Today I’m going to try to explain to you all about photosynthesis. It is something that is happening in my flat right now with all the new herbs and flowers I have started growing in April. I have a lot of information about photosynthesis that I want to share with you so this might end up being a 2 part post. There’s no time to waste so let’s get going!
Photosynthesis is one of the most vital functions on earth and we would not be alive without photosynthesis and the plants produced as a result of it. It is a chemical reaction that takes place in the leaves of green plants and although the name is long, the equation for photosynthesis is quite simple.
It’s carbon dioxide, water, and light energy that reacts together to produce glucose and oxygen. If you read my previous post about ATP, you will have read that the equation for photosynthesis is the reverse equation for cellular respiration. And that is so interesting and fascinating to me that our bodies need the products of photosynthesis to create energy and survive, and plants also need the products from our bodies creating energy to survive and grow! How cool is that?
Photosynthesis occurs in the chlorenchyma tissue in a plant. The chlorenchyma tissues are mostly found in the leaves and the 2 layers of the chlorenchyma tissues are the upper palisade mesophyll and the lower spongy mesophyll. The upper palisade mesophyll layer contains about 80% of the chloroplasts, which is specifically where photosynthesis occurs. The lower spongy mesophyll layer has loosely arranged cells with abundant air spaces as you can see in the diagram below.
Going back to the photosynthesis equation, water is a crucial part of plant growth and you probably know that if you’ve even ever attempted to grow a plant yourself. You would be surprised though that only 1% of the water absorbed by plants is used for photosynthesis. However, that 1% of water used for photosynthesis is quite important in that a shortage of water for the plant will end up indirectly limiting photosynthesis to occur. This is because the lack of water will make the stoma/stomata(pores) of the plant to close which decrease the amount of carbon dioxide the plant is being supplied with for photosynthesis.
Light energy is just as important to water, for photosynthesis and again, you probably know that if you have any experience with growing plants yourself. The sun’s energy reaches the earth in the form of electromagnetic radiation which consists of waves and particles. Those particles are known as photons, the smallest divisible unit of light and can also be referred as a single light quantum. We are going to go a bit into physics to talk a bit about electromagnetic radiation but bare with me now.
Each photon carries a certain amount of energy which determines how much the photon vibrates. The distance of how much a photon moves during one of its vibration is called a wavelength, measured in nanometers. Electromagnetic radiation spans a wide spectrum of wavelengths with gamma rays at one end of the spectrum with wavelengths of 5-10nm, and radio waves at the other end of the spectrum with wavelengths of 1012nm.
The interesting part about electromagnetic radiation is that only a small part of this huge spectrum can be seen with the human eyes. This is spectrum is between the wavelengths 380-750nm, different wavelengths of visible light being perceived as color. And in plants, the chlorophyll does not absorb the green color part of the spectrum which is then reflected from the plant, visible for us humans to see.
Where are the chlorophylls in plants then? They’re in this tiny bean looking sacs called the chloroplast mostly found in the upper palisade mesophyll layer of the chlorenchyma tissues. There are 2 membranes enclosing a gelatinous matrix called the stroma, not be confused with the stoma.
As you can see, suspended in the stroma are these membranous sacs called thylakoids, that can be stacked into piles called grana. It is estimated that there are approximately 500 thousand chloroplasts per square millimeter of a leaf, so we’re talkin’ really small here. Now let’s dig deeper into how photosynthesis occurs in this tiny organelle!
There are 2 phases to photosynthesis, the light reaction phase, and the Calvin cycle phase. The light reaction phase occurs in the thylakoid membranes of the chloroplast where light energy produces energy in the form of ATP and NADPH2. The Calvin cycle phase, however, occurs in the stroma using the energy from the light reaction phase to capture atmospheric carbon dioxide to produce glucose.
The light reaction is a light dependent reaction which means it cannot function without light energy. There are 2 photosynthetic units called the photosystem 2(P680) and the photosystem 1(P700) in the light reaction phases. These units collect light energy and pass on the energy as a stream of electrons. The electrons originate from water molecules where they get split into electrons and hydrogen ions which help release oxygen as a waste product from the plant. The reason we can breathe. Once oxygen is released, the concentration of hydrogen ions in the thylakoid membrane start to increase. This makes the hydrogen ions want to leave the thylakoid membrane making their way towards the ATP synthase where the energy from the hydrogen ions is used to produce ATP. This energy is used to drive the next phase, the Calvin cycle phase in the stroma of the chloroplast.
The Calvin cycle/ Calvin-Benson cycle is also known as the dark reaction cycle because light energy is not directly used in this cycle. I know, so many different names for one cycle! But it’s quite stupid to call this the dark reaction because the energy received from the light dependent phase is very short lived and therefore this cycle only happens during the day when the sun it up. So calling the Calvin cycle the dark reaction may confuse people into thinking the dark reaction happens at night. But anyways, the energy from the light reaction phase processes carbon dioxide to combine with 5 carbon sugars, resulting in a net gain of glyceraldehyde 3-phosphate.
This is what the two phases of photosynthesis look like and I know there are lots of details I did not go through but there are some more details on the Calvin cycle that I would like to go through. The process of Calvin cycle I have explained is the C3 photosynthesis. This is because the 1 carbon molecule from the carbon dioxide and the 5 carbon sugar molecules from the sugars combine together with the help of a catalyst, RuBisCo, to form rubilose which is a compound that is unstable and quickly splits into a 3 carbon compound, hence the C3. But sometimes the RuBisCo will attach oxygen instead of carbon dioxide to the sugar molecule, resulting in a series of reactions that loses carbon and energy. RubisCo is also an inefficient catalyst in that it can only catalyze 3 molecules per second compared to other catalysts that catalyze thousands per second. Therefore plants contain a high amount of RuBisCo which is thought to be the most common protein in the world.
But how does photosynthesis occur for hot weather plants? These plants have to close their stomata to conserve water so they cannot get as much carbon dioxide needed for photosynthesis. Well, not to worry, there are plants called C4 plants that have their own ways of photosynthesis. C4 plants have an enzyme called PEP carboxylase to help capture carbon dioxide which gets stored in the plant’s bundle sheath cells. The RuBisCo can then work with the carbon dioxide that has been isolated from oxygen, avoiding the tendency to wastefully attach oxygen to the sugar molecules.
There is another form of photosynthesis for plants such as cacti, orchids, and succulents that live in very hot climates. It’s similar to C4 photosynthesis but the stomata will stay closed during the day and stay open at night to gather carbon dioxide. This carbon dioxide then gets stored as malic acid in the vacuoles, getting converted back to carbon dioxide when needed.
I know this has been long a post but hang in there, I’ve just got a few more things I want to share because I want to finish this post in 1 part. Maybe go make yourself a little green smoothie with greens that have gone through photosynthesis?
In the thylakoid membrane and grana of chloroplasts, there are several types of chlorophyll molecules. The structure of those molecules are similar to hemoglobins found in our blood, but instead of the central atom of that molecule being iron, it’s magnesium instead. There is a range of different pigments which allow plants to harvest a broader spectrum of light. The most common are chlorophyll a, then chlorophyll b and carotenoids. Carotenoids give off autumn colors such as yellow, orange, and red which can be seen in dead tree leaves, carrots, and tomatoes etc. Carotenoids protect plants against photo-oxidation which is why lots of herbicides contain high levels of carotenoids.
And that is it guys! Whew. Thanks for sticking around if you got to this point. I hope this post was useful in that it gave you a new appreciation for plants and the food we get to eat thanks to photosynthesis. See you in my post! Till then!