The Cell PDF

The Cell And Cellular organelles  Introduction Structural and Functional unit of all living organisms – Complex organisms –multiple cells – Simple organisms e.g bacteria, algae have a single cell EUKARYOTIC CELLS(have a distinct membrane- limited nuclei surround...

The Cell And Cellular organelles  Introduction Structural and Functional unit of all living organisms – Complex organisms –multiple cells – Simple organisms e.g bacteria, algae have a single cell EUKARYOTIC CELLS(have a distinct membrane- limited nuclei surrounded by cytomplasm) PROKARYOTIC CELLS (lack an envelope around their DNA)  Eukaryotes include cells in plants, fungi, and animals – May be UNICELLULAR OR MULTICELLUAR Prokaryotic cells are those cells of bacteria, archae, algae.  Cell differentiation Cells of multicellular organisms e.g Humans, show a great variety of functional and morphological specialisation. Cells become specialised by concentrating specific organelles. This process by which cells adopt a specialised structure and function is called DIFFERENTIATION  Cell differentiation cont.. Following fertilization, ZYGOTE , the single cell is formed Its first cellular divisions produce cells called BLASTOMERES ( has embryonic stem cells which give rise to all tissues) During their specialization (cell differentiation), cells synthesize increased quantities of specific proteins and become very efficient often changing their shape accordingly.  Cell differentiation cont… Functions that are performed by specialised cells can be done by most cells of the body Specialised cell have greatly expanded their capacity for one or more of these functions. Despite this range of morphological forms, all eukaryotic cells conform to a basic structural model  Cell structure Composed of two basic parts – Cytoplasm– surrounds the nucleus and other organelles and is a site of many enzymatic reactions of the cells – Plasma membrane (plasmalemma) – separates the cytoplasm from its extracellular millieu A continuum exists between the interior of the cell and the extracellular macromolecules ( these two spaces are linked by plasma membrane proteins called INTEGRINS)  Cytoplasm consists largely of a fluid component- CYTOSOL – Cytosol contains hundreds of enzymes, oxygen, Carbon dioxide, electrolytes, metabolites, waste products. Also has – Metabolically active structures (ORGANELLES) – Protein components e.g cytoskeleton – Inclusions (generally contains deposits of carbohydrates, lipids or pigments)  PLASMA MEMBRANE Limiting membranes that envelop the cells. Composed of lipids (phospholipids) ,proteins, cholesterol, and oligosacharide It separates the interior of the cell from external milieu It is selectively permeable. Carry out a number of signaling functions and specific recognition The immiscibility of lipids with water leads to formation of lipid bilayers  Plasma membrane cont… Range from 7.5 to 10 nm in thickness Membrane phospholipids are amphipathic – Has two nonpolar (hydrophobic )long chain fatty acids – a polar (hydrophilic) head that bears a phosphate group  Plasma membrane cont.. Phospholipids are most stable when organized into a bilayer with hydrophobic fatty acids chains directed toward the middle away from water and hydrophilic polar heads facing water. Molecules of cholesterol are inserted at varying densities among the closely packed phospholipids – This modulates the fluidity of the membrane  Plasma membrane cont Phospholipids in each half are different Some of the outer lipids (GLYCOLIPIDS), include oligosacharide chanins that are part of the cell surface coating (GLYCOCALYX) With TEM cell membranes exhibit a trilaminar appearance after fixation  Membrane proteins Major constituents of membranes (50% by weight) Integral proteins- incorporated within the lipid bilayer Peripheral proteins- found on membrane surfaces Multipass transmembrane proteins- span the membrane several times  Integral membrane proteins have ability to float (like icebergs in the sea) of phospholipids, this model is referred to as the fluid mosaic model of membrane structure  Membrane proteins cont.. Glycoproteins contribute to glycocalyx Also are important receptors Many proteins are restricted in their lateral diffusion by attachment to cytoskeletal components Various functions – As components of large enzyme complexes – Involved in trasnduction of signals – Membrane transport  Transport across a plasma membrane MEMBRANE TRANSPORT Small lipophilic (fat-soluble) molecules can pass through lipid bilayers by simple diffusion Ions such Na+, K+, and calcium cross membranes by passing through the integral membrane proteins that act as Ion channels or Ion pumps  Water molecules passively diffuse by osmosis through multipass transmembrane proteins called aquaporins Carrier proteins (integral proteins) helps many molecules to cross Note; carrier proteins, ion pumps require energy (active transport) VESICULAR TRANSPORT This involves bulk movement of materials across the plasma membranes Bulk movement of materials into the cell is achieved by a general process called ENDOCYTOSIS – This process involves folding and fusion of membrane to form vesicles that enclose the material transported  Three general types of endocytosis – Phagocytosis (cell eating) – Pinocytosis (cell drinking) – Receptor-mediated endocytosis  Bulk movement of large molecules from inside the cell to outside is achieved by EXOCYTOSIS – A membrane-limited vesicle fuses with the plasma membrane resulting in the release of its contents into the extracellular space – Often triggered by transient increases in cytosolic calcium  Summary of Functions of plasma membrane Physical barrier – Flexible boundary, protects cellular contents, and supports cell structure Selective permeability – Regulates entry and exit of ions, nutrients, and waste molecules Electrochemical gradients – Establishes and maintains an electrical charge difference across the plasma membrane Communication – Contains receptors Signal Reception and transduction Cell communicate with one another – To regulate tissue and organ development – To control their growth and division – To coordinate their functions  Many cells form communicating junctions that couple adjacent cells and allow the exchange of ion and small molecules (gap junctions) Cells also use receptors to detect and respond to stimuli (signaling molecules) Cells have different types of receptors – Hydrophilic signaling molecues Channel-linked receptors Enzymatic receptors G-protein-coupled receptors – Hydrophobic signaling molecules E.g Steroids and thyroid hormones These are actually lipophilic and will cross the membrane by diffusion and bind to specific intracellular receptor proteins  Cell organelles Are metabolically active structures that may be membranous or non membranous protein complexes Membrane-bound structures; – rough and smooth endoplasmic reticulum, – Golgi apparatus, – lysosomes, peroxisomes, – mitochondria and vesicles for transport, – secretion and storage of cellular components Non-membranous include – ribosomes and cytoskeleton  Organelles The Nucleus – Largest organelle and most obvious feature seen by light microscopy – Contains genetic material of the cell (Deoxyribonucleic acid, DNA) in form of chromosomes Protein called Nucleoprotein Ribonucleic acid (RNA) – The genetic blueprint for all proteins is contained within the nucleus of every cell except RBCs – Substance of the nucleus is known as NUCLEOPLASM and it is surrounded by nuclear membrane (envelope)  Nuclear Membrane (envelope) Forms selectively permeable barrier between the nuclear and cytoplasmic compartments Electron microscopy reveals that the envelope has two concentric membranes separated by a narrow perinuclear space (30-50nm) the outer membrane is continuous with rough endoplasmic reticulum  Nuclear envelope has pores which permit molecular transfer in both directions  Nucleoproteins – Two major types Histones Non-histones – Histone proteins form a protein core around which the chromosome is coiled to form nucleosomes – Non-histone proteins include all enzymes for the synthesis of DNA and RNA and other regulatory proteins  Chromatin In non-dividing nuclei, chromatin consists of the DNA and its attendant proteins Two types – Heterochromatin- electron-dense areas consisting of coiled inactive chromatin – Euchromatin- electron-lucent areas representing part of DNA that is active in RNA synthesis In females, the inacvtivated X-chromosome may form a small discrete mass called BARR BODY  Nucleolus Site of ribosomal RNA synthesis and ribosomal assembly Transfer RNA is also processed in the nucleolus  Ribosomes – Small electron dense particles – Approximately 20nm * 30nm in size – Composed of different proteins and 4 segments of ribosomal RNA (rRNA). – Function as a surface for the synthesis of proteins. – Acts in alignment of messenger RNA and transport RNA – Each is composed of a large subunit and a small subunit, both of which are synthesised in the nucleolus and released as separate entities into the cytosol  Endoplasmic Reticulum (ER) – largest membranous system of the cell, comprising approximately ½ of the total membrane volume. – It is a system of interconnected tubules and vesicles whose lumen is referred to as the cistern. – ER has two components: smooth endoplasmic reticulum (SER) rough endoplasmic reticulum (RER); bear polysomes appearing rough, only the RER participates in protein synthesis,  Rough endoplasmic reticulum – Covered by polyribosomes – Prominent in cells specialised for protein secretion e.g. Fibroblasts, plasma cells – Consists of sac like and parallel stacks of flattened cisternae – enveloped by a membrane which continuous with outer membrane of nucleus – Membranes have integral proteins that function in recognizing and binding ribosomes – participates in the synthesis of all proteins that are to be packaged or delivered to the plasma membrane. – Initial glycosylation of glycoproteins – Post translational modification newly formed polypeptides and assembly of multichain proteins – has a highly regulated system  Smooth Endoplasmic Reticulum – system of anastomosing tubules and occasional flattened membrane-bound vesicles; – lumen of SER is continuous with that of the RER Functions of SER – Synthesis of phospholipids – Contains enzymes for synthesis of steroid hormones – Abundant in liver cells; contains enzymes for oxidation, conjugation, and methylation reactions that neutralise toxic molecules like alcohol, barbiturates etc – SER become specialized in some cells (e.g., skeletal muscle cells), where it is known as sarcoplasmic reticulum; functions in sequestering calcium ions from the cytosol, assisting in the control of muscle contraction. Golgi Apparatus/ Golgi complex – a series of parallel stacks of flattened saccules and associated vesicles – is composed of one or more series of flattened, slightly curved membrane-bounded cisternae, the Golgi stack – Smooth membraneous sacules contain enzymes that complete posttranslational modification of proteins from RER, they also important for glycosylation, sulfation. – Enzymes initiate packing, concetration and storage of secretory product – Has 2 distinst sides structurally and functionally From RER transport vesicles to Golgi receiving region (cis face) to trans face  Mitochondria – Membrane bound – Usually enlongated with a diameter of about 0.5 to 1µm – highly plastic, rapidly changes shape , fuse with another and divide (fission) – Has 2 membranes that form 2 compartments; Inner matrix Narrow intermembrane space Mitochondria Outer membrane contains trans-membrane proteins called porins that form transport channels Inner membrane is folded forming series of long infoldings called cristae (increase surface area) Contains enzymes specialised for aerobic respiration and production of ATP, energy supplied for all cellular activities, some energy is dissipated as heat that maintains body temp  Mitochondria – Matrix contains a small circularDNA, ribosomes, mRNA, tRNA – Mitochondria originate by growth and division (fission) of pre-xisting mitochondria – Synthesis proteins in small amounts – Plays a role in apoptosis during cell stress cytochrome c is released into cytoplasm which activitesa set proteases to degrade cellular activities  Peroxisomes – are self-replicating organelles that contain oxidative enzymes – are small (0.2 to 1.0 μm in diameter), spherical to ovoid membrane-bound organelles that contain more than 40 oxidative enzymes, especially urate oxidase, catalase, and D-amino acid oxidase – present in almost all animal cells and function in the catabolism of long-chained fatty acids (beta oxidation),  Lysosomes – Spherical about 0.05 to 0.5µm – Membrane bound organelle formed from Golgi apparatus – Contain hydrolases function in hydrolytic degratdation of endocytosed materials and removal of excess ornonfunctional organelles  Centrosomes – located near the nucleus, consists of 2 components: a pair of centrioles and pericentriolar material – centrioles are cylindrical structures, each composed of 9 clusters of 3 microtubules (triplets) arranged in a circular pattern – Surrounding the centrioles is pericentriolar material (protein) tubulin – tubulin complexes are organizing centres for growth of the mitotic spindle, plays a critical role in cell division, and for microtubule formation in nondividing cells – During cell division, centrosomes replicate for next cell division  Inclusion bodies Inclusions are nonliving components of the cell that do not possess metabolic activity and are not bounded by membranes. most common inclusions are glycogen, lipid droplets, pigments, and crystals  Cytoskeleton Cytoskeleton Animal cells contains cytoskeleton an intricate 3-dimensional meshwork of protein filaments that are responsible for the maintenance of cellular morphology providing mechanical strength locomotion chromosome separation in mitosis and meiosis intracellular transport of organelles has 3 major components: thin filaments, intermediate filaments, and microtubules.  Cytoskeleton Microtubules Are tubular and hollow structures when organised in larger arrays are known as axonemes in the cytoplasmic extensions called cilia and flagella Each is hollow, a structure that confers significant rigidity to help maintain cell shape Vary in length, 2 more are often linked together side by side by proteins  Cytoskeleton Microtubules are built by the assembly of dimers of alpha tubulin and beta tubulin; originate at the centrosome participate in a wide variety of cell activities, most involve motion, provided by protein "motors" that use the energy of ATP to move along the microtubule. 2 major groups of microtubule motors: – Kinesins – Dyneins Cilia and flagella are built from arrays of microtubules  Functions Structural cell morphology and polarity subcellular localization of organelles Tracks for kinesin and dynein motors intracellular transport (protein, vesicles, organelles) Motility cilia and flagella (specialized structures) Mitosis Mitotic spindle  Cytoskeleton Clinica application Vincristine, a drug binds to tubulin dimers preventing the assembly of microtubules halts cells in metaphase of mitosis.  Cytoskeleton Microfilaments Composed of actin allow cellular motility and most contractile activity in cells Actin filaments are thin polarised polymers shorter and more flexible Abundant in all cells, concentrated as a network of actin filaments  Cytoskeleton Microfilaments Functions of actin filaments: form a band just beneath the plasma membrane that – provides mechanical strength to the cell – Links transmembrane proteins (e.g., cell surface receptors) to cytoplasmic proteins – pinches dividing animal cells apart during cytokinesis generate locomotion in cells such as white blood cells and the amoeba interact with myosin ("thick") filaments in skeletal muscle fibres to provide the force of muscular contraction  Cytoskeleton Intermediate filaments Are more stable than microtubules, are composed of different proteins subunits in differents these include – Keratin found in epithelial cells and also form hair and nails; – Neurofilament strengthen the long axons of neurons – Lamins form a meshwork that stabilizes the inner membrane of the nuclear envelop – vimentins provide mechanical strength to muscle (and other) cells.  References Mescher AL Junqeira’s Basic histology 13th Edition Young B. et al Wheaters functional histology 5th edition Standring S. Grays Anatomy 39th Edition