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.

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