Cell Membrane Proteins

Membrane Protein Functions

Students often get mixed up here.  This little lecture is on the proteinsLarge molecules made of amino acids with various functions in the body. that are embedded in cell membranes.  So, these functions listed here apply ONLY to proteins that are embedded in a cell membrane.  That is why you see the gray phospholipid bilayer in each picture.  And that picture on the bottom right even labels the extracellular matrix on the outside of the cell. We will encounter all types of proteins embedded in cell membranes.  The most often encountered is the first listed here: channelsProtein passages in the cell membrane that allow specific molecules to pass through..  Pathways for moleculesGroups of atoms bonded together. to cross the phospholipid bilayer barrier.  The rest of this minilecture goes into much more detail about the various types of channels we will encounter in this class.  Before we list those, let’s just be sure to take a look at these other functions. 

We are familiar with enzymesProteins that speed up chemical reactions in the body. from a previous minilecture and there are enzymes embedded in cell membranes instead of free floating in the cell.  They usually have some part dangling off into the cell where they catalyze their reaction

When we get to speaking about hormones and the immune system, we will talk about receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter that are embedded in cell membranes and respond to chemical signals.  For example, your brain makes a hormone called the antidiuretic hormoneADH A hormone that increases water reabsorption in the kidneys to concentrate urine and reduce water that helps you conserve waterThe universal solvent essential for life. when you are dehydrated.  This hormone floats around in your blood but only communicates with cellsThe basic structural and functional units of life. in the kidney.  No other cells have the specific receptors in their cell membranes that would help them receive the signal. 

Your immune system has cells that are capable of recognizing your own cells. Your cells have specific proteins embedded in the cell membranes. But those proteins are almost like a sticker that says hello my name is Amy. In many autoimmune diseases your immune system cells stop recognizing the identity proteins or the identity proteins begin to be made malformed.

Leaky Channels

All your cells have protein channels embedded in their membranes. The various protein channels contain a type called a leaky channel because it is always open. Channels are specific to the size and shape of 1 molecule. So this pink protein embedded in the cell membrane here allows only one type of molecule to move through it. Other molecules with different shape and different size usually have their own protein that allows them to move from one side to the other. Which direction the molecule moves is determined by a gradient which we speak about in another mini lecture. But you have these leaky channels in all your cells allowing sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. cation potassium(K⁺): Major ICF cation; essential for muscle and nerve function. cation calcium cation the bicarbonate(HCO₃⁻) – A crucial buffer in blood that helps maintain pH balance; formed when carbon dioxide anionA negatively charged ion. to just enter or leave according to a gradient.

Chemically (Ligand)-Gated Channel Proteins

There are channels that will only open when a certain molecule connects with it. I have a picture of a Cell membrane protein here. On the left the channel is closed So despite a gradient the chosen molecule would not move if the channel is closed. On the right I have an open channel. Of course the chosen molecule can move through this open channel.

Notice the molecule that acts as a key opening the channel is different than the molecule that is allowed to move through it. Correct. So the shape of this protein channel determines what molecule moves through it. But the shape of this active siteThe specific region of an enzyme where a substrate binds and a reaction occurs. right here determines which molecule will act as a key opening up this protein channel. These types of channels are called chemically gated channels. The term chemical there just indicates molecule or item or thing. Depends on what book I’m teaching from but sometimes they’re also called ligand gated channels.  Ligand is like an old word in chemistry that just means thing. There were some people sitting around a table and they’re like I don’t know what to call this and they’re like ligand it just means nothing.

Voltage-Gated Channel Proteins

Whereas the previous channel would open with a physical molecule making connection acting like a key that’s inserted into a lock, these types of channel proteins open due to a change in voltage. You may not realize this but that is how key cards work at hotels. There is actually an active magnetic field running through the lock and when you insert the key card you break the field opening the door. You are literally opening the door and walking through it by using a change in voltage. As far as cells are concerned voltage or electricity is just the movementA fundamental property of life involving motion of the body or its parts. of ionsCharged atoms or molecules.. We’ve been talking about that as we’ve been talking about these channel proteins. Positively or negatively charged ions are moving from outside to inside or inside to outside the cell and that movement creates a difference in charges and that’s what voltage is. These channels here have a thresholdThe minimum voltage needed to trigger an action potential. or a set point of voltage At which they will open. If enough sodium ions or enough potassium ions move from one side of the cell to another these voltage gated channel proteins will open allowing their specific ion to flow according to a gradient.

Double Transport

We’ve simplified things in our examples, but in reality, transport is much more complicated.  For example, the management of glucoseA simple sugar that is the main source of energy for cells. by the kidney is linked to the management of sodium.  Cotransport is like when someone hitches a ride with someone else.  You can see cotransport in these examples on the left with the blue membrane protein.  Two or even three molecules are being moved at the same time.  On the right side of this picture is antiportA type of membrane transport in which two substances move in opposite directions across a membrane..  This is when one molecule or ion moves in one direction and another ion moves in the opposite direction.  There is this little protein called the sodium potassium pump, which we will constantly be reminded of in this class.  This pump is an antiporter, transporting sodium and potassium in opposite directions.  And, it’s worthwhile to mention that cotransport and antiport transport can be active, using ATPThe energy currency of cells used for muscle contraction., or passive.