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Transcript
Ch 2, CELLS: The Living Units of Organisms Human embryonic stem cells, S. Huerta Sonya Schuh-Huerta, Ph.D. The Inner Life of the Cell… Introduction to Cells • Several important scientists made discoveries about cells – Antonie van Leeuwenhoek – one of 1st to observe cells; father of microscopy & microbiology – Robert Hooke – coined the term “cell” as the basic unit of life – Matthias Schleiden & Theodor Schwann – Rudolf Virchow • Cells – the smallest living units in our bodies – Organelles “little organs” – carry on essential functions of cells Introduction to Cells • Cells have 3 main components – Plasma membrane = the outer boundary – Cytoplasm = contains most organelles – Nucleus = controls cellular activities Structure of a Generalized Cell Chromatin Nuclear envelope Nucleolus Nucleus Plasma membrane Smooth endoplasmic reticulum Cytosol Mitochondrion Lysosome Centrioles Rough endoplasmic reticulum Centrosome matrix Ribosomes Golgi apparatus Cytoskeletal elements Microtubule Intermediate filaments Secretion being released from cell by exocytosis Peroxisome The Plasma Membrane • Defines the extent of the cell • Structure of membrane – Fluid mosaic model (lipid bilayer) – Types of membrane proteins: • Integral proteins = firmly imbedded in, or attached to lipid bilayer • Short chains of carbohydrates attach to integral proteins - form the glycocalyx • Peripheral proteins = attach to membrane surface – Support plasma membrane from cytoplasmic side The Plasma Membrane Polar head of phospholipid molecule Extracellular fluid (watery environment) Glycolipid Cholesterol Nonpolar tail of phospholipid molecule Glycoprotein Carbohydrate of glycocalyx Bimolecular lipid layer containing proteins Outwardfacing layer of phospholipids Inward-facing layer of phospholipids Cytoplasm (watery environment) Integral proteins Filament of cytoskeleton Peripheral proteins The Plasma Membrane • Functions – relate to location at the interface of cell’s exterior and interior – Acts as barrier against substances outside cell – Cell-to-cell recognition & communication (receptors) – Determines which substances enter or leave the cell • Plasma membrane is selectively permeable! The Plasma Membrane in Action Membrane Transport • Simple diffusion – tendency of molecules to move down their concentration gradient • Osmosis – diffusion of water molecules across a membrane – down their gradient Membrane Transport Mechanisms • Facilitated diffusion = movement of molecules down their concentration gradient through an integral protein • Active transport = integral proteins move molecules across the plasma membrane against their concentration gradient (often using energy in the form of ATP) Membrane Transport Mechanisms Extracellular fluid Lipidsoluble solutes Water molecules Solutes Lipid bilayer Cytoplasm Simple diffusion Osmosis Facilitated diffusion & active transport Endocytosis • Endocytosis – Mechanism by which particles enter cells • Phagocytosis = “cell eating” • Pinocytosis = “cell drinking” Receptor-Mediated Endocytosis • Receptor-mediated endocytosis = plasma proteins bind to certain molecules & bring them into the cell • Invaginates and forms a coated pit – Pinches off to become a coated vesicle • NOTE: This process is very specific to certain molecules. • It is the method by which insulin and cholesterol enter cells! 3 Types of Endocytosis Pinocytosis Phagocytosis Vesicle Phagosome Receptor-Mediated Endocytosis Vesicle Receptor recycled to plasma membrane Exocytosis • Exocytosis = mechanism that moves substances out of the cell – Substance is enclosed in a vesicle – The vesicle migrates to the plasma membrane – Proteins from the vesicles (v-SNAREs) bind with membrane proteins (t-SNAREs) – The lipid layers from both membranes bind & the vesicle releases its contents to the outside Exocytosis Extracellular fluid Plasma membrane SNARE (t-SNARE) 1 The membranebound vesicle Vesicle SNARE migrates to the (v-SNARE) plasma membrane. Secretory vesicle Fused v- and t-SNAREs 2 There, proteins at the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins). Molecule to be secreted Fusion pore formed 3 The vesicle and plasma membrane fuse and a pore opens up. Cytoplasm 4 Vesicle contents are released to the cell exterior. Exocytosis in Action The Cytoplasm • Cytoplasm = lies internal to plasma membrane & consists of cytosol, organelles, & inclusions • Cytosol = jelly-like fluid in which other cellular elements are suspended – Consists of water, ions, & enzymes Organelles – Ribosomes • Ribosomes – constructed of proteins and ribosomal RNA; not surrounded by a membrane – Site of protein synthesis • Assembly of proteins is called translation – Are the “assembly line” of the manufacturing plant Endoplasmic Reticulum • Endoplasmic reticulum—“network within the cytoplasm” Rough ER – ribosomes stud the external surfaces Smooth ER – consists of tubules in a branching network • No ribosomes are attached; therefore no protein synthesis The ER & Ribosomes (b) Electron micrograph of smooth and rough ER (85,000) Smooth ER Nuclear envelope Rough ER Ribosomes Cisternae (a) Diagrammatic view of smooth and rough ER Golgi Apparatus • Golgi apparatus = a stack of 3–10 disk-shaped envelopes – Sorts products of rough ER and sends them to proper destination – Products of rough ER move through the Golgi from the convex (cis) to the concave (trans) side – “Packaging & shipping” division of manufacturing plant Golgi Apparatus Many vesicles in the process of pinching off from the membranous Golgi apparatus Electron micrograph of the Golgi apparatus (90,000) New vesicles forming Transport vesicle from rough ER Cis face— “receiving” side of Golgi apparatus Cisternae New vesicles forming Transport vesicle from trans face Secretory vesicle Trans face – “shipping” side of Golgi apparatus Golgi apparatus Transport vesicle from the Golgi apparatus Lysosomes • Lysosomes = membrane-walled sacs containing digestive enzymes – Digest substances – Cells of gut many lysosomes Lysosomes Sequence of Events From Protein Synthesis to Their Final Distribution Rough ER ER membrane Phagosome Proteins in cisterna 1 Protein-containing vesicles pinch off rough ER and migrate to fuse with membranes of Golgi. Pathway C: Lysosome containing acid hydrolase enzymes 2 Proteins are modified within the Golgi compartments. 3 Proteins are then packaged within different vesicle types, depending on their ultimate destination. Plasma membrane Vesicle becomes lysosome Golgi apparatus Pathway A: Vesicle contents destined for exocytosis Secretory vesicle Secretion by exocytosis Pathway B: Vesicle membrane to be incorporated into plasma membrane Extracellular fluid Mitochondria • Mitochondria = generates most of the cell’s energy; most complex organelle; Arose from early animal cell engulfing a prokaryote – they then lived together happily ever after! – More abundant in energyrequiring cells, like muscle cells & sperm – “Power plant” of the cell Mitochondria Outer mitochondrial membrane Ribosome Mitochondrial DNA Inner mitochondrial membrane Cristae Matrix Enzymes Peroxisomes • Peroxisomes = membrane-walled sacs of oxidase enzymes – Enzymes neutralize free radicals and break down poisons (ie. alcohol, hydrogen peroxide) – Break down long chains of fatty acids – Are the toxic waste removal system – Are numerous in the liver and kidneys!!! The Cytoskeleton • Cytoskeleton = “cell skeleton” – an elaborate network of protein rods – Contains 3 types of rods: • Microtubules = cylindrical structures made of tubulin protein subunits; form mitotic spindle and are the tracks that motor proteins transport materials on • Intermediate filaments = protein fibers; provide structure and rigidity to cell • Microfilaments = filaments of the contractile protein actin; perform contractile functions of cell Cytoskeletal Elements (a) Microfilaments (b) Strands made of spherical protein subunits called actins Intermediate filaments Tough, insoluble protein fibers constructed like woven ropes (c) Microtubules Hollow tubes of spherical protein subunits called tubulins Tubulin subunits Fibrous subunits Actin subunit 7 nm 10 nm 25 nm The Cytoskeleton in Action Centrosome • Centrosomes & centrioles: – Centrosome = a spherical structure in the cytoplasm • Composed of centrosome matrix and centrioles – Centrioles = paired cylindrical bodies made of tubulin • Each consists of 27 short microtubules • Act in forming cilia & flagella • Gives rise to mitotic spindle - necessary for karyokinesis (nuclear division) Centrosome & Centrioles Centrosome matrix Centrioles Microtubules Cytoplasmic Inclusions • May consist of pigments, crystals of protein, and food stores (temporary structures, not present in all cell types) – Lipid droplets – found in liver cells & fat cells – Glycosomes – store sugar in the form of glycogen The Nucleus • Nucleus = “little nut” or “kernel” – control center of cell – DNA directs the cell’s activities & production of proteins – ~5 µm in diameter The Nucleus • Nuclear envelope = two parallel membranes separated by fluid-filled space • Nuclear pores penetrate the nuclear envelope – Pores allow large molecules to pass in and out of the nucleus (like messenger RNA) The Nucleus • Nucleolus = “little nucleus” – in the center of the nucleus – Contains parts of several chromosomes – Site of ribosome subunit assembly The Nucleus Fracture line of outer membrane Nuclear pores Nuclear envelope Nucleus Chromatin (condensed) Nucleolus Cisternae of rough ER Nuclear pore complexes. Nuclear lamina. The netlike Each pore is ringed by protein particles lamina composed of intermediate filaments formed by lamins lines the inner surface of the nuclear envelope. Chromatin & Chromosomes • DNA double helix is composed of a sugar phosphate backbone and four subunits: – Thymine (T), adenine (A), cytosine (C), & guanine (G) • DNA is coiled around proteins (histones) – DNA + proteins = chromatin Chromatin & Chromosomes Hydrogen bond • Each cluster of DNA & histone proteins is a nucleosome Nucleotides Sugar-phosphate backbone Adenine (A) Thymine (T) Cytosine (C) Guanine (G) Deoxyribose sugar Phosphate Chromatin & Chromosomes • Extended chromatin – Is the active region of DNA where DNA’s genetic code is duplicated (in replication) or copied into messenger RNA (mRNA) in the process of transcription • Condensed chromatin – Tightly coiled nucleosomes – Inactive form of chromatin – no transcription occurring • Chromosomes = highest level of organization of chromatin – Contains a long molecule of DNA – Typical human cell has 46 chromosomes! Chromatin & Chromosome Structure 1 DNA double helix (2-nm diameter) Histones 2 Chromatin (“beads on a string”) structure with nucleosomes Linker DNA Nucleosome (10-nm diameter; 8 histone proteins wrapped by 2 winds of the DNA double helix) 3 Tight helical fiber 4 Looped domain (30-nm diameter) structure (300-nm diameter) 5 Chromatid (700-nm diameter) Metaphase Chromosome = 2 sister chromatids (at midpoint of cell division) The Cell Life Cycle • The cell life cycle = the series of changes a cell goes through, including dividing & producing daughter cells – Interphase: • G1 phase = Growth 1 or Gap 1 phase – – – – The first part of interphase Cell metabolically active grows & makes proteins Variable in length from hours to YEARS! (egg cell) Centrioles begin to replicate near the end of G1 The Cell Life Cycle • S (synthesis) phase = DNA replicates itself – Ensures that daughter cells receive identical copies of the genetic material (chromatin extended) • G2 phase = Growth 2 or Gap 2 – Centrioles finish copying themselves (now 2 pairs!) – Enzymes needed for cell division are synthesized • During S & G2 phases of interphase cell carries on normal activities The Cell Life Cycle • Cell division = mitosis – M (mitotic) phase = cells divide during this stage • Comes after interphase (G1, S, & G2) Cell Division • Cell division involves: – Mitosis = division of the nucleus • Chromosomes are distributed to the 2 daughter nuclei – Cytokinesis = division of the cytoplasm • Occurs after the nucleus divides The Cell Life Cycle G1 checkpoint (restriction point) G1 Growth Interphase S Growth and DNA synthesis G2 Growth and final preparations for division M Mitosis Mitotic phase (M) G2 checkpoint Stages of Mitosis • Prophase = 1st & longest stage of mitosis – Early prophase chromatin threads condense into chromosomes • Chromosomes = 2 threads called chromatids (sister chromatids) • Chromatids held together by the centromere • Centriole pairs separate from one another • Mitotic spindle forms – Late prophase centrioles continue moving away from each other • Nuclear membrane breaks down • Chromosomes bound by mitotic spindle Early & Late Prophase Interphase Centrosomes (each has 2 centrioles) Early Prophase Plasma membrane Late Prophase Early mitotic spindle Spindle pole Aster Nucleolus Chromatin Nuclear envelope Chromosome consisting of two sister chromatids Polar microtubule Fragments of nuclear envelope Centromere Kinetochore Kinetochore microtubule Stages of Mitosis • Metaphase = 2nd stage of mitosis – Chromosomes cluster at middle of the cell • Centromeres are aligned along the equator • Anaphase = 3rd & shortest stage of mitosis – Centromeres of chromosomes split apart Stages of Mitosis • Telophase = 4th stage of mitosis – – – – Begins as chromosomal movement stops Chromosomes at opposite poles of the cell uncoil Resume threadlike extended-chromatin form A new nuclear membrane forms • Cytokinesis = completes the division of the cell into 2 daughter cells Metaphase, Anaphase,Telophase & Cytokinesis Metaphase Anaphase Nuclear envelope forming Spindle Metaphase plate Telophase & Cytokinesis Daughter chromosomes Contractile Nucleolus forming ring at cleavage furrow Mitosis in Action Cellular Diversity • Cellular Differentiation – Cells in different regions of developing embryo are exposed to different chemical signals – Cells differentiate into different cell types & become specialized • Specialized functions of cells relates to: – Shape of cell morphology – Arrangement, types, & numbers of organelles Cellular Diversity • Cells that connect body parts or line & cover organs: – Fibroblast = makes and secretes protein component of fibers – Erythrocyte = concave shape provides surface area for uptake of the respiratory gases – Epithelial cell = hexagonal shape allows maximum number of epithelial cells to pack together Cellular Diversity • Cells that MOVE organs & body parts: – Skeletal & smooth muscle cells • Elongated & made of actin & myosin • Contract forcefully! Skeletal muscle cell Smooth muscle cells Cellular Diversity • Cells that store nutrients: – Fat cell = stores fat; shape is Fat cell produced by large fat droplet in its cytoplasm • Cells that fight disease: – Macrophage = moves through tissue to reach infection sites & engulfs foreign particles Macrophage Cellular Diversity • Cells that gather & transmit information: – Neuron = has long processes for receiving & transmitting messages neuron Cellular Diversity • Cells of reproduction: *Sperm (male) = specialized cell with a long tail (flagellum) for swimming to and fertilizing the egg *Oocyte (female) = largest cell of the body, contains extra stores of RNA and protein, specialized outer shell Developmental Aspects of Cells • Aging = complex process that happens over time; caused by a variety of factors: – Free radical theory • Damage from byproducts of cellular metabolism • Radicals build up & damage essential molecules of cells – Mitochondrial theory • A decrease in production of energy by mitochondria weakens & ages our cells – Genetic theory = proposes that aging is programmed by genes • Telomeres = “end caps” on chromosomes • Telomerase = prevents telomeres from degrading Questions…? What’s Next? Lab: The Microscope & Cells Wed Lecture: Embryology & Tissues Wed Lab: Cells & Tissues