Cell Biology--Section III

Dr. Carol Carter

Office Hour: Wednesdays 2-3

Room 314, Sci. Bldg. 1


“Cell Ecology”
Interactions between cells and their environment.


    Extracellular space

    Interactions between cells

    Sealing the extracellular space

    Gap junctions and plasmodesmata

    Cell walls

Tissue Organization
in animals





All are associated with ECM, but connective has high ratio of ECM to cells


Extracellular Matrix (ECM)

    Defined: the organized material beyond the plasma membrane.

    Organization by physical, enzymatic factors:

         self assembly--free energy changes

         binding specificity

         pH, redox

         enzyme activity


         force fields (mechanical, electrical)


(The important role of
 carbohydrates in cell-cell

 “The information-carrying potential of oligosaccharides is far greater than that of proteins and nucleic acids of equivalent molecular weight
, and their presence on cell surfaces and on many proteins suggests an importance which was previously unrecognised. Oligosaccharides are now thought to mediate cell-cell recognition, including the infection of cells by bacteria and viruses, moderate the behaviour of enzymes and other proteins, and play a variety of rôles in the immune response.”

(Fig. of amphiphilic hexose not in text)

An overview of the macromolecular organization of ECM and the plasma membrane in some animals
(fig 7.5)

Some components of the ECM of animals: collagen, fibronection, proteoglycan.  (Fig 7.5)

Fig. 7.5.
Fig. Of fibronectin gene not in text)

     Evolutionarily conserved protein

     Has functional domains; interacts differentially

     Has arg-gly-asp (RGD) sites

     Tissue variable forms from differential RNA splicing

Fig 7.6

     Fibrous protein, abundant.

     Gene family; allows variations per tissue

     Secreted as procollagen, enzyme cleaves, then self-assembly into triple helix

     Fibroblasts may organize

     Tethered by fibronectin, integrin, actin

     Examples: tendons (fibrillar), cornea (crossed), basement membrane (random)





Fig. 7.9

     Protein core with glucosamines (GAG) side chains.

     GAGs neg. charged

     GAGs bind cations

     Cations bind water

     H  O  H    = gel

     If water can’t flow it resists compression.

     Therefore function of proteoglycans is…..?



Cell-Cell Interactions
Fig 7.19

     Depends on distinguishing self, nonself

     Mediated by recognition factors

     selectins, Ig, integrins, cadherins (all glycoproteins)


Integrins, Fig. 7.5

     Integral gycoprotein- spans membrane. (cytoplasmic and extracellular head)

     Subunits: ", $-- not covalently bound

     Gene family for each ", $ unit--specificity, 22 known

     Inside/outside communication

     Requires Ca

     Binds to arginine/glycine/ aspartate residues (RGD)

     Conserved among animals.



Selectins, Fig. 7.20

     Transmembrane molecule


     Lectins (binds to specific carbohydrate)

     Requires Ca ion

     Mediates transient interactions with blood



Immunoglobulin Superfamily (IgSF
 Fig. 7.21


     Members of SF share similar domains (aa sequence, conformation)

     Most are antibodies (immune response)

     Others have generalized cell adhesion or neural development functions

     Can link with integrins




The immunoglobulin superfamily (IgSF) is the largest known group of related proteins. In a single species, the IgSF includes hundreds of members believed to have arisen from a single primordial Ig-like domain during evolution of multicellular metazoans (Williams and Barklay, 1988; Doolittle, 1995). (Fig. 7.21)

Fig. 7.22

     Family of glycoproteins

     Mediate Ca++ dependent cell adhesion

     Transmit signals from ECM to cytoplasm

     Self recognition (same type of cell)

Cadherins, con’t
Fig 7.27a

     In humans, 40 different kinds known

     Most variety found in brain

     Holds epithelial-derived tissues together (e.g., kidney, skin)

     Ca++ integrated between repeats--holds rigid

     Zips together with like cadherins from another cell

     Bound to cytoskeleton

Sealing Extracellular Space

    In animals “tight junctions” isolate regions of extracellular space

    In plants, impregnation of matrix with solids isolates regions.

    Reason: to control the pathway of diffusion of solutes

Tight Junctions, Fig. 7.30a


     Membrane proteins shared or conjoined.

     Forms ring around cells

     Solutes are restricted in ECM.

     Diffusion is restricted within plane of membrane.

     Common in epithelial tissues

Gap Junctions, Fig. 7.32

     Allow intercellular communication

     Found in verts and inverts, but different proteins

     “Connexons” are found in verts.

     Allow direct channel between cells

     Ion exchanges, etc.


The ECM of Plant Cells--plant cell walls
Fig. 7.35

     Cellulose microfibrils - give strength, shape

     Hemicellulose - cross links

     Pectins: neg. charged - gel, glue

     Glycoproteins - allow changes, recognition, and ?

     Other polymers: lignin, suberin, wax

Function of Cell Walls



    Translocation of water, ions, nutritive substances, hormones

    Specializations can direct transport --lignin, suberin, wax

Wall Specialization: Casparian Strip
(Fig. not in text)

     An impervious strip (suberin) within cell walls of cells surrounding the root vascular tissue.  Directs solutes from apoplast to symplast.


Plasmodesmata, (Fig. 7.34)

     Symplastic channel across cell walls between cells

     ER tube goes through

     Cytoplasmic continuum

     amino acids, other charged molecules can pass through; some viruses regulate by “movement protein”

Cellulose Synthesis, Fig. 7.37b

     Cellulose synthase rosettes extrude cellulose polymer.

     Cellulose polymer joins with others to form cellulose microfibril.

     Orientation is related to microtubules


Concept Summary

     Carbos give specificity by conformation

     Glycoproteins carry carbos, bind, allow communication

     Cations (especially Ca++) regulate, bridge, form gels,

     ECM organized by physical factors (e.g. collagen)

     ECM supports, protects, allows cell recognition, interaction

     Fibronectin binds to many types of molecules- specificity by RNA splicing

     Cell-cell interactions mediated by glycoproteins, e.g. integrins, selectins, IgSF, cadherins-- specificity by gene families, gycomoieties

     Tight junctions, Casparian strips-- partition solutes

     Plasmodesmata/ Gap junctions allow cytoplasm continuum

     Integral proteins (cellulose synthase, connexons, integrins) allow inside/out communications & connections