Lecture 1 Vision - Cell Structure and cell membranes PDF

Structure and function of the cell and its organelles Dr Catherine Wright 1  Learning objectives At the end of these lectures you should appreciate: Understand the structure, organisation and contents of the cell Composition of cell...

Structure and function of the cell and its organelles Dr Catherine Wright 1  Learning objectives At the end of these lectures you should appreciate: Understand the structure, organisation and contents of the cell Composition of cell membranes Phospholipid structure and function Mosaic models of membranes Structure and function of membrane proteins And understand: Osmosis and membranes Active vs. passive transport across membranes The functions of bulk transport 2  Cells of the eye Conjunctiva Lens  Cell Diversity Smooth muscle photoreceptor Nerve cell epithelial Red blood cell Differ in appearance and structure but have same characteristics  Eukaryotic Cells Nucleus Nuclear envelope Ribosomes Nucleolus Rough endoplasmic reticulum Nuclear pore Smooth endoplasmic reticulum Have a membrane-bound Intermediate filament Microvilli nucleus Cytoskeleton Actin filament (microfilament) Microtubule Ribosomes Intermediate filament More complex than prokaryotic cells Centriole Hallmark is Cytoplasm Lysosome compartmentalisation – Achieved through use of Exocytosis membrane-bound organelles Vesicle and endomembrane system Golgi apparatus Plasma membrane Peroxisome Mitochondrion Possess a cytoskeleton for support and to maintain cellular structure 5  Video https://vimeo.com/117588519 BBC Our Secret Universe - The hidden Life of the Cell 6  The Nucleus Size: 5 – 7 µm - where genetic information is stored In eukaryotes, the DNA is divided into multiple linear chromosomes – Chromatin is chromosomes plus protein (histones) – Chromosomes are folded in a double helix so huge amounts of info can be stored in a tiny space Most eukaryotic cells possess a single nucleus Nucleolus – region where ribosomal RNA synthesis takes place – ribosomes make proteins Nuclear envelope – 2 phospholipid bilayers (7-8 nm) – Nuclear pores (0.1 µm) – control passage in and out 7  The Nucleus Nuclear pores Nuclear envelope Nucleolus Chromatin Nucleoplasm Nuclear lamina Inner Nuclear membrane basket Outer membrane Cytoplasmic filaments Nuclear pore Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8  Ribosomes The protein synthesis machinery Found in all cell types - essential Ribosomal RNA (rRNA) Messenger RNA (mRNA) Transfer RNA (tRNA) rRNA-protein complex Protein synthesis also requires mRNA and tRNA Ribosomes may be free in cytoplasm or associated with internal membranes 9  Endoplasmic Reticulum Rough endoplasmic reticulum (RER) Ribosomes – Attachment of ribosomes to the Rough membrane gives a rough appearance endoplasmic reticulum – Synthesis of proteins to be secreted, sent to lysosomes or plasma membrane Smooth Smooth endoplasmic reticulum endoplasmic reticulum (SER) Rough endoplasmic – Relatively few bound ribosomes reticulum – Variety of functions – synthesis, stores Ca2+, detoxification Smooth Ratio of RER to SER depends on endoplasmic reticulum 0.08 µm cell’s function 10  Golgi apparatus Flattened stacks of interconnected membranes (Golgi bodies) Transport vesicle 20 -100 nm x 15-20 nm with 20-30 nm gaps cis face Fusing Functions in packaging vesicle Forming vesicle and distribution of trans face Secretory molecules synthesized at vesicle one location and used at another Has cis and trans faces Vesicles transport 1 µm molecules to destination 11  Vesicles Fluid enclosed by a lipid bilayer membrane Can form during exocytosis, phagocytosis and endocytosis For transport of materials within the cytoplasm Vesicles are used by the cell for organizing cellular substances: involved in – metabolism – transport – enzyme storage 12  Lysosomes Nucleus Small (0.5 µm) membrane- Ribosome Nuclear pore bounded digestive vesicles Rough endoplasmic reticulum Arise from Golgi apparatus Membrane protein Hydrolytic enzyme Enzymes catalyze Transport vesicle breakdown of cis face Golgi membrane protein Smooth endoplasmic macromolecules Cisternae reticulum Golgi Destroy cells or foreign trans face Apparatus Old or damaged Lysosome Digestion organelle matter that the cell has Lysosome Food vesicle engulfed by phagocytosis Involved in Autophagy Breakdown of organelle Lysosome aiding in the Phagocytosis Lysosome aiding in the breakdown of an old organelle digestion of phagocytized particles 13  Mitochondria Found in all types of eukaryotic cells Bound by membranes – Outer membrane – Intermembrane space – Inner membrane has cristae – Matrix On the surface of the inner membrane, and also embedded within it, are proteins that carry out oxidative metabolism Have their own DNA and ribosomes 14  Mitochondria Ribosome Matrix DNA Crista Intermembrane space Inner membrane Outer membrane 0.2 µm Copyright © The McGraw-Hill Companies, Inc. Permission 15 (inset): © Dr. Donald Fawcett & Dr. Porter/Visuals Unlimited required for reproduction or display.  Mitochondria A “cell within a cell” Mitochondria are the cell’s power house - essential Metabolise sugars to make ATP which the cell uses for energy Mitochondrial division is separate during cell division – double and partition Mitochondrial dysfunction diseases 16  Cytoskeleton - 3 types of cell fibers Microfilaments (actin filaments) – Two protein chains loosely twined together – Movements like contraction, crawling, “pinching” Microtubules – Largest of the cytoskeletal elements – Dimers of α- and β-tubulin subunits – Facilitate movement of cell and materials within cell Intermediate filaments – Between the size of actin filaments and microtubules – Very stable – usually not broken down 17 Cell fibres within the cell Microtubule Intermediate filament Actin filament Cell membrane a. Actin filaments b. Microtubules c. Intermediate filament 18 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Centrosomes Region surrounding nucleus in almost all animal cells – specialised units = centrioles Microtubule-organizing center – Can ‘nucleate’ the assembly of microtubules Animal cells and most protists have centrioles – pair of organelles Plants and fungi usually lack centrioles 19 Real-Time Microscopy of Microtubules during mitosis 20  Cell membranes Cell membranes surround all living cells and are only 4 nm thick Essential for life – defines cell boundary Also called: Plasma membrane Supports the cell Regulates what moves in and out of cell Allows cell signalling and cell recognition Defines organelle boundaries inside cell  Membrane Structure The membrane is made up of: 1. Phospholipids arranged in a bilayer (~ 50%) 2. Proteins inserted in the bilayer (~ 50%) Integral proteins (e.g. connexins) – sometimes glycosylated Peripheral proteins (e.g. clatherin adaptor protein) – can separate from membrane 3. Cholesterol (and other sterols) – make the membrane less permeable to hydrophilic molecules Hydrogen bonding of water holds the 2 layers together Fluid structure – proteins and lipids in membrane can move easily 22  Fluid mosaic model of membranes Extracellular matrix protein Glycoprotein Glycolipid Integral Integral proteins proteins Glycoprotein Glycoprotein Cholesterol Actin filaments of cytoskeleton Peripheral protein Intermediate filaments of cytoskeleton Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fluid mosaic model: mosaic of proteins floats in or on the fluid lipid bilayer like icebergs in the sea 23  Membranes are fluid They are not static They are active – Move against each other – In relation to Extra Cellular Matrix (ECM) Cell migration, cell-cell contact through ECM – Membrane proteins move in and out constantly Mostly phospholipids move about laterally in membrane but can sometimes move transversely (flip-flop) Fatty acid composition and temperature affect fluidity Hydrophilic / hydrophobic charges allow phospholipids to form membranes Cell 1 Extracellular fluid Polar hydrophilic heads Plasma membrane of cell 1 Nonpolar hydrophobic tails Polar hydrophilic heads Cell 2 Intracellular fluid (cytosol) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 26  Membrane Proteins Various functions: 1. Transporters 2. Enzymes 3. Cell-surface receptors 4. Cell-surface identity markers 5. Cell-to-cell adhesion proteins 6. Attachments to the cytoskeleton 27  Function of plasma membrane proteins Outside Inside cell cell Transporter Enzyme Cell surface receptor Cell surface identity marker Cell-to-cell adhesion Attachment to the cytoskeleton 28 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Transmembrane proteins Proteins need only a single transmembrane domain to be anchored in the membrane, but they often have more than one such domain Copyright © The McGraw- Hill Companies, Inc. a. Many transmembrane domains b. Single transmembrane Permission required for reproduction or display. domain 29  Membrane pores Extensive nonpolar regions within a transmembrane protein can create a pore through the membrane Cylinder of  sheets in the β-pleated sheets protein secondary structure called a -barrel – Interior is polar and allows water and small polar molecules to pass through the membrane 30  Membrane lipids are distributed asymmetrically Lipids are distributed more to inner or outer side of membrane Membrane glycoproteins and glycolipids usually have their carbohydrate portions outward facing Lipids can be laterally organised: can form lipid rafts – Act as micro-domains – Lipid and protein enriched and highly dynamic – Apical and basolateral domains – Platforms for cell signalling  Review questions 1. What are the components of cell membranes? 2. How do phospholipids form membranes? 3. Describe the fluid mosaic model of membranes 4. What are the three main fibres present in cells? 32  Transportation of materials across the plasma membrane 33  Membrane Structure 34 http://images.tutorvista.com/cms/images/38/plasma-membrane.png  Transport across cell membranes: Passive Transport or Diffusion Passive transport: Simple Diffusion [High] Diffusion O2 CO2 non-polar molecules Passive transport: Facilitated Diffusion [Low] ionic (charged) polar molecules, water, glucose movement from high to low concentration 35  Transport across cell membranes: Active Transport Active transport: Charged ions e.g. Na+, K+ [Low] Metabolites, glucose Transport Movement from low to high concentration Uses carrier proteins / cell channels – (Na+-K+)-ATPase [High] Requires energy ATP 36 Osmosis video 37  Osmosis – Key Terms Cytoplasm of the cell is an aqueous solution – Water is a solvent – Dissolved substances are solutes Osmosis: net diffusion of water across a membrane toward a higher solute concentration 38  Osmosis Occurs when: – a membrane separates two solutions with different concentrations of solutes – the concentrations of free water molecules on the two sides of the membrane are different 39  Osmosis This difference creates the concentration gradient: water moves down the [High] gradient Diffusion The concentration of all solutes in a solution determines the osmotic concentration of the solution [Low] 40  Osmosis Water Urea molecule molecules Semipermeable membrane Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 41  Osmotic concentration When 2 solutions have different osmotic concentrations – Hypertonic solution has a higher solute concentration – Hypotonic solution has a lower solute concentration When two solutions have the same osmotic concentration, the solutions are isotonic 42  Osmotic pressure Force needed to stop osmotic flow Cell in a hypotonic solution gains water causing cell to swell – creates pressure If membrane strong enough, cell reaches counterbalance of osmotic pressure driving water in with hydrostatic pressure driving water out – Cell wall of prokaryotes, fungi, plants, protists If membrane is not strong, it may burst – Animal cells must be in isotonic environments – https://www.youtube.com/watch?v=0c8acUE9Itw 43 44  Effects of osmosis Hypertonic Isotonic Hypotonic Solution Solution Solution Human Red Blood Cells Shriveled cells Normal cells Cells swell and eventually burst 5 µm 5 µm 5 µm Plant Cells Cell body shrinks Flaccid cell Normal turgid cell from cell wall Copyright © The McGraw-Hill Companies, Inc. © David M.Phillips/Visuals Unlimited 45 Permission required for reproduction or display.  Maintaining osmotic balance Some cells use Paramecium vacuole extrusion in which water is ejected through contractile vacuoles Isosmotic regulation involves keeping cells isotonic with their environment – Extrusion – Isomotic regulation – Turgor 46  Transport across cell membranes Active Transport Active transport: Charged ions e.g. Na+, K+ [Low] Metabolites, glucose Transport Movement from low to high concentration Uses carrier proteins / cell channels – (Na+-K+)-ATPase [High] Requires energy: ATP 47  Active Transport Active transport uses energy (ATP) to move materials against a concentration gradient Carrier proteins used in active transport include – Uniporters – move one molecule at a time – Symporters – move two molecules in the same direction – Antiporters – move two molecules in opposite directions Terms can also be used to describe facilitated diffusion carriers 48 Cotransport: Symport and antiport: animation 49  ATP-dependent membrane transporters P-type ATPases: – undergo phosphorylation – transport Na+, K+ and Ca2+ F-type ATPases: – proton-transporters in mitochondria and bacteria V-type ATPases: – in lysosomes A-type ATPases: – anion transporters ABC transporters: – have an ATP-binding cassette and transport ions, metabolites and drug molecules 50 51  Sodium–potassium (Na+–K+) pump The (Na+-K+)-ATPase Direct use of ATP for active transport Is an antiporter - moves 3 Na+ out of the cell and 2 K+ in to the cell against their concentration gradients ATP energy is used to change the conformation of the carrier protein Affinity of the carrier protein for either Na+ or K+ changes so the ions can be carried across the membrane Uses a lot of cellular energy More than 1/3 of all cell energy in a cell that is not actively dividing is used in the active transport of Na+ and K+ 52  Sodium–potassium (Na+–K+) pump Na+ Extracellular K+ P Intracellular + ADP 1. Carrier in membrane binds ATP intracellular sodium. 6. Dephosphorylation of protein triggers change back to original conformation, 2. ATP phosphorylates protein with with low affinity for K+. K+ diffuses into bound sodium. the cell, and the cycle repeats. P P P 3. Phosphorylation causes P conformational change in protein, 5. Binding of potassium causes reducing its affinity for Na+. The Na+ dephosphorylation of protein. 4. This conformation has higher affinity then diffuses out. for K+. Extracellular potassium binds to exposed sites. 53 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sodium–potassium (Na+–K+) pump 1. 3 x Na+ bind to cytoplasmic side of the protein → conformation change 2. Protein binds 1 x ATP → ADP and phosphate (Pi). ADP released, but Pi is linked to protein. The protein is phosphorylated 3. Protein phosphorylation → conformational change. This translocates the 3 x Na+ outwards across the membrane. The 3 x bound Na+ break away and diffuse into the extracellular fluid 4. The new conformation has a high affinity for K+ - 2 x K+ bind to the extracellular side of the protein 5. → conformational change again, and hydrolysis of the phosphate group 6. Protein is dephosphorylated and reverts to its original shape - 2 x K+ enters the cytoplasm. The original conformation has a high affinity for Na+. When these ions bind, they initiate another cycle In every cycle, 3 x Na+ leave the cell 54 How the sodium potassium pump works: animation 55  Coupled transport Uses ATP indirectly Uses the energy released when a molecule moves by diffusion to supply energy to active transport of a different molecule Symporter is used Glucose–Na+ symporter captures the energy from Na+ diffusion to move glucose against a concentration gradient 56  Coupled transport Outside of cell Na+ Glucose Na+/ K+ pump Coupled transport protein ATP ADP + Pi Inside K+ of cell Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 57  Review questions 1. What is the difference between active transport and diffusion? 2. How does the Na+/K+ pump work? 3. Explain how coupled transport uses cellular energy 58  Bulk Transport Endocytosis – Movement of substances into the cell – Phagocytosis – cell takes in particulate matter – Pinocytosis – cell takes in only fluid – Receptor-mediated endocytosis – specific molecules are taken in after they bind to a receptor Exocytosis – Movement of substances out of cell – Requires energy 59  Endocytosis Bacterial cells Plasma membrane Cytoplasm a. Phagocytosis 1 m Solute Plasma membrane Cytoplasm b. Pinocytosis 0.1 m Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a: © CDC/Dr. Edwin P. Ewing, Jr. b: © BCC Microimaging, Inc.Reproduced with permission 60  Endocytosis Target molecule Receptor protein Coated pit Clathrin Coated vesicle c. Receptor-mediated endocytosis 90 nm Cholesterol can enter the cell via endocytosis. Familial hypercholesterolemia: – Faulty LDL receptors – Do not trigger vesicle formation. – Cholesterol accumulates inside arteries. – Cholesterol plaques are formed - Atherosclerosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (both): © Reproduced with permission from M.M. Perry and A.B. Gilbert, “Yolk transport in the ovarian follicle of the hen (Gallus domesticus): lipoprotein-like 61 particles at the periphery of the oocyte in the rapid growth phase,” Journal of Cell Science, 39:257-72, October 1979. © The Company of Biologists  Exocytosis Plasma membrane Secretory product Secretory vesicle Cytoplasm a. b. 70 nm Discharge of materials out of the cell Used in animals to secrete hormones, neurotransmitters, digestive enzymes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. b: © Dr. Brigit Satir 62  Multiple mechanisms of movement across cell membranes 1. Diffusion 2. Osmosis 3. Active transport 4. Endocytosis 5. Exocytosis 63  Review questions 1. What sort of molecules are taken into cells via bulk transport? 2. What happens in pinocytosis? 3. Does bulk transport require energy input? 64