Lipids
• Highly diverse structures
• Unifying property
• Hydrophobic: little to no affinity to water
• Contains hydrocarbons, which form nonpolar covalent bonds
• Do not form polymers
• Most important in biological context
– Fats
– Phospholipids
– Steroids
Fats
• Structure
– Composed of two different molecules bonded by ester linkage
• glycerol and fatty acids
– Glycerol
• Three-carbon alcohol
– Each carbon attached to hydroxyl group
- Fatty acid
Hydrocarbon chain with a carboxyl group at one end
LE 5-11a
Fatty acid
(palmitic acid)
Glycerol
Dehydration reaction in the synthesis of a fat
LE 5-11b
Ester linkage
Fat molecule (triacylglycerol)
Properties of Fats
• Hydrophobic
• H2O molecules form hydrogen bonds with each other and
EXCLUDE fatty acid chains-->
Fats separate from H2O
Fatty acids
variable chain length (often 16-18 carbons)
Saturated fats
- refer to saturated fatty acid chains
-contain maximum number of hydrogens
- result in 100% single bonds
- more linear, pack tightly to form solid
Tend to be from animals
LE 5-12a
Stearic acid
Saturated fat and fatty acid.
Unsaturated fats
-contain unsaturated fatty acids
-carbon-carbon double bonds (>1)
-irregular hydrocarbon conformation
- poor packing
- form liquids (oils) at room temperature
Tend to be from plants and fish
LE 5-12b
Oleic acid
cis double bond
causes bending
Unsaturated fat and fatty acid.
Phospholipids
• Structure
– two fatty acids bonded glycerol through ester
linkage
– Phosphate bonded to third hydroxyl group
• Fatty acids= Hydrophobic tail
• Phosphate and other groups= hydrophilic head
Draw schematic
LE 5-13
Choline
Phosphate
Glycerol
Fatty acids
Hydrophilic
head
Hydrophobic
tails
Structural formula
Space-filling model
Phospholipid symbol
If many phospholipids were mixed in H2O
into what structures would they self-assemble?
1. Micelle (draw)
- Detergents
2. Bilayer (draw)
-Cell membranes
LE 5-14
Hydrophilic
head
Hydrophobic
tails
WATER
WATER
Steroids
• Structure
- Hydrophobic molecules made of 4 fused rings
• Examples and Diverse Functions
– Cholesterol, an important steroid, is a component in animal
cell membranes
– High levels--> contribute to heart disease
– Building block for steroid sex hormones such as
• Estrogen, testosterone, progesterone
Cholesterol
estradiol
testosterone
Pardon me. I think I missed
something…
Ch 7 Membrane Structure and Function
Cellular membrane
-overall functions
- Boundary between intracellular compartments,
living cells, and abiotic environment
– Selectively permeable
– Some molecules cross membranes easier than others
Membrane Structure
Predominant constituent: phospholipids
• Amphipathic molecules: hydrophobic AND hydrophilic
properties
Dispersed protein components
Membrane organization and properties described by:
Fluid Mosaic Model
Singer and Nicolson 1972
LE 7-2
Organization of membrane phospholipids
WATER
Hydrophilic
head
Hydrophobic
tail
WATER
Mosaic: something made of small pieces
LE 7-3
Mosaic: Proteins dispersed among phospholipids in membrane:
Hydrophilic region
of protein
Phospholipid
bilayer
Hydrophobic region of protein
• Freeze-fracture studies of the plasma membrane
• Splits a frozen membrane along the middle of the
phospholipid bilayer using a knife
• Imaged by EM
Supports mosaic part of model
LE 7-4
Extracellular
layer
Proteins
Knife
Plasma
membrane
Extracellular layer
Cytoplasmic
layer
Cytoplasmic layer
The Fluidity of Membranes
• Phospholipids move within the bilayer
• Most of the lipids, and some proteins, drift laterally
• Rarely does a molecule flip-flop transversely across
the membrane
LE 7-5a
Lateral movement
(~107 times per second)
Movement of phospholipids
Flip-flop
(~ once per month)
Effects of Temperature on membranes
• Cool temp: membranes switch from fluid to
solid state
• Solidifications depends on the types of lipids
• What propertyof lipids would favor solid versus
liquid state?
LE 7-5b
Degree of saturation of fatty acid tails
Fluid
Unsaturated hydrocarbon
tails with kinks
Membrane fluidity
Viscous
Saturated hydrocarbon tails
Steroid cholesterol
also component of membranes
• Tends to moderate effects of temp on membrane state
• At warm temperatures (such as 37°C), restrains
movement of phospholipids
• At cool temperatures, maintains fluidity by preventing
tight packing
LE 7-5c
Cholesterol
Cholesterol within the animal cell membrane
Movement of membrane proteins
Can drift within the bilayer
– Proteins much larger than lipids--> move more
slowly
• Cell fusion studies done to test fluidity of
membrane proteins
LE 7-6
Membrane proteins
Mouse cell
Human cell
Hybrid cell
Mixed
proteins
after
1 hour
Membrane Proteins and Their Functions
• Proteins determine most of the membrane’s
specific functions
• Peripheral membrane proteins
–not embedded
• Integral membrane proteins
– penetrate the hydrophobic core of bilayer
– often span the membrane
LE 7-7
Fibers of
extracellular
matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE