Skeleton: Movement and Locomotion — ICSE Class 9 Biology Important Questions

Answer: (c) Ball-and-socket joint.

The shoulder joint allows movement in all directions (rotation), which is characteristic of a ball-and-socket joint.

Answer: (b) Vertebral column.

The axial skeleton consists of the skull, vertebral column, ribs and sternum. The femur, humerus and girdles belong to the appendicular skeleton.

Answer: (a) 206.

An adult human skeleton has 206 bones; a newborn has around 300, some of which fuse together as the body grows.

Answer: (b) Triceps contracts, biceps relaxes.

The triceps is the extensor; its contraction straightens (extends) the arm while the antagonistic biceps relaxes.

Answer: (a) The cranial bones meet at fixed joints called sutures where they are firmly interlocked, which is exactly why no movement is possible, so R correctly explains A.

Movement: The change in position of a part of the body of an organism, for example bending the arm, blinking the eye or beating of the heart; the whole body need not move.

Locomotion: The movement of the whole body of an organism from one place to another, for example walking, running or swimming. All locomotion involves movement, but not all movement is locomotion.

(i) It provides a rigid framework that gives shape and support to the body. (ii) It protects delicate internal organs (e.g. the skull protects the brain, ribs protect the heart and lungs). (iii) It provides surfaces for the attachment of muscles, enabling movement and locomotion. (iv) The red bone marrow manufactures blood cells (red and white blood cells).

(i) Ball-and-socket joint: The rounded head of one bone fits into the cup-shaped socket of another, allowing movement in all directions. Example: shoulder joint, hip joint.

(ii) Hinge joint: Allows movement in only one plane, like the hinge of a door (bending and straightening). Example: elbow joint, knee joint.

(iii) Pivot joint: Allows rotational movement about a central axis. Example: the joint between the atlas and axis vertebrae, which lets the head turn side to side.

The biceps and triceps are an antagonistic pair attached across the elbow (a hinge joint). To bend (flex) the arm, the biceps contracts (shortens) and pulls the forearm up while the triceps relaxes. To straighten (extend) the arm, the triceps contracts and the biceps relaxes.

Because of these differences bone gives firm support and protection, while cartilage provides flexible support at places such as the nose tip, ear pinna and between vertebrae, and acts as a cushion at joints.

A synovial joint is a freely movable joint such as the knee, elbow or shoulder. Its parts are:

(i) Articular cartilage: A smooth layer of cartilage covering the ends of the two bones; it reduces friction and absorbs shock during movement.

(ii) Synovial cavity: A space between the two bones that permits movement.

(iii) Synovial membrane: Lines the joint cavity and secretes the synovial fluid.

(iv) Synovial fluid: A slippery fluid that lubricates the joint, reduces friction between the moving bone surfaces and nourishes the articular cartilage.

(v) Ligaments: Tough, fibrous bands that hold the two bones together and keep the joint stable while still allowing movement.

(vi) Capsule: A fibrous covering that encloses and protects the whole joint.

Together, the smooth cartilage and lubricating synovial fluid allow the bones to glide over each other with minimal friction, while ligaments and the capsule keep the joint firm and prevent dislocation.

Why muscles work in antagonistic pairs: A muscle can only pull by contracting (shortening); it cannot push or lengthen itself back. Therefore, to move a bone in two opposite directions, two muscles are needed that pull in opposite directions. Such a pair is called an antagonistic pair - when one contracts, the other relaxes, and vice versa.

At the elbow: The biceps is the flexor. When it contracts (and the triceps relaxes) the forearm is raised - the arm bends. The triceps is the extensor. When it contracts (and the biceps relaxes) the arm straightens.

At the knee: The muscles on the front of the thigh (extensors) contract to straighten the leg, while the muscles at the back (flexors, hamstrings) relax; to bend the knee the flexors contract and the extensors relax.

Why one muscle cannot do both: Since a muscle only produces force while contracting and returns passively, a single muscle could pull the bone in only one direction. Once contracted it cannot push the bone back to the original position. Hence a second, oppositely acting muscle is essential to reverse the movement - which is why antagonistic pairs are needed for complete to-and-fro movement.

(i) P (shoulder) is a ball-and-socket joint; Q (elbow) is a hinge joint.

(ii) R (between skull bones) is an immovable / fixed joint; such joints are called sutures.

(iii) S (atlas-axis in the neck) is a pivot joint, which allows the head to rotate side to side.

(iv) All joints are places where two or more bones meet; they hold the bones together and (except fixed joints) allow controlled movement of body parts.