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Example Questions
Example Question #1 : Glycosidic Linkages
Glucose polysaccharides are linked together at branch points in glycogen by what type of bond?
Alpha-1,6 linkages
Beta-1,4 linkages
Peptide linkages
Alpha-1,4 linkages
Beta-1,6 linkages
Alpha-1,6 linkages
In glycogen, glucose molecules are attached one after the other by alpha-1,4 linkages. However, in order to make glycogen more compact for storage, branch points are created to created links between many shorter glucose polysaccharides. These branch points connect glucose molecules by alpha-1,6 linkages.
Example Question #1 : Carbohydrate Structures And Functions
Why does glycogen have more branches than starch?
There are more alpha-1,4 linkages
There are less alpha-1,4 linkages
There are less alpha-1,6 linkages
There are more alpha-1,6 linkages
None of these answers; glycogen is not more branched than starch
There are more alpha-1,6 linkages
Lots of alpha-1,4 linkages allow for longer chain lengths in carbohydrates like starch and glycogen. However, it is the amount of alpha-1,6 linkages that determine the number of branches - since glycogen has many more alpha-1,6 linkages than starch does, it has more branches. This allows for easy breakdown of glycogen into glucose in the liver should there not be enough glucose in the body to supply the body's demand for energy production. Recall that glycogen phosphorylase can only break terminal alpha-1,4 glycosidic bonds; hence, with more branches there are more terminal glucose molecules that are substrates for this catabolic enzyme.
Example Question #3 : Glycosidic Linkages
During times of glucose deprivation in the human body, the liver is able to supply glucose to the bloodstream by breaking down a large, branched polysaccharide that it holds in reserve until it is needed. Which of the following lists the correct type of glycosidic bonds found in this polysaccharide.
for branch points and
for straight chain
for branch points and
for straight chain
for branch points and
for straight chain
for branch points and
for straight chain
for branch points and
for straight chain
for branch points and
for straight chain
For this question, we're told some background information about the liver's role in providing glucose homeostasis. We're told that when blood glucose levels are lowered, the liver is able to help restore glucose levels by keeping a large polysaccharide of glucose in store. In times of need, the liver can break this compound down to provide glucose to the bloodstream.
Even though the question doesn't explicitly tell us what the polysaccharide is, we should be able to infer that the compound in question is glycogen. Therefore, to answer the question, we need to know which kind of glycosidic bonds are found in glycogen.
First, let's recall that an individual glucose molecule is composed of six carbon atoms. In its ring form, glucose can exist as one of two epimers, depending on how its ring closes when transitioning from its straight chain form to its closed ring form. The anomeric carbon of the glucose molecule can be arranged in one of two ways when its ring closes. The anomeric carbon is the one that goes from being achiral to chiral as the ring closes. In the alpha configuration, the hydroxyl group attached to the anomeric carbon faces down, while in the beta configuration it faces up.
In addition to existing as either an alpha or a beta epimer, glucose also participates in glycosidic linkages using its first, fourth, and sixth carbon atom.
In glycogen, each individual glucose molecule is in the alpha configuration. Thus, we can rule out both answer choices that include beta. Moreover, the fourth carbon atom of each glucose molecule is attached to the first carbon atom (the anomeric carbon) in the next glucose molecule in the straight chain. To make branch points at various points along the straight chain, some of the glucose molecules have their sixth carbon atom attached to the anomeric carbon of other glucose molecules.
In conclusion, glycosidic bonds are responsible for branch points and
glycosidic bonds are responsible for the straight chain.
Example Question #1 : Glycosidic Linkages
Glycogen is not a single chain of glucose units, but many chains branching off of one another. Why is the branching of glycogen important?
All of these are reasons why glycogen branching is important
Branching increases the rate of glycogen synthesis
Branching increases glycogen solubility
Branching makes glycogen more compact
Branching increases the rate of glycogen degradation
All of these are reasons why glycogen branching is important
Because glycogen is so heavily branched, it is able to pack more glucose units together in a small space, thus it is more compact and has a greater solubility. Moreover, the branching allows for glycogen enzymes to act more efficiently on the chains of glucose, and so both degradation and synthesis have increased rates.
Example Question #1 : Glycosidic Linkages
You discover that your patient is lactose intolerant, having a mutation that does not allow them to produce an enzyme that cleaves the disaccharide lactose. If they had the lactase enzyme, which glycosidic bond would it cleave?
Glucose-alpha 1,2-fructose
Galactose-alpha-1,4-glucose
Glucose-alpha-1,4-glucose
Galactose-beta-1,4-glucose
Glucose-beta 1,4-glucose
Galactose-beta-1,4-glucose
lactose is made up of galactose and glucose and is bound via a beta 1,4 glycosidic bond.
the enzyme lactase cleaves this bond to break down the sugar lactose. Maltose is glucose- alpha 1,4- glucose, and sucrose is glucose- alpha, 1,2- fructose.
Example Question #1 : Carbohydrate Structures And Functions
What is an aldotriose?
A monosaccharide that contains both an aldehyde and three carbons
A disaccharide that contains three aldehydes and one carbon
A monosaccharide that contains three aldehydes and one carbon
A disaccharide that contains an aldehyde and three carbons
A monosaccharide that contains both an aldehyde and three carbons
Aldotrioses are monosaccharides that contain both an aldehyde (an aldose) and three carbons (a triose). Knowing the definition of the word, and the breakdown of parts of the word, can help you recognize the molecule. The simplest aldotriose is glyceraldehyde.
A related concept involves ketotrioses, which are monosaccharides that contain both a ketone (a ketose) and three carbonds (a triose). Dihydroxyacetone is an example of a ketotriose.
Example Question #1 : Carbohydrate Structures And Functions
Mirror-image stereoisomers are called __________.
diastereomers
epimers
anomers
enantiomers
enantiomers
Enantiomers are chiral molecules that are non-superimposable mirror images of each other. Diastereomers result when two or more stereoisomers of a compound have different configurations at one or more (but not all) of the equivalent stereocenters and are not mirror images of each other. An epimer is one of two stereoisomers that differ in configuration at only one stereocenter. An anomer is a type of epimer; it is one of two stereoisomers of a cyclic sugar that differs only in its configuration at the hemiacetal or acetal carbon (the anomeric carbon).
Example Question #201 : Biochemistry
Why is it that reducing sugars can be metabolized in humans, but non-reducing sugars cannot?
Because only reducing sugars can traverse the cell membrane in order to enter cells where they can be metabolized, whereas non-reducing sugars cannot
Because reducing sugars can open their cyclic structure into the straight chain form, whereas non-reducing sugars cannot
Because reducing sugars can bind to the proteins needed for metabolism, whereas non-reducing sugars cannot
Because humans lack the enzyme that degrades beta glycosidic linkages
Because reducing sugars can open their cyclic structure into the straight chain form, whereas non-reducing sugars cannot
When it comes to metabolizing sugars, only reducing sugars are able to undergo breakdown. This is because reducing sugars are able to be converted from their closed chain form into their open chain form. It is only in the open chain form that sugars such as glucose can be metabolized.
Only reducing sugars can be converted into their open chain form. The reason for this is that the anomeric carbon for these sugars is not occupied. In their ring form, such sugars exist as hemiacetals that can readily and reversibly undergo chain opening. Additionally, some hemiketals can be converted into their open chain form, but they need to be able to tautomerize into their aldose form first.
Non-reducing sugars have their anomeric carbon tied up in a bond, and thus are locked in an acetal or ketal form. Consequently, they cannot convert into their open chain form, meaning that they cannot be metabolized.
Example Question #1 : Structural Carbohydrates
Which of the following statements about carbohydrates is true?
None of the other statements is true
Sucrose is a reducing sugar
Amylose makes up the major component of starch by mass
Glucose is a sugar with six hydroxyl groups and an aldehyde
Polysaccharides have glycosidic bonds
Polysaccharides have glycosidic bonds
Glucose has five (not six) hydroxyl groups. Reducing sugars either have an aldehyde group or can form one through isomerism; sucrose doesn’t fit either description. Although there are more amylose molecules than amylopectin in starch, amylose is a minor component by mass; amylopectin makes up 70-80% of starch by mass. Polysaccharides are indeed joined in the union of two oses, which form glycosidic bonds.
Example Question #1 : Carbohydrate Structures And Functions
What functional groups are present on carbohydrates?
Phosphate
Amide
Alcohol
Carboxyl
Hydroxyl
Carboxyl
Carbohydrate chains contain aldehyde or ketone functional groups, which are types of carboxyl groups. Remember the general formula for a carbohydrate is: since they are hydrates (water) of carbon.
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