Cell Structure and Function PDF

Faculty of Medical Sciences HESC1006: Health Sciences Cell Structure and Function Dr. N. Ojeh  Lecture Objectives By the end of this lecture, you should be able to: By the end of this lecture, you should be able to: Describe the ultra-structure a...

Faculty of Medical Sciences HESC1006: Health Sciences Cell Structure and Function Dr. N. Ojeh  Lecture Objectives By the end of this lecture, you should be able to: By the end of this lecture, you should be able to: Describe the ultra-structure and functions of the major organelles and surface features of eukaryotic cells. List the eukaryotic cell organelles and explain their importance.  The Cell Basic unit of structure and function in living organisms i.e. “the building block of life” The cell theory was first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann Cell derived from Latin ‘cellula’ –small room named by Robert Hooke in 1665 Rudolf Virchow (1821–1902) German pathologist developed the cell theory further and was credited with initiating the study of disease at the cellular level  Some of the cells and their specific functions: Muscle cell – movement Nerve cell – conductivity Pancreatic acinar cells – synthesis of enzyme Mucous gland cells – secretion of mucous Cells of adrenals, testes, ovaries – secretion of steroids Cells of kidney – ion transport Macrophages – intracellular digestion Cells of intestine – metabolic absorption Prokaryotic cell (e.g. bacteria) Eukaryotic cell (e.g. animal) 10-100 µm size 1-10 µm size Membrane-bound nucleus present Nucleus absent (anucleated) Multi-cellular e.g. approx 100 Single-celled trillion cells (1014 ) in humans Cell wall & plasma membrane Plasma membrane only present Organelles present Organelles absent Multiple, linear chromosomes Single circular chromosomes with wound on nucleosomes no nucleosomes Eukaryotic Cell Structure  Cell (plasma) membrane 6-10 nm thick and acts as boundary to contain cell contents and maintain cell integrity Consist of a double layer of phospholipids in which proteins are embedded (fluid mosaic membrane) Comprised of Lipid bilayer; phopholipids and cholesterol (for rigidity) The cell's membrane system includes the plasma membrane, endoplasmic reticulum, the Golgi complex, and lysosomes.  The Plasma Membrane Outside cell Inside cell Cell (plasma) Membrane Selective barrier Transport Communication Recognition  Cytoplasm Makes up 70% of the cell Composed of water, salts and inorganic molecules (High concentrations of potassium, magnesium, phosphate (osmotic pressure within cells is similar to that of the extracellular fluid) Approx. 8% of protein Contains organelles (Membrane-bound structures) Filaments and granules Metabolic pathways e.g. glycolysis, HMP  Organelles – Nucleus – Mitochondria – Rough endoplasmic reticulum – Smooth endoplasmic reticulum – Golgi apparatus – Lysosomes – Peroxisomes – Vacuole – The Cytoskeleton  Nucleus Two major functions: it stores the cell's hereditary material, or DNA, and it coordinates the cell's activities, which include growth, intermediary metabolism, protein synthesis, and reproduction (cell division). Nuclear envelope (double membrane; inner and outer) continuous with the endoplasmic reticulum and is perforated by thousands of pores called nuclear pore complexes (NPCs) that control the passage of molecules in and out of the nucleus. Nuclear lamina (inner membrane) is stabilized by intermediate filament proteins called lamins. Nucleolus Contains: – DNA (genetic codes) – RNA (essential molecules for protein synthesis)  Nucleus DNA (deoxyribonucleic acid) is double helix composed of 4 deoxyribonucleotides – Complementary base pairing of cytosine (C) with guanine (G) and adenine (A) with thymine (T) held together by hydrogen bonds – Attached to sugar phosphate backbone Chromatin = chromosomal material = DNA, histones, non-histone proteins, RNA Central dogma of Molecular Biology DNA DNA replication Transcription (in nucleus) mRNA Translation (in cytoplasm) tRNA + ribsomes PROTEIN  Nucleolus Discrete densely-stained structure found in the nucleus. Organized from specific chromosomal regions called "nucleolar organizer regions" (NOR). ribosomal DNA (rDNA) is transcribed to ribosomal RNA (rRNA) at this site. Role: synthesis of rRNA and assembly of the large and small subunits of ribosomes. Normal Cancer  Mitochondria Main function is the conversion of the potential energy of food molecules into ATP (adenosine triphosphate) Products of carbohydrate, fat and protein metabolism are oxidized to produce energy. The number of mitochondria is indicative of the cell’s metabolic activity  Mitochondria Mitochondria have: – an outer membrane that encloses the entire structure – an inner membrane that encloses a fluid-filled matrix – Between the two is the intermembrane space – Cristae (formed by infoldings of the inner membrane) – Own genetic material  Endoplasmic reticulum (ER) – Interconnected membrane network of flattened sheets or elongated tubules and cisternal spaces (lumen) – Double layer of lipid molecules and associated proteins – Proteins responsible for reactions that occur within the cytoplasm – Biosynthesis and transport of proteins and lipids  Two types of ER Smooth ER Series of membranes without attached ribosomes that function in synthesis of lipids and processing of proteins (including steroid, carbohydrate and drug metabolism) Rough ER Series of membranes studded with ribosomes that are the site of protein production.  Ribosomes Made up of 60 percent RNA and 40 percent protein. Involved in protein synthesis using genetic information in mRNA originally transferred from DNA in nucleus in a process known as translation. Two types of ribosomes exist – free and fixed Free ribosomes- scattered in the cytoplasm Fixed ribosomes- attached to the ER No. of ribosomes reflects amount of protein synthesis This electron micrograph (courtesy of Keith Porter) shows the RER in a bat pancreas cell. The clearer areas are the lumens.  The Golgi Apparatus aka Golgi body, Golgi complex Series of flattened membrane discs called saccules with secretory granules on its surface. Functions in the modification, sorting and packaging of macromolecules synthesized in the ER and prepares them for export. Examples: Modification of proteins e.g. addition of carbohydrate chains to form glycoproteins (Glycosylation) Transport of lipids Creation of lysosomes Site of carbohydrate synthesis The Golgi Apparatus  Lysosomes Spherical bodies bounded by a single membrane. Formed by budding from the golgi apparatus. Site for intracellular digestion. They fuse with vesicles containing food particles ingested by cell. Contain hydrolytic enzymes that come from the golgi that breakdown large molecules and worn out cell parts into simple compounds. Also termed “suicide bags”  Lysosomes Lysosomal enzymes function at acid pH (acid hydrolases) in the lysosome (pH 5 c/f to cytosolic pH of 7.2) Enzymes Include nucleases, proteases, lipases, phosphatases, polysaccharidases Lysosomal storage diseases (LSD): Genetic defects can lead to LSD where there is deficiency of acid hydrolyses causing accumulation of substrates within lysosomes, resulting in enlargement of these organelles that interfere with the normal function of the cell. Liver and spleen are often enlarged in LSD as they are abundant with lysosomes.  Peroxisomes Also called microbodies. Special vesicles that contain enzymes important for the removal of toxic substances and the breakdown (by oxidation) of excess fatty acids. They are about the size of lysosomes (0.5-1.5 µm) and are bound by a single membrane. They also resemble lysosomes in being filled with enzymes. The enzymes and other proteins destined for peroxisomes are synthesized in the cytosol.  The Cytoskeleton Internal scaffold consisting of a system of filaments and microtubules: maintains cell shape provides mechanical strength for protection movement within the cell (e.g. excretion of material, intracellular transport of organelles) and locomotion Cell division (e.g. chromosome separation in mitosis and meiosis)  The Cytoskeleton 3 types of Protein filaments: Microfilaments (e.g. actin) - 5 nm Microtubules (e.g. tubulin) - 25 nm e.g. centrioles Intermediate filaments - 10 nm – Cytokeratin- epithelial cells – Desmin – smooth/skeletal muscles – Glial fibrillary acidic protein – astrocytes – Neurofilament protein – neurone – Nuclear lamin - nucleus – Vimentin – mesenchymal cells The Cytoskeleton  Centrioles Left photo: electron micrograph of a cross-section of a centriole with its array of 9 triplets of microtubules. Right: Schematic diagram of centriole  Centrioles Pair of self-replicating organelles found within centrosomes (microtubule organising centre) located just outside the nucleus Duplicates just before mitosis Produce microtubules Role in spindle formation- chromosome separation during cell division Centrioles are situated at right angle to each other, made up of nine bundles of microtubules and are found only in animal cells. Mature red blood cells and skeletal muscle cells lack centrioles. Absent in nerve cells also. They appear to help in organizing cell division by forming the spindles to separate chromosome pairs during mitosis, but are not essential to the process.  Cilia and Flagella Cilia- short, numerous complex, Flagella- long, fewer complex Both are constructed from microtubules Both are motile cellular appendages found in most microorganisms and animals, but not in higher plants. For single-celled eukaryotes, cilia and flagella are essential for the locomotion of individual organisms e.g. sperm In multicellular organisms, cilia function to move fluid or materials past an immobile cell. The respiratory tract in humans is lined with cilia that keep inhaled dust, smog, and potentially harmful microorganisms from entering the lungs.  Cilia and Flagella Each cilium (or flagellum) is made of a cylindrical array of 9 evenly- spaced microtubules, each with a partial microtubule attached to it. This gives the structure a "figure 8" appearance when view in cross section. This electron micrograph shows the 9+2 pattern of microtubules in a single cilium seen in cross section. 2 single microtubules run up through the center of the bundle, completing the so-called "9+2" pattern. The entire assembly is sheathed in a membrane that is simply an extension of the plasma membrane.  Cilia and Flagella Motion of cilia and flagella is created by the microtubules sliding past one another. This requires: – motor molecules of Dynein, which link adjacent microtubules together, and – the energy of ATP.  Epithelial cells Epithelium lines both the outside (skin) and the inside cavities of the body (respiratory tract, gastrointestinal tract) Functional units of secretory glands  Epithelial Cell Specialization Cell surface projections – Cilia: facilitate transport along cell surface – Microvilli: increase surface area for absorption Secretory adaptations – Well developed ER and Golgi apparatus Cell junctions  Cell junctions Specialized junctional areas between epithelial cells allow for: – Adherence to each other – Communication channels Three types of junction: – Occludens type/tight junction (barrier) – Nexus/gap junction (2nm; communication) – Adherens type (20nm; adhesion) - e.g. desmosomes Cell junctions  CLASS QUIZ!! (1/3) 1. A centriole is an organelle that is: A. present in the centre of a cell's cytoplasm B. composed of microtubules and important for organizing the spindle fibres C. surrounded by a membrane D. part of a chromosome 2. The rough endoplasmic reticulum is: A. an intracellular double-membrane system to which ribosomes are attached B. an intracellular membrane that is studded with microtubular structures C. a membranous structure found within mitochondria D. only found in prokaryotic cells 3. Which of the following statements does not apply to the nuclear envelope? A. It is a double membrane B. It has pores through which material enters and leaves C. It is continuous with the endoplasmic reticulum D. It has infoldings to form cristae  CLASS QUIZ!! (2/3) 4. Lysosomes are formed by budding from which cellular organelle? A.smooth endoplasmic reticulum B.Golgi apparatus C.rough endoplasmic reticulum D.nucleus 5. All peroxisomes carry out this function: A.break down fats and amino acids into smaller molecules that can be used for energy production by mitochondria B.digest macromolecules using the hydrolytic enzymes they contain C.synthesize membrane components such as fatty acids and phospholipids D.control the flow of ions into and out of the cell 6. Which of the following is the energy supplier for the cell; this organelle contains its own DNA: A.Lysosome B.Mitochondria C.Golgi apparatus D.Ribosome  CLASS QUIZ!! (3/3) 7. Which of the following is an enzyme filled organelle for breaking down waste in the cell? A.Lysosome B.Mitochondria C.Golgi body D.Endoplasmic reticulum 8. The major job of the ribosome is to: A.Make fats B.Make proteins C.Breakdown proteins D.Make sugars 9. The cell membrane is composed of: A.a single layer of proteins B.a phospholipid bilayer C.a carbohydrate bilayer D.a single layer of lipids  Web Information https://biology.arizona.edu/cell_bio/cell_bio.html https://www.savemyexams.com/dp/biology/sl/25/revision- notes/unity-and-diversity/cell-structure/eukaryotic-cell- structure/