IV. Cell Transport A. Diffusion Through Cell Boundaries *In solution, particles move constantly, collide, and spread out randomly. Diffusion - process by which molecules move from high concentration to lower concentration. -Once the concentration is the same throughout a system, equilibrium is reached, but molecular movement doesn’t stop. -Random movement lets substances diffuse across cell membranes without using a cell’s energy, a process called passive transport.

B. Osmosis Osmosis – the diffusion of water through a selectively permeable membrane. -Water will move from an area of high water concentration to an area of low water concentration, Ex : if a beaker has many sugar molecules on one side of a semi-permeable membrane, water will move towards the sugar until it reaches equilibrium.

Movement of water can create 3 cell conditions : 1. Hypertonic cells – cell’s environment has a higher concentration of solutes than inside the cell. “Hyper” meaning higher. Result : Water rushes out of cell & into the environment, causing the cell to shrivel (plasmolysis). 2. Isotonic cells – cell’s environment has the same concentration of solutes as inside the cell. “Iso” meaning same. Result : Perfect equilibrium, with no movement of water in or out of the cell. 3. Hypotonic cells – cell’s environment has a lower concentration of solutes than inside the cell. “Hypo” meaning lower. Result : Water rushes into the cell, causing it to swell & experience lysis (explosion/breaking apart).

C. Facilitated Diffusion Facilitated Diffusion – diffusion of certain molecules across cell membranes via protein channels, ex : glucose moving into red blood cells. -Is like a short-cut. Glucose can’t diffuse across the lipid bilayer on its own, so it moves through a protein channel instead (see pg. 209). The process uses no energy.

D. Active Transport *Sometimes cells must move materials in the opposite direction, against a concentration difference. This is done by active transport. Active transport – energy-using process that moves materials across a cell membrane against a concentration difference. -Is done by transport proteins found in the membrane itself.

Active vs. Passive Transport

Types of Active Molecular Transport *Larger molecules are transported across the cell membrane by 2 processes : 1. Exocytosis – packaging & removal of wastes from the cell. The membrane of a vacuole containing waste fuses with the cell membrane and forces the contents out of the cell. 2. Endocytosis – the process of taking outside material into the cell by means of infoldings, or pockets, of the cell membrane.

Overview of Endo & Exocytosis

2 Types of Endocytosis A. Pinocytosis – “Cellular drinking”. Is when a cell takes in liquid from the surrounding environment. B. Phagocytosis – “Cellular eating”. Is when a cell engulfs an outside food particle or some other solid substance, ex ; amoebas & white blood cells.