Sunday, September 7, 2014

Plant Cell Wall Synthesis

Plants have some things animals don't, including a cell wall surrounding their cells.

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Primary Cell Wall

Here's a diagram of the primary cell wall, along with the middle lamella that lies between adjacent plant cells with their respective primary cell walls.  Also the regular-old plasma membrane lies internally to all that.
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The middle lamella is made of pectins which are the perfect sticky thing to attach a primary wall made of cellulose microfibrils to!  The primary wall also has some other stuff to hold it together in a nice meshy business.

Synthesis of Primary Cell Well
This is the coolest part...

When a cell splits and becomes two cells, a new cell wall must be built between them.  I'll go into the details of the cytokinesis itself in another post, but after that is done, all that is there is is a middle lamella (again, made of pectins), and a plasma membrane on either side.  How does the primary wall end up BETWEEN the plasma mebrane and the middle lamella?!?

Cellulose synthase, that's how.  And it's brilliant.  In the plasma membrane, there is a complex of proteins embedded that make cellulose.  They look like little rosettes, like the ones depicted in blue, below:

The cellulose microfibrils get put together and come out of the external end of the rosettes (closer to the middle lamella).  The long cellulose molecules that strengthen the primary wall adhere to the middle lamella, add some cross-linking stuff (pectin, glycans) and there you have it.

The cool part is those rosettes actually move through the plasma membrane, (like wading through mud) guided by microtubules which are on the internal side of the plasma membrane, leaving the trail of cellulose as it goes.

Here are some other diagrams of how this works.

In this one, the blue arrows indicate the direction the rosettes are "wading" through the plasma membrane, "walking" along the orange microtubules beneath.

This shows how the glucose subunits come in from the cytoplasm (purple circles) and are put together into the complex polymer of cellulose.  Again, the arrow shows the rosette is moving to the left, leaving a trail of cellulose to the right.
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And here we see the yellow plasma membrane cut away partly so we can see the rosettes that pass through and synthesize the microfibrils of cellulose.  Each section of the rosette is an enzyme in its own right that puts together the long chains from glucose, which is then wound together into larger and larger units.
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Here's what the structure of cellulose looks like broken down, so you can see it's a complex, tightly packed polymer.
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The strands of cellulose are arranged pretty randomly in a primary cell wall.  The cellulose synthases don't move very quickly, so the cellulose that is spit out goes around rather randomly, much like squeezing a bunch of toothpaste out of a tube- it goes every which way.

Secondary Cell Wall Synthesis
The secondary cell wall is lain down internally to the primary cell wall.  It is thick and has 3 layers that are put down one at a time.  Each layer has all its cellulose going parallel to each other.  But the layers each have different directions/ orientations than one another, as seen in the bottom part of this diagram:

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This provides a lot of extra strength, because it is protecting against compression, stretching, tension, etc. in all directions once you have all 3 layers put down.  When the cellulose rosettes are laying down cellulose for a secondary cell wall layer, they move more quickly and in regular, straight lines.

That's all she (I) wrote.  Stay curious!

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