Extracellular fluid (ECF) is the fluid environment outside the enclosure of the cell membrane (see above Figure). The phosphate groups are also attracted to the extracellular fluid. Intracellular fluid (ICF) is the fluid interior of the cell. Since the phosphate groups are polar and hydrophilic, they are attracted to water in the intracellular fluid. A similar process occurs in your digestive system when bile salts (made from cholesterol, phospholipids and salt) help to break up ingested lipids. The hydrophilic portion can dissolve in the wash water while the hydrophobic portion can trap grease in stains that then can be washed away. In fact, soap works to remove oil and grease stains because it has amphipathic properties. An amphipathic molecule is one that contains both a hydrophilic and a hydrophobic region. Phospholipids are thus amphipathic molecules. The lipid tails, on the other hand, are uncharged, or nonpolar, and are hydrophobic-or “water fearing.” A hydrophobic molecule (or region of a molecule) repels and is repelled by water. The phosphate heads are thus attracted to the water molecules of both the extracellular and intracellular environments. The phosphate group is negatively charged, making the head polar and hydrophilic-or “water loving.” A hydrophilic molecule (or region of a molecule) is one that is attracted to water. The polar heads contact the fluid inside and outside of the cell. The hydrophobic tails associate with one another, forming the interior of the membrane. The phospholipid bilayer consists of two adjacent sheets of phospholipids, arranged tail to tail. Unsaturated fatty acids result in kinks in the hydrophobic tails. The phospholipid heads face outward, one layer exposed to the interior of the cell and one layer exposed to the exterior ( Figure 3.1.1).įigure 3.1.1 – Phospholipid Structure and Bilayer: A phospholipid molecule consists of a polar phosphate “head,” which is hydrophilic and a non-polar lipid “tail,” which is hydrophobic. The lipid tails of one layer face the lipid tails of the other layer, meeting at the interface of the two layers. Cholesterol and various proteins are also embedded within the membrane giving the membrane a variety of functions described below.Ī single phospholipid molecule has a phosphate group on one end, called the “head,” and two side-by-side chains of fatty acids that make up the lipid “tails” ( Figure 3.1.1). The cell membrane is an extremely pliable structure composed primarily of two layers of phospholipids (a “bilayer”). Structure and Composition of the Cell Membrane This cell membrane provides a protective barrier around the cell and regulates which materials can pass in or out. Just as the outer layer of your skin separates your body from its environment, the cell membrane (also known as the plasma membrane) separates the inner contents of a cell from its exterior environment. Compare and contrast different types of passive transport with active transport, providing examples of eachĭespite differences in structure and function, all living cells in multicellular organisms have a surrounding cell membrane.Describe how molecules cross the cell membrane based on their properties and concentration gradients.Relate structures of the cell membrane to its functions. Describe the molecular components that make up the cell membrane.By the end of this section, you will be able to:
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