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Support and movement: Class 12 Biology
The need for support and movement, the human skeleton, types of bone and cartilage, joints, muscles and the sliding filament mechanism, antagonistic muscles, skeletal disorders and support in plants.
Key points
Support and movement are needed to keep the shape of the body, protect organs, allow locomotion, and for feeding and escape from danger. In animals the skeleton and muscles work together. Plants have no muscles, but they need support to hold leaves to light, and they use turgor pressure and strong tissues.
Types of skeleton: a hydrostatic skeleton is fluid under pressure in a body cavity, as in earthworms. An exoskeleton is a hard outer covering, as in insects, and it must be shed to allow growth. An endoskeleton is inside the body, as in vertebrates, and it grows with the animal. The human skeleton has 206 bones.
Parts of the human skeleton:
- Axial skeleton: the skull, the vertebral column (33 vertebrae at first, with some fused), the ribs and the sternum.
- Appendicular skeleton: the pectoral (shoulder) girdle and arms, and the pelvic (hip) girdle and legs.
- The vertebral column has cervical (7), thoracic (12), lumbar (5), sacral and coccygeal regions. The first vertebra (atlas) lets the head nod, and the second (axis) lets it turn.
- The limb bones: humerus, radius and ulna in the arm, and femur, tibia and fibula in the leg.
Functions of the skeleton are support, protection (skull protects the brain, ribs protect the heart and lungs), movement (bones act as levers), making blood cells in red bone marrow, and storing calcium and phosphate. Bone is living tissue with a matrix of collagen fibres hardened by calcium phosphate, so it is strong and slightly flexible.
Bone is made of compact bone on the outside and spongy bone inside. Compact bone is made of units called Haversian systems, each with a central canal containing blood vessels and nerves, surrounded by rings of matrix with bone cells (osteocytes) in small spaces. Osteoblasts build new bone and osteoclasts break it down. Bone grows in length at the growth plates (epiphyseal plates) until adulthood.
Cartilage is a firm but flexible tissue with a matrix of collagen and chondroitin, and it has no blood supply. It is found at the ends of bones, in the nose, the ear flap, between vertebrae (discs) and in the trachea rings. In the human embryo much of the skeleton is first cartilage and is later replaced by bone.
A joint is where two bones meet. Fixed (immovable) joints, such as those between skull bones, do not move. Slightly movable joints are those between vertebrae. Synovial joints are freely movable. The ball and socket joint (shoulder, hip) allows movement in all directions. The hinge joint (elbow, knee) moves in one plane. A synovial joint has cartilage on the bone ends, a capsule, and synovial fluid that reduces friction. Ligaments join bone to bone, and tendons join muscle to bone.
Muscle tissue is of three types. Skeletal (striated) muscle is voluntary, is attached to bones and contracts fast but tires. Smooth muscle is involuntary, found in the gut and blood vessels, and contracts slowly. Cardiac muscle is found only in the heart, is involuntary and does not tire. Skeletal muscle fibres are long cells with many nuclei, made of myofibrils which contain thin actin and thick myosin filaments. The repeating unit is the sarcomere.
Sliding filament theory. When a nerve impulse arrives, calcium ions are released inside the muscle fibre. The myosin heads attach to actin and bend, pulling the actin filaments inward over the myosin. ATP is needed to detach the heads and to reset them, and then they attach again further along. The filaments slide, the sarcomere becomes shorter and the muscle contracts. The filaments themselves do not shorten.
Muscles pull but cannot push, so they work in antagonistic pairs. In the arm, the biceps contracts to bend (flex) the elbow while the triceps relaxes. To straighten (extend) the arm, the triceps contracts and the biceps relaxes. The bone acts as a lever, and the joint is the fulcrum. Disorders include arthritis (inflamed joints), osteoporosis (bones become thin and weak, especially in older women), rickets (soft bones in children caused by lack of vitamin D) and fractures.
Support in plants. Herbaceous plants depend on turgor: cells full of water press on their walls and keep the stem firm, and wilting follows when water is lost. Woody plants rely on xylem, whose walls are thickened with lignin, and on sclerenchyma fibres. Collenchyma, with walls thickened at the corners, supports young stems and leaf stalks. Movement in plants includes tropisms, which are growth responses to a stimulus, such as phototropism toward light.
Study tip: Practise the labelled diagrams of the human skeleton, a synovial joint and a sarcomere. Write the sliding filament mechanism as numbered steps in the correct order, since the sequence earns the marks.
Definitions and flashcards
Say the meaning first, then open the card to check.
Endoskeleton
A skeleton found inside the body, as in humans.
Ligament
A strong band of tissue that joins one bone to another.
Tendon
A tough cord that joins a muscle to a bone.
Synovial fluid
The fluid in a freely movable joint that reduces friction.
Sarcomere
The basic contractile unit of a muscle fibre between two Z lines.
Antagonistic muscles
A pair of muscles that produce opposite movements.
Osteoblast
A cell that builds new bone.
Cartilage
A firm, flexible connective tissue with no blood supply.
Turgor
The pressure of water inside plant cells against their walls.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
Differentiate between a tendon and a ligament. (2 marks)
A tendon joins muscle to bone and transfers the pull of the muscle. A ligament joins bone to bone and holds the joint together.
State two functions of cartilage in the human body. (2 marks)
Cartilage at the end of bones in a joint reduces friction and absorbs shock. Cartilage in the nose, ear and trachea gives shape and support while remaining flexible.
Describe the structure of a synovial joint. (3 marks)
The ends of the bones are covered with smooth cartilage. A fibrous capsule surrounds the joint and its inner membrane secretes synovial fluid, which lubricates the surfaces. Ligaments join the bones and keep them in position. This allows free movement with little friction and wear.
Why do muscles work in antagonistic pairs? Give an example. (3 marks)
A muscle can only pull when it contracts and cannot push to return the bone. A second muscle that does the opposite job is needed. In the arm, the biceps contracts and the triceps relaxes to bend the elbow. For straightening, the triceps contracts and the biceps relaxes.
Describe the sliding filament mechanism of muscle contraction. (5 marks)
A nerve impulse reaches the muscle fibre and calcium ions are released. This exposes binding sites on the thin actin filaments. The heads of the thick myosin filaments attach to actin forming cross bridges. The heads then bend and pull the actin toward the centre of the sarcomere. ATP provides energy to detach the heads, which return to their original position and attach again further along. The repeated action slides the filaments over each other, so the sarcomere shortens and the whole muscle contracts. The lengths of the filaments do not change.
How are herbaceous plants supported? (3 marks)
In non-woody plants the cells are full of water and are turgid. They press against the cell walls and keep the stem and leaves firm. Collenchyma and sclerenchyma add some strength, and xylem also helps. If water is short, turgor falls and the plant wilts.
Common mistakes
- Saying the filaments shorten during contraction. They slide, and the sarcomere gets shorter.
- Confusing tendons with ligaments.
- Writing that muscles push bones. Muscles can only pull.
- Forgetting the role of ATP in detaching the myosin heads.
- Saying that cartilage has blood vessels, or that bones are dead tissue.