Photosynthesis Notes (PDF)

Photosynthesis is a chemical process with the following equation: H2O + CO2 → C6H12O6 + O2. Both plants and photosynthetic bacteria are capable of this complex conversion process. The overall reaction is spurred by the energy from a photon of light striking a pigment in the chloroplast. It is thoug...

Photosynthesis is a chemical process with the following equation: H2O + CO2 → C6H12O6 + O2. Both plants and photosynthetic bacteria are capable of this complex conversion process. The overall reaction is spurred by the energy from a photon of light striking a pigment in the chloroplast. It is thought that photosynthesis first evolved in prokaryotic cells. Photosynthesis is broken down into two major steps which are dependent on one another: light- dependent reactions and light-independent reactions (Calvin Cycle). Both of these processes occur in the chloroplast of a photosynthetic organism. Light-dependent reactions occur in the thylakoid membranes of the chloroplast. These are the “pancakes” of the chloroplast, as they look like a stack of flattened disks. The thylakoid membranes possess important pigments called chlorophyll. This pigment has electrons in it that are excited when energy is input by a photon of light. When light strikes the chloroplast, an electron from a molecule of chlorophyll is excited and travels through the electron transport chain. In the process, a concentration gradient of hydrogen ions is formed. This will be used later to produce ATP through ATP synthase. The electron lost from chlorophyll is replaced by an electron from water. This creates more hydrogen ions and the production of oxygen, which is released from the plant. When light hits the pigments, it'll hit Photosystem II first, which is embedded in the internal membrane of the chloroplast and excites electrons. This causes H+ ions to move into the thylakoid space and to replenish electrons, the light splits water, called photolysis, into two H+ ions and 1/2 of O2 and electrons, which replaces the missing electrons in Photosystem II. Why are the electrons missing then? Well, it's because the electrons continue to jump down the thylakoid membrane, bumping into Photosystem I, and thus leaving Photosystem II. With the electrons going down the thylakoid membrane, hydrogen ions continue to be pumped into the membrane. Because there a lot of hydrogen ions inside the thylakoid space, it's natural for the H+ ions to want to leave the thylakoid space. But the only way for these ions to leave is to go through a transport protein called ATP (adenosine triphosphate) synthase, where ADP (adenosine diphosphate) is phosphorylated (add another phosphate) when H+ goes through it. Other electrons from Photosystem I bind to an electron carrier, such as NADPH. Electron carriers transport electrons in the form of a hydrogen ion. These electrons can then be used in other processes. In this case, the electrons will be used to form bonds in the Calvin Cycle. The ATP and electron carriers produced during the light-dependent reactions are essential to the production of glucose in the light-independent reactions. The production of oxygen is toxic to the plants but provides the rest of the world with the opportunity to breathe. The light-independent reactions are named due to the fact that they do not require light in order to proceed. This set of reactions is also referred to as the Calvin Cycle. These reactions take place in the stroma of the chloroplast, or the gooey space in between the thylakoid pancakes. With the help of ATP and NADPH, CO2 is turned into sugar. In the Calvin Cycle, carbon dioxide is converted into an organic carbon source, most often modeled by glucose. The first step of this reaction involves the enzyme ribulose bisphosphate carboxylase, abbreviated as rubisco. This enzyme is responsible for carbon fixation, taking carbon dioxide from the air and converting it into an organic, usable form. After carbon dioxide has been fixed, the process begins to convert it into glucose. This involves the creation of a lot of bonds. In order to make bonds, electrons and energy are required. This is where the electron carriers and ATP from the light-dependent reactions come into play. By using the energy from ATP and the electrons from the electron carriers, a number of enzymes are able to convert organic carbon into glyceraldehyde-3-phosphate, or G3P. G3P is a precursor for a number of carbohydrates such as starch, cellulose, and glucose. The cell can use this to create a number of important energy and structural components. Also important, the ATP that is used is broken down into ADP and a phosphate group which can be recycled and rebonded in the light-dependent reactions. Similarly, the electron carrier NADPH becomes NADP+ after dropping off the hydrogen. This can then be refilled with an electron in the light-dependent reactions. Cellular Respiration is a chemical process with the following equation: C6H12O6 + O2 → H2O + CO2. All organisms, including those capable of photosynthesis, go through the process of cellular respiration. The overall reaction breaks down a carbohydrate, most frequently modeled by glucose, and converts the energy stored in that molecule into the most basic cellular energy, ATP. Respiration is almost the complete opposite of photosynthesis. So if you understood photosynthesis, understanding respiration should be relatively easy. Cellular Respiration is broken down into three major steps which are dependent on one another: glycolysis, the Krebs cycle, and the electron transport chain. While glycolysis takes place in the cytoplasm of the cell, the Krebs cycle and the electron transport chain take place inside of the mitochondria. Glycolysis is the most evolutionarily conserved process in cellular respiration. The process takes place in all living organisms in almost the exact same way. Fundamentally, glycolysis involves breaking down glucose, which possesses 6 carbons, into two 3-carbon molecules of pyruvate. In the process, a small amount of energy is released due to the breaking of bonds. This is captured as 2 molecules of ATP. Similarly, the breaking of bonds releases a few electrons that are picked up by electron carriers, NADH. These electrons will be dropped off to the electron transport chain later. Before pyruvate can continue on into the mitochondria to enter the Krebs cycle, pyruvate oxidation takes place. Oxidation is the loss of electrons. In this process, pyruvate becomes a 2-carbon molecule called acetyl CoA. A molecule of carbon dioxide is released from each pyruvate molecule that is oxidized. The Krebs Cycle takes place in the mitochondria. In this cycle, similarly to the Calvin Cycle, a number of enzymes process a number of reactions The moral of the story is that a number of highly specific enzymes break down acetyl CoA in reactions that create a number of electrons and a little bit of energy. The process results in the creation of a lot of electron carriers (around 8) such as NADH and FADH2. These electron carriers will allow a lot of ATP production in the electron transport chain. 2 ATP are also produced in the Krebs Cycle. The cell cycle is the sequence of steps prior to cell division. Cell division is crucial to survival because it replaces bad cells, and plays a role in growth and tissue repair. Mitosis (the process of cell division) is an asexual reproduction, which means the parent cell will produce two identical daughter cells that are also identical to the parent cell. This does not bring diversity within the cells, but it is an efficient way to create cells that will replace old cells. In eukaryotic cells, the cell cycle is highly regulated through the growth and reproduction of cells. If this process is not regulated, the cell will continue to divide non-stop, which is what a cancer cell is. Therefore, there are checkpoints and signals to regulate the cell cycle throughout each phase. Depending on the cell type, the cell division can happen frequently or nearly never. The cell cycle consists of 5 phases: interphase (G1, S, and G2), mitosis, and cytokinesis. Interphase contains the phases G1, S, and G2. Over 90% of the cell cycle is spent in interphase! During interphase, the chromatin of the cell is threadlike so when looking at a cell undergoing interphase, a centrosome can be spotted with 2 centrioles. During the S phase, the centrosome is duplicated. Now, let’s break down the phases. 1 ️⃣ G1 is a period of intense growth and activity. 2 ️⃣ S is used to stand for the synthesis of DNA. The DNA is replicated so the cell now has two sets of the same DNA. 3 ️⃣ G2, the cell continues to grow in order to finish cell division. Keep in mind, a cell can go into the G0 phase, which is a phase where a cell never divides. These G0 phase cells can reenter the cell cycle when they receive appropriate signals, and dividing cells can exit the cell cycle at any given time. Because the cell cycle is highly regulated, cells are only told to divide when it receives a growth factor via the signal-response pathway. Mitosis can also be contagious, because a currently-mitosis-cell can activate mitosis in another cell nearby. Mammalian cells are able to sense cells around it and tell others to divide and stop dividing when there are enough cells. Mitosis is the part of the cell cycle when the nucleus of the cell is divided. Even though mitosis is one process, it is broken down into prophase, metaphase, anaphase, and telophase. Prophase is the first phase of mitosis. In prophase, the nuclear membrane begins to disintegrate, chromosomes condense, and the spindle begins to form. Because DNA is a bunch of tangled mess, the DNA is nicely wrapped into chromosomes so equal dividing will become a little easier. During metaphase, chromosomes begin to line up in the middle of the cell. Also, the centrosomes move to the ends of the cell. Anaphase is when the centromeres finally separate. The spindle pulls apart the now sister chromosomes (identical copies). Telophase begins when the chromosomes move to opposite ends of the cell. The chromosomes begin to uncoil and return to their threadlike shape. Mitosis is complete once the cell separates and 2 separate nucleoli form. The process of cytokinesis is different in plant and animal cells. After mitosis occurs, cytokinesis begins. Cytokinesis is when the cytoplasm is divided. The process of cytokinesis is different in plant and animal cells. For plant cells, a cell plate made of stiff sugars is formed and surrounds the cell membrane. In plant cells, the daughter cells do not separate from each other. Instead, a new cell wall is created. For animal cells, a cleavage furrow is formed. A cleavage furrow is a groove that is created in the middle of the cell surface. The cytoplasm then begins to separate.