He Cytoplasmic movement , Also called protoplasmic flow or cyclosis, is the movement of the fluid substance (cytoplasm) within a plant or animal cell. The movement transports nutrients, proteins and organelles into cells.
Discovered for the first time in the 1830s, the presence of cytoplasmic flow helped convince biologists that cells were the fundamental units of life.
Although the mechanism of cytoplasmic transmission has not been fully understood, it is thought to be mediated by"motor"proteins, molecules made up of two proteins that use adenosine triphosphate to move one protein relative to the other.
If one of the proteins remains fixed in a substrate, such as a microfilament or a microtubule, the motor proteins can move organelles and other molecules through the cytoplasm.
Motor proteins often consist of actin filaments, long protein fibers aligned in rows parallel to the current within the cell membrane.
The molecules of Myosin Attached to cellular organelles move along the actin fibers, towing the organelles and sweeping other cytoplasmic contents in the same direction.
Cytoplasmic transmission, or cyclosis, is an event that consumes energy in plant cells and is used to distribute nutrients in the cytoplasm. It is common in larger cells, where diffusion is not adequate for the distribution of the substance.
In plants, it can also be used to distribute chloroplasts for maximum light absorption for photosynthesis . Scientists still do not understand how this process occurs, although it is hypothesized that microtubules and microfilaments play a role, interacting with the motor proteins of the organelles.
In some plant cells there is a rapid rotating cytoplasmic movement, limited to the peripheral parts of the cell next to the cell wall, carrying chloroplasts and granules.
This movement can be increased by light, and depends on temperature and pH. Auxins, or plant growth hormones, can also increase the speed of movement. In some protozoa, like the ciliates, the slower cyclic movements carry digestive vacuoles through the cell body.
Cytoplasmic transmission
Cytoplasmic transmission in plant cells arises naturally through the self-organization of microfilament
Many cells exhibit a large-scale active circulation of all of their fluid content, a process called cytoplasmic flow or movement. This phenomenon is particularly prevalent in plant cells, often with markedly regulated flow patterns.
In the mechanism of activation in such cells, myosin-coated organelles entrain the cytoplasm as they are processed along beams of actin filaments fixed at the periphery. This process is the development process that builds the ordered actin configurations necessary for a coherent flow on a cellular scale.
It has been observed that the basic paradigm underlying motor proteins that interact with polymeric filaments possesses many patterns-forming behaviors in both theoretical and experimental environments.
However, these studies are usually drawn from the context of particular biological systems, and in particular no direct connection has been made with the development of cytoplasmic transmission.
To understand the fundamental dynamic that drives the formation of ordered flows and to connect the microscopic with the macroscopic, an alternative"top-down"approach is justified.
To do this, we approach the problem through a concrete prototype system. We adopt perhaps the most striking example, the aquatic alga Chara corallina.
Chara's giant cylindrical internodal cells measure 1 mm in diameter and up to 10 cm in length. Its rotational flow called"cyclosis"is driven by vesicles (endoplasmic reticulum) coated with the myosin motor protein that slides along two longitudinally directed bands of oppositely directed many continuous parallels and actin filaments.
Each wire is a beam of many individual actin filaments, each of which has the same intrinsic polarity. The myosin motors move on a filament in a directed way, from its smaller end, to its larger (barbed) end.
These cables are attached to the chloroplasts cortically fixed at the periphery of the cell, generating flow rates of 50-100 μm / s. It is not clear how this simple but striking pattern is formed during morphogenesis, although it can be inferred that they are the result of complex chemical patterns.
The mechanism of cytoplasmic flow in the cells of the characeae algae: the gliding of the endoplasmic reticulum along actin filaments
Directly frozen giant cell electron microscopy of characeous algae shows a continuous three-dimensional network of anastomosed tubes and rugged endoplasmic reticulum cisterns that penetrate the flow region of their cytoplasm.
Parts of this endoplasmic reticulum are brought into contact with the parallel beams of actin filaments at the interface with the stationary cortical cytoplasm.
Mitochondria, glycosomes, and other small cytoplasmic organelles entangled in the endoplasmic reticulum network show Brownian motion as they flow.
The binding and sliding of membranes of the endoplasmic reticulum along the actin leads can also be visualized directly after the cytoplasm of these cells is dissociated in an ATP-containing buffer.
The shear forces produced at the interface with the dissociated actin cables move large endoplasmic reticulum aggregates and other organelles. The combination of rapid freezing electron microscopy and video microscopy of live cells and dissociated cytoplasm demonstrates that cytoplasmic transmission depends on the membranes of the endoplasmic reticulum sliding along stationary actin cables.
Therefore, the continuous endoplasmic reticulum network provides a means of exerting driving forces in the deep cytoplasm within the distal cell of the cortical actin leads where the driving force is generated.
Role in intracellular transport
Although a great deal of work has been published on the molecular basis and hydrodynamics of cytoplasmic movement, relatively few authors venture into a discussion of their function.
It has long been suggested that this flow aids molecular transport. However, the specific hypotheses regarding the mechanism by which transmission speeds up metabolic rates has hardly been analyzed.
The diffusion is not able to explain many transport phenomena in the cells and the degree of homeostasis along the pathways can only be explained by assuming that they are forms of active transport.
The highly symmetrical topology of the stream in the characeae algae seems to have evolved at a considerable evolutionary cost, as reflected also in the fact that the myosin found in this organism is the fastest known in existence.
Based on what we know about characeae algae, we see that transmission is implicated in a multitude of roles in cellular metabolism. It aids transport between cells and is therefore essential to deliver a constant flow of cell building blocks to newly formed cells at the tip of the outbreak.
It also seems important to maintain the alkaline bands that facilitate absorption of inorganic carbon from the surrounding water. However, a key question that remains largely unanswered is precisely what the role of cytoplasmic movement can play in eliminating the diffusion bottlenecks that seem to limit the size of cells in other organisms.
In fact, flow may help homeostatic regulation during the rapid expansion of cell volume, but the precise mechanisms by which it does remain an open area of investigation.
The most important contributions in terms of a quantified discussion of the effect of cytoplasmic flow on intracellular transport are undoubtedly Pickard's. This scientist talked about the escalation of flow velocity and diffusion time scales with cell size as well as the interaction between the stagnant periplasm layer surrounding the rows of chloroplast and the mobile layer of endoplasm.
He pointed to the possibility that the advection of a point source may help homeostasis by smoothing the fluctuations in the concentration field. He also raised the notion that cytoplasmic flow as such does not necessarily have to confer a benefit to the cell if its real purpose is the transport of particles along the cytoskeleton.
Cytoplasmic movement allows the distribution of molecules and vesicles in large plant cells
Recent studies of aquatic and terrestrial plants show that similar phenomena determine the intracellular transport of organelles and vesicles. This suggests that aspects of cell signaling involved in the development and response to external stimuli are conserved across species.
The movement of molecular motors along the cytoskeleton filaments directly or indirectly entrains the fluid cytosol, leading to cyclosis (cytoplasmic movement) and affecting the molecular species gradients within the cell, with potentially important metabolic implications as a force Motor for cell expansion.
Research has shown that myosin XI functions in the movement of organelles that promote cytoplasmic flow in aquatic and terrestrial plants. Despite the conserved cytoskeletal machinery, which propels the movement of the organelle between aquatic plants and the soil, speeds of cyclosis in plant cells vary according to cell types, cell development stages, and plant species .
References
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