Anyone who has studied biology or watched a nature documentary understands that photosynthesis is the process whereby plants create food for themselves. In these times of climate concern, most people focus on the oxygen production aspect that benefits humans the most rather than the amazing feat of an organism creating food and feeding itself.
Rather odd that that oxygen is the photosynthetic process's waste product, isn't it? Doesn't that truly drive home the idea that one organism's trash is another's treasure? Nay, not treasure; a necessity for life!
Even if you're neither an eco-warrior nor a fan of biology, it's not hard to guess the meaning of photosynthesis, especially if you know words. Photo, from Greek, meaning 'light', added to synthesis - combining elements to make a whole, informs us that this light-driven process combines elements to create a compound element.
What are those elements? How does this process happen and what regulates it - besides the presence of light, of course? How does a light-driven process cause plants to grow?
As you've likely been studying plant structures and their functions ahead of your GCSE exams, you may have some idea but, perhaps, some concepts might still elude you.
Let Superprof break it all down for you.
An Overview of Photosynthesis
Autotrophs use photosynthesis to produce chemical energy, in the form of sugar and carbohydrates, from light energy. Although plants are known as the primary users of this process, plenty of other organisms make their own food in a similar way.
Whether cyanobacteria, salamander or the fern in your sitting room, the photosynthetic process starts with light.
Photosynthesis is a series of chemical reactions, for all that its general equation makes it appear as though it were a single, repeated reaction. That formula is: 6Co2 + 6H20 + light energy = C6H12O6 + 6O2 (six carbon dioxide molecules plus six water molecules, subjected to light energy, resulting in one molecule of sugar and six oxygen atoms).
For the plant, the six oxygen atoms that result from the process are a waste product to be expelled through leaves' stomata. The sugar molecule is the desired end-product; these molecules may undergo further processing or be consumed in that form. We'll cover those aspects more in later sections of this article.
Six is the magic number.
You'll note that the formula is written to reflect 6 molecules of carbon dioxide and six of water. We didn't choose that number arbitrarily. Glucose is a comparatively large molecule that requires that amount of gas and liquid to produce it.
Different plants use different actions to get the desired outcomes of their photosynthetic processes. For instance, plants with plenty of water available to them employ a process called C3 while for plants in more arid climates, such as cacti, C4 photosynthesis is the more efficient process.
C3 photosynthesis relies on an enzyme called ribulose bisphosphate (RuBP) carboxylase to drive the reaction with carbon dioxide. C4 photosynthesis employs a phosphoenolpyruvate (PEP) carboxylase enzyme. Another difference between the two processes: photosynthesis does not happen in the same cells.
Speaking of cells and photosynthesis, how familiar are you with leaves' functions and structure?
The Difference Between Photosynthesis and Cellular Respiration
Some students get confused over these processes because they both result in molecules that are used for food. Let's set the record straight before moving on.
Organisms that perform photosynthesis also engage in cellular respiration. However, one process makes sugar and oxygen while the other releases energy, water and carbon dioxide through its use of sugar and oxygen.
The molecule capable of storing energy is sugar, which is produced through photosynthesis. Cellular respiration converts sugar into products the organism - the autotroph can use.
Hopefully, that explanation made it easier for you to distinguish one process from the other. If not, don't hesitate to let us know in the comments section.
Now, before we carry on explaining the process of photosynthesis, let's get familiar with the language associated with it.
Photosynthesis Glossary of Terms
As you review components of the photosynthetic process, you will likely run across some familiar terms: oxygen, carbon dioxide, glucose and others. These are terms commonly bandied about in a variety of disciplines from chemistry to diet and nutrition; even athletes concern themselves with their glucose and oxygen levels.
It's nice to have a sense of familiarity; of knowing you're not entering a field of study where you have absolutely no point of reference, isn't it?
Once you get past those well-known words, though, you might end up scratching your head and running for your dictionary to tease out the meaning of some of the more specific terminology related to photosynthesis.
Maybe you won't have to, after reading this article.
The following are some of the terms you need to know to understand how the process of using light energy to power chemical reactions works.
- autotroph: an organism that makes the food it needs to grow and reproduce
- Calvin cycle: a set of light-independent chemical reactions
- carbon fixation (carbon assimilation): the process by which inorganic carbon converts to organic compounds
- chlorophyll: the reason leaves are green; the pigment used in photosynthesis
- chloroplasts are the organelles in the plant cells where photosynthesis happens
- light: a type of electromagnetic radiation
- lumen: the region within the leaf where water molecules undergo their separation into hydrogen and oxygen
- photosystem: a cluster of molecules including chlorophyll that harvests light energy for the photosynthetic process
- oxidation: losing electrons
- reduction: gaining electrons. These two processes generally complement each other
- stroma: a watery fluid contained in the chloroplast
- thylakoid: a piece of chloroplast shaped like a disc
- granum (pl: grana): a stack of thylakoids
This is, of course, an abbreviated list of terms related to photosynthetic processes. It includes none of the enzymes, isomers or molecules needed for photosynthesis, nor does it detail any of the plant hormones that regulate the process.
Still, this short list of words should see you through the rest of this breakdown of photosynthesis.
Photosynthetic Light Reactions
These processes take place when light is present and rely on light energy to drive them. The ongoing reaction spurs the creation of ATP.
The very first step is to split water molecules into their component parts, hydrogen and oxygen. For the hydrogen atoms, the break is clean but the oxygen atom loses two electrons in the process. These electrons take their place on the thylakoid membrane in the chloroplast.
This process sets up an electron flow dubbed cyclic phosphorylation, which generates ATP (and P700, a type of photochemical reaction centre).
These excited electrons flow in a loop, and they could flow to another electron chain to create NADPH for later use in making carbohydrates. In that circumstance, P700 loses an electron from a different photochemical reaction centre called P680.
In that step of the process, an excited electron creates a proton gradient between thylakoids and the stroma. Noncyclic photophosphorylation is the outcome of this reaction.
The continuous supply of water ensures the availability of electrons to keep the process going. To make two molecules of ATP, four photons and two electrons are used.
Keep in mind that this is a single sequence of what is a continuously looping and ongoing process.
Photosynthetic Dark Reactions
It might be tempting to think that light reactions happen during the day and dark reactions must take place at night. That's not exactly how things work.
Light reactions must absolutely have light because they need light energy to drive them. Dark reactions also take place when the sun shines but these processes can happen whether there is light or not; they don't need that energy to function.
This activity takes place in the chlorophyll-laden chloroplasts; specifically, in the stroma. It is the process of carbon fixation, otherwise known as the Calvin cycle. The ATP and NADPH produced in the light reaction are essential to these processes.
Simply put, the ATP makes energy available while NADPH gives up the required electrons for carbon fixing, in which a 5-carbon sugar combines with carbon dioxide to result in a 6-carbon sugar. This molecule then gets broken down into fructose and glucose.
Some organisms may then further the process, resulting in sucrose.
Throughout this article, you've encountered lots of technical terms and an exploded view of photosynthesis, the process whereby plants and other autotrophs make their food. Wouldn't it be nice to have some key takeaways?
Plants and other single- and multi-celled organisms synthesise their food using light energy. It is generally a two-step, ongoing process in which light energy is used to make an organic compound called ATP and a chemical compound called NADPH.
During this process, electrons are given up (oxidation) and gained (reduction); a constant supply of water is necessary to ensure that there will be enough oxygen atoms from which to gain electrons.
With the continuous production of ATP and NADPH, the organism can maintain its Calvin cycle, the process of fixing carbon. Carbon fixation involves combining one carbon dioxide molecule with a 5-carbon sugar molecule, which yields a 6-carbon sugar.
Remember we said six is the magic number?
This carbon-rich sugar may then be further processed into glucose and fructose, and ultimately into sucrose.
There, doesn't that make the concept of photosynthesis much simpler to grasp?
Now, discover how water and nutrition travel through plants' transport systems...