It used to be thought that a clear distinction between prokaryotic and eukaryotic cells was the presence of membrane-bound organelles in eukaryotic cells (organelles are defined as specialized structures that are separated from the rest of the cell by a phospholipid bilayer). However, we now know that some prokaryotes contain rudimentary organelles. Despite this blurring of the lines between defining prokaryote and eukaryote, eukaryotic cells have a more complex and well-developed subcellular architecture. These membrane-bound structures play important roles in the normal functioning of eukaryotic cells and they participate in everything from constructing and exporting newly synthesized biological molecules to protecting the cell from invading pathogens. The main structures that will be examined in this tutorial can be found in this image.
The nucleus is one of the most visible organelles in the cell. While it is usually only about 5 microns in diameter (approximately 1/20th the thickness of a human hair), it plays a central role in providing genetic information to the cell. The nucleus is surrounded by a double membrane called the nuclear envelope.
The nucleus also houses the nucleolus (depicted above). The nucleolus contains a very active group of genes that encode and transcribe ribosomal RNA (the RNA component of ribosomes).
The most notable function of the nucleus is to store the main genetic material within every eukaryotic cell, namely the nuclear chromosomes (additional genetic information is also found within mitochondria and chloroplasts). In addition to storage, the nucleus is also the site of all gene expression. If a new protein is to be made, a gene must first be transcribed to messenger RNA in the nucleus (you will learn more about this in the tutorial titled "From Gene to Protein: Transcription and Translation"). Once transcribed, messenger RNA exits the nucleus through nuclear pores in the nuclear envelope.
Ribosomes are structures made of ribosomal RNA molecules and proteins and are the sites of protein synthesis in cells. Ribosomes are not enclosed in a membrane and thus are not considered organelles. Each cell has a large number of ribosomes.
Each ribosome has a large component and a small component that together form a single ribosome. A messenger RNA molecule travels from the nucleus to a ribosome to provide the instructions for synthesizing proteins from amino acids.
Ribosomes are found in all cell types – both prokaryote and eukaryote. However, eukaryotic and prokaryotic ribosomes differ from each other enough that many of the antibiotics we use disrupt prokaryotic ribosomes but not eukaryotic ribosomes. Thus antibiotics kill bacterial cells but do not damage our cells.
In eukaryotic cells, ribosomes are either free or bound. Free ribosomes float freely in the cell's cytoplasm and make proteins that are used within the cell. Bound ribosomes are bound to the rough region of the endoplasmic reticulum and produce proteins that are either excreted to the outside of the cell or used within the cell membrane or a membrane-bound organelle. Bound and free ribosomes are structurally identical and can play either role at different times. In other words, free ribosomes become bound ribosomes and bound ribosomes become free ribosomes according to the needs of the cell.
Building a functional protein is a complex process. Most proteins require a number of post-production modifications before they become functional. These modifications generally require specialized cellular structures. Protein production and modification usually takes place in a network of membrane-bound chambers called the endomembrane system . The process begins in a network called the endoplasmic reticulum , which literally means "network within the cytoplasm." There are two types of endoplasmic reticula in the cell: a smooth endoplasmic reticulum (SER) and a rough endoplasmic reticulum (RER) . The smooth endoplasmic reticulum plays a major role in synthesizing lipids, storing calcium ions, and in degrading toxins. The SER does not play a role in producing proteins, but it is a component of the endomembrane system.
The rough endoplasmic reticulum houses ribosomes on its surface, which is where many of the proteins targeted for export outside the cell are synthesized.
This short video (from Ricochet Science) shows the workings of the endomembrane system:
To watch this video on YouTube (and see closed captioning) - press the arrow icon in the bottom right corner of the video player.
We will be discussing how proteins are produced later in the course. For now, it is important to know that proteins typically fall into one of two broad categories: proteins to be retained and used inside the cell and proteins to be exported and used to the outside of the cell.
The mRNA molecules that provide the instructions for making proteins that are to be exported to the outside of the cell have special signal sequences that direct the mRNA to bind to a free ribosome and then move to the rough ER. The ribosome is now bound to the rough ER and is considered a "bound" ribosome.
Once the protein is produced in the rough ER it is packaged in a vesicle and transported to the Golgi apparatus.
The Golgi apparatus is a component of the endomembrane system involved in processing proteins. Named for its discoverer, Camillo Golgi, the Golgi apparatus is where carbohydrates are added to proteins in a process called glycosylation . Proteins move from the rough endoplasmic reticulum in small membrane-bound transport vesicles to the Golgi apparatus. The side of the Golgi apparatus where proteins enter is referred to as the cis face; it can be thought of as the receiving end of the Golgi apparatus. Transport vesicles, containing partially processed proteins, fuse with the folds of the Golgi apparatus on the cis face and bud from the trans face. Proteins move through the Golgi apparatus as they are prepared for transport to their final destinations (either within the cell or for export outside of the cell).
When a protein is fully processed, glycosylated and ready for export, it next makes the trip from the endomembrane system to the cell's plasma membrane or other internal compartments (e.g., a lysosome). The final transport vesicle that buds from the trans face of the Golgi apparatus is termed a secretory vesicle . Secretory vesicles bind and fuse with the internal face of the plasma membrane by interacting with specific membrane proteins.