A gene gun accelerates DNA-coated gold or tungsten microcarriers with a burst of compressed helium, firing them through a stopping screen into plant tissue — used to transform species like corn, soybean and wheat that resist Agrobacterium-mediated delivery.
Impact speed: v_impact ≈ v0(P) · exp(−d / λ_He)
Drag ODE: m dv/dt = −½ Cd ρ(x) A v²
Momentum: m = ρ_Au · (4/3)π r³, A = π r²
Deceleration inside the tissue depends on the local resistance profile ρ(x): a tough cell-wall layer near the surface, a softer mesophyll layer beneath it, then the watery vacuole. A particle only delivers usable DNA if it stops inside the transformation-competent window, roughly where the cytoplasm and nucleus sit within the mesophyll:
- Too shallow — the particle is deflected by the cell wall or stays extracellular; no DNA release into a living cell.
- In window — the particle punches through the wall, decelerates inside the cytoplasm, and its DNA coating disperses near the nucleus — a successful transient (and occasionally stable) transformation event.
- Too deep — excess momentum carries the particle through the vacuole and out the far membrane, rupturing the cell.
Because larger, denser microcarriers carry more momentum per unit drag area (mass grows with r³, drag area only with r²), bigger particles penetrate deeper for the same impact speed — exactly the trade-off breeders tune with M6–M17 particle sizes. Higher helium pressure raises v_impact; a longer flight distance lets air drag bleed more speed off before the particle ever reaches the leaf.