Chapter 5: Carbohydrates

Enzymes hydrolyze carbohydrates to release glucose.

Starch and glycogen are good storage molecules because they contain -glycosidic linkages, which are easily broken down by enzymes. *Phosphorylase* catalyzes the hydrolysis of glycogen in many cells. *Amylase* catalyzes the breakdown of -glycosidic linkages in starch and is found in saliva, as well as the pancreas and intestine

Starch: a storage polysaccharide in plants

Starch consists of alpha-glucose monomers joined by alpha-glycosidic linkages. (Table 5.1) Linkages result in a coiled helix structure. Starch is a mixture of unbranched amylose containing only alpha-1,4-glycoside linkages and branched amylopectin with glycosidic linkages between C-1 carbon of glucose with C-6 carbon on another strand. Branches occur in amylopectin once in every 30 monomers.

Alpha and Beta forms of Glucose (form and function theme) starch and cellulose

Starch is a polysaccharide of alpha glucose monomers. Cellulose is a polysaccharide of beta glucose monomers, making every other glucose monomer upside down with respect to its neighbors. The differing glycosidic links in starch and cellulose give the two molecules distinct three-dimensional shapes. While polymers built with alpha glucose form helical structures, polymers built with beta glucose form straight structures. The straight structures built with beta glucose allow H atoms on one strand to form hydrogen bonds with OH groups on other strands. In plant cell walls, parallel cellulose molecules held together in this way are grouped into units called microfibrils, which form strong building materials for plants (and for humans, as lumber).

Alpha form of Glucose

Starch is a polysaccharide of alpha glucose monomers. The differing glycosidic links in starch and cellulose give the two molecules distinct three-dimensional shapes. While polymers built with alpha glucose

glycosidic linkage structure and function

Storage polysaccharides use one type of glycosidic linkage alpha-1,4-glycosidic linkage (starch, glycogen). Structural polysaccharides use beta-1,4-glycosidic linkages (cellulose, chitin and peptidoglycan).

Each monosaccharide has a unique structure and function

Aldose or ketose placement of the carbonyl group Variation in carbon number Different arrangements of hydroxyl groups in space Alternative ring forms

glyocosidic linkages

Between any two hydroxyl group The location and geometry of these bonds vary widely among polysaccharides

Photosynthesis (previous card)

Carbohydrates store sunlight as chemical energy using the process of photosynthesis. Photosynthesis can be summarized as: CO2 + H2O + sunlight (CH2O)n + O2.

Amylase

Catalyzes the break down of alpha-glycosidic linkages in starch and is found in saliva, as well as the pancreas and intestine

Phosphorylase

Catalyzes the hydrolysis of glycogen in many cells

Chitin

Chitin: a structural polysaccharide in fungi and animals Chitin stiffens the cell walls of fungi and some algae and is an important component of the exoskeletons of insects and crustaceans. Chitin has beta-1,4-glycosidic linkages between monosaccharides called N-acetylglucosamine (NAG). Like cellulose, every other chitin monomer is flipped, allowing for the formation of H bonds and making a strong, tough sheet resulting in stiffness and protection.

Aldose/ketose

Depending on the location of the carbonyl group, the sugar is an aldose or a ketose.

Carbohydrates and cellular identity storage

Electrons in reactants have unequally shared electrons and, hence, *low potential energy* (C=O and C-O have electrons held tightly). (Fig. 5.7) C-H bonds in product have equal sharing of electrons and are therefore of *higher potential energy* than the unequally shared electrons in the reactants. Even C-C bonds have equally shared electrons and relatively *high potential energy* compared to C-O or C=O bonds.

Fatty acids store greater amount of energy than carbohydrates

Fats and carbohydrates store fuel, but fat has twice the amount of energy per gram compared to carbohydrates.

Glycogen: a highly branched storage polysaccharide in animals

Glycogen is stored in liver and muscles. Like starch, glycogen consists of alpha-glucose monomers joined by alpha-linkages, branching is once in every 10 monomers.

Microfibrils

In plant cell walls, parallel cellulose molecules held together with hydrogen bonds between H and OH on different strands on beta glucose are held together in this way and grouped into units called microfibrils, which form strong building materials for plants (and humans, as lumber)

Linear and Ring Form Monosachharides

Linear and alternative ring forms Sugars tend to form ring structures in aqueous solutions

Cellulose: a structural polysaccharide in plants

Many cells surrounded by cell wall; a protective sheet outside the membrane; plants, bacteria, fungi and other cells have cell walls composed of polysaccharides. Cellulose is a major component of plant cell walls. Cellulose consists of beta-glucose monomers joined by beta-1,4-glycosidic linkages. Every other glucose is flipped compared to the previous monomer. Flipping every other monomer results in a linear molecule (not helical). Linear molecule has H bonds between adjacent parallel strands. The structure provided due to flipping every other monomer results in cellulose fibers being straight and strong, requirements for cell structural support.

Symbiosis (refer to outline)

Many eukaryotic herbivores, from cows to termites, have symbiotic relationships with cellulolytic microbes, providing the microbe and the host animal access to a rich source of energy.

Number of carbon atoms present

Monosaccharides can have three, five, or six carbons (trioses, pentoses, and hexoses, respectively), numbered consecutively starting with the one closest to the carbonyl group. Triose: three Pentose: five Hexose: six

Monosaccharide formula

Monosaccharides generally have molecular formulas that are some multiple of the unit CH2O. For example, glucose has the formula C6H12O6.

Peptidoglycan

Peptidoglycan: a structural polysac-charide in bacteria Most bacteria have cell walls composed of peptidoglycan (a polysaccharide). It has two different alternating monosaccharides joined by beta-1,4-glycosidic linkages. It has a short chain of amino acids attached to every other monomer, allowing for the formation of peptide bonds between adjacent strands. The peptide bonds play a similar role to the H-bonds of chitin and cellulose by providing structural polysaccharides that are long, straight, parallel strands bound to each other, making these polysaccharides strong and stable.

glycosidic linkage (ether).

Simple sugars polymerize when A condensation reaction (dehydration synthesis) occurs between two hydroxyl groups resulting in a covalent bond called a glycosidic linkage (ether).

disaccharides

The simplest polysaccharides are disaccharides Comprised of two monosaccharide monomers The monomers can be identical or different

Carbohydrates in cellular identity

Wasserman et al., examined binding of the sperm to the egg during fertilization and demonstrated that sperm only recognize eggs of the same species. (Fig. 5.6) a. Competitive binding assay added purified egg-surface glycoproteins to sperm and blocked binding of sperm to eggs. b. Purified carbohydrates but not proteins caused the block in binding.

Aldose

found at the end of the monosaccharide

Ketose

found in the middle of the monosaccharide

Monomer (simple sugars)

monosaccharide (simple sugars), Carbon number: pentoses (5C), hexoses (6C) Carbonyl groups: aldose , ketose