Cell Bio Lecture 4.pdf

Plasma Membrane and Internal Membranes ★ Outside of the nucleus is called the cytoplasm and the liquid is called the cytosol ★ Cytosol is the aqueous environment between the nuclear membrane and plasma membrane ○ Doesn’t contain membrane-enclosed organelles ○...

Plasma Membrane and Internal Membranes ★ Outside of the nucleus is called the cytoplasm and the liquid is called the cytosol ★ Cytosol is the aqueous environment between the nuclear membrane and plasma membrane ○ Doesn’t contain membrane-enclosed organelles ○ Abundant, closely-packed molecules results in a water-based gel ○ Site of many chemical reactions ○ Contains ribosomes- protein synthesis ★ Membranes are abundant and their function compartmentalizes things/ sets things apart ★ Plasma membrane separates the inside of the cell from everything else- has to be able to get information from outside of cell and relay that information to the insides ★ Mitochondrial membrane separates the mitochondria from everything else ★ Compartmentalization is a big deal as it leads to specialization to accomplish certain things ★ Membrane has to be able to move and expand- malleable Cellular Membranes are Bilayer Structure That Contain Embedded or Associated Proteins ★ Membrane is a bilayer ★ Made from Phospholipids with proteins embedded in it ★ Phospholipids are the basis of the membrane as you can get rid of the proteins and it will still be a membrane, but not vice versa Naming Membrane Layers ★ Mitochondria has two membranes ★ Two leaflets of a membrane that touch the same compartment must have the same name ○ So if the inner leaflet touches the cytosol then it will be called the cytosolic leaflet ○ If you touch the same compartment you have to have the same name Ex. 1st Outer- Exoplasmic Leaflet, 1st Inner- Cytosolic Leaflet, 2nd Outer- Cytosolic Leaflet, 2nd Inner- Exoplasmic Leaflet, 3rd Outer- Exoplasmic Leaflet, 3rd Inner- Cytosolic Leaflet ○ Once you establish which one is the cytosolic leaflet and which one is the exoplasmic leaflet, you can’t change it ★ When the exoplasmic leaflet is on the inside, the way it makes sense is by when a vesicle is coming off the ER, it will fuse to the plasma membrane so it needs the exoplasmic leaflet to help it fuse and dump its contents into the plasma membrane as they are both plasmic and it will take in plasmic material ★ Once a protein is in the ER membrane, the protein will be held in the membrane, so if you want it outside of the cell then you put it in the exoplasmic leaflet so it is where it needs to be (plasma membrane) Membrane Lipid and Protein ★ Lipids ○ Provide structural diversity All membranes are the same and different phospholipids help with that  ○ Provide structural stability ○ Impart a fluid nature to the membrane Malleable ○ Precursor of signaling molecules Membrane has to be able to respond to signals that come from outside the cell ○ Imparts a permeability barrier to water soluble molecules ○ Allow the passage of hydrophobic molecules ★ Proteins ○ Transport ○ Anchoring ○ Chemical Reaction ○ Signaling Intermediates Phospholipid Provide the Fundamental Molecular Structure of Membranes ★ Phospholipids are made of Hydrophilic Head Group and Hydrophobic tails ★ Hydrophilic Head Group: ○ Glycerol (non-polar)- connected straight to the tail, think three carbons, nothing but hydrocarbons ○ Phosphate ○ Choline ○ Together it is polar ★ Phospholipids are amphipathic molecules- meaning they have both nonpolar and polar ends All Membrane Lipids are Amphipathic ★ Glycol lipids are all amphipathic Membranes are Composed of Glycerophospholipids and Sphingolipids ★ Most lipids are made from a Sphingosine Backbone ★ Some Have a Glycerol Backbone ★ Sphingomyelin is often found in the exons of the brain ★ Sphingosine can also be used to build glycolipids ★ Sphingolipids are built from sphingosine rather than glycerol ★ Glycolipids resemble sphingolipids, but, instead of a phosphate-linked head group, they have sugars attached ★ Phosphatidylcholine is more bulky phosphatidylethanolamine as it’s head group is more branched and had a head group just like sphingomyelin ★ Sphingosine has a high affinity for cholesterol thus so do the tails of sphingomyelin Packing Arrangement of Lipids in an Aqueous Environment ★ To keep the fatty acid tails away from an aqueous environment would be to create a ball form - micelle (cone like) or create a bilayer (cylinder like) ★ The space occupied by a head group is the same space as the hydrocarbon chain, so in humans they can’t be packed in a ball ★ The problem with the bilayer, the ends of the bilayer is exposed (edge effect), so they aren’t protected from the aqueous environment ○ A way to solve the edge problem is to take the edges and put them together- creating a liposome ○ At a basic level, a cell is a liposome Sealed Compartments are the Most Energetically Favorable Arrangement of Bilayers ★ Sealed structures are stable because they avoid exposure of hydrophobic hydrocarbon tails to water, thus the sphere-like shape would be like a shield. Individual Membrane Lipid Composition Varies ★ There are big differences in composition of membranes because sometimes you need one with a negative charge, having two with the same head group, but different backbone as one has a high affinity of cholesterol and it is also the reason why there is four phospholipids instead of just one ★ We need all four Why is the Lipid Composition of Membrane so Diverse? ★ Diversity allows our cells do things that prokaryotic cells can’t do ★ Composition affects structure and function ○ Shape ○ Thickness ○ Fluidity ★ Provide correct environment for proteins to function ★ Provides the correct environment for protein localization ★ Allows different membranes to face different external environments ★ Localized production of signaling molecules Cholesterol Structure ★ OH- polar head group ★ Nonpolar hydrocarbon tail ★ Rigid planar steroid ring structure ★ Amphipathic b/c it’s a lipid ★ Inserts itself with the tail going into the middle and polar head group is on the phospholipid Second Messengers Derived From Phosphatidylinostiol ★ How does the length of the side chain affect the membrane? ○ Longer side chains means the membrane will be thicker ★ Fatty ACL can be saturated. What does that mean? ○ No double bonds as it is filled with hydrogens and tend to have a straight structure ○ If it is unsaturated, then there is a kink (in the double bond) in the fatty acid tail, which makes the membrane thinner and affects viscosity due to it taking up more vertical space than straight up ○ The four major phospholipids in the membrane 1. Phosphatidylcholine - Has the same head group as sphingomyelin 2. Phosphatidylethanolamine - Small head group 3. Phosphatidylserine - Negative charge 4. Sphingomyelin ★ Phosphatidylinositol is also a major phospholipid ○ A glycerol lipid ○ Inositol sugar  ○ Its polar head group is always sticking into the cytosol and this matter because it turns out the head group can be clipped off (Inositol) and be used as an important signaling protein ○ All the carbs had the ability to be phosphorylated as it has a OH group on it- can be on 4 (4-phosphate), 4 & 5 (4,5-bisphosphate), or when Inositol had been clipped off 1,4,& 5 (1,4,5-triphosphate) Phosphatidylinositol and Phosphoinositides ★ If the head group isn’t clipped off then it is called a phospholipotide ○ Serve as binding sites for proteins Lipid Composition Affects Membrane Shape ★ Phosphatidylethanolamine the head group is small enough that its shape is a cylinder, so it is more curved to fit in sharped spaces