Trioses, pentoses, and hexoses have three, five, and six carbon backbones, respectively. The chemical formula for glucose is C 6 H 12 O 6. In humans, glucose is an important source of energy. During cellular respiration, energy is released from glucose, and that energy is used to help make adenosine triphosphate ATP. Plants synthesize glucose using carbon dioxide and water, and glucose in turn is used for energy requirements for the plant. Excess glucose is often stored as starch that is catabolized the breakdown of larger molecules by cells by humans and other animals that feed on plants.
Galactose and fructose are other common monosaccharides — galactose is found in milk sugars and fructose is found in fruit sugars. Although glucose, galactose, and fructose all have the same chemical formula C 6 H 12 O 6 , they differ structurally and chemically and are known as isomers because of the different arrangement of functional groups around the asymmetric carbon; all of these monosaccharides have more than one asymmetric carbon Figure 2. Figure 2. Glucose, galactose, and fructose are all hexoses.
They are structural isomers, meaning they have the same chemical formula C6H12O6 but a different arrangement of atoms.
Monosaccharides can exist as a linear chain or as ring-shaped molecules; in aqueous solutions they are usually found in ring forms Figure 3.
Figure 3. Five and six carbon monosaccharides exist in equilibrium between linear and ring forms. Fructose and ribose also form rings, although they form five-membered rings as opposed to the six-membered ring of glucose. During this process, the hydroxyl group of one monosaccharide combines with the hydrogen of another monosaccharide, releasing a molecule of water and forming a covalent bond.
A covalent bond formed between a carbohydrate molecule and another molecule in this case, between two monosaccharides is known as a glycosidic bond Figure 4. Glycosidic bonds also called glycosidic linkages can be of the alpha or the beta type. An alpha bond is formed when the OH group on the carbon-1 of the first glucose is below the ring plane, and a beta bond is formed when the OH group on the carbon-1 is above the ring plane. Figure 4. Sucrose is formed when a monomer of glucose and a monomer of fructose are joined in a dehydration reaction to form a glycosidic bond.
In the process, a water molecule is lost. By convention, the carbon atoms in a monosaccharide are numbered from the terminal carbon closest to the carbonyl group. In sucrose, a glycosidic linkage is formed between carbon 1 in glucose and carbon 2 in fructose. The ketose triose is called dihydroxyacetone, and has the oxygen double-bonded to the center carbon:. The aldose is called glyceraldehyde, and can have the oxygen double-bonded to the first or third carbon of the molecule:.
Increase Font Size. Biology Go to home Biological Macromolecules 4. Previous: Synthesis of Biological Macromolecules. Next: Lipids. Learning Objectives By the end of this section, you will be able to do the following: Discuss the role of carbohydrates in cells and in the extracellular materials of animals and plants Explain carbohydrate classifications List common monosaccharides, disaccharides, and polysaccharides. Molecular Structures The stoichiometric formula CH 2 O n , where n is the number of carbons in the molecule represents carbohydrates.
Scientists classify monosaccharides based on the position of their carbonyl group and the number of carbons in the backbone.
Aldoses have a carbonyl group indicated in green at the end of the carbon chain, and ketoses have a carbonyl group in the middle of the carbon chain. Trioses, pentoses, and hexoses have three-, five-, and six- carbon backbones, respectively. Art Connection.
Glucose, galactose, and fructose are all hexoses. They are structural isomers, meaning they have the same chemical formula C 6 H 12 O 6 but a different atom arrangement.
Five and six carbon monosaccharides exist in equilibrium between linear and ring forms. Fructose and ribose also form rings, although they form five-membered rings as opposed to the six-membered ring of glucose. Sucrose forms when a glucose monomer and a fructose monomer join in a dehydration reaction to form a glycosidic bond.
In the process, a water molecule is lost. By convention, the carbon atoms in a monosaccharide are numbered from the terminal carbon closest to the carbonyl group.
In sucrose, a glycosidic linkage forms between carbon 1 in glucose and carbon 2 in fructose. Common disaccharides include maltose grain sugar , lactose milk sugar , and sucrose table sugar. Amylose and amylopectin are two different starch forms. Because of the way the subunits are joined, the glucose chains have a helical structure. Glycogen not shown is similar in structure to amylopectin but more highly branched. Because of the way the glucose subunits are joined, every glucose monomer is flipped relative to the next one resulting in a linear, fibrous structure.
Insects have a hard outer exoskeleton made of chitin, a type of polysaccharide. Career Connections. Benefits of Carbohydrates Are carbohydrates good for you? Link to Learning. Section Summary Carbohydrates are a group of macromolecules that are a vital energy source for the cell and provide structural support to plant cells, fungi, and all of the arthropods that include lobsters, crabs, shrimp, insects, and spiders. Art Connections Figure What kind of sugars are these, aldose or ketose?
Free Response Describe the similarities and differences between glycogen and starch. Why is it impossible for humans to digest food that contains cellulose? The ketose triose is called dihydroxyacetone, and has the oxygen double-bonded to the center carbon: The aldose is called glyceraldehyde, and can have the oxygen double-bonded to the first or third carbon of the molecule:.
Glucose in a ring form can have two different arrangements of the hydroxyl group OH around the anomeric carbon carbon 1 that becomes asymmetric in the process of ring formation. During this process, the hydroxyl group of one monosaccharide combines with the hydrogen of another monosaccharide, releasing a molecule of water and forming a covalent bond.
A covalent bond formed between a carbohydrate molecule and another molecule in this case, between two monosaccharides is known as a glycosidic bond Figure.
Glycosidic bonds also called glycosidic linkages can be of the alpha or the beta type. Common disaccharides include lactose, maltose, and sucrose Figure. Lactose is a disaccharide consisting of the monomers glucose and galactose.
It is found naturally in milk. Maltose, or malt sugar, is a disaccharide formed by a dehydration reaction between two glucose molecules. The most common disaccharide is sucrose, or table sugar, which is composed of the monomers glucose and fructose. The chain may be branched or unbranched, and it may contain different types of monosaccharides. The molecular weight may be , daltons or more depending on the number of monomers joined.
Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides. Starch is the stored form of sugars in plants and is made up of a mixture of amylose and amylopectin both polymers of glucose. The starch in the seeds provides food for the embryo as it germinates and can also act as a source of food for humans and animals. The starch that is consumed by humans is broken down by enzymes, such as salivary amylases, into smaller molecules, such as maltose and glucose.
The cells can then absorb the glucose. The numbers and refer to the carbon number of the two residues that have joined to form the bond. Glycogen is the storage form of glucose in humans and other vertebrates and is made up of monomers of glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen is broken down to release glucose in a process known as glycogenolysis.
Cellulose is the most abundant natural biopolymer. The cell wall of plants is mostly made of cellulose; this provides structural support to the cell.
Wood and paper are mostly cellulosic in nature. As shown in Figure , every other glucose monomer in cellulose is flipped over, and the monomers are packed tightly as extended long chains. This gives cellulose its rigidity and high tensile strength—which is so important to plant cells. They consist of a glycerolphosphate unit that is esterified with fatty acid tails on their C1 and C2 positions.
The C3 position can house a variety of functional groups, such as ethanolamine, choline, or carbohydrates such as inositol. These Glycerophospholipids have a variety of functions and can be very complex in their structure. Each of these structures leads to a variety of key functions for the cell. A clinical correlate to glycerophospholipid structure that illustrates a complex lipid is Dipalmitoylphosphatidylcholine DPPC , which is the major lipid of lung surfactant.
It is a protein-lipid mixture that is essential for normal pulmonary function, where it resides on the surfaces of the cells that form the alveoli. Premature-born infants are at risk of developing respiratory distress syndrome , characterized by difficulty breathing due to alveolar collapse.
Biopolymers can be built from constituent monomers or broken down into constituent monomers through the process of anabolism or catabolism, respectively. Condensation reactions are the chemical process by which two molecules are joined with the loss of water , and is the process by which carbohydrates, proteins, nucleic acids, and proteins are synthesized from simpler subunits.
Because water is lost, this process can also be called dehydration synthesis. Hydrolysis reactions are those in which the addition of water allows for essentially to the opposite process of condensation to occur, thereby cleaving a larger molecule into smaller substituent molecules. Aoki-Kinoshita, K. Spiro, R. Ninfa, A. Tanford, C. Bloomfield, V.
Watson, J. Edidin, M. Cell Biol. Gurr, M.
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