Electrons are going to enter a membranous pinball machine in episode 57.
Electrons are going to enter a membranous pinball machine in episode 57. The electron transport chain is found in the process of photosynthesis (2:10) and the process of cellular respiration.(4:30) This recap will first focus on structure and function for each respective organelle before drawing similarities (7:34) and pointing out differences. The episode concludes with unit connections and exam tips! (8:29)
The Question of the Day asks (9:43) True or False?Sequentially, photosystem I precedes photosystem II in the thylakoid membrane ETC.
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap Electron Transport Chains
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Unit 3 - Cellular Energetics
Topics 3.5 Photosynthesis and 3.6 Cellular Respiration
Big idea - Energetics
The ETC - or electron transport chain as it's called - is not very creatively named. It involves moving electrons in a row, down a chain. But we have to keep in mind that when we hear the word transport it doesn't always only mean across a membrane. In fact the electron transport chain involves movement of ions across a bilayer through embedded protein as well as movement of electrons within a bilayer. Electrons are going to enter a membranous pinball machine, energetically bouncing to new levels, before reaching the final electron acceptors at the end of the chain.
Let’s Zoom in:
The electron transport chain is found in the process of photosynthesis and the process of cellular respiration. This recap will first focus on structure and function for each respective organelle before drawing similarities and pointing out differences.
Recall that mitochondria and chloroplasts have double membranes and are theorized to have once been independent prokaryotic organisms. Not only do they have their own circular DNA, ribosomes, and replicate independently by binary fission – but they are also approximately the size of existing prokaryotic cells. PLUS, prokaryotes, like bacteria, synthesize ATP across their own plasma membrane with an electron transport chain! Wonder where chloroplasts and mitochondria learned it all.
Photosynthesis is the process by which light energy is converted into chemical energy. It is anabolic and endergonic. The star of the show will be the chloroplast. This organelle contains a double membrane, internal membranous thylakoids, stacked into grana and surrounded by fluid stroma. These flat membranous stacks increase the surface area to volume ratio and allow for small internal volumes to quickly accumulate ions - which will be incredibly important for the ETC. Photosynthesis can be divided into two stages - light dependent, which is where the ETC is located, and the Calvin cycle.
The light reaction occurs in the thylakoid membrane, where energy from sunlight is captured by the photosystems - large protein complexes with pigment molecules, like chlorophyll. When light strikes chlorophyll, it splits water into hydrogen ions, excited electrons and Oxygen. These excited electrons enter the ETC at photosystem II and are shuttled down (not across) the membrane through additional proteins to PSI, losing energy along the way. This action contributes in the formation of an electrochemical gradient, as protons are actively pumped into the thylakoid space, or lumen. The hydrogen ions later flow back down their concentration gradient into the stroma through another protein called ATP Synthase, assisting in the formation of ATP in a process called chemiosmosis. The electrons still in the ETC at PSI are re-excited to a higher energy level by additional light absorption. They then exit the chain, reducing coenzyme NADP+ into NADPH. The entire process is referred to as photophosphorylation since it involves the formation of ATP using light.
The reactants of the light reaction are water, and well light. The products of the light reaction are oxygen (which exits the process at this stage), ATP and NADPH, which will act as energy shuttles to the Calvin cycle for carbon fixation. We’ll revisit these key players during ETC compare and contrast time at the end of the episode.
Cellular respiration is the process by which organic molecules are broken down to release usable energy in the form of ATP. It is catabolic and exergonic. Glucose and oxygen yield water, carbon dioxide and energy. These players should sound familiar, as the reverse reaction (with sunlight) is photosynthesis! The star of the show will be the mitochondria. This organelle contains a double membrane with internal folds called cristae and fluid component called the matrix.
Cellular respiration can be divided into four stages: Glycolysis, Pyruvate oxidation, the Krebs Cycle, and the electron transport chain. The first three steps completely oxidize glucose, with electrons transferred to the coenzymes NADH and FADH2. Now it is time for the big ETC show - and justification for why the process is called respiration.
The final step takes place across the large surface area of the cristae, the inner mitochondrial membrane. Like all membranes, this is composed of phospholipids with embedded proteins and has a role in the formation of an electrochemical gradient. Electrons from the coenzymes NADH and FADH2 enter the Electron transport chain. As they move along the phospholipid membrane through a series of proteins and redox reactions, they lose energy. This lost energy pumps hydrogen ions from the matrix into the intramembranous space. The build up of an electrochemical gradient causes hydrogen ions to diffuse back into the matrix through an additional protein called ATP Synthase. Under ideal conditions, approximately 30 ATP are produced from ADP and an inorganic phosphate through chemiosmosis. Electrons exit the chain and finally, oxygen has its big moment! Oxygen is the final electron acceptor, and combining with hydrogen ions, form water in the mitochondrial matrix. Collectively, this ETC and chemiosmosis are called oxidative phosphorylation.
Let’s bring it all together. The ETC was the first step in photosynthesis and the last in cellular respiration. They each occur in their organelles respective internal membrane - thylakoid and cristae. Electrons are donated to the ETC in chloroplasts by the splitting of water while water forms at the end of the mitochondrial ETC after oxygen accepts electrons. Both chains involve active protons pumps, electrochemical gradients, and chemiosmosis through ATP synthase. ATP is formed in each organelles fluid portion - stroma and matrix. Coenzyme for photosynthesis is NADPH and cellular respiration has NADH and FADH2 - serving the same function. Remember, plant cells have chloroplasts and mitochondria - so they have both electron transport chains!
Time for unit connections. You’ll use this information in Unit 6 when studying how cells use energy to fuel life’s processes as well as in Unit 8 - connecting to ecosystems and the carbon cycle.
Alright - what about the exam? It's important to focus on the difference between these two processes, how it works within an ecosystem, and the possible consequence on several levels if the process is disrupted (dysfunctional proton pump perhaps?). Know your inputs and outputs, so that if a reactant changes -you can predict a possible outcome. You might need to graph experimental data and calculate reaction rates, so brush up on DRY MIX TAILS. Oh, and don’t memorize specific structures or names of enzymes in these processes - you only need to know ATP Synthase. When in doubt, follow the electrons.
To recap……
The ETC - shuttling electrons and transferring energy within membranes. From their humble prokaryotic beginnings, chloroplasts and mitochondria know all the tricks with the same ingredients - water, oxygen, coenzymes, proton pumps, ATP synthase and voila.
Coming up next on the Apsolute RecAP Biology Edition: Enzymes and Pacman
Today’s question of the day is about reactions
Question of the day: True or False. Sequentially, photosystem I precedes photosystem II in the thylakoid membrane ETC.