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What is the primary energy source in the cells of mammals?
Which of the following substances is considered a monosaccharide?
Why does the body store excess glucose as glycogen?
What type of carbohydrate is formed by the condensation of two monosaccharides?
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Which carbohydrate serves as a precursor for the synthesis of other carbohydrates in the body?
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Which of the following carbohydrates is a storage form of energy in plants?
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What percentage of energy in cells comes from carbohydrates?
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What term is used for carbohydrates that cannot be hydrolyzed into simpler carbohydrates?
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What are D and L enantiomers primarily determined by?
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What is the defining feature of α- and β-anomers?
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Which type of isomerism involves monosaccharides that differ by the position of the hydroxyl group on one specific carbon?
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Which two structures do monosaccharides predominantly exist in when they cyclize in solution?
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Which statement about D and L configurations of monosaccharides is true?
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Which of the following monosaccharides is often referred to as dextrose?
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What is the significance of ribose in the body?
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What condition arises from the deficiency of an enzyme needed to metabolize galactose?
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Which type of sugar is known as a reducing sugar?
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What type of isomerism is exhibited by aldoses and ketoses?
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What is the main physiological importance of fructose?
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Which monosaccharide is derived from the hydrolysis of lactose?
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Which sugar is known as levulose?
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What is one way monosaccharides are classified?
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Which prefix is used for monosaccharides with three carbon atoms?
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Which type of monosaccharide contains a carbonyl group at the end of the carbon chain?
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How are ketoses categorized in terms of carbonyl group position?
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What is a common feature of almost all monosaccharides?
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What type of structure can glucose be represented in?
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Which type of monosaccharide is an example of a pentose?
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What distinguishes an aldotriose from a ketotriose?
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Which compound is an exception to the rule about monosaccharides containing a chiral center?
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Which of the following best describes 'optical activity' in monosaccharides?
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What does the oxidation at carbon 6 produce?
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Which of the following conditions can lead to cataract formation due to high levels of a specific sugar alcohol?
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What type of bond connects the two components in a glycoside?
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What is the composition of maltose?
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Which enzyme is responsible for hydrolyzing sucrose into glucose and fructose?
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What type of bond connects galactose and glucose in lactose?
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Which of the following symptoms is commonly associated with lactose intolerance?
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What are oligosaccharides typically found as?
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Study Notes
Monosaccharide Classification
- Monosaccharides are classified based on the number of carbon atoms and the type of carbonyl group present.
- Three-carbon monosaccharides are called trioses, four-carbon monosaccharides are called tetroses, five-carbon monosaccharides are called pentoses, six-carbon monosaccharides are called hexoses, and seven-carbon monosaccharides are called heptoses.
- A carbonyl group at the end of a monosaccharide chain forms an aldehyde, resulting in an aldose.
- A carbonyl group in the middle of a monosaccharide chain forms a ketone, resulting in a ketose.
- Examples include aldotriose and ketotriose.
Glucose Structure
- Glucose structure can be represented in three ways: straight chain form, Haworth projection, and chair form.
Asymmetric Carbon Atom
- Monosaccharides, except for dihydroxyacetone (DHA), contain at least one asymmetric carbon atom (chiral center).
- This leads to optical activity, the ability to rotate plane-polarized light.
Carbohydrate Definition
- The term "carbohydrate" literally means hydrates of carbon.
- Initially, carbohydrates were thought to have the molecular formula (CH2O)n, suggesting a ratio of H to O as in water (H2O).
- However, not all carbohydrates obey this rule, as there are derivatives that do not follow this formula.
Biochemical Definition of Carbohydrates
- Carbohydrates are poly-hydroxy carbonyl compounds (aldehyde or ketone), their derivatives, and the polymers that yield these compounds upon hydrolysis.
Biomedical Importance of Carbohydrates
- Carbohydrates play crucial structural and metabolic roles.
- Glucose is a critical carbohydrate serving as the major metabolic fuel for mammals, a universal fuel for the fetus, and the primary form in which dietary carbohydrates are absorbed into the bloodstream.
Specific Roles of Carbohydrates
- Energy source: Carbohydrates are a primary energy source for plants and animals, with glucose being a significant contributor.
- Cell membrane components: Carbohydrates are components of glycolipids and glycoproteins, which are integral parts of cell membranes.
- Structural tissues: Carbohydrates contribute to structural tissues in plants and microorganisms, including cellulose, chitin, and murein.
- Precursors for other carbohydrates: Glucose is the precursor for the synthesis of other carbohydrates in the body, including galactose in milk, glycogen for storage, ribose, and deoxyribose in nucleic acids.
- Disease mechanisms: Understanding carbohydrates is crucial for understanding diseases related to carbohydrate metabolism, such as diabetes mellitus, lactose intolerance, glycogen storage diseases, and galactosemia.
Classification of Carbohydrates
- Carbohydrates are classified into four groups:
- Monosaccharides: The simplest form of carbohydrates, they cannot be hydrolyzed into simpler carbohydrates. Examples include glucose.
- Disaccharides: Formed by the condensation of two monosaccharide units. Examples include lactose.
- Oligosaccharides: Composed of three to ten monosaccharide units. An example is maltotriose.
- Polysaccharides: Composed of more than ten monosaccharide units. Examples include glycogen and starch.
Monosaccharide Isomerism
- Monosaccharides exhibit various forms of isomerism due to the presence of asymmetric carbon atoms.
- D and L Enantiomers: Monosaccharides exist in either D or L configurations, determined by the orientation of the hydroxyl group on the asymmetric carbon farthest from the carbonyl group.
- Pyranose and Furanose ring structures: Five- or six-carbon monosaccharides tend to cyclize in solution, forming ring structures resembling pyran or furan.
- Alpha and Beta Anomers: Isomers differing in the position of the hydroxyl group around the anomeric carbon (originally the carbonyl carbon) when the ring is formed.
- Epimers: Monosaccharides differing in the position of the hydroxyl group on a single carbon atom (e.g., carbon 2, 3, and 4 of glucose). For example, glucose and galactose are epimers at carbon 4.
- Aldose and Ketose isomerism: This type of isomerism distinguishes between aldoses and ketoses based on the position of the carbonyl group.
- Optical activity: Monosaccharides have the ability to rotate plane-polarized light either to the right (dextrorotatory) or to the left (levorotatory).
Physiological Importance of Pentoses
- Ribose is a key component of nucleic acids.
Physiological Importance of Hexoses
- D-Glucose: Also known as grape sugar or dextrose, it is the primary sugar used by tissues and is a building block for disaccharides (such as sucrose, lactose, and maltose) and polysaccharides (starch, cellulose, and glycogen). It is also used for the synthesis of ribose and deoxyribose for DNA and RNA.
- D-Galactose: Not found in free form, it is formed by lactose hydrolysis. It is converted to glucose and used for milk synthesis and is found in cell membranes of the brain and nervous system. Galactosemia is a deficiency disease characterized by the absence of an enzyme for metabolizing galactose, leading to high levels in the blood, mental retardation, cataracts, and cirrhosis.
- D-Fructose: Also known as levulose or fruit sugar, it is found in fruit juice and honey and is formed by the hydrolysis of cane and beet sugar. It is converted to glucose in the liver and is a source of energy for spermatozoa. Due to its sweetness, it is used as an artificial sweetener.
Monosaccharide Derivatives
- Amino sugars: A hydroxyl group is replaced by an amino group (e.g., glucosamine). They are found in glycoproteins, glycolipids, and glycosaminoglycans (heteropolysaccharides), and some antibiotics like erythromycin.
- Phosphate esters: They are involved in metabolic processes.
- Aldonic acids: Oxidation of glucose at carbon 1 forms gluconic acid, the basis of Benedict and Fehling's reactions for detecting glucose in urine. Aldonic acids, along with ketoses, are reducing sugars. Non-reducing sugars have anomeric carbons bonded to another molecule, preventing their oxidation.
- Uronic acids: Oxidation at carbon 6 forms glucuronic acid, crucial for detoxification reactions and in glycosaminoglycans.
- Sugar alcohols: Reduction of glucose forms sorbitol, which can lead to cataracts in diabetes.
Glycosides
- A glycoside is formed when two substances are connected by a glycosidic bond.
- Their importance lies in:
- The formation of disaccharides and other derivatives.
- Cardiac glycosides, drugs used for heart conditions, such as digitalis and ouabain.
- Certain antibiotics, such as streptomycin.
Disaccharides
- The physiologically important disaccharides are maltose, sucrose, and lactose.
Disaccharide Formation
- Disaccharides form via a condensation reaction between two monosaccharides, releasing a water molecule.
Maltose
- Also called malt sugar, maltose is composed of two glucose monomers linked by an α-(1,4) glycosidic bond.
- It is a major degradation product of starch.
- It is hydrolyzed by the enzyme maltase to produce two glucose molecules.
Sucrose
- Sucrose is table sugar found in cane and beet sugar.
- It is composed of glucose and fructose linked by an α-(1,2)b-glycosidic bond.
- Hydrolyzed by the enzyme sucrase (invertase) to produce glucose and fructose.
- It is important to note that the product of sucrose hydrolysis, known as invert sugar, reverses the optical activity from dextrorotatory to levorotatory.
Lactose
- Lactose is milk sugar found exclusively in the milk of mammals.
- It is composed of galactose and glucose linked by a β-(1,4) glycosidic bond.
- It is hydrolyzed by the enzyme lactase to produce glucose and galactose.
Lactose Intolerance
- This condition occurs due to lactase deficiency, leading to bloating, abdominal discomfort, and diarrhea.
Oligosaccharides
- They are usually not found free but in the form of glycoproteins and glycolipids.
- Notable examples include maltotriose and raffinose.
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Description
This quiz covers the classification of monosaccharides based on their carbon atoms and carbonyl groups, along with the structural representations of glucose. You will also explore the concept of asymmetric carbon atoms and their significance in optical activity. Test your knowledge on these essential components of carbohydrates.
