Membrane Modifications and Cellular Connections
Looking at the figure of simple columnar epithelium in your textbook, let's examine some cellular structures.
FOLDING INCREASES SURFACE AREA.
STRUCTURE AND FUNCTION ARE COMPLEMENTARY (FORM FOLLOWS FUNCTION).
If you remember from our study of carrier proteins that they can become saturated, then you will understand how folding the membrane can increase the rate of transport of carrier-dependent solutes.
As an analogy, let's consider a trip to a busy store - the checkout lines are long, and the manager is concerned, because he sees people leaving just at the sight of the long lines, without shopping first! We could say that the cash registers are saturated - that is, the checkers are moving as fast as they can, and they still can't keep up with the number of shoppers - so shoppers leave without going through the lines. Now, if the manager can just add more registers (and cashiers) he can get more people through the lines in the same amount of time. BUT, what if there's no more room? He has cash registers set up all the way across the front of the store already? Visit your local super-wonder-mart, and you'll see the solution - stack the checkout lanes 2 and 3 deep, and you can add more checkers in the same amount of store front - not wider, but deeper.
A typical store . A store with twice as many checkers.
This is what cells do - they fold the membrane so that they can load up more channels and carriers - deeper, not wider. Take a look:
A typical cell . . A cell with many more carriers!
Let's look closer to see where the carriers are - then you'll understand why there are more of them:
So, imagine how many carriers and channels can be packed into the membrane folds of a cell with microvilli!
Now, let's reconsider what we learned about the 3 kinds of simple epithelia - the kinds thin enough to be involved in transport:
We said that a thinner membrane is better at transport - the shorter the distance, the faster substances can move
Also, don't forget which end is which in each of these tissues:
And, dont forget where these tissues are located all are required to allow the passage of some material IN (absorption) and/or some material OUT (secretion). While the thicker tissue types are used in areas with greater stress, they still must have a way to allow rapid transport.
This is not a problem for the amazingly thin simple squamous epithelium rapid diffusion of oxygen and carbon dioxide through the air sacs of the lungs is easy.
Lets look at the other extreme simple columnar epithelium, which is found lining the small intestine, where absorption of dietary nutrients is a major function. You absorb through your 2 meter length of small intestine, daily, about 8 liters of fluid volume. This is not a continuous process it occurs over the course of only a few hours, several times a day. Imagine the rate of absorption of nutrients, minerals, and water that occurs when the intestine is well stocked with food and digestive juices! Its easy to see why there are microvilli on the surface of intestinal absorptive cells.
How about simple cuboidal epithelium? We find this tissue type busily absorbing nutrient-laced fluid from the filtrate produced in the kidneys. Your produce (by filtration) about 180 liters (yes, thats one hundred and eighty liters) of fluid EVERY DAY. Considering you only have about 3 liters of fluid (the plasma) in your blood stream, its obvious that most of that fluid must be reabsorbed. Since urine output is only about 1 liter, the kidney tubules reabsorb 179 liters of fluid in a 24-hour period. Clearly, this is a job for microvilli! And thats not all the kidney cells responsible for almost all of this reabsorption also have specialized folds on their basal surfaces (called, conveniently enough, basal folds). Again, this increases the surface area this time, of the basolateral membrane to increase the space available for carrier molecules. Look again at the figure on page 108. Can you see the basal folds? The folds on kidney cells go much deeper than the ones in the figure.
While were looking at this figure, lets turn our attention to something else. Look where all the mitochondria are and remember what they do. There are a couple of good reasons for mitochondria to be situated at the basal pole of the cell: (1) thats as close to the blood supply as they can get, and that gives them ready access to the oxygen that they absolutely must have to produce ATP; and (2) it puts them close to the basolateral surface, where they can conveniently supply ATP to power all the solute pumps found there. Yes almost all (if not all) solute pumps (remember, that means active transport) are found on the basolateral membrane, actively pumping solute to the tissue fluid, so that there is a high solute gradient to move passively into the blood. The other effect of pumping solute OUT of a cell is to lower that solutes concentration INSIDE the cell so typically there is a good diffusion gradient between the lumen and the cells interior. SO, solute is pumped out at the basal pole, and diffuses in at the apical pole you only have to pump it once to get it to move through BOTH the basal AND the apical membranes.
Cell to Cell: Structural Connections in Epithelia