Homeostasis is the maintenance of a stable internal environment

Keeping the body's internal conditions — such as temperature, blood glucose concentration, and water potential — within narrow, optimal limits is essential for enzymes and cells to function correctly; this process is called homeostasis. It relies on negative feedback, a control mechanism in which any deviation from the set point (the ideal level) is detected and corrected by effectors, returning conditions back to normal. Understanding homeostasis explains how the nervous and endocrine systems work together as a coordinated response network, and underpins topics across the specification from diabetes to kidney function.

Organisms respond to changes in their internal and external environments

Nervous coordination

Skeletal muscle contraction depends on a precise molecular mechanism in which the proteins actin and myosin slide past one another — shortening the muscle fibre — driven by calcium ions and the breakdown of ATP. Understanding this sliding filament model explains not just how muscles contract, but how the nervous system translates an electrical signal into physical movement. This underpins everything from reflex arcs to the sustained activity of slow muscle fibres and the explosive power of fast muscle fibres.

Organisms respond to changes in their internal and external environments

Skeletal muscles are stimulated to contract by nerves and act as effectors

Muscles can only pull — they cannot push. So two muscles work as a pair: one contracts to create movement, and the other contracts to reverse it.

Organisms respond to changes in their internal and external environments

Skeletal muscles are stimulated to contract by nerves and act as effectors

Skeletal muscle has a layered structure, from the whole muscle you can see down to tiny fibres visible only under a microscope. Each level of organisation helps the muscle generate and transmit force.

Organisms respond to changes in their internal and external environments

Skeletal muscles are stimulated to contract by nerves and act as effectors

A myofibril is a long, cylindrical strand inside a muscle fibre. It contains repeating units called sarcomeres, which are the sections that actually shorten when a muscle contracts.

Organisms respond to changes in their internal and external environments

Skeletal muscles are stimulated to contract by nerves and act as effectors

Muscle fibres contract when two proteins, actin and myosin, repeatedly grab and pull past each other. Calcium ions start the process, and ATP powers each pull.

Organisms respond to changes in their internal and external environments

Skeletal muscles are stimulated to contract by nerves and act as effectors

Calcium ions remove a blocking protein called tropomyosin from actin filaments. This exposes binding sites and allows myosin heads to attach and pull the filament, causing contraction.

Organisms respond to changes in their internal and external environments

Skeletal muscles are stimulated to contract by nerves and act as effectors

AQA does not ask you to explain what troponin does. You only need to know the role of tropomyosin and calcium ions in muscle contraction.

Organisms respond to changes in their internal and external environments

Skeletal muscles are stimulated to contract by nerves and act as effectors

Muscles need ATP to contract. They store a backup molecule called phosphocreatine, which rapidly rebuilds ATP when supplies run low during exercise.

Organisms respond to changes in their internal and external environments

Skeletal muscles are stimulated to contract by nerves and act as effectors

Skeletal muscles contain two fibre types. Slow fibres sustain low-level effort for a long time. Fast fibres produce powerful, rapid contractions but tire quickly.

Organisms respond to changes in their internal and external environments

Skeletal muscles are stimulated to contract by nerves and act as effectors

Skeletal muscle is the effector — the structure that carries out a response — when the nervous system detects a stimulus, and understanding how it works means tracing that response all the way down to the molecular level. Inside each muscle fibre, contractile units called sarcomeres contain two proteins, actin and myosin, which repeatedly bind and pull past one another in a process driven by ATP and triggered by calcium ions. Mastering this sliding filament mechanism unlocks your ability to explain not just how muscles contract, but why energy supply, fibre type, and antagonistic muscle pairs all determine how the body moves and responds.

Organisms respond to changes in their internal and external environments

Stimuli, both internal and external, are detected and lead to a response

Survival depends on an organism's ability to detect changes — called stimuli — in both its external surroundings and its internal environment, and then produce a coordinated response. Receptors are specialised cells or proteins that convert a specific stimulus into an electrical or chemical signal, which is then communicated to effectors — the muscles or glands that carry out the response. Understanding this stimulus–response framework is the foundation for everything that follows in this section, from how nerve impulses are transmitted to how skeletal muscle contracts and how the body maintains a stable internal state.

Organisms respond to changes in their internal and external environments