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The Plant Cell and the Cell Cycle
Chapter 3
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Cell
• Basic unit of plant structure and function
• Robert Hooke– Looked at cork tissue under microscope– “little boxes or cells distinct from one another
….that perfectly enclosed air”
• Nehemiah Grew– Recognized leaves as collections of cells filled
with fluid and green inclusions
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Cell Theory
Statement Year Contributor
All plants and animals are composed of cells. 1838
Matthias Schleiden and Theodor Schwann
Cells reproduce themselves. 1858 Rudolf Virchow
All cells arise by reproduction from previous cells.
1858Rudolf Virchow
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Basic Similarities of Cells
• Cells possess basic characteristic of life– Movement– Metabolism– Ability to reproduce
• Organelles– “little organs”– Carry out specialized functions within cells
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Light Microscope
• View cells 20-200 µm in diameter
• Can view living or stained specimens
• Resolution (resolving power)– Ability to distinguish separate objects– Limited by lenses and wavelengths of light
used– Smallest object that can be resolved is ~ 0.2
µm in diameter
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Confocal Microscope
• Laser illumination
• Detecting lens focuses on single point at a time– Scans entire sample to assemble picture
• No reduction in contrast due to scattered light
• Can generate 3-D images
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Transmission Electron Microscope
• Responsible for discovery of most of smaller organelles in cell
• Greater resolution
• Uses beams of electrons rather than light
• Magnets for lenses
• Ultrathin section examined in vacuum
• View image on fluorescent plate or photographic film
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Scanning Electron Microscope
• Collected electrons used to form picture in television picture tube
• High resolution view of surface structures
• Requires vacuum
• Recent refinements – Can operate in low vacuum– Can view live plant cells and insects
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Microscope ComparisonsSource for illumination
Nature of lenses
Condition of specimen
Image formation
Light microscopeWhite light Glass
Living or killed stained specimen
View directly through microscope
Confocal microscope
Laser Glass
Killed stained specimens
Image analyzed on digital computer screen
Transmission electron microscope Electrons Magnets
Ultrathin section of killed specimen contained within vacuum
View on fluorescent plate or photographic film
Scanning electron microscope Electrons Magnets
Surface view of killed specimen contained within vacuum, with low vacuum can view living cells
Television picture tube
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Generalized Plant Cell
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Boundaries Between Inside and Outside the Cell
Plasma Membrane
and
Cell Wall
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Plasma Membrane
• Surrounds cell
• Controls transport into and out of cell
• Selectively permeable
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Plasma Membrane
• Composed of approximately half phospholipid and half protein, small amount of sterols– Phospholipid bilayer– Separates aqueous solution inside cell from
aqueous layer outside cell– Prevents water-soluble compounds inside cell
from leaking out– Prevents water-soluble compounds outside
cell from diffusing in
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Plasma Membrane
• Proteins in bilayer• Perform different functions
– Ion pumps• Move ions from lower to higher concentration• Require ATP energy• Proton pump – moves H+ ions from inside to
outside of cell• Ca+2 pump – moves Ca2+ to outside of cell
– Channels – allow substances to diffuse across membrane
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Plasma Membrane
• Plasmodesmata– Connects plasma membranes of adjacent
plant cells– Extends through cell wall– Allows materials to move from cytoplasm of
one cell to cytoplasm of next cell
• Symplast – name for continuous cytoplasm in set of cells
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Plasma Membrane
• Apoplast – – Space outside cell– Next to plasma membrane within fibrils of cell
wall– Area of considerable metabolic activity– Important space in plant but questionable as
to whether it is part of the plant’s cells
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Cell Wall
• Rigid structure made of cellulose microfibrils• Helps prevent cell rupture
– Process of osmosis allows water to enter cell– Inflow of water expands cell– Expansion forces cell membrane against cell wall– Resistance of cell wall to expansion balances
pressure of osmosis– Stops flow of water into cell– Keeps cell membrane from further expansion
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Cell Wall
• Osmotic forces balanced by pressure exerted by cell wall– Creates turgor pressure– Causes cells to become stiff and
incompressible– Able to support large plant organs– Loss of turgor pressure – plant wilts
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Cell Wall
• Place cell in salt solution– Water leaves cytoplasm– Protoplast (space inside plasma membrane)
shrinks– Plasma membrane pulls away from cell wall– Cell lacks turgor pressure - wilts
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Cell Wall Structure
• Primary cell wall– Cell wall that forms while cell is growing
• Secondary cell wall– Additional cell wall layer deposited between
primary cell wall and plasma membrane – Generally contains cellulose microfibrils and
water-impermeable lignin– Provides strength to wood
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Cell Wall Structure
• Specialized types of cell walls– cutin covering cell wall or suberin imbedded in
cell wall– Waxy substances impermeable to water– Cutinized cell walls
• Found on surfaces of leaves and other organs exposed to air
• Retard evaporation from cells• Barrier to potential pathogens
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Organelles of Protein Synthesis and Transport
Nucleus, Ribosomes, Endoplasmic Reticulum, and Golgi
Apparatus
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Nucleus
• Ovoid or irregular in shape
• Up to 25 µm in diameter
• Easily stained for light or electron microscopy
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Nucleus
• Surrounded by double membrane – nuclear envelope– Protein filaments of lamin line inner surface of
envelope and stabilize it– Inner and outer membranes connect to form
pores
• Nucleoplasm – Portion of nucleus inside nuclear envelope
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Nucleus
• Nucleoli (singular, nucleolus)– Densely staining region within nucleus– Accumulation of RNA-protein complexes
(ribosomes)– Site where ribosomes are synthesized– Center of nucleoli
• DNA templates• Guide synthesis of ribosomal RNA
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Nucleus
• Chromosomes– Found in nucleoplasm– Contain DNA and protein– Each chromosome composed of long
molecule of DNA wound around histone proteins forming a chain of nucleosome
– Additional proteins form scaffolds to hold nucleosomes in place
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Nucleus
• DNA in chromosomes– Stores genetic information in nucleotide sequences– Information used to direct protein synthesis
• Steps in protein synthesis– Transcription – DNA directs synthesis of RNA– Most RNA stays in nucleus or is quickly broken down– Small amount of RNA (mRNA) carries information
from nucleus to cytoplasm
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Nuclear Components
Component Structure and Function
Nuclear envelopeDouble layered membrane, filaments of protein lamin line inner surface and stabilize structure, inner and outer membranes connect to form pores
Nucleoplasm Portion inside the nuclear envelope
NucleoliDark staining bodies within nucleus, site for ribosome synthesis
Chromosomes
Store genetic information in nucleotide sequences, each chromosome consists of chain of nucleosomes (long DNA molecule and associated histone proteins)
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Ribosomes
• Small dense bodies formed from ribosomal RNA (rRNA) and proteins
• Function in protein synthesis
• Active ribosomes in clusters called polyribosomes– Attached to same mRNA– All ribosomes in one polyribosome make
same type of protein
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Ribosomes
• In living cell, ribosomes are not fixed– Move rapidly along mRNA– Read base sequence– Add amino acids to growing protein chain– At end of mRNA, ribosome falls off, releasing
completed protein into cytoplasm
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Endoplasmic Reticulum
• ER• Branched, tubular structure• Often found near edge of cell• Function
– Site where proteins are synthesized and packaged for transport to other locations in the cell
– Proteins injected through membrane into lumen
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Endoplasmic Reticulum
• Packaging of proteins by ER– Considered to be packaged when separated
from cytoplasm by membrane– Sphere (vesicle) of membrane-containing
proteins may bud off from ER– Vesicle carries proteins to other locations in
cell
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Endoplasmic Reticulum
• Types of ER– rough ER – ribosomes attached to surface– smooth ER – does not have attached
ribosomes
• Carbohydrate transport– Often attached to proteins in ER– Helps protect carbohydrate from breakdown
by destructive enzymes
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Golgi Apparatus
• Also called a dictyosome
• Consists of stack of membranous, flattened bladders called cisternae
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Golgi Apparatus
• Directs movements of proteins and other substances from ER to other parts of cell– Cell wall components (proteins, hemicellulose, pectin)
pass through cisternae– Move to plasma membrane inside membranous
sphere– Sphere joins with plasma membrane– Membrane of sphere becomes part of plasma
membrane– Protein, hemicellulose, and pectin contents released
to outside the cell
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Endomembrane System
• Complex network that transports materials between Golgi apparatus, the ER, and other organelles of the cell
• Movement– Rapid – Continuous
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Organelles of Energy Metabolism
Plastids
and
Mitochondria
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Plastids
• Found in every living plant cell– 20-50/cell– 2-10 µm in diameter
• Surrounded by double membrane
• Contain DNA and ribosomes– Protein-synthesizing system similar to but not
identical to one in nucleus and cytoplasm
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Plastids
• Proplastids– Small plastids always found in dividing plant
cells– Have short internal membranes and
crystalline associations of membranous materials called prolamellar bodies
– As cell matures, plastids develop• Prolamellar bodies reorganized• Combined with new lipids and proteins to form
more extensive internal membranes
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Plastids
• Types of plastids– Chloroplasts– Leukoplasts– Amyloplasts– Chromoplasts
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Plastids
• Chloroplasts– Thylakoids
• Inner membranes• Have proteins that bind to chlorophyll
– Chlorophyll• Green compound that gives green plant tissue its
color
– Stroma• Thick solution of enzymes surrounding thylakoids
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Plastids
• Chloroplasts– Function
• Convert light energy into chemical energy (photosynthesis)
• Accomplished by proteins in thylakoids and stromal enzymes
• Can store products of photosynthesis (carbohydrates) in form of starch grains
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Chloroplast
Component Description
ThylakoidsInner membranes of chloroplast, contain proteins that bind with chlorophyll
StromaThick enzyme solution surrounding thylakoids
ChlorophyllGreen pigment that gives plant tissue its green color
Starch grainsStorage form of carbohydrates produced during photosynthesis
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Leukoplasts
• leuko – “white”
• Found in roots and some nongreen tissues in stems
• No thylakoids
• Store carbohydrates in form of starch
• Microscopically appear as white, refractile, shiny particles
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Amyloplasts
• amylo – “starch”
• Leukoplast that contains large starch granules
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Chromoplasts
• chromo – “color”
• Found in some colored plant tissues– tomato fruits, carrot roots– High concentrations of specialized lipids –
carotenes and xanthophylls– Give plant tissues orange-to-red color
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PlastidsPrefix Meaning Function
Chloroplast “chloro –”“yellow-green”
Photosynthesis, convert light energy into chemical energy, store carbohydrates as starch grains
Leukoplast “leuko –” “white”Store carbohydrates in form of starch
Amyloplast “amylo –” “starch”Leukoplasts that contain large granules of starch
Chromoplast “chromo –” “color”
Stores carotenes and xanthophylls, give orange-to-red color to certain plant tissues
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Mitochondria
• Double-membrane structure
• Contain DNA and ribosomes
• Inner membrane infolded– Folds called cristae– Increase surface area available for chemical
reactions
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Mitochondria
• Matrix – Viscous solution of enzymes within cristae
• Function– source of most ATP in any cell that is not
actively photosynthesizing– Site of oxidative respiration– Release of ATP from organic molecules– ATP used to power chemical reactions in cell
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Other Cellular Structures
Vacuoles, Vesicles, Peroxisomes, Glyoxysomes, Lysosomes, and
Cytoskeleton
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Vacuoles
• Large compartment surrounded by single membrane
• Takes up large portion of cell volume
• Tonoplast – Membrane surrounding vacuole– Has embedded protein pumps and channels
that control flow of ions and molecules into and out of vacuole
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Vacuole
• Functions– May accumulate ions which increase turgor
pressure inside cell– Can store nutrients such as sucrose– Can store other nutritious chemicals– May accumulate compounds that are toxic to
herbivores– May serve as a dump for wastes that cell
cannot keep and cannot excrete
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Vesicles
• Small, round bodies surrounded by single membrane– Peroxisomes and glyoxysomes
• Compartments for enzymatic reactions that need to be separated from cytoplasm
– Lysosomes• Contain enzymes that break down proteins,
carbohydrates, and nucleic acids• May function in removing wastes within living cell• Can release enzymes that dissolve the entire cell
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Cytoskeleton
• Collection of long, filamentous structures within cytoplasm
• Functions– Keeps organelles in specific places– Sometimes directs movement of organelles
around the cell • Cyclosis – cytoplasmic streaming
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Cytoskeleton
• Structures in cytoskeleton– Microtubules– Motor proteins– Microfilaments
• Specialized proteins connect microtubules and microfilaments to other organelles– Connections thought to coordinate many cell
processes
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Microtubules
• Relatively thick (0.024 µm in diameter)
• Assembled from protein subunits called tubulin
• Fairly rigid but can lengthen or shorten by adding or removing tubulin molecules
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Microtubules
• Functions– Guide movement of organelles around
cytoplasm– Key organelles in cell division– Form basis of cilia and flagella
• Cilia and flagella never found in flowering plants• Important to some algae and to male gametes of
lower plants
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Microfilaments
• Thinner (0.007 µm in diameter) and more flexible than microtubules
• Made of protein subunits called actin
• Often found in bundles
• Function– Serve as guides for movement of organelles
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Motor Proteins
• Powered by ATP molecules
• Microtubule motor proteins– Kinesins, dyneins– Move along microtubule making and breaking
connections between tubulin subunits
• Microfilament motor proteins– myosin
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Cytoskeleton
SubunitsMotor proteins
Function
MicrotubulesTubulin (protein)
Kinesins, dyneins
Key organelles in cell division, form basis of cilia and flagella, serve as guides for movement of organelles within cell
MicrofilamentsActin (protein)
Myosin
Serve as guides for movement of organelles within cell
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Cell Cycle
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Cell Cycle
• Interval of time between formation of a cell and its division to form two new cells
• Process occurs in special regions called meristems
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Cell Cycle
• Four phases of cell cycle– G1 (“gap”)– S – (“synthesis” of DNA)– G2 – M – (“mitosis”)
• Karyokinesis – nuclear division• Cytokinesis – cytoplasmic division
* G1, S, and G2 phases of cycle typically grouped together and called interphase.
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Cell Cycle
• Time needed to progress through phases of cell cycle depends on amount of DNA per nucleus– Could range from a few hours to a few days– Plants with more DNA in nuclei have longer
cell cycle times than plants with less DNA
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Cell Cycle
• Must synthesize enzymes and other proteins needed for specific cell cycle events– DNA polymerase
• Essential to synthesis of new DNA• DNA polymerase and histones increase at end of
G1 phase and during S phase
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Metabolic Events of Cell Cycle
G1• Cell prepares itself
metabolically for DNA synthesis– Accumulation and
synthesis of specific enzymes to control DNA synthesis and production of DNA subunits
S phase• Cell duplicates its
DNA molecules
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Regulation of Cell Cycle
Principal Control Point Hypothesis• Developed by Jack Van’t Hof• Proposes control points exist at G1 and
G2 phases• G1
– If cell forms critical macromolecules during G1 phase progresses through S phase
– If cell does not form critical macromolecules during G1 phase cell is arrested in G1
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Regulation of Cell Cycle
• G2– If cell forms critical macromolecules during G2
phase progresses through M phase– If cell does not form critical macromolecules
during G2 phase cell is arrested in G2
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Microtubules and Cell Division
G1 and S
•Microtubules located around periphery of cell next to plasma membrane and cell wall
•Involved in deposition of cellulose in cell wall
G2
•Microtubules form band around nucleus pressed close to cell wall (called preprophase band PPB)
•Precedes mitosis by hours
•Orientation of **PPB marks position of new cell wall
•New cell wall defines plane of division
**Chemicals such as hormone gradients influence position of PPB
•Regulate start of events such as mitosis
•Induce movement of microtubules to PPB
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Mitosis
• Purpose – separate doubled DNA in nucleus of cell
• Four phases– Prophase– Metaphase– Anaphase– Telophase
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Prophase
• DNA condenses• Nucleolus disappears• Late prophase
– Two intertwined DNA molecules of each chromosome are connected at region called centromere
– Nuclear membrane breaks down• Nucleoplasm does not mix with general cytoplasm
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Metaphase
• Chromosomes arranged on equatorial plane of cell
• Centromeres usually aligned• Spindle fibers can now be seen
– Bundles of microtubules– Extend from poles near ends of cell to kinetochore
(attachment point on chromosome)– Collectively called spindle apparatus
• Each half chromosome now called a chromatid
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Anaphase
• Chromatids separate– Move toward opposite poles of cell
• Mechanisms of chromosome movement– Assembly-disassembly mechanism
• Microtubules move chromosomes by losing tubulin molecules from one end of spindle
– Motor protein mechanism• Chromosomes slide over microtubules of spindle• Sliding force provided by motor proteins at
kinetochore
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Telophase
• Begins when chromosomes reach opposite poles of cell
• Chromosomes aggregate and begin to uncoil
• Nuclear envelope and nucleolus reform
• Cells return to G1 of cell cycle
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Mitosis
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Cytokinesis
• Begins before telophase is complete– New cell wall called cell plate forms between
separated nuclei– Process involves phragmoplast
• Band of microtubules • Reforms perpendicular to cell plate• Small membrane-bound vesicles containing cell
wall material
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Cytokinesis
• Cell wall material deposited in center of cell first and outward until new wall attaches to side walls
• Point of attachment of wall previously “marked” by PPB