The movement of materials through the endomembrane system is consistent through all eukaryotic cells.
The movement of materials through the endomembrane system is consistent through all eukaryotic cells. The episode begins with a wording deep dive (1:12) before some molecular story time (2:25). The endomembrane system includes the nuclear envelope, lysosomes, vesicles, endoplasmic reticulum, Golgi apparatus and plasma membrane (4:16). Melanie concludes the episode with unit and exam connections (5:50).
The Question of the Day asks (7:00) What is the internal space of the folded Golgi cisternae called?
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap the endomembrane system.
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Unit 2 - Cell Structure and Function
Topics 2.1 and 2.2
Big idea - Systems Interactions
Since all living things are made of the same macromolecules (proteins, lipids, nucleic acids, and carbohydrates), there is a lot of opportunity for integration, cooperation, and familiar molecular exchanges. Recall that phospholipids have polar heads and non polar fatty acid tails and since both the intracellular and extracellular environments are aqueous, all membranes are bilayered with tails facing inwards. This causes organelles to be compartmentalized in their function and the endomembrane system to integrate seamlessly.
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Maximize the understanding, minimize the need for memorization. So let’s break down the words first - endomembrane: Endo meaning within and membrane implying a bilayer of phospholipids. System - just like all systems we should expect this to include multiple parts that work together to perform specific tasks. And so, the endomembrane system is composed of several organelles bound by phospholipids to complete a process. These membranes will also be highly folded!! - and why? All about that surface area to volume ratio. Go back and listen to episode 7 for a recap.
The endomembrane system is exclusively found in eukaryotic cells since only those cell types include membrane-bound organelles. The included structures are the nuclear envelope, lysosomes, vesicles, the endoplasmic reticulum, and the Golgi apparatus. The plasma membrane isn't technically within the cell, although it does have a role in the endomembrane system to transport products into and out of the cell. Other membrane-bound organelles like the mitochondria and chloroplast aren’t included in this system since they are endosymbionts. Check out Episode 34 for more details.
Once upon a time there was a small protein product assembled at a ribosome on the rough endoplasmic reticulum. This protein was destined to become an integral membrane protein within the plasma membrane. Packaged in a vesicle which buds off of the rough ER, the protein travels to the cis face of the Golgi apparatus, merging with its folds. The Golgi apparatus further processes the protein as it passes through the folded cisternae, adding on a carbohydrate chain. Once fully synthesized, the glycoprotein is again packaged in a vesicle, this time on the trans face of the Golgi, as it heads toward the plasma membrane. The transport vesicle fuses with the intracellular phospholipid bilayer, completing exocytosis with the newly-formed integral membrane protein now part of the extracellular structure. The End.
Ok lets back up a bit. The first structure is the endoplasmic reticulum – A series of interconnected folded membranes sacs and tubules continuous with the nuclear envelope. There are two types. The smooth ER, or SER and the RER or rough ER. The rough ER is named such because it's studded with ribosomes. We already said that ribosomes produce proteins, so it should be no surprise that the RER is involved in this process as well. It will additionally compartmentalize the cell and be involved in intracellular transport. Proteins undergo structural modifications, like folding, within the rough ER. The smooth ER is primarily involved in detoxification and lipid synthesis. You do not need to know how lipids are synthesized for the exam.
Next is the Golgi Complex, also known as the Golgi Apparatus, or Golgi Body. This membrane bound organelle is a series of flattened stacks involved with packaging, processing, sorting, and shipping of lipids and proteins. This organelle is typically seen with vesicles adjacent or merging with it. I’ve always imagined a lava lamp when thinking of the Golgi with its outward bubbling vesicles. The receiving side of the golgi is called the cis face while the opposite side is called the trans face, where secretory vesicles bud off. Not all cellular products are destined for exocytosis and those not made correctly may meet up with a Lysosome, which has hydrolytic enzymes, is involved in cell recycling and apoptosis, or scheduled cell death.
Time for unit connections. The endomembrane system is a large part of unit 2 with transport and has a role in Unit 3, cell communication (endocrine, paracrine and autocrine signaling). You may find reference to this system in Unit 6 as well with gene expression, with translation at ribosomes and the consequence of errors. Alright - what about the exam? It is highly unlikely you will be asked a question like “What is the role of the rough ER?” Instead, you may be given a scenario, or a disrupted scenario, and asked to make predictions and justify your reasoning. For example, if a cell is involved in detoxification, you can expect it to have a greater concentration of smooth ER while muscle cells, which are almost exclusively made of protein, have a greater concentration of Rough ER.
To recap……
The movement of materials through the endomembrane system is consistent through all eukaryotic cells. It includes the nuclear envelope, lysosomes, vesicles, endoplasmic reticulum, Golgi apparatus and plasma membrane. Products are packaged, processed, and transported throughout the system.
Coming up next on the Apsolute RecAP Biology Edition: Passive Transport
Today’s question of the day is about structure
Question of the day: What is the internal space of the folded Golgi cisternae called?