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 waterproof layer. Once the water has diffused into the epidermal cells of the root through the hairs, there are three paths it can take to the xylem vessels:
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Around 90% of water takes the apoplast pathway. This goes through the cell walls and water-filled dead cells in the cortex, which are very absorbent and water can diffuse through and in between them. However, when the pathway takes the water to the endodermis and it hits the Casparian strip, it cannot penetrate the waterproof layer and has to join one of the other paths.
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Some water also travels through the living parts of the root cells – the cytoplasm. It crosses the gaps between cells through cytoplasmic connections called plasmodesmata and diffuses straight into the xylem after passing the endodermis (where it collects water from the apoplast pathway). This route is called the symplast pathway.
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A small amount of water also passes into cell vacuoles by osmosis, then from individual cell to individual cell to xylem vessel. This is called the vacuolar pathway and is accountable for very little of the water that reaches the xylem vessels.
The reason the water moves at all is due to a comparatively high ψcell near the epidermal cells and a lower ψcell further inside the root, around the vascular tissues. This may be due to less turgid cells, lower wall pressure or more dissolved substances due to water being removed by transpiration.
The water has reached the xylem, the ‘business end’ of water transportation in plants. Like their solute-carrying counterparts, phloem, xylem are found throughout the plant – the xylem tissue transports water and mineral ions up through plants. The xylem are also important in maintaining the structure of the plant body, despite their thin appearance. This unexpected property is entirely representative of the complex nature that the variety of cells present in xylem tissue bring to this ‘transport vessel’.
The xylem is made up of five types of cell:
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Vessels, unique to angiosperms, are long tubes consisting of a lot of dead vessel elements placed one on top of the other. They are thickened with lignin secondary walls except on their end walls, which are very thin and perforated to allow water to pass through them easily in an uninterrupted flow. Vessel elements are generally wide in diameter and can be looked at as a more advanced version of the tracheid cell although, like tracheids, they still contain pits to allow substances in and out of the vessel.
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The tracheid cells are long, lignified cells that do the same the job as vessel elements. The difference is that tracheid cells overlap each other so any water flow happens through the pits and bordered pits at each end of it.
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Parenchyma cells are the only living cells in the xylem. Their walls are not lignified so they perform little as far as structure maintenance goes. Their role, however, is important; they store food and allow radial transport (movement of substances across the stem, from the outside to the middle, rather than up and down it).
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One of the cells that do perform a structural function are the fibre cells. They are actually a type of sclerenchyma, along with sclereids. They are elongated, thick, lignified cells, which give mechanical support to tissues when they are bundled together. Their cell walls are very thick, but they can afford a thin lumen because they do not carry water.
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The other sclerenchyma cell is the shorter scelreid cell. It differs from fibre cells in its varying shapes and abundance of pitting. They are found around the parenchyma cells.
The vessel elements and tracheids are the cells that do the transporting. They are surrounded by extra cells for exchange and storage (parenchyma cells) and support (fibre and scelreid cells). The diagram below shows the arrangement of the dead cells in vascular tissue…
The water travels some way up the xylem by the force of root pressure. This is created by the very negative water potential of the root cells, which are constantly expending energy to pump in ions. Water therefore floods into the root cells, which become turgid enough for them to exert a pressure on water in the xylem, forcing it up as far as a meter. In very small plant, this can lead to water being forced out through the leaves (guttation). After the effects of root pressure, the transpiration stream takes over in transporting the water to the leaves.
Plants lose water all the time through their stomata, despite adaptations that reduce the loss (e.g. waxy cuticle). This rapid loss of water from the leaves creates a pull on the water in the leaf cells, since all water molecules cohere to each other due to their special polar properties. So, water is drawn from the leaf cells, creating a negative tension proportional to the diameter of the curvatures in the cell walls. This is compensated for by drawing up water from the xylem in the leaf, which is replaced by water from the xylem in the stem, which is replaced with water from the roots etc. etc…. This process creates a continuous stream of water, from roots to shoots, which is made possible by cohesion between water molecules, adhesion between water and the dead xylem cells and the decreasing concentration gradient as you go further up the plant or tree (from around ψ = -10kPa in the soil to as much as ψ = -30,000kPa in the air).
The rate of transpiration is at its greatest in low humidity (the air water concentration is low), high temperature (speeds up particle movement and therefore, evaporation), heavy winds (disperse water vapour well, maintaining the concentration gradient) and with a considerable, consistent water supply (so the cells aren’t flaccid and the stomata don’t close as a consequence).
The one negative aspect of this process is that any gases in solution are drawn out as bubbles, or embolisms, which can block the flow of water, creating cavitation. However, it is a sign of the specialisation of vessel elements that, with their pits, they can simply re-route the water around the embolisms until the night time, when they dissipate as the negative pull on the xylem water deceases when the stomata close.
Overall, it is the combined efforts of individual cells and the conductive tissue of the vascular system that enable the plant to have an important, constant supply of water to the paradoxical situation that exists between photosynthetic gases and water in the stomata.
To understand the uptake and use of ions in plants, the principles of facilitated diffusion and active uptake (ion pumps) have to be understood…
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Facilitated diffusion is, like diffusion, a passive process of substance uptake. It works by selectively increasing the diffusion rate for important larger molecules (e.g. glucose) and charged molecules (ions) that cannot otherwise permeate the phospholipid bilayer. They are allowed to pass through channel proteins (pore-forming) or carrier proteins (shape changing). The latter are specific to certain molecules, thus effectively increasing their rate of diffusion. However, facilitated diffusion can only transport molecules across the concentration gradient.
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Active transport and, more specifically, ion pumps are a form of transport that move substances against a concentration gradient when the substances gained by facilitated diffusion do not suffice. The process harnesses ATP, which releases a phosphate group onto a carrier protein, giving it the energy to change shape and transport the molecule of glucose, sodium etc. across the membrane and across the concentration gradient.
Ions are needed by plants to produce a lot of the more important substances in plant life – proteins, lipids, growth factors and others. The path of ions through the plant is very simple, as part of it takes place in the water content in the xylem…
Plants need to take up only certain ions and need to get rid of others. Many of these ions can’t simply diffuse across the concentration gradient because the plant either needs to hold on to them even though there is a high concentration of them in the root cells already, or there is a low concentration of them inside but the plant doesn’t even require that. So, after the few useful ions enter and exit passively, through facilitated diffusion, active uptake needs to occur. There are three main ways in which ions enter the roots using ATP:
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The sodium-potassium pump is a specific carrier protein that uses ATP to exchange sodium (Na+) for potassium (K+).
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ATP driven proton pumps use this energy to remove hydrogen ions (H+) to the soil solution. This results in an imbalance of protons either side of the membrane and the plasma membrane becomes negatively charged.
The large concentration gradients created by the above two pumps can be used to drive the transport of other molecules. For instance, without the high concentration of H+ ions outside the root created by the proton pump, the following transport wouldn’t have enough energy to work:
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In coupled transport, specific carrier proteins allow H+ ions back in to the root, but they are now coupled with important nutrients such as sucrose, which separate from each other inside the root and go their separate ways.
After they are in the roots, the ions that need to be used in the stem and leaves are transported as dissolved mineral ions in the water travelling in the same direction, using exactly the same pathway as described previously in the ‘water transport’ section. The ions are deposited at the points they are needed, exiting the vessel elements and tracheid cells.
The phloem deals with solute transportation in plants, mainly amino acids and sugars, produced in the root tips and leaves, respectively. There is some entry of sugars through the roots, and I have already used it as an example of a secondary active pump. Sucrose enters roots through the coupled transport method and then travels to the phloem sieve tubes using both apoplastic and symplastic pathways. However, because it is the translocation method that will be explained, I will ignore this sucrose so as to avoid confusion.
To understand solute transport in the phloem, its structure has to be understood first. The phloem tissue contains very different specialised cell types to the xylem, the four of which are listed below. The main difference, of course, is that the phloem cells are living, as opposed to the xylem cells, which are (apart from the parenchyma cells) dead:
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The sieve tube elements are the living cells that, joined end to end, transport the solutes through the phloem. Their name comes from the end walls of the cells, which have a lot of small perforations to let the sap through (end walls are know as ‘sieve plates’). They have a very thin layer of cytoplasm, but no nucleus.
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Because of their lack of a nucleus, sieve tube cells cannot control what goes in and out of them. This job falls to the companion cells, which are adjacent to the sieve elements. These have a dense and active cytoplasm, which tells part of the story because it does the living for both itself and the sieve tube elements. For instance, it controls the loading and unloading of solute into and out of the sieve cells during translocation.
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Fibres are associated with phloem as they are with xylem, in strictly supportive roles. Their characteristic very think cell wall remains here.
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Parenchyma cells also exist alongside the phloem and their roles have remained as storage cells, although to a lesser importance than in xylem vessels because some of the other live cells, like the companion cells, can take care of this function, to an extent.
To move on to the function of the phloem… Translocation is the method by which the plant transports the organic products of photosynthesis (sugars) from the site they are created, to the sites they are needed, in the phloem. The theory to explain this transport is that of pressure-flow…
Sugar is transferred from a source cell (where it is made by photosynthesis) into sieve tube cells in the phloem, decreasing the ψ there. This ‘loading’ is an active process, requiring a certain amount of energy from respiration. This causes the sieve cells to take up water from the xylem vessels, which travel across the vascular bundle and into the phloem by osmosis. The water absorption creates turgor pressure that gradually builds up as more solute is produced and more water enters, until it is high enough to generate a bulk flow which forces the phloem sap, laden with photosynthates, in the direction of a sink cell (i.e. an area of higher ψ). As the sap moves, photosynthates are transported out of the phloem into the sink cells. Their leaving the phloem causes the ψ of the phloem sap to rise in the positive direction until it becomes more positive than the xylem (which is carrying ions as well as water), at which time the water moves back into the xylem by osmosis, and continues once more on its journey with the transpiration stream.
The reason the phloem sap can deposit most of its solute load, is because the sinks don’t quickly attain a lower water potential than the phloem sap as they receive the photosynthates, as might be expected, because enzymes maintain the large concentration gradient by modifying organic substances at the sink.
It has to be said that this explanation is very simple; indeed, only now are scientists beginning to discover the subtle details of phloem movement in plants. So much for thinking we know how translocation really works - this essay will be invalid very soon! At least it can be said that the issues of water and ion transportation have been explained, along with the structures of the plant root, xylem and phloem system, to give a deep knowledge of why, not just how, water, ions and organic solutes are moved through plants.