Wednesday, 9 November 2011

A Weekend's Worth of Plant Nutrient Transport

Nature's cup of nectar. Photo: Ryan Pearson
By Ryan Pearson
Plant nutrient transport is a hard thing to understand - especially for those of us who tuned out during the lectures. But after reading up on it, here's my understanding of how it works.

Basically, there's three levels to it. The initial uptake of water and minerals via the roots, the short distance transport from cell to cell, and the long distance transport from the roots to the shoots. Let's look at it in terms of a trip to the bottle-o.


In the big picture, the cold-room of the bottleshop is just like the soil. It has lots of fluid and minerals (booze), and you want to get that liquidy goodness to your house to provide you "nourishment". There's a few steps in the process for getting it there, the initial selection, getting it through the door & past the security, and finally the long-distance travel to your house. What the hell does that have to do with plants, you ask? 

Well, first up, the initial uptake of water and minerals is just like taking a carton from the cold-room to your trolley. The process is marginally selective (you need to choose which beer you want, but at the end of the day it doesn't really matter which one ends up in there). This happens because you're thirsty, just like the cells in the tips of a plants roots - they're permeable to water just like you're permeable to beer.

Once you have it in your trolley (cell), you need to decide the best way to get it to the long distance transport you have waiting (your car). In plants, there are a couple of options. The apoplast route, or the symplast route. The apoplast route is taken by nutrients that have 'snuck in' without having their ID's checked (i.e. they didn't cross a selectively permeable plasma membrane), and so they travel through the cell walls (without crossing the plasma membrane into a cell). The symplast route is for those nutrients that had their ID's checked at the door, so they travel cell to cell via pores that join the plasma membrane of each cell together. The end result of this is that the symplast route heads straight out the door of the bottleshop into the stele (which is the term for the core tissue of the plant). For those that take the apoplast route, the bouncer is waiting to check their ID. The bouncer in this case is something called the Casparian strip, and it prevents anything from entering the stele directly through the apoplast route. So the bouncer checks the ID, and the nutrients get to cross the plasma membrane and head on into the main channel for long-distance transport.

In this case, the long-distance transport we're talking about is you in your car... but when transporting this special plant booze, there's heavy traffic. You see, in plants the long distance transport of water and minerals occurs in the Xylem as what's called xylem sap, and sugars are transported as phloem sap... in the Phloem. Using our metaphore, the leaves are your house, i.e. the goal destination. But because of the super heavy traffic the only way to get there is for someone else's car to move out of the way. That's a very basic way of understanding how water and nutrients can be moved up a tree as fast as 15m per hour. You see, what happens is by increasing the surface area of the tree, the leaves heat up and allow water to escape through a process called transpiration. As the water leaves the leaves (cough), it creates a negative pressure in the xylem which needs to be filled by water and minerals from further down the trunk. Therefore, it effectively pulls the water up the xylem. Can you see the relationship? As cars way down the road move out of your way, it clears space for other cars to move into their spot, and a chain reaction allows you to move a little further forward until eventually you reach your house, and the goodness you picked up so far away starts getting used up.

Oh crap! You forgot to pay for it! You need to get the products of your hard work, money (for plants, this is sugars produced by photosynthesis), and rush back down to the bottle shop. The phloem will take you there...


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