BIOS 100 Exam I: Carbohydrates

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Unmodified sugars (those with the formula C n H2 n O n ) can have ...

(a) H covalently bound to the C of a C=O group.
(b) A C atom that is covalently bound to three H atoms.
(c) The formula C3H6O3.
(d) Both (a) and (c).
(e) Both (b) and (c).

(d) Both (a) and (c).

The best choice! Glucose is one of many sugars that have H bound to C=O; they're called aldoses. The simplest sugar has the formula C3H6O3.

What is the LEAST that two molecules can differ, and still be called different kinds of sugars?

(a) The number of C atoms differs.
(b) One is a ring, the other is linear.
(c) The orientation of an OH group differs.
(d) The location of the carbonyl group differs.
(e) None of the above.

(c) The orientation of an OH group differs.

In the open-chain form, two sugars may differ in the orientation (right or left) of just one OH group.

A molecule has the formula C n H2 n O n . What else does it need to be an unmodified monosaccharide?

(a) Nothing.
(b) A carboxyl group.
(c) O bound to every C.
(d) At least 5 carbon atoms.
(e) Both (b) and (c).

(c) O bound to every C.

If a monosaccharide has C without O attached, it's been modified.

The alpha and beta forms of glucose, found in starch and cellulose, differ in ...

(a) Whether the molecule is a ring or an open chain.
(b) Whether the ring contains five or six C atoms.
(c) Whether the first C atom is bound to one or two O atoms.
(d) How one of the -OH groups is oriented.
(e) None of the above.

(d) How one of the -OH groups is oriented.

One particular -OH group points to one side of the ring in alpha glucose; the other side in beta glucose.

What happens when glucose forms a ring?

(a) The molecule loses its carbonyl group.
(b) The molecule loses one O atom.
(c) The sixth C atom binds to the first C atom.
(d) Both (b) and (c).
(e) Both (a) and (c).

(a) The molecule loses its carbonyl group.

The C=O group becomes -O-C-OH.

A sugar can have ...

(a) H covalently bound to the C of a C=O group.
(b) a C atom that is covalently bound to three H atoms.
(c) the formula C5H8O5.
(d) Both (a) and (c).
(e) Both (b) and (c).

(a) H covalently bound to the C of a C=O group.

This occurs in linear aldoses such as glucose.

What is an enantiomere?

They're mirror images, and that makes them enantiomers.

Which feature is shared by all monosaccharides?

(a) In their linear forms, they all contain a carbonyl and several hydroxyl functional groups.
(b) They are all pentoses.
(c) In their linear forms, they all contain a carboxyl and several hydroxyl functional groups.
(d) They all contain more than one sugar.

(a) In their linear forms, they all contain a carbonyl and several hydroxyl functional groups.

Note that in aqueous solution monosaccharides tend to exist in cyclic, not linear, forms.

A sugar that has the chemical formula C5H10O5 would be characterized as a(n) _____.

(a) Hexose
(b) Triose
(c) Oligosaccharide
(d) Pentose

(d) Pentose

Pentose is the general term for a five carbon sugar such as ribose.

The _____ functional group can always be found in a carbohydrate molecule.

(a) Amino
(b) Hydroxyl
(c) Phosphate
(d) Water

(b) Hydroxyl

Carbohydrates are aldehydes or ketones with multiple hydroxyl groups.

How do the α and β forms of glucose differ?

(a) Their ring structures differ in the location of a hydroxyl group.
(b) Their linear structures differ in the location of a hydroxyl group.
(c) The α form can be involved in 1,4- and 1,6-glycosidic linkages; the β form can participate only in 1,4 linkages.
(d) The oxygen atom inside the ring is located in a different position.

(a) Their ring structures differ in the location of a hydroxyl group.

What is the difference between an aldose sugar and a ketose sugar?

(a) The position of the hydroxyl groups
(b) The position of the carbonyl group
(c) The number of carbons
(d) One is a ring form, the other is a linear chain

(b) The position of the carbonyl group

Glycogen is _____.

(a) A source of saturated fat
(b) A polysaccharide found in animals
(c) A transport protein that carries oxygen
(d) A polysaccharide found in plant cell walls
(e) The form in which plants store sugars

(b) A polysaccharide found in animals

Animals store energy in the form of glycogen.

glucose + glucose —> _____ by _____.

(a) Lactose + water ... hydrolysis
(b) Sucrose + water ... dehydration synthesis
(c) Starch + water ... dehydration synthesis
(d) Cellulose + water ... hydrolysis
(d) Maltose + water ... dehydration

(d) Maltose + water ... dehydration

Maltose is the disaccharide formed when two glucose molecules are linked by dehydration synthesis.

Which of these is a source of lactose?

(a) Milk
(b) Potatoes
(c) Sugar cane
(d) Sugar beets
(e) Starch

(a) Milk

Lactose is the sugar found in milk.

Which of these is a polysaccharide?

(a) Cellulose
(b) Galactose
(c) Glucose
(d) Sucrose
(e) Lactose

Cellulose is a carbohydrate composed of many monomers.

_____ is the most abundant organic compound on Earth.

(a) Cellulose
(b) Glucose
(c) Starch
(d) Glycogen
(e) Lactose

(a) Cellulose

Cellulose, a component of plant cell walls, is the most abundant organic compound found on earth.

Which statement is true of sucrose?

(a) It's a disaccharide.
(b) It contains glucose.
(c) It's table sugar.
(d) Both (a) and (c).
(e) - (a), (b), and (c).

(e) - (a), (b), and (c)

We use sucrose as our table sugar because plants make plenty of it. They use it as their main circulating fuel, and sugar beets and sugar cane store a lot of it. It's a disaccharide made of glucose and fructose.

Cellulose is ...

(a) Made with glucose that has the beta ring form.
(b) A component of crab shells.
(c) The third most abundant organic compound in the world.
(d) A branching polymer.
(e) All of the above.

(a) Made with glucose that has the beta ring form.

That's what makes it impossible for humans to digest cellulose. Fungi, plants, and many microbes can digest it, though.

Glycogen ...

(a) Contains several kinds of sugars.
(b) Occurs in animal cells and has branches.
(c) Is stronger, weight for weight, than steel.
(d) Occurs in chloroplasts and stores energy.
(e) None of the above.

(b) Occurs in animal cells and has branches.

This branched polymer is the main energy-storing carbohydrate in animals and humans.

Which fact is most important in explaining why cellulose is a better structural material than starch?

(a) Polymers made of fructose are stronger than polymers made of glucose.
(b) Branched polymers can form more hydrogen bonds than unbranched polymers.
(c) Alpha-linkages make it easier for the polymer to coil into a helix.
(d) C-C links are stronger than C-O-C links.
(e) Links between alpha glucoses are stronger than links between beta glucoses.

(c) Alpha-linkages make it easier for the polymer to coil into a helix.

The alpha links let starch coil into compact helices that pack into dense granules. For structural use, you want polymers that stretch out straight and form cables, as in beta-linked cellulose.

A cotton shirt is chemically most similar to a ...

(a) Sugar cube.
(b) Slice of ham.
(c) Pat of butter.
(d) DNA double helix.
(e) Fingernail.

(a) Sugar cube.

Both cotton and a sugar cube are carbohydrates, made of sugars.

The bond that joins two monosaccharides into a disaccharide is a(n) _____.

(a) Van der Waals bond
(b) Glycosidic bond
(c) Ionic bond
(d) Hydrogen bond

(b) Glycosidic bond

The glycosidic bond is a covalent bond that joins two sugars together via a condensation reaction.

Which of the following polysaccharides contains peptide bonds?

(a) Starch
(b) Peptidoglycan
(c) Glycogen
(d) Chitin

(b) Peptidoglycan

Peptidoglycan is a structural polysaccharide found in bacterial cell walls. The sugars are modified with amino acids that can form peptide bonds to covalently link adjacent strands.

Which of the following polysaccharides composes the cell wall of fungi?

(a) Peptidoglycan
(b) Chitin
(c) Starch
(d) Glycogen

(b) Chitin

Chitin is a structural polysaccharide that forms the cell wall of fungi and is also found in the exoskeleton of insects.

Cells of which organisms lack cell walls?

(a) Plants
(b) Fungi
(c) Animals
(d) Bacteria

(c) Animals

Animal cells do not have a cell wall external to the plasma membrane.

Which of the following linkages would you expect to find at a branch point in glycogen or amylopectin?

(a) α-1,4-glycosidic linkage
(b) β-1,6-glycosidic linkage
(c) α-1,6-glycosidic linkage
(d) β-1,4-glycosidic linkage

(c) α-1,6-glycosidic linkage

What is the major structural difference between starch and glycogen?

(a) The amount of branching that occurs in the molecule
(b) Whether glucose is in the α or β form
(c) The types of monosaccharide subunits in the molecules
(d) The type of glycosidic linkages in the molecule

(a) The amount of branching that occurs in the molecule

Which of the following do starch and cellulose have in common?

(a) The type of glycosidic linkage used
(b) Their main function in plants
(c) The size of their monosaccharide subunits
(d) The amount of hydrogen bonding that occurs between parallel strand

(c) The size of their monosaccharide subunits

Which of the following structural features is common to cellulose, chitin, and peptidoglycan?

(a) They can all form bonds between polymer chains that create parallel strands.
(b) They are all composed of glucose in either the α or β form.
(c) They are all composed of highly branched fibers.
(d) They all contain peptide bonds.

(a) They can all form bonds between polymer chains that create parallel strands.

Dairy cattle were unknown in Thai culture until recently, and 97 percent of Thai people are lactose intolerant as adults. Which explanation for such widespread lactose intolerance is most likely correct?

(a) Evolutionarily, producing an enzyme to break down a sugar that will never be encountered is wasteful.
(b) The ability to digest sugar in milk is determined by environment, and most humans are not exposed to milk as a food source beyond childhood years.
(c) There is no good explanation for this situation in humans.
(d) Allergies are becoming more common in humans as more chemicals are being encountered during longer lifetimes.

(a) Evolutionarily, producing an enzyme to break down a sugar that will never be encountered is wasteful.

The enzyme amylase can break glycosidic linkages between glucose monomers only if the monomers are the α form. Which of the following could amylase break down?

(a) Chitin
(b) Starch
(c) Starch and chitin only
(d) Cellulose
(e) Starch, cellulose, and chitin

(b) Starch

Starch and cellulose _____.

(a) Are cis-trans isomers of each other
(b) Can be digested by humans
(c) Are polymers of glucose
(d) Are structural components of the plant cell wall
(e) Are used for energy storage in plants

(c) Are polymers of glucose

Cellulose is _____.

(a) A monomer of starch.
(b) A major structural component of plant cell walls
(c) A polymer composed of fructose monomers
(d) Used by plants to make glycogen
(e) A storage polysaccharide for energy in plant cells

(b) A major structural component of plant cell walls

What type of bond is formed between two sugars in a disaccharide?

(a) Peptide bond
(b) Hydrogen bond
(c) Glycosidic linkage
(d) Phosphodiester bond

(c) Glycosidic linkage

Which statement best summarizes a key difference in the structure of polysaccharides that function in energy storage versus those used in structural support?

(a) Polysaccharides that function in energy storage are built from β glycosidic linkages, whereas polysaccharides used in structural support form straight chains that bond with adjacent chains.
(b) Polysaccharides that function in energy storage are built from α glycosidic linkages, whereas polysaccharides used in structural support form straight chains that bond with adjacent chains.
(c) Polysaccharides that function in energy storage form straight chains that bond with adjacent chains, whereas polysaccharides used in structural support are built from α glycosidic linkages.
(d) Polysaccharides that function in energy storage are built from α glycosidic linkages, whereas polysaccharides used in structural support form helical chains that are highly branched.

(b) Polysaccharides that function in energy storage are built from α glycosidic linkages, whereas polysaccharides used in structural support form straight chains that bond with adjacent chains.

α linkages are readily hydrolyzed to release glucose; straight chains bond to adjacent chains to form tough fibers.

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