Macromolecules• Macromolecules: large molecules make up cells or

carry out the cellular vital processes (~26% of bacterial cell mass)

– Structural or functional roles

• Macromolecules fall into 4 groups,– Carbohydrates– Lipids– Nucleic acids– Proteins

Fig. 2-26, Alberts et al. 2004(K. Gilmour lecture series)

• Dynamically synthesized and replenished• Macromolecules are constructed by

linking together similar or dissimilar subunits (monomers)

• Role in cell:– Informational: order of monomeric units is not

random and required for proper function ie. proteins and nucleic acids

– Structural/storage: a single repeating subunit or alternating subunits; order of subunits does not carry information e.g. polysaccharides (cellulose, starch, glycogen)

Macromolecules

Macromolecules

• Recycling of mocromolecules is controlled enzymatically through biochemical pathways

• Are synthesized from monomers by dehydration reactions

• condensation of monomers by losing a water molecule)

• Degraded by hydrolysis reactions (addition of a water molecule)

• Carbohydrates include simple sugars and

polysaccharides.

• Monosaccharides (simple sugars) are the major

nutrients of cells. The basic formula is (CH2O)n

• Glucose (C6H12O6) is especially important since it

provides the principal source of cellular energy

• Ribose and deoxyribose are five carbon sugars that are

found in RNA and DNA

Carbohydrates

• MonosaccharidesMonosaccharides– (CH2O)n; n = 3 to 7, with n = 5 (pentose) or 6

(hexose) most common– Aldose sugars (functional group is aldehyde -

CHO)– Ketose sugars (ketone C=O as functional group)

• Isomers: same formula but structurally differenteg. glucose, galactose (aldohexoses) and fructose (Ketohexose) all are C6H12O6 but are structurally different

MonosaccharidesKeto Aldo

Isomers (C6H12O6)

Galactose (C6H12O6)

HO H

Monosaccharides

D-glyceraldehyde L-glyceraldehyde

StereoisomerismStereoisomerism• Enantiomers: optical isomers, same folmula, same groups bonding

with carbon skeleton, but groups on asymmetric carbon are mirror images of each other, eg in glyceraldehyde– D-form (Dexter=right) the –OH is facing to the right of backbone– L-form (Laevus=left) the –OH is facing to the left of backbone; D and L are mirror images and cannot be superimposed– D-form is important for cells? Why?

Click to see a 3D model

• Glyceraldehyde and Dihydroxyacetone are the 3 carbon aldo- and keto-trioses that result from the breakdown of glucose in glycolytic pathway

Monosaccharides: Trioses

Linear vs Ring

• C5 and 6 carbone sugars exist in linear and ring forms

• Penultimate carbon and the C of aldehyde or Ketone

• α or β configurational isomers may form

• Different with respect to the position of –OH relative to plane of ring

• In α isomer -OH is below the plane

• In β the -OH is above the plane

• α or β isomers are important for polysaccharide formation

Linear vs Ring

Formation of a glycosidic bond

• Dehydration reaction joins two simple sugars by glycosidic bond

• α 1→4 bond or β1→4 (C1 and C4) is common

• Main polysaccharide backbone

• α 1→6 (C1 and C6) causes branches in the chain

O+

H2O

O+

H2O

Courtesy of K. Gilmour

Polysaccharides

• Some polysaccharides are short term energy storage molecules.– Plants store glucose as starch– Animals store glucose as

Glycogen in liver

• Some have structural role– Cellulose in plants

– Chitin in arthoropod exoskeleton

Polysaccharide: chemical composition

• Glycogen and Starch: composed entirely of glucose molecules in the αα configuration

((αα1→6)1→6)

((αα1→4)1→4)

Polysaccharide: chemical composition• Cellulose is the most abundant polysaccharide; plant cell wall• Unbranched glucose polymer with β1→4 glycosidic bond

• H-bonding between many parallel polymers forms strong fibers

• Chitin: polymer of N-acetylglucoseamine with β1→4 linkage; is a polysaccharide that forms the exoskeleton of crabs, lobsters, and insects

((ββ1→4)1→4)

NH

C=O

CH3

NH

C=O

CH3

NH

C=O

CH3

NH

C=O

CH3

((β1→4))

Lipids• Lipids: diverse group of macromolecules that are

insoluble in water• Include

– Fats and oils are well-known lipids used for energy storage and other purposes

– Phospholipids are components of the membranes that surround cells

– Glycolipids consist of two hydrocarbon chains linked to polar head groups that contain carbohydrates

– Steroids, which have a different structure from most lipids, are used as hormones and for other purposes.

Lipids

• Lipids have three main roles:– Energy storage

– Major components of cell membranes

– Important in cell signaling: as steroid hormones and messenger molecules

FatsFats and oils contain two subunits

– Glycerol is a polyalcohol with three polar –OH groups– Fatty acids long hydrocarbon chains (16 or 18 carbons) with a carboxyl

group (COO−) at one end

GlycerolUnsaturatedSaturated

Triglycerides (Fats)

• TG: dehydration reaction adds fatty acids to the –OH groups of glycerol and broken down by hydrolysis reactions

• insoluble in water; accumulate as fat droplets in the cytoplasm• When required, break down for use in energy-yielding reactions• produce twice as much energy as sugars per unit weight

Ester bond

Phospholipids• Phospholipids: principal

components of cell membranes• Glycerol phospholipids: 2 fatty acids

are bound to carbon in glycerol. The third carbon of glycerol is bound to a phosphate group

• Molecule is hydrophilic at phosphate end and hydrophobic at fatty acid tails

• This is called amphipathic property; important for formation of bilayer biological membranes

Phospholipids

• Other polar groups is added to phosphate to make the molecule more polar, including,

– Ethanolamine (phosphatidyl ethanolamine)

– Choline (phosphatidyl choline)

– Serine (phosphatidyl serine)

– Inositol (phosphatidyl inositol)

Glycolipids

• Many cell membranes also contain glycolipids and cholesterol

• Glycolipids are amphipathic• Consist of 2 FA, serine instead of

glycerol and one or more sugar instead of phosphate

• Also function as cell surface markers used for cell recognition

Cholestrol and steroids

• Cholesterol: amphipathic four ringed hydrocarbon– Abundant in eukaryotic

membranes– Increases membrane fluidity

• Cholesterol derivatives – estrogens and testosterone – Steroid hormonal

messengers

Can you meet these objectives?

• Distinguish among monomers, polymers and macromolecules?

• Discuss the structure and function of carbohydrates?

• Briefly describe the types of lipids, and outline the basic structure and function of each?