Photosynthesis

Photosynthesis is an incredible process - complementary to cellular respiration. Although they are different, these processes are generally similar to each other. I will be brushing the surface of this process in this page.

First of all, do you remember NADH from cellular respiration? Photosynthesis has its own version of this - NADHPH. Just to remind you, it is an electron carrier. 

Photosynthesis has two steps: the light reactions and the calvin cycle. The chemical equation for photosynthesis is, right below, where H2O is oxidized to become O2 and CO2 is reduced to become C6H12O6.

The chloroplast consists of an inner and outer membrane, of which the inner membrane consists of grana (multiple granum stacks, which are stacks of thylakoids) surrounded by stroma, a thick fluid which also fills the inner and outer membrane.

The materials required for photosynthesis, as we can see, are CO2, H2O, and light. H2O is absorbed through the roots of the plant, and CO2 is brought in through pores in the plant called stomata, the same pores that let out oxygen.

Light, meanwhile is absorbed by several pigments called chlorophyll a, chlorophyll b, and carotenoids. Chlorophyll a participates in light reactions and absorbs mainly blue and orange light. Chlorophyll b absorbs mainly blue and orange light and is similar to chlorophyll a. Carotenoids reflect yellow and orange, and absorb excessive light. These are the reasons for the reds, oranges, and yellows you see in fall.

With that, let's begin the process of photosynthesis! Here is a diagram to get you started.

Here is a photosystem with its anatomy, so you understand its structure (because structure and function have an important relationship!!!!!!!!)

Here is a picture of the first parts of light reactions - H+ being volleyed across the membrane through NADPH and the electrons from water and PSll and PSl.

Did you know that although PSll is sort of the leader in the light reactions, it is the second photosystem because PSl was discovered first?

Now we're showing the events after H+ is volleyed across - the electrons are taken to the Calvin cycle by NADPH. Below, we can see the entire light reactions process, and that ATP is also given to the Calvin cycle from the light reactions. Isn't that interesting? Is the process making more sense now? If not, please do email me at twisha.sharma30@gmail.com!!

Light Reactions

The light reactions of photosynthesis occur in the thylakoids - which are little membranous sacs found inside the inner membrane of the chloroplast. In these reactions, water is split, which provides a source of electrons, leaving an oxygen byproduct. Light energy is absorbed by chlorophyll a and b in the photosystems, which are functional units used in photosynthesis. These photosystems (of which there are two kinds) consist of two complexes: a reaction center surrounding light harvesting complexes. These light harvesting complexes consist of the pigment molecules I mentioned earlier. In total, PSll and PSl work together using NADPH and the electron transport chain to make ATP and inevitably power the Calvin cycle. In the light reactions, the sunlight energy enters PSll and excited the electrons in the chlorophyll particles. Electrons leave the chlorophyll (oxidation) and are accepted by a primary acceptor. From there, they are donated to the electron transport chain (just like oxidative phosphorylation from cellular respiration). The electrons in PSl and PSll are both excited by light and used to move H+ across the membrane with NADPH (again, just like oxidative phosphorylation). PSll electrons replace those lost from PSl, and water electrons replace those lost from PSll. Finally, chemiosmosis makes ATP using the concentration gradient created by the H+ atoms on either side of the membrane and an ATP synthase (for the third time, oxidative phosphorylation).
Now, that was probably really confusing. If you're confused like I am, look at the pictures to the left - it will make a lot more sense. But we haven't done any glucose producing, if you've noticed. Not to fear! That's what the Calvin cycle is for. It needs to be powered by the light reaction to work, after all.

Calvin Cycle

The Calvin cycle occurs in the stroma of a chloroplast. This process utilizes something called carbon fixation. Carbon fixation is "the incorpotation of carbon from CO2 into organic compounds," according to the textbook I cited on the welcome page. In this cycle, which I have included a picture of, carbon is fixated into glucose. However, there is one important thing about this process - there are three different carbon fixation pathways that plants use. These pathways are known as C3, C4, and CAM. C3 is a pathway where CO2 is immediately fixed into three-carbon sugars to be used by an enzyme called rubisco, or RuBP. The stomata are always open the plant is usually working. C4 is another pathway where CO2 is fixed into a four-carbon sugar and the stomata is half-open. It is used for plants in drier weather to avoid wasting water. CAM is used in plants in even drier weather and involves only opening the stomata at night and fixing CO2 into 4-carbon sugars to wait for the day. In my opinion, it's amazing that plants have these pathways - nature is built for maximum efficiency. 

This picture should demonstrate the carbon fixation that happens in the Calvin Cycle that allows for the creation of glucose. This is the fixation for C3 plants. Keep in mind that for every 3 CO2, only one G3P is output. The other five G3P molecules are used to regenerate the enzyme rubisco, so even though 6 G3P are created, only one is usable.

Here is a helpful depiction of C4 and CAM plant cycles to help understand the differences.

Light intensity, carbon dioxide levels, and temperature all impact photosynthesis. While light intensity and carbon dioxide levels are fairly obvious with their impact (too much or too little light or CO2 will result in a plant having more or less material/energy to use for photosynthesis), temperature is less obvious. Temperature is actually quite important though, because as temperature rises, photosynthesis increases, due to the increased collisions between enzymes and substrates.

Either way, photosynthesis is an amazing process that helps the whole world have life. The pictures I used above to help you understand are all screenshotted from the biology textbook I cited on the welcome page. I would have drawn something myself, but I felt like these pictures were very informative and useful for understanding photosynthesis. Please go check the textbook out! I get lots of information for it and it is just an interesting and wonderful resource.

The next page, mitosis and meiosis, will go over sexual and asexual cellular reproduction. I hope to see you in the next one!