chapter 6

Question 1

carbohydrates get their name because many of them fit the formula of:

Answer 1

C_x(H₂O)_y - composed of carbon and water

• they contain a carbon backbone, a carbonyl group (C=O) and at least one hydroxyl group (-OH)
  
  • if the molecule contains a terminal carbonyl group, it is known as an aldose
  • if the carbonyl group is non-terminal it is known as a ketose
    
    • depending on the number of carbons, carbohydrates can be referred to as trioses (3), tetroses (4), pentoses (5), hexoses (6), heptoses (7)

Question 2

glyceraldehyde is an important?

Answer 2

• triose
  
  • it is the simplest aldose
  • derivatives of glyceraldehyde are important intermediates for glycolysis

Question 3

the one major exception is a pentose known as?

Answer 3

• ribose
  
  • it is one of 3 structural components of nucleic acids
  • ribonucleic acid is formed with the ribose itself, whereas its derivative, deoxyribose, is used to form DNA
  • ribose-5-phosphate is also derived from glucose as part of the pentose phosphate pathway, from which it is used in nucleotide synthesis

Question 4

the most important hexose is?

Answer 4

• glucose which is the main source of energy for the body under normal circumstances
  
  • glucose is an aldose

Question 5

other important hexoses are?

Answer 5

• fructose which is present in many food sources and is metabolized by the liver
  
  • it is a ketose
• galactose is found in dairy products and sugar beets
  
  • it is an aldose that can be rapidly converted to glucose

Question 6

pentoses and hexoses can exist in?

Answer 6

• cyclic forms
  
  • the idea here is that the carbonyl carbon that forms the aldehyde or ketone group (C=O) in the carbohydrate can react with the hydroxyl group of either C5 or C6 of the linear chain to form a hemiacetal or hemiketal

Question 7

what are pyranoses and furanoses?

Answer 7

• 6 membered rings are pyranoses
• 5 membered rings are furanoses

Question 8

what is a hemiketal vs hemiacetal?

Answer 8

• the achiral C=O carbon has now turned into a chiral center with 4 different substituents

Question 9

what is an anomer?

Answer 9

• on one structure, the alpha carbon points up (beta) and the other points down (alpha)

Question 10

the key mechanism through which monosaccharides combine with each other is known as?

Answer 10

• a glycosidic bond
  
  • a glycosidic bond is formed when the anomeric carbon of one sugar reacts with a hydroxyl group in another sugar
  • this is a dehydration reaction, in which an H₂O molecule is lost as the 2 monosaccharides condense to form a disaccharide
  • the formation of a glycosidic bond transforms the hemiacteal or hemiketal found at the anomeric carbon into an acetal or ketal

Question 11

what is sucrose?

Answer 11

• aka as table sugar
• a disaccharide
• formed by the combination of glucose and fructose
  
  • in a sucrose molecule, a glycosidic bond is formed between C1 of the alpha-anomer of glucose and C2 of the beta-anomer of fructose

Question 12

what is lactose?

Answer 12

• a disaccharide found in milk
• characterized by a beta(1-4) glycosidc bond between galactose and glucose
  
  • lactose requires a specific enzyme, lactase, to be hydrolyzed

Question 13

what is maltose?

Answer 13

• formed by 2 glucose molecules jpined by an alpha(1-4) glycosidic bond
• it is produced when amylase breaks down the polysaccharide known as starch, which is formed by lonfer chains of glucose molecules

Question 14

what are polysaccharides?

Answer 14

• consist of long chains of carbohydrates
  
  • their most relevant function is energy storage but also play some important structural roles
    
    • the glucose polymer starch is one of the most important ways in which we consume carbohydrates
    • composed of amylose and amylopectin
      
      • amylose is a linear polymer of glucose molecules connected by alpha(1to4) glycosidic bonds, makes up 20-30% of starch
      • amylopectin makes up the remaining 70-80% of starch; it likewise contains glucose molecules connected by alpha(1-4) glycosidic bonds, but it has branches due to alpha(1-6) glycosidic bonds every 24-30 units

Question 15

humans store energy in the form of?

Answer 15

• glycogen
  
  • in humans it is stored in liver and muscle cells
    
    • the glycogen stored in hepatocytes can be mobilized into the bloodstream to regulate blood glucose levels and provide cells with energy, while the glycogen used in muscle cells is generally brokein down to power glycolysis
    • strucutrally similr to amylopectin in that it contains chains of glucose molecules connected by alpha(1-4) glycosidic bonds, with intervening alpha(1-6) glycosidic bonds that create branches; the main difference is that glycogen is more heavily branched than amylopectin, with branches occuring every 8-12 units

Question 16

which polysaccharide plays a major structural role in the cell wall of plants?

Answer 16

• cellulose
  
  • it is a polyner of glucose
  • it incorporates the beta-anomer of glucose
  • humans lack the necessary enzymes to digest cellulose

Question 17

what is peptidoglycan?

Answer 17

• a relevant polaysaccharide that is a mjor consituent of bacterial cell walls, is a polyner composed of carbohydrates that have been modified with amino acids

Question 18

what is chitin?

Answer 18

• a polysaccharide composed of N-acetylglucosamine molecule connected by beta(1-4) glycosidic bonds
  
  • chitin is a major component of cell walls in the exoskeelton of crustaceans, insects, and fungi

Question 19

what is an epimer?

Answer 19

• refers to pairs of carbohydrates that differ at only one stereocenter

Question 20

each aldohexose has a ( ) enantiomer

each aldohexose has ( ) epimers

Each aldohexose has ( ) diasteremoers

Answer 20

• single
• epimers
• diastereomers

Question 21

what a re conformations?

Answer 21

• spatial arrangements that are interchangeable without breaking bonds (unlike anomers)
  
  • pyranoses exist in either a chair or a boat form
  • furanoses exist in the envelope form
    
    • the least sterically hindered form will be favoured, but which form is the most/least sterically hindered will depend on which substituents may be present

Question 22

how are glycosidc bonds broken?

Answer 22

through hydrolysis and is heavily regulated through enzymes

Question 23

carbohydrates also particpate in some important?

Answer 23

REDOX reactions

• aldehydes in general can be oxidized to carboxylic acids, and this is particularly easy to accomplish for aldoses. relatively straightforward oxidizing agents such as Ag+, Cu2+ and Fe3+, can be used to carry out this reaction
  
  • the resulting change in the oxidizing agent can be monitored to visualize the progress of such a reaction
    
    • for ex. the Tollen’s test, the oxidation of an aldose is coupled with the reduction of Ag+ to Ag, which precipitates to form a visually notable silver film
    • Benedict’s reagent uses a similar principle, but involves Copper(II) and sulfate. this can br vosualized as a change from a blue solution to the formation of a red precipitate
    • sygars can participate in this reaction are known as reducing sugars, because they reduce the reagents that are used to monitor the reaction

Question 24

when an aldose is oxidized, something else gets reduced, making the aldose a ?

Answer 24

reducing agent, more specifically known as a reducing sugar