One of the easiest ways to understand the structures and functions of theorganelleshoused in a cell, and cell biology as a whole, is about comparing it to things in the real world.
For example, does it make sense to describe theGolgi apparatusas a packer or post office, since its function is to receive, modify, separate and dispatch cellular loads.
The neighboring organelle of the Golgi body, theEndoplasmatisches Retikulum, can be better understood as the production site of the cell. This organelle factory builds the biomolecules necessary for all life processes. This includes proteins and lipids.
You probably already know how important membranes are toeukaryotic cells; the endoplasmic reticulum, which includes both therough endoplasmic reticulummismooth endoplasmic reticulum, occupies more than half the membrane area in animal cells.
It's hard to overstate the importance of this membrane-like organelle that produces biomolecules for the cell.
Endoplasmic reticulum structure
The first scientists to observe the endoplasmic reticulum, while taking the first electron micrograph of a cell, were amazed by the appearance of the endoplasmic reticulum.
To Albert Claude, Ernest Fullman, and Keith Porter, the organelle appeared "embedded" because of its folds and voids. Modern observers tend to describe the appearance of the endoplasmic reticulum as being like a folded ribbon or even candy ribbon.
This unique structure ensures that the endoplasmic reticulum can fulfill its important function within the cell. The endoplasmic reticulum is best understood asphospholipid membranefolded in on itself to create its maze-like structure.
Another way to think of the structure of the endoplasmic reticulum is as a network of flat sacs and tubes connected by a single membrane.
This folded phospholipid membrane forms what are known as curves.cisterns. These flat discs of phospholipid membrane appear stacked when viewed in cross section of the endoplasmic reticulum under a powerful microscope.
The seemingly empty spaces between these pockets are just as important as the membrane itself.
These areas are calledlumens. The internal spaces that make up the lumen are filled with fluid and actually make up about 10% of the total cell volume, thanks to the way the fold increases the total surface area of the organelle.
Two types of RE
The endoplasmic reticulum contains two main sections, named for their appearance: therough endoplasmic reticulumIt's insmooth endoplasmic reticulum.
The structure of these areas of the organelle reflects its specific function within the cell. Under the lens of a microscope, the phospholipid membrane of the rough endoplasmic membrane appears covered with spots or bumps.
These areribosomes, which give the rough endoplasmic reticulum an irregular or rough texture (hence its name).
These ribosomes are actually separate organelles from the endoplasmic reticulum. Large numbers (even millions!) of them are found on the surface of the rough endoplasmic reticulum because it is vital to their job of protein synthesis. The RER consists of stacked sheets that are twisted with helical edges.
The other side of the endoplasmic reticulum, the smooth endoplasmic reticulum, looks very different.
Although this section of the organelle still contains the folded labyrinthine cisterns and the fluid-filled lumen, the surface on this side of the phospholipid membrane appears smooth or smooth because the smooth endoplasmic reticulum does not contain ribosomes.
This part of the endoplasmic reticulum synthesizesLIPIDSinstead ofProteinTherefore, it does not require ribosomes.
Or rough endoplasmic reticulum (rough ER)
The rough endoplasmic reticulum, or RER, gets its name from its distinctive rough or studded appearance, thanks to the ribosomes that cover its surface.
Remember that the entire endoplasmic reticulum acts as a site for the production ofvital biomoleculesas proteins and lipids. The RER is the area of the factory dedicated exclusively to the production of proteins.
Some of the proteins produced in the RER remain in the endoplasmic reticulum forever.
That is why scientists call these proteinsresident proteins. Other proteins are modified, classified, and transported to other areas of the cell. However, a large number of proteins incorporated into the RER are targeted for secretion by the cell.
This means that, after modification and sorting, these secretory proteins travel through the body via vesicle transporters.cell membranefor work outside the cell.
The location of the RER within the cell is also important to its function.
The RER is next door.Kernthe cell. In fact, the phospholipid membrane of the endoplasmic reticulum is actually connected to the membrane barrier that surrounds the nucleus of the cell, the so-calledatomic envelopethe nuclear membrane.
This tight arrangement ensures that the RER receives the genetic information it needs to build proteins directly from the nucleus.
It also allows the RER to send a signal to the nucleus when protein assembly or protein folding fails. Thanks to its proximity, the rough endoplasmic reticulum can simply send a message to the nucleus to slow production while the RER catches up.
Protein synthesis in the rough ER
protein synthesisIn general, it works like this: The nucleus of each cell contains a complete set of DNA.
This DNA is like the blueprint that the cell can use to build molecules like proteins. The cell transfers the genetic information needed to build a single protein from the nucleus to ribosomes on the surface of the RER. Scientists call this processtranscriptionbecause the cell copies or copies this information from the original DNA using messenger substances.
Ribosomes attached to the RER receive the messengers carrying the transcribed code and use this information to create a specific chain.amino acids.
This step is calledtranslationbecause the ribosomes read the data code in the messenger substance and use it to determine the order of the amino acids in the chain they build.
These chains of amino acids are the building blocks of proteins. Eventually, these strands fold into functional proteins and can even be tagged or modified to help them do their job.
Protein folding in the rough ER
Protein folding generally takes place within the RER.
This step gives the proteins a unique three-dimensional shape calledInformation. Protein folding is critical because many proteins interact with other molecules, using their unique shape to connect like a key fits a lock.
Misfolded proteins may not work properly, and this malfunction can even cause disease in humans.
For example, researchers now believe that problems with protein folding can cause health disorders like type 2 diabetes, cystic fibrosis, sickle cell anemia, and neurodegenerative problems like Alzheimer's and Parkinson's.
DifficultThey are a class of proteins that enable chemical reactions within the cell, including processes involved in metabolism, which is the cell's way of accessing energy.
Lysosomal enzymes help the cell break down unwanted cell contents, such as old organelles and misfolded proteins, to repair the cell and use the waste material for energy.
Membrane proteins and signaling proteins help cells communicate and work together. Some tissues require small amounts of protein, while others require large amounts. These tissues normally devote more space to the ER than other tissues with a lower need for protein synthesis.
The smooth endoplasmic reticulum (smooth ER)
The smooth endoplasmic reticulum, or SER, lacks ribosomes, so its membranes look like smooth or smooth tubules under the microscope.
This makes sense because this part of the endoplasmic reticulum produces lipids or fats instead of proteins and therefore does not need ribosomes. These lipids may includefatty acids, phospholipids and cholesterol molecules.
Phospholipids and cholesterol are necessary to build the cell's plasma membranes.
The SER produces lipid hormones that are necessary for proper lipid hormone function.hormonal system.
These include steroid hormones made from cholesterol such as estrogen and testosterone. Due to the important role that the SER plays in the production of hormones, cells that require a lot of steroid hormones, such as those of the testes and ovaries, tend to devote more cellular space to the SER.
The BEING is also involved in metabolism and detoxification. Both processes occur in liver cells, so liver tissue generally has a higher amount of SER.
When hormones signal that energy stores are running low, the kidneys andliver cellsStart a path of power generation calledgluconogenesa.
This process produces glucose, a major supplier of energy, from non-carbohydrate sources in the cell. The SER in liver cells also helps these liver cells to eliminate toxins. To do this, the SER digests parts of the dangerous compound to make it water soluble so that the body can excrete the toxin in the urine.
O Sarcoplasmic Reticulum Muscle Cells
Some show a highly specialized form of the endoplasmic reticulum.muscle cells, calledmyositis. This form, calledsarcoplasmic reticulum, is normally found in the heart (cardiac) and skeletal muscle cells.
In these cells, the organelle manages the balance of calcium ions, which the cells use to relax and contract muscle fibers. The stored calcium ions are carried into the muscle cells as the cells relax and are released from the muscle cells during this time.Muscle contraction. Problems with the sarcoplasmic reticulum can lead to serious medical problems, including heart failure.
The unfolded protein response
You already know that the endoplasmic reticulum is part of protein synthesis and folding.
Proper protein folding is critical for proteins to be able to do their jobs correctly, and as mentioned above, misfolding can cause proteins to function incorrectly or not at all, which can lead to serious diseases such as type 2, which can lead to diabetes.
Because of this, the endoplasmic reticulum must ensure that only correctly folded proteins are transported from the endoplasmic reticulum to the Golgi apparatus for packaging and transport.
The endoplasmic reticulum ensures protein quality control through a mechanism known asunfolded protein response, our UPR.
This is basically very fast cell signaling that allows the RER to communicate with the nucleus of the cell. When unfolded or misfolded proteins begin to accumulate in the lumen of the endoplasmic reticulum, the RER triggers the unfolded protein response. This does three things:
- It tells the kernel thatdecrease the rate of protein synthesislimit the number of messenger molecules sent to ribosomes for translation.
- The unfolded protein response also increases the capacity of the endoplasmic reticulumfold proteins and degrade misfolded proteins.
- If none of these steps resolve protein aggregation, the unfolded protein response also contains a failsafe. When all else fails, the affected cells self-destruct. This is programmed cell death, also known as cell death.Apoptosisand it is the last option that the cell has to minimize the damage that unfolded or misfolded proteins can cause.
The shape of the ER is related to its functions and can change as needed.
For example, increasing the layers of the RER sheets helps some cells to secrete more protein. On the other hand, cells such as neurons and muscle cells that do not secrete as many proteins may have more SER tubules.
Operipheralos ER, that is, the part that is not connected to the nuclear envelope, can even be moved if necessary.
These reasons and mechanisms for this are under investigation. It can slide along the SER tubulesmicrotubulesdoZitoskeleton, which the ER pulls along with other organelles and even rings of ER tubules that move around the cell like little motors.
The shape of the ER also changes during some cellular processes, such asMitosis.
Scientists are still studying how these changes occur. Several proteins maintain the general shape of the ER organelle, including the stabilization of its sheets and tubules, and help determine the relative amounts of RER and SER in a given cell.
This is an important area of study for researchers interested in the relationship between ER and disease.
ER and human disease
Protein misfolding and ER stress, including stress from frequent UPR activation, may contribute to the development of human disease. These can include cystic fibrosis, type 2 diabetes, Alzheimer's disease, and spastic paraplegia.
virusYou can also hijack the ER and use protein assembly machinery to expel viral proteins.
This can change the shape of the ER and prevent it from performing its normal functions for the cell. Some viruses, such as dengue and SARS, form protective double-membrane vesicles within the ER membrane.