B3.3 - Muscles and Motility
Higher level only
3.3.1 Adaptations for movement as a universal feature of living organisms
Movements can take place within the body but also by moving an organism from one place to another (locomotion).
Sessile organism (e.g. barnacles or corals) do not perform locomotion – they remain in a fixed position but move individual body parts.
Motile organism move around while feeding within their territory – some further distances than others
3.3.2 Sliding filament model of muscle contraction
Muscle fibers contain many parallel myofibrils, which consist of a series of sarcomeres. A sarcomere is the repeating unit of a muscle cell.
The contraction of sarcomeres, and therefore a muscle, is due to the sliding of actin and myosin filaments. ?
The pattern of light and dark bands in sarcomeres is due to a precise and regular arrangement of actin and myosin filaments.
A muscle contraction shortens each sarcomere and therefore the overall length of the muscle fibre. The contraction occurs by sliding of actin and myosin filaments.
When muscle fibers contract the sarcomere length shortens and the Z-discs move closer together. When the muscle is relaxed, the sarcomere length is longer.
The sarcolemma and sarcoplasma are specific adaptations of the cell membrane and cytoplasm, respectively.
3.3.3 Role of the protein titin and antagonistic muscles in muscle relaxation
Titin is the largest polypeptide being composed of 34350 amino acids. It is elastic and acts as a molecular spring.
Titin connects myosin filamints to the Z-discs. It stores potential energy when it is stretched and releases it when it recoils - thereby adding to the force of contraction. It also prevents overstretching of the sarcomere and it holds the mysoin filaments in place surrounded by 6 actin filaments.
Energy is needed to stretch titin and therefore lengthen the muscle when relaxing. Energy cannot be supplied by the muscle itself because muscles can only exert force when contracting. The energy is therefore provided by another muscle called the antagonist. In the elbow joint, the triceps and biceps muscles are an antagonistic pair. (biceps contracted, triceps relaxed or extended - triceps contracted, biceps relaxed)
3.3.4 Structure and function of motor units in skeletal muscle
Sketetal muscles are composed of bundles of striated muscle fibers, each of them stimulated by a motor neuron.
The thin filament is made up of two twisted chains of the globular protein actin.
The thick filament is made of many molecules of the protein myosin. Each myosin molecule has a fibrous tail and a globular head. It has an actin binding site and an ATPase binding site.
3.3.5 Roles of skeletons as anchorage for muscles and as levers
Muscles - apply force or effort
Tendons - connect muscles to bone
Bones - act as levers for structural support
Joints - pivot for movements
An exoskeleton takes over the function of bones for muscle attachment in arthropods
The skeleton of an animal, together with the muscles attached across the joints, functions as a system of levers. Each joint acts as a pivot point (fulcrum). By acting as levers, bones can change the size and direction of a force.
The force applied (when the muscle contracts) is called the effort. A lever is a beam or rigid rod pivoted at a fixed hinge (fulcrum). The further away from the fulcrum the effort is applied, the greater the leverage; that is, the smaller the force that is required to raise the load
3.3.6 Movement at a synovial joint
Most joints in the body are synovial joints. This is where two bones with cartilage and synovial fluid in between joints. Cartilaginous joints are joints in which the bones are connected by cartilage
Elbow joint - hinge joint (type of joint), angular movement in only one direction (range of directional movement), flexion + extension (types of movement)
Radio ulnar joint - pivot joint, movement in all rotational directions (360), inner rotation + outer rotation
Hip joint - ball and socket joint, angular movement + rotational movement, flexion + extension + inner & outer rotation + ab-, adduction
Knee joint - hinge joint, angular movement in only one direction, flexion + extension
3.3.7 Range of motion of a joint
The range of motion at a joint can be measued and investigated using a goniometer. This simple device can be used to measure the angle of movement at a given joint. Muscle stretching and other factors (hormones, biological sex, age…) can all affect the range of motion.
3.3.8 Internal and external intercostal muscles as an example of antagonistic muscle action to facilitate internal body movements
The intercostal muscles are a good example for an antagonistic pair. While the external muscles contract to move the ribcage up, the internal muscles stretch, storing potential energy in the sarcomere protein. When the internal muscles contract the ribcage moves down, stretching the external muscles.
3.3.9 Reasons for locomotion
Foraging for food:
To avoid competition
To establish niches
To source a variety of nutrients
To avoid predation
Migration:
Species might have to migrate during different seasons to avoid food scarcities
Some species migrate to their breeding grounds
Environmental factors or pressure causing migration
Escaping from danger:
Prey is moving from potential predators (selective pressure)
Animals have a roosting site that they return to as a safe place
Searching for a mate:
Populations which are dispersed must travel to find a mate
Some animals move to displace the dominant male
Courtship dances or displays
3.3.10 Adaptations for swimming in marine mammals
Airways:
Marine mammals have a blowhole with openings at the top of the head. There is no shared passageway between trachea and esophagus like in humans to avoid water from entering
Streamlining:
Shape is wide near the front and smaller towards the rear
Body surface is smooth
No body hair, friction is reduced
All attachments have teardrop profile to reduce drag
Dorsal fin - provides stability by preventing rolling
Blubber - provides buoyancy and allows floating (preserving energy)
Flukes - increase thrust and propel the animal forward
Flippers - used for steering