2.5 Carbohydrates

Carbohydrates are vital molecules in living organisms, serving as energy sources and structural components. They come in various forms, from simple sugars like glucose to complex polysaccharides like starch and cellulose, each with unique functions in biological systems.

In the chemistry of life, carbohydrates play crucial roles in energy storage, cellular structure, and metabolic processes. Their ability to form diverse structures and participate in key reactions makes them essential for understanding how living organisms function and interact with their environment.

Carbohydrate Structure and Classification

Composition and Types of Carbohydrates

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• Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen atoms, typically in a ratio of 1:2:1 respectively
• Monosaccharides are the simplest form of carbohydrates and serve as the building blocks for more complex carbohydrates
  • They contain three to seven carbon atoms and cannot be hydrolyzed into simpler sugars
  • Glucose, fructose, and galactose are common examples of monosaccharides with six carbon atoms (hexoses)
  • Ribose and deoxyribose are pentose monosaccharides with five carbon atoms and are essential components of nucleic acids
• Disaccharides are formed when two monosaccharides are joined together by a glycosidic bond, which is formed through a dehydration synthesis reaction
  • Maltose (glucose + glucose), lactose (glucose + galactose), and sucrose (glucose + fructose) are common examples of disaccharides, each composed of specific monosaccharide combinations
• Polysaccharides are long chains of monosaccharides linked together by glycosidic bonds, often containing hundreds or thousands of monosaccharide units
  • Polysaccharides can be linear or branched and serve various functions in living organisms, such as energy storage (starch, glycogen) and structural components (cellulose, chitin)

Monosaccharide Structure and Examples

• Monosaccharides are classified based on the number of carbon atoms they contain and the functional group present
  • Aldoses have an aldehyde group at the end of the carbon chain (glucose, galactose, ribose)
  • Ketoses have a ketone group within the carbon chain (fructose)
• The carbon atoms in a monosaccharide are numbered starting from the end closest to the aldehyde or ketone group
• Monosaccharides can exist in different isomeric forms, such as D- and L-isomers, which differ in the spatial orientation of their hydroxyl groups
  • Most naturally occurring monosaccharides are D-isomers (D-glucose, D-fructose)
• Monosaccharides can also form cyclic structures when the aldehyde or ketone group reacts with a hydroxyl group on the opposite end of the molecule, creating a ring structure (pyranose or furanose)

Carbohydrate Functions in Organisms

Energy Storage and Production

• Carbohydrates serve as the primary source of energy for living organisms, providing 4 kilocalories (kcal) of energy per gram when oxidized during cellular respiration
• Glucose is the most common monosaccharide used for energy production in cells and is the primary substrate for cellular respiration and fermentation processes
• Polysaccharides like starch in plants and glycogen in animals serve as long-term energy storage molecules, allowing organisms to store excess glucose for later use
  • When energy is needed, these polysaccharides can be broken down into their constituent monosaccharides and used in cellular respiration

Structural and Functional Roles

• Structural polysaccharides, such as cellulose in plant cell walls and chitin in fungal cell walls and arthropod exoskeletons, provide mechanical support and protection for cells and organisms
  • Cellulose is the most abundant organic polymer on Earth and is a major component of wood, cotton, and paper
• Some carbohydrates, such as glycoproteins and glycolipids, play crucial roles in cell signaling, cell-cell recognition, and immune system function
  • Glycoproteins are proteins with attached carbohydrate chains that can function as receptors, hormones, or structural components (mucins, antibodies)
  • Glycolipids are lipids with attached carbohydrate chains that are important components of cell membranes and can participate in cell signaling and recognition (blood group antigens)

Starch, Glycogen, and Cellulose: Structures and Functions

Starch: Plant Energy Storage

• Starch is a plant polysaccharide composed of two types of glucose polymers: amylose (linear) and amylopectin (branched)
  • Amylose consists of long, unbranched chains of glucose monomers connected by α-1,4 glycosidic bonds
  • Amylopectin has a branched structure with α-1,4 glycosidic bonds forming the linear chains and α-1,6 glycosidic bonds forming the branch points
• Starch serves as the primary energy storage molecule in plants, particularly in seeds, roots, and tubers
  • When energy is needed, starch can be hydrolyzed by enzymes (amylases) into glucose monomers for use in cellular respiration
• Starch is an important component of the human diet, found in foods such as potatoes, rice, wheat, and corn

Glycogen: Animal Energy Storage

• Glycogen is an animal polysaccharide composed of highly branched chains of glucose monomers
  • Similar to amylopectin, glycogen has α-1,4 glycosidic bonds forming the linear chains and α-1,6 glycosidic bonds forming the branch points
  • However, glycogen is more highly branched than amylopectin, with branch points occurring more frequently
• Glycogen is the primary energy storage molecule in animals, found mainly in the liver and skeletal muscle cells
  • The highly branched structure of glycogen allows for rapid release of glucose monomers when energy is needed
  • Glycogen breakdown (glycogenolysis) is regulated by hormones such as glucagon and epinephrine to maintain blood glucose levels

Cellulose: Plant Structural Support

• Cellulose is a structural polysaccharide found in plant cell walls, composed of linear chains of glucose monomers linked by β-1,4 glycosidic bonds
  • The β-1,4 linkages in cellulose result in a linear, fibrous structure that provides strength and rigidity to plant cell walls
  • Cellulose molecules are arranged in parallel, forming microfibrils that are further bundled into macrofibrils
• Cellulose is the most abundant organic polymer on Earth and is a major component of wood, cotton, and paper
• Most animals cannot digest cellulose due to the lack of enzymes capable of breaking the β-1,4 glycosidic bonds, but it serves as an important dietary fiber for herbivores and omnivores
  • Some microorganisms, such as bacteria and fungi, can produce cellulase enzymes that break down cellulose into glucose monomers

Carbohydrates in Cellular Respiration and Photosynthesis

Cellular Respiration: Glucose as an Energy Source

• Glucose is the primary substrate for cellular respiration, a process that generates ATP (adenosine triphosphate) to power cellular functions
  • During glycolysis, glucose is broken down into pyruvate, releasing a small amount of ATP and reducing power in the form of NADH
  • In the presence of oxygen, pyruvate enters the citric acid cycle and electron transport chain, generating a substantial amount of ATP through oxidative phosphorylation
• The overall equation for the complete oxidation of glucose during cellular respiration is:
  • $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)}$
• Cellular respiration is a highly efficient process, yielding approximately 30-32 ATP molecules per glucose molecule oxidized

Photosynthesis: Carbohydrate Synthesis

• In photosynthesis, light energy is used to convert carbon dioxide and water into glucose and oxygen, storing energy in the chemical bonds of carbohydrates
  • The overall equation for photosynthesis is:
    • $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$
  • The light-dependent reactions of photosynthesis generate ATP and NADPH, which are used in the Calvin cycle to reduce carbon dioxide and synthesize glucose
• The glucose produced during photosynthesis can be used directly for energy, converted into other organic compounds, or stored as starch for later use by the plant
• Photosynthesis is the primary source of organic compounds and energy for most ecosystems, as it converts light energy into chemical energy that can be used by other organisms

Interconnectedness of Cellular Respiration and Photosynthesis

• The interplay between photosynthesis and cellular respiration in ecosystems is crucial for the cycling of carbon and the flow of energy through food chains and food webs
  • Photosynthetic organisms (producers) convert light energy into chemical energy stored in carbohydrates, which are then consumed by heterotrophs (consumers) and used in cellular respiration
  • The carbon dioxide released by cellular respiration is used by photosynthetic organisms to produce more carbohydrates, completing the carbon cycle
• The balance between photosynthesis and cellular respiration helps maintain the oxygen and carbon dioxide levels in the atmosphere and is essential for the sustainability of life on Earth