bio #243 destroyer

[queenskillers]

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hey i was wondering if anybody would be able to clarify what is going on when they say that proteins and lipids can move across laterally but they cannot move from one face to another. what face are they talking about? and does laterally (horizontally) mean laterally across from outside to inside of cell or what? thanks

 

[joefosho315]

Hey, that problem is basically testing your understanding of the fluid mosaic model which is the idea that lipids can move freely in a horizontal fashion around the cell membrane, but NOT vertically. To get a better idea, imagine the cell membrane phospholipid bilayer. It contains TWO layers of phospholipids, with the fatty acid tails of the extracellular phospholipids touching the fatty acid tails of the intracellular phospholipids. The lipids from the extracellular side can move around freely on the extracellular side, but cannot all of a sudden jump down to the intracellular side. If you look at a diagram of a cell membrane, it would really help illustrate this idea. The reason for the fluid mosaic model is that all of the lipids on either extra/intra cellular side associate with each other through hydrophobic interactions (due to the hydrocarbon fatty acid tails). It would be very energetically unfavorable for the polar, thus hydrophilic, head of the lipid, to go through all of the fatty acid tails so that it could reach the other side of phospholipids. The only way that lipids can actually do this is with the catalytic work of flipases, but that's a different subject. Hope that made sense!

 

[marianirj]

hey i was wondering if anybody would be able to clarify what is going on when they say that proteins and lipids can move across laterally but they cannot move from one face to another. what face are they talking about? and does laterally (horizontally) mean laterally across from outside to inside of cell or what? thanks

There are two types of movement in a plasma membrane. There is flip-flopping of phospholipids from the extracellular surface of the membrane to the intracellular surface or vica versa. There is also what you are talking about, lateral diffusion. This is where a lipid or more likely protein just shifts it's position from one place on the membrance to another place without flip flopping it's domains. By domains I mean that an integral protein that spands from the intracellular matrix to the extracellular one has a hydrophobic domain located next to the hydrophobic tails of phospholipids and two hydrophilic domains next to the cytoplasm and extra-cellular fluid. These two extracellular domains cannot flip flop, but they can move laterally, keeping their hyrophilic domains next to hydrophilic solutions.

Hope that helps!