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Membranes

Membranes. 0 153 153. The Plasma Membrane. Selectively permeable means that the membrane is selective as to which substances can pass through it. The Plasma membrane is approximately 8-nm thick and is selectively permeable. The following example will demonstrate this point:.

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Membranes

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  1. Membranes 0 153 153

  2. The Plasma Membrane • Selectively permeable means that the membrane is selective as to which substances can pass through it • The Plasma membrane is approximately 8-nm thick and is selectively permeable. • The following example will demonstrate this point: Selective Membrane Permeable Pore Not permeable Substances Permeable

  3. Larger molecules, such as polysaccharides, proteins, and lipids cannot fit, because they are too big.

  4. Carbohydrates (sugars) Lipids (fats) Proteins The Plasma Membrane Cellular membranes are composed primarily of…

  5. Inner layer Outer layer In a cell’s plasma membrane, back to back phospholipid molecules form two distinct layers.

  6. Water is the main component inside and outside the cell A descriptive word meaning “watery” is the term aqueous. Aqueous environment outside the cell Aqueous inside the cell

  7. The outside of the cell is known as the extracellular side. The inside of the cell is the cytoplasmic side. Extracellular side Cytoplasmic side

  8. Phospholipid bilayer The two layers together are known as a phospholipid bilayer. Extracellular Side (aqueous) Cytoplasmic Side (aqueous)

  9. Phosphate head (hydrophilic) Phosphate head (hydrophilic) The yellow phosphate heads of the lipids are hydrophilic. Hydrophilic means “water loving.” Extracellular Side (aqueous) Cytoplasmic Side (aqueous)

  10. Phosphate head (hydrophilic) Phosphate head (hydrophilic) Lipid Tails (hydrophobic) The blue colored lipid tails are hydrophobic. Hydrophobic means “water fearing.” Extracellular Side (aqueous) Cytoplasmic Side (aqueous)

  11. Hydrophilic “water loving” Hydrophobic “water fearing” Hydrophilic “water loving” A Phospholipid bilayer is an amphipathic structure, meaning it has both a hydrophilic region and a hydrophobic region.

  12. Plasma membrane The picture on the right is a tunneling electron micrograph of a cell’s plasma membrane. The diagram on the left is a model which interprets the two lines in the micrograph as a bilayer of phospholipids. Phospholipids are the most abundant lipids in most membranes.

  13. Membrane Dynamics Phospholipids are very dynamic. They constantly vibrate and make rapid lateral movements many times a second. Phospholipids move laterally at a distance of about 2 μm per second, which is about the length of a typical bacterial cell.

  14. Phospholipids also make transmembrane flip-flops at a rate of about once per month. Cell plasma membranes are usually about as fluid as salad oil.

  15. Cholesterol This bilayer also contains many molecules of the steroid cholesterol. Extracellular Side (aqueous) Cholesterol slows the movement of phospholipids in cell membranes. At lower temperatures however, cholesterol helps to keep the membrane from solidifying. Cytoplasmic Side (aqueous)

  16. Unsaturated hydrocarbon tails cause kinks in the lipid bilayer Bilayers with kinks have fewer cholesterol molecules than those without kinks. The fewer the number of cholesterol molecules, the more fluid the membrane.

  17. Saturated hydrocarbons have straight tails Bilayers without kinks have more cholesterol molecules than those with kinks. The greater the number of cholesterol molecules, the more viscous the membrane.

  18. The phospholipids, steroids and proteins of the membrane form a mosaic of substances that float about. This is known as the Fluid Mosaic Model.

  19. The Fluid Mosaic Model was proposed by Singer and Nicolson in 1972. This is a 3-D model of what the membrane might look like. http://www.goldiesroom.org/Note%20Packets/06%20Transport/00%20Transport--WHOLE.htm

  20. The website below has an animation which demonstrates how the phospholipids and proteins move about in the membrane. http://home.earthlink.net/~shalpine/anim/Life/memb.htm

  21. Parting Shots • Lipids have about twice the energy content of both proteins and carbohydrates. • Some lipid derivatives act as hormones (testosterone, estrogen, and progesterone). • Fat under the skin insulates the body against temperature changes (e.g. seal blubber) • Fat is a hydrophobic storage molecule and thus is lightweight. • Fat acts as a shock absorbent for some organs of the body (e.g. kidneys)

  22. Fluid Mosaic Model

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