Introduction to Biochemistry: Carbohydrates

Introduction to Biochemistry

• Course name: Introductory biochemistry
• Lecturer: Nashwa W. Yassa, PhD
• Institution: Alexandria University, Faculty of Science

Carbohydrates

• Carbohydrates are organic molecules found in nature
• These molecules are aldehyde or ketone compounds with multiple hydroxyl groups.
• The basic molecular formula is (CH₂O)n, where n = 3 or more.
• Carbohydrates are polyhydroxylated compounds with at least 3 carbon atoms and a potentially active carbonyl group (aldehydic or ketonic).
• They can be classified as aldoses (containing an aldehyde group) or ketoses (containing a ketone group)
• Often comprised of polyhydroxy compounds
• Carbohydrates are widespread in both animal and plant tissues.

Where do carbohydrates come from?

• Carbohydrates are produced by photosynthesis in plants.
• Photosynthesis uses light energy, carbon dioxide (CO₂), and water (H₂O) to produce sugars (C₆H₁₂O₆) and oxygen (O₂).
• Cellular respiration in animals and plants uses sugars and oxygen to produce carbon dioxide, water, and energy (ATP).

Process of Photosynthesis

• Sunlight, carbon dioxide, and water are used to create sugars and oxygen by plants.

Structure of Photosynthesis

• The equation for photosynthesis is: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
• This means that 6 molecules of carbon dioxide and 6 molecules of water, reacting with light energy, produce one molecule of glucose and 6 molecules of oxygen.

Importance of Carbohydrates

• Carbohydrates are the most abundant organic molecules in nature.
• They serve many functions in living organisms, most notably as an energy source (forming the majority of dietary calories).
• Also components of nucleic acids, crucial for cell membranes and communication, are key structural elements in plants (e.g, cellulose and lignin) and microorganisms (e.g, murein).
• Issues with their metabolism can lead to diabetes.

Classification of Carbohydrates

• Monosaccharides: simplest carbohydrates (e.g., glucose). They cannot be hydrolyzed into simpler units; the most abundant is D-glucose
• Disaccharides: form 2 molecules of monosaccharides when hydrolyzed. (e.g, maltose, lactose, sucrose)
• Oligosaccharides: form 3 to 10 monosaccharide units when hydrolyzed.
• Polysaccharides: form more than 10 monosaccharide units when hydrolyzed.

Nomenclature of Carbohydrates

• Carbohydrates can be named based on the active group (aldehyde or ketone) and the number of carbon atoms.
• Aldoses contain aldehyde groups; ketoses contain ketone groups.
• The number of carbon atoms determines the prefix (e.g., triose, tetrose, pentose, hexose, heptose).

Monosaccharides

• Aldoses (e.g. glucose) typically have an aldehyde group at one end
• Ketoses (e.g. fructose) usually have a keto group at carbon 2

Disaccharides

• Two monosaccharides covalently joined by a glycosidic linkage.
• Can be classified as homo-disaccharides (two similar units, e.g, Maltose) or hetero disaccharides (two different units, e.g., lactose, sucrose).

Oligosaccharides

• Short chains of 3 to 10 monosaccharides

Polysaccharides

• Long chains of monosaccharides
• e.g Starch, Amylopectin, Cellulose

Structural Representations of Sugars

• Straight-chain (open-chain) formula
• Fisher projection formula
• Haworth projection formula

Cyclic Structure of Sugars

• The cyclic structure of glucose accounts for more of its chemical properties, which includes:
• the reaction of the aldehyde group with a specific alcohol group in the same molecule to form a hemiacetal
• The cyclic structure of glucose can be represented as pyranose and furanose rings
• The alpha and beta forms of glucose differ in the position of the hydroxyl group at carbon 1

Isomers

• Isomers have the same chemical formula but a different arrangement of atoms in the structure
• Optical isomers are mirror images (e.g. D and L forms)
• Epimers differ in the configuration around a single carbon atom.

Anomeric Carbon Atom

• The carbon atom part of the carbonyl group is termed as anomers, a specific form of isomerism in cyclic carbohydrates which differ ONLY in the stereochemistry of this carbon.
• the alpha and beta forms differ only in the stereochemical arrangement around the anomeric carbon

Biologically Important Glucose Derivatives

• Sugar acids, (e.g. gluconic acid, glucuronic acid, glucaric acid)
• Sugar alcohols (e.g., sorbitol, mannitol)
• Deoxy sugars (e.g., deoxyribose)
• Amino sugars (e.g., glucosamine, galactosamine)

Glycosidic Bond

• A covalent bond formed between a cyclic sugar molecule hydroxyl and another sugar or other compound molecule.
• A glycosidic bond is formed when the anomeric carbon of one molecule is bonded to the hydroxyl group of another molecule, releasing water.

Starch

• Starch is made up of amylose and amylopectin
• Composed of chains of glucose monomers joined by glycosidic bonds
• Storage polysaccharide in plants.

Reducing Properties of Monosaccharides

• The ability of monosaccharides to act as reducing agents is due to the presence of a free anomeric carbon atom
• Some monosaccharides can reduce certain oxidizing agents, which is used in laboratory tests like Fehling's.

Properties of Monosaccharides

• Optical isomerism
• Epimerism
• Hemiacetal and hemiketal formation
• L and D forms
• Anomers
• Formation of glycosidic bonds
• Reducing properties

This quiz covers the fundamental concepts of carbohydrates within the introductory biochemistry course. It explores their structure, classification, and roles in living organisms, particularly focusing on their production through photosynthesis and their significance in both plant and animal tissues.