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Cell Architecture
Classification of organisms
“Five kingdom system:”
Divide all organisms into groups (kingdoms)-different groups have different combinations of cell
parts (organelles)
! Procaryota
! Animalia
! Plantae
! Fungi
! Protista
Classification by molecular biology
Modern systems divide organisms into groups-(domains, kingdoms)—on the basis of DNA
sequences
Prokaryotes
! Bacteria, Archaea
! Simplest structure
(Protobionts: a step
toward living cells?)
Plasma Membrane
Thin sheet 7-9 nm (0.007-0.009 !m) thick
Composition: lipid (fat) and protein
Found in all cells
Functions:
! serves as platform for biochemical reactions
! recognizes outside stimuli
! controls transport of molecules (and larger
things) into and out of cell.
Cell Wall
Thick net (variable width) surrounding the plasma
membrane; rigid
Found in most prokaryotes (bacteria, archaea); also
plants, fungi, some protists
Composition: peptidoglycan, cellulose, chitin, etc.
(long molecules, porous net)
Functions
! limits the size of cell
! controls cell growth
Nucleoid
Nucleic acid (long, stringy molecule) in center of cell
Function: contains information (DNA)
! cell functions (metabolism)
! growth of cell
! reproduction
Ribosomes
Small, solid spheroids (0.02 !m diameter)
Nucleic acid (RNA) and protein, two subunits each
Ave. 30,000 per cell
Function
! synthesis of proteins
Eukaryotes
! Plants, Animals, Fungi, Protists
! Complex cell structures
! Multicellular organisms, with specialized cells
Organelles in prokaryotes that are the
"same" in eukaryotes
Plasma membrane
Cell wall (in plants, fungi, some protists)
Ribosomes
Nucleus (nuclei)
Spheroid, 10-20 !m diameter
Present in all eukaryotes: usually
one per cell (more? coenocytic or
syncitial)
Surrounded by nuclear envelope
Nucleolus: makes ribosomes
Chromatin:
! Nucleosomes
! Chromosomes
! Centromeres
! Telomeres
Function: like nucleoid of prokaryotes
Mitochondrion (-dria)
Spheroid ca. 1 !m diameter
Surrounded by a double membrane
Crista (-ae): inner membrane folds
Matrix: liquid gel center
Function: directs energy transformation,
metabolism
Found in all eukaryotic cells: 1 to 10,000 (ave.
100) per cell
Mitochondria in cornial cells are long and tubular
a and d and green in c and f is autofluorescence
b and e and red in c and f is a fluorescent dye absorbed by mitochondria
Plastid
Spheroid 5 !m diameter
Found in plants and some protists
! Double membrane
! Thylakoids
! Stroma
Functions depend on type:
! Chloroplast (green)
! Leukoplast (white)
! Chromoplast (colored)
! Proplastids
Organelles in prokaryotes and eukaryotes
! Plasma membrane
! Cell wall (in prokaryotes, plants, fungi, some
protists)
! Ribosomes
! Nucleoid/nucleus
! Mitochondrion
! Plastids
Endoplasmic reticulum (E.R.) (“network
between organelles”)
Interconnected flattened or tubular sacs,
cisterna(e)
Functions: synthesis and packaging
! Rough: proteins
! Smooth, cell wall carbohydrates, glycogen
(starch), glycoproteins, steroid hormones.
In all eukaryotic cells
Golgi apparatus (dictyosome, in plants)
Dense complex of cisternae, several per cell
Functions:
! synthesis of carbohydrates on proteins
(glycoproteins),
! transport of proteins to different compartments
in cell
Found in all eukarotic cells
Vesicles
Small sacs
Single membrane
Functions:
! storage, isolation
! transport
Vacuoles
Larger sacs
Single membrane
In protists, storage organs, digestive organs,
related to water pumping (contractile vacuole)
In plants and fungi, large storage organs
salts, sugars, red and blue pigments in flowers,
wastes, strange chemicals (poisons )
Cytoskeleton
Microtubules
Stiff helical rods of protein (tubulin); 25 nm diam
Functions:
! Structural support (e.g. mammalian nerve cell).
! Support for movement (e.g., vesicles, cilia,
flagella, spindles).
Motor proteins:
! Kinesin toward + end
! Dynein toward - end
Microfilaments
Flexible rods of protein (actin); 7 nm thick
Functions:
! Structural support
! Support for movement (e.g., muscles,
streaming).
Motor protein:
! Myosin
Intermediate filaments
Stiff protein rods, 8-12 nm thick
Function: hold organelles in place
Figure 4. IF-associated proteins provide flexible intracytoplasmic resilience in response to external stresses by reversibly cross-linking IFs
to other cytoskeletal and membrane sites. (A through C) BPAG1n/dystonin, a 280-kD linker protein with actin- and neurofilament-binding
domains, aligns neurofilaments along actin stress fibers when a cultured mammalian cell is triply transfected to express BPAG1n, NF-L,
and NF-H. (A) Neurofilament; (B) BPAG1n/dystonin; (C) actin. [Reproduced with permission from Yang et al. (50).] (D) Electron
micrograph of the residual cytoskeleton of a rat embryo fibroblast after dissolution of actin filaments with gelsolin. Linked to 10-nm gold
particles, antibodies against the 500-kD cross-linker protein plectin reveal bridges between microtubules and IFs. Pseudocolored elements
are microtubules (orange), IFs assembled from vimentin (green), plectin (red), and gold particles marking plectin (yellow). (E) Model
suggesting that plectin mediates linking between IFs, microtubules, actin, myosin, and membrane-bound adhesion sites (focal contacts or
plasma membrane components) (51, 53). (Images kindly provided by T.ﾊSvitkina and G.ﾊBorisy.)
Dynamic relationships between membranous
organelles
! E.R. formed by outbudding of nuclear envelope
! E.R. produces vesicles: substances produced
in E.R. (e.g. proteins, tagged with
carbohydrates; glycoproteins) appear in vesicles
! Golgi accepts materials from E.R.
! Golgi vesicles modify materials (e.g. altered
glycogroups)
! Golgi vesicles can (a) stay as storage vesicles
(e.g. lysosomes), (b) fuse with plasma
membrane to secrete their substance, (c) fuse
with another membrane (e.g. vacuolar
membrane) to transfer material into an
organelle.
! Secretion vesicles can be “constitutive” or
“regulated”—Golgi targets different materials
into each.
Cells in various organisms
Plants: have all the organelles described for
eukaryotes (except some specialized cells)
Fungi: like plant cells in number and type of
organelles, but without plastids (also many other
biochemical, structural, and behavioral differences)
Animals: no cell wall, no plastids, no large vacuole
Protists: +/- cell wall, pellicle, contractile vacuole
Bacteria, archaea: +/- cell wall, no nucleus,
mitochondria, plastids, E.R., Golgi; but
cyanobacteria have thylakoids (like inside of
plastids)