Episode 34 finds the similarities and differences between chloroplasts and mitochondria.
Episode 34 finds the similarities and differences between chloroplasts and mitochondria. The endosymbiosis theory states that these energy transducers were once independent prokaryotes (1:38). Chloroplasts capture light energy in photosynthesis (2:20). Mitochondria break down sugars in cellular respiration (4:00). There are many commonalities between each organelle’s structure and function.
The Question of the Day asks (6:35) “What are the alternate names for the Krebs cycle”?
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Hi and welcome to the APsolute Recap: Biology Edition.
Today’s episode will recap Chloroplasts and Mitochondria
Lets Zoom out:
Unit 3 - Cellular Energetics
Topic - 3.5 and 3.6
Big idea - Energetics
Chloroplasts and mitochondria are organelles found in eukaryotic cells. They are energy transducers, meaning they transform energy from one form into another through a series of reactions. The main forms of energy are light energy and chemical energy. This episode will recap their origin story as well as compare and contrast each structure and function.
Lets Zoom in:
Once upon a time, small prokaryotic microbes were minding their own business when a unicellular eukaryote came over and tried to engulf them through phagocytosis. Now instead of digesting the little microbes, they formed a symbiosis relationship. You scratch my back and I’ll scratch yours. The microbes were protected from other external predators and the eukaryote now had an intracellular energy converting factory. Not a bad gig.
This is known as the endosymbiosis theory. It is thought that both chloroplasts and mitochondria were once independent prokaryotes. 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. Fun fact - both mitochondrial and chloroplast DNA follow maternal lineages through sexual reproduction - as eggs and ovules contribute these organelle structures to the embryo. Furthermore - they have double membranes - the inner membrane representative of their original cell and the outer membrane deriving from the eukaryote that tried to consume them during endocytosis vesicle formation. It’s all compatible since membranes are composed of the same phospholipids and proteins.
Photosynthetic processes allow organisms to capture and transfer light energy. Carbon dioxide, water and light energy yield glucose and oxygen. The chloroplast contains a double membrane, internal membranous stacks and fluid. The individual membranous fold is a thylakoid, collectively called a granum as a stack. These flat membranous stacks increase the surface area to volume ratio and allow for small internal volumes to quickly accumulate ions. The surrounding organelle fluid is called the stroma. The compartmentalization of the chloroplast divides this process into light and dark reactions.
The light reaction occurs in the thylakoid membrane, where energy from sunlight is captured by the pigment chlorophyll. When light strikes chlorophyll, it splits water and excites electrons. These excited electrons are passed down the thylakoid membrane in an electron transport chain, contributing to the formation of an electrochemical gradient of protons into the thylakoid space. The hydrogens flow back down their concentration gradient through ATP Synthase, assisting in the formation of ATP in the stroma. The products of the light reaction are oxygen, ATP and NADPH. The dark reaction, or Calvin Cycle, occurs in the stroma and through a series of steps and unique structural molecules, carbon dioxide is rearranged to form carbohydrates using chemical energy from ATP and NADPH. We’ve captured light energy, transferred it, and stored it in sugars.
Switching sides now. Glucose and oxygen yield water, carbon dioxide and ATP. A familiar process of converting energy from one form into another - this time with cellular respiration and the mitochondria. The mitochondria has a double membrane and matrix fluid. The internal membrane is highly folded into cristae, which increases the surface area to volume ratio and allows for small internal volumes to quickly accumulate ions (just like chloroplast thylakoids). This means that more ATP can be synthesized by the mitochondria. The compartmentalization of the mitochondria divides this process into the Kreb’s cycle and the ETC.
The first step doesn’t even occur in the mitochondria, which is why it is universal to aerobic and anaerobic pathways. Glycolysis occurs in the cytosol of cells, splitting 6 carbon glucose in half and producing two 3 carbon pyruvates, ATP and the coenzyme NADH. Quick contrast - NADH is a coenzyme in cellular respiration while NADPH is in photosynthesis. Their function as an electron carrier is similar, but remember the P in NADPH for photosynthesis.
Next, the Krebs cycle - yes the mitochondria also has a cyclic stage for rearranging carbon molecules and it also occurs in the liquid portion of the organelle. Through a series of steps, intermediate molecules, and enzymes - pyruvate is oxidized releasing carbon dioxide. A few more ATP are produced as well additional coenzymes of NADH and now FADH2. Electrons from the coenzymes NADH and FADH2 enter the Electron transport chain in the cristae. Whatttttt!? Another electron chain!? Yep. As electrons move along, hydrogen ions are transported into the intramembranous space, forming an electrochemical gradient. Hydrogen ions diffuse back into the matrix through ATP Synthase (doesn’t that sound familiar?). Electrons exit the chain, joining with oxygen and hydrogen ions to form water.
To recap….
Chloroplasts and mitochondria are energy transducers may have once existed as independent prokaryotes. What is formed in photosynthesis is undone in cellular respiration. The ETC starts photosynthesis in the thylakoid membrane and ends cellular respiration in the cristae. Electrochemical gradients and ATP synthase in both. The Calvin cycle forms sugars in the stroma and the Krebs cycle disassemble them in the matrix. Electrons split from water in photosynthesis and join in cellular respiration. Both exchange the gases oxygen and carbon dioxide.
Today’s Question of the day is about cellular respiration.
Question “What are the alternate names for the Krebs cycle?”
Coming up next on the Apsolute RecAP Biology Edition: Energy and Graphing