Context: Youve probably blown soap bubbles like the child in Figure 4.1. In some ways, the thin film of soap molecules that forms a bubble resembles the cell membrane. Like the soap film, the cell membrane consists of a thin skin of molecules. You can see a model of the cell membrane in Figure below. The molecules that make up the cell membrane are mainly phospholipids. There are two layers of phospholipids. They are arranged so the lipid tails are on the inside of the membrane. They make the interior of the membrane hydrophobic, or "water fearing". The lipid heads point toward the outside of the membrane. The make the outer surfaces of the membrane hydrophilic, or "water loving". Different types of proteins are embedded in the lipid layers. The proteins are needed to help transport many substances across the membrane. The passage of a substance through a cell membrane is called transport. There are two basic ways that transport can occur: passive transport and active transport. For a good video introduction to passive and active transport, click on this link: . MEDIA Click image to the left or use the URL below. URL: Passive transport occurs when a substance passes through the cell membrane without needing any energy to pass through. This happens when a substance moves from an area where it is more concentrated to an area where it is less concentrated. Concentration is the number of particles of a substance in a given volume. Lets say you dissolve a teaspoon of salt in a cup of water. Then you dissolve two teaspoons of salt in another cup of water. The second solution will have a higher concentration of salt. Why does passive transport require no energy? A substance naturally moves from an area of higher to lower concentration. This is known as moving down the concentration gradient. The process is called diffusion. Its a little like a ball rolling down a hill. The ball naturally rolls from a higher to lower position without any added energy. You can see diffusion if you place a few drops of food coloring in a pan of water. Even without shaking or stirring, the food coloring gradually spreads throughout the water in the pan. Some substances can also diffuse through a cell membrane. This can occur in two ways: simple diffusion or facilitated diffusion. Simple diffusion occurs when a substance diffuses through a cell membrane without any help from other molecules. The substance simply passes through tiny spaces in the membrane. It moves from the side of the membrane where it is more concentrated to the side where it is less concentrated. You can see how this happens in Figure 4.2. Substances that cross cell membranes by simple diffusion must squeeze between the lipid molecules in the mem- brane. As a result, the diffusing molecules must be very small. Oxygen (O2 ) and carbon dioxide (CO2 ) are examples of molecules that can cross cell membranes this way. When you breathe in, oxygen is more concentrated in the air in your lungs than it is in your blood. So oxygen diffuses from your lungs to your blood. The reverse happens with carbon dioxide. Carbon dioxide is more concentrated in your blood than it is in the air in your lungs. So carbon dioxide diffuses out of your blood to your lungs. Hydrophilic molecules and very large molecules cant pass through the cell membrane by simple diffusion. They need help to pass through the membrane. The help is provided by proteins called transport proteins. This process is known as facilitated diffusion. There are two types of transport proteins: channel proteins and carrier proteins. They work in different ways. You can see how they work in Figure 4.3. A channel protein forms a tiny hole called a pore in the cell membrane. This allows water or hydrophilic molecules to bypass the hydrophobic interior of the membrane. A carrier protein binds with a diffusing molecule. This causes the carrier protein to change shape. As it does, it carries the molecule across the membrane. This allows large molecules to pass through the cell membrane. Osmosis is the special case of the diffusion of water. Its an important means of transport in cells because the fluid inside and outside cells is mostly water 
 Question: The sodium-potassium pump involves
Output: carrier proteins.