17-20 amino acids to make transmembrane chain coz of the length of the fatty acid. Made up of mixture of polar and non-polar amino acids. Coz doesn"t have exclusively polar amino acids. There are hydrophobic interactions between hydrophobic amino acids. Most of these are on cytoplasmic side of a membrane. Hydrophobic nature of a cell membrane restricts free movements of many molecules and substances which are essential for life. Movement of a substance across a membrane without need to expand energy such as atp. Diffusion: net movement of a substance from a region of higher to lower concentration. Primary mechanism of solute movement within the cell and b/w celluer compartments separated by a membrane. Rate of diffusion depends of the concentration gradient. Higher the concentration faster the reaction- bigger the gradient, faster the diffusion rate. Driving force is an increase in entropy. Passive transport of substances across lipid portion of membranes with their concentration gradients.