Cell membrane
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The cell membrane (also called the plasma membrane or plasmalemma) is a semipermeable lipid bilayer common to all living cells. It contains a variety of biological molecules, primarily proteins and lipids, which are involved in a vast array of cellular processes. It also serves as the attachment point for both the intracellular cytoskeleton and, if present, the cell wall. Robert Hooke was the first one to name the cells parts including the plasma membrane[citation needed].
The cell membrane surrounds the cytoplasm of a cell and physically separates the intracellular components from the extracellular environment, there by serving a mechanical function similar to that of skin. This barrier is able to regulate what enters and exits the cell as it is selectively permeable - cells require a variety of substances to survive and the cell membrane serves as "gatekeeper" to what, and how much, enters and exits. The movement of substances across the membrane can be either passive, occurring without the input of cellular energy, or active, requiring the cell to expend energy moving it across the membrane.
Specific proteins embedded in the cell membrane act as molecular signals which allow cells to communicate with each other. Protein receptors are found ubiquitously and function to receive signals from both the environment and other cells. These signals are transduced into a form which the cell can use to directly effect a response. Other proteins on the surface of the cell membrane serve as "markers" which identify a cell to other cells. The interaction of these markers with their respective receptors forms the basis of cell-cell interaction in the immune system.
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[edit] Composition
[edit] Lipids
The cell membrane consists of three classes of amphipathic lipids: phospholipids, glycolipids, and steroids. The relative composition of each depends upon the type of cell, but in the majority of cases phospholipids are the most abundant.[1]
The fatty acid chains in phospholipids and glycolipids usually contain an even number of carbon atoms, typically between 14 and 24. The 16- and 18-carbon fatty acids are the most common. Fatty acids may be saturated or unsaturated, with the configuration of the double bonds nearly always cis. The length and the degree of unsaturation of fatty acids chains have a profound effect on membranes fluidity[citation needed].
The entire membrane is held together via non-covalent interaction of hydrophobic tails, however the structure is quite fluid and not fixed rigidly in place. Phospholipid molecules in the cell membrane are "fluid" in the sense that they are free to diffuse and exhibit rapid lateral diffusion along the layer they are present in. However, movement of phospholipid molecules between layers is not energetically favourable and does not occur to an appreciable extent. Lipid rafts and caveolae are examples of cholesterol-enriched microdomains in the cell membrane.
In animal cells cholesterol is normally found dispersed in varying degrees throughout cell membranes, where it confers a stiffening and strengthening effect on the membrane[citation needed]. It resides in the irregular spaces between the hydrophobic tails of the membrane lipids.
[edit] Cell membrane proteins
The cell membrane plays host to a large amount of protein which is responsible for its various activities. The amount of protein differs between species and according to function, however the typical amount in a cell membrane is 50%.[2] These proteins are undoubtedly important to a cell: approximately a third of the genes in yeast code specifically for them, and this number is even higher in multicellular organisms.[1] Three groups of membrane proteins can be identified:[1]
| Type | Description | Examples |
| Integral proteins | Located spanning the membrane and thus have a hydrophilic cytosolic domain which interacts with internal molecules, a hydrophobic membrane-spanning domain which anchors it within the cell membrane, and a hydrophilic extracellular domain which interacts with external molecules. The hydrophobic domain consists of one, multiple, or a combination of α-helices and β sheet protein motifs. Also known as transmembrane proteins. | Ion channels, proton pumps, G protein-coupled receptor |
| Lipid anchored proteins | Located covalently bound to single or multiple lipid molecules, which hydrophobically insert into the cell membrane and anchor the protein. The protein itself is not in contact with the membrane. | G proteins |
| Peripheral proteins | They are attached to integral membrane proteins, or associated with peripheral regions of the lipid bilayer. These proteins tend to have only temporary interactions with biological membranes, and once reacted the molecule dissociates to carry on its work in the cytoplasm. | Some enzymes, some hormones |
The cell membrane, being exposed to the outside environment, is an important site of cell communication. As such, a large variety of protein receptors and identification proteins, such as antigens, are present on the surface of the membrane.
[edit] Structure
[edit] Lipid bilayer
The cell membrane consists of a thin layer of amphipathic lipids which spontaneously arrange so that the hydrophobic "tail" regions are shielded from the surrounding polar fluid, causing the more hydrophilic "head" regions to associate with the cytosolic and extracellular faces of the resulting bilayer. This forms a continuous, spherical lipid bilayer containing the cellular components approximately 7 nm thick[3] which is barely discernible with a transmission electron microscope.
One disadvantage of this arrangement is that hydrophilic solutes cannot passively diffuse across the band of hydrophobic tail groups, allowing the cell to control the movement of these substances via transmembrane protein complexes such as pores and gates.
[edit] Fluid mosaic model
According to the fluid mosaic model of Singer and Nicolson, the biological membranes can be considered as a two-dimensional liquid where all lipid and protein molecules diffuse more or less freely. This picture may be valid in the space scale of 10 nm. However, the plasma membranes contain different structures or domains that can be classified as (a) protein-protein complexes; (b) lipid rafts, (c) pickets and fences formed by the actin-based cytoskeleton; and (d) large stable structures, such as synapses or desmosomes.
[edit] Large membrane structures
These structures can be visualized by electron microscopy or immunofluorescence microscopy. They include synapses, desmosomes, clathrin-coated pits, caveolaes, and different structures involved in cell adhesion.
[edit] Membrane skeleton
The cytoskeleton is found underlying the cell membrane in the cytoplasm and provides a scaffolding for membrane proteins to anchor to, as well as forming organelles which extend from the cell. Anchoring proteins restricts them to a particular cell surface — for example, the apical surface of epithelial cells that line the vertebrate gut — and limits how far they may diffuse within the bilayer. The cytoskeleton is able to form appendage-like organelles, such as cilia, which are covered by the cell membrane and project from the surface of the cell. The apical surfaces of the aforementioned epithelial cells are dense with finger-like projections, called microvilli, which increase cell surface area and thereby increase the absorption rate of nutrients.
[edit] Functions
In animal cells the cell membrane alone establishes a separation between interior and environment, whereas in fungi, bacteria, and plants an additional cell wall forms the outermost boundary. However, the cell wall plays mostly a mechanical support role rather than a role as a selective boundary. One of the key roles of the membrane is to maintain the cell potential. The functions of the cell membrane include, but are not limited to:
- Controlling what goes in and out of the cell.
- Anchoring of the cytoskeleton to provide shape to the cell
- Attaching to the extracellular matrix to help group cells together in the formation of tissues
- Transportation of particles by way of ion pumps, ion channels, and carrier proteins
- Containing receptors that allow chemical messages to pass between cells and systems
- Participation in enzyme activity important in such things as metabolism and immunity
New material is incorporated into the membrane, or deleted from it, by a variety of mechanisms:
- Fusion of intracellular vesicles with the membrane not only excretes the contents of the vesicle, but also incorporates the vesicle membrane's components into the cell membrane. The membrane may form blebs that pinch off to become vesicles.
- If a membrane is continuous with a tubular structure made of membrane material, then material from the tube can be drawn into the membrane continuously.
- Although the concentration of membrane components in the aqueous phase is low (stable membrane components have low solubility in water), exchange of molecules with this small reservoir is possible.
In all cases, the mechanical tension in the membrane has an effect on the rate of exchange. In some cells, usually having a smooth shape, the membrane tension and area are interrelated by elastic and dynamical mechanical properties, and the time-dependent interrelation is sometimes called homeostasis, area regulation or tension regulation.
[edit] See also
[edit] References
- ^ a b c Lodish, Harvey; et al., (5th edn) (2004). Molecular Cell Biology.
- ^ Jesse Gray, Shana Groeschler, Tony Le, Zara Gonzalez (2002). Membrane Structure (SWF). Davidson College. Retrieved on 2007-01-11.
- ^ Alberts, Bruce; et al., (4th edn). Molecular Biology of the Cell.
[edit] External links
- Lipids, Membranes and Vesicle Trafficking - The Virtual Library of Biochemistry and Cell Biology
- Cell membrane protein extraction protocol
- Membrane homeostasis, tension regulation, mechanosensitive membrane exchange and membrane traffic
- 3D structures of proteins associated with plasma membrane of eukaryotic cells
Acrosome - Cell wall - Cell membrane - Chloroplast - Cilium/Flagellum - Centrosome - Cytoplasm - Endoplasmic reticulum - Endosome - Golgi apparatus - Lysosome - Melanosome - Mitochondrion - Myofibril - Nucleus - Nucleolus - Parenthesome - Peroxisome - Plastid - Ribosome - Vacuole - Vesicle
