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How are cells ableto move? List some different techniques of cell locomotion.
Actin Microfilaments
• two gently twisted strands of actin subunits– long but only 7nm in diameter– maintain shape of the cell– bearing tension (pull) rather than resistance (push)– instrumental in major changes of cell shape such as
• Pseudopodia• muscle contractions• cleavage that occurs during cell division.
Tubulin - Microtubules
• The microtubule subunits wind around in a continuously growing strand and can be added to lengthen the hollow tube that is formed– 25nm in diameter– Hollow tube part is 15 nm in diameter
• used in cell motility by flagella and cilia • Maintains the shape of a cell by resisting compression• used to move around items inside the cell, such as
organelles or chromosomes during cell division.
Dividing newt lung cell seen under a light microscope and colored using fluorescent dyes: chromosomes in blue, intermediate filaments in red, and spindle fibers (bundled microtubules assembled for cell division) in green.
Fig. 6-21
VesicleATP
Receptor for motor protein
Microtubuleof cytoskeleton
Motor protein (ATP powered)
(a)
Microtubule Vesicles
(b)
0.25 µm
Intermediate Filaments
• Keratin protein fibers – arranged in cords of differing diameters of 8-12nm
(depending on the function in the cell)
• Serve as permanent structures, unlike the microtubules and microfilaments– part of a permanent scaffolding – serve to maintain rigid cell shape– anchor organelles in fixed positions when necessary
• the nucleus is fixed in a position with rigid intermediate filaments
Name the three types of structures which make up the cytoskeleton.
– Microtubules– Microfilaments
– Intermediate filaments
• Describe the differences in size between microtubules, microfilaments and intermediate filaments.
Components of the Cytoskeleton
– Microtubules are the thickest of the three components of the cytoskeleton
– Microfilaments, also called actin filaments, are the thinnest components
– Intermediate filaments are fibers with diameters in a middle range
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Table 6-1
10 µm 10 µm 10 µm
Column of tubulin dimers
Tubulin dimer
Actin subunit
25 nm
7 nm
Keratin proteins
Fibrous subunit (keratins coiled together)
8–12 nm
Table 6-1a10 µm
Column of tubulin dimers
Tubulin dimer
25 nm
Table 6-1b
Actin subunit
10 µm
7 nm
Table 6-1c
5 µm
Keratin proteins
Fibrous subunit (keratinscoiled together)
8–12 nm
Centrosomes and Centrioles
• In many cells, microtubules grow out from a centrosome near the nucleus
• The centrosome is a “microtubule-organizing center”
• In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring
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Fig. 6-22Centrosome
Microtubule
Centrioles
0.25 µm
Longitudinal section of one centriole
Microtubules Cross sectionof the other centriole
Cilia and Flagella
• Microtubules control the beating of cilia and flagella
• Cilia and flagella differ in their beating patterns
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Fig. 6-23
5 µm
Direction of swimming
(a) Motion of flagella
Direction of organism’s movement
Power stroke Recovery stroke
(b) Motion of cilia15 µm
• Cilia and flagella share a common ultrastructure:
– A core of microtubules sheathed by the plasma membrane
– A basal body that anchors the cilium or flagellum
– A motor protein called dynein, which drives the bending movements of a cilium or flagellum
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Fig. 6-24
0.1 µm
Triplet
(c) Cross section of basal body
(a) Longitudinal section of cilium
0.5 µm
Plasma membrane
Basal body
Microtubules
(b) Cross section of cilium
Plasma membrane
Outer microtubule doublet
Dynein proteins
Central microtubuleRadial spoke
Protein cross-linking outer doublets
0.1 µm
• How dynein “walking” moves flagella and cilia:
− Dynein arms alternately grab, move, and release the outer microtubules
– Protein cross-links limit sliding
– Forces exerted by dynein arms cause doublets to curve, bending the cilium or flagellum
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Fig. 6-25a
Microtubule doublets
Dynein protein
(a) Effect of unrestrained dynein movement
ATP
Fig. 6-25b
Cross-linking proteins inside outer doublets
Anchorage in cell
ATP
(b) Effect of cross-linking proteins
(c) Wavelike motion
1 3
2
https://www.youtube.com/watch?v=SlZtSDalef0
Hypothesis of Flagellar Beating
• Microfilaments that function in cellular motility contain the protein myosin in addition to actin
• In muscle cells, thousands of actin filaments are arranged parallel to one another
• Thicker filaments composed of myosin interdigitate with the thinner actin fibers
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Fig, 6-27a
Muscle cell
Actin filament
Myosin filamentMyosin arm
(a) Myosin motors in muscle cell contraction
• actin and myosin drive amoeboid movement
• Pseudopodia (cellular extensions)
• extend and contract through the reversible assembly and contraction of actin subunits into microfilaments
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Fig. 6-27bcCortex (outer cytoplasm): gel with actin network
Inner cytoplasm: sol with actin subunits
Extending pseudopodium
(b) Amoeboid movement
Nonmoving cortical cytoplasm (gel)
Chloroplast
Cell wall
Streaming cytoplasm (sol)
Parallel actin filaments
(c) Cytoplasmic streaming in plant cells
Vacuole
• Cytoplasmic streaming
– a circular flow of cytoplasm within cells
– speeds distribution of materials within the cell
– In plant cells, actin-myosin interactions and sol-gel transformations drive cytoplasmic streaming
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Extracellular components and connections between cells help coordinate cellular activities
• Most cells synthesize and secrete materials that are external to the plasma membrane
• These extracellular structures include:
– Cell walls of plants
– The extracellular matrix (ECM) of animal cells
– Intercellular junctions
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Cell Walls of Plants
• cell wall
– an extracellular structure that distinguishes plant cells from animal cells
– Prokaryotes, fungi, and some protists also have cell walls
– protects the plant cell, maintains its shape, and prevents excessive uptake of water
– Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein
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• 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
• Plasmodesmata are channels between adjacent plant cells
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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
• Animal cells lack cell walls but
– covered by an elaborate extracellular matrix (ECM)
– made up of glycoproteins such as collagen, proteoglycans, and fibronectin
• bind to receptor proteins in the plasma membrane called integrins
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Fig. 6-30
EXTRACELLULAR FLUIDCollagen
Fibronectin
Plasmamembrane
Micro-filaments
CYTOPLASM
Integrins
Proteoglycancomplex
Polysaccharidemolecule
Carbo-hydrates
Coreprotein
Proteoglycanmolecule
Proteoglycan complex
Fig. 6-30a
Collagen
Fibronectin
Plasma membrane
Proteoglycan complex
Integrins
CYTOPLASMMicro-filaments
EXTRACELLULAR FLUID
Fig. 6-30b
Polysaccharide molecule
Carbo-hydrates
Core protein
Proteoglycan molecule
Proteoglycan complex
• Functions of the Extracellular matrix (ECM):
– Support
– Adhesion
– Movement
– Regulation
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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
– Tight junctions
– Desmosomes
– Gap junctionsCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Plasmodesmata in Plant Cells
• Plasmodesmata
– channels that perforate plant cell walls
– water and small solutes (and sometimes proteins and RNA) can pass from cell to cell
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Fig. 6-31
Interior of cell
Interior of cell
0.5 µm Plasmodesmata Plasma membranes
Cell walls
Tight Junctions, Desmosomes, and Gap Junctions in Animal Cells
• At 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
1 µmDesmosome
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
Fig. 6-32aTight junctions prevent fluid from moving across a layer of cells
Tight junction
Intermediate filaments
Desmosome
Gap junctions
Extracellular matrixSpace
between cells
Plasma membranes of adjacent cells
Fig. 6-32b
Tight junction
0.5 µm
Fig. 6-32c
Desmosome1 µm
Fig. 6-32d
Gap junction
0.1 µm
The Cell: A Living Unit Greater Than the Sum of Its Parts
• Cells rely on the integration of structures and organelles in order to function
• For example, a macrophage’s ability to destroy bacteria involves the whole cell, coordinating components such as the cytoskeleton, lysosomes, and plasma membrane
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Fig. 6-33
5 µ
m
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).
Fig. 6-UN1a
Cell Component Structure Function
Concept 6.3 The eukaryotic cell’s geneticinstructions are housed inthe nucleus and carried outby the ribosomes
Nucleus Surrounded by nuclearenvelope (double membrane)perforated by nuclear pores.The nuclear envelope iscontinuous with theendoplasmic reticulum (ER).
(ER)
Houses chromosomes, made ofchromatin (DNA, the geneticmaterial, and proteins); containsnucleoli, where ribosomalsubunits are made. Poresregulate entry and exit osmaterials.
Ribosome Two subunits made of ribo-somal RNA and proteins; can befree in cytosol or bound to ER
Protein synthesis
Fig. 6-UN1b
Cell Component Structure Function
Concept 6.4 The endomembrane systemregulates protein traffic andperforms metabolic functionsin the cell
Endoplasmic reticulum
(Nuclearenvelope)
Golgi apparatus
Lysosome
Vacuole Large membrane-boundedvesicle in plants
Membranous sac of hydrolyticenzymes (in animal cells)
Stacks of flattenedmembranoussacs; has polarity(cis and transfaces)
Extensive network ofmembrane-bound tubules andsacs; membrane separateslumen from cytosol;continuous withthe nuclear envelope.
Smooth ER: synthesis oflipids, metabolism of carbohy-drates, Ca2+ storage, detoxifica-tion of drugs and poisons
Rough ER: Aids in sythesis ofsecretory and other proteinsfrom bound ribosomes; addscarbohydrates to glycoproteins;produces new membrane
Modification of proteins, carbo-hydrates on proteins, and phos-pholipids; synthesis of manypolysaccharides; sorting ofGolgi products, which are thenreleased in vesicles.
Breakdown of ingested sub-stances cell macromolecules, and damaged organelles for recycling
Digestion, storage, wastedisposal, water balance, cellgrowth, and protection
Fig. 6-UN1c
Cell Component
Concept 6.5Mitochondria and chloro-plasts change energy fromone form to another
Mitochondrion
Chloroplast
Peroxisome
Structure Function
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
Cellular respiration
Photosynthesis
Contains enzymes that transferhydrogen to water, producinghydrogen peroxide (H2O2) as aby-product, which is convertedto water by other enzymesin the peroxisome
You should now be able to:
1. Distinguish between the following pairs of terms: magnification and resolution; prokaryotic and eukaryotic cell; free and bound ribosomes; smooth and rough ER
2. Describe the structure and function of the components of the endomembrane system
3. Briefly explain the role of mitochondria, chloroplasts, and peroxisomes
4. Describe the functions of the cytoskeleton
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
5. Compare the structure and functions of microtubules, microfilaments, and intermediate filaments
6. Explain how the ultrastructure of cilia and flagella relate to their functions
7. Describe the structure of a plant cell wall
8. Describe the structure and roles of the extracellular matrix in animal cells
9. Describe four different intercellular junctionsCopyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
How might cells from various parts of the body differ in the number and types of cellular organelles they contain due to their specific function?
• stomach cell – rough ER for secretion
• muscle cell – mitochondria for ATP
• liver storage cell – vacuoles for storage
• Liver detoxification cell– peroxisomes and smooth ER to break down toxins
• Adipose cell– vacuoles for storage
• white blood cell – lysosomes to break down engulfed pathogens
• mesophyll cell—plant leaf cell– chloroplasts for photosynthesis
• potato cell – vacuoles for starch storage
Recycling
• Cells are wonderful at recycling!– the endomembrane system
• cycles phospholipids
– lysosomes, peroxisomes and the smooth ER • break down macromolecules to component parts and
reassemble them
– the cytoskeleton• constant flow of assembling and de-assembling
subunits
Why are cells so efficient at recycling?
• Limited resources• limited energy• efficiency