Chapter 1 Summary 1 Membranes 1.
Membranes enclose all cells and all intracellular organelles.
Their integrity is essential to the life of the cell and the life of the organism. Membranes, which consist primarily of phospholipids and proteins, separate the cytosol from the extracellular fluid, and separate and the cytosol from the contents of the organelles. Selective membrane permeability compartmentalizes the functions of the cell and their organelles. Diffusion always follows a concentration gradient, i.e., molecules always move from a space where their concentration is higher to where it is lower. Fick's first law of diffusion describes the net rate of diffusion on the basis of the diffusion constant.