Cell Structure and Functions PDF

Cell Structure and functions By Dr B Hottor  Aim To introduce you to the structure of the cell  Specific learning objectives By the end of the session you should be able to: – Appreciate the level of organization of the living organism – D...

Cell Structure and functions By Dr B Hottor  Aim To introduce you to the structure of the cell  Specific learning objectives By the end of the session you should be able to: – Appreciate the level of organization of the living organism – Define a cell – List the types of cells and their contrasting features – Discuss the regions of a generalized human/mammalian cell – Describe the organelles and their function – Discuss cell movement and transport across the cell membrane.  Levels of organization of organisms Chemical level-Atom- molecules- which will form organelles- Cell: Organelles together form Cells Tissue: Different cells come together to form tissue-group of similar cells with common function Organs level-discrete structure composed of at least 2 different tissues Organ system level- made up of 2 or more organs performing similar functions Organism level- made up a number of organ systems  What is a cell? The cell is defined as the functional unit of living organisms Cells vary in shape, size and functions Two cell types: – Prokaryotic – Eukaryotic cells  Prokaryotic vrs Eukaryotic cells Prokaryotic (In – Eukaryotic Bacteria) – Larger – 1-5 micrometres long – Have cell membrane – Have cell wall – Distinct nuclear – Lack nuclear envelop envelop – Have no histones – Histones associated (basic proteins) with genetic material – No membranous – Have membrane- organelles limited organelles  General characteristics of mammalian cell The shapes of mammalian cells vary widely Cell size is limited by rates of diffusion Most cells are between 5–50 μm in diameter Cell motility Cell communication  Cellular functions Movement (muscle cells) Synthesis and secretion of enzymes, mucous substances, and steroids Ion transport Intracellular digestion Transformation of physical and chemical energy into nervous impulse Metabolite absorption  Parts of a human cell There are 200 different types of cells in the human body, each performing different functions All cells however have the same basic structure (generalized or composite cells) The cell is composed of – Plasma membrane ( plasmalemma, cell membrane): – Cytoplasm(PM to nuclear envelop) – Nucleus  Plasma/Cell membranes The cell membrane forms a selectively permeable barrier between the cytoplasm and the external milieu Composed of lipids (mainly phospholipids, cholesterol[20 %] and glycolipids [5 %]) and proteins- usually in 1:1 ratio  Membranes lipids The lipid bilayer Phospholipid molecule is composed of polar heads and nonpolar fatty acyl tails Hydrophilic head and hydrophobic tails=amphipathic  Plasma membranes proteins Integral/intrinsic – Transmembrane or embedded in the internal or external leaflet Peripheral proteins Fluid mosaic model Function as enzymes, transport proteins(pores or channels and carriers) and receptors Plasma membranes carbohydrates Some of the membrane proteins or lipids on the external surface have polysaccharides (Carbohydrate) attached-glycoproteins or glycolipids Synthesized by rough endoplasmic reticulum  Function of cell membranes Transport( Diffusion, Endocytosis and Exocytosis- R-mediated endocytosis, Phagocytosis, pinocytosis) Signal receptors- recognize via R antigens, foreign materials and altered cells Generation of electrochemical gradient and potential Site of attachment of enzymes Maintains structural integrity of the cell Cell surface contact- regulating cell-cell interactions Compartmentalise the cell  Cell surface contacts General adhesive contact Specialised adhesive contact occluding jxns (tight jxns) Adhesive jxns e.g. Intermediate jxns, fascial adherens, Desmosomes, hemidesmosomes) Gap jxns Cell structure  cytoplasm It is made up of organelles and cytosol – Cytosol forms the bulk of the cytoplasm – It is the fluid suspension of organic and inorganic substances – It also contains organelles, cytoskeleton and inclusions  Cytosol Is the fluid part Suspended in the cytosol are: – Organelles – Inclusions – Cytoskeleton  Organelles Membrane-bound – Golgi apparatus (Golgi complex) – Endosomes, lysosomes, proteasomes,and peroxisomes – Mitochodria – Endoplasmic reticulum- RER, SER Non-membrane-bound organelles – Cytoskeletons – Centrioles – Ribosomes  Mitochondria They are flexible rod-shaped organelles. Most cells process large number of mitochondria Has double membrane Has smooth outer and folded (cristae) inner membrane The space b/n the membranes is intermembrane space; the space enclosed b/n the inner membrane is the matrix space Outer membrane possess porins (multipass transmembrane proteins).  Mitochondria The inner membrane is endowed with cardiolipin, a PL that possesses 4 acyl fatty acid chains making it impermeable They perform oxidative phosphorylation and lipid synthesis Principal source of chemical energy (ATP) Site of citric acid (Krebs’ or tricarboxylic acid) cycle and electron transport pathway Contain their own DNA, RNA and ribosomes  Ribosomes Small electron dense particles- 12 nm wide and 25 nm long Composed of ribosomal RNA (rRNA) and proteins Each has 2 globular subunits- a small and large Site of synthesis of proteins Free ribosomes synthezise protein for use within the cell Attached ribosomes- synthezise protein for export and for membrane synthesis  Endoplasmic reticulum(ER) System of interconnecting membrane- lined channels within the cytoplasm Largest membranous system in the cell Two types-rER (studded with ribosomes) and sER rER site for synthesis of secretory proteins, cell membrane protein and PL sER-play a role in lipid metabolism, chol synthesis, steroid hormone synthesis, absorption, synthesis and transport of fat  Golgi It consists of stacks of closely folded flattened membranous sacs It is the site for packaging of export protein and synthesis of carbohydrate  Endosomes and lysosomes Endosomes are divided into two compartments: – Early endosomes, near the periphery of the cell – Late endosomes, situated deeper within the cytoplasm. Lysosomes have an acidic pH and contain hydrolytic enzymes. Each lysosome is round to polymorphous in shape It contains at least 40 different types of acid hydrolases A lysosomal storage disorder e.g Tay-Sachs disease  Peroxisomes 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 They contain more than 40 oxidative enzymes, especially urate oxidase, catalase, and D-amino acid oxidase Function in the catabolism of long-chained fatty acids (beta oxidation) to form acetyl coenzyme A (CoA) as well as hydrogen peroxide (H2O2) H2O2 detoxifies ethanol and kills microorganisms  Proteosomes Are multi-subunit protein complexes that are responsible for proteolysis of malformed and ubiquitin-tagged proteins.  INCLUSIONS Non-living components of the cell possess no metabolic activity Not bounded by membranes. They are mostly glycogen, lipid droplets, pigments, and crystals  Cytoskeleton An intricate three-dimensional meshwork of protein filaments Functions: – Maintain cellular morphology – Cellular motion It has three components: – Thin filaments (microfilaments) e.g Actin – Intermediate filaments keratins, desmin, vimentin – Microtubules  Nucleus Largest intracellular organelle Spherical or ellipsoid in shape Contains almost all deoxyribonucleic acid DNA And the mechanisms for ribonucleic acid (RNA) synthesis The nucleolus is the location for the assembly of ribosomal subunits  Nucleus It bounded by two lipid membranes (Nuclear membrane/ envelop) Consists of: – Chromatin, the genetic material of the cell – Nucleolus, the center for ribosomal RNA (rRNA) synthesis – Nucleoplasm, containing macromolecules and nuclear particles involved in the maintenance of the c  Nucleus Usually spherical, but maybe ellipsoid, oblong, twisted, lobulated or discoid Usually centrally located but may be peripheral in some cells Single nucleus per cell but some cells e.g. osteoclast have several(mature RBC have none) Usually uniform size, shape and form for each cell type