In the earlier installments in this series, we have looked at the formation of the cell membrane, a couple of membrane bound protein ion pumps, the different parts of the cytoskeleton and molecules that walk. Starting with today’s piece, we are going to spend some time looking at some of the organelles found inside the cell.
When you look at a cell under a microscope, you will see what looks like a bunch of thin threads lying near the nucleus of the cell. In reality, the threadlike structures are a complex series of branching tubules and flattened sacs that extend throughout the cytosol (the fluid portion of the cytoplasm). This netlike labyrinth is known at the Endoplasmic Reticulum (ER).
If you were able to follow all of the folds and branches of the endoplasmic reticulum, you would discover that it is an enclosed saclike structure. The inside of the endoplasmic reticulum sac is called the lumen. The endoplasmic reticulum is so extensive that its membrane accounts for more than half of total membrane surface associated with the cell.
A closer look at endoplasmic reticulum reveals that some of them have numerous tiny dots associated along the membrane. These dots are known as ribosomes. The endoplasmic reticulum containing the ribosomes is knows as the rough endoplasmic reticulum. The plain looking endoplasmic reticulum without the ribosomes is known as smooth endoplasmic reticulum.
The primary responsibility of the endoplasmic reticulum is the production of protein and lipid components that are used by most of the cell’s other organelles. It is also responsible for moving proteins and carbohydrates to the Golgi apparatus, the plasma membrane, the lysosomes and wherever else they are needed.
The ribosomes in the rough endoplasmic reticulum are the structures that actually synthesize the proteins. Once the ribosome initiates the synthesis of a specific protein it will attach itself to a location along the membrane of the rough endoplasmic reticulum and remain there until the protein synthesis has been completed. Then the ribosomes will detach from the membrane and the rough endoplasmic reticulum will direct the transportation of the protein to its needed location.
Now you may ask, ‘how do the proteins get to the right location’? As it turns out, the endoplasmic reticulum has its own package delivery system, kind of like FedEx. Before leaving the rough endoplasmic reticulum, a few specific amino acids are attached to one end of the protein. Those amino acids are referred to as a signal sequence and act like the shipping labels you place on packages. Once the signal sequence has been attached, the protein is placed into a transport vesicle, like a shipping box. Just like your FedEx package will be placed on trucks and planes and shipped for delivery, the transport vesicle containing the new protein is placed on the cytoskeleton which then transports it to the location coded for by the signal sequence. When FedEx delivers a package to your house, you open the box, remove the item and discard the packaging and shipping label. In the cell, once the transport vesicle has been delivered to the right location, the transport vesicle is opened up, the protein removed and the signal sequence is then discarded from the end of the protein. The main difference between the endoplasmic reticulum transport system and FedEx is that the cellular system is far more efficient as very few proteins are ever lost along the way.
The smooth endoplasmic reticulum has a number of functions different from those of the rough endoplasmic reticulum. It is involved with the metabolism and detoxification of a number of drugs, the conversion to glucose in glucogenesis, the regulation of the concentration of calcium, the synthesis of steroids and lipids, the metabolism of carbohydrates, the production and storage of certain enzymes and the attachment of receptors on cell membrane proteins.
There is another form of reticulum found in muscle cells which is known as the sarcoplasmic reticulum. It is very similar in appearance to smooth endoplasmic reticulum. The main difference between the two is the primary function of each. The main function of the sarcoplasmic reticulum is to store calcium ions and then to pump them out when needed when muscle cells become active. The more active the muscle cells become, the more calcium ions the sarcoplasmic reticulum will pump out to supply that increased activity.
There are a number of additional functions that take place in the rough and smooth endoplasmic reticulum that we did not explore. The main goal of today’s installment on the Simple Cell is to once again show some of the intricate complexity that occurs within the cell to show that the cell is anything but simple.
As you read each part of this series, I hope you realize that you need to add all of these features and structures and functions together in order to have a living cell. Think of all of these cell parts as different parts to the motor of an automobile. Every week we are adding more parts to the engine. Each part is important and necessary to the overall operation of the motor. Remove any one part and the motor no longer functions properly and may not even run at all. Then realize the microscopic scale that this is taking place at.
It makes you wonder how anyone can believe that the complexity of the cell, especially with a molecular FedEx system, could have just evolved without any intelligence behind it. More and more it points in only one direction, to our infinitely wise Creator God of the Bible.