Lecture 5 The cell The Cell membrane and Membrane Proteins Ameoba- single celled organism A single human cell The Cell is the simplest collec4on of ma9er that can live Cells 4ssue organ Cellular membranes A cell is the simplest collection of matter that can live Phospho-lipids Plasma membrane separates the living cell from its non-living environment Cell membranes are fluid mosaics of lipids and proteins Exhibit selective permeability allowing some substances to cross more easily than others 1
Plasma membrane is made up of a phospholipid bilayer Model of the cell membrane Cell membrane Membrane proteins Phospholipids- fabric of the cell membrane Proteins determine membranes specific functions Different types of cells-different types of proteins Different organelles within a cell-different proteins Lipids are always moving sideways Proteins drift slowly Cholesterol act as a temperature buffer keeping the membrane fluid at moderate temperatures Membranes must be fluid to work 2
Membrane proteins 2 types of membrane protein 1. Integral proteins penetrate the hydrophobic core, many are transmembrane Transmembrane protein eg Bacteriorhodopsin 2. Peripheral proteins are not embedded in the membrane, they are often bound to exposed integral proteins or loosely to the surface of the membrane Membrane proteins Membrane protein func4ons Hydrophilic channel or Carrier protein Tight junc4ons formed between cells Team of enzymes Glycoprotein- ID tag Stablises loca4on of certain proteins 3
Transport across the membrane Transport across a membrane Regulation of transport across the cell membrane-essential its to existence Eg. Muscle cells Sugars, amino acids, oxygen, ions in Carbon dioxide, ions, metabolic waste Cell membrane is selectively permeable out Selective Permeabilty Membrane proteins key role in regulating transport Diffusion-passive transport Diffusion- tendency for molecules of any substance to spread evenly into available space Any substance will diffuse down its concentration gradient- spontaneous process no energy required Small hydrophobic molecules will diffuse across the membrane Eg. Oxygen crosses into cells performing cellular respira4on 4
Selective permeability Charged and polar molecules can cross the membrane by passing through transport proteins Transport proteins span the membrane Transport proteins are very specific 2 types- channel and carrier Channel protein eg. aquaporin Carrier protein eg glucose Facilitated Diffusion-Passive Transport Diffusion of hydrophilic solutes across the membrane must be facilitated by transport proteins Transport proteins are very specific channel transporter eg. aquaporin Ion channel eg. Sodium chanel Channel proteins provide hydrophilic corridors Ion channels Gated ion channelsstimulus regulated Ligand gated ion channel eg. neurotransmi9ers Facilitated Diffusion-Passive Transport Carrier proteinschange shape in order to translocate substances across the membrane Changes in shape triggered by the binding and release of the transported molecule Carrier protein eg. Glucose transporter Diffusion-passive transport Uniport-movement of one solute from one side to the other Co-transport Symport-two solutes in the same direction Antiport-two solutes in opposite directions Solutes will diffuse down their concentration gradient Facilitated Diffusion is s4ll passive because the solute moves down the concentra4on gradient 5
Active transport-uses energy Movement of substances against their concentration gradient across the membrane From a low concentration to a high concentration Active transport ATP supplies the energy for active transport ATP can transfer its terminal phosphate group directly to the transport protein-transfer of energy Eg of a carrier protein that requires ATP to transport is the sodium-potassium pump Requires work- cell uses energy- active All active transporters are carrier proteins Active transport enables a cell to maintain higher internal concentrations of molecule compared to the external environment Eg. Low [Na+] and high [K+] compared with [extracellular] Sodium-potassium pump Membrane potential is the difference in voltage (or electrical potential difference) between the interior and exterior of a cell Resting membrane potential -inside of the cell has a negative voltage (millivolts; mv) with respect to the outside of the cell (0 mv). Sodium- potassium pump = electrogenic pump = Generates a voltage across the membrane - More nega4vely charged inside a cell than outside - creates an electrochemical gradient 6
Membrane potential Voltage across the membrane = membrane potential -storage of electrical potential energy This potential energy can be tapped into by the cell to carry out work eg.: the diffusion of H+ ions back down their electrochemical gradient can be coupled to the active transport of Sucrose against its concentration gradient Membrane potential Upon stimulation of the cell, this negative voltage inside the cell (negative membrane potential) may become positive-depolarisation More negative-hyperpolarisation Important in Neurons-Generation of a membrane potential will induce electric current (action potential) to flow rapidly to other points in the membrane Mitochondria-the polarisation state determines life or death. Depolarisation =activation of cell death In Summary...Transport across the membrane Summary 1. Proteins - What are they? And why are they important? 2. The building blocks (aa s) and how they are connected 3. The hierarchy of protein structure 4. Three-dimensional protein structure 5. Proteins in disease 6. Enzymes as an example of complex proteins 7. Cell membranes and the role of proteins 7
Biology Campbell and Reece Chapters 5, 7 and 8 http://www.tcd.ie/biology_teaching_centre/ BY1010/modules/BY1011 8