BIOL 150 Lecture Exam II Study Guide

- **Key Molecules in Membrane Structure:** - Lipids: Various types (phospholipids, cholesterol, glycolipids) contribute to the structure and function. - Proteins: Integral and peripheral proteins perform many functions. - Carbohydrates: Glycolipids and glycoproteins are involved in cell recognition. - **Types of Lipids:** - Phospholipids: The most abundant lipid in the plasma membrane. - Consist of a hydrophilic (polar) head and two hydrophobic (nonpolar) tails. - **Importance:** - Amphipathic nature – both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions allow them to form bilayers in aqueous solutions. - Arranged in bilayers with hydrophobic tails facing inward and hydrophilic heads facing outward. - **Membrane Proteins:** - Proteins are inserted into the lipid bilayer. - Have both hydrophilic and hydrophobic regions. - Hydrophobic regions often consist of nonpolar amino acids coiled into alpha helices. - **Current Model of Membrane Structure:** - Fluid mosaic model: The membrane is a mosaic of protein molecules drifting laterally in a fluid bilayer of phospholipids. - **Properties of Membranes:** - Fluidity – components like lipids and proteins can move laterally within the membrane. - Hydrophobic interactions hold membranes together. - Most of the lipids and some proteins can drift laterally. - **Factors Affecting Membrane Fluidity:** - Phospholipids with double bonds maintain fluidity at lower temperatures because kinks prevent tight packing. - Cholesterol reduces fluidity at body temperature by restraining movement of phospholipids. - **Phospholipid Composition:** - Phosphate head group (hydrophilic) interacts with water. - Fatty acid tails (hydrophobic) avoid water, form bilayer interior. - **Types of Membrane Proteins:** - **Integral Proteins:** - Embedded in membrane, potentially spanning entire membrane (transmembrane proteins). - Have hydrophobic regions that interact with the hydrophobic core of the lipid bilayer. - Examples: channels, carriers. - **Peripheral Proteins:** - Attached to inside or outside of membrane, not embedded. - Often attached to integral proteins. - Examples: enzymes, structural proteins. - **Importance of Membrane Carbohydrates:** - Cell-cell recognition: Carbohydrates on the external surface of the plasma membrane are important for cell-cell recognition. - Can distinguish one cell from another. - **Properties of Membrane Oligosaccharides:** - Typically less than 15 residues in length. - Attached to lipids (glycolipids) or proteins (glycoproteins). - Glycolipids contain carbohydrates covalently bonded to lipids. - Glycoproteins contain carbohydrates covalently bonded to proteins. - **Functions of Membrane Proteins:** - Transport: Provide channels for ions and polar molecules to pass through the membrane. - Examples: aquaporins (water channels), ion channels. - Enzyme activity: Some membrane proteins are enzymes that catalyze reactions. - Examples: ATPases. - Signal transduction: Membrane proteins may have binding sites for hormones or other chemical messengers. - Examples: G protein-coupled receptors. - Intercellular joining: Membrane proteins of adjacent cells may hook together. - Examples: tight junctions, gap junctions. - Cell-cell recognition: Glycoproteins serve as ID tags that are recognized by membrane proteins of other cells. - Examples: immune system cells. - Attachment to cytoskeleton and extracellular matrix: Help maintain cell shape and stabilize the location of certain membrane proteins. ### MEMBRANE TRANSPORT - **Selective Permeability:** - Hydrophobic molecules pass easily; ions and polar molecules do not cross the phospholipid bilayer. - Lipid bilayer is permeable to small nonpolar molecules. - **Hydrophilic Substance Transport:** - Via protein channels and protein carriers. - Transport proteins are specific for the substances they move. - **Active vs. Passive Transport:** - *Active Transport:* Requires ATP, moves against concentration gradients. - ATP hydrolysis provides energy. - *Passive Transport:* No energy needed, moves from high to low concentration. - Driven by concentration gradients. - **Driving Force of Passive Transport:** - Thermal motion or heat. - Diffusion is driven by the intrinsic kinetic energy (thermal motion) of molecules. - **Solutions:** - **Hypertonic:** Higher solute concentration. - **Hypotonic:** Lower solute concentration. - **Isotonic:** Equal solute concentration. - **Osmosis:** - Movement of water across selectively permeable membrane. - Water diffuses from a region of higher water concentration to a region of lower water concentration. - Water moves from hypotonic to hypertonic. - **Water Movement in Isotonic Solutions:** - No net movement of water, equal movement in both directions. - **Cell Reactions in Different Solutions:** - *Hypertonic:* Cell shrinks (water moves out). - Plasmolysis occurs in plant cells. - *Hypotonic:* Cell swells and may burst (water moves in). - Lysis occurs in animal cells. - *Isotonic:* No volume change. - **Mechanisms for Stable Cell Volume:** - Contractile vacuoles in fresh water cells pump out excess water; cell walls in plant cells provide structural support. - **Plant Cell Reactions:** - Isotonic: Cell is limp (flaccid) due to no net water movement. - Hypertonic: Cell shrinks (plasmolysis) as water moves out. - Hypotonic: Cell fills (turgor pressure) as water moves in, cell becomes turgid. - **Facilitated Diffusion:** - Polar molecules and ions move passively with transport proteins; aquaporins facilitate water movement. - Transport proteins include channel proteins and carrier proteins. - Aquaporins are channel proteins that facilitate the diffusion of water. - **Active Transport:** - Energy required to pump solutes against gradients. - ATP hydrolysis is used to power the transport. - **Large Molecule Transport:** - Use vesicles: exocytosis for export, endocytosis for import. - **Types of Endocytosis:** - Phagocytosis (large particles in). - Cell engulfs a particle by wrapping pseudopodia around it and packaging it within a membrane-enclosed sac. - Pinocytosis (drops of fluid in). - Cell continually gulps droplets of extracellular fluid into tiny vesicles. - Receptor-mediated endocytosis (specific receptor proteins required). - Enables the cell to acquire bulk quantities of specific substances.