A2/A-level Biology - Support and Locomotion

• Function of the human skeleton: support, structure, protection.
• The human skeleton is made mostly of bone, with cartilage covering the end of moveable joints to provide lubrication, so it is easier to move.
• As a foetus develops, the skeleton is made only of cartilage.
• During growth and development, this is mostly replaced by bone. This process is ossification.
• Bone and cartilage are living tissues – they contain living cells.
• They require nutrients and oxygen, which are supplied by the blood.

• Compact bones are found in most of the long shafts of bone in the body, i.e. the limbs.
• Compact bone has many closely packed concentric circular arrangements.
• Each circle is called a Haversian system.
• The lacunae, which can be seen above, and the ‘thread-like’ canaliculi which branch from them are filled with osteocytes (which begin their lives as bone forming cells called osteoblasts).
• These cells synthesise and secret the fibrous protein tropocollagen outside their plasma membranes.
• Tropocollagen molecules link together end-to-end and side-to-side to form collagen fibres.
• An inorganic mineral containing calcium phosphate is deposited amongst these fibres.
• The mineral also contains magnesium and carbonate.
• Cells become completely surrounded by and trapped within a matrix collagen and calcium phosphate.

• Collagen gives the bone tensile strength since collagen fibres are very strong, whereas calcium phosphate gives rigidity and compressive strength.
• This is important in leg bones - they carry the weight of the body.

• Living bone is broken down by osteoclasts, allowing bone structure to be altered, for example to repair damage.

• Hyaline cartilage is a translucent tissue found covering the ends of bones at moveable joints, in the ribs where they join to the sternum, and in the C-shaped cartilage rings that surround and support the trachea.
• The living cells in cartilage are called chondrocytes, they produce and maintain the matrix in which they lie.
• Being protein secreting cells, they have a well developed Golgi apparatus and endoplasmic reticulum.
• Unlike bone, the matrix does not contain calcium phosphate or blood vessels.
• The chondrocytes gain nutrients by diffusion from surrounding tissue fluids.
• The matrix consists of water, collagen and glycoproteins.
• The surface of the hyaline cartilage at joints is very smooth and slippery.
• Fine collagen fibres are closely packed and arranged parallel to the surface.
• This is then covered by a very smooth, glycoprotein layer, making it slippery to prevent bones from wearing.

• The human skeleton consists of the axial skeleton and the appendicular skeleton.
• The axial skeleton comprises of the middle of the skeleton - the skull, vertebral column, ribs and sternum.
• The appendicular skeleton comprises of the limb bones, the pectoral girdle.

The Axial Skeleton

• There are 33 vertebrae, 7 cervical (support head/neck), 12 thoracic (support chest/rib articulation), 5 lumbar (large muscles of lower back attach to), 5 fused sacral vertebrae (supports and articulates with hip bones), and the 4 fused tail vertebrae - the coccyx.
• Joints between vertebrae are known as intervertebral joints.
• Each vertebra is joined to the next by ligaments.
• Between each vertebra there are disks of cartilage, intervertebral disks, which act as shock absorbers

Lumbar Vertebra:

• Lumbar consists of

- Centrum: Load-bearing part.

- Neural Arch: Holds neural canal - spinal column.

- Transverse processes: Project outwards/sideways - powerful lower back muscles attach here, requiring the transverse process.

- Neural spine - Backwards pointing process, larger than in thoracic vertbrae.

- Articular processes - On the upper and lower surface, in contact with the vertebrae either side.

Thoracic Vertebra:

• Thoracic vertebrae are similar, except the neural spines are much longer and more slanted than in a lumbar vertebra.

The Appendicular Skeleton

• In each limb there is a similar basic structure - a single bone which articulates with the girdle, then two bones in the lower limb.
• Numerous small bones make up the wrist and ankle, hand and foot.
• In both hand and foot there are 5 digits- this is known as a pentadactyle limb.

That's the end of the topic!

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