There are many reasons to be interested in photosynthesis: Maybe you are just taking the subject at school through? Or you want to brush up on what you once learned in biology class. Maybe you just want to understand how light leads to growth in plants.
Then this article is just right for you.
Because maybe you have asked yourself:
Why is a plant the way it is?
To approach the answer, let’s look at photosynthesis from a slightly different angle here:
1) Photosynthesis is the splitting of water by light
What? Sounds strange, or? But one thing at a time.
In school you usually learn that photosynthesis makes sugar and oxygen out of light energy, water and carbon dioxide.
This is correct, but let’s start a bit differently:
When you live on a planet that is hit by huge amounts of light energy, like Earth, it’s certainly a good idea to use that energy to build molecules. Before this was possible by evolution of living beings, organisms (e.g.B. Bacteria) namely to resort to inorganic substances such as iron sulfide or other as a form of energy.
But how can solar energy be used to build cells?? Well, for example by somehow managing to make longer-chain carbon skeletons from the carbon dioxide present in the atmosphere. That’s what pretty much everything in a cell is made of.
That’s what photosynthesis is all about: building chains (sugars, for example, are chains of this kind). rings of carbon) from individual chain links (z.B. from carbon dioxide).
So far, so good. But how do you do that? And what does it take?
The answer is quite simple: you need a form of energy and machinery that allows carbon dioxide to be incorporated into a carbon chain.
The sunlight itself does not help at first: When photons, i.e. light particles, hit carbon dioxide, nothing happens. In any case, nothing that would help in the synthesis of carbon chains. So in the air, when the sun shines, something is not simply built up.
To make chemical bonds, you need at least two things: first, favorable conditions, and second, something often described as "reduction power. What is meant by this?
Favorable circumstances here means nothing else than that a chemical reaction can actually take place by itself. This is basically a matter of probability.
For example, a house of cards almost collapses by itself. The circumstances are therefore favorable, that exactly this happens.
Conversely, it is more than unlikely that a collapsed house of cards will rebuild itself by wind, right?? This is why chemical reactions have a direction, so to speak, that is, they follow a path in which disorder increases in the aggregate.
This is not only true for kitchens or children’s rooms, but for the whole universe. The famous heat death of the universe is simply the most probable state in the end.
Then how can there be any order at all? Now, by paying for order in the small with greater disorder in the large.
So in nature, every increase of order, for example the synthesis of a protein molecule from single amino acid building blocks in a cell, is paid for with an increase of disorder, namely more heat in space.
But that doesn’t matter for the time being, because the latter is huge and we surely have a few billion years until everything is nice and evenly warm.
Back to photosynthesis:
We have said that to build carbon chains, you need "favorable circumstances" and not just sunlight.
So somehow you have to pay for this buildup of order with disorder.
The currency, i.e. the "paying" in the process, is nothing other than a reaction that takes place voluntarily and with the release of heat (when, for example, you let a ball roll down a slope or burn gasoline).
If you couple this with a non-voluntary reaction (here the building of carbon chains), you can force the non-voluntary reaction to take place!
This is what is meant by "favorable circumstances": The coupling of a reaction that takes place voluntarily with one that takes place involuntarily.
And now pay attention: In living cells, the "currency" described above is a molecule that can drive just about anything: Adenosine triphosphate, or ATP for short!
ATP is a fuel: when it reacts with water, work can be done in the cell (e.g. movement or building something).
Let’s keep in mind: To increase order in the plant cell (e.g. to build up molecules for the next cell division), the fuel ATP is needed. We will see later how photosynthesis can produce exactly this ATP.
In many cases, however, in addition to this form of energy (or, to be more precise, in addition to this generation of order on a small scale and disorder on a large scale), electrons are needed to form new bonds. So now the above described reduction force comes into play.
What is "reduction force?
The reduction force describes the readiness of a substance to give up electrons. Some substances give off electrons very easily, for example iron. It then rusts. Other substances (e.B. Gold) hardly ever give off electrons, they rather take up electrons easily compared to other substances. Such and other "noble metals" therefore do not rust or corrode. only under very special circumstances.
The electrons of a substance are also given off more easily if they are reduced by other energies (z.B. Light) can be stimulated. They then reach a state from which they can separate from their original atom and transfer to another atom.
And here’s the thing:
This is exactly what happens when light hits water during photosynthesis: The electrons of the water are transferred to other molecules in the green chloroplasts. Water becomes: oxygen!
This doesn’t just happen, but requires complicated machinery in the cell. Maybe you understand now, why photosynthesis is the splitting of water by light!
Let’s make clear again what this is all about:
- To build compounds, that is, to grow, you first need a fuel that helps make improbable reactions possible: In the cell, this is ATP. All energy stored in the form of ATP ultimately comes (we’ll ignore certain bacteria that can get energy from minerals) from the sun.
- Furthermore one needs: Reduction force to transfer electrons, i.e. to form new bonds between atoms.
- Both ATP and reducing power are produced in plants by photosynthesis, as light raises electrons to a higher energy level. This does not happen by itself, but needs u.a. the thing that makes plants green: chlorophyll!
- When water is split by light, electricity is generated: Chloroplasts are therefore machines, so to speak, that generate electricity from light.
Have you ever thought about what a solar cell on the roof of a house does? It generates electricity from sunlight! Solar cells don’t look very pretty, and they don’t look much like plants, but in fact a leaf in sunlight does something very similar: it generates electricity (namely the flow of electrons) that can be used to do work or to charge batteries (they look a little different in the cell, of course).
The reason why forests and meadows are so green is, as you probably already know, that a molecule (the chlorophyll) in the leaves can absorb certain parts of the white sunlight. But what is a molecule that only absorbs parts of the white light?? A dye! √
The part of the light that is not absorbed by the chlorophyll, but is reflected back, is green!
Why is this so? Couldn’t the entire color spectrum of light be absorbed by plants? Such leaves would indeed be black to our eyes, and in principle there is nothing to prevent such a thing from occurring in nature. Obviously, the system that plants use today developed at an early stage of evolution, using especially the red and blue parts of light. The chlorophyll molecule may not be ideal, but it is sufficient. By the way, certain algae species use molecules that can absorb light exactly in these "gaps".
We talked at the beginning about fuel and reducing power being the two essential elements for creating growth.
We have also claimed that the reduction force is generated by light, which raises electrons to a higher energy level. What does the electricity do in this story? Well, it flows. And just as a river always flows downhill, so do excited electrons: Downwards, i.e. to a less excited level. In the process, as we said, they do work, just as, for example, a river can be used to drive a water mill.
We won’t go into detail here about the complicated molecules involved in these processes for now: Just as it is more important to understand the principle of a solar cell than to know its exact chemical composition, what you have read will give you all the essentials you need to understand how sunlight can be turned into wood, for example.
But, wait: Is that all??
By no means, because what we have discussed so far is often described as a "light reaction" to distinguish it from another mechanism of photosynthesis, called the "dark reaction".
2) Electricity can also be used to work in the dark?
The so-called "light reaction" is indeed light dependent. If, for example, a plant leaf is darkened, oxygen is immediately no longer produced from water. Nevertheless, the energy stored in the form of ATP and reduction force (i.e. excited electrons) can be used to build molecular chains. So the whole thing works (at least theoretically) also in the dark (therefore "dark reaction"), but this does not mean that it MUST be dark to do so! More about this in a moment.
What is the dark reaction all about??
Something amazing happens here: a gas is transformed into solids. So solid material is produced from the air, so to speak. A process that even today is a formidable challenge even for complicated processes and machines (> see z.B. Haber-Bosch process). The gas we all know is, of course, carbon dioxide. We breathe it out (and in) all the time.
In the dark reaction, a plant cell now manages to incorporate this gaseous carbon dioxide into a carbon chain, to "fix" it, so to speak, and make it part of a solid (sugar, for example). So ultimately, all the carbon that is in the sugars, proteins, or fats of a (z.B. Your!) body, from the carbon dioxide in the air.
While the process is extraordinarily complicated in detail (and is called the "Calvin-Benson cycle" after its discoverers, you will encounter it everywhere if you read anything about photosynthesis) but the step that is essential for understanding it is handled by only one, but very astonishing molecule, an enzyme with the long name "ribulose-1,5-bisphosphate carboxylase/oxigenase", which is often abbreviated to "RuBisCO".
Amazing for several reasons: First, it is the most abundant water-soluble protein on Earth, and second, it can incorporate a gas into a solid, as noted. But above all, from today’s point of view, it can be said that it was enormously underestimated by the very researchers who investigated it in the beginning.
How did this come about? Well, soon after the discovery of RuBisCO, it was found that the enzyme was an astonishingly poor catalyst for the fixation of carbon dioxide: enzymes accelerate chemical reactions, and RuBisCO is apparently not particularly good at this. At the same time, and to make matters worse, it also fixes a significant amount of oxygen instead of carbon dioxide! Strange, or? This process is also called "photorespiration", so to say "light-dependent respiration". The plants have just produced oxygen, and now they consume it again? What sense should this make??
In fact, this process seems enormously wasteful. From today’s point of view, the best way to understand this is to assume that RuBisCO arose and was evolutionarily optimized at a time when the oxygen concentration in the atmosphere was comparatively low.
Apparently, the price of this waste seems to be tolerable for many plants, others have developed mechanisms to compensate for this disadvantage in an oxygen-rich atmosphere by some biochemical tricks. Such plants are called "C4 plants" or "CAM plants", depending on the biochemical pathways the plants have chosen to follow.
So one finds here, as incidentally in many places in nature, that the phenomena are expressions of their history and by no means ideal (i.e., as an engineer with sufficient knowledge might have designed them to work). They are often a compromise that has to be made once a path has been taken and conditions change. To choose an appropriate image: You also arrive with a used car that has been patched up a bit, albeit perhaps a bit slower.
Crazy world: The dark reaction can only take place in the light!
While whole generations of students have dutifully learned that the described "dark reaction" can (or even must) take place in the dark, the Calvin cycle does not work at all. ), it is now known that the opposite is the case: rather, the Calvin cycle does not work at all in the dark or at night, u.a. because the activity of the enzymes required for this depends on the reduction force obtained in the light reaction!
In the test tube, some steps of the cycle do work without light: if you simply add the reducing power in the form of a dissolved salt (called NADPH, but that is not important for the moment).
What do we learn from this? Research is always in motion and what was true yesterday is no longer true today or tomorrow (loosely based on Hannes Wader).
We will now turn to what might be called an anatomical part of photosynthesis, because we have already understood the following:
- Water is split by light with the help of photosynthesis. It produces oxygen (which we breathe!), hydrogen (whose electrons are available as a reducing force for the synthesis of carbon chains), and ATP (as a kind of universal energy currency).
- In a subsequent step, carbon dioxide is incorporated into pre-existing carbon chains by an enzyme, RuBisCO. Light is also necessary here, contrary to what the name "dark reaction" suggests.
- So we have understood what is necessary to turn sunlight into something that grows!
- We have understood chlorophyll a little better as a dye that can capture light and somehow pass on that energy in the form of reductive power and ATP.
But how does it look more exactly in the plant cell, when light is transformed into another form of energy (you remember the solar cell)??) will?
Don’t be afraid, no complicated formulas are coming now. But it will become clear to you what the principles are for the conversion of light into another form of energy.
Namely, we can qualitatively understand what happens when a chlorophyll molecule is excited:
An example from music helps us here: Maybe you have heard of the phenomenon of resonance? This is about vibrations, oscillations, being put into the air, for example when a guitar string is struck and then swings to and fro. The vibration is transferred from the string to the air, which in turn makes your eardrum vibrate in the ear, where the whole thing is then perceived by the brain as sound.
Oscillations can be transferred from one system to another in a practical way. But this system has to be able to swing as well! Such systems have a so called "natural oscillation", so once you push them, they oscillate with a typical frequency (for example a swing).
Resonance occurs when this system is excited with its natural frequency, i.e. when the swing is pushed exactly in the rhythm in which it would oscillate by itself after being pushed once. Then the rocking system will oscillate much stronger than with any other excitation frequency. That’s why you don’t need much power to bring a fat person up on a swing: Just nudge a little bit in the rhythm of the swing!
We come back to the guitar string again. For them the same is true: if you, let’s say with a tuba, produce a tone (i.e. an air vibration) that has the same frequency (i.e. the same tone, for example a low A) as the natural vibration of the guitar string, the guitar string will start to vibrate with its natural vibration without our intervention. It would then probably buzz and generate a low A itself.
What does all this have to do with photosynthesis??
Well, it’s the same here: photons ("light particles") and electrons also oscillate! As you probably know, monochromatic light for example has a very specific frequency. Electrons also have one, depending on where and in which molecule they are located.
The electrons of the chlorophyll are now excited to oscillate by light of a certain wavelength (and thus also frequency) due to the physical-chemical properties (which we will not go into here: if you want to know more, read something about "conjugated double bonds") of this chlorophyll molecule. Namely, as we have already seen, of blue and red light.
Similar to the way the fat child on the swing is pushed into the next treetop (no! Do not imitate!) by nudging it with the natural frequency of the swing, electrons can also reach a physically higher level through light. They actually also move away from the center of the atom in the process.
From "up there" they can now "jump" to places which were not accessible to you before.
This is exactly what happens when we talk about "reduction power" being generated. It would be comparable to someone who has been brought up the tree by the swing and can now jump with a good thump onto a seesaw on the ground, where the next child can then jump (No!)! Also do not imitate!) as a rocket shoots into the air.
Very practical: light energy funnel
How do you actually have to imagine this, is every light particle actually immediately converted into another form of energy?
We encounter here an interesting mechanism that serves to capture light and transport it from A to B within plant cells. In doing so, plants possess something remotely reminiscent of a funnel, one might even say a trap. A light trap!
Photons, i.e., light particles of different colors, are "captured" by different dyes; in addition to chlorophyll, there are z.B. so-called carotenoids (not only does it sound like that, but it actually gives the carrot its characteristic orange appearance) before.
These dyes act as antennas, so to speak, absorbing photons and then passing on the excited state of their electrons to neighboring molecules. This happens until they land at a pair of chlorophyll molecules in the so-called "reaction center", which then conserves the original energy of the captured photon by irreversibly transferring its own electrons (and not just the excited state) to another molecule.
So the deeper purpose of this light trap is, on the one hand, to be able to capture light with a slightly larger spectrum than can be captured by chlorophyll alone, and on the other hand, a much larger area can be used to deliver photons to the limited number of reaction centers.
The best way to understand this is to imagine what happens when a cloth is stretched out in the rain and a small stone is placed in its center. Where does most of the water end up?!
So, that was a lot of stuff. Therefore, we now repeat the most important things in a meaningful order:
- Light is collected from plant leaves by a type of light trap. The energy of the light particles is used to excite electrons. These electrons are then passed on to a place where they are stably available as a reducing force.
- The reduction force allows carbon dioxide to be incorporated into carbon chains with the help of enzymes and ATP (the "fuel"). So: light enables the multiplication of biomass = growth!
- The electron gap created by the transfer of excited electrons in chlorophyll is replenished by electrons from water. This produces oxygen as a waste product of the photosynthetic splitting of water!
You have now understood the essential mechanisms of photosynthesis and can really join in the conversation! You don’t need to learn any chemical formulas by heart.
Of course, the details of these mechanisms are highly interesting! But they make sense especially when you have grasped the essence of photosynthesis. If, for example, you simply learn the Calvin-Benson cycle by heart, you will soon be lost when it comes to recognizing the beauty and significance of these processes.
After all, if you don’t use this knowledge with an "Aha!!-experience, you will have completely forgotten it after a short time, I bet?
We hope this article has helped you a little bit so that you can still say in a year’s time: "I’ve understood what photosynthesis is all about!"