Deep breath! Episode 11 will recAP photosynthesis - the process of converting light energy into chemical energy. Do you know why plants are green?
Deep breath! Episode 11 will recAP photosynthesis - the process of converting light energy into chemical energy. Do you know why plants are green? (1:18) Melanie provides an overview of the chemical equation (1:35) before drawing an analogy between the chloroplast and a favorite breakfast treat (2:10). Photosynthesis is divided into two stages - the light and dark reactions (2:40). She warns you of a common student error with remembering coenzymes (3:10). Don’t get lost in all the details with a check in on reactants and products (4:12). Melanie gives a study tip to compare light and dark reactions (5:53).
The Question of the Day (6:08) asks “How is the light reaction dependent on the Calvin Cycle?”
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap Photosynthesis.
Lets Zoom out:
Unit 3 - Cellular Energetics
Topic - 3.5
Big idea - Energetics
Introduction:
It should be noted that photosynthesis is not the only strategy of autotrophs on Earth, but it is where our focus will be for this episode. A photon checks into a hotel and is asked if he needs help with his luggage. No thanks, he replies, I’m traveling light. Get it? Photons. I didn’t write that, but the point still holds true
Photosynthesis is the process by which light energy is converted into chemical energy. We are building something (synthesis) using light (photo). It is anabolic and endergonic. Stopping by in physics land again - why are plants green? The electromagnetic spectrum has a portion called visible light (ROYGBIV - the colors of the rainbow). Chlorophyll absorbs several colors of light but primarily reflects green.
Lets Zoom in:
Photosynthesis: 6 carbon dioxides and 6 waters, rearranged with the addition of sunlight to yield 6 oxygens and glucose - C6H12O6. This is an oversimplification, but a good place to start our understanding. When observing the balanced chemical equation for photosynthesis, why are 6 carbon dioxides required?There are 6 atoms of carbon in glucose, therefore 6 molecules of Co2 are required. We cannot have more matter in the products than we started with.
The star of the show will be the chloroplast. This organelle contains a double membrane, internal membranous stacks and fluid. Think of stacks of pancakes with syrup (where the individual pancake is a thylakoid, the stack is a grana and syrup is the stroma). These flat membranous stacks increase the surface area to volume ratio and allow for small internal volumes to quickly accumulate ions. Photosynthesis can be divided into two stages (light and dark or light dependent and light independent, or further still the light reaction and the Calvin cycle) - sorry for the skitsofrenic naming.
The light reaction occurs in the thylakoid membrane, where energy from sunlight is captured by the pigment chlorophyll. The products of the light reaction are oxygen (which exits the process at this stage) and ATP and NADPH, which will act as energy shuttles to the Calvin cycle. Watch out for this common student error! NADPH is a coenzyme in photosynthesis while NADH is a coenzyme in cellular respiration. Their function as an electron carrier is similar, but remember the P in NADPH is for photosynthesis.
The thylakoid membrane (like all membranes) is made of phospholipids and embedded proteins. When light strikes chlorophyll, it splits water and excites electrons. These excited electrons are passed down the membrane in an electron transport chain through proteins called photosystems. This action contributes in the formation of an electrochemical gradient of protons across the membrane and into the thylakoid space (inside the pancake). And you know what can happen when a concentration gradient is high? Of course! Passive transport. The hydrogens flow back down their concentration gradient through a protein called ATP Synthase, assisting in the formation of ATP in the stroma.
Ok, let's touch base on what has happened so far, because I thought we were supposed to be building sugars? Light energy has been captured, transferred down the ETC, and stored temporarily as chemical energy in ATP and NADPH. We have used one reactant (water) and produced one product (O2). Now it is time for the big show.
The Calvin Cycle occurs in the stroma and will use the other reactant (CO2) and produce the intended product (sugars). Through a series of steps and unique structural molecules, Carbon dioxide is rearranged to form carbohydrates using chemical energy from ATP and NADPH. Diagrams of the Calvin cycle can be incredibly overwhelming, but remember that you are not required to know the specific steps, molecular structures or enzymes involved. Spoiler alert - we haven’t actually made glucose, but a 3 carbon molecule called PGAL or G3P. This three carbon molecule can be used by the cell to make glucose or other biological molecules. We are organic, after all, and need carbon chains. The Carbon cycle is able to continue through the regeneration of a 5 carbon sugar, ready to once again join up with carbon dioxide.
To recap….
Photosynthetic processes allow organisms to capture and transfer and light energy. The compartmentalization of the chloroplast divides this process into light and dark reactions. The formation of biological molecules store this energy for later use. I highly recommend making a summary table as a study tool. A light reaction column and Calvin cycle column - comparing purpose, location, reactants, products, and molecular machinery.
Today’s Question of the day is about reciprocity.
Question: How is the light reaction dependent on the Calvin Cycle?