Mass transport is required in multicellular plants due to their increased size, the variation in their metabolism as they photosynthesise and respire at different rates throughout the day or in different seasons, and the decreased surface area to volume ratio coming from their larger size.
Water enters a plant through the roots. In order to understand how water gets in the root, you should definitely check out the root structure:
What you can see above is a delicious slice of pineapple. OK, it’s not. That is a slice of a root. Roots, as you may have seen in real life, are hairy. All those tiny and not so tiny root hairs buried into the soil greatly increase the surface area of the root. This exposes it to more water molecules which can be taken up. The hairs are nothing like human hairs; they are extensions of the outer layer of the root, made up of cells. This layer is called the epidermis.
Why does water move inside the root? Simple: osmosis. The cell sap (i.e. cell juice) has a lower water potential than the fluid found in the soil, so the water in the soil kindly makes its way into the thirsty awaiting root. Once the water reaches the first cell in its path, the water potential of that cell is increased compared to the cell next to it. Therefore, water moves into the next cell, leaving the current cell. This in turn results in the previous cell taking up water all over again, and so forth, until water makes its way across all cells of the cortex.
Reaching the endodermis, water then enters the xylem. The xylem is a tissue of dead cells which contributes to the vascular system of plants by being the transportation medium for water and dissolved mineral ions. The xylem brings them to the leaves and plants’ other organs.
The vascular system of a plant differs between monocotyledonous (monocots) plants such as cereal crops, and dicotyledonous (dicots) plants like carrots and potatoes. The cotyledon is the baby-leaf contained in seeds that gives rise to the plant leaves as it develops. Monocotyledons have one while dicotyledons have two.
Root differences in the vascular systems of monocots and dicots exist. Monocots have a much larger vascular cylinder than dicots, with respective larger xylem and phloem vessels.
The stems of monocots have dispersed vascular bundles, while those of dicots have ordered bundles around the circumference of the stem. They are also much larger, swapping around the pattern seen for roots where monocots had larger vessels.
Leaf differences are what originally divided monocots from dicots. Monocots have leaves with longitudinally arranged vessels that follow the leaf length in parallel e.g. grass, while dicots have net-like vessels spread throughout the leaf in all directions. They also have larger vessels that smaller ones branch out of.
Within similar vascular bundles, monocots have larger xylem and phloem vessels. The vascular bundle also spans the width of the leaf, while in dicots it is separated from the leaf lower epidermis by ground tissue (collenchyma) that provides extra support.
There are three different pathways that water uses in order to reach the xylem:
1. The apoplast pathway whereby water slaloms between cell walls and the spaces in between, without passing directly through live tissue; this accounts for ~90% of water uptake.
2. The symplast pathway whereby water goes straight through living tissue i.e. the cells in the cortex, and into the xylem; this accounts for only ~10% of water uptake.
Basically, the symplast pathway is just way simpler.
3. The vacuolar pathway whereby water goes through the plasma membrane and cytoplasm, into the vacuole.
The endodermis acts as a selective barrier by only allowing water entry by the symplast pathway, as well as preventing any air bubbles from entering and travelling further into the plant.
The symplast pathway means that water has to pass through one plasma membrane checkpoint into the cell, and another one out of the cell. This helps filter out unwanted water contents such as certain ions, as well as somewhat control the entry and speed of water into the plant.
Transpiration is water loss through the parts of a plant which are found above soil level i.e. not the roots. As water streams through a plant, transpiration affects the speed of the stream. Increased transpiration will lead to a quicker uptake of water through the roots to maintain the water flow throughout the plant. So what affects transpiration?
1. Light causes stomata to open, resulting in increased water loss (transpiration).
2. Temperature going up also raises the rate of transpiration, as more water molecules evaporate.
3. Humidity. An increase in humidity around the leaves means that there is less space for water molecules from the plant to evaporate into, so transpiration is decreased.
4. Air movement (wind) can displace water molecules from around the stomata, so that more space becomes available for additional water molecules to go into. Transpiration increases.
Hydrophytes and xerophytes show special adaptations to deal with extreme water conditions. Hydrophytes live in or on water, while xerophytes live where water is very scarce.
Hence, their root and leaf adaptations mirror their conditions. Hydrophytes (aquatic plants) such as water lilies have no need for roots or stomata, sometimes presenting vestigial stomata. Their leaves are waxy and shiny, something xerophytes couldn’t afford. In fact, xerophytes have leaf surfaces that don’t catch the light much, in order to minimise transpiration and water loss in an already dry environment.
Additionally, xerophytes such as marram grass have rolled leaves with hairs. These trap air pockets with moisture in order to prevent a steep water vapour concentration gradient such as those created by winds or exposure to surrounding air (that is low in water vapour due to the dry weather). Their stomata may only open at night. They have needle-like leaves which minimise the surface area over which water could be lost.
Looking at sample leaves under a microscope reveals these adaptations. Hydrophytes present large air pockets in their waterproof, flat leaves. Leaf flatness and air pockets both serve to ensure the plant can float on water. The aforementioned xerophyte marram grass adaptations in the leaf can also be seen under the microscope.
Root pressure. The cohesion-tension hypothesis
These are the two ways in which the stream of water through a plant can work. Root pressure is the water being pushed into the roots, while cohesion-tension is the water being pulled up.
When a plant doesn’t transpire much, mineral ions can get accumulated at the bottom in the roots. This decreases the water potential inside the roots, so that water moves in by osmosis from the soil into the roots.
A key property of water is cohesion. Cohesion refers to the way in which water molecules stick to one another. A good example of this is when water moves up a very narrow plastic tube, all by itself. This is due to water sticking to itself and hence pulling itself upwards. This happens in plants too.
Arguing the mass-flow hypothesis of sugar movement through phloem
The mass-flow hypothesis states that sugar moves through a plant from its production site (such as a leaf, termed a source site) to other areas (such as a root, termed a sink site) as a result of the pressure built up from the accumulation of sugar in the phloem. It is so far the best supported theory regarding transport of sap in plants.
More specifically, it follows that:
1. As the sugar molecules are produced at the photosynthesis site, they accumulate and must be transported via active transport into the phloem sieve tube
2. This decreases the water potential in the phloem, resulting in water moving from the xylem to the emerging sap (the sap is the sugary solution)
3. The hydrostatic pressure in the phloem reaches a critical high, resulting in pressure flow of the sap further through the phloem
4. At the sink site sugars are actively transported from the sap, increasing the water potential in the phloem and allowing the water movement back into the xylem
This theory has been supported by experiments showing multiple things. Firstly, shaded leaves in plants versus well-illuminated leaves showed different outcomes of markers such as growth chemicals or viruses being introduced in the plant leaves. The well lit plants had these in their roots, while the shaded ones didn’t. This indicates that in the absence of photosynthesis, transport didn’t occur, and hence it isn’t carried out by diffusion alone which is passive and wouldn’t require a photosynthesising leaf producing energy for active transport.
Secondly, investigating the concentration of solutes between the source and sink has revealed concentration gradients such as an increasing concentration in the sink-source direction and a decreasing concentration in the source-sink direction. This indicates that transport is continuous between source and sink. Such experiments can be carried out with autoradiographs that create visualisation through radioactive chemicals.
Thirdly, upon puncturing the phloem, sap is released outwards, for example into the mouthparts of an aphid, indicating the sap is under pressure. Classic.
So far so good? Not so fast. Counterarguments are raised against the mass-flow hypothesis, including solutes travelling at different rates through plants, such as various amino acids and sugars, and bi-directionality of solutes travelling in opposite direction. Experiments to investigate bi-directionality are hard to perform because you would need to load two different substances at different points in transport and track them.
Moreover, transport is affected by temperature and metabolic inhibitors. All these points indicate that pressure flow can’t be taking place, as it would be a uniform process. Uniformity can’t happen at the same time as opposite direction, speeds, and changes in speed caused by environmental factors.
An alternative to the mass flow hypothesis is the cytoplasmic strands hypothesis.
Phloem tissue contains sieve tube cells which are connected via openings in the sieve plates. They lack a nucleus and other organelles (which are present in their companion cells), but could contain cytoplasmic strands that may contract via peristalsis to create movement along the sieve tube cells. These strands haven’t been observed in sieve tube cells, so this hypothesis remains unconfirmed.