As you've been preparing to sit GCSE Biology, you are surely aware of the breadth of the subject - your lessons have touched on everything from cell biology to treating and preventing disease. Amidst review study materials, all of your coursework and on exam board past papers, plant biology is a relatively small sliver.
Since you selected Biology as your mandatory science component, as a single science or as a part of your combined science exam, you must have some interest in the subject.
But maybe you're not particularly interested in plant biology. Perhaps you have your sights set on a career in medicine - either veterinary or human. Indeed, the whole topic of plant functions and structures may drive you mad.
Nobody could fault you for feeling that way but your Superprof urges you to set those notions aside because, like it or not, you will be tested on aspects of plant biology. That's why it's best to brush up on those concepts.
Superprof helps you do that.
Leaf Structure and Function
Leaves come in all shapes and sizes, from the spines on a cactus to the astounding size some lily pads can reach. Regardless of how big or small and no matter their shape - from the scales that feed asparagus to the needles that pine trees grow, they all serve the same purpose and they all work the same way.
Leaves' primary function is to produce food.
Of course, leaves feed a variety of organisms from insect to human but that brand of food production is secondary to making food for the plant itself.
The chlorophyll-laden leaves make them uniquely capable of absorbing the light needed to make the food plants consume. Even other-shaped leaves, those that are not relatively flat and blade-like fulfil this function. Here's how the process works.
The leaves take in carbon dioxide from the air; it mixes with the water present in the leaf. Photon energy - the 'photo' part of the photosynthesis process splits and rearranges the molecules, resulting in a 'food' (sucrose) and oxygen, which the plant does not need and thus, expels.
The equation for this process looks like this: 6CO2 + 6H2O + light energy = C6H12O6 + 6O2 - where the resulting six oxygen atoms are expelled and the sucrose molecules are transported throughout the plant, to feed it.
Leaves don't only produce sucrose; they also produce starch. Plants don't immediately consume everything they produce through photosynthesis; these carbohydrates store energy for later use - say, for new growth or as a last burst of sustenance before senescence.
To fully understand how leaves can make and transport food, you have to know how they're built.
The outermost layer of a leaf consists of a waxy substance called a cuticle. It protects the leaf by helping it keep water sealed in, as well as keeping leaves' delicate inner components shielded from the environment.
The cuticle protects leaves from predators by making the leaves both less accessible and harder to eat. If a hardy herbivore should claim a particular leaf as its meal, the cuticle will further ensure that digesting what it eats will be difficult.
Under the cuticle layer lies the epidermis. It is rather thin, only about one cell layer deep, sometimes thinner on the abaxial (lower) side than the adaxial side. And, sandwiched between those two epidermal layers, we find the mesophyll - where the real work of leaves takes place.
There's much more to be said about the anatomy, structure and function of leaves; why not dive deeper into the subject?
Photosynthesis and Plant Growth
As touched on in the previous segment, photosynthesis is the process by which light energy is converted into chemical energy. Although not all species conduct photosynthesis in the same way, the process always starts when light enters chlorophyll-laden reaction centres.
Some of this light energy splits water molecules into their component atoms. The freed hydrogen is used to create two types of energy stores: adenosine triphosphate (ATP) and nicotinamide adenine nucleotide phosphate (NADPH). These are only for short-term energy storage.
Plants' long-term energy storage vehicle is sugar, which is produced in the Calvin cycle. This process does not depend on light energy; rather, it is a chemical reaction.
As you well know, leaves naturally take in carbon dioxide. The Calvin cycle involves incorporating that gas into other organic carbon compounds already present, after which ATP and NADPH essentially undertake a refining role that ultimately results in the process's end product: glucose.
This is, of course, a simple breakdown of the photosynthetic process. For an in-depth look at every step, we refer you to our full-length article.
Transport in Plants
Tempting as it might be to believe that photosynthesis takes place all over the plant or that the food produced in that process is only consumed by the leaves that make it, those ideas are a far cry from how plants shuttle critical nutrients around.
Besides the sugar plants manufacture, they also need minerals. They're not a photosynthate; they have to be absorbed in the roots and make their way through the plant. Also, even though leaves contain some water, the rest of the plant - the stems and stalks and trunks and roots have to have water, too.
Enter the vascular system, a network of tube-like structures that work in tandem to deliver water and minerals to plants' farthest reaches and carry nutrients everywhere they're needed.
These vascular bundles consist of xylem and phloem pairs. The xylem transport water and minerals upwards from the roots while the phloem move photosynthesised product out of the leaves and throughout the plant.
Xylem is made up of two types of non-living cells: tracheids and vessel elements. Every plant has tracheid cells but only the angiosperms' xylem contains vessels.
By contrast, phloem is made up of a collection of living cells including conducting cells, fibres and sclereids, companion cells and two different types of parenchymal cells. Phloem's comparatively complex, sturdy structure is due to this 'return' part of the system bearing more of a load.
How much of a load? If you've ever seen or felt tree sap, you've seen/felt the substances transported in the phloem. Wouldn't you agree that such a transport system has to be durable?
Transpiration causes a negative pressure - a tension to form, that draws water and minerals up through the xylem. However, the phloem demands translocation, a system of positive pressure that essentially pushes the product along. This process of phloem loading and unloading is the essence of translocation.
Phloem doesn't simply decide things on their own and there's a whole lot more to the process. You will need our companion article to understand it all.
Explaining Plant Hormones
When do flowers know to bloom? How does the grass turn green? Why do trees grow so tall, bear fruit and shed leaves? The answer to all of these questions is hormones.
Hormones - or phytohormones, as they're also called when discussing plants, control virtually every aspect of a plant's life, from its embryonic development to its dormant stage (senescence). How big a plant grows, how it reproduces, how it defends itself against predators... all of these aspects and others are regulated by hormones.
Unlike animals, plants do not have centralised glands that make and store hormones until they're needed - and then, they're sent out. Plants have to rely on more localised hormone production in the cells themselves. These cell-produced hormones produced are simple chemicals that can move around fairly easily.
Of course, if a hormone has to travel some distance, it can hitch a ride on the plant's vascular system, in the phloem sieve tubes. Otherwise, cytoplasmic streaming is the most efficient way for a hormone to get to where it needs to be.
For having such an important function in plant development and growth, phytohormone activity seems pretty disorganised. But that's only until you discover that there's no hormone free-for-all going on; there is order amidst the flow.
Every cell has receptors specifically meant for a corresponding hormone. As an example, guard cells that flank a leaf's stomata have receptors for nitric oxide - a hormone that signals for defence and other hormonal responses but they have no receptors for growth hormones.
Some cells have multiple receptors for certain hormones, meaning that that hormone's signals are stronger, but may have only one receptor for another type of hormone, meaning that process is not as urgent or vital - but must nevertheless be carried out.
Science has identified nine classes of hormones, along with other regulators such as:
- peptide hormones that play a big role in growth and defence
- polyamines impact both types of cell division - mitosis and meiosis; they also play a role in (programmed) cell death and senescence
- triacontanol helps to stimulate growth; it is integral to waxy cuticle leaf covering
- nitric oxide: besides signalling defence responses, it also helps regulate organelle function in the cells
Our main article about plant hormones describes each class of hormone in-depth: their composition and uses in various plants.
Before we close this article out, let's present just one amazing plant hormone fact.
Salicylic acid, an important class of plant hormone, is found in tree bark - particularly in willow bark. For centuries, people have chewed on the bark of trees, not out of some strange inclination but because it offered relief from pain.
You can also find salicylic acid in skincare products, especially those meant to treat psoriasis and acne.
Feel free to rush to your medicine cabinet to read your skin product labels; all of this information will still be here when you get back.