Transcript Slide 1
Maintaining a Balance • All living cells must maintain a balance regardless of internal and external conditions • Survival depends on the cell’s ability to maintain the proper conditions within itself • Why cells must control materials • The plasma membrane is the flexible boundary between the cell and its environment • The plasma membrane’s job is to allow a steady supply of these (glucose, amino acids, and lipids) nutrients to come into the cell no matter what the external conditions are • However, too many of any of these nutrients or other substances, especially ions, can be harmful to the cell • The plasma membrane relives the cell of excess nutrients as well as waste from the cell • Remember that the process of maintaining balance in the cell’s environment is called homeostasis • How does the plasma membrane maintain homeostasis 1. By having a selective permeable membrane that allows some molecules to pass through while keeping others out 2. Some molecules, such as water, freely enter the cell through the plasma membrane 3. Other particles, such as sodium and calcium ions, must be allowed into the cell only at certain times, in certain amounts, and through certain channels The plasma membrane must be selective in allowing these ions to enter Structure of the Plasma Membrane • We need to remember that lipids are large molecules that are composed of glycerol and three fatty acids • If a phosphate group replaces a fatty acid, a phospholipid is formed • So a phospholipid has a glycerol backbone, two fatty acid chains, and a phosphate group • The plasma membrane is composed of a phospholipid bilayer, which has two layers of phospholipids back – to – back The phospholipid bilayer • The phosphate group is critical for the formation and function of the plasma membrane • The 2 fatty acid tails of the phospholipids are nonpolar Phospholipid Bilayer Cont. • The head of the phospholipid (which contains the phosphate group) is polar • The polar phosphate group allows the cell membrane to interact with its water environment because, water is also polar • The fatty acid tails, on the other hand, avoid water • When put together the phospholipid molecules make a barrier that is water soluble at its outer surfaces and water – insoluble in the middle • The model of the plasma membrane is called the fluid mosaic model Other Components of the Plasma Membrane • Cholesterol is also found in the plasma membrane where it helps to stabilize the phospholipids by preventing their fatty acid tails from sticking together • Transport Proteins move needed substances or waste materials through the plasma membrane • Proteins at the inner surface of a plasma membrane play an important role in attaching the plasma membrane to the cell’s internal support structure, giving the cell its flexibility