a tour of the cell. eukaryotic cells have internal membranes that compartmentalize their functions...
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A Tour of the Cell
Eukaryotic cells have internal membranes that compartmentalize their functions
• Basic features of all cells: plasma membrane, cytosol, chromosomes, ribosomes
• Prokaryotic vs Eukaryotic cells
– prokaryotic cells Bacteria and Archaea
• No nucleus
• DNA in an unbound region called the nucleoid
• No membrane-bound organelles
• Cytoplasm bound by the plasma membrane
– eukaryotic cells Protists, fungi, animals and plants
• DNA in a nucleus bounded by nuclear envelope
• Membrane-bound organelles
• Cytoplasm in the region between the plasma membrane and nucleus
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Fig. 6-6
Fimbriae
Nucleoid
Ribosomes
Plasma membrane
Cell wall
Capsule
Flagella
Bacterialchromosome
0.5 µm
Prokaryotic cell
Fig. 6-9a
ENDOPLASMIC RETICULUM (ER)
Smooth ERRough ERFlagellum***(not in plants)
Centrosome
CYTOSKELETON:
Microfilaments
Intermediatefilaments
Microtubules
Microvilli
Peroxisome
MitochondrionLysosome***(not in plants/prokaryotes)
Golgiapparatus
Ribosomes
Plasma membrane
Nuclearenvelope
Nucleolus
Chromatin
NUCLEUS
Eukaryotic Animal Cell
Fig. 6-9b
NUCLEUS
Nuclear envelopeNucleolusChromatin
Rough endoplasmic reticulum
Smooth endoplasmic reticulum
Ribosomes
Central vacuole***
MicrofilamentsIntermediate filamentsMicrotubules
CYTO-SKELETON
Chloroplast***
Plasmodesmata***Wall of adjacent cell
Cell wall***
Plasma membrane
Peroxisome
Mitochondrion
Golgiapparatus
Eukaryotic Plant Cell
***specific to plants
Organelles to know
• Nucleus – Genetic information
• Ribosomes – protein factories
• Endoplasmic Reticulum – protein trafficking and metabolic functions
• Golgi Apparatus – shipping and receiving center
• Lysosomes – digestive compartments
• Vacuoles – maintenance compatments
• Mitochondria – chemical energy conversion (site of cellular respiration)
• Chloroplasts – light energy conversion (site of photosynthesis)
• Peroxisomes - oxidation
• Cytoskeleton – support, motility and regulation
• Extracellular Matrix – support, adhesion, movement, regulation
• Intercellular Junctions – facilitate contact between cells
Nucleus: Information Central
– nuclear envelope encloses the nucleus, separating it from the cytoplasm
– nuclear membrane is a double membrane; each membrane consists of a lipid bilayer
– Pores regulate the entry and exit of molecules from the nucleus
– The shape of the nucleus is maintained by the nuclear lamina, which is composed of protein
– In the nucleus, DNA and proteins form genetic material called chromatin
– Chromatin condenses to form discrete chromosomes
– The nucleolus is located within the nucleus and is the site of ribosomal RNA (rRNA) synthesis
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Fig. 6-10
NucleolusNucleus
Nuclear lamina
Close-up of nuclear envelope
1 µm
1 µm
0.25 µm
Ribosome
Pore complex
Nuclear pore
Nuclear envelope
Chromatin
Surface ofnuclear envelope
Pore complexes
Ribosomes: Protein factories
– particles made of ribosomal RNA and protein
– Protein synthesis occurs here
• Free ribosomes are localized to the cytosol
• Bound ribosomes are on the ER or the nuclear envelope
Fig. 6-11
Cytosol
Endoplasmic reticulum (ER)
Free ribosomes
Bound ribosomes
Large subunit
Small subunit
Diagram of a ribosome
Endoplasmic Reticulum (ER): Protein Trafficking
– The ER membrane is attached to the nuclear envelope
– There are two distinct regions of ER:
• Smooth ER lacks ribosomes
– Synthesizes lipids
– Metabolizes carbohydrates
– Detoxifies poison
– Stores calcium
• Rough ER with ribosomes
– Secretes glycoproteins (proteins covalently bonded to carbohydrates)
– Distributes transport vesicles, proteins surrounded by membranes
– membrane factory for the cell
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Fig. 6-12Smooth ER
Rough ER Nuclear envelope
Rough ERSmooth ER
Transport vesicle
Ribosomes
Golgi apparatus: Shipping and Receiving Center
– shipping and receiving center
– consists of flattened membranous sacs called cisternae
– Functions:
• Modifies products of the ER
• Manufactures certain macromolecules
• Sorts and packages materials into transport vesicles
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Cisternae
Lysosome: Digestive Compartment
– a membranous sac of hydrolytic enzymes that can digest macromolecules
– Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids
• After phagocytosis (engulfing of another cell) lysosomes fuse with the food vacuole and digests the molecules
• Autophagy uses enzymes to recycle the cell’s own organelles and macromolecules
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Digestiveenzymes
Lysosome
Plasmamembrane
Food vacuole
Digestion
Peroxisome
Vesicle
Lysosome
Mitochondrion Digestion
(a) Phagocytosis (b) Autophagy
– Diverse Maintenance Compartments
– Food vacuoles are formed by phagocytosis
– Contractile vacuoles pump excess water out of cells
– Central vacuoles (in many mature plant cells) hold organic compounds and water
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Fig. 6-15
Central vacuole
Cytosol
Central vacuole
Nucleus
Cell wall
Chloroplast
Vacuoles: Maintenance Compartment
The Endomembrane System: A Review
• The endomembrane system is a complex and dynamic player in the cell’s compartmental organization
– Nuclear envelope
– ER
– Golgi apparatus
– Lysosomes
– Vacuoles
– Plasma membrane
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Fig. 6-16-3
Smooth ER
Nucleus
Rough ER
Plasma membrane
Golgi
•Nuclear envelope is connected to rough ER•Proteins produced by the ER flow in transport vessicles to the Golgi•Golgi pinches off vessicles that give rise to lysosomes, vessicles and vacuoles•Lysosomes can fuse with another vessicle for digestion•Transport vessicle carries proteins to plasma membrane for secretion•Plasma membrane expands by fusion of vessicles; proteins are secreted from the cell
Transport vessicle
Transport vessicle
Lysosome
Mitochondria: Chemical Energy Conversion
– sites of cellular respiration, a metabolic process that generates ATP
– Have a double membrane
– Contain their own DNA
– Mitochondria are in nearly all eukaryotic cells
– They have a smooth outer membrane and an inner membrane folded into cristae
• Cristae present a large surface area for enzymes that synthesize ATP
– The inner membrane creates two compartments: intermembrane space and mitochondrial matrix
• Some metabolic steps of cellular respiration are catalyzed in the mitochondrial matrix
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Fig. 6-17
Free ribosomesin the mitochondrial matrix
Intermembrane space
Outer membrane
Inner membraneCristae
Matrix
0.1 µm
Chloroplasts: Light Energy Conversion
– Capture light energy, are the sites of photosynthesis
– found in plants and algae
– Have a double membrane (similar to mitochondria)
– Contain their own DNA (similar to mitochondria)
– contain chlorophyll and other molecules that function in photosynthesis
– found in leaves and other green organs of plants and in algae
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Ribosomes
Thylakoid
Stroma
Granum
Inner and outer membranes
1 µm
– oxidative organelles
– specialized metabolic compartments bounded by a single membrane
– produce hydrogen peroxide and convert it to water
– Oxygen is used to break down different types of molecules
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1 µm
ChloroplastPeroxisome
Mitochondrion
Peroxisome: Oxidation
Cytoskeleton: Support, Motility and Regulation
– a network of fibers extending throughout the cytoplasm
– organizes the cell’s structures and activities, anchoring many organelles
– composed of three types of molecular structures:
• Microtubules are the thickest of the three components
• Microfilaments, also called actin filaments, are the thinnest components
• Intermediate filaments are fibers with diameters in a middle range
– helps to support the cell and maintain its shape
– interacts with motor proteins to produce motility
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• Functions
– Shaping the cell
– Guiding movement of organelles
– Separating chromosomes during cell division
• Centrosomes
– The centrosome is a “microtubule-organizing center”
– microtubules grow out from a centrosome near the nucleus and attach to chromosomes during mitosis
• Cilia and Flagella
– Microtubules control the beating of cilia and flagella
– A core of microtubules sheathed by the plasma membrane
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Microtubules
Fig. 6-22
Centrosome
Microtubule
centrosome
microtubules
Microtubules separate chromosomes during mitosis
Fig. 6-23
(a) EX: Motion of flagella in sperm (b) EX: Motion of cilia in aquatic life
Microtubules assist in motility
Fig. 6-24
(c) Cross section of basal body
(a) Longitudinal section of cilium
Plasma membrane
Basal body
Microtubules
(b) Cross section of cilium
Plasma membrane
Microtubule structure in cilia
Microfilaments (Actin Filaments)
• Microfilaments are solid rods built as a twisted double chain of actin subunits
• The structural role: bear tension and resist pulling forces within the cell
• Cellular function: cellular motility
– Myosin and actin contribute to this
• Examples
– Muscle contraction
– Ameoboid movement occurs through Pseudopodia
– Cytoplasmic streaming
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Microvillus
Microfilaments (actin filaments)
Intermediate filaments
Intermediate Filaments
• They support cell shape and fix organelles in place
• more permanent cytoskeleton fixtures than the other two classes
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Cell Walls of Plants
• Prokaryotes, fungi, and some protists also have cell walls
• Functions: protects the plant cell, maintains its shape, prevents excessive uptake of water
• made of cellulose fibers
• Plant cell walls may have multiple layers:
– Primary cell wall: relatively thin and flexible
– Middle lamella: thin layer between primary walls of adjacent cells
– Secondary cell wall (in some cells): added between the plasma membrane and the primary cell wall
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Fig. 6-28
Secondary cell wall
Primary cell wall
Middle lamella
Central vacuoleCytosol
Plasma membrane
Plant cell walls
Plasmodesmata
1 µm
The Extracellular Matrix (ECM) of Animal Cells
• Functions
– Support
– Adhesion
– Movement
– Regulation
• made up of glycoproteins (collagen, proteoglycans, and fibronectin)
• ECM proteins bind to receptor proteins in the plasma membrane called integrins
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EXTRACELLULAR MATRIXCollagen
Fibronectin
Micro-filaments
CYTOPLASM
Integrins
Proteoglycancomplex
Intercellular Junctions
• Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact
• Intercellular junctions facilitate this contact
• There are several types of intercellular junctions
– Plasmodesmata – channels that perforate cell walls
– Tight junctions - membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid
– Desmosomes (anchoring junctions) fasten cells together into strong sheets
– Gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells
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Fig. 6-32
Tight junction
0.5 µm
Desmosome
Gap junction
Extracellularmatrix
0.1 µm
Plasma membranesof adjacent cells
Spacebetweencells
Gapjunctions
Desmosome
Intermediatefilaments
Tight junction
Tight junctions preventfluid from movingacross a layer of cells
Desmosomes fasten cells together in sheets
Gap junctions all cells to communicate with one another via cytoplasmic channels
Fig. 6-UN1Cell Component Structure Function
Houses chromosomes, made ofchromatin (DNA, the geneticmaterial, and proteins); containsnucleoli, where ribosomalsubunits are made. Poresregulate entry and exit ofmaterials.
Nucleus
(ER)
Concept 6.3 The eukaryotic cell’s geneticinstructions are housed inthe nucleus and carried outby the ribosomes
Ribosome
Concept 6.4 Endoplasmic reticulum The endomembrane systemregulates protein traffic andperforms metabolic functionsin the cell
(Nuclearenvelope)
Concept 6.5 Mitochondria and chloro-plasts change energy fromone form to another
Golgi apparatus
Lysosome
Vacuole
Mitochondrion
Chloroplast
Peroxisome
Two subunits made of ribo-somal RNA and proteins; can befree in cytosol or bound to ER
Extensive network ofmembrane-bound tubules andsacs; membrane separateslumen from cytosol;continuous withthe nuclear envelope.
Membranous sac of hydrolyticenzymes (in animal cells)
Large membrane-boundedvesicle in plants
Bounded by doublemembrane;inner membrane hasinfoldings (cristae)
Typically two membranesaround fluid stroma, whichcontains membranous thylakoidsstacked into grana (in plants)
Specialized metaboliccompartment bounded by asingle membrane
Protein synthesis
Smooth ER: synthesis oflipids, metabolism of carbohy-drates, Ca2+ storage, detoxifica-tion of drugs and poisons
Rough ER: Aids in synthesis ofsecretory and other proteins frombound ribosomes; addscarbohydrates to glycoproteins;produces new membrane
Modification of proteins, carbo-hydrates on proteins, and phos-pholipids; synthesis of manypolysaccharides; sorting of Golgiproducts, which are then released in vesicles.
Breakdown of ingested substances,cell macromolecules, and damagedorganelles for recycling
Digestion, storage, wastedisposal, water balance, cellgrowth, and protection
Cellular respiration
Photosynthesis
Contains enzymes that transferhydrogen to water, producinghydrogen peroxide (H2O2) as aby-product, which is convertedto water by other enzymesin the peroxisome
Stacks of flattenedmembranoussacs; has polarity(cis and transfaces)
Surrounded by nuclearenvelope (double membrane)perforated by nuclear pores.The nuclear envelope iscontinuous with theendoplasmic reticulum (ER).