Membrane Structure and Function

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Cell Organization. Many metabolic reactions occur at the same time in a cell so control and organization are vital.Organization within the cell comes from teams of enzymes, but for cells to operate properly enzymes must be in the right place at the right time.Membranes provide the structural bas

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Membrane Structure and Function

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1. Membrane Structure and Function E:\bc_campbell_concepts_5\0,11761,4089848-,00.htmlE:\bc_campbell_concepts_5\0,11761,4089848-,00.html

2. Cell Organization Many metabolic reactions occur at the same time in a cell so control and organization are vital. Organization within the cell comes from teams of enzymes, but for cells to operate properly enzymes must be in the right place at the right time. Membranes provide the structural basis for metabolic order and many enzymes are built right into the membranes of organelles.

3. Plasma Membrane Function The cell’s outer membrane made up of two layers of phospholipids with embedded proteins. Functions of the plasma membrane: To form a boundary between the living cell and its surroundings To control the traffic of molecules into and out of the cell To take up substances the cell needs and to dispose of cell wastes

4. Selective Permeability The plasma membrane has selective permeability. Selective permeability – to allow some substances to cross more easily than others

5. Plasma Membrane Structure Phospholipids are the main structural components of membranes. Phospholipids have a phosphate group and two fatty acids. Polar, hydrophilic head and nonpolar, hydrophobic tails. Hydrophilic vs. hydrophobic

6. Phospholipids in Membranes Phospholipids form a two layer sheet in membranes called a phospholipid bilayer. The hydrophilic heads face outward, toward the water which is on both sides of the membrane and the hydrophobic tails face inward away from the water.

7. Effects of the Bilayer The hydrophobic interior of the bilayer results in the membrane being selectively permeable. Molecules soluble in lipids can pass easily through the membrane. Soluble - pass easily Polar molecules and ions are not soluble in lipids so the passing of these molecules is dependent on protein molecules being present in the phospholipid bilayer. Nonsoluble – need assistance to pass through membrane

8. Question Why do phospholipids organize into a bilayer in an aqueous environment?

9. Fluid Mosaic The plasma membrane is described as being a fluid mosaic. Fluid mosaic – the membrane is seen as a mosaic of individually inserted protein molecules drifting laterally in a fluid bilayer of phospholipids

10. Proteins in the Membrane Function to support the membrane and link the membrane to the cytoskeleton and to fibers of the extracellular matrix.

11. Glycoproteins, Glycolipids and Carbohydrate Chains Glycoprotein – protein with attached sugars Glycolipid – lipid with attached sugars Carbohydrate chains – function as cell identification tags that are recognized by other cells enabling cells to be differentiated Vary in species, individuals, cells Allow cells in an embryo to sort themselves into tissues and organs What allows cells of the immune system to recognize and reject foreign cells

12. Video and Activity – 5E Membrane Structure

13. Question Why are cellular membranes described as a fluid mosaic?

14. Membrane Mosaic Membrane acts as a mosaic in both positioning of proteins and functions of proteins. Different cells have different membrane proteins and different membranes within cells have proteins that are specific and unique to them.

15. Functions of Proteins within the Plasma Membrane Membrane proteins are enzymes. Functions: Catalytic teams for molecular assembly lines (enzyme activity) Receptors for chemical messengers from other cells (signal transduction) Moves substances across the membrane (transport)

16. Function 1: Enzyme Activity Proteins function in catalytic teams for molecular assembly lines. Allows for enzyme activity.

17. Function 2 – Signal Transduction Receptor - a protein that binds selectively to a specific molecule and initiates a biological response A receptor protein has a shape that fits the shape of a specific messenger (i.e. hormone) just like an enzyme fits its substrate. Binding of the messenger to the receptor triggers a chain reaction involving other proteins which relays the message to a molecule that performs a specific activity in the cell . Message-transfer process is called signal transduction Signal transduction – mechanism linking a mechanical or chemical stimulus to a cellular response Video – 5F Signal Transduction

18. Function 3 - Transport Membrane proteins move substances across the membrane. Small, nonpolar molecules pass easily through the membrane but many essential molecules (i.e. glucose and water) need assistance from proteins to leave the cell. Movement of substances across the membrane is known as transport. Two types of molecular transport: Passive Active

19. Review Why are phospholipids well suited to be the main structural components of membranes? They are completely insoluble in water.   They form a single sheet in water.   They form a structure in which the hydrophobic portion faces outward.   They form a selectively permeable structure.   They are triglycerides, which are commonly available in fatty foods.

20. The lipids in a cell membrane are arranged _____. between two layers of protein   on either side of a single layer of protein   so that the polar parts of two lipids point toward each other   so that the nonpolar parts of two lipids point toward each other   none of the above

21. Which of the following best describes the general structure of a cell membrane? proteins sandwiched between two layers of phospholipid   proteins embedded in two layers of phospholipid   a layer of protein coating a layer of phospholipid   phospholipids sandwiched between two layers of protein   phospholipids embedded in two layers of protein

22. The membrane is referred to as a "fluid mosaic" structure. Which of the following statements is true? The fluid is phospholipid and the mosaic is carbohydrate.   The fluid is protein and the mosaic is phospholipid.   The mosaic comprises the carbohydrate chains on the inner surface of the membrane.   The fluid is phospholipid and the mosaic is protein.   The term refers to the appearance of cells in a tissue.

23. Which of the following functional processes results from the presence of protein within the plasma membrane? signal transduction   attaching the membrane to the cytoskeleton   forming junctions between adjacent cells   enzymatic activity   all of the above

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