The Movement of Water, Ions and Organic Solutes in Plants.

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The Movement of Water, Ions and Organic Solutes in Plants.

Green plants can deceive. As we bustle past them in their hundreds, on our way to school or work, we see them standing in formation or swaying in the breeze in people’s front gardens and they could be mistaken for very idle organisms. However, we couldn’t be more wrong – as transport vessels, plants are busy all the time; they transport the products of photosynthesis (mainly sucrose) all over their internal surface and amino acids, water, ions and other solutes travel from the roots, upwards.

With so much to transport, and a lot of it up the plant, Nature is posed a problem; a plant has no pumping organ, so how can vital substances possibly reach the upper levels of plants without expending a lot of energy? And when the products get there, how does the plant know what to deposit, and where?

In this essay, I will seek to answer these questions while increasing my understanding of the cells and mechanisms associated with the movement of water, ions and organic solutes in plants.

 To understand water movement through the plant, the key principles of osmosis (and therefore diffusion) and water potential must be understood…

  • The molecules that make up substances are constantly moving about in a random way. This causes them to move from an area of higher concentration to an area of lower concentration, along a concentration gradient. Diffusion occurs in cells and across cell membranes when the membrane is fully permeable to the solute present or the pores in the membrane are big enough to allow the solute through. Diffusion is a passive process – it requires no energy other than the kinetic energy of the continuously moving molecules. A greater concentration gradient, over a shorter distance, over a larger area and through a thinner membrane (preferably with pores) will increase the rate of diffusion.
  • Osmosis is a particular kind of diffusion that occurs when water diffuses across partially permeable membrane, from an area of higher water potential (higher concentration of ‘free’ water molecules) to an area of lower water potential. The water molecules are smaller than the solute molecules, so only they can fit through this type of membrane. NB. A hypertonic solution has a higher concentration of solute molecules and a hypotonic solution has a lower concentration of the solute. When two solutions have the same solute concentration, they are isotonic. This is the ultimate goal of both osmosis and diffusion.
  • The water potential of a solution (ψ) is the potential of water molecules to diffuse out of a solution. So, water is more likely to diffuse out of a solution with a higher water molecule concentration, a hypotonic solution, and it therefore has a high water potential. From this, we can infer that pure water will have the highest water potential, set at 0kPa (zero). Dissolving a solute into a solution reduces the water potential, making it negative. Water potential in a cell is affected by solute potential (the amount by which solute molecules reduce water potential, ψs) and pressure potential (the pressure exerted on the water by the cell membrane, ψp).

        

So, having cleared up the key areas, we can start on the journey of a water molecule through a plant and why not begin at the journey’s very commencement, in the roots…

        The roots of plants are important organs in the process of water uptake because they provide the link between the soil and the xylem, thus allowing water into the plant in the first place. The images on the next page show the structures of the root cells in typical dicotyledonous roots. They show the main characteristics of the large cortex made up of the parenchyma cells (used for storing starch and some other substance), with small gaps in between them (important for the aeration of the root tissue, which is non-photosynthetic). The xylem and phloem are at the centre of the stele, surrounded by a ring of cells called the pericycle (from which lateral roots arise), which is itself surrounded by the endodermis (a single layer of cells bearing a band of waterproof suberin, called the Casparian Strip).

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        The uptake of the water occurs by osmosis as there is a lower water potential in the root hair cells than in the soil due to the continuous loss of water through the stomata in the leaves. The root hairs are all around the outside of young dicot roots, which is why they don’t feature in the above images (they are older dicot roots). They increase the surface area for absorption and provide less of a barrier for the water to cross because they have no ...

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