Amoeba Phagocytosis: Chasing Food Up a Chemical Gradient

A cell that navigates without a nervous system

An amoeba solves a genuinely hard navigation problem — find and reach a food source in a fluid environment — using nothing but its own cell membrane and cytoskeleton, no specialized sensory organs at all. The process runs in two coupled phases: chemotaxis, directed movement up a concentration gradient of a diffusing chemical cue (a chemoattractant, often a molecule leaking from prey — bacteria, other microbes, organic debris), and phagocytosis, the mechanical act of engulfing that food once contact is made. Both phases run on the same underlying machinery: a dynamic, reshaping actin cytoskeleton just beneath the plasma membrane.

Reading a gradient with receptor occupancy, not a sensor organ

Chemoattractant molecules diffuse outward from their source, so their concentration is naturally slightly higher on the side of the amoeba nearer the source than on the far side — a difference that, for a cell only tens of micrometres across, can be a few percent at most. Cell-surface receptors bind the chemoattractant, and this small spatial difference in binding is amplified internally: a signalling cascade (in the well-studied social amoeba Dictyostelium discoideum, involving G-protein-coupled receptors and downstream PI3-kinase activity) translates a shallow external gradient into a much steeper internal asymmetry, concentrating the machinery needed to build a pseudopod specifically on the up-gradient side of the cell rather than letting it form randomly all around.

Pseudopods: pushed forward by polymerizing actin

A pseudopod ("false foot") is not pulled forward by a motor protein the way muscle contracts — it is pushed. Free actin monomers polymerize rapidly into a dense, branching filament network right at the leading edge of the membrane, nucleated and branched by protein complexes like Arp2/3, and the physical force of that growing polymer meshwork against the inside of the membrane bulges it outward. Behind the advancing front, myosin motors and cortical actin contract to squeeze the cell body forward, so the whole crawling motion is a coordinated push-at-the-front, pull-at-the-back cycle repeated many times a second, steered continuously by whichever side currently has the strongest gradient signal.

for each patch of membrane around the cell:
  c        = chemoattractant concentration just outside this patch
  bias     = c - averageConcentration(wholeCellSurface)     // local gradient signal
  actinRate(patch) ∝ max(0, bias)                            // only up-gradient patches build actin
  membrane(patch).push(actinRate(patch) * dt)                // polymerization pushes membrane out

on contact with a food particle:
  wrap membrane around particle (pseudopods extend on both sides, meet, fuse)
  pinch off → forms a phagosome (an internal membrane-bound vacuole)
  phagosome fuses with lysosome → digestive enzymes break the food down

Engulfment: wrap, pinch, digest

Once the leading edge makes contact with a food particle, the same actin-pushing mechanism extends pseudopods around the particle from multiple directions rather than just forward — a "zippering" process where receptor binding along the particle's surface progressively recruits more actin polymerization, wrapping the membrane tightly around it until the extending edges meet on the far side and fuse. That seals the particle inside a membrane-bound compartment, the phagosome, which then fuses with a lysosome carrying digestive enzymes, breaking the captured material down into nutrients the cell can absorb and use.

The same trick, reused throughout the immune system

This exact combination — gradient-sensing chemotaxis followed by actin-driven engulfment — is one of the most evolutionarily conserved behaviours in eukaryotic biology. Free-living amoebae like Amoeba proteus use it to hunt for food; the social amoeba Dictyostelium uses the identical chemotactic machinery to aggregate into a multicellular slug when food runs out; and in the human body, neutrophils and macrophages — key cells of the innate immune system — use essentially the same receptor-gradient-sensing and actin-pushing mechanism to chase down and engulf invading bacteria at a wound or infection site. Studying amoeba behaviour in a dish has directly informed models of how immune cells navigate tissue and clear pathogens.

Frequently asked questions

Try it live

The gradient-sensing and engulfment loop above runs live in Amoeba Phagocytosis. Scatter several food particles at once and watch the cell weigh competing gradients as it decides which pseudopod to commit to.