Published on February 27, 2014
Cell and Sub-cellular Structures Dr.GANESH
Cell Cell is the Basic Structural and Functional Unit of all Living Organisms • Therefore, Evolution of cell is a crucial milestone in the evolution of life.
Cell structure •A eukaryotic cell contains a)plasma membrane, b)cytosol and c)subcellular organelles. • Cell is a closed compartment containing aqueous fluid called – cytosol surrounded by cell membrane plasma membrane called –
Cell • Subcellular organelles are bathed by cytosol and include – nucleus, mitochondria, endoplasmic reticulum, ribosomes golgi apparatus (golgi complex), lysosomes, peroxisomes, and cytoskeleton.
Cell • Cell = Plasma membrane + cytoplasm • Cytoplasm = Cytosol + Subcellular organelles
Cell All subcellular organelles, except ribosomes and cytoskeleton, are compartments within the cell, surrounded by cell membrane and containing their own aqueous fluid
Cell • Diversity of cell types serves the function of the particular tissues and organs in which they are present. • Depending on the function, different cell types differ in their organelle content, or their organelles may contain different amounts of particular enzymes or structural molecules.
Cell Membrane Contents: • Structure • Function • Transport Across Cell Membrane
Cell Membrane • thin hydrophobic sheet in fluid state which envelopes the cell • made up of lipid bilayer (two layers) • containing also proteins • Lipid and Protein molecules are bound to each other by non-covalent bonds • Carbohydrates, found in lesser amounts are bound to lipid and protein molecules by covalent bonds
Cell Membrane Membranes • Define the external boundaries of cells and organelles • Maintain their integrity and • Serve to compartmentalize functions within the cells.
Salient features of Cell Membrane • Membranes are flexible (because they are fluid), elastic and self-sealing • flexibility permits the shape changes that accompany cell growth and movement of cells, gives stability Also enables the cell to perform exocytosis and endocytosis
• Membranes are selectively permeable to molecules. -Being hydrophobic membranes are permeable to only lipid soluble/hydrophobic substances and impermeable to hydrophilic/polar substances. -However, membranes have transport systems (made of proteins) to permit and regulate the movement of polar compounds across its thickness.
Functions of Cell Membrane 1. Cell Membranes maintain the shape and size of the cells. 2. Protects the cytoplasm and the cell organelles from the external environment 3. The intracellular membranes serve to compartmentalize functions within the cells. 4. Membranes regulate the transport of substances like nutrients, ions, gases, water, various products, wastes into and out of cells and their organelles.
5.Membranes bound enzymes carry out metabolic reactions near the inner surface of the cell membrane. Egs:Succinate dehydrogenase . 6.Membranes are involved in signal transduction; i.e. proteins in membranes detect specific signals transmit such signals to the cell interior by specific chemical events. 7. Membrane mediates cell-to-cell communication between adjacent cells by gap-junctions. 8. Membrane regulates the flow of information between cell and its environment
Structure of Cell Membrane Membranes are sheet-like complex structures composed of • Lipids • Proteins and • Carbohydrates
Structure of Cell Membrane • Lipid bilayer conformation is the basic structure of all biological membranes. • lipid bilayer is made up of amphipathic lipid molecules (having both hydrophilic or polar part and a hydrophobic or non-polar part) crucial in the formation of membrane structure
Structure of Cell Membrane Fluid Mosaic Model proposed by Singer and Nicolson to explain the structure of cell membrane According to this, • membrane is a fluid lipid bilayer with a mosaic of embedded proteins
Membrane Components Phospholipids Proteins (peripheral and integral) Cholesterol Carbohydrates
In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer It’s like a fluid… It’s like a mosaic… It’s the Fluid Mosaic Model! AP Biology
Structure of Cell Membrane In other words, the model is compared to icebergs (membrane proteins) floating in a sea ( predominantly phospholipid molecules)
Structure of Cell Membrane Membranes are 5-8 nm thick and appear trilaminar when viewed through an electron microscope
Structure of Cell Membrane • Different membranes within the cell and between cells have different compositions • This difference reflects the diversity of biological roles of these membranes
Structure of Cell Membrane For e.g. • myelin sheath of neurons, which acts as an electrical insulator, is rich in lipids whereas • inner mitochondrial membrane in which many enzyme-catalyzed processes take place contains more proteins than lipids
Lipids All Lipids present in cell membrane are – Amphipathic Lipids Compound Lipids and Cholesterol (Free Cholesterol) Phospholipids Glycolipids conceived as having a – polar head and a non-polar tail.
Amphipathic Lipid Polar Head Non-polar Tails
Lipids Amphipathic lipids self-assemble in aqueous medium into bilayer sheets (lipid bilayer) with their hydrophobic parts (non-polar tails) of each layer facing and interacting with each other forming a hydrophobic membrane core and hydrophilic parts (polar heads) facing towards the two surfaces interacting with the aqueous medium
Lipid Monolayer Aqueous Medium Non-polar Medium
Lipid Bilayer Aqueous Medium Aqueous Medium Hydrophobic Core
Lipids of cell membrane Ampipathic lipids such as • phospholipids (e.g. lecithin,cephalin,sphingomyelin) • glycolipids and • cholesterol.
Lipids Specific type of lipid may be present in particular tissues Example: • Nerve tissues have large quantity of glycolipids and sphingomyelins. • Mitochondrial membrane rich in cardiolipin.
Proteins • Make up about 50% of total membrane mass In a typical cell • Distributed Asymmetrically in the lipid bilayer
Proteins • Membrane proteins are of 2 types. 1.Peripheral membrane proteins attached to the lipid bilayer on either surface E.g. succinate dehydrogenase (TCA Cycle), endoplasmic reticulum enzymes, etc 2. Integral membrane proteins deeply embedded in the lipid bilayer. – Some integral membrane proteins may completely span the lipid bilayer – –transmembrane proteins e.g. receptor proteins, transport proteins, channel proteins, etc).
Cell Membrane proteins Peripheral membrane proteins Integral Transmembrane proteins membrane proteins Examples Integral membrane proteins •Receptor proteins • Transport proteins • Channel proteins
Many Functions of Membrane Proteins Outside Plasma membrane Inside Transporter AP Biology Enzyme activity Cell surface receptor Cell surface identity marker Cell adhesion Attachment to the cytoskeleton
Membrane Carbohydrates • relatively minor components 5-8% of the total membrane mass. • covalently linked to lipids and proteins as glycolipids and glycoproteins. • Located on the extra cellular side of the membrane, which forms a loose outer carbohydrate coat called Glycocalyx
Functions of Glycocalyx • Gives a net negative surface charge and repel from other electrically negative particles. • Cell to cell attachment is possible. • Part of receptor substances for binding hormones such as insulin • Some of the carbohydrate moieties enter into immune reaction.
Cell Membrane Carbohydrates Glycolipid Glyccalyx Glycoprotein
Oligosaccharide Peripheral Membrane Protein Glycolipid Glycoprotein Integral Membrane Proteins
Cell Membrane Structure Glycolipid Lipid Bilayer Peripheral membrane proteins Glycoprotein Polar head Non polar tail Integral Transmembrane proteins membrane proteins
Membrane Components Phospholipids Proteins (peripheral and integral) Cholesterol Carbohydrates
Fluidity of Membranes Membrane consists of a mosaic of lipids and proteins that can move laterally (so fluid) in the plane of the membrane. fluidity makes the membrane • flexible (which in turn permits the shape changes that accompany cell growth and cell movements), • increases permeability ,gives stability and enables them to • invaginate or evaginate allowing them to ingest or to expel materials.
Factors Affecting the Fluidity • Unsaturated cis-fatty acids • Short chain fatty acids and • High temperature Increase the membrane fluidity. Whereas, Cholesterol decreases the membrane fluidity
Specialised Membrane Structures • Tight Junction • seen in epithelial cells, where the lateral membrane of a cell is fused with lateral membrane of adjacent cell. This prevents the movement of molecules through the gap between the cells. This ensures that, molecules move only through the luminal side to the serosal side. E.g.: Seen in gastrointestinal epithelial cells. • Myelin Sheath Specialized structure for the conduction of nerve impulse, rich in lipids.
Specialised Membrane Structures • Synaptic membranes: Cell membranes associated with synapses. Required for the release or reception of neurotransmitters. • Microvilli: Hair like projections produced by the membrane evagination, which increases absorptive surface area. Eg: intestinal epithelial cells.
Specialized Membrane Structures Tight Junction Tight Junction Eg: gastrointestinal epithelial cells For cell to cell communication Microvilli Eg: gastrointestinal epithelial cells Enhance absorption of food Myelin sheath Myelin Sheath Eg: neurons For conduction of nerve impulse Synaptic membrane Eg: neurons Transmit information between neurones
Any Questions?? AP Biology
Cell membrane is the boundary between inside & outside… separates cell from its environment Can it be an impenetrable boundary? NO! OUT IN food carbohydrates sugars, proteins amino acids lipids salts, O2, H2O AP Biology OUT IN waste ammonia salts CO2 H2O products cell needs materials in & products or waste out
Transport Across Cell Membrane • Membranes act as effective barrier for the passage of molecules, thereby keeping some substances inside the cell and others out. • Yet they also contain transport systems which confer on membranes the important property of selective permeability by allowing specific molecules to be taken up and unwanted compounds to be removed from the cell
Transport Across Cell Membrane • As the membrane core is hydrophobic in nature hydrophobic molecules move more readily across the membrane than hydrophilic ones. • As the membrane fluidity increases, permeability to hydrophilic substances also increases
Transport across cell membrane What molecules can get through directly? fats & other lipids inside cell NH3 What molecules can lipid salt NOT get through directly? polar molecules H 2O outside cell sugar aa H 2O ions salts, ammonia large molecules starches, proteins AP Biology
Transport across cell membrane Membrane becomes semi-permeable with protein channels specific channels allow specific material across cell membrane inside cell NH3 AP Biology salt H 2O aa sugar outside cell
Classification of Transport Across Cell Membrane Membrane Transport Small Molecules Passive transport (Energy independent) Simple Diffusion Macromolecules & Particles Active transport (Energy dependent, Carrier mediated) Facilitated Transport (Carrier mediated) Ion-channels Endocytosis Eocytosis
Another Way to Classify Transport of Small Molecules Non-mediated transport (no carrier proteins) Simple Diffusion Facilitated Transport (Passive transport) Carrier mediated Ion-channels Active transport (Energy dependent,)
Contents •Transport of Small Molecules o Non-mediated transport (no carrier proteins) -- Simple Diffusion -- Ion-channels o Carrier mediated -- Facilitated Transport(Passive) -- Active transport(Energy dependent) •Macromolecules & Particles
Non-mediated Transport (no carrier proteins) Simple Diffusion Very Small molecules (like water) and gases (CO2,O2) enter the cell by this method. It is a very slow process. Doesn’t require energy (energy independent/passive). It is a non-mediated transport (no carrier proteins involved). Molecules diffuse from a region of higher concentration to a region of lower concentration (down the concentration gradient) diffusion occurs through a membrane opening or through intermolecular spaces.
Simple diffusion Eg: a) Respiratory exchange of gases between pulmonary alveolar membrane and tissue capillary wall b) Intestinal absorption of pentoses, some mineral ions and water-soluble vitamins and c) renal reabsorption of urea
Ion-channels • specialized protein molecules that span the membranes & permit the rapid transport of ions such as Na+, K+, Cl-. • The channels generally remain closed but in response to stimulus, open allowing rapid flux of ions down the gradient
Carrier Mediated Transport • specific carrier molecules are required • protein in nature. • Have specific binding sites for the molecules to be transported • Transport is dependent on availability of free binding sites on the carrier protein • more rapid than simple diffusion.
Classification of Carrier Mediated Transport There are 2 Ways of Classification 1. Depending upon Number of Molecules Transported and Direction of Transport 2. Depending upon Whether Energy is Required or not
Classification of Carrier Mediated Transport 1. • Uniport • Co-transport - Symport and - Antiport 2. • Facilitated Transport(Passive) • Active transport(Energy dependent)
Carrier Mediated Transport • Uniport Movement of one molecule from one side to another E.g.: movement of glucose from the cells of GIT to ECF. • Co-transport Movement of one molecule depends on simultaneous or sequential transfer of another molecule Co-transport may be - Symport Two molecules move in the same direction E.g.: Na+/Glucose transport. -Anti-port Two molecules move in opposite directions E.g.: Cl- – HCO3- exchange in RBCs
Carrier Mediated Transport • Based on energy need, carrier-mediated transport are of two types: --Facilitated Transport (energy independent/passive) and -- Active transport (energy independent/active)
Facilitated Transport • It is passive transport and carrier mediated. • Here transport is down the concentration gradient. • Once the molecule binds to the biding site, a conformational change occurs in the carrier making the binding site exposed to the opposite direction Now the molecule is released from the carrier. Another conformational change in the carrier leads to the exposure of the binding site to the region where free molecules to be transported are present. • Structurally similar solutes can inhibit the entry of one another by competitive inhibition. E.g. There are four different facilitated carrier systems for carbohydrates and five for amino acids.
Facilitated transport Diffusion through protein channels channels move specific molecules across cell membrane facilitated = with help no energy needed open channel = fast transport high low AP Biology “The Bouncer”
Transport Across Cell Membrane Facilitated transport Mechanism : Ping pong model
Active Transport • Transport is carrier mediated against the concentration gradient and hence energy-dependent. • Transport occurs only in one way, against the concentration gradient. • The energy comes usually from hydrolysis of ATP molecules • About 40% of the total energy of the cell is used for the active transport.
Active Transport Cells may need to move molecules against concentration gradient shape change transports solute from one side of membrane to other protein “pump” conformational change “costs” energy = ATP low ATP high AP Biology “The Doorman”
Active Transport E.g. • Na+–K+ pump or Na+-K+ ATPase is the best example for active transport because virtually all cells have it. Other examples • Ca+–dependent ATPase (in sarcoplasmic reticulum of skeletal muscles), • H+–dependent ATPase located in the membrane of epithelial cell lining the stomach and has the function of acid (H+) secretion
Na+-K+ ATPase • Na+-K+ ATPase establishes and maintains a high intracellular K+ concentration and a low Na+ concentration compared to their concentrations in ECF. • The Na+ – K+ ATPase, expels 3 Na+ ions and brings 2K+ ions from outside to inside with a concomitant hydrolysis of ATP. Drugs like digitalis (a cardiac glycoside) and ouabain inhibit Na+ – K+ ATPase.
Active Transport ATP 2 K+ 2 K+ Na+-K+ ATPase 3 Na+ ADP + Pi 3 Na+
Sodium pump or Na+-K+ ATPase
Classification of active transport • based on the source of energy 1. Primary active transport - Transport of molecules is directly linked to the hydrolysis of ATP, which provides energy. E.g. Na+–K+ pump or Na+-K+ ATPase 2.Secondary active transport- Transport of molecules is indirectly linked to the hydrolysis of ATP. Eg: Glucose and galactose are absorbed from the intestine by secondary active transport. Concentration gradient of Na+ is maintained by Na+ – K+ ATPase.
Physiological importance of active transport • -Responsible for the generation of the resting membrane potential, basis for excitability in nerve and transmission of nerve impulse • -Na+ pump is driving force for several secondary active transport of nutrients into the cell. For example, glucose is cotransported with sodium into the cell • -Calcium pump (Ca++ dependent ATPase): found in sarcoplasmic reticulum of skeletal muscles. It transports calcium from the cytosol to the sarcoplasmic reticulum. It regulates muscle contraction • -Proton pump (H+ dependent ATPase): located in the parietal cells of the stomach . It is responsible for the secretion of Hcl into stomach lumen, to maintain the highly acidic pH essential for gastric digestion.
• Clinical application: Cardiotonic rugs like digitalis (a cardiac glycoside) and ouabain inhibit Na+ – K+ ATPase. They are used in treatment of heart failure.
Transport of Macromolecules & Particles -transported by Endocytosis and Exocytosis -Macromolecules such as proteins, polysaccharides, hormones and particles like viruses, bacteria etc are transported by these mechanisms.
Endocytosis -internalize extra-cellular macromolecules by invagination of cell membrane - Endocytosis may be either Phagocytosis or Pinocytosis.
Transport Across Cell Membrane Endocytosis: Process by which cells take up the large molecules
Phagocytosis (Gk: Phagein = to eat) • • • • Occurs in specialized cells such as macrophages and granulocytes. ingestion of large particles such as viruses, bacteria, cells or debris. • endocytic vesicle (phagosome) fuses with the lysosome. hydrolytic enzymes of lysosomes break down the macromolecular contents released in to the cytosol reused or further catabolized
Pinocytosis(cell drinking) • Cellular uptake of fluid and fluid contents containing small particles. E.g.: -Intake of chylomicron by the hepatocytes; -internalization of LDL by LDL receptor
Exocytosis extrusion of particulate or macromolecular materials, which can’t pass out through the intact membrane. • secretory vesicle is pinched off from the Golgi apparatus; • moves towards and fuses with the plasma membrane. • E.g. a) Release of Trypsinogen by pancreatic acinar cells. b) Release of Insulin by -cells of Langerhans. • c) Release of acetylcholine by pre-synaptic cholinergic nerves.
Transport Across Cell Membrane Exocytosis: Process of extrusion of a macromolecule from the cell
Disorders of Membrane Structure and Transport Abnormality in membrane structure or transport can cause diseases. • Respiratory distress syndrome Defect in biosynthesis of dipalmitoyl lecithin (lung surfactant) • Familial hypercholesterolemia Mutations in the gene encoding LDL receptor Cystic fibrosis Mutations in the gene encoding Cl- transporter
Sub-cellular Organelles include -- nucleus, Ribosomes endoplasmic reticulum, golgi apparatus/complex, mitochondria, lysosomes, peroxisomes and cytoskeleton.
Sub-cellular Organelles All eukaryotic cells contain all these organelles RBC is not a true cell and contain only the plasma membrane and cytoskeleton
Nucleus • largest sub-cellular organelle. • double membrane – nuclear membrane, surrounds it. • At intervals nuclear membrane has nuclear pores, permit the passage of molecules in and out of the nucleus. • nucleus of eukaryotic cell contains a dense body known as nucleolus rich in rRNA.
Nucleus • Nucleoplasm – ground material of nucleus rich in enzymes such as, DNA polymerases, RNA polymerases, etc. • Nucleus of an interphase (non-dividing) cell filled with a diffuse material – chromatin. – During the cell division, chromatin condenses to form chromosomes. – Humans have 23 pairs of chromosomes compactly packed in the nucleus.
Nucleus –Functions • Nuclear DNA --the repository of genetic information serves two purposes -i) By DNA replication provides genetic information to offspring or daughter cells during cell division., thus it is blue print of life. ii) By transcription (RNA synthesis) provides information for the synthesis of all protein molecules of the cell. Both replication and transcription take place in the nucleus. Function of nucleolus: -Synthesis of rRNA and ribosomes
Mitochondria spherical, oval or rod like bodies. have two membranes – outer and inner membrane. outer membrane is smooth while the inner membrane is for folded to form cristae components of electron transport chain (ETC) and oxidative phosphorylation buried in the inner mitochondrial membrane.
Mitochondria • The central cavity of the mitochondrion contains the matrix • Matrix contains enzymes and chemical intermediates of -TCA cycle Heme synthesis Urea cycle, etc. Also present in the matrix are, mitochondrial DNA, RNA and ribosomes.
Functions • ETC and oxidative phosphorylation-- situated in inner mitochondrial membrane are involved in ATP synthesis, hence mitochondria are regarded as ‘powerhouse of the cell’ • Some of the major pathways operate in the mitochondria. They are, TCA cycle, -Oxidation of fatty acid, ketone bodies formation, gluconeogenesis (partly), urea cycle (partly), heme synthesis (partly), pyrimidine synthesis (partly) . • Mitochondrial DNA codes for some of the mitochondrial proteins involved in oxidative
Endoplasmic Reticulum (ER) network of membrane-enclosed spaces extends throughout the cytoplasm. • classified into rough and smooth ER rough appearance (when observed under electron microscope) is due to ribosomes attached to the cytoplasmic side of the membrane. smooth ER does not have ribosomes.
Functions of ER • Rough ER : involved in synthesis of proteins (lipoproteins, glycoproteins) • Smooth ER: I. Metabolism of drugs and toxic compounds (cyt P450 monooxygenases are present in liver cell smooth ER) II. Synthesis of lipids (TAG, phospholipids, cholesterol) and III. Ca2+ storage in skeletal and cardiac muscle.(note- sarcoplasmic reticulum of muscle is a modified ER)
Golgi Complex/Golgi Apparatus • group of membrane bound flattened tubes or sacs placed one over another in a pile or stack.
Golgi Apparatus - Functions Main functions of Golgi apparatus are protein sorting, packaging and secretion. • newly synthesized proteins are handed over to the Golgi apparatus, which catalyze the addition of carbohydrates, lipids or sulfate moieties to the proteins.
Lysosomes membrane bound vesicle containing various hydrolytic enzymes (hydrolases. • Lysosomal enzymes are capable of digesting proteins, carbohydrates, lipids and nucleic acids • pH inside the lysosomes is less than that of cytosol necessary for its digestivse function
Lysosomes -Functions • hydrolases breakdown complex molecules brought into the cell by endocytosis, phagocytosis or worn-out organelles from the cells own cytoplasm. Lysosomes - termed as ‘suicide-bags’ as their lysis can lesad to digestion and death of the cell Sphingolipidosis – group of disorders in which excess of sphingolipids accumulates in lysosomes
Peroxisome small spherical or oval membranous bodies • contain enzymes -peroxidases and catalase
Peroxisome - Functions Free radicals formed by peroxidation of PUFA capable of damaging cell membranes, tissues, and genes Such reactions are implicated in inflammatory diseases, ageing process malignant transformation. and • Catalase and peroxidase enzymes destroy such unwanted peroxides and other free radicals
Ribosomes: nucleoproteins present either freely in cytosol or bound to ER • Function: provide necessary infrastructure for mRNA, tRNA & amino acid to interact with each other for translation process.
Cyto skeleton Made up of microtubules and actin filaments role in maintaining the cellular structure, mobility and cell division. Hereditary spherocytosis due to mutations in genes encoding spectrin or other structural proteins in red blood cell membrane, leading to excessive hemolysis
Organelle Nucleus Function Provides genetic information to offspring RNA transcription, directs protein synthesis Mitochondria Energy production from the oxidation of food substances and the release of adenosine triphosphate Endoplasmic Translation and folding of new proteins reticulum (rough endoplasmic reticulum), synthesis of lipids (smooth endoplasmic reticulum)
Golgi appartus Sorting, packaging, and modification of proteins Endoplasmic Translation and folding of new proteins reticulum (rough endoplasmic reticulum), synthesis of lipids (smooth endoplasmic reticulum) Lysosome Breakdown of large molecules Peroxisome breakdown of metabolic hydrogen peroxide and free radicals
Organelle Function Ribosome Translation of RNA to form proteins Cytoskeleton Maintaining the cellular, shape, motility and cell division.
Sub-Cellular Fractionation isolation of an organelle in a relatively pure form in order to study its functions Cell membrane is disrupted usually by mechanical means called homogenization • subcellular organelles can then be separated from the homogenate by differential centrifugation using the instrument ultracentrifuge
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