Photosynthesis Notes PDF

The light-dependent phase of photosynthesis. Light-Dependent Reactions Just as the name implies, light-dependent reactions require sunlight. The light-dependent reactions take place in the thylakoid membranes in the granum (stack of thylakoids), within the chloroplast. The light-dependent reactio...

The light-dependent phase of photosynthesis. Light-Dependent Reactions Just as the name implies, light-dependent reactions require sunlight. The light-dependent reactions take place in the thylakoid membranes in the granum (stack of thylakoids), within the chloroplast. The light-dependent reactions begin in a grouping of pigment molecules and proteins called a photosystem. Photosystems exist in the membranes of thylakoids. A pigment molecule in the photosystem absorbs one photon, a quantity or “packet” of light energy, at a time. A photon of light energy travels until it reaches a molecule of chlorophyll. The photon causes an electron in the chlorophyll to become “excited.” The energy given to the electron allows it to break free from an atom of the chlorophyll molecule. Chlorophyll is therefore said to “donate” an electron A molecule of water splits to release an electron, which is needed to replace the one donated. Oxygen and hydrogen ions are also formed from the splitting of water. Generating an Energy Carrier: ATP In the light-dependent reactions, energy absorbed by sunlight is stored by two types of energy- carrier molecules: ATP and NADPH. The energy that these molecules carry is stored in a bond that holds a single atom to the molecule. For ATP, it is a phosphate atom, and for NADPH, it is a hydrogen atom. When these molecules release energy into the Calvin cycle, they each lose atoms to become the lower-energy molecules ADP and NADP+. Generating Another Energy Carrier: NADPH The remaining function of the light-dependent reaction is to generate the other energy-carrier molecule, NADPH. As the electron from the electron transport chain arrives at photosystem I, it is re- energized with another photon captured by chlorophyll. The energy from this electron drives the formation of NADPH from NADP+ and a hydrogen ion (H+). Now that the solar energy is stored in energy carriers, it can be used to make a sugar molecule. The Dark reactions of photosynthesis 1. The Calvin cycle in C3 plants. The Calvin cycle reactions can be divided into three main stages: carbon fixation, reduction, and regeneration of the starting molecule. Carbon fixation A carbondioxide molecules combine with three molecules of the five-carbon acceptor molecule (RuBP), yielding a three-carbon compound (3-PGA). This reaction is catalyzed by the enzyme rubisco. Reduction In the second stage, six ATP and six NADPH are used to convert the six 3-PGA molecules into six molecules of a three-carbon sugar (G3P). This reaction is considered a reduction because NADPH must donate its electrons to a three-carbon intermediate to make G3P. Regeneration One G3P molecule leaves the cycle and will go towards making glucose, while five G3Ps must be recycled to regenerate the RuBP acceptor. Regeneration involves a complex series of reactions and requires ATP. The “dark” reactions of photosynthesis in C4 plants In C4 plants, the light-dependent reactions and the Calvin cycle are physically separated, with the light-dependent reactions occurring in the mesophyll cells (spongy tissue in the middle of the leaf) and the Calvin cycle occurring in special cells around the leaf veins. These cells are called bundle- sheath cells. First, atmospheric CO2 is fixed in the mesophyll cells to form a simple, 4-carbon organic acid (oxaloacetate). This step is carried out by a non-rubisco enzyme, PEP carboxylase, that has no tendency to bind oxygen. Oxaloacetate is then converted to a similar molecule, malate, that can be transported in to the bundle-sheath cells. Inside the bundle sheath, malate breaks down, releasing a molecule of CO2. The CO2 is then fixed by rubisco and made into sugars via the Calvin cycle, exactly as in C3 photosynthesis. The “dark” reactions of photosynthesis in CAM plants Some plants that are adapted to dry environments, such as cacti and pineapples, use the crassulacean acid metabolism (CAM) pathway to minimize photorespiration. Instead of separating the light-dependent reactions and the use of CO2 in the Calvin cycle in space, CAM plants separate these processes in time. At night, CAM plants open their stomata, allowing CO2 to diffuse into the leaves. This CO2 is fixed into oxaloacetate by PEP carboxylase (the same step used by C4, then converted to malate or another type of organic acid. The organic acid is stored inside vacuoles until the next day. In the daylight, the CAM plants do not open their stomata, but they can still photosynthesize. That's because the organic acids are transported out of the vacuole and broken down to release CO2 which enters the Calvin cycle.