C2006/F2402 '04 -- Outline Of Lecture #3 -- Last update 01/26/2004 05:52 PM

2004 Deborah Mowshowitz, Department of Biological Sciences, Columbia University, New York NY


3A -- Epithelial cells (Examples of Cell-Cell connections) & RBC Membrane;
3B -- Cell-Cell Junctions, Glycoproteins & Proteoglycans, & Types of Transport

I. Membrane Structure, cont.

    A. Fluid mosaic model -- where are the proteins? Is it a "unit membrane?" 

For "unit membrane" See Becker fig. 7-4 ; for fluid mosaic model see Becker fig. 7-5 or Purves 5.1.

1. Use of freeze fracture/freeze etch procedure 

a. E vs P faces of bilayer = surfaces you see if you crack bilayer open = inside of bilayer

(1). E face = inside of the monolayer that is closer to extracellular space 

(2). P face = inside of the monolayer that is closer to protoplasm 

b. What do you see on inside? (see handout 2A, bottom & Becker fig. 7-16 or Purves 5.3). 

(1) Inside is not smooth -- shows proteins go through bilayer (implies "mosaic" model not unit membrane)

(2). More bumps (proteins) on P face than E face -- shows more proteins anchored on cytoplasmic side.

    B. Types of Membrane Proteins

1. Peripheral membrane proteins vs. integral membrane proteins

Type of Membrane Protein

Alt. terminology

Protein Removed From Membrane By

Location/Attachment of Protein



salt, pH changes 

On one 1 side of bilayer; non covalently attached to lipid



disrupting lipid bilayer

Goes through bilayer* or Covalently attached to lipid on one side (Lipid-anchored)**

* A small number of integral proteins do not go all the way through the membrane; they will be largely ignored in this course. For examples see Becker fig. 7-19 (first protein on left) or Purves 5.1 (last protein on right).

**Note that lipid-anchored proteins can be considered a type of integral protein or a separate category. See Becker fig. 7-19.

2. Transmembrane proteins (See Purves 5.5 and/or Becker fig. 7-19 & 7-21)

a. Single pass vs multipass 

b. Domains -- intracellular, extracellular, transmembrane 

c. Location of carbohydrates -- all in extracellular domain

d. Anchorage -- Some proteins are anchored to cytoskeleton; some float in lipid bilayer

e. Functions -- bridge the membrane -- facilitate transport of materials & signals across membrane; physically connect cytoskeleton (inside of cell) to materials on outside of cell.

II.  The Red Blood Cell (RBC) Membrane -- The best studied example of a Membrane. 

    A. Why RBC's

1. Easy to get

 2. No internal membranes  -- all organelles lost during maturation of RBC -- see Becker fig. 7-20 (a). Only membrane = plasma membrane.

 3. Can make ghosts = resealed plasma membranes. Can be resealed in either orientation -- "right" or "wrong" side out.

    B. RBC membrane proteins -- Structure & Function. See Purves fig.4-23 or Becker fig. 7-20 (b) & 22-22. (Handout 3A -- bottom)

1. Peripheral proteins -- spectrin, ankyrin, (band 4.1), actin.  Comprise peripheral cytoskeleton, which supports membrane.

2.  Intrinsic proteins 

a. Examples

    (1). Multipass (band 3/anion exchanger) -- Catalyzes reversible exchange of the anions HCO3- (bicarb) and Cl- . Exchange allows max. transport of CO2 in blood (as bicarb in solution). See Purves 48.17 or Becker 8-3. Details FYI (to be discussed at length later):

(a). In tissues -- Bicarb out/chloride in: Where CO2 is high, in tissues, CO2 enters RBC and is converted to bicarb inside the RBC. Then bicarb leaves RBC in exchange for chloride. Bicarb is much more soluble in plasma than CO2, so lots of bicarb (but not much CO2) can be carried in the blood.  Blood carries bicarb to lungs.

(b). In lungs -- Chloride out/bicarb in: In lungs,  the process is reversed --  bicarb reenters the RBC in exchange for chloride. The bicarb is converted back to CO2 inside the RBC and then the CO2 is exhaled.

    (2). Single pass (glycophorin) -- function of protein not entirely clear.

(a). Large amount of (-) charged carbohydrate -- sialic acid -- may cause RBC to repel each other and prevent clumping of RBC.

(b). Loss of terminal sugars may occur with age and trigger destruction of "old" RBC.

(c). Protein = malaria receptor; variations in protein responsible for MN blood type differences.

Try problems 1-2 & 1-3. To review membrane structure, try 1-15 to 1-18.

III. Extracellular Matrix (ECM)

    A. Major Structural Proteins -- collagen (nice picture in Becker fig. 11-2) and elastin

    B. Adhesive Proteins -- fibronectin, laminins, etc. Connect other materials in ECM with each other and/or membrane proteins

    C. Proteoglycans -- structure and function (as compared to ordinary glycoproteins). See Becker fig. 11-5 & 11-6. This table is for reference purposes so you can understand the terminology. See http://www.indstate.edu/thcme/mwking/glycans.html for a nice web site with a summary of structure, function, and medical significance of proteoglycans and GAGs.

Proteoglycan (mucoprotein)


General description

Lots of carbohydrate attached to a protein Core;* Can be 95% carbohydrate

A protein with some Carbohydrate Attached

Are sugar chains branched? 



Length of Sugar Chain



Are sugars repeating?

Yes (repeating disaccharide)


Type of Carbohydrate

Mucopolysaccharide or GAG (glycosoaminoglycan)
Examples: heparin** , chondroitin sulfate



See Becker fig. 11-5.

Band 3 protein or glycophorin


Extracellular matrix (form gel)

Integral membrane protein (carbohydrates on extracellular domain)

* Multiple proteoglycans can be attached to a core carbohydrate chain (GAG or  mucopolysaccharide)  to form a giant aggregate as shown in Purves fig. 4-28 or Becker fig. 11-6.  (An individual proteoglycan is shown in Becker fig. 11-5.)

** Widely used as an anticoagulant. Inhibits factor required for blood clotting.

    D. Basal Lamina (see Becker pp 297-298 & epithelial cells below.)

1. Structure -- Solid layer found in parts of ECM. Main components are networks of laminin & collagen.

2. Location -- surrounds some cells (muscle, fat) and underlies some epithelial layers (on basal side).

3. Terminology -- Also called basement membrane especially in older literature. Has no lipid & is not a real membrane.

4. Function -- physical barrier, support and/or filter.

5. How Connected to cells -- see integrins and hemidesmosomes below.

    E. Integrins -- How ECM connected to Cells

        Components of ECM often connected to transmembrane proteins called integrins. Integrins link ECM and cytoskeleton. More details below.

IV. Cell-Cell Connective Structures  -- For Pictures see Purves 5.6, & Becker Ch. 11 (exact figures listed below).  For comparisons, see table below and/or Becker table 11-3.

    A. Gap junctions -- Becker fig. 11-23 (11-20); Purves 15.18. 

    B. Plasmodesmata -- Purves 15.19 or Becker 11-28 (11-25). The plant's equivalent of a gap junction. 

    C. Adhesive junctions or desmosomes. Spot (with IF) vs belt (with MF). Becker fig. 11-19 (11-15) & 11-14 for molecular details (for your interest only). For a nice EM picture see http://trc.ucdavis.edu/mjguinan/apc100/modules/Integument/_index.html (click on generic desmosome).

    D. Tight Junctions -- Becker fig. 11-20 (11-17)

Summary Table of Animal Cell-Cell Junctions:

Name of Junction

Important Structural Features


Gap Junction

Connexons;  small gap between cells (2-4 nm)

Passage of small molecules and ions (signaling & nutrition)

Tight Junction

Fusion of ridges of membrane -- no gap at ridge.

Water Tight Seal between cells; divide membrane into regions

Desmosome or Adhesive 
(or adherens) Junction

Intracellular Plaques with filaments. Classified by:
(a) Shape of plaque: spot vs belt
(b) Type of filaments:
IF (desmosomes) vs MF (adherens junctions)

Cell/cell protein connectors = cadherins

Larger gap between cells (25-35 nm)


    E. Cell - ECM junctions.  Connect cytoskeleton to ECM or solid support.  Resemble half of an adhesive junction (adherens junction or desmosome).(Becker Fig. 11-12. I think diagrams are not quite right but pictures are good.) For a better diagram see http://celljunctions.med.nyu.edu/images/figure1.gif. For a nice EM picture see http://trc.ucdavis.edu/mjguinan/apc100/modules/Integument/skin/hemidesmosome/hemidesmosome2.html

1. Similarities to adhesive junctions

a. Plaque (thickening) forms on inside of cell near junction

b. Plaque connects transmembrane protein to IF or MF on inside of cell.

2. Differences from adhesive junctions

a. Transmembrane protein is an integrin, not a cadherin

b. Transmembrane protein connects to ECM, or to solid support cell is growing on,  instead of to another cell. 

3. Two Types.

a. Hemidesmosomes  = "half desmosome";  connect to IF on inside of cell. Connect to basal lamina on outside.

b. Focal Adhesions  = "half adherens junction" --  connect to MF on inside of cell.

 V.  Examples of specialized cells with cell-cell junctions -- epithelial cells -- Becker Fig. 11-18 (11-14) or handout 3A, top.

    A. Cells & Tissues

a. Specialization. All cells in multicellular organism are specialized; there is no "typical cell."

b. Types. About 200 different cell types per human.

c. Tissue = Group of cells with similar structure & function that work as a unit.

d. 4 Major cell/tissue types -- muscle, nerve, connective, epithelial

e. Terminology Note: "tissue" is also used in a nonspecific way to mean a group of cells derived from an organ or system as in "kidney tissue." A kidney is an organ made up of many different tissue types.

    B. The Four major Tissue Types  (See Purves 40.2)

a. Muscle -- specialized for contraction. 

b. Nervous -- individual cell is called a neuron. Specialized for conduction of messages.

c. Connective -- cells dispersed in a matrix. Extracellular matrix can be solid (as in bone), liquid (as in blood) or semi-solid (gel like) as in cartilage, adipose. (Note fat in adipose is stored inside the adipose cells in vesicles, not between cells in the matrix.) See Purves 40.4

d. Epithelial -- see handout 3A -- example of cells with many types of junctions

1. Cells tightly joined 

2. Make up linings of external and internal surfaces

3. Usually sheets. Can have one or more layers

4. Often rest on noncellular support material = basal lamina = part of ECM secreted by cells

5. Usual functions: selective absorption (transport), protection, secretion.

6. An example: epithelial layer surrounding the gut. See handout 3A and Becker fig. 11-18 (11-14) or Purves 40.3 .

This leads to the next topic: How does the intestinal epithelium (& various types of junctions) function in transport?

Now try problems 1-12 to 1-14. By now you should be able to do all the problems in problem set #1.

VI. Types of Transport Across Membranes (of small molecules/ions). For an overall summary, see bottom of handout 3B. Additional information/handouts will be provided next time. For reference, types of transport are numbered 1-5 on handout 3B. Also see Becker, fig. 8-2.

    A. Basic Types of transport -- classified by type of protein (or none) involved (See handout 3B)

 1. No protein involved -- Simple Diffusion (case 1). Effective only for hydrophobic molecules (such as steroid hormones), gases, and very small molecules that can diffuse across lipid bilayers. See Becker table 8-1 & figure 8-5.

 2. Protein involved -- protein is a channel, permease (carrier or exchanger) or pump. Cases 2-5.

a. channel (case 2) -- protein forms a pore allowing passage of hydrophilic materials across the lipid bilayer. (Passage through a channel may be referred to as diffusion,  facilitated diffusion, or neither, depending on the text.) 

b. transporter -- permease, carrier or pump -- protein binds to substance(s) on one side of bilayer, protein changes conformation and releases substance on other side of bilayer. Cases 3-5.

    B. Other ways of classifying transport

1. Active vs. passive -- whether substances flow down their gradients (passive transport -- cases 1-3) or are pushed up their gradients by using energy (active transport -- cases 4 & 5). See Becker table 8-2 . 

2. Direction things move -- do all things being moved go in the same direction? Opposite directions? (See Becker fig. 8-7 or Purves 5.9.)

Next Time: Transport, cont. How you study transport and how you tell all the types of transport apart.