41) Endosome and lysosome play an important role in uptake of cholesterol (3) A.

Question

41) Endosome and lysosome play an important role in uptake of cholesterol (3) A. Explain what happens in endosome and lysosome during LDL transport? B. What type of vesicles carries LDL molecules? 42) What is the function of cholesterol in the membrane? Explain how it works. (2) 43) Where are flippases and scramblases found? What are their functions? (4) 44) ATP, Pyruvate and Pi are transported through coupled pumps in and out of mitochondria, name the molecules that drive the transport of ATP, Pyruvate and Pi and the type of coupled transport (symport/antiport). (3) Coupled molecule type of transport ??? Pyruvate

Explanation / Answer

41.A. Most cholesterol is transported in the blood as cholesteryl esters in the form of lipid-protein particles known as low-density lipoproteins (LDL). When a cell needs cholesterol for membrane synthesis, it makes transmembrane receptor proteins for LDL and inserts them into its plasma membrane. Once in the plasma membrane, the LDL receptors diffuse until they associate with clathrin-coated pits that are in the process of forming . After shedding their clathrin coats, the vesicles deliver their contents to early endosomes, which are located near the cell periphery. Once the LDL and LDL receptors encounter the low pH in the endosomes, LDL is released from its receptor and is delivered via late endosomes to lysosomes. There the cholesteryl esters in the LDL particles are hydrolyzed to free cholesterol, which is now available to the cell for new membrane synthesis. If too much free cholesterol accumulates in a cell, the cell shuts off both its own cholesterol synthesis and the synthesis of LDL receptor proteins, so that it ceases either to make or to take up cholesterol.

B. Pinocytic Vesicles Form from Clatherin-Coated Pits in the Plasma Membrane.Once in the plasma membrane, the LDL receptors diffuse until they associate with clathrin-coated pits that are in the process of forming  Since coated pits constantly pinch off to form coated vesicles, any LDL particles bound to LDL receptors in the coated pits are rapidly internalized in coated vesicles.

42. Functions of cholesterol in the membrane:

43.Flippases are transmembrane lipid transporter proteins located in the cell membrane responsible for aiding the movement of phospholipid molecules between the two leaflets that compose a cell's membrane. They are transmembrane lipid transporter enzymes responsible for aiding the outward-in movement of phospholipid molecules between the two leaflets that compose a cell's membrane

Scramblase is a enzymatic protein found in cell membrane responsible for the translocation of phospholipids between the two monolayers of a lipid bilayer of a cell membrane.

44.

Coupled Molecule

Type of transport

ATP

Na+/glucose

Symport

Pyruvate

H+

Symport

Pi

H+

Symport

Coupled Molecule

Type of transport

ATP

Na+/glucose

Symport

Pyruvate

H+

Symport

Pi

H+

Symport

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