Study Guide: Structure and Function of Large Biological Molecules, The Cell, and Membrane Structure

Chapter 5: The Structure and Function of Large Biological Molecules

Key Concepts and Vocabulary

• Polymer: A long molecule consisting of many similar or identical building blocks (monomers) linked by covalent bonds.

• Dehydration Reaction: A chemical reaction in which two molecules are covalently bonded to each other with the removal of a water molecule. Responsible for the synthesis of polymers.

• Hydrolysis: A chemical reaction that breaks bonds between two molecules by the addition of water; functions in the disassembly of polymers to monomers.

• Carbohydrate: A biological molecule consisting of carbon, hydrogen, and oxygen; includes sugars and polymers of sugars.

• Glycosidic Linkage: A covalent bond formed between two monosaccharides by a dehydration reaction.

• Starch: A storage polysaccharide in plants consisting entirely of glucose monomers joined by glycosidic linkages.

• Cellulose: A structural polysaccharide of plant cell walls, composed of glucose monomers joined by β glycosidic linkages.

• Lipid: A diverse group of hydrophobic molecules, including fats, phospholipids, and steroids.

• Fatty Acid: A carboxylic acid with a long carbon chain; a component of many lipids.

• Ester Linkage: The bond formed between a hydroxyl group and a carboxyl group, as in the formation of a fat.

• Triacylglycerol (Triglyceride): A lipid consisting of three fatty acids linked to one glycerol molecule.

• Saturated Fatty Acid: A fatty acid in which all carbons in the hydrocarbon tail are connected by single bonds, maximizing the number of hydrogen atoms.

• Unsaturated Fatty Acid: A fatty acid that has one or more double bonds between carbons in the hydrocarbon tail, reducing the number of hydrogen atoms.

• Protein: A biologically functional molecule consisting of one or more polypeptides folded and coiled into a specific three-dimensional structure.

• Polypeptide: A polymer of amino acids joined by peptide bonds.

• Amino Acid: An organic molecule with both an amino group and a carboxyl group; monomer of polypeptides.

• Peptide Bond: The covalent bond between the carboxyl group on one amino acid and the amino group on another, formed by a dehydration reaction.

• Nucleic Acid: A polymer (DNA or RNA) consisting of nucleotide monomers.

• Nucleotide: The building block of a nucleic acid, consisting of a five-carbon sugar, a nitrogenous base, and a phosphate group.

• Phosphodiester Linkage: The covalent bond that joins the phosphate group of one nucleotide to the sugar of another in nucleic acids.

Polymer Synthesis and Breakdown

• Dehydration Reaction: Joins monomers by removing a water molecule. Each time a monomer is added to a polymer chain, a water molecule is released.

• Hydrolysis: Breaks polymers into monomers by adding a water molecule, reversing the dehydration reaction.

• Example: Digestion in the human body uses hydrolysis to break down food polymers into monomers for absorption.

Carbohydrates

• Definition: Sugars and their polymers; serve as fuel and building material.

• Functions: Energy storage (e.g., starch, glycogen), structural support (e.g., cellulose, chitin).

• Types:

  • Monosaccharides: Simple sugars (e.g., glucose, fructose). General formula: $C_nH_{2n}O_n$

  • Disaccharides: Two monosaccharides joined by a glycosidic linkage (e.g., sucrose, lactose).

  • Polysaccharides: Polymers of many monosaccharides (e.g., starch, cellulose, glycogen).

• Glycosidic Linkage: The covalent bond between monosaccharides in a carbohydrate.

Lipids

• Definition: Hydrophobic molecules, not true polymers; include fats, phospholipids, and steroids.

• Functions: Energy storage, membrane structure, signaling molecules.

• Examples:

  • Triacylglycerol (Triglyceride): Three fatty acids linked to glycerol by ester linkages.

  • Phospholipids: Two fatty acids and a phosphate group attached to glycerol; major component of cell membranes.

  • Steroids: Lipids with a carbon skeleton of four fused rings (e.g., cholesterol).

• Subunits of Fats: Glycerol and fatty acids.

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

• Saturated vs. Unsaturated Fatty Acids:

  • Saturated: No double bonds; solid at room temperature (e.g., butter).

  • Unsaturated: One or more double bonds; liquid at room temperature (e.g., olive oil).

Proteins

• Definition: Polymers of amino acids; perform most cellular functions.

• Functions: Enzymes, structural support, transport, signaling, movement, defense.

• Monomer Subunits: Amino acids (20 types).

• Structure of Amino Acid:

  • Central carbon (α-carbon), amino group ($-NH_2$), carboxyl group ($-COOH$), hydrogen atom, and R group (side chain).

  • General formula: $H_2N-CHR-COOH$

• Hydrophilic/Hydrophobic Nature:

  • Nonpolar: Hydrophobic side chains.

  • Polar: Hydrophilic side chains.

  • Electrically Charged: Hydrophilic; can be acidic (negatively charged) or basic (positively charged).

• Peptide Bond: Covalent bond between amino acids.

• Polypeptide vs. Protein: A polypeptide is a linear chain of amino acids; a protein is one or more polypeptides folded into a functional shape.

• Levels of Protein Structure:

  1. Primary: Sequence of amino acids.

  2. Secondary: Coils and folds (α-helix, β-pleated sheet) due to hydrogen bonding.

  3. Tertiary: 3D shape due to interactions among R groups.

  4. Quaternary: Association of multiple polypeptides.

Nucleic Acids

• Definition: Polymers of nucleotides; store and transmit genetic information.

• Types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• Monomer Subunits: Nucleotides (phosphate group, five-carbon sugar, nitrogenous base).

• Structure of Nucleotide: Phosphate group attached to 5' carbon of sugar; nitrogenous base attached to 1' carbon.

• Phosphodiester Linkage: Covalent bond between nucleotides in a nucleic acid chain.

• Functions:

  • DNA: Stores genetic information.

  • RNA: Various roles in gene expression, including carrying instructions from DNA to ribosomes.

Chapter 6: A Tour of the Cell

Cell Organelles and Their Functions

• Nucleus: Contains most of the cell's DNA; controls gene expression.

• Nuclear Envelope: Double membrane enclosing the nucleus; has nuclear pores for transport.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

• Ribosome: Site of protein synthesis; can be free in cytosol or bound to rough ER.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

  • Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs.

• Vesicle: Small membrane-bound sac for transport within cells.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosome: Contains hydrolytic enzymes for digestion of macromolecules.

• Vacuole: Storage, waste disposal, and maintaining cell turgor (especially in plants).

• Mitochondria: Site of cellular respiration; produces ATP.

• Chloroplast: Site of photosynthesis in plant cells.

• Peroxisome: Contains enzymes that break down fatty acids and detoxify harmful substances.

• Cytoskeleton: Network of protein fibers (microtubules, microfilaments, intermediate filaments) that provide structural support, cell movement, and transport.

Key Comparisons and Concepts

• Magnification vs. Resolution:

  • Magnification: Ratio of an object's image size to its real size.

  • Resolution: Measure of the clarity of the image; minimum distance two points can be separated and still be distinguished.

• Prokaryotic vs. Eukaryotic Cells:

  • Prokaryotic: No nucleus, DNA in nucleoid, no membrane-bound organelles (e.g., bacteria, archaea).

  • Eukaryotic: Nucleus, membrane-bound organelles (e.g., plants, animals, fungi, protists).

• Free vs. Bound Ribosomes:

  • Free: Suspended in cytosol; make proteins for use in cytosol.

  • Bound: Attached to ER or nuclear envelope; make proteins for membranes or export.

Endomembrane System

• Includes nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Functions in synthesis, modification, and transport of proteins and lipids.

Cytoskeleton Components

Component	Structure	Function
Microtubules	Hollow tubes of tubulin	Cell shape, organelle movement, chromosome separation
Microfilaments	Two intertwined strands of actin	Cell shape, muscle contraction, cell division
Intermediate Filaments	Fibrous proteins coiled into cables	Cell shape, anchorage of organelles

Cilia and Flagella

• Both are made of microtubules in a "9+2" arrangement.

• Cilia: Short, numerous; move fluid over cell surface.

• Flagella: Longer, usually one or a few; propel cell through liquid.

Plant Cell Wall

• Composed mainly of cellulose fibers embedded in other polysaccharides and proteins.

• Provides structural support, protection, and regulates water intake.

Extracellular Matrix (ECM) in Animal Cells

• Network of glycoproteins (e.g., collagen), proteoglycans, and fibronectin outside the plasma membrane.

• Provides structural support, cell signaling, and anchorage.

Intercellular Junctions

• Plasmodesmata: Channels between plant cells for transport and communication.

• Tight Junctions: Seal cells together to prevent leakage of extracellular fluid.

• Desmosomes: Anchor cells together into strong sheets.

• Gap Junctions: Provide cytoplasmic channels between animal cells for communication.

Chapter 7: Membrane Structure and Function

Fluid-Mosaic Model of Membrane Structure

• Membranes are a fluid structure with a "mosaic" of various proteins embedded in or attached to a double layer (bilayer) of phospholipids.

Structural Components of the Plasma Membrane

• Phospholipids: Form the bilayer; amphipathic with hydrophilic heads and hydrophobic tails.

• Membrane Proteins: Integral (span the membrane) and peripheral (attached to surface); perform transport, signaling, and structural roles.

• Glycolipids/Glycoproteins: Lipids/proteins with attached carbohydrates; involved in cell recognition.

• Cholesterol: Interspersed within the bilayer; modulates membrane fluidity.

Phospholipid Structure and Membrane Formation

• Phospholipids have a hydrophilic (polar) head and two hydrophobic (nonpolar) tails.

• In water, they self-assemble into bilayers, shielding hydrophobic tails from water.

Membrane Fluidity

• Saturated Fatty Acids: Make membranes less fluid (pack tightly).

• Unsaturated Fatty Acids: Increase fluidity (prevent tight packing due to kinks).

• Cholesterol: Reduces fluidity at high temperatures, prevents solidification at low temperatures.

Integral Membrane Proteins

• Amphipathic; hydrophobic regions span the membrane, hydrophilic regions exposed to aqueous environments.

Glycolipids and Glycoproteins

• Carbohydrate chains attached to lipids (glycolipids) or proteins (glycoproteins); function in cell-cell recognition.

Extracellular Matrix (ECM)

• Attached to the outside of the plasma membrane in animal cells; provides support and mediates cell signaling.

Membrane Synthesis

• New membrane sections are made in the ER, modified in the Golgi apparatus, and transported in vesicles to the plasma membrane.

Functions of the Plasma Membrane

• Selective permeability, transport, signal transduction, cell-cell recognition, intercellular joining, attachment to cytoskeleton and ECM.

Diffusion and Membrane Transport

• Diffusion: Movement of molecules from high to low concentration.

• Direct Diffusion: Small, nonpolar molecules (e.g., O2, CO2) can diffuse through the membrane; ions and polar molecules cannot.

• Gradients:

  • Concentration Gradient: Difference in concentration across a membrane.

  • Charge Gradient: Difference in electrical charge.

  • Electrochemical Gradient: Combined effect of concentration and charge gradients.

• Facilitated Diffusion: Passive transport aided by proteins (channels or carriers).

Osmosis and Tonicity

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Solution Types:

  • Hypertonic: Higher solute concentration outside cell; water leaves cell.

  • Hypotonic: Lower solute concentration outside cell; water enters cell.

  • Isotonic: Equal solute concentration; no net water movement.

• Effects:

  • Crenation: Shrinking of animal cells in hypertonic solution.

  • Lysis: Bursting of animal cells in hypotonic solution.

  • Plasmolysis: Shrinking of plant cell cytoplasm in hypertonic solution.

Active Transport

• Movement of substances against their concentration gradient using energy (usually ATP).

Bulk Transport

• Exocytosis: Vesicles fuse with plasma membrane to release contents outside the cell (e.g., secretion of proteins).

• Endocytosis: Cell takes in substances by forming vesicles from the plasma membrane.

  • Phagocytosis: "Cell eating"; uptake of large particles.

  • Pinocytosis: "Cell drinking"; uptake of extracellular fluid.

  • Receptor-Mediated Endocytosis: Uptake of specific molecules via receptor proteins.