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Explores the structure of sugars and how they are linked to provide energy and structural support to cells.
Why can a marathon runner power through 26 miles on a bowl of pasta, yet a cow can grow to 1,000 pounds eating nothing but grass—something humans can't even digest?
Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically in a ratio. The simplest units are monosaccharides, or single sugars. The most biologically significant is glucose (), which serves as the primary fuel for cellular respiration. Monosaccharides can exist as linear chains but often form ring structures in aqueous solutions. Interestingly, molecules like glucose and galactose share the same chemical formula but differ in the spatial arrangement of their atoms; these are called isomers. This subtle difference in shape determines how they interact with enzymes and other molecules.
Quick Check
If two sugars have the same chemical formula but different shapes, what are they called?
Answer
Isomers
When two monosaccharides join, they form a disaccharide through a process called dehydration synthesis (or a condensation reaction). During this reaction, a hydroxyl group () from one sugar combines with a hydrogen atom from another, releasing a molecule of and forming a covalent bond known as a glycosidic linkage. Common examples include sucrose (table sugar), formed from glucose and fructose, and lactose (milk sugar), formed from glucose and galactose. The type of linkage (labeled or ) depends on the orientation of the group on the first carbon.
1. Take two molecules of glucose (). 2. Remove one molecule of during the bonding process. 3. The resulting formula for the disaccharide maltose is .
Polysaccharides are long chains of monosaccharides. In the biological world, they serve two main purposes: storage and structure. Starch is the storage form of glucose in plants, consisting of two types: amylose (unbranched) and amylopectin (somewhat branched). In contrast, animals store glucose as glycogen in the liver and muscle tissues. Glycogen is highly branched, which provides more 'ends' for enzymes to attach to, allowing for the rapid release of glucose when energy demands spike during exercise.
Quick Check
Why is the high degree of branching in glycogen advantageous for animals compared to the lower branching in plant starch?
Answer
It allows for faster mobilization and release of glucose because more enzymes can work on the multiple 'ends' of the branches simultaneously.
While starch uses -glucose, cellulose is built from **-glucose monomers. This small change in the glycosidic bond means every other glucose molecule is 'upside down' relative to its neighbor. This allows cellulose chains to remain straight and pack tightly together. These parallel chains form hydrogen bonds with one another, creating tough, insoluble fibers called microfibrils**. These microfibrils provide the incredible tensile strength found in plant cell walls, allowing trees to grow hundreds of feet tall without a skeleton.
1. Enzymes are shape-specific. Human amylase can break -1,4-glycosidic bonds found in starch. 2. However, the -1,4-glycosidic bonds in cellulose require a specific enzyme called cellulase. 3. Because humans lack cellulase, cellulose passes through our system as 'insoluble fiber,' while cows rely on symbiotic bacteria in their gut to break it down.
Which of the following describes the formation of a glycosidic linkage?
What structural feature makes cellulose different from starch?
Glycogen is more highly branched than amylopectin.
Review Tomorrow
In 24 hours, try to sketch the difference between an alpha-linkage (found in starch) and a beta-linkage (found in cellulose) and explain how they affect the molecule's shape.
Practice Activity
Look at a nutrition label for a 'high fiber' cereal. Identify which ingredients likely provide the cellulose (fiber) and which provide the starch (sugars/carbs).