Plant Cell Wall Lab

In this lab we explored various anatomical features of plant cell walls.

Mitosis & Cytokinesis
Not going to repost that, I did this last year and it is still beautiful: Mitosis post
Here's another great pic of a cell plate and phragmoplast on the edges I got today though:

Cytokinesis in plants is a rather involved process since a new middle lamella must be made, followed by primary wall and sometimes secondary wall.


Primary pit fields
This is a TERRIBLE picture taken of a picture shown to me on another camera.  But you can kind of see the primary wall and the thinner parts that indicate the primary pit fields (pointer is on the thicker part of the primary cell wall).

More primary pit fields from a more 3D outside view of the cell (this is from #6 of the lab).

The cell walls are stained pink and you can see that there are lighter/ white spots on it.  That's where the wall is thinner, so those are the primary pit fields.

Plasmodesmata
By definition, plasmodesmata are only in primary cell walls, and they are channels between adjacent cells, through which the cytoplasm of each cell is continuous.  These occur more often in primary pit fields where the membrane is thinner, but they can happen anywhere in the primary wall.

You can also see these in the tomato cells we looked at last week:

Intercellular Air Spaces
These get formed often at the edges of cells as they are dividing, and are important to gas exchange for the plant. (#6)

Secondary Cell Wall
Here you can see the distinction between the secondary wall and the compound middle lamella (which includes middle lamella and 2 adjacent primary walls).

Just a picture to orient- vascular bundle, and the bundle cap at the top.

The lighter lines between all the cells are the compound middle lamella, the darker parts are secondary cell wall.

Pits
These are pear stone cells, and they have many long simple pits running through their secondary cell walls.  I thought they look rather similar to plasmodesmata, but the distinction is plasmodesmata are ONLY in primary walls, and pits are ONLY in secondary walls.  (Lab #9)

Bordered pits
Bordered pits from above look like little donuts.  Here there are a whole bunch in some dense pine wood.  These types of pits are common in water conducting cells, and they act to help prevent clogs from air bubbles. (Lab #11)

Stained microscope slides are pretty.  That is all.

Stay curious.

Plant Cell Wall Synthesis

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

 

Source of picture

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.

Link to source

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.

Source of image

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.

Source of image

Here's what the structure of cellulose looks like broken down, so you can see it's a complex, tightly packed polymer.

Source of image

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:

Source of image

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!

Directional Terms - Plant

I couldn't find a good diagram of this online, so I did what any logical person would - made my own!

Basal- closer to the base of the organ (note the "base" of the root system is next to the soil line)
Apical- closer to the apex of the organ (the "apex" of the root system is the bottom-most/ inferior-most part)
Proximal - closer to the base of/ attachment point of a lateral organ like a leaf.  For a leaf, this term is the same as basal.
Distal - farther from the base of/ attachment point of a lateral organ like a leaf.  For a leaf, this term is the same as apical.
Adaxial- this is closer to the axis of the organ ("adding" to the axis).  Axis is the very center of a cylindrical organ like a stem.
ABaxial- farther from the axis of the organ (similar to aBduction of a limb at the joint in human anatomy- taking it away from)
Periclinal- along the perimeter/ outer edge of an organ.  They follow the perimeter.
Anticlinal- anti to the perimeter (at a right angle to the outer edge, like the scar in the drawn cross section)

Directional Terms - human

Here is a handy reference on directional terms used for human anatomy.  I was nice enough to choose the pictures of people with clothes on.  You're welcome.

Here are some specific to the brain, because for humans our brain as we are standing is tilted compared to the way we hold up our heads, like so:

Source on Studyblue

So simply using superior/inferior and anterior/posterior doesn't quite work for how we normally think of orienting the brain, which is why we use dorsal/ventral and rostral/caudal.

Here is a memory aid for this.  I had a hard time keeping dorsal/ventral straight, so this is what helped me.

When I think of dorsal, a shark comes to mind with its iconic dorsal fin on its back or in this case top:

For ventral being the bottom or down, I had to think about stingrays.  They take water in on the top of their bodies and then shoot the water out the bottom over their gills.  So they vent the water out the bottom side of their body.  Hope that helps.

Left image: top/ dorsal side of stingray.  Right image: bottom / ventral side of stingray

There ya go, have fun in anatomy or whatever class brought you to find this blog!

Stay curious.

Blood Brain Barrier

No one would argue blood is very important to our bodies!  It carries very important things to all parts of the body we need such as glucose and oxygen.  It also takes out the trash by removing wastes like lactic acid and carbon dioxide.

Blood is the highway by which our immune system cells gets around our body to take care of anything that invades.  Blood is also how medications, drugs, poison and toxins, hormones, etc. can get around our bodies.

But let's talk about blood and the brain.  Our brain is a very special organ that deserves special protection.  It's the only part of our body that is protected by a 7 mm thick covering of bone, in addition to cerebrospinal fluid cushioning and protective layers of meninges.  That protects from the outside in, but we also have protection from the inside out, called the blood-brain barrier (I will abbreviate it BBB).

Blood supply is very important to the brain so it has a constant supply of energy and waste removal.  Here are some diagrams showing the blood vessels supplying the brain.

Notice the arch at the bottom of this diagram is the aorta which comes right off the heart itself

The Common carotid artery is the one you are feeling when you take your pulse on your neck

This "Circle of Willis" shows the blood supply on the inferior/ ventral side of the brain.  You can see in the image on the right where this is in relation to the brain.

Alright, so we need that blood and it definitely is there.  But how to protect it?  Some may think the "Blood-brain Barrier" is some kind of a gate the blood goes through when it enters the vicinity of the brain, but that isn't the case.  There isn't a particular spot for the BBB, but rather, it exists as protection on the capillaries (smallest blood vessels where material exchanges happen) themselves in EVERY location within the area of the brain.  It's not a matter of filtering all the blood as it travels through your head, but it's a matter of being more selective about what things cross over FROM that blood into the brain tissue.

We have special gate-keepers to protect things from getting into our brains.  Here's a cross-section of what a blood vessel in the brain looks like compared to a regular one elsewhere in the body:

Not only are the capillary cells (red in the diagram) closed more tightly so things can't leak through, but the entire blood vessel is covered with the "feet" of astrocytes.  (My favorite glia!  Here's a post about them.)

Here's a more 3D view:

See how is it a gatekeeper?  Anything in the blood must go through the astrocyte in order to get to the neuron.  Astrocytes are like the bouncer, protective big brother, or best friend: "if you want to get to the neuron, you have to [quite literally] go through me first!"

Astrocytes are really integral to the chemical integrity in the brain and are a bit of the "unsung heroes" of the brain.  Not only are they gatekeepers, but they act as a kind of mop-up crew and storage unit for any leftovers the neurons leave around (like ions, some neurotransmitters), and they serve to make sure the neuron stays well-fueled, like a mother who keeps snacks in her purse for her toddler.  No wonder astrocytes far outnumber neurons in the brain.

How do these tight blood vessels and "feet" of the astrocytes actually protect it? They are cells, which means they are surrounded by membrane- a phospholipid bilayer, which looks like this up close:

Because of this configuration, stuff that is polar (charged) or water-soluble can't get through the membrane- it can't get past all those hydrophobic fatty tails. 

YOU SHALL NOT PASS!!!
Water-soluble stuff such as nutrients (Amino Acids, Glucose, vitamins)
Polar stuff
Chemicals & toxins
Viruses
Bacteria

Stopping viruses and bacteria for the win.  Stopping nutrients? FAIL.

So to fix that, we have special transporters to let the good stuff in.  They can be super specific, so a glucose transporter will ONLY let glucose in.

Okay, you can go in...
Non-polar/ uncharged/ fat soluble stuff: this includes oxygen going in and carbon dioxide going out
Drugs that are fat-soluble
Other important stuff with special transporters embedded into the membrane to let them in, like water, glucose, amino acids, vitamins, etc. (Glucose has a wicked-awesome backstage pass, AND it knows the lead singer of the band.)


Whew!  That's a big job and an important one for the BBB.

Stay curious!