Muscle and motility — IB Diploma Biology HL

Revision notes

6 short notes, in the order of the specification. Each one in short:

1. Movement and reasons for locomotion

   Movement is a universal feature of living organisms: even sessile organisms, which stay fixed in one place, move parts of their bodies, while motile organisms move from place to place. Locomotion is movement of the whole organism, and animals use it for foraging for food, escaping from danger, searching for a mate and migration.

2. Sliding filament model of muscle contraction

   The sliding filament model explains muscle contraction: myosin heads bind to actin filaments, forming cross-bridges, and pull the actin towards the centre of the sarcomere using energy from ATP. The actin and myosin filaments slide past each other without changing length, so the sarcomere, and the whole muscle, shortens.

3. Titin, antagonistic muscles and the intercostal muscles

   Titin is an immense protein in the sarcomere that acts like a spring: it helps sarcomeres recoil after stretching and prevents overstretching. Antagonistic muscles are needed because muscle tissue can only exert force when it contracts. The internal and external intercostal muscles are an antagonistic pair that move the ribcage in opposite directions.

4. Motor units in skeletal muscle

   A motor unit is a single motor neuron together with all the skeletal muscle fibres it stimulates, connected by neuromuscular junctions. When the motor neuron fires, acetylcholine released at the junctions stimulates every fibre in the unit to contract together. Recruiting more motor units increases the force of a muscle contraction.

5. Skeletons, the hip joint and range of motion

   Practical

   Skeletons provide anchorage for muscles and act as levers: arthropods have exoskeletons and vertebrates have endoskeletons. At a synovial joint such as the human hip, between the femur and pelvis, cartilage and synovial fluid reduce friction, ligaments hold the bones together and muscles pull on bones through tendons. Range of motion is measured in degrees.

6. Adaptations for swimming in marine mammals

   Marine mammals such as dolphins and whales are adapted for swimming by a streamlined body that reduces drag, forelimbs modified into flippers for steering, and a tail with a horizontal fluke moved up and down for propulsion. Changes to the airways, such as a blowhole that closes, allow periodic breathing at the surface between dives.