ATP is produced in the cell in two ways:

    1.  Substrate-Level Phosphorylation
    2.  Chemiosmosis

Substrate-Level Phosphorylation (SLP) involves the transfer of phosphate from a high energy molecule to ADP, with the aid of an enzyme.

Chemiosmosis requires a phospholipid bilayer, a proton pump, protons and ATPase.

Energy from food molecules is used to pump protons out through a proton pump (intrinsic protein).  This creates a concentration of protons (H+) outside.

ATP Synthase has a channel that allows for the facilitated diffusion of protons back in through the membrane.  This activates ATP Sythase, which in turn catalyzes the formation of ATP.

In summary, the energy stored in food molecules is transferred to a proton pump which creates a proton gradient across the membrane.  This gradient is used to activate the enzyme ATP Synthase.  ATP Synthase catalyzes the formation of ATP