Biological Macromolecules and Lipids: Carbohydrates

Foundations of Biological Macromolecules

All living organisms are composed of four primary classes of large biological molecules: carbohydrates, lipids, proteins, and nucleic acids. These macromolecules are characterized by their large size and structural complexity. The unique properties of these large biological molecules arise from the precise and orderly arrangement of their atoms. In the study of biology, the relationship between molecular structure and function is considered inseparable; the way a molecule is shaped directly dictates how it behaves and performs within a biological system.

Polymers and the Mechanisms of Polymerization

Most macromolecules are polymers, which are long molecules consisting of many similar or identical building blocks linked by covalent bonds. These individual building blocks are called monomers. Of the four classes of organic molecules essential for life, three are true polymers: carbohydrates, proteins, and nucleic acids. Lipids are the exception, as they are not composed of true monomers that form long polymeric chains.

The chemical reactions involved in the making and breaking of polymers are facilitated by enzymes, which are specialized macromolecules that act as catalysts to speed up chemical processes. The synthesis of a polymer occurs through a dehydration reaction, also known as a condensation reaction. This process takes place when two monomers bond together, resulting in the loss of a water molecule ($H_2O$). Conversely, polymers are disassembled into monomers through hydrolysis. Hydrolysis is essentially the reverse of a dehydration reaction, where the addition of a water molecule ($H_2O$) breaks the covalent bond between monomers. One part of the water molecule (the hydroxyl group, $-OH$) attaches to one monomer, while a hydrogen atom ($H$) attaches to the adjacent monomer.

Structure and Classification of Monosaccharides

Carbohydrates serve as a primary source of fuel and building material for cells. They include both simple sugars and the polymers of these sugars. Monosaccharides are the simplest carbohydrates, often referred to as simple sugars. These molecules generally have molecular formulas that are multiples of the unit $(CH_2O)$. Glucose ($C_6H_{12}O_6$) is the most common and significant monosaccharide in biological systems.

Monosaccharides are classified based on two main criteria: the location of the carbonyl group ($C=O$) and the number of carbons in their skeletal structure. If the carbonyl group is at the end of the carbon chain, the sugar is an aldose (aldehyde sugar). If the carbonyl group is within the carbon skeleton, the sugar is a ketose (ketone sugar). Sugars are further categorized by the number of carbons they contain:

• Trioses (3-carbon sugars, $C_3H_6O_3$): Examples include Glyceraldehyde (an aldose) and Dihydroxyacetone (a ketose). These are initial breakdown products of glucose during cellular respiration.
• Pentoses (5-carbon sugars, $C_5H_{10}O_5$): Examples include Ribose (an aldose), which is a crucial component of RNA, and Ribulose (a ketose), which serves as an intermediate in the process of photosynthesis.
• Hexoses (6-carbon sugars, $C_6H_{12}O_6$): Examples include Glucose, Galactose (aldoses), and Fructose (a ketose). These serve as major energy sources for cells.

While sugars are frequently depicted as linear skeletons, they typically form ring structures in aqueous solutions. Only sugars in solution that exist in their linear form are capable of reducing other molecules due to the presence of a free aldehyde group. Monosaccharides also act as raw materials for the synthesis of other small organic molecules, such as amino acids and fatty acids.

Synthesis and Properties of Disaccharides

A disaccharide is formed when a dehydration reaction joins two monosaccharides through a specific covalent bond known as a glycosidic linkage. Common examples of disaccharides include:

• Maltose: Formed by the union of two glucose molecules via a $1-4$ glycosidic linkage.
• Sucrose: Formed by the union of glucose and fructose via a $1-2$ glycosidic linkage. Sucrose is unique as it is a non-reducing sugar because the linkage involves the reducing groups of both monomers, leaving no free aldehyde or ketone group available.
• Lactose: Formed by the union of glucose and galactose.

Storage Polysaccharides: Starch and Glycogen

Polysaccharides are polymers of hundreds to thousands of monosaccharides joined by glycosidic linkages. They serve two primary roles: storage and structural support. The specific architecture and function of a polysaccharide are determined by its sugar monomers and the positions of its glycosidic linkages.

Plants store surplus glucose in the form of starch, a storage polysaccharide consisting entirely of glucose monomers. Starch is stored as granules within cellular structures known as plastids, including chloroplasts. The simplest form of starch is amylose, which is unbranched. A more complex form is amylopectin, which is a branched polymer with $1-6$ linkages at the branch points. In contrast, animals store glucose in the form of glycogen. Glycogen is highly branched and is stored primarily in the liver and muscle cells. When the demand for sugar increases, hydrolysis of glycogen releases glucose into the bloodstream to fuel cellular activity.

Structural Polysaccharides: Cellulose and Chitin

Cellulose is a structural polysaccharide that serves as a major component of the rigid cell walls in plants. Like starch, cellulose is a polymer of glucose, but the nature of the glycosidic linkages differs significantly. This difference stems from two distinct ring forms for glucose: alpha ($\alpha$) and beta ($\beta$). In starch, all glucose monomers are in the $\alpha$ configuration, leading to a helical shape. In cellulose, the glucose monomers are in the $\beta$ configuration, resulting in a straight and unbranched molecule.

In cellulose, some hydroxyl groups ($-OH$) on the glucose monomers can form hydrogen bonds with the hydroxyl groups of parallel cellulose molecules. These parallel molecules are grouped into units called microfibrils, which provide great strength to plant tissues. Enzymes that digest starch by hydrolyzing $\alpha$ linkages are unable to hydrolyze the $\beta$ linkages of cellulose. Consequently, cellulose passes through the human digestive tract as "insoluble fiber." However, some microbes possess enzymes to digest cellulose, and many herbivores, such as cows and termites, maintain symbiotic relationships with these microbes to process plant material.

Chitin is another important structural polysaccharide found in the exoskeletons of arthropods (such as insects, spiders, and crustaceans). Chitin provides structural support for the cell walls of many fungi as well. In practical applications, chitin is used to manufacture strong and flexible surgical thread that eventually decomposes as the wound heals.

Questions & Discussion

Process Analysis:

• Which picture (A or B) represents dehydration synthesis vs. hydrolysis? In dehydration, water is removed to form polymers. In hydrolysis, water is added to break polymers back into monomers.
• Regarding trioses ($C_3H_6O_3$), pentoses ($C_5H_{10}O_5$), and hexoses ($C_6H_{12}O_6$): What type of molecules are Glyceraldehyde, Ribose, Glucose, and Galactose? These are all Aldoses (aldehyde sugars). How are Dihydroxyacetone, Ribulose, and Fructose different? These are Ketoses (ketone sugars) because their carbonyl group is located within the carbon chain rather than at the end.

Structural Comparisons:

• What is the difference between $\alpha$ and $\beta$ glucose ring structures? The difference lies in the orientation of the hydroxyl group attached to the number 1 carbon. In $\alpha$ glucose, the hydroxyl group is situated below the plane of the ring; in $\beta$ glucose, it is above the plane.
• What shape does cellulose have and why? Cellulose is straight and unbranched. This shape is due to the $\beta$ glycosidic linkages and the resulting hydrogen bonding between parallel strands, forming rigid microfibrils.
• What basic shape do starch (amylose/amylopectin) and glycogen share? They are generally helical/curved in structure due to their $\alpha$ glycosidic linkages.
• Summary Question: What type of linkage do starch and glycogen share in common? Both utilize $\alpha-1,4$ glycosidic linkages for the main chain. Which is more branched? Glycogen is significantly more branched than amylopectin (starch).

Chemical Properties:

• Where is the free aldehyde group in sucrose? Why is sucrose non-reducing? Sucrose does not have a free aldehyde or ketone group because the glycosidic bond forms between the reducing ends of both glucose and fructose. Without a free carbonyl group to open into a linear form, it cannot reduce other molecules.