Saturday, February 21, 2009

taking a break

I am taking a break from this blog for a while. I am still trying to consolidate the feedback I got.

I'll be back soon.

Sunday, February 8, 2009

Why the cell membrane model is called “fluid mosaic model”

Yeah, why “fluid”, why “mosaic”?

Let’s look at our model of the membrane again. The basic membrane is the double phospholipid layer and the proteins and carbohydrates are just add-ons.

So what makes this a fluid mosaic model?

Well, the fluid that is being referred to is the fluidity of the phospholipid bilayer. Each of the phospholipid molecule here can change places with each other side to side. Sometimes, they can even change places in to out or vice versa.

Because of the fluidity of the phospholipid bilayer, the proteins embedded in them can also change places. They can be clumped in one section or they can be spread out in another. Since the membrane is very dynamic, there is constant motion of molecules making it up. Thus, the membrane that one sees at one time is not exactly the same membrane at another time. This is the reason why the membrane is “mosaic”.

So the “fluidity” of the membrane is due to the phospholipid molecules while the “mosaicity” is due to the proteins.

Saturday, February 7, 2009

the genetic alphabet - A,T or U, C and G

Can we write instructions with only 4 (or 5) letters?

The answer to that is “yes”! Our cells’ genetic instructions are written using only the letters A, C, T or U and G. Can you believe that?

If that is the case, how come there are so many different cells? How come we are different from each other?

Well, these letters vary in number as well as in their sequence in every cell. Also, sometimes not all the sequences of these letters are active at the same time. If for example we designate numbers to the various sequence of these letters in a cell, it’s possible that only sequences 1, 2, and 3 are active in one cell while it can be sequences 1, 5, and 8 in another cell.

So if you think about it, just varying the number of A, T, C and G as well as their sequence in a cell are enough to produce almost limitless variety of combinations.

By the way, in the genetic alphabet, A always pairs with T or U and C always pairs with G. So that actually limits the combinations because there will always be the same number of A and T or U and equal number of C and G. Still even with this constraint, the possibilities are almost limitless.

Thursday, February 5, 2009

Why do eukaryotic cells keep their genetic material inside a nuclear membrane?

If you recall, the major difference between eukaryotic and prokaryotic cells is the presence of an internal membrane system in the former (Nov 3, cell design 101.1).

This internal membrane system encloses the various metabolic centers and separates them into organelles. It also encloses the genetic material and we now recognize a nucleus in eukaryotic cells.

So, what is the advantage of this kind of design?

Well, as Sherlock Holmes would say, “elementary my dear Watson, elementary”. Just think about it, the genetic material contains the ‘blueprint’ of the cell’s life. So it’s but natural to ensure its safety, right?

The cell cannot leave its 'blueprint' lying around, exposed to all the enzymes and activities going on in the various metabolic centers. What if an enzyme will act on it and split it into pieces? What if it suddenly gets entangled in all the activities going on? The information in the blueprint might be destroyed or lost.

Now, do you wonder why prokaryotic cells mutate so fast? Their genetic material is not protected like that of eukaryotic cells.

Tuesday, February 3, 2009

Junctional complex – all together now

Source of image:
A.Diagrammatic representation of junctional complex in intestinal cells
B.Electron micrograph of of actual intestinal cells

Let’s put together all the components of our junctional complex.

In epithelial cells lining the small intestine for example, the components of the junctional complex are usually arranged in the following sequence, starting from the free surface or exposed surface:
Tight junction
Adhesive junction
Gap junction

The adhesive or adherens junction that is labelled in the image above is what we mentioned last time as the belt desmosome while the one labelled as desmosome is what we called as spot desmosome.

There is logic to this kind of arrangement based on the nature and function of the components of the junctional complex.

The tight junction is always at the topmost or most exposed part of the cells because it is supposed to prevent any entrance or exit of materials between cells. The scientific term for tight junction is actually zonula occludens, meaning ring-like occlusion.

The adhesive junctions are usually located below the tight junction because they glue cells together and provide mechanical support to the tight junction.

Finally, gap junctions are at the lowermost part or the least exposed part of the cells. There is actually some space between cells here such that there is rapid exchange of information between cells.

By the way, there may be a 3rd kind of desmosome, the hemidesmosome. As the name implies, it is half of a desmosome. This kind of adhesive junction usually glues epithelial cells to the basal lamina which is in contact with the connective tissues underneath epithelial tissues.

Monday, February 2, 2009

Carbon fixation - why plants need carbon dioxide

In my Jan 23 post, ‘why is it better to water plants in the morning”, we talked about the first phase of photosynthesis or the light dependent phase. Today, we will talk about the second phase of photosynthesis or the light-independent phase, also known as carbon fixation.

If sunlight and water are needed in the 1st phase, carbon dioxide and the energy produced from the 1st phase are the ones needed in this 2nd phase.

The end result of this carbon fixation phase is the formation of organic molecules especially carbohydrates.

Different plants use different ways of fixing carbon dioxide into organic molecules. The difference is dictated by the surrounding temperature.

Plants found in temperate regions or what are called C3 plants, generally use a 3-Carbon compound as their first molecule in the process. This process is also called Calvin cycle.

Plants in tropical regions on the other hand, use a preliminary 4-Carbon compound before it proceeds to the Calvin cycle. Plants using this process are therefore called C4 plants.

Finally, plants in desert areas cannot open their stomata (passage way for carbon dioxide) during daytime because they will lose too much water this way. They can thus take in carbon dioxide only at night. They store the carbon dioxide in organic acids at night and just transform these acids into carbohydrates during daytime. Plants belonging to this group include various cacti (singular, cactus) and are called CAM plants or Crassulacean Acid Metabolism plants.

Anyway, whichever process is used by plants to fix carbon dioxide, they still end up making organic compounds especially carbohydrates. These compounds are what we use as food.

So this gives us more reasons to thank green plants. Don’t you agree?

Sunday, February 1, 2009

Adhesive junction – let’s stick together

As the name implies, adhesive junction glues adjacent cells together. There are two kinds of adhesive junction in cells: the belt desmosome and the spot desmosome.

Both belt and spot desmosomes are made up of adhesion proteins that form a kind of bridge between cells and anchor this bridge to cytoplasmic elements inside cells. The two differ only on how the bridge is formed.

Belt desmosomes form a complete ring of bridge around cells while spot desmosomes form the bridge only at certain points or spots between cells.

If the desmosome between cells do not form well, then the cells can come off in layers and fluid will accumulate between them. This is what happens when a blister is formed. Notice that a piece of skin (layer of cells) separates from the underlying cells and fluid fills the space created. Eventually, the blister dries up and the separated piece of skin dies and peels off.

In this case, the adhesive junction or bridge between cells collapses.

Tight junction – holding on tight

Epithelial cells by the nature of their function as linings or as glands must always “hold on to each other”. Cells lining our intestines for example, must hold on to each other to prevent any material from passing between them. This ensures that anything that goes through our bodies passes through them (the intestinal cells) for proper processing, and not between them. So how is this possible? Well, intestinal cells as well as most other epithelial cells are attached to each other by junctional complexes, a primary component of which is the tight junction.

Tight junctions function as seals between and around epithelial cells. These consist of closely apposed plasma membranes of adjacent cells with no space at all between them. Thus, nothing escapes between cells.

For example, urine remains inside our urinary bladder and does not leak through it because of tight junctions. The contents of our intestines and stomach are kept inside our bodies because of tight junction. Food is absorbed by the cells but once absorbed cannot leave the cells. Contents of our body do not leak through our skin because of tight junction.

These are just some examples of the importance of tight junctions between cells.

By the way, gap junctions mentioned in my Nov 17 (intercellular communication) post, also are components of junctional complexes. Another component is the adhesive junction and this will be the subject of a future post.
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