<p>Name ______Test Date______UNIT IV – CELL TRANSPORT I. Cell Membrane (pgs. 182-183) Every cell is surrounded by a cell membrane. The cell membrane is _selectively permeable_ which means it only allows certain substances in and certain substances out. This characteristic is critical in helping the cell maintain _homeostasis__. A. The Cell Membrane is composed of three biomolecules: 1. Lipids – There are two lipids that make up the cell membrane: __phospholipids_ and _cholesterol (steroid)__. a. phospholipid bilayer - _Non-polar_ glycerol backbone with two fatty acid tails (hydrophobic) and a _polar_ (hydrophilic) phosphate group on the other end. The bilayer consists of 2 layers of phospholipid molecules. b. cholesterol – found in the cell membranes of _animal_ cells to help stabilize them. Cholesterol is wedged between the non- polar fatty acid tails of the phospholipid bilayer to maintain stability and fluidity. 2. Carbohydrates – found on the outside of the cell membrane and Serve as “ID tags” to _identify the cell__. 3. Proteins – Proteins are embedded in the phospholipids bilayer. Serve as __channels__ and _pumps_ for transport of specific molecules across the cell membrane. II. Traffic Across the Cell Membrane A cell is a working unit and thus, needs to move materials in and out of the cell. A. Passive Transport (pgs. 183 – 187) Movement of molecules across a membrane without using energy. There is a movement of substances from a HIGH concentration to a LOW concentration. There are 3 types of passive transport:</p><p>1. Diffusion – random movement of molecules in a liquid or gas from high to low. Ex. perfume, food coloring Many substances move across the cell membrane (phospholipid bilayer) by diffusion. Ex. CO2, O2 DIFFUSION 2. Facilitated Diffusion – diffusion of specific substances from high to low through a protein channel. Protein channels provide for larger openings for molecules like glucose, fructose, galactose to get through.</p><p>3. Osmosis – diffusion of water from a high water concentration to a low water concentration through a selectively permeable membrane. Cells must have a mechanism for counteracting the pressure osmosis can create, otherwise a cell could swell & burst or explode when it comes in contact with a dilute water solution. Cells control this osmotic pressure in one of three ways: a. cell wall – physically prevents the cell from expanding. Ex. bacteria, plants, fungi b. contractile vacuole – actively pumps water out of the cell. Ex. protists c. blood – cells are bathed in blood which is mostly water, but yet also has other components to equalize the pressure on both sides of the cell membrane. Ex: Human cells</p><p>**Comparative Terms: These terms describe the solution outside the cell compared to inside the cell.</p><p>Hypertonic – increased concentration of solute (sugar, salt), and decreased concentration of water. Hypotonic – decreased concentration of solute (sugar, salt), and increased concentration of water. Isotonic – equal concentration of water & solute inside the cell compared to outside the cell. </p><p>(crenated)</p><p>B. Active Transport – requires energy (ATP) (pgs. 188-189) 1. Protein Pumps – energy requiring process during which membrane proteins pump molecules across a membrane against a concentration gradient from a LOW concentration to a HIGH concentration. Ex. Pumping of ions such as which are important in nerve transmission and muscle contraction.</p><p>2. Endocytosis – “ sucking IN” Import of macromolecules INTO the cell from the exterior by forming a vesicle around the molecule from the existing cell membrane. </p><p>Specific types of endocytosis are: a. phagocytosis – “cellular eating” Large, solid particles. Ex. Amoeba eating; WBC’s in our immune system b. pinocytosis – “cellular drinking” Small, fluid droplets</p><p>3. Exocytosis – “spitting OUT” Export of macromolecules FROM the inside of the cell to the exterior of the cell by a vesicle fusing with the cell membrane. Ex. Insulin, neurotransmitters </p>