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Transcript
Lecture 5 The Cell The cell membrane and Membrane Proteins The cell Ameoba-‐ single celled organism A single human cell The Cell is the simplest collec4on of ma9er that can live Cells 4ssue organ The cell chromosomes ribosome Plasma membrane Ameoba-‐ single celled organism A single human cell The Cell is the simplest collec4on of ma9er that can live Cells 4ssue organ The cell chromosomes ribosome Cytoplasm Plasma membrane Ameoba-‐ single celled organism A single human cell The Cell is the simplest collec4on of ma9er that can live Cells 4ssue organ 2 Main cell types: Prokaryotic and Eukaryotic 3.5 billion yrs ago 1.5 billion yrs ago No organelles Only ribosomes Thick cell wall made of peptidoglycan Genetic material floats free Eukaryotic cells are ~ ten times larger than prokaryotic cells. No cell wall (animals) Or Cell wall made from cellulose (plants) eukaryotic cell An organelle is a specialized part of a cell Analogous to the organs of mul4cellular animals. All eukaryotic cells have a number of features in common. These include: nucleus cytoplasm cell membrane mitochondria endoplasmic reticulum Golgi apparatus ribosomes cytoskeleton centriole eukaryotic cell The three main parts of every eukaryo4c cell are: • The cell membrane (its outside or coa4ng) • The nucleus (its center or brain) • The cytoplasm (everything in between), which is also called cytosol The cell membrane: -‐ keeps the cell together. Separates living cell from non-‐living surrounds -‐ Is not solid. It has 4ny openings that let things in and out. -‐ exhibits selec4ve permeabilty = allowing some substances to cross more readily than others This is very important for life – allows nutrients in -‐ allows waste out Macrophage Macrophage T-cells Nerve cells Cellular membranes • Cell membranes are fluid mosaics of lipids and proteins • Cell membrane is made up of a phospholipid bilayer Phospho-lipids Amphipathic molecules Plasma membrane is made up of a phospholipid bilayer Creates a stable boundary between 2 aqueous compartments Discovered in the 1900’s Model of the cell membrane A plasma membrane will adhere to water Two schools of thought on how 1930’s 1960’s Model of the cell membrane 1. not all membranes have the same % of proteins associated with them 2. membrane proteins are not very soluble in water Singer and Nicholson Cell membrane Mosaic of proteins • Lipids are always moving sideways • Proteins driL slowly • Cholesterol act as a temperature buffer keeping the membrane fluid at moderate temperatures • Membranes must be fluid to work 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 Membrane proteins • 2 types of membrane protein1. Integral proteins penetrate the hydrophobic core, many are “transmembrane” 2. Peripheral proteins are not embedded in the membrane at all, they are oLen bound to exposed integral proteins or loosely to the surface of the membrane Membrane proteins Transmembrane protein eg Bacteriorhodopsin Transmembrane, hydrophobic regions are usually alpha helical 6 major Membrane protein func4ons Hydrophilic channel or Carrier protein Tight junc4ons formed between cells Team of enzymes Glycoprotein-‐ ID tag Stablises loca4on of certain proteins Transport across the membrane 3 modes of transport: Transport across a membrane • Regulation of transport across the cell membrane-essential its to existence Eg. Muscle cells Sugars, amino acids, oxygen, ions in out Carbon dioxide, ions, metabolic waste • Cell membrane is selectively permeable Diffusion-passive transport • What dictates the direction of 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 Selective Permeability • Membrane proteins – key role in regulating transport } Need protein transporters 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 High conc. of solute • Transport proteins are very specific • Channel proteins provide hydrophilic corridors Eg.s • Ion channels• Gated ion channelsstimulus regulated channel transporter: eg. Aquaporin for water or Sodium channel for Na+ Low conc. of solute Na+ Na+ Na+ Na+ Na+ Ligand gated ion channel eg. neurotransmi<ers 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 Facilitated Diffusion is s@ll passive because the solute moves down the concentra@on gradient Active transport-uses energy • Movement of substances against their concentration gradient across the membrane • From a low concentration to a high concentration • 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 Active transport • ATP supplies the energy for active transport • ATP can transfer its terminal phosphate group directly to the transport protein: transfer of energy phosphorylation • Eg of a carrier protein that requires ATP to transport is the sodium-potassium pump Adenosine triphosphate (ATP) is the cells energy molecule Sodium-potassium protein pump High in Na+ High in K+ Sodium and Potassium are being pumped against their concentration gradient Sodium is already higher outside the cell and potassium is already higher inside the cell Sodium-potassium protein pump Sodium-‐potassium pump = electrogenic pump = Generates a voltage across the membrane -‐More nega@vely charged inside a cell than outside -‐creates an “electrochemical” gradient 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, instead of using ATP • eg.: the diffusion of H+ ions back down their electrochemical gradient can be coupled to the active transport of Sucrose against its concentration gradient In Summary...Transport across the membrane Biology Campbell and Reece • Chapters 5, 7 and 8 http://www.tcd.ie/Biology_Teaching_Centre/local/junior-freshman/