The plasma membrane functions as a physical and electrical barrier that restricts the entry of foreign material into the cell and prevents cellular contents from leaking out. As the primary interface between the cell and its microenvironment, the plasma membrane mediates communication with neighboring cells as well as the extracellular matrix , via a number of cell adhesion proteins residing on this cellular structure.
It serves as a hub for highly dynamic and diverse cellular biochemistry, as a number of external signals mediating fundamental cellular functions are relayed by diverse classes of membrane-bound proteins and lipids . It also regulates the mechanical properties of the cell, as it determines cell shape by conforming to the arrangement of cytoskeletal structures inside the cell and aids in cell motility by undergoing restructuring concomitant to the cytoskeletal rearrangements .
In addition, cell membranes also play a role in compartmentalizing the cell and defining its organelle spaces, so that correct molecular components are recruited to distinct organelles for carrying out organelle-specific functions. The various functions of the lipid membranes are outlined in this schematic.
Channels and transporters
Biological membranes are known to exist in different structures and phases, such as bilayers, micelles, hexagonal, and cubic phases . However, the most common structure of membranes is described in the classic fluid mosaic model that was put forth by SJ Singer and GL Nicolson in . According to this model, membranes are depicted as two-dimensional fluids made up of lipid bilayers that are interspersed with proteins in a mosaic-like fashion.
The hydrophilic phosphate head groups of the lipid molecules are in contact with the aqueous environment at the outer surface of the bilayer, whereas the two hydrophobic lipid chains are sequestered to the inner side of the bilayer away from any contact with the fluidic environment. The fluid mosaic model incorporates the dynamic nature of bilayer membrane organization that occurs due to the constant rotational and lateral motion of the integral lipid and protein molecules.
Schematic showing the structure of a lipid bilayer membrane. The molecular composition of cell membranes is responsible for determining its physiochemical properties. For instance, a number of membrane properties including its phase behavior, viscosity ability to flow like a liquid , rigidity, and thickness are determined by the types of lipids and their densities found in the membrane . Similarly, the accumulation of several curvature-sensing lipids and proteins tend to induce curvature in localized regions of the membrane that, at a larger scale, determines its shape .
Dynamic lipid-protein interactions on the cell membrane influences the distribution of these molecular components in the bilayer and results in the formation of distinct membrane domains, such as lipid rafts. Such membrane domains serve as precursor sites for the formation of vesicles, invaginations, and protrusions that are essential for the transport of cellular cargo, as well as for internalization of pathogens. Furthermore, the lipids and proteins on the cell membrane diffuse freely within the bilayer unless they are tethered to cytoskeletal structures, extracellular matrix or to other membrane proteins.
The continuous diffusion of molecular components within the bilayer, coupled to the loose packing of hydrocarbon tails of membrane lipids, is responsible for the viscous nature not solid-like nature of cell membranes . Similarly, the rigid ring structures of cholesterol molecules inserted in between the phospholipids produces temperature-dependent effects on membrane structures.
A diverse population of lipids, which mainly vary in size and chemical properties of their head groups and fatty acid chains, and the type of linkages formed between these two structural domains, constitute the cell membrane. These include a number of phosphatidylcholines, phosphatidylethanolamines, phosphatidylserines, sphingolipids, phosphoinositides, glycolipids, and cholesterol . While phosphatidylcholine and sphingolipids predominate the outer leaflet of the lipid bilayer, the inner leaflet is primarily constituted by phosphatidylethanolamine, phosphatidylinositol and phosphatidylserine.
This chemical diversity is further magnified by the action of several membrane-bound enzymes that possess different lipid metabolizing functions. However, highly regulated pathways of lipid synthesis and transport across the cell influences the molecular composition of lipids in different cell organelles as well as within the different leaflets of the same bilayer that determines its physicochemical properties, and thus its biological functions . Membrane lipids consist of phosphatidylcholine and sphingolipids on the outer leaflet, and phosphatidylethanolamine, phosphatidylinositol and phosphatidylserine on the inner leaflet.
While lipids contribute primarily to the structural characteristics of cell membranes, an equally diverse set of proteins that localize to the membranes, including receptors, transporters, cell adhesion molecules, enzymes, and energy transducing proteins are involved in diverse biological functions associated with membranes.
Proteins are recruited to membranes either through their interactions with membrane lipids or with other membrane proteins, and these interactions are closely regulated by post translational modifications of either of the interacting partners. At the membrane, proteins are either attached to a single leaflet of the lipid bilayer or they extend across the bilayer, in which case they are known as transmembrane proteins.
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Archive Journals. All Journals. However, some cells of normal size, such as cells in stinging nettle, parenchymal cells in onion or leaf cells in Elodea , exhibit slow circulation streaming. Fountain streaming can be observed in root hairs and pollen tubes of various higher plants. The details of the actin-myosin driven streaming were elucidated in characean cells, because their cell compartments can be manipulated. In large celled characean thalli streaming is crucial for intercellular transport of both nutrients and organic compounds see Radioactive or Substitution Tracers.
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The pH banding that aids carbon fixation does not occur without streaming. There may be further roles of streaming in cell metabolism and improving homeostasis by enhancing vacuolar mixing Goldstein and van de Meent, In land plants growth and development is directed by auxin indoleacetic acid IAA concentration minima, maxima and gradients.
IAA research in Characeae can elucidate some of the developmental steps in auxin signaling and metabolic pathways from origins in chlorophyte algae De Smet et al. Hori et al. In Characeae with more complex morphology, effects of external IAA and its transport through the thallus and rhizoids were investigated. Klambt et al. The polar growth of rhizoids was inhibited by explant decapitation or by addition of NPA.
Protein Transport Across Cell Membranes: Mechanism, Structure, and Regulation
If IAA was added to the medium, the inhibition of growth was reversed. The rhizoid development in mosses is also affected by IAA Eklund et al. Clabeaux and Bisson decapitated C. Greater number of axillary branches was observed in decapitated explants and below the tied thread: clear demonstration of apical dominance.
However, unlike higher plants, the tied explants also produced greater number of rhizoids and addition of IAA to the medium had no effect. Boot et al. The middle chamber was labeled by addition of 3 H-IAA and the appearance of the label was then monitored in the outer chambers. After 1 hr the shoot to rhizoid directed transport of IAA was times greater than that in the opposite direction. The polarity was lost upon exposure to NPA. Initially, the IAA transport through the thallus was attributed to cytoplasmic streaming, as the rate was comparable.
When no rate decrease resulted from streaming inhibition by cytochalasin, Raven proposed involvement of other mechano-chemical motors such as dynein-tubulin or kinesin-tubulin. If the label was added to one of the outer compartments, large amount of IAA was detected in the middle compartment.
The cortication of C.