Episode 12 begins with a Latin lesson, *spirare.* Melanie pulls the oldest three stooges trick in the book to remind students of the products of cellular respiration
Episode 12 begins with a Latin lesson, spirare. Melanie pulls the oldest three stooges trick in the book to remind students of the products of cellular respiration (1:25). The star of the show is introduced (2:20) with a reminder of the importance of surface area to volume ratios. Cellular respiration is explored in three steps: Glycolysis (2:50), the Krebs Cycle (3:55), and ETC (5:08). Melanie checks in on the tallies of reactants and products (4:43). Don’t forget that oxygen is the final electron acceptor! (5:40). Deep breath in, deep breath out - Repeat as needed (6:35).
The Question of the Day (7:13) asks “Which step of cellular respiration will still occur under anaerobic conditions?”
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap Cellular Respiration.
Lets Zoom out:
Unit 3 - Cellular Energetics
Topic - 3.6
Big idea - Energetics
The words respiration and inspiration have the same Latin root, spirare, which means “to breathe.” So - does this mean our cells are breathing? All aerobic life requires oxygen - yes even photosynthetic ones like plants. There are several varied methods and structures to obtain and transfer oxygen within an organism. Humans, for example, have external respiration when the air we breathe diffuses from our lungs into our bloodstream - internal respiration when the gases are exchanged from the bloodstream to a tissue - and lastly, cellular respiration.
Lets Zoom in:
Place the palm of your hand in front of your mouth, take a deep breath in, and exhale. What did you feel? Your palm had a breeze, maybe slightly damp, and probably warm. Why? You exhale the products of cellular respiration! Cellular respiration - the eukaryotic process by which organic molecules are broken down to release usable energy. Glucose and oxygen yield water, carbon dioxide and energy. These players should sound familiar, as the reverse reaction (with sunlight) is photosynthesis!
So oxygen comes from the environment and glucose? Heterotrophic organisms must consume their glucose through food whereas autotrophic organisms assemble it themselves in photosynthetic or chemosynthetic pathways. The star of the show will be the mitochondria. This organelle contains a double membrane and matrix fluid. The internal membrane is highly folded (called cristae). These folds increase the surface area to volume ratio and allow for small internal volumes to quickly accumulate ions.
Cellular respiration can be divided into three stages: Glycolysis, the Krebs Cycle, and the electron transport chain. You’ve heard the suffix lysis before - hydrolysis “to cut with water.” In glycolysis, the 6 carbon sugar is cut into two equal halves, called pyruvate. This process occurs in the cytosol of cells and requires an input of energy. What, an input of energy? How can we use energy if we are trying to form energy? You’ve got to spend money to make money. And if you’ve received your first paycheck, you may have noticed that there is a gross income and a net income. The net income is what you can actually deposit at the bank after taxes have been taken out. The same is true for glycolysis. An input of 2 ATP are required while 4 ATP are formed. So the net gain of energy is 2 ATP. You will see variation in the quantity of energy molecules produced at each step, and even variation in different resources/tables/diagrams. Don’t get lost in the number details - stay focused on the big picture. Glycolysis has also produced NADH, the coenzyme and electron carrier which will be needed in later steps.
Next up, the Kreb’s cycle, also known as the Citric Acid Cycle. Pyruvate enters the mitochondrial matrix for further oxidation, or breaking down. Through a series of steps, intermediate molecules, and enzymes - carbons are removed from pyruvate and released as carbon dioxide. A few more ATP are produced as well additional coenzymes of NADH and now FADH2. Excluded from the exam are specific steps, enzymes, and intermediate molecules of the processes. This can make it really overwhelming when looking at diagrams, but zoom out. Follow the carbons, follow the energy.
Ok, let's touch base on what has happened so far, because I thought we were supposed to be forming ATP, and so far there has only been about 4 molecules. Oxygen hasn’t even done anything yet! We have used one reactant (glucose) and produced one of the products (CO2). Essentially, glucose has been completely oxidized, with electrons transferred to the coenzymes NADH and FADH2. Now it is time for the big 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, hydrogen ions are transported from the matrix into the intramembranous space. Electrons exit the chain and finally, oxygen has its big moment! Oxygen is the final electron acceptor, and combining with hydrogen ions, forms water in the mitochondrial matrix.
So we’ve used all the reactants and formed all the products of cellular respiration - when are we finally going to get these ATP molecules that were promised? At the very end. The hydrogens that accumulated in the intramembranous space during the ETC are crowded. The build up of an electrochemical gradient causes hydrogen ions to diffuse back into the matrix through a protein called ATP Synthase. Under ideal conditions, approximately 30 ATP are produced from ADP and an inorganic phosphate.
Deep breath in, deep breath out. Exhale the products of cellular respiration: CO2, water, and energy. Repeat as needed. The products of cellular respiration have a variety of uses. Some of the carbon skeletons formed are used in biosynthesis of other biological polymers. ATP formed can be used in cellular work, such as movement, active transport, animal nerve impulses, and maintenance and organization.
To recap….
Cells obtain energy from biological macromolecules in order to power cellular functions. In the presence of oxygen, eukaryotic organisms perform cellular respiration. Each step occurs in a different cellular location, with the most ATP formed in the inner mitochondrial membrane with the Electron Transport Chain.
Today’s Question of the day is about fermentation.
Question: Which step of cellular respiration will still occur under anaerobic conditions?