Another simple case of diffusion is the Brownian motion of one particle. The particle's Mean squared displacement from its original position is: MSD ≡ ≡ ⟨ ⟨ ( x − − x 0 ) 2 ⟩ ⟩ = 2 n D t where n is the dimension of the particle's Brownian motion. For example, the diffusion of a molecule across a cell membrane 8 nm thick is 1-D diffusion because of the spherical symmetry; However, the diffusion of a molecule from the membrane to the center of a eukaryotic cell is a 3-D diffusion. For a cylindrical cactus, the diffusion from photosynthetic cells on its surface to its center (the axis of its cylindrical symmetry) is a 2-D diffusion.