Chapter 04

Lecture Outline

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Cell StructureChapter 4

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Cells

• Cells were discovered in 1665 by Robert Hooke

• Early studies of cells were conducted by– Mathias Schleiden (1838)– Theodor Schwann (1839)

• Schleiden and Schwann proposed the Cell Theory

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Cell Theory

1. All organisms are composed of cells2. Cells are the smallest living things3. Cells arise only from pre-existing cells• All cells today represent a continuous line

of descent from the first living cells

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Cell size is limited

• Most cells are relatively small due reliance on diffusion of substances in and out of cells

• Rate of diffusion affected by– Surface area available– Temperature– Concentration gradient– Distance

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4–3 πr3)

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Cell radius (r)

Surface Area / Volume

Surace area (4πr2)

Volume ( 4.189 unit3

12.57 unit2

1 unit 10 unit

1257 unit2

4189 unit3

0.3

Surface area-to-volume ratio

• Organism made of many small cells has an advantage over an organism composed of fewer, larger cells

• As a cell’s size increases, its volume increases much more rapidly than its surface area

• Some cells overcome limitation by being long and skinny – like neurons

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Microscopes

• Not many cells are visible to the naked eye– Most are less than 50 μm in diameter

• Resolution – minimum distance two points can be apart and still be distinguished as two separate points– Objects must be 100 μm apart for naked eye

to resolve them as two objects rather than one

2 types

• Light microscopes– Use magnifying lenses with visible light– Resolve structures that are 200 nm apart– Limit to resolution using light

• Electron microscopes– Use beam of electrons– Resolve structures that are 0.2 nm apart

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• Electron microscopes– Transmission electron

microscopes transmit electrons through the material

– Scanning electron microscopes beam electrons onto the specimen surface

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Hum

an E

ye

Ligh

t Mic

rosc

ope

Elec

tron

Mic

rosc

ope

Hydrogen atom

Amino acid

Logarithmic scale

Protein

Ribosome

Large virus (HIV)

Human red blood cell

Prokaryote

Human egg

Paramecium

Chicken egg

Adult human

Frog egg

ChloroplastMitochondrion

1 m

10 cm

1 cm

1 mm

1 nm

0.1 nm (1 Å)

10 nm

100 nm

1 m

10 m

100 m

10 m

100 m

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Basic structural similarities

1. Nucleoid or nucleus where DNA is located2. Cytoplasm

– Semifluid matrix of organelles and cytosol

3. Ribosomes– Synthesize proteins

4. Plasma membrane– Phospholipid bilayer

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Prokaryotic Cells

• Simplest organisms• Lack a membrane-bound nucleus

– DNA is present in the nucleoid• Cell wall outside of plasma membrane• Do contain ribosomes (not membrane-

bound organelles)• Two domains of prokaryotes

– Archaea– Bacteria

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Cytoplasm

Ribosomes

Nucleoid (DNA)

Plasma membrane

CapsuleCell wall

Pili

Flagellum

Pilus

0.3 µm

© Phototake

Bacterial cell walls

• Most bacterial cells are encased by a strong cell wall– composed of peptidoglycan– Cell walls of plants, fungi, and most protists different

• Protect the cell, maintain its shape, and prevent excessive uptake or loss of water

• Susceptibility of bacteria to antibiotics often depends on the structure of their cell walls

• Archaea lack peptidoglycan

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Flagella

• Present in some prokaryotic cells– May be one or more or none

• Used for locomotion• Rotary motion propels the cell

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Outer protein ring

Hook

Filament

Inner protein ringH+ H+

Peptidoglycanportion ofcell wall

Outermembrane

Plasmamembrane

a. b. c.

0.5 µm

a: © Eye of Science/Photo Researchers, Inc.

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Eukaryotic Cells

• Possess a membrane-bound nucleus• More complex than prokaryotic cells• Hallmark is compartmentalization

– Achieved through use of membrane-bound organelles and endomembrane system

• Possess a cytoskeleton for support and to maintain cellular structure

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Nucleus

Nucleolus

Nuclear pore

Intermediate filament

Ribosomes

Ribosomes

Cytoplasm

Cytoskeleton

Microtubule

Centriole

Plasma membrane

Mitochondrion

Golgi apparatus

Exocytosis

Peroxisome

Smooth endoplasmic reticulum

Rough endoplasmic reticulum

Microvilli

Nuclear envelope

Actin filament(microfilament)

Intermediatefilament

Lysosome

Vesicle

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Nucleus

Nucleolus

Nuclear pore

Intermediate filament

Ribosome

Cytoplasm

Cytoskeleton

Microtubule

Plasma membrane

Mitochondrion

Peroxisome

Smooth endoplasmic reticulum

Rough endoplasmic reticulum

Nuclear envelope

Central vacuole

Plasmodesmata

Adjacent cell wall

Cell wall

Chloroplast

Golgiapparatus

Vesicle

Actin filament(microfilament)

Intermediatefilament

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Nucleus

• Repository of the genetic information• Most eukaryotic cells possess a single nucleus• Nucleolus – region where ribosomal RNA

synthesis takes place• Nuclear envelope

– 2 phospholipid bilayers– Nuclear pores – control passage in and out

• In eukaryotes, the DNA is divided into multiple linear chromosomes– Chromatin is chromosomes plus protein

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Nuclearbasket

Nuclear pores

Nuclearenvelope

Nucleolus

Chromatin

Nucleoplasm

Nuclearlamina

Innermembrane

Outermembrane

Cytoplasmicfilaments

Nuclear porea.

Ribosomes

• Cell’s protein synthesis machinery• Found in all cell types in all 3 domains• Ribosomal RNA (rRNA)-protein complex• Protein synthesis also requires messenger

RNA (mRNA) and transfer RNA (tRNA)• Ribosomes may be free in cytoplasm or

associated with internal membranes

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Endomembrane System

• Series of membranes throughout the cytoplasm

• Divides cell into compartments where different cellular functions occur

• One of the fundamental distinctions between eukaryotes and prokaryotes

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Endoplasmic reticulum

• Rough endoplasmic reticulum (RER)– Attachment of ribosomes to the membrane gives a

rough appearance– Synthesis of proteins to be secreted, sent to

lysosomes or plasma membrane• Smooth endoplasmic reticulum (SER)

– Relatively few bound ribosomes– Variety of functions – synthesis, store Ca2+,

detoxification• Ratio of RER to SER depends on cell’s function

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Ribosomes

Smoothendoplasmic

reticulum

Smoothendoplasmicreticulum

0.08 µm(inset): © Dr. Donald Fawcett & R. Bolender/Visuals Unlimited

Roughendoplasmicreticulum

Roughendoplasmicreticulum

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Golgi apparatus

• Flattened stacks of interconnected membranes (Golgi bodies)

• Functions in packaging and distribution of molecules synthesized at one location and used at another within the cell or even outside of it

• Has cis and trans faces• Vesicles transport molecules to destination

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1 µm

Secretoryvesicle

Formingvesicle

trans face

cis face

Fusingvesicle

Transport vesicle

(inset): © Dennis Kunkel/Phototake

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1.

2.

3.

Golgi membrane protein

Cisternae

Secretory vesicle

Cell membrane

Extracellular fluid

Roughendoplasmicreticulum

Membraneprotein

Newlysynthesizedprotein

Vesicle containingproteins buds fromthe rough endo-plasmic reticulum,diffuses through thecell, and fuses tothe cis face of theGolgi apparatus. Smooth

endoplasmicreticulumcis face

GolgiApparatus

trans faceThe proteins aremodified andpackaged intovesicles fortransport. Secreted

protein

The vesicle maytravel to the plasmamembrane,releasing itscontents to theextracellularenvironment.

Transportvesicle

Nucleus

Nuclear poreRibosome

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Lysosomes

• Membrane-bounded digestive vesicles• Arise from Golgi apparatus• Enzymes catalyze breakdown of

macromolecules• Destroy cells or foreign matter that the cell

has engulfed by phagocytosis

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Lysosome aiding in thebreakdown of an old organelle

Lysosome aiding in the digestion of phagocytized particles

Golgi membrane protein

Cisternae

Roughendoplasmicreticulum

Smoothendoplasmic

reticulum

cis face

GolgiApparatustrans face

Nucleus

Ribosome

Membrane protein

Hydrolytic enzyme

Transport vesicle

Breakdownof organelle

Lysosome

Lysosome DigestionOld or damaged

organelle

Food vesicle

Phagocytosis

Nuclear pore

Microbodies

• Variety of enzyme-bearing, membrane-enclosed vesicles

• Peroxisomes– Contain enzymes

involved in the oxidation of fatty acids

– Hydrogen peroxide produced as by-product – rendered harmless by catalase

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0.2 µm(inset): From S.E. Frederick and E.H. Newcomb, “Microbody-like organelles in leaf cells,” Science,

163:1353-5. © 21 March 1969. Reprinted with permission from AAAS

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Vacuoles

• Membrane-bounded structures in plants• Various functions depending on the cell

type

• There are different types of vacuoles:– Central vacuole in plant cells– Contractile vacuole of some fungi and protists– Storage vacuoles

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(inset): © Henry Aldrich/Visuals Unlimited

Nucleus

ChloroplastTonoplast

Centralvacuole

1.5 µm

Cellwall

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Mitochondria

• Found in all types of eukaryotic cells• Bound by membranes

– Outer membrane– Intermembrane space– Inner membrane has cristae– Matrix

• On the surface of the inner membrane, and also embedded within it, are proteins that carry out oxidative metabolism

• Have their own DNA

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IntermembranespaceInner membrane

Outer membrane

RibosomeMatrix

DNACrista

0.2 µm(inset): © Dr. Donald Fawcett & Dr. Porter/Visuals Unlimited

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Chloroplasts

• Organelles present in cells of plants and some other eukaryotes

• Contain chlorophyll for photosynthesis• Surrounded by 2 membranes• Thylakoids are membranous sacs within

the inner membrane– Grana are stacks of thylakoids

• Have their own DNA

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Ribosome DNA

Stroma

Stroma

ThylakoidmembraneOutermembraneInnermembrane

Thylakoid diskGranum

Granum

1.5 µm(inset): © Dr. Jeremy Burgess/Photo Researchers, Inc.

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Endosymbiosis

• Proposes that some of today’s eukaryotic organelles evolved by a symbiosis arising between two cells that were each free-living

• One cell, a prokaryote, was engulfed by and became part of another cell, which was the precursor of modern eukaryotes

• Mitochondria and chloroplasts

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Chloroplast

Chloroplast

Modern Eukaryote

Unknown Bacterium

Unknown ArchaeonMitochondrion

Protobacterium

Modern EukaryoteCyanobacterium

Cyanobacterium

Unknown Archaeon

ProtobacteriumMitochondrion

Nucleus

Nucleus

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Cytoskeleton

• Network of protein fibers found in all eukaryotic cells– Supports the shape of the cell – Keeps organelles in fixed locations

• Dynamic system – constantly forming and disassembling

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3 types of fibers

• Microfilaments (actin filaments)– Two protein chains loosely twined together– Movements like contraction, crawling, “pinching”

• Microtubules– Largest of the cytoskeletal elements– Dimers of α- and β-tubulin subunits– Facilitate movement of cell and materials within cell

• Intermediate filaments– Between the size of actin filaments and microtubules– Very stable – usually not broken down

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Microtubule

Intermediate filament

Actin filament

Cell membrane

a. Actin filaments

b. Microtubules

c. Intermediate filament

Centrosomes

• Region surrounding centrioles in almost all animal cells

• Microtubule-organizing center– Can nucleate the assembly of microtubules

• Animal cells and most protists have centrioles – pair of organelles

• Plants and fungi usually lack centrioles

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Centrioles

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Microtubule triplet

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Cell Movement

• Essentially all cell motion is tied to the movement of actin filaments, microtubules, or both

• Some cells crawl using actin microfilaments

• Flagella and cilia have 9 + 2 arrangement of microtubules– Not like prokaryotic flagella– Cilia are shorter and more numerous

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Flagellum

Basal body

Microtubuletriplet

Centralmicrotubule pair

Plasmamembrane

Radial spokeDynein arm

Doublet microtubule

0.1 µm

0.1 µm(top & bottom insets): © William Dentler, University of Kansas

• Eukaryotic cell walls– Plants, fungi, and

many protists– Different from

prokaryote– Plants and protists

– cellulose– Fungi – chitin– Plants – primary

and secondary cell walls

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Plant cell

Plasmodesmata Primary wallSecondary wall

Cell 1

Cell 2

Primary wall

Secondary wall

Plasma membrane

Middle lamella

Middlelamella

Plasmamembrane

© Biophoto Associates/Photo Researchers, Inc.0.4 µm

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Extracellular matrix (ECM)

• Animal cells lack cell walls• Secrete an elaborate mixture of

glycoproteins into the space around them• Collagen may be abundant• Form a protective layer over the cell

surface• Integrins link ECM to cell’s cytoskeleton

– Influence cell behavior

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Cytoplasm

Actin filament

Integrin

Fibronectin

Collagen Elastin

Proteoglycan

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Cell-to-cell interactions

• Surface proteins give cells identity– Cells make contact, “read” each other, and

react– Glycolipids – most tissue-specific cell surface

markers– MHC proteins – recognition of “self” and

“nonself” cells by the immune system

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Cell connections

• 3 categories based on function1.Tight junction

– Connect the plasma membranes of adjacent cells in a sheet – no leakage

2.Anchoring junction– Mechanically attaches cytoskeletons of neighboring

cells (desmosomes)

3.Communicating junction– Chemical or electrical signal passes directly from one

cell to an adjacent one (gap junction, plasmodesmata)

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a.

Tight junction

Adjacent plasmamembranes

Tight junctionproteins

Intercellularspace

Microvilli

Basal lamina

Adhesivejunction(desmosome)

Tightjunction

Intermediatefilament

Communicatingjunction

2.5 µm

Intercellular space

b.

Adjacent plasmamembranesCadherin

Cytoplasmicprotein plaque

Cytoskeletal filamentsanchored to plaque

Anchoring junction (desmosome)

0.1 µm

c.

Intercellular space

Channel (diameter 1.5 nm)

Communicating junction

ConnexonTwo adjacent connexonsforming an open channelbetween cells

Adjacent plasmamembranes

1.4 µm

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a: Courtesy of Daniel Goodenough; b: © Dr. Donald Fawcett/Visuals Unlimited; c: © Dr. Donald Fawcett/D. Albertini/Visuals Unlimited

Plasmodesmata

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•Plant cells• Plasmodesmata

• Specialized openings in their cell walls

• Cytoplasm of adjoining cells are connected

• Function similar to gap junctions in animal cells

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PrimaryCell wall

Middle lamella Plasmamembrane

PlasmodesmaSmoothER

Centraltubule

Cell 2Cell 1