2.8
Chemistry of Carbohydrates
Carbohydrates are an essential part of the diet in humans and animals. Grains, fruits, and vegetables are natural sources of carbohydrates that provid…
Carbohydrates are compounds composed of carbon, hydrogen, and oxygen. These molecules get their name from the empirical formula of many monosaccharides that have two atoms of hydrogen and one atom of oxygen for every carbon.
Simple carbohydrates are monomers called monosaccharides and dimers called disaccharides. Complex carbohydrates are polymers called polysaccharides.
A monosaccharide can be classified as an aldose or a ketose by its functional group. Those having an aldehyde group are called aldoses and those having a ketone group are called ketoses.
Monosaccharides usually contain three to seven carbon atoms in their chains. Each of the carbons can be numbered starting with the number one from the end closest to the carbonyl carbon.
Sugars can form non-planar ring structures and in nature mostly exist in this cyclic form. Ring structures form when the carbonyl group reacts with a hydroxyl group at the opposite end of the molecule through a condensation reaction. The carbon attached to the reactive aldehyde or ketone is also known as the anomeric carbon.
These carbon-oxygen ring structures can exist as two anomers: alpha and beta. Anomers are structural isomers that differ in the configuration at the carbonyl carbon. In the alpha form, the hydroxyl group on the anomeric carbon and the CH2OH group on the last carbon within the ring are located on opposite sides of the ring, and in the beta form, they are located on the same side.
A similar naming convention is used for bonds linking the monosaccharides to other molecules.
Bonds formed below the ring structure are known as alpha linkages, whereas those above the ring are known as beta linkages. The numbers in the name of glycosidic linkages indicate the carbon numbers from the monosaccharides that are involved in the linkages.
When two monosaccharides link together through glycosidic linkages, they form disaccharides.
Glycosidic linkages are covalent bonds that are formed when the hydroxyl group of one monosaccharide reacts with the anomeric carbon of another monosaccharide with the elimination of a water molecule.
Polysaccharides or complex carbohydrates are a chain of monosaccharides held together by covalent bonds. Variation in the glycosidic linkages present in these molecules can result in different types of polysaccharides from the same monosaccharide.
Glucose molecules linked through β-1,4-glycosidic bonds form cellulose, a water insoluble polysaccharide. Glucose molecules linked through α-1,4-glycosidic bonds form amylose, a water-soluble polymer and a component of starch. Starch and cellulose are polysaccharides found in plants.
Glycogen is a water-insoluble molecule made of glucose linked through α-1,4-glycosidic bonds and intermittent branching thorough α-1,6-glycosidic bonds. Glycogen serves as a storage polysaccharide in animals.
View the full transcript and gain access to JoVE Core videos
Q1: What are the main structural components of carbohydrates?
Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen atoms arranged in specific ratios. They contain carbonyl groups and multiple hydroxyl groups that define their structure. These functional groups enable carbohydrates to form various configurations, from simple sugars to complex polymers used for energy storage and structural support in cells.
Q2: How do monosaccharides link together to form polysaccharides?
Monosaccharides join through condensation reactions that form glycosidic bonds between adjacent sugar units. This process removes water molecules and creates covalent linkages. Polysaccharides like starch and cellulose result from repeating this bonding pattern, creating long chains that function as energy storage or structural materials through polymers addition and condensation polymerization.
Q3: What distinguishes glucose, fructose, and galactose from each other?
Glucose, fructose, and galactose are isomers with identical molecular formulas but different structural arrangements. Glucose is an aldose sugar with an aldehyde group, while fructose is a ketose with a ketone group. Galactose differs from glucose in the spatial orientation of one hydroxyl group, creating distinct chemical properties and metabolic roles despite their shared composition.
Q4: Why are carbohydrates important for cellular energy and structure?
Carbohydrates serve dual roles in cells: they provide rapid energy through glucose metabolism and form structural frameworks in cell walls and connective tissues. Polysaccharides like glycogen store energy efficiently, while cellulose provides mechanical strength. Their abundant hydroxyl groups enable noncovalent attractions in biomolecules, facilitating interactions with proteins and other cellular components.
Q5: What is the difference between disaccharides and polysaccharides?
Disaccharides consist of two monosaccharide units joined by a single glycosidic bond, such as sucrose or lactose. Polysaccharides contain many monosaccharide units linked through multiple glycosidic bonds, forming long chains like starch or cellulose. This structural difference determines their solubility, digestibility, and biological function in energy storage versus structural support.
Q6: How do carbohydrates interact with other biomolecules in cells?
Carbohydrates interact with proteins through hydrogen bonding and van der Waals forces, forming glycoproteins essential for cell recognition and signaling. Their hydroxyl groups participate in noncovalent interactions that stabilize three-dimensional structures. These interactions enable carbohydrates to function in cell surface markers, enzyme cofactors, and regulatory molecules throughout cellular processes.
Q7: What role do carbohydrates play in photosynthesis and cellular respiration?
Carbohydrates are the primary products of photosynthesis, where plants convert light energy into glucose. During cellular respiration, glucose is oxidized to release energy stored in chemical bonds, producing ATP for cellular work. This cyclical relationship makes carbohydrates central to energy flow through living systems, connecting photosynthetic organisms to consumers.