Chapter 3 - sugars (polysaccharides)

what are polysaccharides?

• long chain polymers of sugars and sugar derivatives

• function: in structure and storage

• consist of a single kind of repeating unit or sometimes an alternating pattern of two kinds

oligosaccharides

• short polymers sometimes attached to cell surface proteins

• identification tag (glycoprotein)

what are the monomers of sugars

• monosaccharides

what can a sugar be? two types

• aldehyde (aldosugars) — terminal carbonyl group

• ketone (ketosugars) — with an internal carbonyl group

• (named based on how many carbon atoms they contain)

aldosugar

ketosugar

classification of sugars

• most sugars have bn 3 and 7 carbons:

• trioses (3 C)

• tetroses (4 C)

• pentoses (5 C)

• hexoses (6 C)

• heptoses (7 C)

What is glucose?

• most common monosaccharide is aldohexose D-glucose (c6h12o6)

formula for sugars and its name

• CnH2nOn

  • carbohydrate

how are carbons of glucose numbered?

from the more oxidized, carbonyl end

for every molecule of ___ incorporated into a sugar, one ___ molecule is consumed

CO2; H2O

• structure of alpha D-Glucose

• repeating unit of starch and glycogen

what is the 2 alternative forms of the ring of D-glucose? why do they happen?

• depend on spatial orientation of the hydroxyl group of carbon number 1

• forms designated (alpha if pointed downward) or beta (hydroxyl group upward)

• beta glucose

• repeating unit in cellulose

disaccharides and bond type

• two monosaccharides are covalently linked

• linkage of disac glycosidic bond

  • formed bn 2 monos by the elimination of water

• alpha form of bond called: alpha glycosidic bonds (maltose)

• beta form of bond called: beta glycosidic bonds (lactose)

what is the most familiar storage polysaccharides? what is the bond type?

• starch in plant cells

• glycogen in animal cells and bacteria

• consist of alpha D-glucose units linked by alpha glycosidic bonds, involving 1→4

• sometimes alpha(1→6) bonds may form, allowing for branching!

glycogen and branching and location

• HIGHLY BRANCHED

  • occurs every 8-10 glucose units along the backbone

• mainly stored in liver (as a source of glucose) and muscle tissue (as a fuel source for muscle contractions) of animals

• bacteria store glycogen as glucose reserve

starch and branching

• glucose reserve found in plant tissues

• can be

  • unbranched = amylose (10-30% of time)

  • branched = amylopectin (70-90% of time)

amylopectin branching

• has alpha 1→6 branches once every 12-25 glucose units

• has longer side chains than glycogen

starch storage

• stored as starch grains within plastids

  • chloroplasts: the sites of carbon fixation and sugar synthesis in photosynthesis

  • amyloplasts: specialized for starch storage

  • both are membrane bound organelles

cellulose

• the best known structural polysaccharide

  • found in plant cell walls

• composed of repeating monomers of beta-D-glucose, which is v abundant in plants

• mammals cannot digest cellulose “insoluble fiber”

beta (1→4) bond

• microfibrils — made of many long, unbranched cellulose chains

• each chain made up of beta D-glucose molecules arranged in linear manner and linked by beta (1 to 4) glycosidic bonds

how does the cellulose of fungal walls differ from that of plants?

• they may contain either beta(1-4) or beta(1-3) linkages

what kind of sugars does bacterial cell walls contain?

• GlcNAc and MurNAc

• both are derivatives of beta-glucosamine and are linked alternatively in cell walls

Chitin

• a polysaccharide

• consists of GlcNAc units only, joined by beta(1-4) bonds

• found in insect exoskeletons, crustacean shells, and fungal cell walls

what does polysaccharide structure depend on?

• the type of glycosidic bonds involved

alpha and beta glycosidic bonds are associated with ___ ___ ___

marked structural differences

what do starch and glycogen form?

• they are alpha polysaccharides

• form loose helices that aren’t highly ordered bc of side chains

cellulose forms what kind of linkages? what is structure?

• beta linkages

• exists as rigid linear rods that aggregate into microfibrils, about 5-20 nm in diameter

what contains the rigid microfibrils? what other things do they contain?

• plant and fungal cell walls in a noncellulose matrix

• contains other polymers (hemicellulose and pectin)

• and a protein called extensin (glycoprotein)