Membranes 0 153

The Plasma Membrane 1. The Plasma membrane is approximately 8-nm thick and is selectively permeable. 1. The Plasma membrane is approximately 8-nm thick and is selectively permeable. Selective Membrane Pore Substances Permeable Permeable Not permeable 2. Selectively permeable means that the membrane is selective as to which substances can pass through it 2. Selectively permeable means that the membrane is selective as to which substances can pass through it The following example will demonstrate this point: The following example will demonstrate this point:

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

The Plasma Membrane 3. Cellular membranes are composed primarily of… Lipids(fats) Proteins Carbohydrates (sugars)

The Plasma Membrane 4. Lipids are a family of compounds including FatsLard Waxes Parafin wax steroidsCholesterol

The Plasma Membrane 5. Lipids are insoluble in water 6. Insoluble means that they don’t dissolve in water.

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

8. Water is the main component inside and outside the cell Aqueous inside the cell Aqueous environment outside the cell 9. A descriptive word meaning “watery” is the term aqueous.

10. The outside of the cell is known as the extracellular side. Cytoplasmic side Extracellular side 11. The inside of the cell is the cytoplasmic side.

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

13. The yellow phosphate heads of the lipids are hydrophilic. Extracellular Side (aqueous) Cytoplasmic Side (aqueous) Phosphate head (hydrophilic) 14. Hydrophilic means “water loving.” Phosphate head (hydrophilic)

15. The two blue colored lipid tails are hydrophobic. Extracellular Side (aqueous) Cytoplasmic Side (aqueous) Phosphate head (hydrophilic) Lipid Tails (hydrophobic) 16. Hydrophobic means “water fearing.” Phosphate head (hydrophilic)

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

The picture on the right is a tunneling electron micrograph of a cell’s plasma membrane. 18. Phospholipids are the most abundant lipids in most membranes. The diagram on the left is a model which interprets the two lines in the micrograph as a bilayer of phospholipids. Plasma membrane

Membrane Dynamics Phospholipids are very dynamic. They constantly vibrate and make rapid lateral movements many times a second. 19. Phospholipids move laterally at a distance of about 2 μm per second, which is about the length of a typical bacterial cell.

20. Phospholipids also make transmembrane flip- flops at a rate of about once per month. 21. Cell plasma membranes are usually about as fluid as salad oil.

Extracellular Side (aqueous) Cytoplasmic Side (aqueous) 22. This bilayer also contains many molecules of the steroid cholesterol. Cholesterol 24. At lower temperatures however, cholesterol helps to keep the membrane from solidifying. 23. Cholesterol slows the movement of phospholipids in cell membranes.

27. The fewer the number of cholesterol molecules, the more fluid the membrane. 25. Unsaturated hydrocarbon tails cause kinks in the lipid bilayer 26. Bilayers with kinks have fewer cholesterol molecules than those without kinks.

29. The greater the number of cholesterol molecules, the more viscous (thick-flowing) the membrane. 28. Saturated hydrocarbons have straight tails Bilayers without kinks have more cholesterol molecules than those with kinks.

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Extracellular Side Cytoplasmic Side 30. In addition to cholesterol, the membrane bilayer is also studded with proteins. 31.These two proteins are integral (internal) proteins, meaning that they are embedded in the bilayer. 32. This protein is a transmembrane protein. It spans the entire bilayer.

Cytoplasmic Side 33. Like the bilayer, internal proteins have water loving and water fearing regions. Hydrophilic region Hydrophobic region Extracellular Side “water loving” “water fearing” AqueousAqueous

Extracellular Side Cytoplasmic Side 34. Other proteins are Extrinsic (peripheral) proteins, meaning they are not embedded, but cling to the outside of the membrane. 35. Extrinsic proteins are hydrophilic, or water loving.

36. Proteins are made up of chains of amino acids 36. Proteins are made up of chains of amino acids The hydrophobic region of the chain is usually shaped like a helix or coil. The hydrophobic region of the chain is usually shaped like a helix or coil. Hydrophobic (“water fearing”) helix or coil shape Hydrophobic (“water fearing”) helix or coil shape Hydrophilic region “water loving” Hydrophilic region

Serine Tyrosine Alanine Leucine Valine Lysine Cysteine This protein example is made up of 250 amino acids. This protein example is made up of 250 amino acids. 37. Here are some example names of amino acids and their locations within the protein. 37. Here are some example names of amino acids and their locations within the protein.

38. The amino acids in the helix or coil tend to be nonpolar. 38. The amino acids in the helix or coil tend to be nonpolar. 39. The amino acids at the ends tend to be polar. 39. The amino acids at the ends tend to be polar.SerineTyrosine Alanine Leucine Valine Lysine Cysteine Nonpolar Polar Polar

40. All amino acids have the following parts: 40. All amino acids have the following parts: H2NH2N H O COH CC R Carboxyl group Hydrogen atom Variable group Central Carbon atom Amino group 41. There are only 20 different amino acids used in the making of proteins.

H2NH2N H O COH CC R Variable group 42. It is the R group that makes all 20 amino acids structurally different from one another. Amino group Hydrogen atom Central Carbon atom Carboxyl group

43. The R group also determines the amino acid’s polarity. Serine Tyrosine Alanine Leucine Valine Lysine Cysteine R H3NH3N H O COH CC

44. For example, the OH at the end of the R group of Serine makes it a hydrophilic (water loving) polar molecule. 44. For example, the OH at the end of the R group of Serine makes it a hydrophilic (water loving) polar molecule. Serine Tyrosine Alanine Leucine Valine Lysine Cysteine R H3NH3N H O COH CC OH Serine is well suited to aqueous (watery) environments. Serine is well suited to aqueous (watery) environments.

The same goes for Tyrosine, which also has an OH at the end of its R group that makes it “water loving.” The same goes for Tyrosine, which also has an OH at the end of its R group that makes it “water loving.” Serine Tyrosine Alanine Leucine Valine Lysine Cysteine R H3NH3N H O COH CC OH CH 2

45. The methyl (CH 3 ) R group of Alanine makes it nonpolar. 45. The methyl (CH 3 ) R group of Alanine makes it nonpolar. Serine Tyrosine Alanine Leucine Valine Lysine Cysteine R H3NH3N H O COH CC Alanine is found in the middle hydrophobic (water fearing) region of the plasma membrane. Alanine is found in the middle hydrophobic (water fearing) region of the plasma membrane. CH 3

Leucine and Valine have several terminal methyl R groups. Because of this, they are also nonpolar and hydrophobic. Leucine and Valine have several terminal methyl R groups. Because of this, they are also nonpolar and hydrophobic. Serine Tyrosine Alanine Leucine Valine Lysine Cysteine R H3NH3N H O COH CC H3CH3C CH 3 C – H

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

47. The Fluid Mosaic Model was proposed by Singer and Nicolson in This is a 3-D model of what the membrane might look like.

The website below has an animation which demonstrates how the phospholipids and proteins move about in the membrane.

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

Fluid Mosaic Model