5.1 Sugars as Monomers

• Three-carbon sugars are called trioses.

• Ribose, which acts as a building block for nucleotides, has five carbons and is called a pentose.

• The glucose that’s coursing through your bloodstream right now is a six-carbon sugar, or a hexose.

• Because the molecular structures of glucose and galactose differ, their functions differ.

• Many distinct monosaccharides exist because so many aspects of their structure are variable: aldose or ketose placement of the carbonyl group, the number of carbons, and the different arrangements of hydroxyl groups in space.

5.2 The Structure of Polysaccharides

• When just two sugars link together, the resulting molecule is known as a disaccharide.

• Monosaccharides polymerize when a condensation reaction occurs between two hydroxyl groups, resulting in a covalent connection called a glycosidic linkage, or glycosidic bond.

• A functional consequence of the structural differences between maltose and lactose is that the enzymes used to hydrolyze maltose will not cleave lactose.

• The variation in how polysaccharides are formed allows organisms to use them in radically different ways.

• Starch consists entirely of glucose joined by glycosidic linkages.

• Glycogen performs the same storage role in animals as starch does in plants.

• All cells are enclosed by a membrane and the cells of many organisms are also surrounded by a protective layer of material called a cell wall.

• Cellulose is a polymer made from !3-glucose monomers joined by B-1,4-glycosidic linkages.

• Chitin is a polysaccharide that stiffens the cell walls of fungi.

• The primary structural component of bacterial cell walls consists of a polysaccharide called peptidoglycan.

5.3 What Do Carbohydrates

• Carbohydrates have diverse functions in cells:

  • They serve as precursors to other molecules

  • Provide fibrous structure materials

  • Mark cell identity

  • Store chemical energy

• A glycolipid is a lipid that has been glycosylated, meaning it has one or more covalently attached carbohydrates.

• A glycoprotein is a protein that is similarly linked to carbohydrates.

• Glycolipids and glycoproteins are key molecules in what biologists call cell-cell recognition and cell-cell signaling.

• Plants harvest the energy in sunlight and store it in the bonds of carbohydrates by the process known as photosynthesis.

• The most important enzyme involved in catalyzing the hydrolysis of α-glycosidic linkages in glycogen molecules is a protein called phosphorylase.

• The enzymes involved in breaking the glycosidic linkages in starch are called amylases.