Cell Ultrastructure & Function PDF

Cell Ultrastructure & Function BIOL1362 – Biochemistry 1 Dr. Nigel Austin Learning Outcomes…By the end of this lecture, you should be able to: State State characteristics of living cells Compare Compare prokaryotic and eukaryotic cells Match Match organelle...

Cell Ultrastructure & Function BIOL1362 – Biochemistry 1 Dr. Nigel Austin Learning Outcomes…By the end of this lecture, you should be able to: State State characteristics of living cells Compare Compare prokaryotic and eukaryotic cells Match Match organelles with their roles or localized events Distinguish between the roles of lysosomes and Distinguish proteasomes Describe Describe the basis of the limits on cell sizes Describe adaptations which developed to facilitate Describe greater cell sizes The Nature of Life Living systems possess: The ability to reproduce Metabolism (400-500 essential biochemical rnx) Mutagenicity – potential for change and to pass these changes to their offspring. Replicate and express genetic information Levels of Organization Like everything else in the physical universe, life is composed of lifeless atoms which form complex macromolecules. These biomolecules interact with each other to: Form organelles and other cellular components Form hundreds of metabolic pathways which metabolize compounds necessary for life processes such as: growth, repair, homeostasis, reproduction, defense and development. The Chemical Unity Of Diverse Living organisms are Living Organisms composed of lifeless molecules assembled from lifeless atoms C, H, O, N, P & S make up −92% of the dry wt of living things (living things are −70% H2O)  Seawater Human body Earth’s crust H- 66 H- 63 O- 47 O- 33 O- 25.5 Si- 28 Cl- 0.33 C- 9.5 Al- 7.9 Percentage Na- 0.28 N -1.4 Fe- 4.5 Composition Mg- 0.033 Ca- 0.31 Ca- 3.5 of Elements S- 0.017 P- 0.22 Na- 2.5 Ca- 0.0062 Cl- 0.08 K- 2.5 K- 0.0060 K- 0.06 Mg- 2.2 C- 0.0014 N? / P?  Characteristics of Cells Cells are the unit of structure and function They originate from existing cells They transduce energy to maintain life processes They have and maintain a selective barrier (plasma membrane) containing protoplasm Utilize genetic information to guide the synthesis of most cell components Contain catalysts (enzymes) which speed up chemical rxns. The sum of all rxns is referred to as metabolism (catabolism + anabolism). Metabolism Catabolism: Complex molecules broken down into simpler molecules (eg polysaccharides to monosaccharides or decreasing number of carbons) Exergonic – release energy usually by oxidation or hydrolysis Generates ATP and/or reducing equivalents (NADH, NADPH and or FADH2) Anabolism?  All Living Organisms Utilize Genetic Information The Central Dogma of Molecular Biology explains how genetic information is utilized to synthesize RNA and proteins which are not only common to all living things but are necessary to perform and regulate the reactions that maintain life (metabolism). These reactions are responsible for the synthesis of complex molecules which make cellular structures and perpetuate the reproduction of the genetic information. Cell Structure  Classification of Cells Cells are classified based on the presence or absence of: A membrane bound nucleus A membrane bounded organelles Two Cell Types: Prokaryotic cells which lack the above Eukaryotic cells which possess the above  The eubacteria are distinguished from the archaebacteria using phylogenetic methods All cells are surrounded by a plasma membrane (8nm Prokaryotes thick). Membrane lipids of the archaea are ether-linked Prokaryotes Most abundant organisms Lack a complex system of interconnected internal membranes found in eukaryotes #EndomembraneSystem Exist in a variety of shapes spherical (cocci) rodlike (bacilli) helically coiled (spirilla) Prokaryotes There are no membranous organelles – the plasma membrane is usually infolded to form mesosomes The prokaryotic chromosome is found in the cytosol It consists of a single circular molecule of DNA which is condensed to form the nucleoid  Bacterial Cell Walls The cell wall (3-25nm) is made of peptidoglycan - a heteropolymer of substituted glucose (GlcNAc and Mur2Ac) Cross-linked to a short tetrapeptide containing a D-amino acid Each heteropolymer chain is linkedto another via the tetrapeptide by a pentaglycine cross-link  Trait Prokaryotic Eukaryotic Nucleus absent present Diameter 1-10 m 10-100 m Cytoskeleton present present Cytoplasmic absent present Organelles Prokaryotes DNA content 1x106 - 5x106 1.5x107 - 5x109 vs. (base pairs*) Eukaryotes DNA Single circular DNA molecule Multiple linear DNA molecules Ribosomes Smaller 70S Larger 80S Cell wall usually present present/absent Mode of cell Mitosis & cell fission division meiosis Prokaryotic Cytoskeleton The cytoskeleton is an internal network of proteinaceous filaments and tubules that maintains cell shape, internal organization and provide mechanical support. It also plays a major role in cell division and establishing cell polarity. Thought to have been absent in prokaryotes. In the last decade, homologous cytoskeletal structures have been discovered in prokaryotes Tubulin – FtsZ Actin – MreB Intermediate filaments – CreS (cresentin) Eukaryotic Cellular Architecture Eukaryotic Cell Architecture  Eukaryotic Cells Contain membrane bounded organelles Contain a membrane bounded nucleus & system of internal membranes Unicellular (Protists) Multicellular (fungi, plants, animals) Linear DNA  Membranous Organelles Functional components within cytoplasm Bounded by membranes Compartmentalizes cell so specialized activities occur in specific places Many increase surface area to volume ratio  Biological Membranes - Composition All biological membranes are composed primarily of 2 main components – lipids & proteins Composed of a primarily of: a lipid bilayer (made of a double layer of phospholipids) Proteins Sterols eg Cholesterol The various types and ratios of proteins and lipids in the different membranes of a cell are responsible for their varying functions  Plasma Membrane Characteristic of ALL cells Selective barrier that regulates the cell’s internal composition Surrounds and delimits cell Separates external & internal environment Carbohydrates are also found in the plasma membrane but always associated with lipids - glycolipids Bonded with proteins - glycoproteins Location - exterior of the membrane  Membrane Protein Functions Membrane proteins have 6 main functions 3 are characteristic of ALL membranes Transport Enzymatic activity Receptors (Signal transduction) 3 exclusive to the plasma membrane Cell-cell recognition Cell-cell adhesion Attachment to the cytoskeleton  Cell Walls An extracellular matrix found in plants, fungi, many protists and bacteria Found external to and surrounds plasma membrane Plants – mostly cellulose Fungi – contain chitin  Cellular Structures Membrane-bound Organelles Macromolecular Structures Single Double Cellular Inclusions (Non-membranous organelles) membrane membrane Glycogen granules endoplasmic ribosomes reticulum nucleus Phosphate granules centrioles Golgi apparatus mitochondria lysosome Lipid droplets cytoskeleton chloroplast vacuole Pigment granules nucleolus peroxisome proteasome glyoxysome Non-Membranous Organelles - Ribosomes Ribosomes are made in the nucleolus inside in the nucleus They are the site of translation (protein synthesis) Made from ribonucleoprotein (rRNA and proteins) organized into 2 differently sized subunits Occurs in 2 sizes 70S (30S and 50S) – prokaryotes 80S (40S and 60S) – eukaryotes In eukaryotes it is found in 3 places: The outer nuclear membrane The rough ER Free in the cytosol Cytoskeleton Composed of filaments & fibers 3 types of cytoskeletal elements Microfilaments – monomer – G-actin Intermediate filaments Microtubules – monomer - tubulin Functions include: Mechanical support Maintenance of cell shape Whole cell movement Anchor organelles Intracellular movement Organellar movement – eg cyclosis (cytoplasmic streaming)  Proteasome Proteasomes are cylindrical structures which degrade unneeded or damaged endogenous proteins eg transcription factors, cyclins, mutated and misfolded proteins, viral and parasitic proteins Proteins selected for degradation are tagged with a small protein called ubiquitin Ubiquitin targets the damaged protein to the proteasome where it is degraded by proteolysis. Proteasomes are found in both prokaryotes and eukaryotes In eukaryotes they are found in both the cytosol and nucleus  Single Membraned Organelles - Lysosomes Lysosomes contain highly hydrolytic enzymes which function at an acidic pH. Unlike proteasomes, lysosomes degrade primarily: Extracellular proteins brought into the cell by pinocytosis eg plasma proteins Bacteria and other particles brought into the cell by phagocytosis Cell-surface membrane proteins that are used in receptor-mediated endocytosis. Organelles, proteins (and other macromolecules, some viruses etc) engulfed by autophagosomes. Smooth Endoplasmic Reticulum (SER) Site of lipid metabolism and transport Phospholipids, ceramides, cholesterol and steroid hormones are synthesized here Stores and releases cellular Ca2+ Detoxification of lipophilic drugs Glucose homeostasis because it contains glucose-6-phosphatase Along with the RER is responsible for production of all biological membranes  Rough Endoplasmic Reticulum Has ribosomes attached to surface which synthesize proteins (translation) Not all ribosomes in cells attached to rough ER Modifies and transports proteins (post- translational modification) from attached ribosomes in lumen usually for secretion Protein folding Assembly of multisubunit proteins Disulfide bond formation Initial stages of glycosylation Golgi Apparatus Composed of flattened membranous sacks called cisternae Proteins arriving from the RER in vesicles fuse and are further modified in the Golgi Some post-translational modifications which occur include Glycosylation Sulphation Proteolytic cleavage Proteins, lipids and carbohydrates are sorted, packaged and targeted to their desired destinations by the Golgi  Nucleus Surrounded by a double membrane (nuclear envelope) Has pores which regulates the entry and exit of large molecules (namely RNA and proteins) Information Centre of the cell as it is the major repository of genetic material (DNA) in the cell Control centre of the cells as it contains all the genes which makes all of the proteins required for growth, metabolism, differentiation and repair of cells. Site of transcription – synthesis of RNA synthesis using DNA as the template Contains the nucleolus where rRNA is transcribed, as such, it is the site of synthesis and assembly of ribosomes Glyoxysomes and Peroxisomes Lipid metabolism occurs in both organelles Peroxisomes Contain enzymes to degrade H2O2 eg catalase Contains anti-oxidants to scavenge free radicals and reactive oxygen species (ROS) Glyoxysomes Found in plants and filamentous fungi In addition to breakdown of fatty acids, in seeds, it converts lipids to carbohydrates in the glyoxylate cycle  The Endomembrane System The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport mainly lipids and proteins. Consists of: ER Golgi apparatus Lysosomes Endosomes Plasma membrane Vesicles The Endosymbiotic Theory The endosymbiotic theory states that various eukaryotic cell types evolved via symbiosis with specialized prokaryotes First proposed by Lynn Margulis (1970) Explains the origin of eukaryotic cells Explains the origin of mitochondria and chloroplasts in eukaryotic cells  Evidence for the Endosymbiotic Theory Similarities between mitochondria, chloroplasts, & prokaryotes: Multiple copies of a circular genome (DNA molecules) Smaller 70S ribosomes Mode of division - binary fission The starting amino acid for protein synthesis is N-formylmethionine Chloroplasts and mitochondria both phylogenetically related to bacteria Mitochondria Has double membrane. Inner membrane highly invaginated to form cristae Site of oxidative metabolism Many pathways/processes occur in the mitochondria such as: The Krebs Cycle Electron Transport Oxidative phosphorylation (ATP synthesis) Pyruvate decarboxylation -oxidation Amino acid metabolism Heme biosynthesis Urea cycle (partially)  Chloroplasts Chloroplasts are found in plant cells and green algae and are the site of photosynthesis Chloroplasts, like mitochondria, are semi-autonomous, having their own circular extra-nuclear DNA. They also produce some proteins and lipids required for the chloroplast membrane (also mitochondria) Other pathways which occur besides the light dependent (photophosphorylation #ATP and NADPH synthesis) and light independent reactions (Calvin cycle) of photosynthesis include: Carbohydrate synthesis Lipid synthesis (galactolipids and sulfolipids found in thylakoid) Amino acid synthesis Electroscopy – know how organelles look under the electron microscope! Review of Eukaryotic Cells Comparison of Bacterial, Animal and Plant Cells