Light Reactions
Before we get into the first step of photosynthesis, we need to familiarize ourselves with the site of photosynthesis - the chloroplast.
The Chloroplast
Chloroplasts have a double membrane. Inside the membranes, there are thylakoids (flattened sacs resembling pancakes), which contain chlorophyll. This green pigment converts CO2 and water to carbohydrates using the sun's energy. Grana (sing. granum) are stacks of many many thylakoids. Finally, stroma is the fluid-filled space inside the chloroplast, much like what the cytosol is to the cell.
Chloroplasts are mainly found in the mesophyll, which is the tissue that occupies the middle section of the leaf. Chloroplasts are also found elsewhere, but in much smaller quantities. The complete equation of photosynthesis is: 6 CO2 + 12 H2O + light energy -> C6H12O6 + 6 O2 + 6 H2O Unlike the net equation for photosynthesis, there are 6 more H2O's on either side of the equation. This is because in reality 12 H2O's are needed and 6 H2O's are given off. However, we often cancel them out to get the net equation: 6 CO2 + 6 H20 + light energy -> C6H12O6 + 6 O2 |
One more thing we should learn before proceeding is the splitting of water. When a water molecule is split into two hydrogen ions, two electrons, and an oxygen atom, the electrons will be used in photosynthesis to oxidize the CO2, while the oxygen atom will combine with another oxygen atom to form oxygen gas (O2). It is important to keep in mind that the oxygen atoms in O2 come from water and not carbon dioxide. Also note that water is the reducing agent in photosynthesis.
The Absorption of Light
Light powers photosynthesis through photons, which are small "packages" of energy. They are not particles, but they behave in a fashion similar to real particles. When light hits matter, it can be absorbed, transmitted, or reflected. The colour of an object is dependent on what wavelengths of light are reflected. Pigments absorb light. Chlorophyll pigments specifically absorbs red and violet light, and green light is reflected. Therefore, plants appear green because their chloroplasts have a lot of chlorophyll pigments in them.
There are multiple types of chlorophyll pigments, chlorophyll a and chlorophyll b have slightly different absorption spectra. This broadens the range of colours that can power photosynthesis. In addition, there are other accessory pigments such as carotenoids which serves a role in photoprotection by dissipating excessive light energy so it does not harm the plant. Overall, the combined absorption spectrum of all the pigments favours the absorption of red and violet light, while green light is reflected.
The absorbed photon excites an electron to a state of greater potential energy. If this is in an isolated chlorophyll molecule, then the electron will immediately drop back down to its ground state, thereby dissipating its energy in the form of heat and light (fluorescence). However, if the chlorophyll molecule is in a chloroplast, then the energy can be converted to something more useful, as we will see below.
There are multiple types of chlorophyll pigments, chlorophyll a and chlorophyll b have slightly different absorption spectra. This broadens the range of colours that can power photosynthesis. In addition, there are other accessory pigments such as carotenoids which serves a role in photoprotection by dissipating excessive light energy so it does not harm the plant. Overall, the combined absorption spectrum of all the pigments favours the absorption of red and violet light, while green light is reflected.
The absorbed photon excites an electron to a state of greater potential energy. If this is in an isolated chlorophyll molecule, then the electron will immediately drop back down to its ground state, thereby dissipating its energy in the form of heat and light (fluorescence). However, if the chlorophyll molecule is in a chloroplast, then the energy can be converted to something more useful, as we will see below.
Photosystems
Photons of light are absorbed by photosystems in plants. A photosystem consists of light-harvesting complexes and a reaction centre. Many chlorophyll pigments reside in the light-harvesting complexes, and each reaction centre consists of two special chlorophyll a molecules and a primary electron acceptor.
There are two types of photosystems, photosystem I and photosystem II. Photosystem II functions first during photosynthesis, followed by Photosystem I. The most notable difference between them is their reaction centres. The special chlorophyll a molecules in photosystem II is best at absorbing light at at wavelength of 680 nm. Thus they are also called P680. On the other hand, the special chlorophyll a molecules in photosystem I is called P700 because they are best at absorbing light at a wavelength of 700 nm.
There are two types of photosystems, photosystem I and photosystem II. Photosystem II functions first during photosynthesis, followed by Photosystem I. The most notable difference between them is their reaction centres. The special chlorophyll a molecules in photosystem II is best at absorbing light at at wavelength of 680 nm. Thus they are also called P680. On the other hand, the special chlorophyll a molecules in photosystem I is called P700 because they are best at absorbing light at a wavelength of 700 nm.
Noncyclic Electron Flow
When a photon of light is captured by a chlorophyll pigment in photosystem II, its energy is transferred until it reaches P680 in the reaction centre. P680 then uses that energy to boost one of their own electrons to a higher energy level, so the primary electron acceptor can capture it, reducing itself in the process. Water is then split to produce electrons in order to replace the missing electron in P680. After the primary electron acceptor is excited in photosystem II, it sends the electron down an electron transport chain to photosystem I, producing 1 ATP in the process. Then, another photon is harvested by photosystem I. Its energy excites an electron in P700, and the primary electron acceptor in photosystem I captures it. The new electron that just came from photosystem II then replaces it. Finally, the electron goes through another electron transport chain until it joins to NADP+. It requires two electrons to go through the above process in order to fully reduce it into NADPH. Since when water is split it releases two electrons, only one water molecule is required to reduce NADP+ to NADPH. In short, two electrons in noncyclic electron flow can produce 1 ATP and 1 NADPH.
Cyclic Electron Flow
ATP and NADPH are produced in the same proportions in noncyclic electron flow. However, the Calvin Cycle consumes 50% more ATP than NADPH, so the light cycle must somehow make up for that difference by producing more ATP than NADPH. The answer is cyclic electron flow. Depending on how much excess NADPH is present, photosystem I's excited electrons may go to either the first or second electron transport chains. If it goes to the second chain, then NADPH will be produced, but if it goes back to the first, then more ATP will be produced. Here NADPH acts as a control mechanism. Too much of it will cause it to inhibit the second chain, thus allowing the first one to produce more ATP through cyclic electron flow until ATP supply catches up to demand.