Chapter 5: An Introduction to Carbohydrates
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.