Tuesday, 28 May 2019

Cell Membrane Function

Cell Membrane Function

Plant cells as eukaryotes are characterized by their compartmentation into membrane enclosed reaction spaces. This structure separates different metabolic pathways but at the same time allows a grouping of connected biochemical functions. This process allows sophisticated regulations. The borders of compartments are composed of biomembranes.

These thin and highly flexible structures determine the architecture of biological systems. Biomembranes are flat, asymmetrical structures, which are closed and usually topologically equivalent to the surface of a sphere or a torus. Along with the basic composition of lipids and proteins there are variety of individual compositions. The ratio of proteins to lipids varies within a range of 1:4 to 4:1. Both components are held together by hydrophobic interactions. The lipids form a bimolecular layer, which serves as a barrier and prevents the passage of polar molecules. Integral proteins penetrate completely or at least partly the lipid bilayer. Conspicuous examples are the chlorophyll a/b-binding protein of chloroplast membranes and the adenosine triphosphate (ATP) synthase of mitochondrial and chloroplast membranes. Peripheral proteins bind at the membrane surface to integral proteins from which they can be detached easily by appropriate solvents. They include clathrin, spectrin, and ankyrin of the plasma membrane. Usually membrane proteins have mediating functions: they serve as receptors, ion channels, ATP-powered pumps, or transporters. In many cases enzymatic activities are crucial for the function. This explains why the two sides of the membrane bilayer are usually different. 

Transport through a biomembrane uses one of the three following mechanisms (Fig. 2):

1. Passive diffusion: Only a few substances are able to penetrate the lipid bilayer. Examples are gases such as oxygen (O 2 ), nitrogen (N 2 ), or carbon dioxide (CO 2 ) and some small, uncharged molecules such as ethanol, urea, or benzene, which dissolve easily in the lipid bilayer. In principle this also holds true for water (H 2 O) although its diffusion may be accelerated by transport proteins (aquaporins). Passive diffusion of H 2 O through the biomembrane is crucial for osmosis. In contrast, lipid bilayers are practically impermeable to charged molecules.

2. Facilitated diffusion (catalyzed diffusion): Similar to passive diffusion this mechanism does not require energy and leads only to a concentration equilibrium. However, transport proteins (channels and transporters) are required for this type of membrane transport. Three options are available (Fig. 3): 

(a) unidirectional transport (uniport), which moves only one kind of molecule; 
(b) symport (cotransport), in which two molecules or ions are transported in the same direction, and one of them, for instance, H þ , follows a concentration gradient; 
(c) antiport, in which two molecules or ions move in opposite directions, one of them following a concentration gradient. Both antiporters and symporters mediate coupled reactions in which the energetically unfavorable reaction is coupled to an energetically favorable reaction.

3. Active transport: The process pumps ions or small molecules through a membrane against a chemical concentration gradient and/or electric potential. This ATP-powered transport moves ions such as H þ , K þ , Ca 2þ , and Na þ in one direction. It is mediated by adenosine triphosphatases (ATPases). The required energy is released by the hydrolysis of ATP to adenosine disphosphate (ADP) and inorganic phosphate (P i ). This active transport indirectly drives symport and antiport. The compartmentation of cells creates huge membrane areas, which offer extended targets to environmental compounds and toxins approaching and penetrating the cells. Binding of a variety of substances can lead not only to impairment of individual functions, but also to destruction of whole compartments. The abolition of compartmentation always leads to cell death.

Compartmentation entails a separation of protoplasmic and nonprotoplasmic spaces: the plasmalemma separates the protoplast from the cell wall at the outside, the tonoplast from the vacuole at the inside (Fig. 4).

 The plasmalemma (plasma membrane) allows the movement of solutes into the protoplast as well as outward. In addition, it is involved in signal transduction between the outside and insideand mediates hormonal as well as light effects. This allows the cell to detect changes of its environment and react accordingly. For this purpose several receptor types as well as redox chains are available. A prominent component of the plasmalemma is the H þ -ATPase. This transport ATPase plays a crucial role in the uptake of nutrients and pH regulation. By means of this ATP-driven H þ pumping the membrane becomes energized, and the electrochemical potential for the import of solutes through ion channels and carriers is maintained by ATP hydrolysis. This function is taken in animal cells by the sodium pump (Na þ / K þ -ATPase). Whereas in this case the pump is usually coupled to K þ import and Na þ export, proton coupling is used by plants. The proton pump transports a single H þ for each hydrolized ATP and creates in this way a large electrical potential up to ?300mV (inside the membrane negative) as well as a proton gradient. The values reach a pH of c. 7.1 at the inner side of the membrane and at the outside values between 4.5 and 5. A comparison of the different strategies is illustrated by Fig. 5.

Cells with intensive active transport such as root hairs require between 25% and 50% of their total cellular ATP to keep their proton pumps running. In addition hydrogen adenosine triphosphatases (H þ -ATPases) play an important role in cell elongationThe plasma membrane contains a multitude of further membrane proteins, depending on the specific cell. In addition to H þ -ATPases, cation and anion channels as well as carriers for sucrose, nitrate, and amino and a multitude of hormone and blue light receptors have been identified. Even many pollutants act on components of the plasmalemma, especially on H þ ATPases. Many xenobiotics also enter the cell, where they are taken up directly or after conjugation to glutathione into the vacuole. In other cases cytochrome P450–dependent hydroxylations occur, followed by further metabolic steps.

According to the principle of hierarchic structuring the protoplast is divided into several compartments. The membrane-enclosed spaces are organelles in a narrower sense. There are organelles bounded by two membranes (cell nucleus, mitochondria, and plastids) and organelles surrounded by a single-membrane envelope (such as endoplasmic reticulum, dicytosomes, lysosomes, peroxisomes, glyoxysomes, and microbodies). These organelles are embedded into the cytosol, which also contains additional particles (organelles in a wider sense such as ribosomes as well as fibrous elements belonging to the cytoskeleton). Sometimes the traditional grouping of the protoplast into cell nucleus and cytoplasm is used. With respect to cell compartmentation the following rules are important: an envelope composed of two membranes separates plasmatic from other plasmatic spaces (P spaces). The P spaces are characterized by their potential to synthesize nucleic acids and/or proteins. The delimitation of the nucleus, mitochondria, and plastids against the cytosol falls into this group. A delimitation by a single membrane separates plasmatic from endoplasmatic spaces (E spaces). The latter spaces are not capable of nucleic acid or protein synthesis. The cell wall and vacuole are nonplasmatic spaces, as are the lumen of the endoplasmic reticulum (ER), dicytosomes, lysosomes, and microbodies. The symbiont theory of organelle evolution explains these distinctions: mitochondria and plastids are derived from prokaryotic cells that have colonized as endosymbionts, the progenitors of modern eukaryotes.

Cell membranes are subjected to remarkable dynamics, to which membrane flow substantially contributes. Membrane flow is mediated by an intensive vesicle stream within the cytosol. There are several options, depending on the respective cell: the pathway from the ER, which is connected to the nuclear envelope, via the Golgi apparatus to the plasmalemma (exocytosis) or to the lysosomal compartment. On the other hand, components of the plasmalemma, e.g., receptors, are recycled by endocytosis. By means of membrane flow large amounts of newly synthesized fatty acids and lipids are transported from the ER to the plasmalemma. The plant cell differs from the animal cell in the existence of three additional compartments, plastids, vacuole, and cell wall, whereas the other structures are generally very similar. These additional components provide specific targets for pollutants, but on the other hand remarkable indifference to certain xenobiotics that have dramatic effects on animals and humans prevails.

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