What cell controls what goes in or out?

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THE CELL MEMBRANE

History: Up until now you knew that the function of the cell membrane was to separate the inside of the cell from its outside. In some pictures you have seen, the cell membrane may have looked like this: 

Scientists thought that this is what it looked like for a long time.  Then the electron microscope came along and they were able to see the cell membrane.  Below is a picture of how the cell membrane looked to them.  

Scientists were then able to break apart the cell membrane and find out what chemicals were in it.  The current best model of what a cell membrane looks like is in the figure below.  As you can see it is made up of two parts.  They are the phospholipids (magenta and green), the proteins (orange and red), and the carbohydrates (black).  Carbohydrates can be attached to either the phospholipids or the proteins in the cell membrane.

The cell membrane is just like the other organelles of cells in that it serves the cell by having its own specialized jobs.  One of its jobs is to control what enters and exits the cell and thereby to protect the cell.  In the figure above we see a magnified model of a cell membrane.  

        Location: The cell membrane surrounds the cell.

        Structure: Cell membranes are made up of phospholipid molecules (fats) with various large globular protein       molecules suspended in them.  In the figure above, you can see the lipid bi-layer in purple and the proteins in orange. The lipid bi-layer (two layers of phospholipids) is formed because of the chemical structure of a phospholipid.  Since cells are constantly in water, the phospholipids form a double layer, with the heads towards the water and the tails inside so that they can stay away from the water.  These bi-layers have proteins scattered about in them.  Sometimes carbohydrates (sugars) are attached to cell membrane phospholipids and to cell membrane proteins.  A selectively permeable (sometimes called semi-permeable) membrane allows some molecules across but not others.

JOB:  The cell membrane controls what enters and exits the cell and thus protects the cell. 

Q: So how do things enter and exit the cell?  We did say that was one of the jobs of the cell membrane.

A: Well, molecules can only enter if they can go through the phospholipid bilayer (fat) or they have a special protein in the cell membrane that they can use.

Click here to see a real picture of a cell membrane.

Created March 22, 2013
Center for Teaching and Learning

By Ho-Diep Dinh

From the simplest bacteria to the most complex human cell, the plasma membrane has retained such an important function in cells that its structure has been conserved over a wide range of species. The plasma membrane is selectively permeable; it regulates what passes from one side to the other. A selectively permeable membrane will allow water to flow freely while limiting the passage of molecules, especially large molecules or those with a charge. A plasma membrane provides some protection to the insides of cells, but its main life-giving function is to control the passage of substances into and out of cells.

Not all material going in and out of a cell is tightly controlled by the cell membrane; water represents one of those substances. Water moves easily into and out of cells by osmosis. Through a force called osmotic pressure, water moves from an area that has a low concentration of dissolved molecules called solutes to an area with a high solute concentration. Solutes cannot pass through the plasma membrane, only water. When the concentration of solutes is the same on both sides of the membrane, equilibrium is reached, and water no longer flows from one side to the other.

In simple diffusion, small molecules without charges such as oxygen and carbon dioxide flow through a plasma membrane without assistance and without expending energy. Other substances such as proteins, glucose and charged particles called ions cannot pass through the selectively permeable membrane. Oxygen and carbon dioxide move, or diffuse, from an area where they exist in high concentrations to an area where they have low concentrations. This means that in general, oxygen can move from blood vessels into cells, and carbon dioxide moves from inside cells back into the red blood cells inside blood vessels.

For small charged particles called ions, carriers in the membrane will transport them into and out of cells. These carriers are specialized proteins that lie within the plasma membrane. This type of transport, called facilitated diffusion, does not use energy because the carrier proteins merely act as a channel for an ion to move naturally from an area of high to low concentration, also called moving down a concentration gradient. Carrier proteins in the membrane transport larger molecules such as glucose, a simple sugar used for energy in cells, and amino acids, the building blocks of proteins. Carriers belong to three categories. Uniporters allow only one type of ion or molecule to enter or exit the cell. Symporters provide passage for two different ions or molecules going in the same direction. Antiporters bring one substance into the cells and transfer another outside the cell.

Active transport, unlike facilitated diffusion, uses energy and some type of transporter to move substances. These transporters are called pumps and are integrated into the cell membrane. Pumps use energy in the form of a molecule called ATP, or adenosine triphosphate, that comes from the breakdown of glucose.

Another form of active transport involves the use of vesicles. In a particular type of vesicular transport called endocytosis, substances outside the cell meet the plasma membrane. The membrane then envelops the particles, forming a vesicle around them, bringing them into the cell's interior. In exocytosis, substances in vesicles within the cell travel to the plasma membrane, fusing with it. The fusion forms an opening, and the substances are released outside the cell.

Updated March 13, 2019

By David H. Nguyen, Ph.D.

The plasma membrane is an oily layer of fat molecules that prevents water and salts from passing through. So how do water, salts and large molecules like sugars get into cells? These molecules are essential for living things.

The cell membrane controls what goes in and out by having protein channels that act like funnels in some cases and pumps in other cases.

Passive transport does not require energy molecules and happens when a funnel opens in the membrane, letting molecules flow through. Active transport requires energy, because protein machines actively grab molecules on one side of the membrane and push them through to the other side.

Learning more about these processes helps you describe how the plasma membrane controls what goes into and comes out of a cell.

The simplest way that a cell membrane can control what goes in and out is to have a protein channel that fits only one type of molecule. In this way, the cell can control the flow of just water, salts or the hydrogen ions that make a liquid acidic or not acidic.

Aquaporins are protein channels that allow water to pass freely through the cell membrane. Since water does not mix with oil, and the cell membrane is oily, water cannot freely pass in or out of a cell. Aquaporins allow water molecules to flow into cells as a single-file line. In short, an aquaporin controls the level of water coming into the cell.

Diffusion is the random but directional movement of molecules from a place where there are many of them to a place where there are few of them. The flow of molecules down this gradient, or difference in concentration, is like the flow of water down a waterfall. It is a form of energy that can be used to do other things.

Protein pumps in the membrane can exploit the natural flow of salt ions across a membrane to pump in other types of ions or molecules. This is like hitchhiking.

The pumping of a molecule in the same direction as the diffusing molecule is called symport. The pumping of a molecule in the opposite direction of the diffusing molecule is called antiport.

Letting molecules diffuse down their gradient does not require energy, but pumping these molecules in other directions to make the gradient in the first place does require energy. Active transport describes the movement of molecules against their concentration gradients, like stuffing more people into a room that is already overcrowded, and requires pumps that are powered by an energy molecule called ATP (adenosine triphosphate).

ATP is like a rechargeable battery. Each use releases a jolt of energy that turns one ATP into its uncharged state called ADP. ADP can be recharged into ATP. Proteins that pump molecules against their gradient have a pocket into which ATP fits.

Cells can move large molecules or large mixtures of molecules across their membrane. This type of cargo is too big to be pumped or too diverse to be controlled by just one channel. The movement of this type of material across a membrane requires the process of pinching or fusion of membrane pouches.

Endocytosis is the process in which the cell membrane pinches inward to swallow a molecule that is outside the cell. Exocytosis is the transport process in which a membrane pouch inside the cell runs into the cell's surface membrane.

This collision connects the pouch with the surface membrane, causing the pouch to break and release its contents outside the cell. The contents end up on the outside because the broken membrane of the pouch becomes part of the surface membrane – like two droplets of olive oil that fuse to form a larger droplet on top of water.