"As the major component of trees, wood seems useful and even
beautiful in finished products, but scarcely biologically dramatic.
Yet recent research on the development of wood portrays an elaborate ballet which proceeds inside young developing xylem cells.
This performance culminates in their producing molecular ‘tracks’ for the cell-wall-producing machinery. This machinery moves along the microtubules like an asphalt paver and continuously deposits wall material on the outside of cells.
It is apparent that trees need a plumbing system to move water up from the soil to the canopy, the most important part of which are the leaves.
*The leaves have to photosynthesize, and for this they need carbon dioxide and water (as well as sunlight).
This performance culminates in their producing molecular ‘tracks’ for the cell-wall-producing machinery. This machinery moves along the microtubules like an asphalt paver and continuously deposits wall material on the outside of cells.
It is apparent that trees need a plumbing system to move water up from the soil to the canopy, the most important part of which are the leaves.
*The leaves have to photosynthesize, and for this they need carbon dioxide and water (as well as sunlight).
*Since carbon dioxide is a gas, it has to diffuse into the leaves from the air.
*As water is scarce up there, the leaves utilize a protective layer to prevent unnecessary water loss. Thus, the leaves need controlled openings in leaf surfaces to let in the air including carbon dioxide. This also however allows water to evaporate and escape from inside the leaf.
The beauty of this is however that water molecules have a special feature called cohesion. They stick together through forces generated by hydrogen bonding.
--As water evaporates into the air from cells in the leaf interior, the escaping molecules tug at the water molecules behind them, and they tug at the ones farther into the plant.
--This evaporation process is called a Cohesion-Tension mechanism. It creates a negative pressure which holds the water column together under tension, pulling it up through the plant’s woody plumbing all the way from the root tips far below. Thus, as water escapes from photosynthesizing leaves, a steady column of water is drawn up from the soil into the canopy.
--Since the water column is held in such tension inside trees, it is
evident that the plumbing design has to be very sophisticated indeed. The piping must not collapse inward under the negative pressure. The water conducting system (tissue) in plants is called xylem. In trees, a new layer of xylem (called secondary xylem) is added in a ring around the stem each year. Thus the tree progressively grows in diameter as more and more wood is accumulated. The xylem largely consists of two kinds of hollow conducting elements or cells.
--The smaller ones are tracheids,
--the larger are called vessels.
Tracheids are shorter in length and of smaller diameter than vessels. These smaller elements are connected with cells above and below by means of overlapping tapering ends and numerous border pits.
Vessels are much larger cells. They too have border pits in the side walls, They are stacked end to end with ends entirely open or with
perforation plates.
--Vessels exhibit diameters approximately that of a human hair. These water conduits may be about 2 inches long or as long as 33 ft. The water moves upward through the tracheids and vessels, and sideways through border pits to new vessels and tracheids as upward progress through individual extensions ends.
A future xylem vessel or tracheid initially is an ordinary living cell equipped with all the usual interior organelles.
The outer boundary of the cell is a typical plasma membrane made up of a phospholipid bilayer with various proteins situated in the expanse.
--Lying immediately below the plasma membrane and parallel to it are microtubules. These tiny protein tubes are dynamic structures, always forming new material at the front end and shrinking at the back end. Thus, the tubules are continuously advancing forward under the plasma membrane....Penetrating through the plasma membrane from the cell interior to its exterior are very large protein structures called cellulose synthetic complex (CSC). According to an article on these structures, current estimates suggest that this is “one of the largest protein complexes” that we know about.
--Electron micrograph images reveal six hexagonally arranged
particles arranged on the surface like a rosette, but they are really rectangular molecules that penetrate the plasma membrane from cell interior to the outside.
Closer examination however reveals that these are elaborate structures: Each of the six lobes of the rosette in turn consists of six cellulose synthases [enzymes expediting formation of cellulose], that each polymerizes a single glucan chain using UDP-glucose as a substrate.
--Those individual chains are then assembled into one crystalline CMF [cellulose microfibril], which by implication consists of 6 X 6 = 36 chains [of cellulose].
The microtubules inside the xylem cell and the rosette CSCs embedded in the plasma membrane, together cooperate to produce strong walls that are deposited in intricate patterns taking the form of rings, spirals, nets, and pitted patterns.
The beauty of this is however that water molecules have a special feature called cohesion. They stick together through forces generated by hydrogen bonding.
--As water evaporates into the air from cells in the leaf interior, the escaping molecules tug at the water molecules behind them, and they tug at the ones farther into the plant.
--This evaporation process is called a Cohesion-Tension mechanism. It creates a negative pressure which holds the water column together under tension, pulling it up through the plant’s woody plumbing all the way from the root tips far below. Thus, as water escapes from photosynthesizing leaves, a steady column of water is drawn up from the soil into the canopy.
--Since the water column is held in such tension inside trees, it is
--The smaller ones are tracheids,
--the larger are called vessels.
Tracheids are shorter in length and of smaller diameter than vessels. These smaller elements are connected with cells above and below by means of overlapping tapering ends and numerous border pits.
Vessels are much larger cells. They too have border pits in the side walls, They are stacked end to end with ends entirely open or with
--Vessels exhibit diameters approximately that of a human hair. These water conduits may be about 2 inches long or as long as 33 ft. The water moves upward through the tracheids and vessels, and sideways through border pits to new vessels and tracheids as upward progress through individual extensions ends.
A future xylem vessel or tracheid initially is an ordinary living cell equipped with all the usual interior organelles.
The outer boundary of the cell is a typical plasma membrane made up of a phospholipid bilayer with various proteins situated in the expanse.
--Lying immediately below the plasma membrane and parallel to it are microtubules. These tiny protein tubes are dynamic structures, always forming new material at the front end and shrinking at the back end. Thus, the tubules are continuously advancing forward under the plasma membrane....Penetrating through the plasma membrane from the cell interior to its exterior are very large protein structures called cellulose synthetic complex (CSC). According to an article on these structures, current estimates suggest that this is “one of the largest protein complexes” that we know about.
--Electron micrograph images reveal six hexagonally arranged
Closer examination however reveals that these are elaborate structures: Each of the six lobes of the rosette in turn consists of six cellulose synthases [enzymes expediting formation of cellulose], that each polymerizes a single glucan chain using UDP-glucose as a substrate.
--Those individual chains are then assembled into one crystalline CMF [cellulose microfibril], which by implication consists of 6 X 6 = 36 chains [of cellulose].
The microtubules inside the xylem cell and the rosette CSCs embedded in the plasma membrane, together cooperate to produce strong walls that are deposited in intricate patterns taking the form of rings, spirals, nets, and pitted patterns.
Q: But how do the microtubules know how to guide the machinery depositing cellulose outside the cell wall?
A: As long ago as 1975 one specialist suggested:
It is proposed that plasmalemma [plasma membrane] located cellulose synthase enzyme complexes are free to move in the plane of the membrane. Their directed movement may …. generate a sliding force which moves the entire complex through the membrane utilizing the microtubule as a rigid guiding track and thus laying down, in the wake of the complex, cellulose fibrils whose orientation mirrors that of the microtubules.
How amazing is that!
Here are large molecular machines able to move through the plasma membrane!
And even more confounding to our imaginations is the idea of microtubules arranging themselves into various elaborate patterns in order to guide the depositing of cellulose outside the cell.
It is proposed that plasmalemma [plasma membrane] located cellulose synthase enzyme complexes are free to move in the plane of the membrane. Their directed movement may …. generate a sliding force which moves the entire complex through the membrane utilizing the microtubule as a rigid guiding track and thus laying down, in the wake of the complex, cellulose fibrils whose orientation mirrors that of the microtubules.
How amazing is that!
Here are large molecular machines able to move through the plasma membrane!
And even more confounding to our imaginations is the idea of microtubules arranging themselves into various elaborate patterns in order to guide the depositing of cellulose outside the cell.
It is just recently that scientists from the Netherlands and Germany have described the elaborate dance of the microtubules which leads to the fancy xylem wall patterns.
Q: How do they do it?
A: You have probably seen dance moves involving two steps forward, one step back, or one step forward and two steps back. Well microtubules exhibit similar patterns of motion which are no less an art form.
Young xylem cells start out with microtubules evenly arranged in multiple directions parallel to the surface of the plasma membrane. The CSC are already arranged in the membrane above the microtubules and a thin primary cellulose wall is deposited with cellulose fibrils randomly arranged.
But microtubules are dynamic. While the initial scene finds the microtubules evenly dispersed and facing in multiple directions, they end up all oriented in the same direction with gaps between bands of microtubules. The result is that the microtubules “directionally and spatially template the cellulose synthesis machinery during cell wall deposition.”
This process has to be directed and coordinated. Microtubules are constantly on the move.
However, this is an elaborate progression involving “catastrophes” and “rescues”.
Q: How do they do it?
A: You have probably seen dance moves involving two steps forward, one step back, or one step forward and two steps back. Well microtubules exhibit similar patterns of motion which are no less an art form.
Young xylem cells start out with microtubules evenly arranged in multiple directions parallel to the surface of the plasma membrane. The CSC are already arranged in the membrane above the microtubules and a thin primary cellulose wall is deposited with cellulose fibrils randomly arranged.
But microtubules are dynamic. While the initial scene finds the microtubules evenly dispersed and facing in multiple directions, they end up all oriented in the same direction with gaps between bands of microtubules. The result is that the microtubules “directionally and spatially template the cellulose synthesis machinery during cell wall deposition.”
This process has to be directed and coordinated. Microtubules are constantly on the move.
However, this is an elaborate progression involving “catastrophes” and “rescues”.
Catastrophes represent the occasion when forward growth of a tubule actually stops and disintegration of the tip (depolymerization) takes place. Rescues result in forward growth starting again.
The nature of trees and their major impact on life on Earth depends upon their ability to manufacture woody stems. The ability of plants to funnel certain cells into the wood developing process, is as important as are the amazing processes that lead to the appearance of wood itself."
By Margaret Helder CEH
The nature of trees and their major impact on life on Earth depends upon their ability to manufacture woody stems. The ability of plants to funnel certain cells into the wood developing process, is as important as are the amazing processes that lead to the appearance of wood itself."
By Margaret Helder CEH
