Lipids

• Highly diverse structures

• Unifying property• Hydrophobic: little to no affinity to water• Contains hydrocarbons, which form nonpolar covalent bonds

• Do not form polymers

• Biologically important lipids containing molecules:– Fats– Phospholipids– Steroids

Fats

• Structure– Composed of two different molecules bonded by ester linkage

• fatty acid & glycerol

– - Fatty acid

Hydrocarbon chain with a carboxyl group at one end

– Glycerol• Three-carbon alcohol

– Each carbon attached to hydroxyl group

Try to Draw

LE 5-11a

Dehydration reaction in the synthesis of a fat

Glycerol

Fatty acid(palmitic acid)

LE 5-11b

Ester linkage

Fat molecule (triacylglycerol)

• Hydrophobic• H2O molecules form hydrogen bonds with each other and

EXCLUDE fatty acid chains-->Fats separate from H2O (like oil

& vinegar)

Properties of Fats

Fatty acids variable chain length (often 16-18 carbons)

Saturated fat- refers to saturated fatty acid chains

- carbons contains maximum number of hydrogens

- results in 100% single bonds (no double bonds)

- more linear, packs tightly to form solid

Tend to be from animals

LE 5-11b

Ester linkage

Fat molecule (triacylglycerol)

Saturated or unsaturated?

LE 5-12a

Saturated fat and fatty acid.

Stearic acid

Butter

Unsaturated fat-contains unsaturated fatty acids-carbon-carbon double bonds (>1)-irregular hydrocarbon conformation-poor packing-forms liquids (oils) at room temperature

Tend to be from plants and fish

LE 5-12b

Unsaturated fat and fatty acid.

Oleic acid

cis double bondcauses bending

Olive oilOther liquid fats (unsaturated)?

Phospholipids• Structure

– two fatty acids bonded to glycerol through ester linkage

– Phosphate bonded to third hydroxyl group of glycerol

• Fatty acids= Hydrophobic tail• Phosphate and other groups= hydrophilic head

Draw schematic

LE 5-13

Structural formula Space-filling model Phospholipid symbol

Hydrophilichead

Hydrophobictails

Fatty acids

Choline

Phosphate

Glycerol

Hyd

rop

ho

bic

tai

lsH

ydr o

ph

ilic

hea

d

If many phospholipids were mixed in H2Ointo what structures would they self-assemble?

1. Micelle (draw)- Detergents

2. Bilayer (draw)-Cell membranes

LE 5-14

WATERHydrophilichead

Hydrophobictails

WATER

Cellular membrane: Phospholipid Bilayer

Steroids

• Structure- Hydrophobic molecules made of 4 fused hydrocarbon rings

• Examples and Diverse Functions– Cholesterol

• Component of animal cell membranes

• Building block for steroid sex hormones such as– Estrogen, testosterone, progesterone

• High levels--> contribute to heart disease

Cholesterol

estradiol

testosterone

Pardon me. Did you missanything?

Ch 7 Membrane Structure and Function

- Boundary between intracellular compartments, living cells, and abiotic environment

– Selectively permeable– Some molecules cross membranes more readily than others

Cellular membrane-overall functions

Predominant constituent: phospholipids• Amphipathic molecules: hydrophobic AND hydrophilic

Membrane Structure

Dispersed protein components

Membrane organization and properties described by:

Fluid Mosaic Model

Singer and Nicolson 1972

LE 7-2

Hydrophilichead

Hydrophobictail

WATER

WATER

Organization of membrane phospholipids

Mosaic: something made of small pieces

LE 7-3

Hydrophilic regionof protein

Hydrophobic region of protein

Phospholipidbilayer

Mosaic: Proteins dispersed among phospholipids in membrane:

• Freeze-fracture studies of the plasma membrane

• Frozen membrane split along the middle of the phospholipid bilayer using a knife

• Imaged by EM

Supports mosaic part of model

LE 7-4

Knife

Cytoplasmic layerExtracellular layer

Cytoplasmic layer

Plasmamembrane

Extracellular layer

Proteins

The Fluidity of Membranes

• Phospholipids move laterally within the bilayer

• Some membrane proteins also drift laterally

• Rarely does a phospholipid flip-flop transversely across the membrane

LE 7-5a

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

Movement of phospholipids

• Cool temp: membranes switch from fluid to more solid state

• Solidification depends on type of lipid

• What property of lipids would favor liquid versus solid state?

Effects of Temperature on membranes

LE 7-5b

ViscousFluid

Unsaturated hydrocarbontails with kinks

Membrane fluidity

Saturated hydro-carbon tails

Degree of saturation of fatty acid tails

• 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

Steroid cholesterolalso component of membranes

LE 7-5c

Cholesterol

Cholesterol within the animal cell membrane

Can drift within the bilayer– Proteins much larger than lipids--> move more

slowly

• Cell fusion studies support fluidity of membrane proteins

Movement of membrane proteins

LE 7-6

Membrane proteins

Mixedproteinsafter1 hourHybrid cell

Human cell

Mouse cell

Membrane Proteins and Their Functions

• Proteins determine most of the membrane’s specific functions

• Peripheral membrane proteins –not embedded–attached to extracellular or cytoplasmic

surface

• Integral membrane proteins– penetrate the hydrophobic core of bilayer– often span the membrane

LE 7-7

Fibers ofextracellularmatrix (ECM)

Glycoprotein

Carbohydrate

Microfilamentsof cytoskeleton

Cholesterol

Integralprotein

Peripheralproteins

CYTOPLASMIC SIDEOF MEMBRANE

EXTRACELLULARSIDE OFMEMBRANE

Glycolipid