Oligosaccharides - Cell Recognitionprofkatz.com/courses/wp-content/uploads/2013/05/oligpolysaccharides.pdfOligosaccharides - Cell Recognition 1) Cells interact with and recognize other

Oligosaccharides - Cell Recognition1) Cells interact with and recognize other cells through a process called cell recognition.

2) Cell recognition is accomplished through saccharides attached to the cell surfaces.

3) These saccharides, usually oligosaccharides, are present as glycolipids and glycoproteins. The lipid or protein part of the molecule is integrated into the cell-membrane structure with the saccharide part located on the external membrane surface.

Oligosaccharides - Cell Recognition

The ABO blood group types are: A, B, AB, and O.

These result from three types of antigens (containing saccharide molecules) A, B, and O.

There are only two types of antibodies: anti-A and anti-B. There is no anti-O.

Characteristic A B AB Oantigen on RBC

surface A B A,B NONE

antibodies produced anti-B anti-A none anti-A, anti-B

can receive RBCs from blood type O, A O, B O,A,B,AB Ocan donate RBCs

to blood type A, AB B, AB AB O,A,B,AB

ABO Blood-Typing System

Blood Types

Polysaccharides

Biological polymers containing large numbers (hundreds or thousands) of monosaccharides residues (repeat units) bonded together.

Starch and glycogen serve as storage forms for D-glucose.

Starch - plants

Glycogen - animals

Cellulose and chitin serve as biological structural materials.

Polysaccharides

Polysaccharides differ in the following ways:

1) The monosaccharides which constitute the residues

2) Which of the hydroxyl groups participate in linking the monosaccharide residues together

3) The type of glycosidic linkage (α or β) involved between the residues

4) The presences or absence of branching

Polysaccharides - StarchFound in Plants

Starch: 10-30% Amylose, 70-90% Amylopectin

Amylose: linear, α(1→4) linkages

Amylopectin:branched, α(1→4) linkages and α(1→6) branches

α -1, 6’ glucosidic linkagebranching point

Polysaccharides -GlycogenFound in Animals

Glycogen is similar to amylopectin but more highly branched.

Polysaccharides

Amylose, amylopectin, and glycogen all contain a single free hemiacetal hydroxyl and hundreds or thousands of acetal glycosidic linkages. The percentage of free hemiacetal –OH groups is so small that none of these molecules give a positive test with Benedict’s solution. They are all non-reducing sugars.

Polysaccharides - 3D Structure of Amylose

Polysaccharides - 3D Structures

Polysaccharides - Starch and Glycogen Digestion

Starch is digested to D-glucose:

Amylase hydrolyzes amylose and parts of amylopectin to maltose in the digestive tract.

Maltase cleaves maltose to D-glucose.

The result of amylase digestion of amylopectin is dextrin which contains the remaining α(1→6) linkages. Dextrin is hydrolyzed by dextrinase to D-glucose.

Polysaccharides - Starch and Glycogen Digestion

Some of the D-glucose from starch is used immediately for energy by cells (glycolysis).

The excess D-glucose is stored in the liver and skeletal muscles as glycogen (glycogenesis). Any D-glucose still in excess is converted to fat and deposited in the fat tissues. (lipogenesis)

When required for energy or biosynthesis, D-glucose is released from a glycogen molecule (glycogenolysis). Removal of D-glucose from glycogen can be very rapid because it can be removed from all of the tips of the glycogen branches simultaneously.

Finally, if the body’s glycogen and glucose stores are depleted, glucose can be supplied by combining smaller molecules in the liver(gluconeogenesis).

Polysaccharides - Cellulose

Cellulose is a β (1→4) Polymer of D-Glucose

Cellulose (wood) is insoluble and retains its shape and most of its physical strength when placed in water. Cellulose molecules exist in an extended chain conformation and pack side to side to form ribbons. The ribbons pack side to side and on top of each other to form fibers. All of the cellulose molecules are held together in a fiber by intermolecular hydrogen bonding.

Comparing Starch and CelluloseUnlike cellulose, starch swells and forms a colloidal suspension when placed in water.

Starch can be differentiated from cellulose by adding a few drops of I2 solution. I2 forms a dark blue solution in the presence of starch which does not form with cellulose.

Humans cannot digest cellulose because we lack the enzyme cellulase which cleaves β(1→4) linkages. Even so, cellulose in the diet has some beneficial effects.

The main nutritional carbohydrate for grazing animals is cellulose (grass and other plants). The main nutritional carbohydrate for termites is found in wood.

These animals cannot digest cellulose directly, but have symbiotic microorganisms in their digestive tracts that secrete cellulase into the animal’s digestive tracts.

Comparing Starch and Cellulose