Cell Membrane Information Worksheet - Dr. Lamia

Composition of the Cell Membrane & Functions The cell membrane is also called the plasma membrane. It is made of a phospholipid bilayer. This double layer of lipids is also known as a fluid mosaic because the phospholipids that make up the membrane lie next to each other but are not connected. This gives the cell membrane a fluid motion. The cell membrane is embedded with many different proteins and other substances that allow some selective materials to enter and leave the cell. The many different substances that are embedded in the cell membrane give it a look similar to a mosaic in the art world which uses pieces of tile to form a picture or design.

The phospholipids have a polar, hydrophilic (water attracting) head and two nonpolar, hydrophobic (water repelling) tails. Phospholipids can move laterally and allow water and other small molecules to pass through into or out of the cell. Since the cell membrane is mostly made of lipids, only lipid soluble substances such as oxygen and carbon dioxide can pass through in this way. Other substances that are water soluble need to pass through openings created by different kinds of proteins which are embedded in the membrane. Water soluble substances include molecules like glucose, amino acids, ions and water.

A concentration gradient is a difference in concentration between the fluid or solution outside a cell and the concentration within the cell. When the movement of molecules does not require energy the molecules are moving down the concentration gradient (from a higher concentration to a lower concentration). This is known as simple diffusion. The special term for the diffusion of water is osmosis.

Membrane proteins that make up the mosaic of the membrane are very important. Some proteins are attached to the surfaces of the cell membrane. These peripheral proteins are located on both the interior surface and the exterior surface of the cell membrane. The proteins that are embedded in the lipid bilayer are called integral proteins. Because the cell membrane is selectively permeable, cells must have mechanisms for transporting molecules through the lipid bilayer. Membrane proteins play an important role in this process. For example, some integral proteins form channels or pores through which certain substances can pass. Other proteins bind to a substance on one side of the membrane and carry it to the other side of the membrane. These proteins are called transport proteins.

Proteins form pores or channels in the cell membrane which allows water, which is not lipid soluble, to move freely across the cell membrane from an area of greater concentration to an area of lesser concentration.

Large molecules like glucose or other carbohydrates us transport proteins with carbohydrate chains to help move across cell membranes. The carbohydrate chains act like ID cards to help cell recognize certain molecules. Because the large molecules are moving down the concentration gradient and do not require energy, it is still a type of diffusion, but with the help of a protein. This process is called facilitated diffusion.

*Color the diagram of the phospholipid bilayer below as follows: Heads of the lipid molecules – red Tails of the lipid molecules – blue Proteins (B, C, and H) - yellow Carbohydrate Chains - orange

Questions:   1. Explain why the cell membrane is called a fluid mosaic. The phospholipids that make up the membrane lie next to each other but are not connected. This gives the cell membrane a fluid motion. Many substances are embedded in the cell membrane giving it a mosaic look.    2. The phospholipids have a polar, ___________head and two nonpolar ____________ tails. 3. Define hydrophilic - _______________________________________________ 4. Define hydrophobic - ______________________________________________ 5. Since the cell membrane is mostly lipids, only lipid soluble substances such as _______________ and _______________ can pass through. hydrophilic hydrophobic Water attracting Water repelling CO2 H2O

6. Water soluble substances need to pass through openings created by different   kinds of _____________ which are embedded in the membrane. 7. Give examples of water soluble substances: ___________, ____________, _______, and water. 8. What is a concentration gradient? 9. When molecules move from a higher concentration to a lower concentration, it is known as simple ___________. 10. The diffusion of water is called __________. proteins glucose Amino acids ions A difference in concentration between the fluid or solution outside a cell and the concentration within the cell. diffusion osmosis

11. Describe the function of a transport protein.   _______________________________________________________________ 12. What type of molecules use transport proteins? __________________________ 13. Carbohydrate chains act like ______ cards to help cells recognize certain _____________________. 14. What is facilitated diffusion? Proteins that bind to a substance on one side of the membrane and carry it to the other side of the membrane. Glucose & other carbohydrates ID molecules When large molecules are moving down the concentration gradient and do not require energy, but move with the help of a transport protein.