Endocytosis, how plant cells utilize large molecules for nutrition
When plants absorb large molecules or microbial cells through the roots (or the leaves), how are those large molecules absorbed into cells and used as a source of nutrition?
Endocytosis is one known mechanism of cellular absorption of large molecules, and has been known to be a significant method of nutrient absorption in animal cells for over 70 years. It has only been in recent decades that this process is also recognized to function in plant cells. There has been much progress in the knowledge of this process in recent years, but I wanted to give credit to an original champion of this idea and share the thoughts she expressed originally in the 1970’s, and then updated in 1993.
From Bargyla Rateaver:
The membrane is a thin layer of mostly protein and lipid (fat) molecules, in constant motion. The layer may be undulating and rough, but always it’s molecules are moving; this movement is essential to the cell’s life, as cessation of movement indicates death.
Imagine a group of large molecules, poised outside this membrane, that are ready to get into the cell. The membrane itself engulfs them and pull them down into the cell.
Such engulfment was only crudely made visible with older equipment; with the modern electron microscope advancements, even molecules can be discerned, at least in outline, so we now know that the engulfment is really a complicated, precisely programmed series of events.
This occurs because of some special activity in certain small, three-legged, protein molecules, called clathrin.These are programmed to fit themselves together into a cage, or basket, resembling a Fuller dome, upside down.
Even if these molecules are isolated, in a solution, they assemble themselves that way, just on their own, as though their mere structure impelled them to do so. It is these clathrin molecules that give the cage its “bristly” surface appearance in sections only: actually it looks like a basket with 12 plane faces (dodecahedron)
They come from somewhere in the cell, maybe the protein factories (ribosomes), and assemble themselves at a spot on the inside of the plasma membrane, where they start to form themselves into the cage.
As they draw themselves together to make this cage, or basket, they draw the membrane down with them, like a lining to the basket. Large molecules and/or aggregates of them on the membrane at this spot, presumably waiting to enter the cell, are caught in this cage. There are several stages of this drawdown.
First, a cup-shaped depression, or pit, is formed. It comes to be lined with the membrane, that therefore must conform to the pit depression. It is called a coated pit, because of the clathrin surface.
Next, the cup becomes deeper, resembling a flask.
Lastly, the neck of the flask closes, and the pit has become a cage, a closed basket, a bag, a ball, and it contains the large, enclosed, entering molecules or particles. It is then called a coated vesicle, because it is a closed, round, ball-shaped cage, with the clathrin surface, a coating of hexagons and pentagons.
The clathrin molecules have completed their task of bringing a bag full of large molecules into the cells. They disassemble themselves, detach, and go off to do the same chore someplace else on the cell’s plasma membrane. (Sometimes the vesicles keep their coat for a while.)
Without the clathrin cage, the naked membrane ball is called a smooth vesicle. It embarks upon its predestined path through the cell, to unload its cargo of large molecules or particles at predetermined locations.
Imagine a ball of yeast dough, into which you press a finger to make an indentation; the pit made by your finger gradually smooths out. You see a kind of dimpling in and out. This is what goes on all over the cell membrane surface, all the time, at a fast pace, measured in seconds or minutes.
Although it takes time to describe this, the actual action is unimaginably rapid. Within minutes the molecular load is found in the various organelles; this means enormous numbers of reactions have taken place to engender the movements.
1. Rateaver, B. & Rateaver, G. Organic Method Primer Update: A Practical Explanation : the how and why for the Beginner and the Experienced. (The Rateavers, 1993). Page 22
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