Membranes211S - Membrane Structure and Function - 1 Cell...

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Membrane Structure and Function - 1 Cell Membranes and Interactions with the Environment Each cell must interact with its environment in a number of ways. Each cell needs to obtain oxygen and other nutrients (carbohydrates, amino acids, lipid molecules, mineral ions, etc.) from the environment, maintain water balance with its surroundings, and remove waste materials from the cell. The plasma membrane separates a cell from its environment. The plasma membrane has a number of functions: The plasma membrane serves as the boundary between the cytoplasm of the cell and the external environment. The plasma membrane maintains the cell's environment by regulating materials that enter or leave the cell. The plasma membrane is differentially, or selectively, permeable. Some materials enter and leave easily through the membrane, some with the assistance of membrane molecules, and some are prohibited from passing through the plasma membrane. The plasma membrane provides mechanisms for cell-to-cell communication. The plasma membrane provides mechanisms for a cell to recognize "self" versus "non-self" (foreign materials), important to the immune system and the development and defense of the organism through genetically unique cell recognition markers. Although the plasma membrane forms the boundary of the cell, and surrounds the cell, many internal structures of eukaryotic cells also have membrane boundaries. Much of what we will learn about membrane structure and function at this time applies to all membranes, not just the plasma membrane. The Fluid Mosaic Membrane Structure The structure and function of a membrane depends of its molecular composition. The foundation of the membrane is its phospholipid bilayer, with a number of associated proteins. Membranes also contain carbohydrates in glycoproteins, proteoglycans and glycolipids. The resultant membrane structure (proteins scattered throughout the fluid phospholipid layers) resembles a mosaic, hence the name "fluid mosaic membrane".
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Membrane Structure and Function - 2 Membrane Phospholipids Phospholipids are amphipathic. They have both hydrophilic (polar) and hydrophobic (non polar) regions. This property makes phospholipids ideal molecules for membrane functions. The fatty acid "tails" of the two phospholipid layers are oriented towards each other so that the hydrophilic "heads", which contain the "charged" phosphate portion, face out to the environment as well as into the cytoplasm of the cell's interior, where they can form hydrogen bonds with surrounding water molecules. The phospholipid molecules of a membrane provide for its physical integrity. Exterior Cytoplasm Phospholipid Bilayer Membrane Properties A membrane is held together, for the most part, by hydrophobic interactions within the phospholipid bilayer. Because individual phospholipid molecules are not bonded to each other, a membrane is flexible, or "fluid", particularly to lateral movement of the fatty acids. Phospholipid molecules easily and rapidly move along
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Membranes211S - Membrane Structure and Function - 1 Cell...

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