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Lipids and Membranes. Chapter 12 (pp. 382 – 454) Read topics covered in lectures. 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

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slide1

Lipids and Membranes

Chapter 12 (pp. 382 – 454)

Read topics covered in lectures

slide2

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
slide3

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/

slide4

Lipid Classification

Fatty acids

Triacylglycerols

Glycerophospholipids

Sphingolipids

Steroids

Other lipids(Example: Eicosanoids)

slide5

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
slide6

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

slide10

What are membranes made of?

Biological membranes are composed of proteins associated with a lipid bilayer matrix

Oligosaccharide

Lipid (bilayer)

Protein

Cholesterol

slide11

Lipid composition of the plasma membrane & organelle of a rat hepatocyte

Note the diversity of various membranes

From Lehninger

Principles of Biochemistry

slide12

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 CH=CH(CH2)7 COOH

Stearic acid

CH3(CH2)16COOH

slide13

Structural formulas of some C18 Fatty acids

Stearic acid

Stearic acid, alternative representation

18

1

Oleic acid

18

9

1

a-Linolenic acid

18

15

12

9

1

slide14

From Lehninger

Principles of Biochemistry

slide15

Packing of fatty acids into stable aggregates

Polar

Presence of one or more cis double bonds interferes with tight packing

Results in less stable aggregates

Pack Efficiently

Stabilized by hydrophobic

interactions

slide16

Melting points of fatty acids

  • Increase with increase in chain length (or molecular mass)
  • For Example:
  • Melting Point (oC)
  • 12:0 Lauric acid 44.2
  • 18:0 Stearic acid 69.1
  • Decrease with the degree of unsaturation
  • (or increase in double bonds)
  • For Example:
  • Melting Point (oC)
  • 18:0 Stearic acid 69.1
  • 18:1 Oleic acid 13.2
  • 18:2 Linoleic acid -9
slide17

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

slide18

The principal class of storage and membrane lipids

From Lehninger

Principles of Biochemistry

slide19

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

slide20

Page 385

Table 12-2 The Common Classes of Glycerophospholipids.

slide21

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

slide22

Sphingolipids

  • Major membrane components
  • Derived from C18 amino

alcohol, sphingosine

  • Double bond of sphingosine

has transconfiguration

From Lehninger

Principles of Biochemistry

slide23

The specificities of phospholipases

hydrolytic enzymes

Phospholipids & sphingolipids are degraded in lysosomes

steroids
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
slide26

Lipids as intracellular signals

From Lehninger

Principles of Biochemistry

slide27

Eicosanoids carry messages to nearby cells

From Lehninger

Principles of Biochemistry

slide28

Steroids derived from cholesterol

Steroids are oxidized derivatives of sterols

From Lehninger

Principles of Biochemistry