Cell membrane , cytoplasm along with its organelles, and the nucleus are the three main structures in a cell. Organelles are specialized structures, which carry out unique functions inside the cell. These structures can be categorized into two as membranous and nonmembranous organelles. Most organelles like mitochondria , plastids, endoplasmic reticulum, and Golgi apparatus are fluid-filled structures.
On that account, fluid-filled organelles are membranous. Nonmembranous organelles are ribosomes , cytoskeleton, nucleolus , and centrosome. All prokaryotic organelles are nonmembranous. The main difference between membranous and nonmembranous is that membranous organelles are surrounded by single or double membranes, which are structurally similar to a cell membrane whereas nonmembranous organelles are not surrounded by any kind of a membrane.
Membranous organelles are surrounded by the cell membrane. Nucleus, endoplasmic reticulum, mitochondria, Golgi apparatus, plastids, and lysosomes are membranous organelles. Membranous organelles are fluid-filled, and are separated from the cytoplasm. The endoplasmic reticulum is a double-membraned system that is involved in the transferring of substances from one part to the other part of the cell.
Microtubules appear to be the principal tracks for long-range movements along the axis of the axon. Plus end—directed kinesin motors propel organelles along microtubules anterogradely, whereas dynein and possibly also minus end—directed kinesin motors propel organelles retrogradely Goldstein and Yang, Each motor binds its cargo by direct interaction with transmembrane proteins on the organelle surface, or by indirect interaction with the organelle surface via scaffolds of cytosolic linker proteins Karcher et al.
Direct observations on the movement of membranous organelles in living axons indicate that many organelles move in a continuous and unidirectional manner at instantaneous rates that are comparable to the maximal rate of fast axonal transport determined by radioisotopic pulse labeling Table I and also to the maximal rates reported for microtubule motors in vitro e. These observations indicate that many membranous organelles move along axons in a highly efficient manner, pausing only infrequently during their journey.
One way to express this efficiency is in terms of the duty ratio. In the context of intracellular transport, the duty ratio is the proportion of time that a cargo structure spends actually moving. Thus, we can say that the axonal transport of many membranous organelles is characterized by a high duty ratio.
One exception to the high duty ratio of membranous organelles are mitochondria Hollenbeck, , Fig. These organelles move less rapidly than most Golgi-derived and endocytic vesicles, forming a kinetically distinct component of axonal transport Table I. The explanation for this transport behavior appears to be that mitochondria move in an intermittent and bidirectional manner, with differences in the balance of anterograde and retrograde movements and pauses giving rise to a broad range of overall rates Morris and Hollenbeck, ; Ligon and Steward, Although some axonal mitochondria may move rapidly and continuously for long distances, resulting in the maximal rate of movement, most exhibit frequent pauses and reversals, resulting in much slower rates of movement Blaker et al.
Though not extensively studied, it appears that endoplasmic reticulum may also be transported along axons in this manner Ellisman and Lindsey, In radioisotopic pulse labeling studies, cytoskeletal and cytosolic proteins can be resolved into two kinetically distinct groups Lasek et al.
The mechanism by which these proteins move has been the subject of intense interest and controversy for more than two decades, due largely to the failure of numerous attempts to observe slow axonal transport directly in living cells Brown, Most attention has been focused on the cytoskeletal subunit proteins, and most of the controversy has centered on the form in which these proteins move.
Some have argued that cytoskeletal proteins move in the form of assembled polymers Baas and Brown, , whereas others have argued that cytoskeletal polymers are stationary and that cytoskeletal proteins move in the form of free subunits or small oligomers Hirokawa et al.
Recently the movement of neurofilaments and microtubules has been observed in axons, and these observations indicate that the solution to the slow axonal transport controversy is disarmingly simple. Cytoskeletal polymers do move in axons, but their movements are not slow after all Fig. Both neurofilaments and microtubules move at fast rates, approaching the rate of movement of membranous organelles, but the average rate of movement is slow because the movements are both infrequent and bidirectional Roy et al.
Remarkably, the key to observing this movement was simply a matter of experimental design; previous studies that did not detect movement were designed with the explicit expectation of a slow and synchronous movement, and it now appears that they were probably not capable of detecting the rapid and asynchronous movement Wang and Brown, Thus, the overall speed and direction of neurofilament and microtubule movement is a temporal summation of anterograde and retrograde movements and pauses, perhaps not fundamentally dissimilar from the behavior of mitochondria in axons described above.
As is the case for mitochondria, the slow overall rate of movement of neurofilaments and microtubules suggests that these structures move with a low duty ratio, spending most of their time not moving. It seems likely that microfilaments may exhibit a similar behavior, but the movement of these cytoskeletal polymers has not yet been observed. The rapid rate of movement of neurofilaments and microtubules in axons indicates that they are transported by fast motors, perhaps similar or identical to motors that move membranous organelles, but the identity of these motors and the tracks along which they move are not known.
Several lines of evidence suggest that dynein may transport axonal microtubules anterogradely, perhaps relative to the microfilament matrix Susalka and Pfister, ; Baas, , and that dynein and kinesin may transport axonal neurofilaments bidirectionally along microtubules by the same mechanism that is thought to move vimentin along microtubules in nonneuronal cells Prahlad et al.
Interestingly, axonal neurofilaments also interact with myosin Va, but the role of this interaction in neurofilament movement is presently unclear Rao et al.
Although much remains to be learned about the movement of cytoskeletal polymers in axons, it seems likely that microtubules, microfilaments, and neurofilaments each interact with a number of different motor proteins and that these motors act cooperatively to translocate and organize these polymers in both the longitudinal and radial dimensions of the axon.
Cytoskeletal proteins have been the exclusive focus of studies on slow axonal transport in recent years, but it is important to remember that several hundred other proteins also move in this rate group, representing the entire spectrum of cytosolic proteins that comprise axoplasm. Some examples include proteins involved in vesicle dynamics such as clathrin and synapsin; regulatory proteins such as calmodulin; metabolic enzymes such as creatine kinase, aldolase, and enolase; cytoskeletal proteins such as spectrin, tau, and dynactin; and motor proteins such as dynein and myosin Lasek et al.
The sheer number and diversity of these proteins and the fact that they move together for days, weeks, or months as they travel down the axon, suggest that they associate with a common class of nonmembranous carrier structures analogous to the membranous organelles of fast axonal transport, but the identity of these carrier structures is not known. One hypothesis, first proposed more than 15 years ago, is that cytoskeletal polymers fulfill this role and that other cytosolic proteins are transported by riding piggyback on the moving polymers Lasek et al.
According to this hypothesis, the transport rate of cytosolic proteins is determined by the proportion of the time that they spend in association with the cytoskeletal polymers, as well as by the rate and frequency of movement of the polymers themselves. The idea that cytoskeletal polymers are carrier structures for slow axonal transport was initially met with skepticism, but it is now clear that cytoskeletal polymers do move in axons and that many of the cytosolic proteins that are conveyed by slow axonal transport can bind, directly or indirectly, to cytoskeletal polymers.
For example, tau protein and spectrin both move in slow component a along with neurofilament proteins and tubulin. Spectrin is known to interact with neurofilaments Macioce et al. Thus, it is possible that neurofilaments and microtubules could be carrier structures for slow component a of axonal transport.
A prediction of this hypothesis is that other proteins that move in this rate component, which have yet to be identified, will also be found to bind to these cytoskeletal polymers.
By the same logic, it is also possible that microfilaments could be carrier structures for slow component b. In support of this hypothesis, microfilaments are known to interact with a wide range of different cytosolic proteins, including many that are not traditionally thought of as cytoskeleton-associated proteins e.
However, it is unlikely that the several hundred different proteins that move in slow component b all bind directly to microfilaments. More probably, many of these proteins form functional complexes that in turn associate with the moving filaments. It is also possible that some cytosolic protein complexes may move by binding directly to motor proteins.
The identification and characterization of these various protein complexes is likely to provide fundamental insights into the supramolecular interactions that organize the cytosolic compartment of cytoplasm, not just in axons, but in all eukaryotic cells.
The existence of distinct fast and slow components of axonal transport has been known for more than 25 years, but the mechanistic significance underlying these different rates of movement has been obscure for most of that time.
The principal reason for this protracted period of uncertainty has been our inability to observe slow axonal transport directly in living cells. The recent discovery that cytoskeletal polymers conveyed by slow axonal transport actually move as fast as membranous organelles indicates that both fast and slow axonal transport may be generated by fast motors; cargoes as diverse as vesicles, mitochondria, and neurofilaments all move at comparable rates but they differ in the proportion of the time that they spend moving.
According to this unified perspective, membranous and nonmembranous cargoes are all transported along axons by the same underlying mechanism but they move at different rates due to differences in their duty ratio.
Membranous organelles on the secretory and endocytic pathways, which function primarily to deliver membrane and protein components to sites along the axon and at the axon tip, move rapidly in a unidirectional manner, pausing for only brief periods of time.
The high duty ratio of these organelles ensures that they are delivered rapidly to their destination. In contrast, cytoskeletal polymers, mitochondria, and possibly also endoplasmic reticulum, move in an intermittent and bidirectional manner, pausing more often and for longer periods of time, and sometimes reversing during their journey along the axon. Although we refer to these structures as cargoes, they are not simply the luggage of intracellular transport; these organelles and macromolecular assemblies are preassembled functional units that fulfill their architectural, physiological, and metabolic roles in the axon during their transit.
For these cargoes, the journey is perhaps more important than the ultimate destination, and this may explain their unique motile behavior. Based on these considerations, a central question underlying the difference between fast and slow axonal transport is the mechanism by which the movement of membranous and nonmembranous cargoes is regulated.
For example, what determines whether a particular cytoskeletal polymer or membranous organelle moves or pauses, or how frequently it does so? And when movement does occur, what determines its direction and duration? Since the motile behavior of axonally transported cargoes determines the efficiency with which they are transported and the manner in which they are distributed along the axon, the regulation of this behavior is likely to be critical for many aspects of axonal structure and function.
For example, in the case of mitochondria, the balance of anterograde and retrograde movements and pauses is regulated during axon growth in order to recruit these organelles to sites of metabolic demand Morris and Hollenbeck, Likewise, in the case of neurofilaments and microtubules, the balance of anterograde and retrograde movements and pauses is likely to be the principal determinant of their steady-state distribution along the axon, and thus the regulation of the axonal transport of these structures is probably essential for local and long-range remodeling of the neuronal cytoskeleton during axon growth and maturation.
Since axonal transport continues throughout the life of the neuron, it is likely that active regulation of the movement of its membranous and nonmembranous components is an ongoing process as fundamental to the biology of axons as metabolism itself.
The cytoplasm contains a network of filaments and microtubules called cytoskeleton which acquires the cell a support for helping it in maintaining its shape and form , it works as passages to transport different substances from a place to another inside the cell.
Non-membranous organelles are not surrounded by a membrane such as Ribosomes and Centrosome , Membranous organelles are surrounded by a membrane such as Endoplasmic reticulum , Golgi body , Lysosomes , Mitochondria , Vacuoles and plastids.
It is round-shaped organelles , Its function is synthesizing protein in the cell , Some of them present free in cytoplasm single or in clusters where they produce and release protein directly into cytoplasm which the cell uses it in its vital processes as growth , regeneration and others.
Most of them are attached to outer surface of rough endoplasmic reticulum where they produce proteins as enzymes that transported by endoplasmic reticulum to outside of the cell after making changes to it in the Golgi body. It is located near the nucleus in the animal cells except the nerve cells and in some fungi cells , It is not present in cells of plants , algae and most fungi , where these cells contain a region of cytoplasm performing the same function instead.
It consists of two tiny particles called centrioles which are named the Centrosome , Each centriole consists of 9 groups of microtubules arranged in triples in a cylinderical shape. Animal cell.
It is a network of membranous canaliculi, It extends through all parts of cytoplasm attached to the nuclear envelope and cell membrane, It forms an internal transferring system that benefits in transferring the substances from a part to another inside the cell , It transfers substances between the nucleus and the cytoplasm.
There are two types of endoplasmic reticulum , which are Rough endoplasmic reticulum and Smooth endoplasmic reticulum. Rough endoplasmic reticulum is characterized by presence of a large number of ribosomes on its surfaces , Its functions are Synthesizing proteins in the cell , Making changes on proteins produced by ribosomes , making new membranes in the cell.
Rough endoplasmic reticulum abundance : Its presence increases in the cells of stomach lining and endocrine glands where these cells are responsible for secretion of enzymes and hormones. Smooth endoplasmic reticulum does have Ribosomes , Its functions are Synthesizing lipids in the cell , Transforming glucose into glycogen , Modifying the nature of some toxic chemicals to reduce their toxicity. Smooth endoplasmic reticulum abundance : Its presence increases in hepatic cells , where : Glucose sugar is converted into glycogen that stored in liver , Some toxic compounds are converted into less toxic ones.
It receives the substances that are secreted by endoplasmic reticulum through a group of transporting vesicles and it classifies and modifies these substances. It distributes these substances into the places where they are used in the cell , or it may pack them inside secreting vesicles lysosomes that move forward to the cell membrane where the cell expels them to outside as secretory products.
A definite boundary is given to the organelles, which are membranous organelles; on the flip side, the nonmembranous organelles are incessant with the cytoplasm. The membranous organelles are only present in eukaryotic cells; on the contrary, the nonmembranous organelles are present in both prokaryotic and eukaryotic cells. Various examples of membranous organelles are endoplasmic reticulum, mitochondria, nucleus, Golgi apparatus, Golgi complex, Golgi vesicles, secretory vesicles, plastids, peroxisomes, phagosomes, pinocytotic vesicles, and lysosomes; on the other hand, the nonmembranous organelles listed as nucleoid, ribosomes, proteasomes, cilia, flagella, centrioles, and some components of the cytoplasm such asmMicrofilaments, microtubules, and intermediate filaments.
The organelles which consist of a plasma membrane all around them are known as membranous organelles. Some organelles like endoplasmic reticulum, Golgi apparatus, nucleus, mitochondria, lysosomes, and plastids are considered as membranous organelles. The membranous organelles are said to be fluid-filled and are generally separated from the contents of the cytoplasm through the defined membrane.
SER does not contain ribosomes on its surface, while RER contains ribosomes on its surface and involve in the production of proteins. Lysosomes consist of digestive enzymes.
Mitochondria is present in different shapes such as spherical, rod-shaped, or oval.
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