Photosynthesis and the processes that allow us to gather energy from the sun are covered in this portion of the AP Biology curriculum. We’ll begin by looking back 2.3 billion years to investigate the origins of photosynthesis. After that, we’ll go through a fast rundown of the whole procedure. Finally, we’ll look at the intricacies of light-dependent processes in the thylakoid membrane (such as photosystems and the electron transport chain) and how the products of these reactions feed the Calvin cycle’s light-independent reactions, which produce glucose. We’ll learn not just how photosystems use pigments like chlorophyll to capture light energy, but also how this energy is transferred to the Calvin cycle through ATP and NADPH!
Humans could not have survived on Earth billions of years ago, even if we had a time machine. There was absolutely no oxygen in the atmosphere back then. The process of photosynthesis did not emerge in primordial cyanobacteria until around 2.3 billion years ago. The atmosphere of the Earth shifted dramatically as this process progressed. Massive quantities of oxygen flowed into the atmosphere as the cyanobacteria proliferated and filled the Earth’s seas. This irreversibly altered the trajectory of evolutionary history, resulting in today’s sophisticated forms of life.
Process of Photosynthesis
Photosynthesis is a process that uses energy from the sun to make glucose. This process not only alters the environment to allow for all the existing forms, but it also gathers enough energy from the sun to feed ecosystems all over the globe. How does this procedure operate, though? How can a solar panel be more efficient than basic cellular components? What’s the difference between reactions that need light and those that don’t? These questions will almost certainly appear on the AP Exam. So bear with us as we go through all there is to know about photosynthesis!
Around 3 billion years ago, the Earth was largely made up of water, rock, and a basic atmosphere of nitrogen and carbon dioxide. If there was any oxygen at all, it was insufficient to support contemporary creatures. Early cells arose from the ocean’s primordial soup, and some of these cells had the capacity to photosynthesize. Cyanobacteria were first discovered in these creatures. The vast quantity of photosynthesis that occurred when these cells reproduced and spread over the world’s waters provided huge quantities of oxygen to the Earth’s early atmosphere! Scientists believe that the early atmosphere became highly oxygenated approximately 2.3 billion years ago, based on the analysis of rocks, fossils, and other ancient data.
Other bacteria that could use the energy gathered by these early cyanobacteria likely arose as a result of this gaseous oxygen, which is a key aspect of the aerobic respiration process. This might have triggered the endosymbiosis process (discussed in section 2.11), which gave rise to the earliest eukaryotic species roughly 2 billion years ago. Early eukaryotic algae developed into the first land-dwelling plants that were likely comparable to ferns after almost a billion years of evolution in the sea. Flowering plants only appeared after several million years, allowing animals to develop into the huge variety we see today!
We know a bit less about the development of the macromolecules that enable photosynthesis. These molecules, such as chlorophyll, have a structure that enables them to take energy from passing photons in order to harvest solar energy. Although numerous pigment molecules may receive energy from photons, cyanobacteria, algae, and vascular plants only have chlorophyll-a. As a result, ancient bacterial populations are thought to have produced the first photosynthetic pigment.
Consider this… While photosynthesis may seem to be an abstract idea when we look at the separate mechanisms that make it possible, it actually feeds the planet. Most life on Earth depends on the process of photosynthesis to gather energy from the sun and store it in the bonds of organic molecules, whether it is a herbivore feeding off the excess energy that plants have accumulated or a carnivore feeding off the energy that herbivores have gathered from plants. So, when we begin to understand how photosynthesis works at a molecular level, bear in mind that this process is physically generating the energy you need to exist.
Let’s take a look at the overall process before diving into the several processes that make photosynthesis possible. Photosynthesis is a process that converts water, carbon dioxide, and solar energy into glucose molecules while also releasing oxygen as a by product. While the overall reaction is simple enough to comprehend, and you can literally count all the atoms involved to ensure that they are all in balance, the actual process of photosynthesis requires dozens of individual reactions and enzymes to slowly recombine carbon atoms from carbon dioxide into much larger glucose molecules.
These many reactions may be separated into two categories: light-dependent and light-independent reactions. Both of these sets of events occur in separate areas of the chloroplast. The light-dependent reactions take place immediately on the thylakoid membranes in the stacks of grana, whereas the light-independent reactions take place inside each chloroplast’s stroma (or space between the grana). The light-dependent processes are responsible for obtaining energy from the sun and storing it in the bonds of ATP molecules and electron carriers like NADPH, as shown in the following slides. This method makes use of water and produces oxygen. The light-independent processes convert carbon dioxide molecules into glucose molecules using the energy from ATP and electron carriers.
Photosynthesis takes place on thylakoid membranes, which contain a variety of critical enzymes and pigments that allow the process of extracting energy from light to take place. Let’s investigate further!
A close examination of the grana stacks inside a chloroplast reveals that they are just a complicated stack of thylakoids. This folded membrane creates a compartment in each thylakoid. But we’ll have to zoom in much further to see the photosystems, which are proteins embedded in the thylakoid membrane that enable photosynthesis.
The photosystems, which include photosystem I and II, are integral membrane proteins that are incorporated directly into the thylakoid membrane. Keep in mind that these photosystems were named in the order in which they were found, but they all work together to create an electron transport chain that begins with photosystem II. Photons strike photosystem II, which begins the process. This enzyme contains chlorophyll pigment molecules, which employ a magnesium atom in the middle of the structure to capture photon energy. Photosystem II, which catalyses a process that splits a water molecule apart into oxygen atoms and hydrogen ions, receives this energy.
Electron Transport Chain
The hydrogen atoms accumulate in the thylakoid lumen as the electrons from this reaction move through the electron transport chain. Before reaching the protein proton pump cytochrome b6f, electrons pass via a molecule of plastoquinol (or simply PQ). The energy from electrons is used to pump additional hydrogen ions into the thylakoid membrane via this proton pump.
Before reaching photosystem I, the electrons pass via another molecule called plastocyanin (or PC). Another pigment-containing enzyme is Photosystem I. Instead of splitting water, photosystem I re-energize electrons with the energy it collects. After that, the re-energized electrons may be combined with the electron transporter molecule NADP to form NADPH. The hydrogen ion gradient created by this electron transport chain is then transmitted via ATP synthase to form ATP molecules as a final step.
Chloroplasts use the energy acquired in light-dependent activities to create glucose in light-independent reactions. These processes take place in the chloroplast’s stroma. The Calvin cycle is a process that concentrates on the most important of these processes.
When an enzyme combines the Calvin cycle’s end product, ribulose biphosphate, with a carbon dioxide molecule, the Calvin cycle begins. Because it “fixes” 1 carbon atom from carbon dioxide with an already-established 5-carbon chain, this process is known as carbon fixation. This produces a 6-carbon molecule, which soon splits in half to yield 3-phosphoglyceric acid, a 3-carbon molecule.
Reduction And Regeneration
The Calvin cycle then rearranges this molecule into glyceraldehyde 3-phosphate using ATP and NADPH from light-dependent processes (also known as G3P). “Reduction” is the name for this stage of the Calvin cycle.
This G3P molecule is what you get when you split glucose molecules in half, and we’ll see it again in the following part when we look at the glycolysis and aerobic respiration processes. Only a small part of G3P is exported from the Calvin cycle to create glucose, leaving the remainder in the cycle. In the “regeneration” stage of the Calvin cycle, the leftover G3P molecules will be utilised to produce additional ribulose bisphosphate. This permits the cycle to continue, with the carbon fixation step being restarted.
While plants complete certain portions of this process in different ways, the Calvin cycle and carbon fixation are the same in all photosynthesizing organisms and are responsible for storing the bulk of the energy used on the planet.