Passive and active transport across cell membranes:- Substances can enter or leave a cell in 2 ways: 1) Passive a) Simple Diffusion b) Facilitated Diffusion c) Osmosis (water only) 2) Active a) Molecules b) Particles I. Passive transport across cell membranes 1. Diffusion Molecules and ions move freely in gases and liquids, each type of these particles tends to spread out evenly within the space available. This is diffusion . Diffusion is: + the net movement of molecules + from a region of its higher concentration to a region of its lower concentration. + down a concentration gradient + no energy is used. Source: northlandcollege.edu Some molecules and ions are able to pass through cell membranes --> The membrane is permeable. Some substances cannot pass through cell membranes --> The membrane is partially permea...