 Welcome to Construction of the Cell Membrane. In this video, you'll learn the makeup and the basics of cell membrane function by examining the molecular structure of the cell membrane. It's a critical step to learning about the fundamental unit of life, the cell. The membrane's main job is to protect the cell from its surroundings. It separates the cell's interior from the outside environment and helps it keep its shape. The membrane is selectively permeable, which means it allows certain organic molecules to pass into and out of the interior. The cell membrane is mainly composed of lipids and proteins. Its framework consists of a double layer of phospholipids. Lipids make up 50% of most membranes, although depending on the location, they may make up up to 80%. The remainder is made from proteins. Lipids make the membrane flexible, and the proteins help transfer molecules across the membrane. There are two major types of proteins, tightly coiled, rod-shaped fibrous proteins, which help give the cell support and shape. And the more compact, globular-shaped, integral proteins and peripheral proteins, these help cells communicate and transport molecules. Examples include glycoprotein, porprotein, and channel protein. Because the cell membrane is mostly lipid, it only allows lipid-soluble substances like oxygen, carbon dioxide, and steroids to pass through water-soluble substances such as glucose, amino acids, ions, and water need the help of the various proteins in order to move across the membrane. Two layers of phospholipid molecules self-assemble so that their water-soluble, or hydrophilic, heads form the surface and interior of the membrane, and the water-insoluble, or hydrophobic, tails face each other. The fibrous proteins may span the entire membrane and serve as receptors for the cell. They communicate with the external environment using hormones and neurotransmitters. One type of globular protein forms pores to allow lipid-insoluble water molecules to pass through. Other integral proteins serve as channels and selectively support ions, including nutrients such as sugars or amino acids. Channel proteins are usually quite specific, so each one specializes in transporting a single substance or group of similar chemical substances. Globular proteins are peripheral, meaning they're associated with the surface of the cell. These include enzymes or glycoproteins. These proteins have carbohydrate associations that identify the cell. Cholesterol molecules are embedded in animal cell membranes, but not in plant cell membranes. They're found dispersed through the cell membrane and help strengthen the membrane wall. In addition, they, along with phospholipids, help make the membrane impermeable to water-soluble substances. Cell membranes are integral to cell health. With their combination of lipids, proteins, and cholesterol, they are able to complete a complex variety of tasks essential to life. Cell membranes keep the outside out and the inside in. You've completed construction of the cell membrane.