The F₀F₁-ATPase molecule is divided into two portions termed F₁ and F₀. The F₀ portion is embedded in the thylakoid membrane, while the F₁ portion projects into the lumen. Each portion is in turn made up of several different proteins or subunits. In F₀, the subunits are named a, b, and c. There is one a subunit, two b subunits, and 9–12 c subunits. The large a subunit provides the channel through which H⁺ ions flow back into the stroma. Rotation of the c subunits, which form a ring in the membrane, is chemically coupled to this flow of H⁺ ions. The b subunits are believed to help stabilize the F₀F₁ complex by acting as a tether between the two portions. The subunits of F₁ are called α, β, γ, δ, and ε. F₁ has three copies each of α and β subunits which are arranged in an alternating configuration to form the catalytic “head” of F₁. The γ and ε subunits form an axis that links the catalytic head of F₁ to the ring of c subunits in F₀. When proton translocation in F₀ causes the ring of c subunits to spin, the γ–ε axis also spins because it is bound to the ring. The opposite end of the γ subunit rotates within the complex of α and β subunits. This rotation causes important conformational changes in the β subunits resulting in the synthesis of ATP from ADP and Pi (inorganic phosphate) and to its release.