Study the following and identify the incorrect statements with respect to cell membrane.
I) The major types of lipids present in cell membranes are phospholipids.
II) The amount of proteins and lipids is always the same in different cell types.
III) Neutral solutes may move across the membrane by the process of simple diffusion against a gradient.
IV) The quasi-fluid nature of lipids enables lateral movement of proteins within the overall bilayer.

The amount of lipids and proteins may vary in the cell membrane. For example, in RBC, 52 percent protein and 40 percent lipids are present. Neutral solutes move across the cell membrane along the concentration gradient (high to low) but not against the concentration gradient(low to high). According to an improved model of the structure of the cell membrane proposed by Singer and Nicolson (1972), the fluid mosaic model was widely accepted. The quasi-fluid nature of lipids enables lateral movement of proteins within the overall bilayer. This ability to move within the membrane is measured as its fluidity.