Chapter 5: The Structure and Function of Large Biological Molecules

Polymers and Monomers

Biological macromolecules are often polymers, which are long chains made from repeating monomer units. The formation and breakdown of these polymers are governed by specific chemical reactions.

• Polymer: A molecule consisting of many similar or identical monomers linked by covalent bonds.

• Dehydration Reaction: A chemical reaction that joins two molecules by removing a water molecule, enabling polymer formation.

• Hydrolysis: A reaction that breaks bonds between monomers by adding water, leading to polymer disassembly.

Carbohydrates

Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen. They serve as energy sources and structural materials in cells.

• Monosaccharides: Simple sugars (e.g., glucose, fructose) with a single ring structure; immediate energy source.

• Disaccharides: Two monosaccharides linked by a glycosidic bond (e.g., sucrose, lactose); transportable energy.

• Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose); energy storage and structural support.

• Glycosidic Linkage: The covalent bond joining monosaccharides.

Lipids

Lipids are hydrophobic molecules including fats, phospholipids, and steroids. They are essential for energy storage, membrane structure, and signaling.

• Triacylglycerol (Triglyceride): Glycerol + 3 fatty acids; main energy storage lipid.

• Phospholipid: Glycerol + 2 fatty acids + phosphate group; major component of cell membranes.

• Steroid: Four fused carbon rings (e.g., cholesterol, hormones); membrane integrity and signaling.

• Ester Linkage: Bond between glycerol and fatty acids in fats.

Saturated vs. Unsaturated Fatty Acids

• Saturated: All single C–C bonds; straight chains; solid at room temperature.

• Unsaturated: One or more C=C double bonds; bent chains; liquid at room temperature.

Proteins

Proteins are polymers of amino acids, folded into specific three-dimensional structures for diverse functions.

• Polypeptide: Single chain of amino acids linked by peptide bonds.

• Protein: One or more polypeptides folded into a functional shape.

• Amino Acid: Monomer unit with amino group, carboxyl group, hydrogen, and variable R group.

• Peptide Bond: Covalent bond between amino acids.

Levels of Protein Structure

• Primary: Linear sequence of amino acids.

• Secondary: Local folding (α-helix, β-sheet) stabilized by hydrogen bonds.

• Tertiary: Overall 3D shape from R-group interactions.

• Quaternary: Assembly of multiple polypeptide chains.

Nucleic Acids

Nucleic acids (DNA and RNA) are polymers of nucleotides, responsible for storing and transmitting genetic information.

• Nucleotide: Monomer with phosphate group, pentose sugar, and nitrogenous base.

• Phosphodiester Linkage: Covalent bond joining nucleotides in DNA/RNA.

• DNA: Double-stranded; long-term genetic storage.

• RNA: Single-stranded; involved in protein synthesis and gene expression.

Chapter 6: A Tour of the Cell

Cell Organelles and Their Functions

• Nucleus: Stores and protects DNA, regulates gene expression, organizes chromosomes, and produces ribosomal components.

• Nuclear Envelope: Double membrane with nuclear pores for selective transport; structural support.

• Nucleolus: Site of ribosome biogenesis and rRNA synthesis.

• Ribosome: Synthesizes proteins by translating mRNA.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids and detoxifies.

• Vesicle: Transports, stores, digests, and removes cellular materials.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosome: Digests macromolecules and recycles cellular components.

• Vacuole: Storage, waste disposal, turgor maintenance, and digestion.

• Mitochondria: Produces ATP, regulates cell growth/death, and integrates metabolism.

• Chloroplast: Performs photosynthesis in plants.

• Peroxisome: Breaks down fatty acids and detoxifies compounds.

• Cytoskeleton: Maintains cell shape, enables movement, and organizes organelles.

Endomembrane System

The endomembrane system is a network of organelles that synthesize, modify, package, and transport proteins and lipids.

• Nuclear envelope

• Endoplasmic reticulum (rough and smooth)

• Golgi apparatus

• Lysosomes

• Endosomes

• Vesicles

• Vacuoles

• Plasma membrane

Cytoskeletal Elements

• Microfilaments: Actin filaments for movement and shape.

• Intermediate Filaments: Stable, rope-like structures for mechanical support.

• Microtubules: Hollow tubules for transport, organization, and cell division.

Cell Junctions

• Tight Junctions: Impermeable seals between cells.

• Gap Junctions: Channels for direct communication.

• Adherens Junctions: Mechanical stability via actin linkage.

• Desmosomes: Strong anchoring via intermediate filaments.

• Plasmodesmata (plants): Channels for intercellular communication.

Chapter 7: Membrane Structure and Function

Fluid-Mosaic Model of Membrane Structure

The fluid-mosaic model describes the plasma membrane as a dynamic, flexible bilayer of lipids, proteins, and carbohydrates, with components moving laterally to maintain fluidity and function.

• Phospholipids: Form the bilayer; hydrophilic heads face outward, hydrophobic tails inward.

• Membrane Proteins: Integral and peripheral proteins for transport, signaling, and structure.

• Glycolipids/Glycoproteins: Carbohydrate chains for cell recognition and adhesion.

• Cholesterol: Regulates membrane fluidity and stability.

Amphipathic Nature and Membrane Formation

Amphipathic molecules, such as phospholipids, have both hydrophilic and hydrophobic regions. In water, they self-assemble into bilayers, with hydrophilic heads facing water and hydrophobic tails hidden, forming the fundamental structure of biological membranes.

Role of Fatty Acids and Cholesterol in Membrane Fluidity

• Saturated Fatty Acids: Straight chains; pack tightly; make membranes rigid.

• Unsaturated Fatty Acids: Bent chains; prevent tight packing; increase fluidity.

• Cholesterol: Acts as a fluidity buffer, preventing excessive rigidity or fluidity.

Integral Membrane Proteins

Integral proteins have hydrophobic regions embedded in the membrane and hydrophilic regions exposed to aqueous environments, enabling transport, signaling, and enzymatic activity.

Glycolipids and Glycoproteins

• Glycoproteins: Proteins with carbohydrate chains; cell recognition and adhesion.

• Glycolipids: Lipids with carbohydrate chains; found on extracellular surface.

Extracellular Matrix (ECM)

The ECM is a network of proteins and carbohydrates outside animal cells, providing structural support, cell adhesion, signaling, and protection.

Membrane Synthesis

New membrane sections are synthesized in the ER, processed in the Golgi, and delivered to the plasma membrane via vesicular transport, ensuring dynamic growth and repair.

Functions of the Plasma Membrane

• Protective barrier

• Selective transport

• Communication and signal reception

• Cell recognition and adhesion

• Structural support

Diffusion and Osmosis

• Diffusion: Passive movement from high to low concentration.

• Facilitated Diffusion: Passive movement via transport proteins.

• Osmosis: Movement of water across a semipermeable membrane.

Tonicity and Cell Responses

• Hypertonic: Cell shrinks (crenation in animals, plasmolysis in plants).

• Hypotonic: Cell swells (lysis in animals, turgid in plants).

• Isotonic: No net water movement; cell remains stable.

Active Transport and Bulk Transport

• Active Transport: Movement against concentration gradient, requiring energy.

• Exocytosis: Release of materials via vesicle fusion with plasma membrane.

• Endocytosis: Uptake of materials via vesicle formation (phagocytosis, pinocytosis, receptor-mediated).