Unit 10 Skeletal systems of organisms
Key unit competence: Identify the structure and role of different types of skeletal systems.
Key ideas
- A skeleton supports the body, keeps its shape, protects soft organs and allows movement. Land animals need more support than water animals, because water helps to hold them up.
- Hydrostatic skeleton: fluid inside the body, squeezed by muscles, keeps a soft body firm (earthworm, slug, jellyfish, leech). Exoskeleton: a hard, non-living covering outside the body made of chitin (insects, spiders, crabs, scorpions); it protects, reduces water loss and has flexible joints, but it cannot grow, so the animal moults and is soft and easy prey for a short time afterwards. Endoskeleton: living bone and cartilage inside the body (fish, frogs, reptiles, birds, mammals); it grows with the animal, muscles attach to its outside, and all these animals are vertebrates.
- The adult human skeleton has 206 bones. Bone is living tissue (cells, blood vessels) made hard by calcium phosphate; there is compact bone (outer, dense) and spongy bone (inner, light). Bones are long (femur, humerus), short (wrist, ankle), flat (skull, ribs, sternum, scapula) or irregular (vertebrae).
- The axial skeleton is the skull (fused bones forming the cranium around the brain, with jaws), the vertebral column (backbone of 33 vertebrae, S-shaped, protects the spinal cord) and the rib cage (12 pairs of ribs joined to the vertebrae and in front to the sternum, protecting heart and lungs).
- The appendicular skeleton is the pectoral girdle (clavicle and scapula), arms (humerus; radius and ulna; carpals, metacarpals, phalanges), the pelvic girdle (hip bones) and legs (femur, the longest bone; patella or kneecap; tibia and fibula; tarsals, metatarsals, phalanges).
- Functions of the human skeleton: support, protection, movement (bones act as levers moved by muscles at joints), production of blood cells (red bone marrow makes red cells, white cells and platelets), storage of minerals (about 99% of the body's calcium, and phosphorus) and body shape.
- Tendons join muscles to bones; ligaments join bones to bones at joints; cartilage covers bone ends to reduce friction and absorb shock.
- Healthy bones need calcium (milk, small fish such as isambaza, green vegetables), vitamin D (eggs, fish, sunlight), regular exercise, good posture, carrying loads correctly, avoiding smoking and alcohol, and protective gear in sport. Lack of calcium and vitamin D causes rickets in children; osteoporosis (porous, fragile bones) is common in old age.
Common misconceptions
- "Bones are dead, like stones." No: bones contain living cells and blood vessels; they grow, repair fractures and make blood cells.
- "Insects grow continuously like humans." No: their exoskeleton cannot grow; they grow in steps by moulting.
- "Tendons join bone to bone." No: tendons join muscle to bone; ligaments join bone to bone.
- "An earthworm has no skeleton." No: it has a hydrostatic skeleton made of fluid under pressure.
Worked examples
Example 1. Explain why a grasshopper is in danger just after moulting.
Solution
Its new exoskeleton is soft, so it gives little protection and support; the grasshopper moves slowly and is easy prey until the new skeleton hardens.
Example 2. Name the bones that protect: the brain, the heart and lungs, the spinal cord.
Solution
Brain: skull (cranium). Heart and lungs: ribs and sternum (rib cage). Spinal cord: vertebrae (vertebral column).
Practice questions
Compare hydrostatic skeletons, exoskeletons and endoskeletons, with an example of each.
Show answer
Hydrostatic: fluid under pressure inside the body (earthworm). Exoskeleton: hard non-living chitin outside the body, must be moulted (grasshopper). Endoskeleton: living bone and cartilage inside, grows with the animal (human, fish).
Common mistake: Saying jellyfish have an exoskeleton.
State five functions of the human skeleton.
Show answer
Support, protection of organs, movement (with muscles), making blood cells in bone marrow, storing calcium and phosphorus; also gives body shape.
Common mistake: Writing "digestion" as a function.
Name the bones of the leg from the hip to the toes.
Show answer
Femur, patella, tibia and fibula, tarsals, metatarsals, phalanges.
Common mistake: Calling the tibia the femur.
Distinguish between the axial and appendicular skeleton.
Show answer
Axial: skull, vertebral column, ribs and sternum. Appendicular: pectoral and pelvic girdles and the bones of the arms and legs.
Common mistake: Putting the girdles in the axial skeleton.
Explain why insects are vulnerable after moulting.
Show answer
The new exoskeleton is soft until it hardens, so it gives less protection and support, and the insect moves slowly; predators can catch it easily.
Common mistake: Saying insects die when they moult.
Give four practices that keep the skeleton healthy.
Show answer
Eat calcium-rich foods; get vitamin D from sunlight and food; exercise regularly; keep good posture and lift loads correctly; avoid smoking and alcohol; wear protective gear in sport.
Common mistake: Listing only "eat well".
Distinguish between a tendon and a ligament.
Show answer
A tendon joins a muscle to a bone and is non-elastic; a ligament joins bone to bone at a joint and is slightly elastic.
Common mistake: Mixing them up.
Name the bones that make up the pectoral girdle and the pelvic girdle.
Show answer
Pectoral girdle: clavicles (collarbones) and scapulae (shoulder blades). Pelvic girdle: hip bones joined to the base of the vertebral column (sacrum).
Common mistake: Putting the femur in the pelvic girdle.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The skull, vertebral column and rib cage form the: a. moulting 2. Animals with an endoskeleton and a backbone are called: b. axial skeleton 3. Shedding of the exoskeleton so that an animal can grow is called: c. Vitamin D 4. Which vitamin helps the body absorb calcium for strong bones? d. vertebrates Show answer
1 → b (axial skeleton) · 2 → d (vertebrates) · 3 → a (moulting) · 4 → c (Vitamin D)
- The axial skeleton is the central axis.
- Fish, frogs, reptiles, birds and mammals.
- The new exoskeleton is soft at first.
- Vitamin D is made in the skin in sunlight.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
chitin · humerus · exoskeleton · cellulose- An exoskeleton is made mainly of .
- The bone of the upper arm is the .
- Insects and spiders have a .
Show answer
1: chitin · 2: humerus · 3: exoskeleton
- Chitin is tough and waterproof.
- The radius and ulna are in the forearm.
- A hard covering outside the body.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The shoulder blade is the: a. scapula 2. Red blood cells are made in the: b. sternum 3. The number of bones in the adult human skeleton is: c. 206 4. The bone at the front of the chest where ribs join is the: d. red bone marrow Show answer
1 → a (scapula) · 2 → d (red bone marrow) · 3 → c (206) · 4 → b (sternum)
- The clavicle is the collarbone.
- Red marrow also makes white cells and platelets.
- Babies have more, which fuse as they grow.
- Also called the breastbone.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
tendons · osteoporosis (bones lose minerals) · femur · patella- Elderly women often break bones easily after a fall because of .
- The kneecap is the .
- Muscles are attached to bones by .
Show answer
1: osteoporosis (bones lose minerals) · 2: patella · 3: tendons
- Bone density falls with age, especially after menopause.
- It protects the knee joint.
- Tendons are strong and non-elastic.