Basic plant science (photosynthesis,

I was reading a thread on here about optimal CO2 levels when I decided it might be helpful to make a thread about photosynthesis, light reactions, Calvin cycle, and some specifics of plants for those that are interested. I'll try to make it as user friendly as possible.

Lets start off with the basic chemical equation for photosynthesis:

6 CO2 + 12 H2O + photons → C6H12O6 + 6 O2 + 6 H2O
(carbon (water) (light energy) (glucose) (oxygen) (water)
dioxide)

Photosynthesis is a crucial aspect in the lives of almost all organisms whether it be direct or indirect. Organisms acquire carbon skeletons and organic compounds used for energy in one of two ways: autotrophic nutrition or heterotrophic nutrition.

The former means to sustain oneself without eating things derived from other organisms. These kinds of organisms (like plants) produce their organic molecules from CO2 and other compounds from the environment. They are the main source of organic compounds for nonautotrophic organisms (like humans) and are therefore known as producers.

Almost all plants are autotrophs and more specifically, photoautotrophs because they use light energy to synthesize organic substances.

With me so far? Good. Now I mentioned light energy, lets see why this is important for plants.

All green parts of a plant, including stems, have chloroplasts, although the leaves harbor the greatest numbers. What are chloroplasts you ask? They are organelles that absorb sunlight and use it to drive the synthesis of organic compounds from CO2 and water. To understand how, we should understand them better.

There are close to half a million chloroplasts on a piece of leaf with a surface area of 1mm squared. They are found in the interior tissue of the leaf known as the mesophyll. A typical mesophyll cell has 30-40 chloroplasts. A chloroplast has an envelope of two membranes surrounding a fluid called the stroma. Within the stroma is a third membrane (thylakoid membrane) made up of tiny sacs called thylakoids. This is where chlorophyll, responsible for the green pigment on plants, can be found. Here is a picture to help with the anatomy:



Light energy is absorbed by chlorophyll that drives the synthesis of organic molecules in the chloroplast. Review the general equation of photosynthesis again. Glucose is not the direct product of photosynthesis, a 3-carbon sugar is, but for simplification we use glucose. Water is on both sides of the equation because while 12 molecules are used, 6 are produced again.

The net equation would become:
6 CO2 + 6 H2O → C6H12O6 + 6 O2
(it becomes clear that this is the reverse of cellular respiration, the reaction that humans use!)

It is important to note that while oxygen is produced, the O2 molecule given off by plants does not come from CO2, it comes from H20. This allows for the extraction of a positively charged hydrogen ion and its incorporation into a sugar molecule (note the chemical formula of glucose again and how many H's it has).

So what exactly happens in photosynthesis and why is water and carbon dioxide used?

Photosynthesis splits water and transfers electrons along with hydrogen ions from the water to carbon dioxide, reducing it to sugar. This is known as a redox reaction and more about them can be learned here: http://en.wikipedia.org/wiki/Redox .

Since the electrons transferred increase in potential energy as they move from water to sugar, the process requires energy, and is therefore known as an endergonic reaction. This energy, of course, comes from light!

Now that we covered the basics, photosynthesis is not actually a single process but two separate processes called the light reactions and the Calvin cycle. I will cover these processes in detail and they will help you understand the life of the plant in light and dark cycles. I will include pictures to help break down the cycles. Stay tuned for that. I will continue this thread as soon as I get a chance. Some of this might be repetitive to the people familiar with it but it never hurts to refresh your mind and to those that are learning I hope this helps you!

TranceAddictT7 said:

The net equation would become:
6 CO2 + 6 H2O → C6H12O6 + 6 O2
(it becomes clear that this is the reverse of cellular respiration, the reaction that humans use!)

Click to expand...

Note that both plants and humans depend on cellular respiration for energy. That's why we worry about roots getting enough oxygen. It's just that plants make their own sugar while humans have to go out and find it.

taged along for the ride

It is a question of balance.

Blackman's law applies to these equations used to calculate peak photosynthis and each individual is unique in its own ability to fluctuate its motabalism as much as is required, as such they all process photosynsthastate differently in response to the environment.

Therefore if one limiting factor should change the whole "balance" is thrown out of rhythm and the other factors must continually be amended.

Crusader Rabbit said:

Note that both plants and humans depend on cellular respiration for energy. That's why we worry about roots getting enough oxygen. It's just that plants make their own sugar while humans have to go out and find it.

Click to expand...

Yes, excellent point. I will touch up on this as I move along.


On to the two stages of photosynthesis, starting with the light reactions. The light reactions convert solar energy to chemical energy by splitting water to gain electrons and protons (in the form of hydrogen ions or H+). Oxygen is given off as a byproduct. When chlorophyll absorbs light it starts the transfer of these electrons and hydrogen ions from water to an acceptor called NADP+ (nicotinamide adenine dinucleotide phosphate). NADP+ serves a similar function to the electron carrier NAD+ (found in cellular respiration), they differ only by an extra phosphate group in the NADP+ molecule. In essence, the light reactions reduce NADP+ to NADPH by attaching a pair of electrons and a hydrogen ion (derived from water). The light reactions also generate ATP using chemiosmosis to phosphorylate ADP to ATP (more will be discussed on this). In this fashion, light energy is converted to chemical energy in the form of NADPH and ATP.

Let's see how NADPH is made first by understanding the nature of light. Light behaves like it is made up of particles, but it is not. These intangible particles are referred to as photons because they have a fixed quantity of energy. Their energy is inversely related to the wavelength of light, with longer wavelengths having lower energy. Violet light has significantly more energy than red light for example. Guess which one has the shorter wavelength?

So all this talk about light but how does it relate? When light hits matter, it can be absorbed, reflected, or transmitted. Pigments are substances that absorb visible light of different wavelengths. If the wavelengths are absorbed that means they disappear or are no longer visible. The color that we see is the color most reflected or transmitted by a pigment. To explain why you use a green light when you go into your grow room at night: chlorophyll absorbs violet-blue and red light while transmitting and reflecting green light. There are ways to measure the ability of a pigment to absorb light using a spectrometer but it's not relevant to our discussion.

I mentioned the colors we don't see have their wavelengths absorbed and disappear. If you're familiar with physics than you are familiar with thelaw of conservation of energy which states that energy cannot be created or destroyed. So how then did these wavelengths disappear? The answer is they didn't, they were merely transferred to another form of energy. When a molecule absorbs a photon of light, one of its electrons is pushed to an orbital that has more potential energy. Briefly, potential energy is the energy that matter possesses as a result of its location or structure. For example, if you pick up a pot, the pot has more potential energy in your hands then it does in its resting state on the floor. If you let go, the pot will fall back to the floor. Electrons behave in this manner as well. When the electron is in its normal orbital, the pigment molecule is said to be in a ground state. When a photon is absorbed, an electron is boosted to an orbital with higher energy (known as the excited state). Photons are only absorbed when their energy is exactly equal to the energy difference between the ground state and an excited state. In other words, photons are only absorbed in specific wavelengths unique to the absorption spectrum of the pigment molecule.

When the absorption of a photon causes a change from the ground state to the excited state, the electron won't remain their long much like a pot won't remain in your hands once you release it. When pigment molecules absorb light, their excited electrons drop back down to the ground state releasing the excess energy as heat. This explains why things get hot on a sunny day, especially black things that absorb all wavelengths of light as opposed to white things that reflect almost all wavelengths. Pigments such as chlorophyll can not only release heat but light as well. This is how fluorescence works.

So far what I've mentioned happens in isolated chlorophyll, meaning when its not a part of a bigger system in a plant. For obvious reasons, it won't be much benefit to a plant if the electrons in the excited state fall back down and release heat or light in the process. For this reason, a complex process happens to the electrons in the excited state within chloroplasts.

In the next post I will discuss this in detail so we can see how NADPH and ATP are finally produced. After that, I will get to the Calvin cycle which I find a lot easier to grasp. I have heaps of work ahead of me so I will try to update as soon as possible.

I will add pictures to the above post when I can edit posts.

We have seen what happens to chlorophyll in isolation when they are hit by photons of light. However, chlorophyll molecules in plants are organized with other small organic molecules and proteins into complexes called photosystems. A photosystem contains a reaction-center complex as well as a light-harvesting complex. The reaction-center complex is an organized association of proteins holding a special pair of chlorophyll pigments referred to as chlorophyll a. The light-harvesting complex consists of many different pigment molecules bound to proteins. The number and variety of pigment molecules is important in photosystems because it allows for the absorption of light over a large surface area and a broad spectrum. When a pigment molecule in the light-harvesting complex absorbs a photon of light, energy is transferred from that pigment molecule to neighboring pigment molecules until it reaches the reaction-center complex. Here is a picture to describe this scenario, imagine the energy being bounced around.

Once the energy reaches the reaction-center complex, it is utilized by the pair of chlorophyll a molecules. The chlorophyll a molecules use the energy from light to boost one of their electrons to a higher energy level and transfer it to another molecule called the primary electron acceptor. This is a redox reaction because as soon as the chlorophyll electron is excited to a higher energy level, the primary electron acceptor captures it. In non-isolated chloroplasts, an electron acceptor is readily available and the potential energy represented by the excited electron is not lost as light or heat but rather captured. The result of this is that each photosystem functions in the chloroplast to convert light energy to chemical energy that will ultimately be used for creating sugars.

It is important to know that there are two photosystems known as photosystem II (PS II) and photosystem I (PS I). PS II functions first in the light reactions. They both have a reaction-center complex with two chlorophyll a molecules next to a primary electron acceptor associated with specific proteins. The chlorophyll a of PS II is known as P680 because this pigment is best at absorbing light having a wavelength of 680 nanometers (red light). The chlorophyll a of PS I is called P700 also for the same underlying reason. The two pigments in PS II and PS I are similar but are associated with different proteins in the thylakoid membrane. They work together in using light energy to create ATP and NADPH.

This is done in one of two ways: linear electron flow or cyclic electron flow. The next post will briefly explain the two with some pictures to help. You will see in greater detail how photosystem II and photosytem I work together.

Thanks!

<3

this is excellent - takes me right back to Bio-chem class, something I have neglected for way too long. Thanks!

wowzers! i can't believe the basics of plants physiology, finally make it to the "advanced" area...

blessss

I apologize for the great delay. I am on a schedule that rarely allows any free time so please bear with me as I update this thread.
I'm glad this information was useful to those that showed their appreciation.

PetFlora said:

So, at the end of the day is the 64,000 question

What is the optimum amount of watts of each corresponding light spectrum needed to optimize chlorofill A & B production?

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Don't quote me, but I would imagine the broader the spectrum of light the greater the easier it will be to provide the full range of wavelengths that a plant can use. In order to know which ones are absorbed most effectively one would have to isolate the chloroplast pigments from a specific strain and run them through a spectrophotmeter to measure the absorbance at different wavelengths. I tend to imagine that the results would vary slightly by strain.

It would actually be interesting to know which wavelengths cannabis plants (generally) absorb light at the most efficiently. We can then look at those light bulb boxes and see if they really are the best fit for our plants or not.

While we're on the topic here is an interesting article on how plants cope with "too much light" to avoid photodamage. Some of the things discussed here are mentioned. http://www.plantphysiol.org/content/125/1/29 .



Back to the discussion at hand...

We now know that light is the energy behind creating ATP and NADPH and that there are two photosystems in the thylakoid membranes of chloroplasts responsible for utilizing this energy. Let us now break down linear electron flow into steps to be able to see what happens to that excited electron after a photon of light strikes a pigment molecule at a light harvesting complex of PS II.

1. As the electron returns back to its ground state, an electron in a nearby pigment molecule is simultaneously raised to an excited state. This process continues as the energy is "bounced" around until it reaches the P680 pair of chlorophyll a molecules in the PS II reaction-center complex. At this point, it excites an electron within the complex to a higher energy state.

2. The electron is then transferred from the excited P680 to the primary electron acceptor. We refer to the P680 as P680+, attributing a "+" sign to indicate the transferred negatively charged electron.

3. Next, an enzyme catalyzes (speeds up) the splitting of a water molecule into two electrons, two hydrogen ions (H+), and an oxygen atom. The electrons replace the ones that were transferred or lost to the primary electron acceptor from the two P680+ pairs. Due to the nature of the P680+ molecule, this replacement of electrons happens extremely quickly. (P680+ is one of the strongest biological oxidizing agents known to date.) The remaining H+ ions from water are released into the thylakoid space. The oxygen atom combines with a second oxygen atom generated by the splitting of another water molecule, forming oxygen in its diatomic state (O2). Now you know exactly how and where our beloved oxygen that plants produce comes from and why they depend on water.

4. Every transferred electron passes from the primary electron acceptor of PS II to PS I via an electron transport chain. The electron transport chain between PS II and PS I contains: the electron carrier known as plastiquinone (Pq), a cytochrome complex, and a protein called plastocyanin (Pc).

5. The fall of electrons to a lower energy level is what provides energy for the synthesis of ATP. As the electrons pass through the cytochrome complex, H+ ions are pumped into the thylakoid lumen, creating a proton gradient that is used in chemiosmosis. Remember that the H+ ion is a proton, and chemiosmosis is the diffusion of ions across a selectively permeable membrane.

6. As the light energy is transferred via the light harvesting complex pigments to the PSI reaction-center complex, an electron from each of the P700 pair of chlorophyll a molecules becomes excited. The photoexcited electron is again transferred in a similar manner as before to PS I's primary electron acceptor, making the two molecules P700+. Each P700+ is now without an electron and would certainly like to get it back. The electrons that will replace the ones it passed on to its primary electron acceptor will again be passed on from PS II as this cycle repeats itself.

7. The photoexcited electrons with the primary electron acceptor of PS I are passed in a series of redox reactions down a second electron transport chain through the protein ferredoxin (Fd). This chain does not produce a proton gradient and therefore does not produce ATP.

8. Finally, the enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+. Two electrons are used to reduce NADP+ back to NADPH. This restored molecule has a higher energy state than water and can provide the electrons necessary for the reactions of the Calvin cycle. In essence, the whole scheme is one big recycling system!

Here is a diagram to help demonstrate the steps above:



Here is a picture with a proton "pump" included so you can get an idea on how it functions:



And to help you understand chemiosmosis better keep in mind that things will naturally go from higher concentrations to lower concentrations. This isn't the only place where H+ flows down a gradient to generate energy. Many different cells use this phenomenon in many places to synthesize ATP. Read this short and easy to comprehend article about the types of diffusion that exist: http://antranik.org/movement-of-substances-across-cell-membranes/

The next post will briefly describe cyclic electron flow which is an alternative way that light is used in these reactions. I will then break down the Calvin cycle. Lastly, we will look at the basics of C3, C4, and CAM plants, how they differ and why it matters. My goal is for the people who read this to have a basic and proper understanding of plants and (hopefully) be able to obtain a greater appreciation for them. Please stay tuned, as always. Thanks.

Thanks for the biology lesson TranceAddict. Very interesting.

this is a great thread. Thanks TransAddictT7

PetFlora said:

So, at the end of the day is the 64,000 question

What is the optimum amount of watts of each corresponding light spectrum needed to optimize chlorofil A & B production?

Click to expand...

This is an interesting question. When I look at charts of PAR ratings for the visible light spectrum, it appears that the absorbent characteristics of Chlorophyll A and Chlorophyll B have been merged as if they both contributed equally to the plant's total photosynthesis. But my first attempts at getting info on this haven't been very successful. On one web page though I read the statement that most plant leaves contain ten times the amount of Chlorophyll A as they do Chlorophyll B.




This suggests that grow lights should maximize the wavelengths absorbed by Chlorophyll A because you'll get ten times the bang for your buck than using those wavelengths favoring Chlorophyll B. But when I read the specs on growlight LED diode selections, it seems that manufacturers aren't doing this. Any info on this situation would be really helpful.


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