What Is Movement in a Biological Context?

Movement in biology spans an enormous range, from single protein molecules stepping along tracks inside a cell to whales crossing oceans and plants bending toward light. At its core, biological movement is any change in position or shape generated by a living system, whether through chemical energy, stored elastic energy, or hydraulic pressure. What makes it fascinating is that life has invented many different engines for the job, and they show up at every scale of organization.

Molecular Motors Are Where It All Starts

The smallest biological movements happen at the molecular level. Proteins called molecular motors convert chemical energy, primarily from a molecule called ATP, into physical motion. These motors do not spin or combust fuel the way a car engine does. Instead, they undergo shape changes during chemical reactions, and those shape changes generate tiny “steps” along a track. Researchers have pieced together how these steps work by imaging the motor proteins at different stages of their chemical cycle, showing how energy from ATP is transformed into directed motion within the tightly constrained structure of the motor itself.¹

Three families of molecular motors do most of the heavy lifting inside cells. Myosin motors walk along actin filaments and are responsible for muscle contraction as well as many other cell movements. Kinesin and dynein motors travel along microtubules, ferrying cargo like organelles and vesicles to where they are needed. These same dynein motors also power the beating of cilia and flagella, the whip-like appendages that propel sperm cells and clear mucus from your airways. The principle is always the same: chemical energy in, mechanical work out, one tiny step at a time.

Movement Inside Cells

Before cells move themselves, there is a world of movement happening within them. Cytoplasmic streaming is a good example. In large plant cells, especially algal cells, the entire fluid interior of the cell circulates in a steady current. This streaming is driven by myosin motors pulling along actin cables, dragging organelles and the surrounding fluid with them. In some algal species, the flow reaches speeds around 100 micrometers per second, which is impressively fast at that scale.² The interaction between the actin cables and myosin generates a steady force-velocity relationship that keeps the current flowing smoothly.³

Another striking example of intracellular movement occurs during cell division. Chromosomes do not just drift passively to opposite ends of a dividing cell. Instead, structures called kinetochores actively move chromosomes along microtubule tracks, switching between phases of motion toward and away from the cell’s poles. These kinetochores are sometimes described as “smart” because they adjust their direction in response to signals within the dividing cell, ensuring that chromosomes end up properly aligned before the cell splits in two.⁴

How Individual Cells Crawl and Swim

Cells themselves can move through their environment, and they use two broad strategies: crawling and swimming. Crawling cells, like immune cells chasing down bacteria or cancer cells spreading through tissue, extend sheet-like protrusions called lamellipodia at their leading edge. The actin cytoskeleton drives this process: actin filaments polymerize rapidly near the membrane, pushing it forward. Research has shown that the actin network in lamellipodia is more complex than previously thought, with three distinct sub-domains of filament orientation, including a small but significant population of filaments pointing in the “wrong” direction, back toward the cell center.⁵ The front of the cell pushes forward through actin polymerization, while the tail retracts with the help of microtubules, and both systems are needed for a cell to actually migrate in a sustained direction.⁶

Swimming cells use appendages: flagella or cilia. Although these structures share names with their bacterial counterparts, eukaryotic cilia are radically different machines. Bacterial flagella are relatively simple rotary motors made of a handful of proteins, spinning from one end like an outboard motor. Eukaryotic cilia, by contrast, contain over 500 proteins and are powered by dynein motors distributed along their entire length, a design called distributed force actuation.⁷ This means every segment of the cilium contributes to the bending wave, rather than the whole structure being driven from the base. The result is a flexible, undulating beat that can be tuned for speed, steering, or fluid transport depending on the organism and tissue.

Muscle and the Sliding Filament

For most animals, the word “movement” calls to mind muscles. Muscle contraction is really just the molecular-motor principle scaled up spectacularly. A muscle fiber is packed with repeating units called sarcomeres, each containing overlapping sets of thick (myosin) and thin (actin) filaments. When the muscle is activated, the myosin heads grab onto actin and row, pulling the filaments past each other. The filaments themselves do not shorten much; the sarcomere shortens because the filaments slide together, like interlocking fingers closing.⁸ Multiply this across millions of sarcomeres arranged in series and in parallel, and you get the force needed to lift a limb, pump blood, or launch a grasshopper into the air.

Muscle needs something to pull against, and that is where skeletons come in. Vertebrates and arthropods use rigid skeletons, either internal bone or external exoskeletons, as lever systems. But a huge number of animals have no rigid skeleton at all. Worms, sea anemones, octopus arms, the tube feet of sea stars, and even elephant trunks rely on hydrostatic skeletons, where muscles squeeze against a volume of incompressible fluid. Because the fluid cannot be compressed, squeezing in one direction forces elongation in another, allowing the structure to bend, extend, and retract.⁹ Spiders use a similar hydraulic principle to extend their legs, since they lack extensor muscles in most of their joints. This makes the hydrostatic skeleton one of the most widespread solutions to the problem of animal movement, even though it gets far less attention than bone and chitin.¹⁰

The Neural Wiring Behind Rhythmic Movement

Having muscles and skeletons is not enough. The movements have to be coordinated, and that job falls to the nervous system. For rhythmic activities like walking, swimming, breathing, and flying, animals rely on neural circuits called central pattern generators, or CPGs. These are networks of neurons, typically in the spinal cord or its equivalent, that can produce rhythmic motor patterns on their own, without needing moment-to-moment instructions from the brain or timing cues from sensory organs.¹¹ Researchers have demonstrated this by isolating spinal cord tissue and showing it still generates the rhythmic signals for walking or swimming even when completely disconnected from the brain and from sensory input.¹²

CPGs set the basic rhythm, but real-world movement constantly adjusts to the terrain, the load, and the animal’s intent. That adjustment depends on sensory feedback, especially from mechanosensory pathways that detect limb position, joint angle, and muscle stretch. Muscle spindles, for instance, are sensory receptors embedded within muscle tissue that continuously report to the central nervous system how much each muscle is being stretched.¹³ This proprioceptive information is woven into the CPG circuits and descending commands from the brain, forming a feedback loop that gives animals the stability to walk on uneven ground or adjust their gait mid-stride.¹⁴ Mammals navigating in the dark can also update their position by integrating proprioceptive feedback and stored motor commands, a process called path integration that lets them return to a starting point even without visual landmarks, though errors accumulate quickly without external reference cues.¹⁵

How Plants Move Without Muscles

Plants obviously lack muscles and nerves, yet they are far from motionless. Plant movement falls broadly into two categories: growth-mediated movements that happen over hours or days, and rapid movements that happen in fractions of a second.

Growth-mediated movements include the familiar tropisms. A seedling bending toward light or roots growing downward in response to gravity are controlled by the redistribution of the hormone auxin. Specialized transport proteins called PINs redirect auxin flow in response to environmental stimuli, creating concentration gradients across a stem or root.¹⁶ The side of a stem with more auxin grows faster, causing the stem to bend away from that side. Molecular and genetic studies have confirmed that these lateral auxin gradients are the core mechanism behind both gravitropism and phototropism.¹⁷ The movement here is real and directed, just achieved through differential growth rather than contraction.

Rapid plant movements are a different story. The Venus flytrap, the sensitive plant (Mimosa), and the explosive seed capsules of some species can move faster than any growth-based mechanism allows. These rapid movements use turgor pressure, the internal water pressure of cells, as their main driving force, sometimes paired with elastic energy stored in cell walls or other tissues. Plants also produce electrical signals similar to the action potentials in animal nerves, and these signals are thought to coordinate rapid cell-to-cell communication during fast movements, though the molecular identity of the channels involved is still not well understood.¹⁸

Ultra-Fast Movements and Elastic Energy Storage

Some of biology’s most extreme movements do not come directly from muscles or turgor at all. A mantis shrimp’s strike, a frog’s tongue, or the spore launch of a fungus can reach accelerations far beyond what any muscle could produce in real time. These movements rely on latch-mediated spring actuation: energy is slowly loaded into an elastic structure (a tendon, a cell wall, a specialized appendage), held in place by a latch that prevents premature release, and then unleashed all at once when the latch is removed.¹⁹ The principle appears across an astonishing range of organisms, from fungi to arthropods to vertebrates. It works because the elastic element releases its energy much faster than the muscle or motor that loaded it, making the output power far greater than the input power at any given instant.

This elastic trick shows up in everyday human movement, too. When you run, your Achilles tendon stores and releases elastic energy with each stride, reducing the metabolic cost of locomotion. The energetics of animal movement in general follow interesting scaling patterns: larger animals tend to use their muscles more efficiently, whether running, flying, or hovering, with the metabolic cost of transport decreasing relative to body mass in a predictable way.²⁰

Collective Cell Movement Shapes Embryos

Not all biologically important movement is about an organism getting from point A to point B. During embryonic development, large groups of cells migrate collectively, moving as coordinated sheets or streams rather than as isolated individuals. These collective migrations reshape the embryo, positioning tissues for organ formation and establishing the body plan. The cells communicate with each other mechanically and chemically, so the group behaves as a cooperative unit rather than a crowd of independent agents.²¹

Tissue folding is a related process. When sheets of epithelial cells need to bend inward to form a tube (as in the early vertebrate neural tube, which becomes the brain and spinal cord), they use a cell shape change called apical constriction. The tops of the cells squeeze together, driven by non-muscle myosin motors pulling on actin, and the resulting wedge shape of each cell causes the whole tissue to buckle inward.²² This constriction is coordinated by signaling pathways that concentrate the motor protein Rho kinase and non-muscle myosin at the apical surface.²³ These developmental movements are among the most ancient forms of multicellular coordination. The actomyosin machinery used for apical constriction is shared between all animals and even their closest single-celled relatives, the choanoflagellates, suggesting it predates the origin of animal life itself.²⁴

Active Versus Passive Dispersal

When biologists talk about movement at the population or ecological level, they often distinguish between active and passive dispersal. Active dispersal means an organism moves under its own power, choosing direction and speed. Passive dispersal means an organism or its propagules (seeds, spores, larvae) are carried by wind, water currents, or other external forces. The two strategies have different evolutionary pressures depending on the environment. In the ocean, passive dispersal by currents is relatively easy, and specific adaptations for active dispersal are uncommon; in fact, there may be more selective pressure to limit dispersal than to promote it. On land, the opposite is true: getting anywhere without specific adaptations for movement is difficult, so dispersal structures like wings, legs, and seed plumes are relatively common.²⁵

The line between active and passive is not always clean. Some marine larvae are technically passive drifters but possess enough swimming ability to influence their fate. In coral relatives, larger larvae with greater energy reserves can swim actively and compensate for the sinking that would otherwise limit their dispersal. Smaller larvae of related species lack swimming ability, settle quickly in still water, and ultimately disperse less despite their lighter weight.²⁶ Active versus passive dispersal also plays out differently in ecological simulations: both strategies cause distortions in species abundance across habitat patches, but active habitat selection tends to cause larger ecological effects than passive drifting.²⁷

Swimming Through Thick Fluids

The physical environment shapes what kind of movement works. For microorganisms, the world is dominated by viscosity rather than inertia. A bacterium swimming in water is, from a physics standpoint, roughly like a person trying to swim through honey: the moment it stops moving, it stops instantly, with no coasting at all. This regime, called low-Reynolds-number swimming, demands very different strategies from the ones that work for fish or birds.

Some microbes encounter environments even thicker than water, such as mucus layers, biofilms, or soil saturated with polymers. You might expect that thicker fluid would slow them down, but modeling and experiments have shown the opposite: certain swimming gaits, including propagating surface waves and rotating helical filaments (the classic bacterial flagellum), actually produce faster propulsion in these heterogeneous, polymer-laden fluids than in pure water, along with better hydrodynamic efficiency.²⁸ The polymers in the fluid appear to provide something for the swimmer to push against, turning a seemingly hostile medium into an advantage. This finding matters for understanding how pathogens navigate mucus barriers and how soil bacteria spread through saturated ground.

Two Families of Contractile Cells and the Deep History of Animal Movement

Looking across the full sweep of evolution, the contractile systems that underpin animal movement fall into two broad lineages defined by which version of the motor protein myosin II they express. “Fast” striated-type myosin II produces the rapid, powerful contractions of skeletal and cardiac muscle. “Slow” smooth-type and non-muscle-type myosin II drives the slower contractions seen in blood vessels, the gut, and in developmental tissue movements like the apical constriction discussed earlier. Both lineages trace back to a deep evolutionary split, and both are built on the same fundamental actomyosin machinery that single-celled eukaryotes use for crawling and dividing.²⁴

The transition from single-celled to multicellular life required two key innovations in movement. First, cells learned to coordinate their actomyosin machinery across multiple cells at once, forming supracellular cables that maintain tissue integrity and permit large-scale morphogenesis. Second, dedicated contractile cell types evolved specifically for adult locomotion, giving rise to the muscle cells that let animals chase prey, flee predators, and pump fluids through their bodies. The molecular toolkit for these innovations was already present in ancestral single-celled organisms; multicellularity rewired it into something new. That reuse of ancient molecular parts is one of the recurring themes of biological movement: a relatively small set of protein systems, above all actin and myosin, has been repurposed into an extraordinary diversity of movement strategies, from the streaming cytoplasm of an algal cell to the contraction of a human heart.