ch 2, cells: the living units of organisms sonya schuh-huerta, ph.d. human embryonic stem cells, s....
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Ch 2, CELLS: The Living Units Ch 2, CELLS: The Living Units of Organismsof Organisms
Sonya Schuh-Huerta, Ph.D.Sonya Schuh-Huerta, Ph.D.
Human embryonic stem cells, S. Huerta
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The Inner Life of the Cell…
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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
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Introduction to Cells
• Cells have 3 main components– Plasma membrane = the outer boundary– Cytoplasm = contains most organelles– Nucleus = controls cellular activities
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Structure of a Generalized Cell
Secretion being released from cell by exocytosis
Peroxisome
Ribosomes
Roughendoplasmicreticulum
Nucleus
Nuclear envelopeChromatin
Golgi apparatus
Nucleolus
Smooth endoplasmicreticulum
Cytosol
Lysosome
Mitochondrion
Centrioles
Centrosomematrix
Microtubule
Cytoskeletalelements
Intermediate filaments
Plasmamembrane
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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
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The Plasma Membrane
Integral proteins
Extracellular fluid(watery environment)
Cytoplasm (watery environment)
Polar head of phospholipid molecule
GlycolipidCholesterol
Peripheralproteins
Bimolecular lipid layercontaining proteins
Inward-facing layer of phospholipids
Outward-facing layer of phospholipids
Carbohydrate of glycocalyx
GlycoproteinNonpolar tail of phospholipid molecule
Filament of cytoskeleton
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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!
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The Plasma Membrane in Action
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Membrane Transport
• Simple diffusion – tendency of molecules to move down their concentration gradient
• Osmosis – diffusion of water molecules across a membrane – down their gradient
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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)
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Membrane Transport Mechanisms
Extracellular fluid
Lipid-solublesolutes
Cytoplasm
Watermolecules
Solutes
Lipidbilayer
Simple diffusion Osmosis Facilitated diffusion & active transport
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Endocytosis
• Endocytosis– Mechanism by which particles enter cells
• Phagocytosis = “cell eating”• Pinocytosis = “cell drinking”
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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!
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3 Types of Endocytosis
Phagosome
Vesicle
Vesicle
Receptor recycledto plasma membrane
Phagocytosis Pinocytosis
Receptor-Mediated Endocytosis
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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
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Exocytosis
The membrane-bound vesicle migrates to the plasma membrane.
There, proteins at the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins).
The vesicle and plasma membrane fuse and a pore opens up.
Vesicle contentsare released to the cell exterior.
Extracellularfluid
Plasma membraneSNARE (t-SNARE)
Secretoryvesicle Vesicle SNARE
(v-SNARE)
Molecule tobe secreted
Fusedv- andt-SNAREs
Fusion pore formed
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2
4
Cytoplasm
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Exocytosis in Action
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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
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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
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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
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The ER & Ribosomes
Nuclearenvelope
Ribosomes
Rough ER
(a) Diagrammatic view of smooth and rough ER
Smooth ER
(b) Electron micrograph of smooth and
rough ER (85,000)
Cisternae
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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
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Golgi Apparatus
Cis face—“receiving” side of Golgi apparatus
Secretory vesicle
Many vesicles in the process of pinching off from the membranous Golgi apparatus
Electron micrograph of the Golgi apparatus (90,000)
Transport vesicle from the Golgi apparatus
Transportvesiclefromtrans face
Trans face –“shipping” side ofGolgi apparatus
New vesiclesforming
New vesicles forming
Cisternae
Transport vesiclefrom rough ER
Golgi apparatus
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Lysosomes• Lysosomes = membrane-walled sacs containing
digestive enzymes– Digest substances– Cells of gut many lysosomes
Lysosomes
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Plasmamembrane
Secretion by exocytosis
Vesicle becomeslysosome
Golgi apparatus
Rough ER ER membrane Phagosome
Proteins in cisterna
Pathway B:Vesicle membraneto be incorporatedinto plasmamembrane
Pathway A:Vesicle contentsdestined forexocytosis Extracellular fluid
Secretoryvesicle
Pathway C: Lysosomecontaining acid hydrolaseenzymes
Protein-containing vesicles pinch off rough ER and migrate to fuse with membranes of Golgi.
Proteins are modified within the Golgi compartments.
Proteins are then packaged within different vesicle types, depending on their ultimate destination.
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2
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Sequence of Events From Protein Synthesis to Their Final Distribution
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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 energy- requiring cells, like muscle cells & sperm
– “Power plant” of the cell
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Mitochondria
Enzymes
Matrix
Cristae
Mitochondrial DNA
Ribosome
Outer mitochondrial membrane
Innermitochondrial membrane
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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!!!
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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
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(a) Microfilaments
Strands made of spherical proteinsubunits called actins
(b) Intermediate filaments
Tough, insoluble protein fibersconstructed like woven ropes
(c) Microtubules
Hollow tubes of spherical proteinsubunits called tubulins
Actin subunit
7 nm 10 nm 25 nm
Fibrous subunitsTubulin subunits
Cytoskeletal Elements
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The Cytoskeleton in Action
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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)
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Centrosome matrix
Centrioles
Microtubules
Centrosome & Centrioles
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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
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The Nucleus
• Nucleus = “little nut” or “kernel” – control center
of cell
– DNA directs the cell’s activities & production of proteins– ~5 µm in diameter
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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)
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The Nucleus
• Nucleolus = “little nucleus” – in the center of the nucleus
– Contains parts of several chromosomes– Site of ribosome subunit assembly
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The Nucleus
Chromatin (condensed)
Nuclear envelope Nucleus
Nuclearpores
Fractureline of outermembrane
Nuclear pore complexes. Each pore is ringed by protein particles
Nuclear lamina. The netlike lamina composed of intermediate filaments formed by lamins lines the inner surface of the nuclearenvelope.
Nucleolus
Cisternae of rough ER
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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
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Chromatin & Chromosomes
• Each cluster of DNA & histone proteins is a nucleosome Deoxyribose
sugar
PhosphateSugar-phosphatebackbone
Hydrogen bond
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Nucleotides
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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!
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Chromatin & Chromosome Structure
MetaphaseChromosome = 2 sister chromatids(at midpoint of cell division)
Nucleosome (10-nm diameter; 8 histone proteins wrappedby 2 winds of the DNA double helix)
Linker DNA
Histones
DNA doublehelix (2-nm diameter)
Chromatin (“beads on a string”) structure with nucleosomes
Tight helical fiber(30-nm diameter)
Chromatid(700-nm diameter)
Looped domain structure (300-nm diameter)
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2
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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
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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
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The Cell Life Cycle
• Cell division = mitosis
– M (mitotic) phase = cells divide during this stage
• Comes after interphase (G1, S, & G2)
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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
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G1
Growth
SGrowth and DNA
synthesis G2
Growth and finalpreparations for
divisionM
G2 checkpoint
G1 checkpoint(restriction point)
Mitotic phase (M)Cytokinesis
Telo
ph
ase
An
aph
aseM
etaphaseProphase
Interphase
Mitosis
The Cell Life Cycle
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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
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Early & Late Prophase
Centrosomes (eachhas 2 centrioles)
Early mitoticspindle
Spindle pole
Kinetochore Kinetochoremicrotubule
Polar microtubule
Nucleolus Centromere
Plasmamembrane Fragments
of nuclearenvelopeAster
Nuclearenvelope
Chromosomeconsisting of twosister chromatids
Chromatin
Interphase Early Prophase Late Prophase
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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
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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
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Metaphase, Anaphase,Telophase & Cytokinesis
Contractilering atcleavagefurrow
Nuclearenvelopeforming
Nucleolus forming
Spindle
Metaphaseplate
Daughterchromosomes
Metaphase Anaphase Telophase & Cytokinesis
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Mitosis in Action
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Cellular Diversity
• Specialized functions of cells relates to:– Shape of cell morphology– Arrangement, types, & numbers of organelles
• Cellular Differentiation– Cells in different regions of developing embryo are
exposed to different chemical signals– Cells differentiate into different cell types &
become specialized
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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
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Cellular Diversity
• Cells that MOVE organs & body parts:
– Skeletal & smooth muscle cells• Elongated & made of actin & myosin• Contract forcefully!
Skeletalmusclecell
Smoothmuscle cells
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Cellular Diversity
• Cells that store nutrients:– Fat cell = stores fat; shape is
produced by large fat droplet in
its cytoplasm
• Cells that fight disease:– Macrophage = moves through tissue to reach
infection sites & engulfs foreign particles
Macrophage
Fat cell
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Cellular Diversity
• Cells that gather & transmit information:– Neuron = has long processes for receiving &
transmitting messages
neuron
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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
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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
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Questions…?
What’s Next?Lab: The Microscope & Cells Wed Lecture: Embryology & TissuesWed Lab: Cells & Tissues