Have you ever looked up to the top of a very tall tree and wondered how food and water make it all the way up there to keep those leaves green and keep the tree growing? Or have you never given any thought to whether plants need to eat and drink?
With being rooted in the ground and having no mouth or stomach, it's understandable that many would simply overlook plants' need for sustenance.
Not you, though. If you're preparing for a major biology exam - say, the biology GCSE or equivalent, you already know that every organism needs food and drink to live, plants included.
How food and water circulate in plants is the question your Superprof investigates and reports on today.
An Overview of Transport Systems
If you have a pet - or even if you don't, it's relatively easy to understand how animals take in and digest food, and expel waste. Food goes into the mouth and down the oesophagus after a bit of chewing, aided along by peristaltic waves.
It then arrives in the stomach, where it stays until the acid there can break it down further and then, it's off to a full course around the big and small intestines, where the bulk of the foods' nutritional value is extracted. In the intestines, too, peristaltic waves drive matter forwards until, after all of that processing, waste products are expelled.
That's the roadmap that food follows as it courses through our bodies. Our blood has a completely separate transport system.
The heart pumps rich, oxygenated blood into our vascular system - the highway of arteries and veins that will carry blood, hormones, enzymes and nutrients throughout our bodies. Those arteries branch into smaller arterioles, which then transition into capillaries.
The capillaries function as gateways, where the blood begins its return journey to the heart.
Those capillaries merge into venules which, in turn, via the system's veins, feed the oxygen-depleted blood into the heart's right ventricle.
Both our digestive and vascular systems are closed-loop, meaning that they are complete onto themselves and none of their contents (should) spill out into the rest of the body. There is some interaction between the two - every organ in our bodies, including the digestive organs, is lousy with blood vessels but, in general, the systems remain distinct from one another.
Plants' transport system is also closed-loop and vascular.
However, in naming the components of plants' food transport system, the vocabulary more resembles the animals' cardiovascular system terminology than their digestive systems'.
There's a good reason for that.
Overview of the Plant Transport System
Upon first study of the subject, you may have been surprised to discover that plants' food transport system is vascular.
It's so-named because it all comes down to the definition of vascular: 'concerning the circulation of fluids' - which both animal and plant vascular systems do, even though, in plants, the vascular system carries food.
By definition, animals' food transport cannot be vascular because their food is solid or, at least, semi-solid.
Transpiration is the proper term for water transport in plants. The process starts when water on the surface of leaf cells diffuses out of the leaf, calling forth water in the xylem to replace it.
Xylem: one half of a plant's transport system.
Xylem cells string themselves together to form a continuous tube that reaches from the plants' roots to its furthest extremities. A drinking straw would be an apt analogy of the xylem's build and function. Water in the xylem is drawn upwards in four different ways:
- by capillary action: water molecules' attraction to the xylem surfaces propels it forward/upward
- water's stronger attraction to other water molecules bind them together, in effect creating an upward-travelling train of molecules drawn forward by a lesser attraction to the xylem surface.
- by transpirational pull: the water expelled by the leaves' stomata creates a vacuum effect that draws 'replacement' water upwards
- by root pressure: water moves into the root from the soil via osmosis. The higher water pressure in the root pushes water upwards
- by pressure-flow: as the solute-heavy products of photosynthesis travel downward through the phloem, that pressure forces the lighter-solute fluid in the xylem upwards.
Phloem: the other half of plants' transport systems.
The phloem transports the products of photosynthesis from the leaves, where that process takes place, to the parts of the plant where photosynthates - sucrose, in particular, are needed.
This half of the transport system has a more complex build consisting of sieve elements (conducting cells), parenchyma cells - with both specialised and albuminous cells, along with a variety of unspecialised cells and, finally, fibres, sclereids and other supportive cells.
Phloem is necessarily hardier than its xylem counterpart because it transports a relatively heavier load.
Also, learn about the hormones that regulate every aspect of plant life...
How Plants Take In and Transport Minerals
Man cannot live by bread alone.
Leaving aside the religious roots of this idiom, let's look at it from a nutrition perspective. If we ate only bread, we would soon be plagued by a host of illnesses caused by nutritional deficiencies. That suggests that a single type of food is not enough to sustain health and, ultimately, life.
The same is true for plants. Water and the products derived from photosynthesis are not enough to sustain plant life. They need a variety of minerals from the soil they're planted in.
The trouble is, mineral ions, charged particles that they are, cannot penetrate roots' cells to be passively absorbed, as water is, via osmosis. Also, the minerals in the soil are less densely concentrated than in the roots, meaning that, if such a flow were possible, minerals would travel from the roots to the soil.
Such a mechanism would harm the plant rather than sustain it.
Adenosine triphosphate or ATP drives the active absorption of minerals into epidermal cells' cytoplasm, a process further aided by root hairs actively pumping ions into those cells.
Specific cells called endodermic cells are embedded with transport proteins. These proteins function as a checkpoint that controls the amount and type of solutes the plant will consume. Note that transport proteins will allow some solutes to pass through the cells' membranes but not all of them.
Once the mineral ions have reached the xylem, the process of translocation begins.
Contrary to animals' digestive system - a direct path from one organ to the next, plants' vascular system carries the food to where it is needed: apical buds, new and young leaves, flowers and so on. Of course, that doesn't mean the rest of the plant starves while those areas feast. They simply get a higher dosage of nutrients to see them through their growth.
Plant transport is essentially a one-way system. Once mineral ions are absorbed, they cannot flow back out. Water travels up the xylem and the products of photosynthesis flow 'downward', into the plant through the phloem.
Join the discussion: where do vascular bundles fit in a leaf's structure?
The Distance Question
All plants are autotrophs, meaning they create their food and feed themselves. Granted, we assist sometimes by watering them and providing plant food but they do the bulk of the work themselves.
For some plants, such as grass, flowers and ground-covering vines, it's relatively easy to get food from the soil to their extremities because it doesn't have to travel so far.
Shrubs, bushes and trees have a harder row to hoe; the farther water and nutrients have to travel and the more energy sinks they have to feed - think of all the leaves on a tall tree, the more complex the transport system has to be.
How do plants manage?
Neither diffusion nor active transport work if the part of the plant needing food is far away from the food source. One reason is that diffusion is a random process wherein food and water travel from areas where they are highly concentrated to places where the concentration is lower. What would have to happen if the substance required is in a high concentration area?
Diffusion requires no energy but active transport does because it forces molecules from areas of low concentration to high concentration ones. Besides, those processes can only take place over very short distances, say, at the cellular level.
To increase the rate of flow for water and nutrients, plants use their system of bulk flow. This system of xylem and phloem, paired into vascular bundles, is the dominant transport system for water and nutrients throughout the plant.
Three phenomena make this system possible: adhesion, cohesion and surface tension.
Adhesion describes water's attraction to the surface of the xylem, cohesion explains how water molecules stick together and surface tension holds those molecules together as they travel through the plant.
Supplemented by other processes, such as diffusion and facilitated diffusion, as well as active transport that uses energy to push molecules from low- to high-concentration areas, plants ensure that they have the means to consume the food that they produce.
Now, read on to discover more about photosynthesis and plant growth.