Cell Surfaces and Membranes • Junctions and Connection Structures between Animal Cells  We will examine the structure/function relationships of the various proteins that allow some cell types to associate and/or communicate. • Cell Membranes  Cells cannot exist in isolation from the external environment.  There has to be ways to control what comes in and what goes out.  Cell membranes and surfaces control this function.  We will examine how cell membranes and their associated proteins etc. enable these functions. • Most cells synthesize and secrete materials that are external to the plasma membrane • These extracellular structures include:  The extracellular matrix (ECM) of animal cells  Intercellular junctions  Cell walls of plantsCell Walls of Plants • The cell wall is an extracellular structure that distinguishes plant cells from animal cells  Prokaryotes, fungi, and some protists also have cell walls • The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water • Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein• The cell wall is secreted by the cell and is made up of 3 components.  a thin primary cell wall.  a middle lamella, which is sticky and made up of polysaccharides called pectins. This helps stick cells together.  as the plant matures, a thicker and less flexible set of secondary cell walls is layered. This is less flexible and can be quite rigid particularly in woody plants.  The plant wall consists of a variety of macromolecules, but cellulose is a primary component. • Plant cells are not isolated from each other, but are linked by cytoplasmic bridges through the wall structure - plasmodesmata. Cell Wall of PlantsThe Extracellular Matrix (ECM) of Animal Cells • Functions of the ECM:  Support  Adhesion  Movement  RegulationExtracellular components and connections between cells help coordinate cellular activities • Animal cells lack cell walls but are covered by an elaborate extracellular matrix (ECM) • The ECM is made up of glycoproteins such as collagen, proteoglycans, and fibronectin • ECM proteins bind to receptor proteins in the plasma membrane called integrinsIntercellular Junctions • Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact • Intercellular junctions facilitate this contact • There are several types of intercellular junctions  Tight junctions  Desmosomes  Gap junctionsTight Junctions, Desmosomes, and Gap Junctions in Animal Cells • Tight junctions  membranes of neighboring cells are pressed together  contain membrane proteins, occludins and claudins  Seals that prevents leakage of extracellular fluid • Desmosomes  anchoring junctions that use intermediate filaments  fasten cells together into strong sheets • Gap junctions  communicating junctions  Formed from proteins called connexons that form a 'tube' between cells.Cell Surfaces and Membranes • For a cell to function efficiently it must be able to do 2 fundamental things: 1. control its internal environment • what goes on inside the cell. 2. control its interactions with the external environment. • Cells cannot exist in isolation from the external environment.  There has to be ways to control what comes in and what goes out. • Cell membranes and surfaces control the second function.Cellular membranes are fluid mosaics of lipids and proteins • Phospholipids are the most abundant lipid in the plasma membrane • Phospholipids are amphipathic molecules  contain hydrophobic and hydrophilic regionsFluid mosaic model • States that a membrane is a fluid structure with a “mosaic” of various proteins embedded in itPlasma membrane • Freeze-fracture studies of the plasma membrane supported the fluid mosaic model  a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayerThe Fluidity of Membranes • Phospholipids in the plasma membrane can move within the bilayer • Most of the lipids, and some proteins, drift laterally • Rarely does a molecule flip-flop transversely across the membrane • Fluidity is affected by its lipid constituency  Unsaturated versus saturated tails  CholesterolMembrane Proteins and Their Functions • A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer • Proteins determine most of the membrane’s specific functions  Peripheral proteins are bound to the surface of the membrane  Integral proteins penetrate the hydrophobic coreMembrane proteins • Integral proteins that span the membrane are called transmembrane proteins  The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices • Six major functions of membrane proteins:  Transport  Enzymatic activity  Signal transduction  Cell-cell recognition  Intercellular joining  Attachment to the cytoskeleton and extracellular matrix (ECM)Transmembrane proteinsTransmembrane proteinsSynthesis and Sidedness of Membranes • Membranes have distinct inside and outside faces • The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatusMembrane structure results in selective permeability • A cell must exchange materials with its surroundings, a process controlled by the plasma membrane • Plasma membranes are selectively permeable, regulating the cell’s molecular traffic  Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly  Polar molecules, such as sugars, do not cross the membrane easily • Thus, most molecules need to be “transported” across the membrane.  Requires energy.Transport Proteins • Transport proteins allow passage of hydrophilic substances across the membrane • Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel  Channel proteins called aquaporins facilitate the passage of water • Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane • A