12. Carbohydrates/Introduction to Membranes

The lecture begins with an introduction to carbohydrates, emphasizing their role as energy storage molecules and their importance in cell biology. The professor discusses the four main classes of biological molecules, highlighting proteins, nucleic acids, carbohydrates, and lipids. Carbohydrates are defined as carbon compounds with a ratio to water (CnH2On), and their forms range from monosaccharides to polysaccharides. Monosaccharides like glucose and disaccharides like sucrose are mentioned, with glucose being a primary sugar in the blood and sucrose being common table sugar. The lecture also touches on the basic membrane structure, setting the stage for a deeper dive into biochemistry.

This paragraph delves into the structure of carbohydrates, specifically focusing on trioses like glyceraldehyde and dihydroxyacetone, which are isomers sharing the same chemical formula (C3H2O3) but different structures. The concept of isomerism is introduced, explaining how these molecules can interconvert through the action of isomerases. Stereochemistry is also discussed, highlighting the importance of stereocenters and the distinction between D- and L-glyceraldehyde. The professor uses models to illustrate these concepts, emphasizing the biological relevance of stereoisomers and their interaction with enzymes.

The lecture continues with an exploration of stereochemistry in sugars, explaining the conventions of Fischer projections and the significance of D- and L-sugars. The professor discusses the importance of stereoisomers in biological processes, noting how enzymes interact differently with various stereoisomers. Examples of four-carbon sugars (tetroses) and their potential isomers are provided, demonstrating the complexity of sugar structures. The focus is on understanding the nomenclature and the biological relevance of these structures, setting the stage for further discussions on metabolism and signal transduction.

This paragraph discusses the diversity of carbohydrates, focusing on the subset of stereoisomers that nature uses. The professor explains that not all possible stereoisomers are found in biology, highlighting examples like D-erythrose and D-threose. The importance of aldehydes and ketones in sugar structures is discussed, leading to the classification of sugars as aldoses and ketoses. The lecture also covers the significance of the carbonyl group's position in ketoses and the prevalence of hexoses and pentoses in biological systems.

The lecture moves on to the major hexoses and pentoses used in nature, discussing their structures and biological roles. Glucose, fructose, galactose, and mannose are highlighted, with an emphasis on their stereochemistry and how it affects their properties. The concept of epimers is introduced, explaining how sugars like glucose and galactose differ in their stereochemistry at a single carbon. The professor also discusses the formation of rings in sugars, such as the pyranose and furanose forms of glucose and fructose, and their implications for solubility and reactivity.

This paragraph explores the complexity of sugar structures, particularly focusing on the different forms of fructose and their impact on taste. The professor explains how the same sugar can have different tastes depending on its structural form, using honey and corn syrup as examples. The difference between pyranose and furanose rings in fructose is discussed, demonstrating how these structural variations can significantly affect the perceived sweetness. The lecture also touches on the role of ribose in nucleic acids and its ring formation, highlighting the importance of sugar structures in biological processes.

The lecture shifts focus to lipids, explaining their general structure and their role in forming cell membranes. Lipids are defined as molecules consisting of a fatty acid esterified to an alcohol. The hydrophobic nature of fatty acids and their esterification to alcohols are discussed, highlighting how these properties allow lipids to form barriers between aqueous compartments. The professor introduces the concept of neutral lipids and their use in energy storage, contrasting them with charged lipids that can form interfaces with water. The lecture sets the stage for a deeper exploration of lipids in cell signaling and energy transduction.

This paragraph delves into the structure of phospholipids, which are key components of cell membranes. The professor explains that phospholipids consist of a glycerol backbone esterified to two fatty acids and a phosphate group. The hydrophilic head group and hydrophobic fatty acid tails of phospholipids are discussed, illustrating how these molecules can spontaneously form bilayers in an aqueous environment. Examples of common phospholipids like phosphatidylcholine, phosphatidylserine, and phosphatidylethanolamine are provided, highlighting their roles in membrane structure and function. The lecture also touches on the use of phospholipids in food emulsification and their importance in cell signaling.

The final paragraph of the lecture discusses the interaction between proteins and lipids in cell membranes. The professor explains how proteins can span the membrane, interacting with both the hydrophobic and hydrophilic regions. Examples of membrane proteins that form channels or float on the membrane surface are provided, demonstrating their role in cell signaling and energy transduction. The lecture concludes with a preview of future discussions on metabolism and the role of membranes in energy storage and transduction, setting the stage for a deeper exploration of these topics after spring break.