While most eukaryotes are multicellular organisms, there are some single-cell eukaryotes. Within a eukaryotic cell, each membrane-bound structure carries out specific cellular functions. Here is an overview of many of the primary components of eukaryotic cells. Other common organelles found in many, but not all, eukaryotes include the Golgi apparatus, chloroplasts and lysosomes. Animals, plants, fungi, algae and protozoans are all eukaryotes. All life on Earth consists of either eukaryotic cells or prokaryotic cells.
Prokaryotes were the first form of life. Scientists believe that eukaryotes evolved from prokaryotes around 2. The primary distinction between these two types of organisms is that eukaryotic cells have a membrane-bound nucleus and prokaryotic cells do not. The nucleus is where eukaryotes store their genetic information. In prokaryotes, DNA is bundled together in the nucleoid region, but it is not stored within a membrane-bound nucleus.
The nucleus is only one of many membrane-bound organelles in eukaryotes. Prokaryotes, on the other hand, have no membrane-bound organelles. Most of these proteins contain a short sequence, called a signal sequence, that determines their intracellular location. Signal sequences can be localized anywhere in a protein but are often found in the N-terminus. Signal sequences that target proteins to the same organelle often do not share the same primary sequence. It is usually the overall biochemical properties of the sequence that determine whether it targets a proteins to an organelle.
Signal sequences are used to import both soluble proteins and integral membrane proteins. Because the membranes that surrounds organelles restricts the passage of proteins, organelles have evolved different mechanisms for importing proteins from the cytoplasm. Most organelles contain a set of membrane proteins that form a pore. This pore allows the passage of proteins with the correct signal sequence.
Some pores ER, mitochondria can only accommodate unfolded proteins, whereas other pores nucleus, peroxisome allow folded proteins to pass.
Proteins destined for secretion, the plasma membrane or any organelle of the secretory pathway are first inserted into the ER. Most proteins cross the ER co-translationally, being synthesized by ribosomes on the ER.
Both soluble proteins proteins that reside in the lumen of organelles or are secreted and integral membrane proteins are targeted to the ER and translocated by the same mechanism. The signal sequence for ER proteins usually resides at the N-terminus. The signal recognition particle SRP , a complex of 6 proteins and one RNA, binds the signal sequence immediately after it is translated. The SRP also interacts with the ribosome and stops translation. Ribosomes on the ER membrane bind to the protein translocator.
The translocator is a transmembrane protein that forms a aqueous pore. The pore is the channel through which the newly synthesized ER proteins will be translocated across the ER membrane. Soluble proteins are completely translocated through the channel; the signal sequence remains in the channel and is cleaved from the rest of the protein by a protease in the lumen of the ER. Integral membrane proteins contain a stop transfer sequence downstream from the signal sequence.
The stop transfer sequence ceases translocation through channel and the portion of the protein after the stop transfer sequence resides outside the ER. Integral membrane proteins can be translocated such that either their N-terminus or C-terminus resides in the lumen of the ER. Proteins with their C-terminus in the lumen tend to have an internal signal sequence.
The translocator appears to open on one side to allow integral membrane proteins to diffuse into the surrounding lipid bilayer. Some proteins span the membrane several times and these proteins contain after the stop transfer sequence a start transfer sequence that reinitiates translocation of the protein through the channel. A protein with a signal sequence, stop transfer and start transfer would span the membrane twice with a loop residing in the cytosol or lumen.
To generate proteins that span the membrane several times, the protein would need several alternating stop and star transfer sequences.
Once proteins enter the ER, they fold into their three dimensional structures. Several mechanisms exist to help fold proteins, including chaperones and glycosylation. The ER also contains mechanisms to handle proteins that fail to fold. Although mitochondria contain their own genome, most mitochondrial proteins are encoded by nuclear genes, necessitating a mechanism to target and import those proteins into mitochondria.
Similar to proteins imported into the ER, mitochondrial proteins contain a signal sequence that targets them to mitochondria. Unlike ER proteins, mitochondrial proteins are imported post-translationally. Because proteins must be unfolded to translocate through channels in the mitochondrial membrane, mitochondrial proteins are kept unfolded in the cytosol by chaperones.
Protein import into mitochondria is similar to import into the ER but is complicated by the presence of two membrane around mitochondria. Mitochondrial proteins can reside in the outer membrane, inner membrane, intermembrane space, or matrix space inside inner membrane. Thus, mitochondria have translocators that allow passage of proteins across the outer membrane and across the inner membrane.
The TOM complex mediates passage across the outer membrane whereas the TIM complex mediates passage across the inner membrane. The signal sequence that targets proteins to the matrix usually resides at the N-terminus. The signal sequence is recognized by proteins in the TOM complex.
The TOM complex passes the proteins into the inner membrane space where the TIM complex in the inner membrane passes the protein into the matrix. Translocation across mitochondrial membranes is energy dependent. Eukaryotic Cells. Figure 1: A mitochondrion. Figure 2: A chloroplast.
What Defines an Organelle? Why Is the Nucleus So Important? Why Are Mitochondria and Chloroplasts Special? Figure 4: The origin of mitochondria and chloroplasts. Mitochondria and chloroplasts likely evolved from engulfed bacteria that once lived as independent organisms. Figure 5: Typical prokaryotic left and eukaryotic right cells. In prokaryotes, the DNA chromosome is in contact with the cellular cytoplasm and is not in a housed membrane-bound nucleus.
Figure 6: The relationship between the radius, surface area, and volume of a cell. Note that as the radius of a cell increases from 1x to 3x left , the surface area increases from 1x to 9x, and the volume increases from 1x to 27x.
Organelles serve specific functions within eukaryotes, such as energy production, photosynthesis, and membrane construction. Most are membrane-bound structures that are the sites of specific types of biochemical reactions. The nucleus is particularly important among eukaryotic organelles because it is the location of a cell's DNA. Two other critical organelles are mitochondria and chloroplasts, which play important roles in energy conversion and are thought to have their evolutionary origins as simple single-celled organisms.
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