The Reactions of Photosynthesis for AP Biology
Practice problems for these concepts can be found at: Photosynthesis Review Questions for AP Biology
The process of photosynthesis can be neatly divided into two sets of reactions: the light dependent reactions and the light-independent reactions. The light-dependent reactions occur first and require an input of water and light. They produce three things: the oxygen we breathe, NADPH, and ATP. These last two products of the light reactions are then consumed during the second stage of photosynthesis: the dark reactions. These reactions, which need CO2, NADPH, and ATP as inputs, produce sugar and recycle the NADP+ and ADP to be used by the next set of light-dependent reactions. Now, I would be too kind if I left the discussion there. Let's look at the reactions in more detail. Stop groaning … you know I have to go there.
Light-dependent reactions occur in the thylakoid membrane system. The thylakoid system is composed of the various stacks of poker chip look-alikes located within the stroma of the chloroplast. Within the thylakoid membrane is a photosynthetic participant termed chlorophyll. There are two main types of chlorophyll that you should remember: chlorophyll a and chlorophyll b. Chlorophyll a is the major pigment of photosynthesis, while chlorophyll b is considered to be an accessory pigment. The pigments are very similar structurally, but the minor differences are what account for the variance in their absorption of light. Chlorophyll absorbs light of a particular wavelength, and when it does, one of its electrons is elevated to a higher energy level (it is "excited"). Almost immediately, the excited electron drops back down to the ground state, giving off heat in the process. This energy is passed along until it finds chlorophyll a, which, when excited, passes its electron to the primary electron acceptor; then, the light-dependent reactions are under way.
The pigments of the thylakoid space organize themselves into groups called photosystems. These photosystems consist of varying combinations of chlorophylls a, b, and others; pigments called phycobilins; and another type of pigment called carotenoids. The accessory pigments help pick up light when chlorophyll a cannot do it as effectively. An example is red algae on the ocean bottom. When light is picked up by the accessory pigments, it is fluoresced and altered so that chlorophyll a can use it.
Imagine that the plant represented in Figure 8.2 is struck by light from the sun. This light excites the photosystem of the thylakoid space, which absorbs the photon and transmits the energy from one pigment molecule to another. As this energy is passed along, it loses a bit of energy with each step and eventually reaches chlorophyll a, which proceeds to kick off the process of photosynthesis. It initiates the first step of photosynthesis by passing the electron to the primary electron acceptor.
Before we continue, there are two major photosystems I want to tell you about—you might want to get out a pen or pencil here to jot this down, because the names for these photosystems may seem confusing. They are photosystems I and photosystem II. The only difference between these two reaction centers is that the main chlorophyll of photosystem I absorbs light with a wavelength of 700 nm while the main chlorophyll of photosystem II absorbs light with a wavelength of 680 nm. By interacting with different thylakoid membrane proteins, they are able to absorb light of slightly different wavelengths.
Now let's get back to the reactions. Let's go through the rest of Figure 8.2 and talk about the light-dependent reactions. For the sole purpose of confusing you, plants start photosynthesis by using photosystem II before photosystem I. As light strikes photosystem II, the energy is absorbed and passed along until it reaches the P680 chlorophyll. When this chlorophyll is excited, it passes its electrons to the primary electron acceptor. This is where the water molecule comes into play. Photolysis in the thylakoid space takes electrons from H2O and passes them to P680 to replace the electrons given to the primary acceptor. With this reaction, a lone oxygen atom and a pair of hydrogen ions are formed from the water. The oxygen atom quickly finds another oxygen atom buddy, pairs up with it, and generates the O2 that the plants so graciously put out for us every day. This is the first product of the light reactions.
The light reactions do not stop here, however. We need to consider what happens to the electron that has been passed to the primary electron acceptor. The electron is passed to photosystem I, P700, in a manner reminiscent of the electron transport chain. As the electrons are passed from P680 to P700, the lost energy is used to produce ATP (remember chemiosmosis). This ATP is the second product of the light reactions and is produced in a manner mechanistically similar to the way ATP is produced during oxidative phosphorylation of respiration. In plants, this process of ATP formation is called photophosphorylation.
After the photosystem I electrons are excited, photosystem I passes the energy to its own primary electron acceptor. These electrons are sent down another chain to ferredoxin, which then donates the electrons to NADP+ to produce NADPH, the third and final product of the light reactions. (Notice how in photosynthesis, there is NADPH instead of NADH. The symbol P stands for photosynthesis. )
Remember the following about the light reactions:
- The light reactions occur in the thylakoid membrane.
- The inputs to the light reactions are water and light.
- The light reactions produce three products: ATP, NADPH, and O2.
- The oxygen produced in the light reactions comes from H2O, not CO2.
Two separate light-dependent pathways occur in plants. What we have just discussed is the noncyclic light reaction pathway. Considering the name of the first one, it is not shocking to discover that there is also a cyclic light reaction pathway (Figure 8.3). One key difference between the two is that in the noncyclic pathway, the electrons taken from chlorophyll a are not recycled back down to the ground state. This means that the electrons do not make their way back to the chlorophyll molecule when the reaction is complete. The electrons end up on NADPH. Another key difference between the two is that the cyclic pathway uses only photosystem I; photosystem II is not involved. In the cyclic pathway, sunlight hits P700, thus exciting the electrons and passing them from P700 to its primary electron acceptor. It is called the cyclic pathway because these electrons pass down the electron chain and eventually back to P700 to complete the cycle. The energy given off during the passage down the chain is harnessed to produce ATP—the only product of this pathway. Neither oxygen nor NADPH is produced from these reactions.
A question that might be forming as you read this is: "Why does this pathway continue to exist?" or perhaps you are wondering "Why does he insist on torturing me by writing about all of this photosynthesis stuff?" I will answer the first question and ignore the second one. The cyclic pathway exists because the Calvin cycle, which we discuss next, uses more ATP than it does NADPH. This eventually causes a problem because the light reactions produce equal amounts of ATP and NADPH. The plant compensates for this disparity by dropping into the cyclic phase when needed to produce the ATP necessary to keep the light-independent reactions from grinding to a halt.
Before moving on to the dark reactions, it is important to understand how ATP is formed. I know, I know … you thought I was finished … but I want you to be an expert in the field of photosynthesis. You never know when these facts might come in handy. For example, just the other day I was offered $10,000 by a random person on the street to recount the similarities between photosynthesis and respiration. So, this stuff is useful in everyday life. As the electrons are passing from the primary electron acceptor to the next photosystem, hydrogen ions are picked up from outside the membrane and brought back into the thylakoid compartment, creating an H+ gradient similar to what we saw in oxidative phosphorylation. During the light-dependent reactions, when hydrogen ions are taken from water during photolysis, the proton gradient grows larger, causing some protons to leave, leading to the formation of ATP.
You'll notice that this process in plants is a bit different from oxidative phosphorylation of the mitochondria, where the proton gradient is created by pumping protons from the matrix out to the intermembrane space. In the mitochondria, the ATP is produced when the protons move back in. But in plants, photophosphorylation creates the gradient by pumping protons in from the stroma to the thylakoid compartment, and the ATP is produced as the protons move back out. The opposing reactions produce the same happy result—more ATP for the cells.