Locomotion and Movement

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 17.

Is your heartbeat locomotion?

Your heart muscle contracts over a lakh times a day, yet you go nowhere. The cilia in your windpipe sweep dust upward all day, and you go nowhere. Yet when your thigh muscles contract in a stride, you cross the road. All three are movements, but only one is locomotion. So what makes something "locomotion"?

This chapter answers that and then goes down to the molecules that do the pulling. It is a favourite for CUET because it is full of exact counts and exact changes, so a few clean tables will carry you a long way.

Movement versus locomotion

Movement is one of the main features of living beings. The streaming of protoplasm in an Amoeba is its simplest form. Locomotion is the voluntary movement that changes your place or location: walking, running, climbing, flying, swimming.

The structures used are often shared. Paramoecium uses its cilia to move food through the cytopharynx and also to swim. Hydra uses its tentacles to capture prey and also to move. So every locomotion is a movement, but not every movement is locomotion.

Human cells show three types of movement.

Three kinds of muscle

A muscle is a specialised tissue of mesodermal origin. It makes up 40–50% of the body weight of an adult and has four properties: excitability, contractility, extensibility and elasticity.

Inside a skeletal muscle

A muscle is built in nested layers. Think of a rope made of ever finer cords.

The layers in one line: muscle → fascicles (bundles wrapped in a collagenous fascia) → muscle fibres → myofibrils → sarcomeres

Each muscle fibre is lined by a plasma membrane, the sarcolemma, enclosing the sarcoplasm. The fibre is a syncytium, because the sarcoplasm contains many nuclei. The endoplasmic reticulum of the fibre, the sarcoplasmic reticulum, is the storehouse of Ca²⁺ ions.

Each myofibril has alternating dark and light bands, which gives the muscle its striped look. A sarcomere is the portion between two successive Z lines, and it is the functional unit of contraction.

Memory hook. The dark A band holds the thick myosin. The light I band holds only thin actin. The H zone sits in the middle of the A band.

The two filaments are built differently.

The sliding filament theory

Muscle does not shrink because its filaments shrink. It shortens because thin filaments slide over thick ones.

Why it works this way. ATP does two jobs. Its hydrolysis energises the myosin head so it can bind actin and pull, and a fresh ATP is what makes the head let go again. Ca²⁺ is the switch: troponin is a lock that keeps the actin sites hidden, and Ca²⁺ is the key.

The exam favourite is what changes and what does not.

The A band is as long as the thick filament, and the thick filament does not change. That is why the A band can't shorten.

Pause and predict: at maximum contraction, what happens to the H zone, and why?

Check your answer: it can disappear. The H zone is the part of the thick filament that the thin filaments do not overlap. When the thin filaments are pulled right to the centre, there is no un-overlapped part left.

Beyond the textbook. For the exam: the cross bridge breaks when a new ATP binds. In reality: this is why a dead body stiffens. After death, cells stop making ATP, Ca²⁺ leaks into the sarcoplasm, and myosin heads attach to actin but can never detach. The muscle locks into the stiffness called rigor mortis. It eases later as the tissue breaks down. Why it matters: it proves that ATP is needed to release the muscle, not just to contract it.

Fuel, fatigue and fibre types

A motor unit is a motor neuron together with all the muscle fibres connected to it. Repeated activation of a muscle can lead to the accumulation of lactic acid from the anaerobic breakdown of glycogen, which causes fatigue.

Muscles differ in how they get their energy.

The human skeleton: 206 bones

The skeleton has 206 bones, split into two groups: 80 axial and 126 appendicular. The sum is worth working out so you can reconstruct it in the exam.

The total is 22 + 1 + 6 + 26 + 1 + 24 = 80. The hyoid is a single U shaped bone at the base of the buccal cavity. The skull articulates with the first vertebra, the atlas, by two occipital condyles, so it is dicondylic.

Pause and predict: which numbers make the 126 appendicular bones?

Check your answer: each limb has 30 bones, and there are four limbs, so 120. Add the two halves of the pectoral girdle (clavicle and scapula, 4 bones) and the two coxal bones of the pelvic girdle (2 bones). That gives 120 + 4 + 2 = 126.

The vertebral column has 26 bones arranged in a series, dorsally placed. It protects the spinal cord, supports the head and gives attachment to the ribs and the back muscles.

Beyond the textbook. For the exam: the cervical vertebrae are seven in almost all mammals, including humans. In reality: a giraffe's long neck has the same seven, with each one hugely elongated. A few mammals, such as manatees and sloths, are exceptions, which is why the standard statement says "almost all". Why it matters: it is an exam trap that rewards the words "almost all" and a good example of how evolution reshapes an existing pattern rather than adding parts.

The rib cage is made of the thoracic vertebrae, the sternum (a flat bone on the ventral midline of the thorax) and 12 pairs of ribs. Each rib is thin and flat, and bicephalic, which means it has two articulation surfaces at its dorsal end.

The limbs and girdles. The forelimb has the humerus, radius and ulna, 8 carpals, 5 metacarpals and 14 phalanges. The hindlimb has the femur (the longest bone), tibia, fibula, 7 tarsals, 5 metatarsals and 14 phalanges, with a cup shaped patella covering the knee.

Joints: where the movement happens

Joints are points of contact between bones, or between bones and cartilages. The force of the muscles is used to carry out movement through the joint, which acts as a fulcrum.

When muscles and bones go wrong

Beyond the textbook. For the exam: tetany is spasm due to low Ca²⁺ in body fluid. In reality: it may seem odd that contraction needs Ca²⁺ yet low blood calcium makes muscles spasm. The Ca²⁺ that triggers contraction is inside the fibre, released from the sarcoplasmic reticulum. Low Ca²⁺ in the fluid outside makes nerve and muscle membranes more excitable, so they fire without proper stimulation. Why it matters: tetany is not the same as tetanus, which is a bacterial disease.

Exam traps

60-second revision

Where this chapter connects

• Before: Excretory Products and their Elimination explains the uric acid that crystallises in the joints in gout.
• Next: Neural Control and Coordination covers neurons and neurotransmitters, the start of every muscle contraction.
• Tissue link: Structural Organisation in Animals describes muscle and connective tissue, including cartilage and bone.
• Cell link: Cell: The Unit of Life covers the cytoskeleton, cilia and mitochondria behind amoeboid and ciliary movement.
• Energy link: Biomolecules covers proteins such as actin and myosin and the ATP that powers them.