Chapter 2: Introduction to Cells Several important scientists made discoveries about cells Robert Hooke Matthias Schleiden and 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 three main components Plasma the outer boundary Cytoplasm contains most organelles Nucleus controls cellular activities NOTE: Important chemicals involved in cell anatomy include: Water and ions (simple charged particles) Macromolecules, such as Proteins (chains of smaller units called amino acids) Lipids (include fat [is 3 fatty acids + glycerol]; steroids; cholesterol; waxes) Carbohydrates (sugars: mono-saccharides, poly-saccharides, glycogen) ATP: adenosine triphosphate Nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) Structure of a Generalized Cell Smooth endoplasmic reticulum Cytosol Chromatin Nucleolus Nucleus Plasma The Plasma Membrane Plasma (or plasmalemma) defines the extent of the cell: separates intracellular fluid from extracellular fluid (watery inside from watery outside with a fatty barrier) Mitochondrion Lysosome Centrioles Centrosome matrix Cytoskeletal elements Microtubule Intermediate filaments Peroxisome Rough endoplasmic reticulum Ribosomes Golgi apparatus Secretion being released from cell by exocytosis Figure 2.1 Structure of the cell Fluid mosaic model (lipid bilayer) Types of 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 surface (in or out) Support plasma from the cytoplasmic side The Plasma Membrane The Plasma Membrane Polar head of phospholipid molecule Extracellular fluid (watery environment) Glycolipid Cholesterol Nonpolar tail Glycoprotein of phospholipid molecule Carbohydrate of glycocalyx Bimolecular lipid layer containing proteins Outwardfacing layer of phospholipids Inward-facing layer of phospholipids Integral Cytoplasm proteins (watery environment) Filament of cytoskeleton Peripheral proteins Functions relate to location at the interface of cell s exterior and interior Provides barrier against substances outside cell Some plasma molecules act as receptors Determines which substances enter or leave the cell Membrane is referred to as being selectively permeable Figure 2.2 1
Figure 2.3 Membrane transport mechanisms. Membrane Transport: moving substances across the cell These processes include: Extracellular fluid Lipidsoluble solutes Water molecules Water soluble solutes Solute Simple diffusion tendency of molecules to move down their concentration gradient through the lipid bilayer Osmosis diffusion of water molecules across a through small channels Facilitated diffusion movement of molecules down their concentration gradient through an integral protein channel Active transport integral proteins move molecules across the plasma against their concentration gradient Endo- and exocytosis Lipid bilayer Cytoplasm Simple diffusion of fat-soluble molecules directly through the phospholipid bilayer down their concentration gradient Osmosis, diffusion of water through the lipid bilayer and down its concentration gradient. Facilitated diffusion An integral protein that spans the plasma enables the passage of a particular solute across the and down its conc gradient. ATP Active transport Some transport proteins use ATP as an energy source to actively pump substances across the plasma against their concentration gradient. Endocytosis Three Types of Endocytosis Endocytosis: Mechanism by which particles (chunks of stuff) enter cells Phagocytosis cell eating Pinocytosis cell drinking Receptor-mediated endocytosis Plasma proteins bind to certain molecules Invaginates and forms a coated pit Pinches off to become a coated vesicle NOTE: This is the method by which insulin and cholesterol and some viruses enter cells! Phagosome Receptor recycled to plasma Vesicle (a) Phagocytosis The cell engulfs a large particle by forming projecting pseudopods ( false feet ) around it and enclosing it within a sac called a phagosome. The phagosome then combines with a lysosome, and its contents are digested. Vesicle may or may not be protein-coated but has receptors capable of binding to microorganisms or solid particles. (c) Receptor-mediated endocytosis Extracellular substances bind to specific receptor proteins in regions of protein-coated pits, enabling the cell to ingest and concentrate specific substances in protein-coated vesicles. The ingested substance may simply be released inside the cell, or combined with a lysosome to digest contents. Receptors are recycled to the plasma in vesicles. Vesicle (b) Pinocytosis The cell gulps drops of extracellular fluid containing solutes into tiny vesicles. No receptors are used, so the process is nonspecific. Most vesicles are proteincoated. Figure 2.4 Exocytosis Exocytosis Exocytosis a mechanism that moves substances out of the cell Substance is enclosed in a vesicle The vesicle migrates to the plasma Proteins from the vesicles (v-snares) bind with proteins (t-snares) The lipid layers from both s bind, and the vesicle releases its contents to the outside of the cell (a) The process Extracellular Plasma of exocytosis fluid SNARE (t-snare) 1 The - Secretory bound vesicle vesicle Vesicle SNARE migrates to the (v-snare) plasma. Molecule to be secreted Cytoplasm Fusion pore formed 3 The vesicle and plasma fuse and a pore opens up. 2 There, proteins Fused at the vesicle v- and surface (v-snares) t-snares bind with t-snares (plasma proteins). 4 Vesicle contents are released to the cell exterior. Figure 2.5 2
The Cytoplasm Cytoplasm lies internal to plasma Consists of cytosol, organelles, and inclusions, in watery environment Cytosol Jelly-like watery fluid in which other cellular elements are suspended Consists of water with dissolved ions (salts) and enzymes Cytoplasmic Organelles Ribosomes constructed of proteins and ribosomal RNA; not surrounded by a Site of protein synthesis Assembly of proteins is a process called translation Are the assembly line of the manufacturing plant Endoplasmic reticulum network within the cytoplasm tow types of ER: Rough ER ribosomes stud the external surfaces, so protein synthesis occurs here Smooth ER consists of tubules in a branching network; No ribosomes are attached, therefore no protein synthesis but lipid and carb synthesis The Endoplasmic Reticulum and Ribosomes Cytoplasmic Organelles Smooth ER Golgi apparatus a stack of three to 10 disk-shaped s 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 Is the packaging and shipping division of the manufacturing plant Rough ER Ribosomes Cisternae (a) Diagrammatic view of smooth and rough ER (b) Electron micrograph of smooth and rough ER (85,000 ) Figure 2.6 Golgi Appartus The sequence of events from protein synthesis on the rough ER to the final distribution of these proteins (1 of 2). New vesicles forming Cis face receiving side of Transport vesicle Golgi apparatus from rough ER Cisternae New vesicles forming Transport vesicle from trans face Trans face shipping side of Secretory vesicle Golgi apparatus Transport vesicle Golgi apparatus from the Golgi apparatus (a) Many vesicles in the process of pinching off (b) Electron micrograph of the Golgi from the membranous Golgi apparatus apparatus (90,000 ) 1 Protein-containing vesicles pinch off rough ER and migrate to fuse with s of Golgi apparatus. 2 Proteins are modified within the Golgi compartments. 3 Proteins are then packaged within different vesicle types, depending on their ultimate destination. Rough ER Golgi apparatus Pathway A: Vesicle contents destined for exocytosis ER Phagosome Plasma Proteins in cisterna Pathway C: Lysosome containing acid hydrolase enzymes Vesicle becomes lysosome Secretory vesicle Pathway B: Vesicle to be incorporated into plasma Secretion by exocytosis Extracellular fluid Figure 2.7 Figure 2.8 3
More Cytoplasmic Organelles... Lysosomes Lysosomes -walled sacs containing digestive enzymes Digest unwanted substances Lysosomes Peroxisomes -walled sacs of oxidase enzymes Enzymes neutralize free radicals and break down poisons Break down long chains of fatty acids Are numerous in the liver and kidneys Are the toxic waste removal system Light areas are regions where materials are being digested. Figure 2.9 Mitochondria Figure 2.10 Mitochondria. Mitochondria generate most of the cell s energy most complex organelle Contain some maternally inherited DNA Believed to have arisen from bacteria Mitochondrial DNA Ribosome Outer mitochondrial More abundant in energy-requiring cells, like muscle cells and sperm Inner mitochondrial Cristae Power plant of the cell: release energy stored in chemical bonds and transfer energy to produce ATP Matrix Enzymes More Cytoplasmic Organelles Cytoskeleton cell skeleton an elaborate network of rods: for support and communication Contains three types of rods: Microtubules cylindrical structures made of proteins Microfilaments filaments of contractile protein actin Intermediate filaments protein fibers Cytoskeleton: Microfilaments (a) Microfilaments Strands made of spherical protein subunits called actins Actin subunit 7 nm Microfilaments form the blue network surrounding the pink nucleus in this photo. Figure 2.11a 4
Cytoskeleton: Intermediate filaments (b) Intermediate filaments Tough, insoluble protein fibers constructed like woven ropes Fibrous subunits Cytoskeleton: Microtubules (c) Microtubules Hollow tubes of spherical protein subunits called tubulins Tubulin subunits 10 nm 25 nm Intermediate filaments form the purple batlike network in this photo. Figure 2.11b Microtubules appear as gold networks surrounding the cells pink nuclei in this photo. Figure 2.11c More Cytoplasmic Organelles... Cytoplasmic Inclusions Centrosomes and centrioles Centrosome a spherical structure in the cytoplasm Composed of centrosome matrix and centrioles Centrioles paired cylindrical bodies Consists of 27 short microtubules Act in forming cilia Necessary for karyokinesis (nuclear division) Temporary structures Not present in all cell types May consist of pigments, crystals of protein, and food stores; examples... Lipid droplets found in liver cell and fat cells Glycosomes store sugar in the form of glycogen The Nucleus The nucleus little nut or kernel : control center of cell DNA directs the cell s activities provides instructions for protein synthesis Nucleus is approximate 5µm in diameter two parallel s separated by fluid-filled space pores --- penetrate the nuclear Pores allow large molecules to pass in and out of the nucleus Nucleolus little nucleus in the center of the nucleus Contains parts of several chromosomes Site of ribosome subunit assembly The Nucleus Chromatin (condensed) Nucleolus Cisternae of rough ER (a) Surface of nuclear. Fracture line of outer pores Nucleus pore complexes. Each lamina. The netlike pore is ringed by protein particles lamina composed of intermediate filaments formed by lamins lines the inner surface of the nuclear. (b) Figure 2.13 5
Chromatin and Chromosomes Chromatin and Chromosomes Hydrogen bond DNA (NOT a protein but another type of macromolecule called a nucleic acid) is a double helix is composed of four subunits: Thymine (T), adenine (A), cytosine (C), and guanine (G) DNA is packed with protein molecules DNA plus the proteins form chromatin ( colored stuff ) Each cluster of DNA and histone proteins is a nucleosome The double-helix structure of the DNA molecule: Sugar-phosphate backbone Nucleotides Deoxyribose sugar Phosphate Adenine (A) Thymine (T) Cytosine (C) Guanine (G) Figure 2.14 Chromatin and Chromosomes Extended chromatin Is the active region of DNA where DNA s genetic code is copied onto mrna (transcription) Condensed chromatin Tightly coiled nucleosomes Inactive form of chromatin Chromosomes highest level of organization of chromatin Contains a long molecule of DNA 46 chromosomes (arranged in 23 pairs) are in a typical human cell 1 DNA double helix (2-nm diameter) Histones 2 Chromatin ( beads on a string ) structure with nucleosomes Linker DNA Nucleosome (10-nm diameter; eight histone proteins wrapped by two winds of the DNA double (a) helix) 3 Tight helical fiber (30-nm diameter) 4 Looped domain structure (300-nm diameter) 5 Chromatid (700-nm diameter) (b) Metaphase chromosome (at midpoint of cell division) Figure 2.15 The Cell Life Cycle The cell life cycle is the series of changes a cell goes through Interphase G 1 phase growth 1 or Gap 1 phase The first part of interphase Cell metabolically active growth make proteins Variable in length from hours to YEARS (egg cell) Centrioles begin to replicate near the end of G 1 S (synthetic) phase DNA replicates itself Ensures that daughter cells receive identical copies of the genetic material (chromatin extended) G 2 phase growth 2 or Gap 2 Centrioles finish copying themselves Enzymes needed for cell division are synthesized in G 2 During S (synthetic) and G 2 phases, cell carries on normal activities The Cell Life Cycle G 1 checkpoint (restriction point) G 1 Growth Cytokinesis Interphase S Growth and DNA synthesis G 2 Growth and final Telophase M Mitosis Anaphase Metaphase Mitotic phase (M) preparations for division Prophase G 2 checkpoint Figure 2.16 6
The Cell Life Cycle Cell division M (mitotic) phase cells divide during this stage Follows interphase (G 1, S, and G 2 ) Cell division involves: Mitosis division of the nucleus during cell division Chromosomes are distributed to the two daughter nuclei Cytokinesis division of the cytoplasm Occurs after the nucleus divides The Stages of Mitosis Prophase the first and longest stage of mitosis Early prophase chromatin threads condense into chromosomes Chromosomes are made up of two threads called chromatids (sister chromatids) Chromatids are held together by the centromere Centriole pairs separate from one another The mitotic spindle forms Late prophase centrioles continue moving away from each other fragments Early Prophase and Late Prophase The Stages of Mitosis Interphase Early Prophase Late Prophase Centrosomes (each Plasma Early mitotic Spindle pole Polar microtubule has 2 centrioles) spindle Fragments of nuclear Aster Metaphase the second stage of mitosis Chromosomes cluster at the middle of the cell Centromeres are aligned along the equator Anaphase the third and shortest stage of mitosis Centromeres of chromosomes split Nucleolus Chromatin Chromosome consisting of two sister chromatids Centromere Kinetochore Kinetochore microtubule Figure 2.17 (1 of 2) Metaphase and Anaphase The Stages of Mitosis Metaphase Anaphase Telophase and Cytokinesis Telophase begins as chromosomal movement stops Chromosomes at opposite poles of the cell uncoil Resume threadlike extended-chromatin form A new nuclear forms Spindle forming Nucleolus forming Contractile ring at cleavage furrow Cytokinesis completes the division of the cell into two daughter cells Metaphase plate Daughter chromosomes Figure 2.17 (2 of 2) 7
Telophase and Cytokinesis Metaphase plate Metaphase Anaphase Telophase and Cytokinesis Spindle Daughter chromosomes forming Nucleolus forming Contractile ring at cleavage furrow Specialized functions of cells relates to: Shape of cell Arrangement of organelles Some types of cells 1. Cells that connect body parts or cover organs or transport gases 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 Figure 2.17 Cells that Connect Body Parts or Cover Organs Fibroblasts Erythrocytes 2. Cells that move organs and body parts Skeletal and smooth muscle cells Elongated and filled with actin and myosin Contract forcefully ( shorten with force ) Epithelial cells (a) Cells that connect body parts, form linings, or transport gases Figure 2.18a Cells that Connect Organs and Body Parts Skeletal muscle cell (b) Cells that move organs and body parts Smooth muscle cells 3. Cells that store nutrients Fat cell shape is produced by large fat droplet in its cytoplasm 4. Cells that fight disease Macrophage moves through tissue to reach infection sites Figure 2.18b 8
Cells that Store Nutrients and Cells that Fight Disease Fat cell 5. Cells that gather and process information Neuron has long processes for receiving and transmitting messages (c) Cell that stores nutrients Macrophage Nerve cell (d) Cell that fights disease (e) Cell that gathers information and controls body functions Figure 2.18c, d Figure 2.18e Developmental Aspects of Cells 6. Cells of reproduction Sperm (male) possesses long tail for swimming to the egg for fertilization Aging a complex process caused by a variety of factors Free radical theory Damage from byproducts of cellular metabolism Radicals build up and damage essential molecules of cells (f) Cell of reproduction Sperm Mitochondrial theory A decrease in production of energy by mitochondria weakens and ages our cells Genetic theory proposes that aging is programmed by genes Telomeres end caps on chromosomes Telomerase prevents telomeres from degrading Figure 2.18f 9