Movement is any change of position of any part of an organism. It is broader than locomotion. CBSE Class 11 treats movement as the introduction to locomotion, distinguishing the three types.
Core Concepts
Three types of movement
Amoeboid — by pseudopodia, using microfilaments. Example — WBCs (phagocytosis), Amoeba. Ciliary — by cilia, in lining of trachea, oviduct and on Paramecium. Muscular — by muscle contraction, used by most animals and all limb movements.
Amoeboid movement relies on the sol-gel transformation of cytoplasm. The inner cytoplasm (endoplasm) is fluid (sol state) and flows forward. At the leading edge, it converts to a stiffer gel (ectoplasm), forming a pseudopodium. Actin and myosin microfilaments drive this transformation. In our body, white blood cells use amoeboid movement to squeeze through capillary walls and reach infection sites — a process called diapedesis.
Ciliary movement uses tiny hair-like projections that beat in coordinated waves. Each cilium has a 9+2 arrangement of microtubules (nine outer doublets and two central singlets) called the axoneme. The motor protein dynein causes the microtubules to slide, producing the beating motion. In the oviduct, cilia push the ovum towards the uterus. In the trachea, cilia sweep mucus and trapped particles upward.
Muscular movement is the most familiar type. Skeletal muscles attached to bones create locomotion. Smooth muscles in the gut create peristalsis. Cardiac muscle keeps the heart beating.
Muscle structure
Skeletal muscle is bundles of fibres. Each fibre is a syncytium (many nuclei) with myofibrils inside. Myofibrils are made of thick (myosin) and thin (actin) filaments arranged in sarcomeres.
Organisational hierarchy:
- Whole muscle → wrapped in epimysium
- Fascicle (bundle of fibres) → wrapped in perimysium
- Single muscle fibre (cell) → wrapped in endomysium
- Myofibril → contains sarcomeres
- Sarcomere → the functional unit of contraction
The sarcomere is bounded by two Z-lines. It contains:
- A-band (dark band): region where thick filaments are present. Its length does not change during contraction.
- I-band (light band): region with only thin filaments. Shortens during contraction.
- H-zone: central part of A-band with only thick filaments. Shortens during contraction.
- M-line: holds thick filaments together at the centre.
Sliding filament theory
Thick and thin filaments do not shorten — they slide past each other. Myosin heads bind actin, pull, release, rebind. The sarcomere shortens, H-zone narrows, A-band stays the same, I-band narrows.
The cross-bridge cycle has four steps:
Myosin head (energised by ATP hydrolysis) binds to exposed actin site, forming a cross-bridge.
Myosin head pivots, pulling the thin filament towards the M-line. ADP and Pi are released.
A fresh ATP molecule binds to the myosin head, causing it to release from actin.
ATP is hydrolysed to ADP + Pi. The energy re-cocks the myosin head back to its high-energy position. The cycle repeats.
Each cycle shortens the sarcomere by about 10 nm. Thousands of cycles per second produce smooth contraction.
Role of calcium and ATP
Calcium from sarcoplasmic reticulum binds troponin, shifting tropomyosin to expose actin binding sites. ATP is needed for myosin head release and re-cocking. Without ATP, rigor mortis sets in.
The sequence of events from nerve impulse to contraction is called excitation-contraction coupling:
- Motor neuron releases acetylcholine (ACh) at the neuromuscular junction
- ACh binds receptors on the muscle fibre membrane (sarcolemma)
- Action potential spreads along the sarcolemma and into T-tubules
- T-tubule depolarisation triggers Ca release from sarcoplasmic reticulum (SR)
- Ca binds troponin C, causing a conformational change
- Tropomyosin shifts, exposing myosin-binding sites on actin
- Cross-bridge cycling begins — muscle contracts
- When nerve impulse stops, Ca is pumped back into SR (requires ATP)
- Tropomyosin re-covers the binding sites — muscle relaxes
Remember: calcium is needed for contraction, ATP is needed for both contraction (power stroke) AND relaxation (detachment + Ca pumping back). This is why rigor mortis happens after death — no ATP means myosin heads cannot detach from actin.
Types of skeletal muscle fibres
| Property | Red (slow-twitch) | White (fast-twitch) |
|---|---|---|
| Myoglobin content | High (red colour) | Low (pale colour) |
| Mitochondria | Many | Few |
| Energy source | Aerobic (oxidative) | Anaerobic (glycolytic) |
| Fatigue resistance | High | Low |
| Contraction speed | Slow | Fast |
| Example | Postural muscles (back) | Sprinting muscles (legs) |
Marathon runners have more red fibres; sprinters have more white fibres. The ratio is largely genetic but can shift somewhat with training.
Disorders of the muscular system
- Myasthenia gravis: Autoimmune disease where antibodies destroy ACh receptors at the neuromuscular junction. Leads to progressive muscle weakness.
- Muscular dystrophy: Genetic disorder (usually X-linked) where the protein dystrophin is defective. Muscles progressively degenerate.
- Tetany: Sustained muscle contraction due to low blood calcium. Without enough Ca for proper regulation, muscles cannot relax normally.
Worked Examples
Cilia are connected at the base and beat metachronously — each one slightly ahead of its neighbour. This creates a wave that moves fluid efficiently, as seen in trachea moving mucus upward.
Motor neuron fires. Acetylcholine releases at NMJ. Action potential spreads along muscle fibre. Calcium floods out of SR. Sliding filaments contract. Bones move at the elbow joint. All in about 20 ms.
During contraction:
- A-band: No change in length (thick filaments stay the same)
- I-band: Decreases (thin filaments slide inward)
- H-zone: Decreases (thin filaments overlap more with thick)
- Sarcomere length: Decreases (Z-lines come closer together)
This is the single most asked question on movement in NEET. Memorise it as: “A stays, everything else shrinks.”
Solved Problems (Exam Style)
Problem 1 (NEET 2021 pattern): During muscle contraction, which of the following remains constant? (a) I-band (b) A-band (c) H-zone (d) Sarcomere length
The A-band is the region containing thick (myosin) filaments. Thick filaments do not change length or position during contraction. Only the thin filaments slide over them. Therefore the A-band length remains constant. Answer: (b)
Problem 2 (NEET pattern): Which mineral ion is essential for muscle contraction? (a) Na (b) K (c) Ca (d) Mg
Ca binds to troponin C, which shifts tropomyosin to expose the actin binding sites. Without Ca, the cross-bridge cycle cannot begin. Na and K are needed for nerve impulse propagation, but the direct trigger for contraction is Ca. Answer: (c)
Problem 3 (CBSE Board): Explain the__(a) role of__(b) in muscle contraction: (i) Troponin (ii) Sarcoplasmic reticulum
(i) Troponin is a regulatory protein bound to tropomyosin on the thin filament. When Ca binds to troponin C subunit, troponin changes shape, pulling tropomyosin away from the myosin-binding sites on actin. This exposes the sites for cross-bridge formation.
(ii) Sarcoplasmic reticulum is a specialised endoplasmic reticulum that stores Ca ions. Upon receiving the nerve impulse (via T-tubules), it releases Ca into the sarcoplasm, triggering contraction. After contraction, it actively pumps Ca back, causing relaxation.
Common Mistakes
Calling locomotion and movement the same thing. Locomotion is movement of the whole organism from one place to another; movement includes any part moving.
Saying muscles push. They can only pull.
Forgetting that ATP is needed for release of myosin heads, not just contraction.
Writing that the A-band shortens during contraction. The A-band stays the same length. Only the I-band and H-zone shorten. This is the most common error in NEET answers on this topic.
Confusing the role of Ca and ATP. Ca is the trigger (removes the tropomyosin block). ATP is the energy currency (powers the cross-bridge cycle and calcium pumping). Both are essential but they do different jobs.
Exam Weightage and Revision
This topic is a repeat performer in board papers and entrance exams. NEET typically asks one to two questions on the core mechanisms, CBSE boards give three to six marks, and state PMT papers often include a diagram-based long answer. The PYQs cluster around a small set of facts — lock those and you clear the topic.
NEET has asked about the A-band in 2018, 2019, and 2021. The sliding filament mechanism appeared in NEET 2020. Muscle fibre types (red vs white) appeared in NEET 2022. This is a high-frequency topic — expect one question every year.
When a question gives a scenario, identify the core mechanism first, then match it to the concepts above. Most wrong answers come from reading the scenario too quickly.
Draw one sarcomere with Z-lines, A-band, I-band and H-zone labelled. That single diagram handles most questions on movement.
Practice Questions
Q1. What is the 9+2 arrangement in cilia?
The axoneme of a cilium has 9 peripheral doublets of microtubules arranged in a circle, with 2 central singlet microtubules. This is called the 9+2 arrangement. The motor protein dynein on the outer doublets causes sliding, producing the beating motion.
Q2. Why does rigor mortis occur after death?
After death, ATP production stops. Without ATP, myosin heads cannot detach from actin filaments. The cross-bridges become locked in place, making the muscles stiff. Rigor mortis begins a few hours after death and lasts until the muscle proteins start to degrade.
Q3. Differentiate between red and white muscle fibres.
Red fibres have high myoglobin, many mitochondria, use aerobic metabolism, resist fatigue, and contract slowly (postural muscles). White fibres have low myoglobin, fewer mitochondria, use anaerobic glycolysis, fatigue quickly, and contract fast (sprinting muscles).
Q4. Name the protein that stores calcium in muscle cells.
Calsequestrin is the calcium-binding protein inside the sarcoplasmic reticulum. It allows the SR to store high concentrations of Ca without creating osmotic problems. When the SR is stimulated, it releases this stored calcium into the sarcoplasm.
Q5. What is the role of acetylcholine in muscle contraction?
Acetylcholine (ACh) is the neurotransmitter released at the neuromuscular junction by the motor neuron. It binds to nicotinic receptors on the sarcolemma, generating an action potential in the muscle fibre. This action potential travels along the T-tubules and triggers Ca release from the SR, starting contraction.
FAQs
What is the difference between isotonic and isometric contraction? In isotonic contraction, the muscle changes length while tension stays roughly constant (e.g., lifting a weight). In isometric contraction, the muscle generates tension but does not change length (e.g., pushing against a wall). Both types use ATP and the cross-bridge cycle.
Why do muscles work in antagonistic pairs? Because muscles can only pull, never push. To move a joint in both directions, you need two opposing muscles. The biceps flexes the elbow (pulls forearm up), and the triceps extends it (pulls forearm down). When one contracts, the other relaxes.
Can smooth muscle be controlled voluntarily? Generally no. Smooth muscle is involuntary and controlled by the autonomic nervous system. However, with training, some degree of control is possible (e.g., bladder control). Skeletal muscle is the only type under full voluntary control.
What causes muscle fatigue? During intense exercise, anaerobic glycolysis produces lactic acid. The accumulation of lactate and H ions lowers the pH inside the muscle fibre, interfering with enzyme activity and cross-bridge cycling. The muscle becomes unable to contract effectively until the metabolic by-products are cleared.
Movement at the molecular level is just myosin walking on actin, powered by ATP. Scale that up and you get a working muscle.