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Table of Contents
Table of Contents
Structure of the Chloroplast
Structure of the Chloroplast
As we investigate the structure of the chloroplast, you will see many parallels to that of the mitochondrion. This is no coincidence - there are two membranes for the same reason and there will be transport across membranes in similar fashions as what we saw in cellular respiration.
There is a lot to a chloroplast, certainly more than is shown in this first image. However, I want to highlight the basics and cover the similarities to what we have seen previously.
The Double Membrane and Intermembrane Space
The Double Membrane and Intermembrane Space
Just like the mitochondrion we've seen before, chloroplasts have a double membrane. This is a relic of their evolutionary history - according the the endosymbiotic theory, the mitochondrion and the chloroplast both originated as simple prokaryotic organisms that were engulfed by a eukaryotic organism (it is important to note that this is not the same organism for the chloroplast as for the mitochondrion). There is other evidence such as their own DNA and ribosomes as well.
This double membrane is important because it effectively creates another barrier across which solutes may cross (or be prevented from crossing). This has essentially created a bubble within the larger bubble that is the organelle.
Because there are two membranes, there is, of course, a space between the two membranes known as the intermembrane space.
The Stroma, Thylakoids, and Granum
The Stroma, Thylakoids, and Granum
The stroma is the fluid found within the innermost level of the chloroplast. It is analogous in location to the the fluid found within the mitochondrial matrix to give you more context as we look into the specific reactions of photosynthesis.
The thylakoid is a membrane-bound compartment (a flat sac, essentially) found within the stroma... This means that this is a bubble (the thylakoid) within a second bubble (the inner membrane) within a third bubble (the chloroplast) within a fourth bubble (the cell). Yes, that is confusing, and it just matters because there are a lot of layers here.
It is important for chemical reactions to occur within the proper compartments. Specifically, the thylakoid compartment is home to the light-dependent reactions of photosynthesis as we will see shortly.
The thylakoid membrane is where chlorophyll is found - the pigment that absorbs light and makes plants green (more on that soon). The inside of the thylakoid is known as the lumen,
Finally, the grana (the singular form is granum) are stacks of thylakoids. These look like a big stack of pancakes, and each pancake is a thylakoid.
Purpose of Chlorophyll
Purpose of Chlorophyll
Chlorophyll in Chloroplasts
Chlorophyll in Chloroplasts
As mentioned, chlorophyll is the pigment responsible for plants' green coloration. Chlorophyll is the molecule that is able to absorb sunlight, and that absorbed sunlight is what powers the entire set of photosynthesis reactions.
In actuality, chlorophyll can absorb some sunlight, but it cannot absorb all sunlight. If it could, plants would be black, not green.
So, much of the light is indeed absorbed by the pigment, but the color we most often associate with chlorophyll, green, actually wants nothing to do with the molecule!
Green is the color of light that chlorophyll cannot absorb. Instead, the green light is reflected, bounced back at our eyes as we look at a plant, making our brain determine it as green. Some of the light does pass through the chloroplast as well (it is transmitted), but that is not visible to us.
Absorption Spectrum of Chlorophyll
Absorption Spectrum of Chlorophyll
As noted, chlorophyll cannot absorb green light. This graph shows the absorption spectrum of some different pigments.
Firstly, notice that there are two different chlorophyll molecules, but we do not have to worry about their differences in this class.
There are also carotenoids shown here. Carotenoids are better at absorbing green light than chlorophyll, but cannot absorb red, orange, or yellow light. These are what are left in leaves during autumn, giving the leaves their vibrant coloration.
You do not need to memorize these different molecules nor the specific wavelengths at which they can or cannot absorb light. You do need to be able to understand an absorption spectrum, however, and answer questions about that data.
Overview of the Process
Overview of the Process
Photosynthesis is broken up into two major parts - the light-dependent reactions (sometimes simply called the 'light reactions') and the light-independent reactions (sometimes referred to as the 'dark reactions', but this is misleading because it can also occur in the daylight. The light-dependent reactions occur in the thylakoids - this makes sense because that is where the chlorophyll molecules can be found. The light-independent reactions occur in the stroma surrounding the thylakoids.
Overall Equation:
6CO2 + 6H2O + sunlight --> C6H12O6 (Glucose) + O2
I. Light-Dependent Reactions - In the Thylakoids
I. Light-Dependent Reactions - In the Thylakoids
Summary of the Light Reactions
Summary of the Light Reactions
The light reactions of photosynthesis are summarized in this image, showing the inputs as sunlight and water and the outputs of ATP, NADPH, and oxygen. The water molecules provide electrons to an electron transport chain much like that which we saw in cellular respiration. In fact, this water as an electron donor is essentially the opposite of how water was the final product of cellular respiration.
In cellular respiration, oxygen was the final electron acceptor, producing water. In photosynthesis, the opposite is occurring - water is being split to produce an electron along with some oxygen.
This image represents the steps occurring in the light reactions. You will notice that these steps span a membrane - this membrane is the thylakoid membrane, the membrane of one of those pancakes.
There are more parallels to cellular respiration here too - NADPH is akin to NADH and will be important later.
There is a lot going on here, so we will break up the light reactions into 3 parts - those occurring in the photosystem II, the electron transport chain, and those occurring in the photosystem I.
Mechanical Analogy for Light Reactions
Mechanical Analogy for Light Reactions
Sometimes it is best to think of these abstract forms of energy as one with which you are a little more familiar. This diagram shows a representation of the light reactions in the form of mechanical energy.
The height of the electron (its gravitational potential energy) represents the electrons potential energy in the electron transport chain as well. The photon excites the electron (shown here as smacking it up into a higher level. Then, it loses some energy (that energy is harvested to produce ATP). Another photon excites the electron again at PS I and the electron eventually reduces NADP+ to for NADPH, the electron carrier that will be used during the Calvin Cycle.
A. Photosystem II (PS II)
A. Photosystem II (PS II)
Photosystem II actually comes first despite the two in its name (that is the result of PS II being discovered second). At PS II, sunlight, in the form of a photon (a little packet of energy making up sunlight) interacts with some molecules within the thylakoid membrane (namely chlorophyll). This excites an electron on this molecule. This excited electron is not stable, so these molecules that make up PS II play hot potato with the excited electron. The electron is then passed off to some electron carriers in an electron transport chain much like we saw in cellular respiration.
So, to summarize, an electron was lost, so PS II (specifically P680, but don't memorize that) was oxidized. We're going to have to replenish that electron if we want to do this again, and that is where water comes in as a reactant for photosynthesis. Water is our electron donor in photosynthesis. It donates an electron to replenish the one taken from PS II. This results in water becoming O2 molecules and H+ ions - this is precisely the reverse of what we saw the electrons do in cellular respiration. (this is actually contributing to a H+ gradient with a high concentration in the thylakoid lumen and a low concentration in the stroma.
B. Electron Transport Chain (ETC) - Linear Electron Flow
B. Electron Transport Chain (ETC) - Linear Electron Flow
The electron transport chain is comprised of a similar set of proteins as those we saw in cellular respiration. You do not need to know the names of any of these structures. The electron transport chain delivers the electron from PS II to photosystem I, or PS I. Energy from this excited electron is used to pump H+ ions from the stroma to the lumen (interior) of the thylakoid. This is going against its concentration gradient, so this required energy from the electron.
C. Photosystem I (PS I)
C. Photosystem I (PS I)
At Photosystem I (PS I), another photon is exciting another electron at a chlorophyll molecule. That electron, just like the last one is passed around like a hot potato. Just like in PS II, this results in an electron being passed off of PS I, so we need to replenish that electron somehow. Where does this replacement electron come from? From the electron transport chain. Follow the orange arrows in the diagram. The electron left the molecule labeled P700 (don't memorize that) - thus, P700 was oxidized. We replace that electron with the one we followed from PS II and through the electron transport chain.
This final electron is eventually passed off to an NADP+ ion, resulting in NADPH. NADPH is akin to NADH in cellular respiration. The easiest way to remember it is that the 'P' could stand for 'photosynthesis'. This electron carrier will be crucial for the light-independent reactions occurring in the stroma. The NADPH is formed in the stroma, which will be important in the next step.
Cyclic Electron Flow - Finally something you can ignore!
Cyclic Electron Flow - Finally something you can ignore!
NOTE: In actuality, there is also a process known as cyclic electron flow (as opposed to the linear electron flow we've seen up until now) that occurs within PS I to generate more ATP to power the Calvin Cycle. The basics of the process is diagrammed in the image below. Again, the purpose of cyclic electron flow is to generate enough ATP (along with the ATP that would be produced from linear electron flow). You do not need to worry about cyclic electron flow much at all for this course - that is why it was not included in the diagrams before. I am including it in the website primarily so that you can be familiar with the terms if you pursue biology or biochemistry in your future.
D. ATP Synthase
D. ATP Synthase
Just like in cellular respiration, there is ATP Synthase spanning this membrane. That H+ gradient that we were forming during the electron transport chain and the splitting of water is now used to power the enzyme to phosphorylate ADP into ATP. This ATP, along with the NADPH from the ETC, will be important in the upcoming Calvin Cycle.
II. Light-Independent Reactions (AKA the Calvin Cycle) - In the Stroma
II. Light-Independent Reactions (AKA the Calvin Cycle) - In the Stroma
Summary of the Light-Independent Reactions
Summary of the Light-Independent Reactions
Part two of photosynthesis is also known as the light-independent reactions - or, occasionally the dark reactions (this is a bit of a misnomer, however, because these can also occur in the daytime). We will also be referring to these reactions as the Calvin Cycle. Try your best not to get that confused with Krebs Cycle! The Calvin Cycle occurs in the stroma - remember, that was where we were making the ATP with ATP Synthase and where the NADPH was synthesized... that will be important shortly.
During the Calvin Cycle, we finally get to see the CO2 molecule that we so often associate with photosynthesis. These 3 CO2 molecules will be 'fixed' with the help of the enzyme RuBisCO. You do not need to know any of the intermediate molecules between the CO2 molecules and the 3-Carbon sugar synthesized at the bottom of this image. This required the energy and electrons from the ATP and NADPH that were synthesized during the light reactions.
The leftover 3 ATP molecules from the light reactions are used to 'regenerate' RuBP, which is a molecule that you do not need to worry about. Just know that this is a cycle, so we need to replenish our inputs from the beginning of the cycle in order to complete this cycle again.
The Calvin Cycle occurs twice for every 6 CO2 molecules. This creates 2 3-carbon sugars. These are combined later into glucose.
The overall reactions within photosynthesis can be represented in this image, which shows the relevant inputs and outputs of each part.
Please note, these are not balanced, so the amount of ATP, NADPH, etc. molecules are not listed here.
Also note that glucose is not shown necessarily as an output in this - instead sucrose is shown.
Sucrose is just two glucose molecules bonded together in a disaccharide. This is just a more commonly formed end product in real plants.
The glucose could also be used to form starch for energy storage or even amino acids or fatty acids.
Impact on the Earth's Atmosphere
Impact on the Earth's Atmosphere
The evolution of photosynthesis some time around 3.5 billion years ago can be linked to a decrease in the amount of CO2 in Earth's atmosphere and an increase in O2 concentration.
This should make sense because CO2 is used up in photosynthesis and O2 is produced.
The higher oxygen levels of earth allowed life as we think of it in our daily life to evolve, including you!
If you look closely, you can see that a little less than 500 million years ago, oxygen concentrations were even over 30% (much higher than today). These higher levels of oxygen allowed insects to be much larger.
The largest insect ever described a species of Meganeuropsis dragonfly. It was estimated that Meganeuropsis had a body roughly 47 centimeters long, with a wingspan of 75 centimeters across. (Source). So life could be a lot worse for you insect-haters!
Photosynthesis and Cellular Respiration
Photosynthesis and Cellular Respiration
Hopefully you have noticed by now that cellular respiration and photosynthesis are reverses of one another. Without the other process, one process would eventually (it might take a very, very long time) run out of reactants if there are no other sources of reactants.
This is why photosynthesis and cellular respiration are quintessential to the carbon cycle - both cycle carbon, but in different directions. Understanding this is more important than ever due to global climate change.
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