Cell Membranes

The Plasma Membrane

Plasma membranes, also called cell membranes are the barrier between the living and non-living.  Seriously.  All life is made of cellsThe basic structural and functional units of life. (well, life as we now know it). This means that the cell membrane separates all cell contents from the outside environment.  Physical separation is what a cell does, but in no way does it isolate cells.  Cells continue to exchange with the environment through the cell membrane.  Just like opening and closing your home’s doors and windows regulates heat and air flowThe movement of air into and out of the respiratory tract, driven by pressure differences between th, cell membranes function similarly. They insert and remove proteinsLarge molecules made of amino acids with various functions in the body.. They do this in the cell membrane.  This leads us to apply the term “fluid mosaic” to cell membranes.  Fluid because they are always changing and mosaic because they are made of many different parts.  Although proteins in the cell membrane seem to be our main focus, other characteristics like cholesterolA lipid molecule that is a key component of cell membranes and a precursor for bile acids and steroi provide structural support.  In this class, we will encounter muscle cells called muscle fibers.  We also call their cell membranes the sarcolemma.  The same goes for neuronsThe functional cells of the nervous system that transmit signals., we specialize their cell membrane name to axolemmaThe cell membrane of an axon..

Phospholipid Bilayer

The cell membrane, at its heart, it really just a double layer of phospholipids.  Recall that phospholipids have a polar or waterThe universal solvent essential for life. loving headRounded proximal end that fits into the acetabulum of the hip bone. of glycerol and phosphate, and nonpolar or water hating tails.  Arranged into a cell membrane requires the phospholipids to line up in two layers.  In each layer, the polar heads face the aqueous inside of the cell. They also face the aqueous outside of the cell.  This creates an area where all the nonpolar tails are concentrated.  This is what creates a barrier for polar moleculesGroups of atoms bonded together..  We will explore this topic in greater detail. We will find that cell membrane proteins are necessary. They are needed for polar molecules such as water and glucoseA simple sugar that is the main source of energy for cells. to cross the cell membrane.

Basic Cell Anatomy

I’m not kidding when I say that this is an anatomical drawing.  It is.  Let’s apply some of the fluid compartment terms from the first chapter.  Inside the cell, inside all cells, is their cytoplasmThe gel-like substance within a cell that contains organelles and cytosol..  We could collect all the cytoplasm from your cells and squeeze it into a glass. That would be your intracellular fluids.  All the fluid that is outside of a cell is called the extracellular fluids.  When we are specifically talking about a grouping of cells, we can use the term interstitial fluids.

Cell Membrane Components

The phospholipids and their bilayer is definitely the most prominent component of a cell membrane, but there are many more.  These little orangey molecules here are cholesterol.  Cholesterol provides that structural support for cell membranes.  This is why lipidsOrganic molecules including fats, oils, and steroids. are important in the diet.  You are always making cells, you always need cholesterol.  On the outside of the cell here are glycolipids, which are just big sugary fat molecules.  These make something like a sugary coating all over your cells.  It makes me think of sour patch kids with all the sugar on the outside.  These glycolipids form something we call the glycocalyxA sugar-rich coating on the surface of cells that provides protection and cell recognition..  It’s basically an outside coating, but it plays a part in your immune system.  It’s kinda like a name tag for your cells.  Everything else labeled on this picture, all of those other things between B and F are all proteins.  Some of them span the entire length of the cell membrane and are calls transmembrane proteins.  This appears like a gated tunnel through the membrane. The structure looks like an alpha helixA common spiral structure in proteins, stabilized by hydrogen bonds..  Transmembrane proteins usually play a part in some kind of transport through the cell membrane.  Ions and molecules can leak across the membrane. They can get pumped across the membrane. They can even act as a key to open the gates on these proteins. There are other proteins that don’t span the length of the cell membrane and these are called peripheral proteins.  Peripheral means “to the side.”  Most peripheral proteins are on the inside of the cell and are enzymatic or anchoring proteins.