared cytoplasm called sarcoplasm.

1.2 The Sarcomere — Structure and Bands

Each myofibril is divided into repeating units called sarcomeres, which are the basic contractile units. The banding pattern of sarcomeres produces the characteristic striations visible under a light microscope.

1.3 Thick and Thin Filaments

Thick filaments are composed of the protein myosin. Each myosin molecule has a globular head that can bind to actin and hydrolyse ATP. Many myosin molecules bundle together with their heads projecting outward.

Thin filaments are composed of:

• Actin — a globular protein (G-actin) that polymerises into two helical strands (F-actin) forming the backbone of the thin filament
• Tropomyosin — a fibrous protein that wraps around the actin helix, blocking the myosin-binding sites when the muscle is relaxed
• Troponin — a globular protein complex bound to tropomyosin at regular intervals; troponin has a binding site for $\text{Ca}^{2+}$ ions

1.4 The Sliding Filament Theory — Step by Step

The sliding filament theory explains how sarcomeres shorten during contraction. The key principle is that filaments do not shorten — actin slides over myosin, pulling the Z-lines closer together.

1.5 Relaxation

When nervous stimulation stops:

1. $\text{Ca}^{2+}$ is actively pumped back into the sarcoplasmic reticulum by $\text{Ca}^{2+}$-ATPase pumps (requires ATP)
2. $\text{Ca}^{2+}$ dissociates from troponin
3. Tropomyosin slides back over the myosin-binding sites on actin
4. Cross-bridges cannot form, so the muscle relaxes passively (antagonistic muscles or elastic recoil return it to original length)

1.6 The Neuromuscular Junction

The neuromuscular junction (NMJ) is the synapse between a motor neuron and a muscle fibre. The signal to contract arrives here.

Sequence of events at the NMJ:

1. An action potential arrives at the motor neuron axon terminal.
2. Voltage-gated $\text{Ca}^{2+}$ channels open; $\text{Ca}^{2+}$ enters the terminal.
3. $\text{Ca}^{2+}$ causes synaptic vesicles containing acetylcholine (ACh) to fuse with the presynaptic membrane and release ACh into the synaptic cleft by exocytosis.
4. ACh diffuses across the cleft and binds to nicotinic ACh receptors on the sarcolemma (postsynaptic membrane of the muscle fibre).
5. These receptors are ligand-gated $\text{Na}^+$ channels — they open, allowing $\text{Na}^+$ to flow in, depolarising the sarcolemma.
6. The depolarisation generates a muscle action potential that spreads along the sarcolemma and into the T-tubules, triggering $\text{Ca}^{2+}$ release from the SR.
7. ACh is rapidly broken down by acetylcholinesterase in the synaptic cleft, preventing continued stimulation. The choline is recycled back into the presynaptic terminal.

1.7 Fast-Twitch vs Slow-Twitch Muscle Fibres

Skeletal muscles contain a mixture of fibre types, but the proportion varies depending on the muscle’s function.

1.8 Anaerobic Cell Respiration in Muscle — The Lactate Pathway

During intense exercise, oxygen supply cannot meet demand. Muscle fibres (especially fast-twitch) switch to anaerobic respiration to regenerate ATP rapidly.

The lactate pathway:

$\text{Glucose} \xrightarrow{\text{glycolysis}} 2 \text{ pyruvate} \xrightarrow{\text{lactate dehydrogenase}} 2 \text{ lactate} + 2 \text{ NAD}^+$

• Glycolysis produces a net gain of 2 ATP per glucose (far less than aerobic respiration’s ~30-32 ATP)
• The conversion of pyruvate to lactate regenerates $\text{NAD}^+$, which is essential for glycolysis to continue when the electron transport chain is not operating
• Lactate accumulation lowers intracellular pH, which inhibits enzyme activity and contributes to muscle fatigue
• Lactate is transported to the liver via the blood, where it is converted back to pyruvate and then to glucose (the Cori cycle) — this requires oxygen, contributing to the oxygen debt after exercise

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Section 2: Adaptation to Environment (B4.1)

2.1 Types of Adaptation

An adaptation is an inherited characteristic that increases an organism’s fitness (ability to survive and reproduce) in a particular environment. Adaptations arise through natural selection over many generations.

2.2 Adaptation vs Acclimatization

2.3 Xerophytes — Adaptations to Dry Conditions

Xerophytes are plants adapted to survive in environments with limited water availability (deserts, sand dunes, exposed rocky habitats).

2.4 Hydrophytes — Adaptations to Aquatic Conditions

Hydrophytes are plants adapted to living in water or very wet conditions. Their challenges are the opposite of xerophytes — they must deal with excess water and limited gas exchange.

2.5 Animal Adaptations to Extreme Environments

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Section 3: Ecological Niches (B4.2)

3.1 The Concept of an Ecological Niche

An ecological niche is not simply where an organism lives (that is its habitat) — it encompasses the totality of an organism’s interactions with its environment, including what it eats, when it is active, what eats it, its tolerance ranges for abiotic factors, and its role in nutrient cycling.

3.2 The Competitive Exclusion Principle (Gause’s Principle)

The competitive exclusion principle states that two species cannot occupy the same niche indefinitely in the same habitat. If two species compete for exactly the same resources, one will inevitably outcompete the other, leading to the local extinction (competitive exclusion) of the inferior competitor.

Gause’s experiment (1934):

• Gause grew two species of Paramecium (P. aurelia and P. caudatum) separately and together
• Grown separately, both species thrived
• Grown together in the same culture (competing for the same food), P. aurelia consistently outcompeted P. caudatum, which declined to extinction
• Conclusion: two species with identical niches cannot coexist

3.3 Resource Partitioning

Resource partitioning occurs when competing species divide resources to reduce niche overlap and avoid competitive exclusion. Each species specialises on a slightly different subset of the available resources.

Types of resource partitioning:

• Spatial partitioning — species use different parts of the habitat (e.g. different heights in a forest canopy; different depths in a lake)
• Temporal partitioning — species use the same resource at different times (e.g. diurnal vs nocturnal foraging; seasonal differences in activity)
• Morphological partitioning — species evolve different body structures to exploit different food sizes or types (e.g. Darwin’s finches with different beak sizes)

3.4 Character Displacement

Character displacement is the evolutionary divergence of competing species in areas where they coexist (sympatry), compared to areas where they occur alone (allopatry).

Where two species overlap geographically and compete, natural selection favours individuals that are most different from the competing species, reducing niche overlap. Over time, this leads to measurable morphological divergence.

Classic example — Darwin’s finches:

• Geospiza fortis and G. fuliginosa on islands where they coexist have significantly different beak sizes (one larger, one smaller)
• On islands where only one species is present, beak size is intermediate
• The divergence in beak size reduces competition for seeds of the same size

3.5 Generalist vs Specialist Species

3.6 Niche Overlap and Interspecific Competition

Niche overlap occurs when two or more species use the same resource or occupy the same environmental conditions. Greater overlap leads to more intense interspecific competition.

Types of interspecific competition:

• Exploitation competition (indirect) — species compete by consuming shared resources, reducing availability for others (e.g. two bird species eating the same insect prey)
• Interference competition (direct) — species directly prevent others from accessing resources through aggression, territory defence, or chemical inhibition (e.g. allelopathy in plants, where chemicals are released into the soil to inhibit competitors)

Outcomes of interspecific competition:

1. Competitive exclusion — one species is eliminated locally
2. Resource partitioning — species evolve to reduce overlap (coexistence)
3. Character displacement — morphological divergence in sympatry

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Mixed Practice — Exam Style

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1. [Sarcomere] During contraction of a sarcomere, which of the following changes occurs?

   A. The A-band shortens and the I-band stays the same

   B. Both the A-band and H-zone shorten

   C. The I-band and H-zone shorten, but the A-band stays the same length

   D. The actin and myosin filaments both shorten

2. [Cross-Bridge Cycle] What is the immediate role of ATP in muscle contraction?

   A. ATP hydrolysis provides the energy for the power stroke

   B. ATP binding to the myosin head causes it to detach from actin, and ATP hydrolysis re-cocks the head

   C. ATP binds to troponin, causing tropomyosin to move

   D. ATP is required for the release of acetylcholine at the neuromuscular junction only

3. [Muscle Fibres] A sprinter’s leg muscles would be expected to contain a higher proportion of:

   A. Slow-twitch fibres with many mitochondria and high myoglobin content

   B. Fast-twitch fibres with few mitochondria and high glycogen stores

   C. Slow-twitch fibres with high glycogen stores and few capillaries

   D. Fast-twitch fibres with many mitochondria and high myoglobin content

4. [Xerophytes] CAM photosynthesis is an adaptation to dry conditions because:

   A. Stomata remain open during the day to maximise $\text{CO}_2$ fixation

   B. $\text{CO}_2$ is fixed at night when stomata are open, then stomata close during the day to conserve water

   C. CAM plants do not use stomata at all, absorbing $\text{CO}_2$ through their roots

   D. CAM plants photosynthesise only at night when temperatures are lower

5. [Adaptation] A population of mice living in a cold environment has shorter tails and smaller ears than a related population in a warm environment. This is best explained by:

   A. Acclimatization — individual mice in the cold grow shorter tails

   B. Natural selection favouring a lower surface area to volume ratio in cold environments, reducing heat loss

   C. Genetic drift randomly producing shorter tails in the cold population

   D. The cold environment directly causing mutations that shorten tail length

6. [Ecological Niche] The realized niche of a species is smaller than its fundamental niche because:

   A. The species cannot tolerate the full range of abiotic conditions

   B. Interspecific competition, predation, and parasitism restrict the species from using all available resources

   C. The species chooses to occupy only part of its fundamental niche

   D. Intraspecific competition forces individuals into a narrower range

7. [Competitive Exclusion] In Gause’s experiment with Paramecium, P. caudatum was driven to extinction when grown with P. aurelia. This result supports the principle that:

   A. Two species can coexist indefinitely if they share the same niche

   B. Two species occupying the same niche cannot coexist — one will be competitively excluded

   C. Predation is the main factor limiting population growth

   D. P. caudatum was unable to reproduce in laboratory conditions

8. [Counter-Current Exchange] Counter-current flow in fish gills is more efficient than concurrent flow because:

   A. It allows water to flow faster than blood, increasing pressure

   B. It maintains a concentration gradient along the entire length of the exchange surface, maximising diffusion

   C. It allows active transport of oxygen from water to blood

   D. It reduces the surface area needed for gas exchange

9. [Neuromuscular Junction] Acetylcholinesterase in the synaptic cleft of the neuromuscular junction prevents:

   A. Calcium ions from entering the presynaptic terminal

   B. Continuous stimulation of the muscle fibre by rapidly breaking down acetylcholine

   C. The release of acetylcholine from synaptic vesicles

   D. Sodium ions from entering the muscle fibre

10. [Resource Partitioning] Five species of warbler coexist in the same spruce forest by foraging at different heights in the trees. This is an example of:

    A. Character displacement

    B. Competitive exclusion

    C. Spatial resource partitioning

    D. Temporal resource partitioning

Show Answers

1. C — The I-band (actin only) and H-zone (myosin only) both shorten as actin slides over myosin. The A-band (defined by myosin length) does not change. D is incorrect — the filaments do not shorten; they slide past each other.

2. B — ATP binding causes myosin to detach from actin. Subsequent hydrolysis of ATP to ADP + $\text{P}_i$ re-cocks the myosin head into the high-energy position. The power stroke is driven by the release of ADP and $\text{P}_i$, not by ATP hydrolysis. A is incorrect — this is the most common exam error. D is partially true (ATP is needed for vesicle fusion) but is not the primary role of ATP in muscle contraction.

3. B — Sprinters rely on short, explosive bursts of power. Fast-twitch fibres contract rapidly, have fewer mitochondria, rely on anaerobic glycolysis, and store abundant glycogen for rapid ATP production. A describes endurance-type fibres. D incorrectly combines fast-twitch with aerobic characteristics.

4. B — CAM (Crassulacean Acid Metabolism) plants open stomata at night when temperatures are lower and humidity is higher, fixing $\text{CO}_2$ into malate (stored in vacuoles). During the day, stomata close to prevent water loss, and malate is decarboxylated to release $\text{CO}_2$ for the Calvin cycle. A is the opposite. C is incorrect — CAM plants have stomata.

5. B — This pattern follows Allen’s rule: in cold environments, natural selection favours smaller extremities (lower SA:V ratio), which reduces heat loss. This is a heritable, evolutionary adaptation — not acclimatization (A), which would be reversible and non-heritable. C (genetic drift) would not consistently produce this pattern across populations. D is Lamarckian and incorrect.

6. B — The fundamental niche represents the theoretical range of conditions a species could occupy without biotic interactions. In reality, interspecific competition, predation, and parasitism reduce the range of resources and habitats a species actually uses. A describes limits on the fundamental niche itself. D describes intraspecific competition, which does not define the realized niche concept.

7. B — Gause’s experiment is the classical demonstration of the competitive exclusion principle: when two species occupy identical niches, the weaker competitor is eliminated. A is the opposite of the principle. C and D are not supported by the experimental results.

8. B — In counter-current flow, blood and water flow in opposite directions. This ensures that at every point along the gill lamella, there is a diffusion gradient favouring oxygen transfer from water to blood. In concurrent flow, the two media would reach equilibrium midway, and net diffusion would stop. A and C are incorrect mechanisms. D is opposite — counter-current allows more exchange, not less surface area.

9. B — Acetylcholinesterase rapidly hydrolyses ACh in the synaptic cleft, ensuring that each nerve impulse produces a single, brief muscle contraction. Without it, ACh would continuously stimulate the muscle, causing sustained contraction (this is the mechanism of nerve agent poisoning). A, C, and D describe other processes not performed by acetylcholinesterase.

10. C — The warblers reduce competition by foraging at different heights (different spatial positions) within the same habitat. This is spatial resource partitioning (MacArthur’s warblers). A (character displacement) involves morphological divergence. B (competitive exclusion) would mean all but one species disappears. D (temporal partitioning) would involve foraging at different times.

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May 2026 Prediction Questions

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IB Biology HL — Muscle Contraction, Adaptation & Ecological Niches — Complete Study Guide — 2025 Syllabus — Good luck!