Lipids and Membranes

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Lipids and Membranes
Chapter 12 (pp. 382 454) Read topics covered in
lectures
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What are Lipids? (Greek lipos, fat)

• Fourth major group of molecules found in cell
• Lipids are not polymeric
• (unlike nucleic acids, proteins
  polysaccharides)
• Water insoluble
• (soluble in organic solvents such as
  methanol chloroform)
• Includes fats, oils, certain vitamins and
  hormones

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• Lipids, a broad class of organic products found
  in living systems.
• Most are insoluble in water but soluble in
  nonpolar solvents. The definition excludes the
  mineral oils and other petroleum products
  obtained from fossil material.
• Major classes of lipids include the fatty acids,
  the glycerol-derived lipids (including the fats
  and oils and the phospholipids), the
  sphingosine-derived lipids (including the
  ceramides, cerebrosides, gangliosides, and
  sphingomyelins), the steroids and their
  derivatives, the terpenes and their derivatives,
  certain aromatic compounds, and long-chain
  alcohols and waxes.
• In living organisms lipids serve as the basis of
  cell membranes and as a form of fuel storage.
• Often lipids are found conjugated with proteins
  or carbohydrates, and the resulting substances
  are known as lipoproteins and lipopolysaccharides.
  
• The fat-soluble vitamins can be classified as
  lipids.
• Liposomes are spherical vesicles formed by mixing
  lipids with water or water solutions. They have
  found applications in the oral administration of
  some drugs (e.g., insulin and some cancer drugs),
  since they retain their integrity until they are
  broken down by the lipases in the stomach and
  small intestine.

http//www.allrefer.com/
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• Lipid Classification
• Fatty acids
• Triacylglycerols
• Glycerophospholipids
• Sphingolipids
• Steroids
• Other lipids (Example Eicosanoids)

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Biological Membranes

• Organized sheet-like assemblies consisting mainly
  of proteins lipids
• Functions carried out by membranes are
  indispensable for life
• Plasma membrane give cells their individuality
• Eukaryotic cells contain internal membranes that
  form the boundaries for organelles

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Common Features of Biological Membranes

• Sheet-like structures with thickness of about 6 -
  10 nm
• Consists of mainly lipids and proteins
  (Carbohydrates may be linked to proteins or
  lipids)
• Membranes are fluid and asymmetric
• Highly selective permeability barriers-- not
  impervious walls
• Control flow of information and material between
  cells or organelles their environments
• Specific proteins mediate distinctive functions
  of membranes
• Proteins in the membrane carry out important
  functions
• Example Mitochondrial inner membrane where
  ATP production takes place

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Schematic drawing of a prokaryotic cell
Page 4
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Schematic diagram of an animal cell
Page 7
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Drawing of a plant cell
Page 11
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What are membranes made of? Biological membranes
are composed of proteins associated with a lipid
bilayer matrix
Oligosaccharide
Lipid (bilayer)
Protein
Cholesterol
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Lipid composition of the plasma membrane
organelle of a rat hepatocyte
Note the diversity of various membranes
From Lehninger Principles of Biochemistry
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• Fatty Acids
• Carboxylic acids with long hydrocarbon side
  group (R-COOH)
• Usually occur in esterified form (R-COOR)
• Can be either saturated or unsaturated (contain
  double bonds)
• Often are polyunsaturated (2 or more double
  bonds)
• Double bonds have cis configuration and are non
  conjugated

Fully extended lowest energy conformation
Oleic acid CH3(CH2)7 CHCH(CH2)7 COOH
Stearic acid CH3(CH2)16COOH
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Structural formulas of some C18 Fatty acids
Stearic acid
Stearic acid, alternative representation
18
1
Oleic acid
18
9
1
a-Linolenic acid
18
12
15
9
1
14
From Lehninger Principles of Biochemistry
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Packing of fatty acids into stable aggregates
Presence of one or more cis double bonds
interferes with tight packing Results in less
stable aggregates

• Pack Efficiently
• Stabilized by hydrophobic
• interactions

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• Melting points of fatty acids
• Increase with increase in chain length (or
  molecular mass)
• For Example
• Melting Point (oC)
• 120 Lauric acid 44.2
• 180 Stearic acid 69.1
• Decrease with the degree of unsaturation
• (or increase in double bonds)
• For Example
• Melting Point (oC)
• 180 Stearic acid 69.1
• 181 Oleic acid 13.2
• 182 Linoleic acid -9

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• Triacyl Glycerols (triglycerides)
• Fatty acid triesters of glycerol
• Non polar, water insoluble
• Fats oils are mixtures of triacyl glycerol
• Function as energy reservoirs in animals
• Usually not part of biological membranes

• Fats are good way to store
• metabolic energy
• More energy than sugars or proteins
• which are partially oxidized
• Fats are non-polar and are stored in
• anhydrous form (unlike glycogen which
• binds twice its weight of water)

From Lehninger Principles of Biochemistry
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The principal class of storage and membrane lipids
From Lehninger Principles of Biochemistry
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• Glycerophospholipids (or phosphoglycerides)
• Major lipid components of biological
  membranes
• Derived from glycerol-3-phosphate whose C1 and C2
  positions are esterified with fatty acids
• Phosphoryl group is linked to a group X
• Amphiphilic molecule
• non-polar hydrocarbon tail
• polar phosphoryl X heads

Glycerol-3-phosphate
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Page 385
Table 12-2 The Common Classes of
Glycerophospholipids.
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Plasmalogen

• Glycerophospholipids
• At C1 position there is a, b-unsaturated ether
  linkage
• instead of ester linkage
• Functions of most plasmalogen not well
  understood

Choline
Ethanolamine, choline serine form the most
common plasmalogen head groups
From Lehninger Principles of Biochemistry
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Sphingolipids

• Major membrane components
• Derived from C18 amino
• alcohol, sphingosine
• Double bond of sphingosine
• has trans configuration

From Lehninger Principles of Biochemistry
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The specificities of phospholipases
hydrolytic enzymes
Phospholipids sphingolipids are degraded in
lysosomes
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Steroids

• Structure consists of three 6-membered rings
  one 5-membered ring, all fused together
• Cholesterol is the most common steroid in
  animals ( precursor for all other steroids in
  animals)
• Steroid hormones serve many functions in
  animals - including salt balance, metabolic
  function and sexual function

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Cholesterol
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Lipids as intracellular signals
From Lehninger Principles of Biochemistry
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Eicosanoids carry messages to nearby cells
From Lehninger Principles of Biochemistry
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Steroids derived from cholesterol
Steroids are oxidized derivatives of sterols
From Lehninger Principles of Biochemistry