The Plant Cell and the Cell Cycle

Chapter 3

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

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

Basic Similarities of Cells

• Cells possess basic characteristic of life– Movement– Metabolism– Ability to reproduce

• Organelles– “little organs”– Carry out specialized functions within cells

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

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

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

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

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

Generalized Plant Cell

Boundaries Between Inside and Outside the Cell

Plasma Membrane

and

Cell Wall

Plasma Membrane

• Surrounds cell

• Controls transport into and out of cell

• Selectively permeable

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

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

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

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

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

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

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

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

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

Organelles of Protein Synthesis and Transport

Nucleus, Ribosomes, Endoplasmic Reticulum, and Golgi

Apparatus

Nucleus

• Ovoid or irregular in shape

• Up to 25 µm in diameter

• Easily stained for light or electron microscopy

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

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

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

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

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)

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

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

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

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

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

Golgi Apparatus

• Also called a dictyosome

• Consists of stack of membranous, flattened bladders called cisternae

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

Endomembrane System

• Complex network that transports materials between Golgi apparatus, the ER, and other organelles of the cell

• Movement– Rapid – Continuous

Organelles of Energy Metabolism

Plastids

and

Mitochondria

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

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

Plastids

• Types of plastids– Chloroplasts– Leukoplasts– Amyloplasts– Chromoplasts

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

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

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

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

Amyloplasts

• amylo – “starch”

• Leukoplast that contains large starch granules

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

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

Mitochondria

• Double-membrane structure

• Contain DNA and ribosomes

• Inner membrane infolded– Folds called cristae– Increase surface area available for chemical

reactions

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

Other Cellular Structures

Vacuoles, Vesicles, Peroxisomes, Glyoxysomes, Lysosomes, and

Cytoskeleton

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

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

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

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

Cytoskeleton

• Structures in cytoskeleton– Microtubules– Motor proteins– Microfilaments

• Specialized proteins connect microtubules and microfilaments to other organelles– Connections thought to coordinate many cell

processes

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

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

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

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

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

Cell Cycle

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

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.

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

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

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

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

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

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

Mitosis

• Purpose – separate doubled DNA in nucleus of cell

• Four phases– Prophase– Metaphase– Anaphase– Telophase

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

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

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

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

Mitosis

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

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