Photosynthesis is a metabolic process which makes stuff using light. How? How can you make anything from light? And why? Living things are made of complex organic molecules such as carbohydrates and proteins, as opposed to simple inorganic molecules such as carbon dioxide and water.
The vast majority of plants on Earth today undergo photosynthesis via a specific route (C3) which is slightly different to two other potential routes (C4 and CAM). The general balanced reaction for photosynthesis is:
H2O + CO2 + energy –> C6H12O6 + O2
…where water, carbon dioxide and energy are the starting materials, and glucose and oxygen the products. Here, glucose is the key product because it is the complex organic molecule made from simple inorganic reactants. The “energy”, as you may have noticed, is where the light comes in.
Photosynthesis is the process by which most plants as well as other organisms e.g. photosynthetic bacteria obtain their energy (glucose) ultimately in the form of ATP upon respiration. So photosynthesis produces the glucose, and the glucose is the substrate for respiration which produces ATP.
All living things undergo respiration to produce ATP from substrates including glucose, but only some (notably plants) undergo photosynthesis to produce the glucose themselves.
So where do other organisms get their respiration substrates – “food” – from? Well, most do directly from the plants by eating them, indirectly from other organisms who ate the plants (herbivores) or even more indirectly from carnivores. Fungi, for example, do neither – they simply digest any organic compounds from their environment, the soil.
That is why plants are considered autotrophs (they make their own “food” via photosynthesis), while humans amongst others are considered heterotrophs (they must obtain their “food” indirectly from other organisms which photosynthesise).
Back to photosynthesis itself now! We know that photosynthesis requires light, however the twist is that the process is split into two: the light-independent and light-dependent reactions. So some parts of photosynthesis don’t actually require light. The very first stages of photosynthesis are the ones which require light, and once those have been accomplished, the subsequent reactions may proceed regardless.
Overview of the Light-dependent and Light-independent Reactions
The LD reactions take place on the thylakoid membranes within chloroplasts, whereas the LI reactions take place in the surrounding space called the stroma.
The LD reactions produce protons, electrons and oxygen, while the LI reactions produce triose phosphate which ultimately is converted to glucose and other organic molecules. So the overall purpose of the LD reactions is to convert light energy into chemical energy, while the overall purpose of the LI reactions is to convert the LD products into useful molecules like glucose.
The light-dependent reaction
As in the overview of photosynthesis, the light-dependent reactions utilise light energy to convert it into more usable chemical energy.
So naturally, it starts with light. This is the brief sequence of events:
1. The electrons present in the chlorophyll of the plant’s chloroplasts are brought to a higher energy level (they enjoy dancing more) by light energy. This takes place on the thylakoid membrane, and more specifically in a conglomerate of proteins/enzymes dedicated to this reaction, called photosystem II. It’s known as photoionisation.
2. To maintain a fresh supply of dancing electrons, light also splits (photolysis) the H2O into… electrons, protons and… wait for it. Wait for it. Wait… Oxygen! So that’s how the oxygen by-product is made.
3. What’s the deal with the dancing electrons? They’re picked up by electron carriers (nightclub bouncers) and thrown out, one by one. This releases energy every time a poor electron is pushed down another flight of stairs (thylakoids are multi-story clubs thank you very much) all the way to photosystem I. Ouch. I sure hope that energy is put to good use.
4. The sweat and blood and tears of electrons passing down the electron transport chain is used to pump the elite clientèle into the thylakoid. Who is this clientèle, I hear you ask. It is none other than the protons! You know, the ones snatched from the H2O.
They rush inside all at once as soon as the electrons are suitably thrown out – just couldn’t stand all that… negativity. They are stuffed inside the thylakoids like sardines on a hot day, to the point where the nightclub is filled with positivity and the outside (the stroma) is totally missing out.
5. The proton gradient formed as a result (lots of protons inside the thylakoid, few outside) enables their movement subsequently in the opposite direction, down their concentration gradient. Unfortunately for them, there are only a few exits back outside. These are gates – enzymes – called ATP synthase. They have the absolute cheek to charge every proton to get out energy currency. This energy makes ATP from ADP + Pi.
6. Meanwhile, what are the electrons doing at photosystem I? They’re electrons, what else are they going to do if not get excited – again – and end up in trouble – again. Light strikes them at PSI, even harder this time, and they roll-rollety-roll along to electron carrier NADP (nicotinamide adenine dinucleotide phosphate, of course you were dying to know) where they are coerced into making friends (?!) with a proton from the stroma and sticking together to form reduced NADP.
Phew. Did I call that a BRIEF sequence of events? Hahahaha sorry, my bad.
On the upside, you now get to see the gorgeous summary diagram of it all happening at once. You wouldn’t have wanted to see that first.
The light-independent reaction
The light-independent reaction of photosynthesis is where the ultimate product, glucose, is made. Given its name, the reactions involved in this step do not require light, since the reactants used are taken from the products of the light-dependent reaction. The LIR occurs in the stroma of chloroplasts (the space around thylakoid stacks which contains lots of enzymes involved in photosynthesis).
All LIR events can be viewed as a cycle termed the Calvin cycle. The starting point is carbon dioxide, CO2, and the ending point is glucose (C6H12O6). Before the carbon atoms in CO2 can be incorporated into glucose, a series of events must take place. As you can appreciate, turning a simple inorganic gas into a complex organic molecule which is at the heart of life today as we know it takes just a little bit of magic.
This magic has 3 chapters, as ordered in the Calvin cycle:
1. Carbon dioxide fixation
2. Carbon dioxide reduction
3. Ribulose bisphosphate regeneration
Firstly, carbon dioxide reacts with ribulose bisphosphate waaaaaaaaaaaaaaaaait. Ribulose bisphosphate. Say it out loud. Ribulose bisphosphate. What does it want from you? Nothing, just remember it’s a 5-carbon sugar. Carbon dioxide has 1 carbon, glucose has 6… catch the drift? 5+1…. 5+1=…! But those goodies are for later. For now, carbon dioxide reacts with RuBP (a reaction catalysed by the enzyme RuBisCO – sounds like a supermarket chain in Paraguay) to produce two 3-carbon molecules called glycerate 3-phosphate, GP. Here’s a carbon atom scheme:
C + CCCCC –> 2CCC
(CO2 + RuBP –> 2GP)
Next: The 2 GP molecules react further to produce triose phosphate, TP (3-carbon molecule also known as glyceraldehyde phosphate). This forms the building block for glucose and other organic compounds. To add up the carbon atoms, 2 TP are needed for 1 glucose.
Expenditure: 2 ATP molecules and 2 NADPH molecules from the LDR. Once NADPH is oxidised back to NADP, it can return to the LDR in the thylakoids. NADP is therefore recycled.
Finally: For the cycle to continue, a supply of RuBP must be kept constant to meet the incoming carbon dioxide back at the beginning of the Calvin cycle. This is actually achieved by most TP molecules produced in the previous step. A phosphate group from ATP is used to convert ribulose monophosphate into ribulose bisphosphate, RuBP.
If photosynthesis had no limiting factors, what would glasshouse growers have to exploit?
Precisely. Photosynthesis, just like all other physiological processes in living things as well as chemicals and beyond, is subject to external influences. The main factors that weigh in on the efficiency and speed of photosynthesis are :
2. CO2 Concentration
3. Light intensity
Both the concentration of carbon dioxide and the intensity of light are similar in that they are both direct ingredients in the overall photosynthesis reaction. But since temperature insists on having the first say, let it be that way…
The optimum for a lot of plants is 25 degrees Celsius – regardless, the rate of photosynthesis forms a bell curve in response to increasing temperature:
The rate of photosynthesis is sluggish at lower temperatures, while at higher temperatures it drops sharply. What’s happening? It’s all in the enzymes. Enzymes are subject to the same laws of thermodynamics as everything else. Put simply, temperature influences the random movement and collisions between molecules; at low temperatures the movement decreases, so the activity of enzymes involved in photosynthesis, among others, also decreases.
Turn up the heat a few notches, and hey presto, photosynthesis speeds up! Turn it up beyond 30 degrees or so, and you kill the party. Just what’s happened now? It’s a very general property of most enzymes that at high temperatures they denature. Photosynthesis enzymes, such as RuBP, are no exception. Denatured enzymes have a misfolded 3D structure.
In this state they cannot bind their substrates and carry out their catalytic activity. Hence, no photosynthesis!
2. CO2 concentration
Needless to repeat, carbon dioxide is one of the prerequisites for photosynthesis. After all, it supplies the carbon (C) atoms without which glucose and other organic molecules wouldn’t exist. This is how plants react to changing CO2 concentration:
As the CO2 concentration increases, so does the rate of photosynthesis, as the much-loved carbon dioxide is becoming more and more plentiful! So why does it have to end so tragically and abruptly? It seems as if the plant has enough CO2 but it’s just not good enough. Why?
Well, it’s simple: CO2 isn’t the only thing needed in photosynthesis. In other words, there are other limiting factors. Perhaps they are the same as the others listed here! When the plant has more CO2 than it can use it’s because it doesn’t have enough light, or heat, or has too much heat (denatured enzymes), or…
3. Light intensity
Temperature has degrees, CO2 concentration has pressure/volume, so what does light have? Would you believe it, there’s a special unit of measurement for light called the lux. Pretty awesome. Around 100,000 lux are available in an average day to a photosynthesising plant. Unsurprisingly, light is very much welcomed.
Just like CO2 concentration, increasing light intensity will only result in so much increased photosynthesis rate before another limiting factor comes in.
Plant growers must take into account all these different factors affecting photosynthesis and know which one becomes limiting when. The environment within a glasshouse, for example, must be optimised by adding extra CO2, increasing the temperature especially during winter, and maximising light exposure including adding artificial light.