The membranes divide the mitochondrion into two compartments, the central matrix, and the intermembrane space. DNA, in the form of a circular or linear molecule, is found in the matrix. The mitochondrial DNA encodes many of the components for mitochondrial function, while nuclear DNA encodes the remaining components. Components of the protein synthesizing machinery specific for mitochondria-ribosomes, tRNAs and specific proteins and enzymes-are also found in the matrix.
All eukaryotic cells have within them a functionally interrelated membrane system, the endomembrane system which consists of the nuclear envelope, endoplasmic reticulum ER , Golgi apparatus, vesicles and other organelles derived from them for example, lysosomes, peroxisomes , and the plasma membrane.
Many materials, including some proteins, are sorted by the functionally cellular membranes of the endomembrane system. The various membranes involved, though interrelated, differ in structure and function. The endomembrane system plays a very important role in moving materials around the cell, notably proteins and membranes the latter is called membrane trafficking. For example, while many proteins are made on ribosomes that are free in the cytoplasm and remain in the cytoplasm, other proteins are made on ribosomes bound to the rough endoplasmic reticulum RER.
The latter proteins are inserted into the lumen of the RER, carbohydrates are added to them to produce glycoproteins, and they are then moved to cis face of the Golgi apparatus in transport vesicles that bud from the ER membrane.
Within the Golgi, the protein may be modified further and then be dispatched from the trans face in a new transport vesicle. These vesicles move through the cytoplasm to their final desinations using the cytoskeleton. We can think of the system as analogous to a series of switching yards and train tracks, where materials are sorted with respect to their destinations at the switching yards and sent to those destinations along specific tracks in the cytoskeleton.
Proteins destined for secretion are made on ribosomes bound to the RER. The proteins move through the endomembrane system and are dispatched from the trans face of the Golgi apparatus in transport vesicles that move through the cytoplasm and then fuse with the plasma membrane releasing the protein to the outside of the cell.
Examples of secretory proteins are collagen, insulin, and digestive enzymes of the stomach and intestine. In a similar way, proteins destined for a particular cell organelle move to the organelle in transport vesicles that deposit their contents in the organelle by membrane fusion.
Like secretory proteins and some other proteins, proteins destined for lysosomes are made on ribosomes bound to the RER and move through the endomembrane system. In this case the lysosomal protein-containing vesicle that buds from the trans face of the Golgi apparatus is the lysosome itself. Inside eukaryotic cells , smaller sub-structures called organelles possess their own membranes. Molecules can diffuse across membranes via transport proteins, or they can be aided in active transport by other proteins.
Organelles such as the endoplasmic reticulum, Golgi apparatus, mitochondria and peroxisomes all play a role in membrane transport. The membrane of a eukaryotic cell is often referred to as a plasma membrane.
The plasma membrane is comprised of a phospholipid bilayer, and is permeable to some molecules, but not all. Components of the phospholipid bilayer include a combination of glycerol and fatty acids with a phosphate group. These yield the glycerophospholipids that generally make up the bilayer of most cell membranes. The phospholipid bilayer possesses water-loving hydrophilic qualities on its exterior, and water-repellant hydrophobic qualities on its interior.
The hydrophilic portions face the outside of the cell as well as the inside of it, and are both interactive and attracted to the water in these environments. Throughout the cell membrane , pores and proteins help determine what enters or exits the cell.
Of the different kinds of proteins found in the cell membrane, some extend only into part of the phospholipid bilayer. These are called extrinsic proteins.
The proteins that cross the entire bilayer are called intrinsic proteins, or transmembrane proteins. While some proteins can move around easily in the bilayer, others are locked in place and need help if they must move. Cells need a way to get necessary molecules into them. They also need a way to release certain materials back out again. Released materials can of course include wastes, but often certain functional proteins must be secreted outside of cells as well.
The phospholipid bilayer membrane maintains a flux of molecules into the cell, by means of osmosis, passive transport or active transport. The extrinsic and intrinsic proteins work to help with this transport biology.
These proteins may possess pores to allow for diffusion, they may work as receptors or enzymes for biological processes, or they might work in immune responses and cellular signaling. There are different types of passive transport as well as active transport that play a role in the movement of molecules across membranes. The endomembrane system was first discovered in the late s when scientist Camillo Golgi noticed that a certain stain selectively marked only some internal cellular membranes.
Golgi thought that these intracellular membranes were interconnected, but advances in microscopy and biochemical studies of the various membrane-encased organelles later made it clear the organelles in the endomembrane system are separate compartments with specific functions. These structures do exchange membrane material, however, via a special type of transport.
Today, scientists know that the endomembrane system includes the endoplasmic reticulum ER , Golgi apparatus , and lysosomes. Vesicles also allow the exchange of membrane components with a cell's plasma membrane. Membranes and their constituent proteins are assembled in the ER. This organelle contains the enzymes involved in lipid synthesis, and as lipids are manufactured in the ER, they are inserted into the organelle's own membranes.
This happens in part because the lipids are too hydrophobic to dissolve into the cytoplasm. Similarly, transmembrane proteins have enough hydrophobic surfaces that they are also inserted into the ER membrane while they are still being synthesized. Here, future membrane proteins make their way to the ER membrane with the help of a signal sequence in the newly translated protein.
The signal sequence stops translation and directs the ribosomes — which are carrying the unfinished proteins — to dock with ER proteins before finishing their work. Translation then recommences after the signal sequence docks with the ER, and it takes place within the ER membrane. Thus, by the time the protein achieves its final form, it is already inserted into a membrane Figure 1. The proteins that will be secreted by a cell are also directed to the ER during translation, where they end up in the lumen, the internal cavity, where they are then packaged for vesicular release from the cell.
The hormones insulin and erythropoietin EPO are both examples of vesicular proteins. Figure 1: Co-translational synthesis A signal sequence on a growing protein will bind with a signal recognition particle SRP.
This slows protein synthesis. Then, the SRP is released, and the protein-ribosome complex is at the correct location for movement of the protein through a translocation channel. Figure Detail. The ER, Golgi apparatus , and lysosomes are all members of a network of membranes, but they are not continuous with one another. Therefore, the membrane lipids and proteins that are synthesized in the ER must be transported through the network to their final destination in membrane-bound vesicles.
Cargo-bearing vesicles pinch off of one set of membranes and travel along microtubule tracks to the next set of membranes, where they fuse with these structures. Trafficking occurs in both directions; the forward direction takes vesicles from the site of synthesis to the Golgi apparatus and next to a cell's lysosomes or plasma membrane.
Vesicles that have released their cargo return via the reverse direction. The proteins that are synthesized in the ER have, as part of their amino acid sequence, a signal that directs them where to go, much like an address directs a letter to its destination. Soluble proteins are carried in the lumens of vesicles.
Together, the total area of a cell's internal membranes far exceeds that of its plasma membrane. Like the plasma membrane, organelle membranes function to keep the inside "in" and the outside "out.
Although each organelle performs a specific function in the cell, all of the cell's organelles work together in an integrated fashion to meet the overall needs of the cell.
For example, biochemical reactions in a cell's mitochondria transfer energy from fatty acids and pyruvate molecules into an energy-rich molecule called adenosine triphosphate ATP. Subsequently, the rest of the cell's organelles use this ATP as the source of the energy they need to operate.
Because most organelles are surrounded by membranes, they are easy to visualize — with magnification. For instance, researchers can use high resolution electron microscopy to take a snapshot through a thin cross-section or slice of a cell.
In this way, they can see the structural detail and key characteristics of different organelles — such as the long, thin compartments of the endoplasmic reticulum or the compacted chromatin within the nucleus. An electron micrograph therefore provides an excellent blueprint of a cell's inner structures. Other less powerful microscopy techniques coupled with organelle-specific stains have helped researchers see organelle structure more clearly, as well as the distribution of various organelles within cells.
However, unlike the rooms in a house, a cell's organelles are not static. Rather, these structures are in constant motion, sometimes moving to a particular place within the cell, sometimes merging with other organelles, and sometimes growing larger or smaller.
These dynamic changes in cellular structures can be observed with video microscopic techniques, which provide lower-resolution movies of whole organelles as these structures move within cells. Of all eukaryotic organelles, the nucleus is perhaps the most critical. In fact, the mere presence of a nucleus is considered one of the defining features of a eukaryotic cell.
This structure is so important because it is the site at which the cell's DNA is housed and the process of interpreting it begins. Recall that DNA contains the information required to build cellular proteins.
In eukaryotic cells, the membrane that surrounds the nucleus — commonly called the nuclear envelope — partitions this DNA from the cell's protein synthesis machinery, which is located in the cytoplasm. Tiny pores in the nuclear envelope, called nuclear pores, then selectively permit certain macromolecules to enter and leave the nucleus — including the RNA molecules that carry information from a cellular DNA to protein manufacturing centers in the cytoplasm.
This separation of the DNA from the protein synthesis machinery provides eukaryotic cells with more intricate regulatory control over the production of proteins and their RNA intermediates. In contrast, the DNA of prokaryotic cells is distributed loosely around the cytoplasm, along with the protein synthesis machinery. This closeness allows prokaryotic cells to rapidly respond to environmental change by quickly altering the types and amount of proteins they manufacture.
Note that eukaryotic cells likely evolved from a symbiotic relationship between two prokaryotic cells, whereby one set of prokaryotic DNA eventually became separated by a nuclear envelope and formed a nucleus.
Over time, portions of the DNA from the other prokaryote remaining in the cytoplasmic part of the cell may or may not have been incoporated into the new eukaryotic nucleus Figure 3. Figure 3: Origin of a eukaryotic cell. A prokaryotic host cell incorporates another prokaryotic cell. Each prokaryote has its own set of DNA molecules a genome.
The genome of the incorporated cell remains separate curved blue line from the host cell genome curved purple line. The incorporated cell may continue to replicate as it exists within the host cell.
Over time, during errors of replication or perhaps when the incorporated cell lyses and loses its membrane separation from the host, genetic material becomes separated from the incorporated cell and merges with the host cell genome. Eventually, the host genome becomes a mixture of both genomes, and it ultimately becomes enclosed in an endomembrane, a membrane within the cell that creates a separate compartment. This compartment eventually evolves into a nucleus.
0コメント