Chapter 5 Macromolecules-Lipids

Lipids • Lipids are composed of C, H, O– long hydrocarbon chains (H-C)

• “Family groups”– fats– phospholipids– steroids

• Do not form polymers– big molecules made of smaller subunits– not a continuing chain

Fats • Structure:– glycerol (3C alcohol) + fatty acid• fatty acid =

long HC “tail” with carboxyl (COOH) group “head”• 12-20 carbons- Always an even number.• These extensive chains make fats very

nonpolar.

dehydration synthesis

H2O

enzyme

Fats (Tricylglycerols)• Fats constructed of two kinds of smaller molecules– Glycerol - Three carbons covalently bound to one

another, each with a single hydroxyl group.

Function of Fats

• Energy storage. 1 gram of fat stores more than 2x the energy of 1 gram of carbohydrate.

• Insulation. Fat is an exceptional thermal and electrical insulator.

Formation of Fats

• Fats form as a result of a dehydration reaction between glycerol and a fatty acid.

• This kind of linkage is called an ester linkage.• Ester linkage = between OH & COOH

Fatty acid(palmitic acid)

(a)Dehydration reaction in the synthesis of a fatGlycerol

(b) Fat molecule (triacylglycerol)

Ester linkage

Fig. 5-11b

Dehydration synthesis

dehydration synthesis

H2O

H2O

H2O

H2O

enzyme

enzyme

enzyme

Fats store energy• Long HC chain– polar or non-polar?– hydrophilic or hydrophobic?

• Function:– energy storage • concentrated– all H-C!

• 2x carbohydrates– cushion organs– insulates body• think whale blubber!

Why do humanslike fatty foods?

Saturated v. Unsaturated Fats• Saturated fats are composed of fatty acid chains that

contain no double bonds. – Saturated with hydrogens.

• Unsaturated fats are composed of fatty acid chains with double bonds. – They are not saturated.

• Naturally occurring fatty acid chains are all cis-fats.

Saturated fats

• All C bonded to H• No C=C double bonds– long, straight chain–most animal fats – solid at room temp.• contributes to

cardiovascular disease (atherosclerosis) = plaque deposits

Unsaturated fats• C=C double bonds in

the fatty acids– plant & fish fats – vegetable oils– liquid at room temperature• the kinks made by double

bonded C prevent the molecules from packing tightly together

mono-unsaturated?poly-unsaturated?

Fig. 5-12b

(b) Unsaturated fat

Structural formulaof an unsaturatedfat molecule

Oleic acid, anunsaturatedfatty acid

cis doublebond causesbending

Fig. 5-12a

(a) Saturated fat

Structuralformula of asaturated fatmolecule

Stearic acid, asaturated fattyacid

Saturated vs. unsaturatedsaturated unsaturated

Phospholipids & cells• Phospholipids of cell membrane – double layer = bilayer– hydrophilic heads on outside

• in contact with aqueous solution outside of cell and inside of cell– hydrophobic tails on inside

• form core– forms barrier between cell &

external environment

Phospholipids • Structure:– glycerol + 2 fatty acids + PO4

• PO4 = negatively charged

It’s just like apenguin…A head at one end& a tail at the other!

Phospholipids• Hydrophobic or hydrophilic?– fatty acid tails =– PO4 head =– split “personality”

interaction with H2O is complex & very important! “repelled by water”

“attracted to water”

hydrophobichydrophillic

Fig. 5-13

(b) Space-filling model(a) (c)Structural formula Phospholipid symbol

Fatty acids

Hydrophilichead

Hydrophobictails

Choline

Phosphate

Glycerol

Hydr

opho

bic t

ails

Hydr

ophi

lic h

ead

20

Phospholipid

Phospholipids in water• Hydrophilic heads “attracted” to H2O

• Hydrophobic tails “hide” from H2O– can self-assemble into “bubbles” • bubble = “micelle”• can also form a phospholipid bilayer

bilayer

water

water

Phospholipids

• Similar to fats in structure.

• Three components– Glycerol– 2 Fatty acids– Phosphate group

• Phosphate group replaces the 3rd fatty acid in tricylglycerol

Phospholipid Properties

• Phospholipids are amphipathic.

• In an aqueous environment the hydrophobic tails of phospholipids are pushed together while the heads interact with water and each other.

• Because of this, phospholipids spontaneously take on a characteristic bilayer arrangement.

Functional Importance of Phospholipids

• Phospholipid bilayer is the primary component of biological membranes

http://academic.brooklyn.cuny.edu/biology/bio4fv/page/lipos.gif

Importance of Phospholipids• Water and ions cannot freely diffuse through the

bilayer.

• Phospholipids keep the inside of the cell inside the cell and the outside of the cell outside the cell.

• There are proteins embedded in the phospholipid bilayer that allow select polar molecules and ions through the bilayer.

Why is this important?• Phospholipids create a barrier in water– define outside vs. inside– they make cell membranes!

Steroids

• Steroids are another class of lipids.

• They are hydrophobic and always have the same root structure.

• Functional groups attached to the steroid structure greatly influence the biological action of the steroid.

Steroids• Structure:– 4 fused C rings

• different steroids created by attaching different functional groups to rings

• different structure creates different function – examples:

• Cholesterol• Sex hormones

cholesterol

Important Steroids

http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch26/ch26-5-1.html

Cholesterol• Important cell component – animal cell membranes– precursor of all other steroids• including vertebrate sex hormones

– high levels in blood may contribute to cardiovascular disease

Cholesterol

helps keep cell membranes fluid & flexible

Important component of cell membrane

From Cholesterol Sex Hormones• What a big difference a few atoms can make!

2006-2007

Lipids!

34

Tricylglycerol

Glycerol

Fatty Acid

Triacylglycerol

35

Review

• What two components make up fats?

• What is an Ester Linkage?

• Triacylglycerol (triglyceride):

36

Saturated Fats Vs. Unsaturated Fats

• Saturated:

• Unsaturated:

37

Function and Storage of Fat

• Function:

• Storage:Adipose Tissue

38

Phospholipids

• Structure:

• How do they behave in water?

• Relation to cell membrane:

39

Steroids

• Structure:

• Ex) Cholesterol:

How are Phospholipids different than fats?

• The phosphate is charged.

• This makes that part of the molecule _________________________

• The fatty acid chains are _________________________

Function of Steroids

• Component of cell membranes (cholesterol).

• Hormones

Questions to Consider

• Steroids including cholesterol are do not usually travel alone in the bloodstream. Why not?

• Suppose a phopholipid membrane surrounded an oil droplet. It frequently does in plant seeds. What form might the membrane take on in these cases?