NCERT Solutions for Class 11 Biology Chapter 17: Locomotion and Movement – Free PDF Download

Class 11 Biology Chapter 17, Locomotion and Movement, explains how movement is a defining feature of living organisms and covers the amoeboid, ciliary and muscular types of movement seen in the human body. The chapter goes on to explore the structure of the sarcomere and the sliding filament mechanism of muscle contraction, the three types of muscle tissue, and the axial and appendicular skeleton along with the classification of joints and common skeletal-muscular disorders such as myasthenia gravis, tetany, arthritis, osteoporosis and gout. These Class 11 Biology Chapter 17 solutions are also useful as quick revision notes before exams.

Exercises

Q1. Draw the diagram of a sarcomere of skeletal muscle showing different regions.

A sarcomere is the functional contractile unit of a skeletal muscle fibre, defined as the portion of a myofibril lying between two successive Z-lines (also called Z-discs). Its main regions, arranged symmetrically on either side of the Z-line, are as follows:

Z-line: An elastic fibre that bisects each I-band and firmly anchors the thin (actin) filaments.
I-band (Isotropic band): The light band containing only thin filaments (actin), which is bisected by the Z-line.
A-band (Anisotropic band): The dark band containing thick filaments (myosin), including the regions where thick and thin filaments overlap. The length of the A-band stays constant during contraction.
H-zone: The central, lighter portion of the A-band that contains only thick filaments and is not overlapped by thin filaments; it narrows and can disappear during contraction.
M-line: A line in the middle of the H-zone that anchors the thick filaments.

A simple linear diagram of one sarcomere would be labelled, from left to right: Z-line – I-band – A-band (with the H-zone and M-line marked at its centre) – I-band – Z-line, with the thin (actin) filaments projecting inward from each Z-line and the thick (myosin) filaments occupying the centre of the A-band.

Q2. Define the sliding filament theory of muscle contraction.

The sliding filament theory states that muscle contraction occurs when the thin filaments (actin) slide over the thick filaments (myosin) using energy from ATP hydrolysis, without any change in the length of the filaments themselves. As the myosin heads (cross-bridges) repeatedly attach to, pull, and detach from the actin filaments, the actin filaments are drawn towards the centre of the sarcomere (the H-zone), pulling the Z-lines closer together. This shortens the sarcomere and, consequently, the whole muscle fibre. During this process, the A-band length stays constant, while the I-band shortens and the H-zone narrows or disappears, causing the muscle to contract and generate tension.

Q3. Describe the important steps in muscle contraction.

Muscle contraction, as explained by the sliding filament theory, occurs through the following sequential steps:

Step 1 – Signal generation: A signal from the central nervous system travels through a motor neuron to the neuromuscular junction (motor end plate), the junction between the neuron and the muscle fibre’s membrane (sarcolemma). This causes the release of the neurotransmitter acetylcholine, which generates an action potential in the sarcolemma.

Step 2 – Calcium release: The action potential spreads through the muscle fibre and the T-tubule system, triggering the release of Ca²⁺ ions from the sarcoplasmic reticulum into the sarcoplasm.

Step 3 – Exposure of active sites: The rise in Ca²⁺ concentration causes calcium to bind to troponin on the actin filaments. This shifts the position of tropomyosin, which was masking the myosin-binding (active) sites on actin, thereby exposing them.

Step 4 – Cross-bridge formation and the power stroke: Using energy from ATP hydrolysis, the myosin heads attach to the exposed active sites on actin, forming cross-bridges. The myosin heads then pull the actin filaments towards the centre of the sarcomere (the power stroke), and the attached Z-lines are pulled inward, shortening the sarcomere.

Step 5 – Cross-bridge detachment: A fresh ATP molecule binds to the myosin head, causing it to release ADP and inorganic phosphate and detach from actin, breaking the cross-bridge.

Step 6 – Repetition and relaxation: This cycle of cross-bridge formation, pulling, and detachment repeats as long as the stimulus and Ca²⁺ ions are available. Once Ca²⁺ is pumped back into the sarcoplasmic reticulum, tropomyosin re-masks the active sites on actin, and the muscle fibre relaxes.

Q4. Write true or false. If false, change the statement so that it is true.

(a) Actin is present in thin filament.
(b) H-zone of striated muscle fibre represents both thick and thin filaments.
(c) Human skeleton has 206 bones.
(d) There are 11 pairs of ribs in man.
(e) Sternum is present on the ventral side of the body.

(a) True.
(b) False. The H-zone of a striated muscle fibre represents only the thick (myosin) filament.
(c) True. The adult human skeleton is made up of 206 bones.
(d) False. There are 12 pairs of ribs in man.
(e) True. The sternum (breastbone) lies on the ventral (front) side of the body.

Q5. Write the differences between:

(a) Actin and myosin

Actin	Myosin
A thin contractile protein found in the I-band (light band) of the sarcomere.	A thick contractile protein found in the A-band (dark band) of the sarcomere.
Made of two helically wound ‘F actin’ strands along with tropomyosin and troponin.	A polymer of meromyosin units, each with a globular head, a short arm and a tail.
Does not itself form cross-bridges; provides the binding site for myosin heads.	The myosin head forms cross-bridges with actin and possesses ATPase activity.

(b) Red and white muscle fibres

Red muscle fibres	White muscle fibres
Contain a large amount of the red, oxygen-storing pigment myoglobin.	Contain very little myoglobin, hence appear pale/white.
Have numerous mitochondria and derive energy mainly by aerobic respiration.	Have comparatively fewer mitochondria and rely more on anaerobic respiration.
Show slow, sustained contractions and fatigue less quickly.	Show rapid contractions but fatigue quickly.

(c) Pectoral and pelvic girdle

Pectoral girdle	Pelvic girdle
Located in the shoulder region; each half is made of a clavicle and a scapula.	Located in the hip region; each half is made of three fused bones — ilium, ischium and pubis.
Provides articulation for the bones of the upper limb (forelimb) at the glenoid cavity.	Provides articulation for the bones of the lower limb (hind limb) at the acetabulum.
Mainly supports movements of lifting and holding.	Mainly supports body weight, standing, walking and running.

Q6. Match Column I with Column II:

Column I	Column II
(a) Smooth muscle	(iv) Involuntary
(b) Tropomyosin	(ii) Thin filament
(c) Red muscle	(i) Myoglobin
(d) Skull	(iii) Sutures

Q7. What are the different types of movements exhibited by the cells of the human body?

Cells of the human body exhibit three main types of movement:

Amoeboid movement: Certain cells, such as leucocytes (white blood cells) and macrophages, move using pseudopodia (temporary cytoplasmic projections) formed by the streaming of cytoplasm and the action of microfilaments — similar to how Amoeba moves. This allows these cells to migrate from the blood into tissues to fight infection.

Ciliary movement: Many internal tubular organs are lined with ciliated epithelium. The coordinated beating of cilia moves particles or fluids in a specific direction — for example, cilia in the trachea move dust-trapping mucus outward, and cilia in the fallopian tube help move the ovum towards the uterus.

Muscular movement: Brought about by the coordinated contraction and relaxation of muscle fibres acting on the skeleton and joints; this includes both movement of body parts and locomotion (movement of the whole body from one place to another).

Q8. How do you distinguish between a skeletal muscle and a cardiac muscle?

Skeletal muscle	Cardiac muscle
Attached to bones and involved in voluntary movements; under conscious/voluntary nervous control.	Found only in the heart wall; contraction is involuntary and myogenic (self-generated), though modulated by the nervous and endocrine systems.
Fibres are long, cylindrical, unbranched and multinucleated.	Fibres are cylindrical, branched and usually uninucleate.
Cells are not joined by intercalated discs.	Cells are joined end-to-end by intercalated discs, which help synchronise contraction.
Contracts rapidly but fatigues relatively quickly.	Contracts rhythmically throughout life without fatigue.

Q9. Name the type of joint between the following:

(a) Atlas/axis
(b) Carpal/metacarpal of thumb
(c) Between phalanges
(d) Femur/acetabulum
(e) Between cranial bones
(f) Between pubic bones in the pelvic girdle

(a) Pivot joint — allows rotation of the head.
(b) Saddle joint — allows a wide range of movement of the thumb.
(c) Hinge joint — allows movement in one plane only.
(d) Ball and socket joint — allows movement in almost all directions.
(e) Fibrous joint — an immovable joint seen as sutures between the flat skull bones.
(f) Cartilaginous joint — the pubic symphysis, where the bones are joined by cartilage and only slight movement is possible.

Q10. Fill in the blank spaces:

(a) All mammals (except a few) have __________ cervical vertebrae.
(b) The number of phalanges in each limb of a human is __________.
(c) Thin filament of myofibril contains 2 ‘F’ actins and two other proteins namely __________ and __________.
(d) In a muscle fibre, Ca²⁺ is stored in __________.
(e) __________ and __________ pairs of ribs are called floating ribs.
(f) The human cranium is made of __________ bones.

(a) Seven cervical vertebrae.
(b) 14 phalanges.
(c) Tropomyosin and troponin.
(d) The sarcoplasmic reticulum.
(e) The 11th and 12th pairs of ribs are called floating ribs.
(f) Eight bones.

Class 11 Biology Chapter 17 – Notes and Extra Questions

The Locomotion and Movement exercise has a fixed set of 10 questions that combine short definitional answers, longer explanatory answers (the sliding filament theory and the sequential steps of muscle contraction are the most exam-important long answers), a true/false correction question, three difference/comparison questions best answered in tabular form, a match-the-column question, a joint-identification question, and a fill-in-the-blanks question. While studying, pay special attention to exact terminology — actin, myosin, troponin, tropomyosin, sarcomere, A-band, I-band, H-zone, Z-line — since NCERT and CBSE examiners award marks strictly for correct terms. Also memorise the key numbers used repeatedly across boards and NEET: 206 bones in the adult human skeleton, 12 pairs of ribs (with the 11th and 12th as floating ribs), 8 cranial bones, 7 cervical vertebrae in almost all mammals, and 14 phalanges per limb. Diagrams of the sarcomere and the neuromuscular junction are frequently asked and should be practised by hand.

Frequently Asked Questions

How many exercise questions are there in Class 11 Biology Chapter 17?

There are 10 exercise questions at the end of Chapter 17, Locomotion and Movement, in the current NCERT Class 11 Biology textbook (2023 rationalised syllabus, 19-chapter structure). This chapter was earlier numbered Chapter 20 (Unit 5: Human Physiology) in pre-2023 editions of the book; only the chapter number has changed due to rationalisation — the exercise questions and their content remain the same.

What is the sliding filament theory in simple terms?

The sliding filament theory explains that a muscle contracts not because its filaments shrink, but because the thin actin filaments are pulled and slide inward over the stationary thick myosin filaments, powered by repeated cross-bridge cycling using ATP. This sliding shortens each sarcomere, and the shortening of millions of sarcomeres in series shortens the whole muscle fibre.

What is the difference between a fibrous, cartilaginous and synovial joint?

A fibrous joint (e.g., sutures between skull bones) allows no movement at all. A cartilaginous joint (e.g., the pubic symphysis, or the joint between adjacent vertebrae) allows only slight movement, since the bones are connected by cartilage. A synovial joint (e.g., the ball and socket joint of the hip, or the hinge joint of the knee) is a freely movable joint in which a fluid-filled synovial cavity separates the bones, allowing a wide range of motion.

What are some common disorders of the muscular and skeletal system covered in this chapter?

The chapter discusses myasthenia gravis (an autoimmune disorder that weakens skeletal muscles and affects neuromuscular junctions), tetany (rapid muscle spasms caused by a fall in blood calcium levels), muscular dystrophy (progressive degeneration of skeletal muscle), arthritis (inflammation of one or more joints), osteoporosis (age-related loss of bone mass and density, often linked to a decline in estrogen after menopause), and gout (inflammation of joints due to the deposition of uric acid crystals).