The cell membrane is a semipermeable phospholipid bilayer.1 The cell or plasma membrane has an active role in signaling, entry of nutrients into the cell and removal of waste, cell recognition, as well as transport of materials between tissues. Cell membranes have both a flexible component (phospholipids) and an abundance of stabilizing molecules (cholesterol and protein) to make sure that the membrane remains intact. As a semipermeable membrane, it chooses which particles can enter and leave the cell at any point in time. This selectivity is mediated not only by the various channels and carriers that are integrated in the membrane, but also by the membrane itself. Composed primarily of two layers of phospholipids, the cell membrane permits fat-soluble compounds to cross easily, while larger and water-soluble compounds do not readily cross it. The fluid mosaic model defines the structure as well as the function of the cell membrane.1
The fluid mosaic model speaks to the presence of certain macromolecules such as lipids, proteins, and carbohydrates in a dynamic, semisolid plasma membrane that encompasses cells. The plasma membrane contains proteins embedded within the phospholipid bilayer and the membrane is not static. Lipids move freely in the plane of the membrane and can assemble into lipid rafts. Proteins and carbohydrates may also move within the membrane, but are slowed by their relatively large size.1
The main function of the cell membrane is to protect the interior of the cell from the external environment. Cellular membranes selectively regulate chemicals into and out of the cell and are involved in both intracellular and intercellular communication and transport. Proteins embedded within the plasma membrane bilayer function as cellular receptors during signal transduction. These proteins play an important role in regulating and maintaining overall cellular activity. The cell membrane functions as a stable semisolid barrier between the cytoplasm and the environment, but it is in a constant state of flux on the molecular level. Phospholipids move rapidly in the plane of the membrane through simple diffusion.11 Lipid rafts are collections of closely related lipid molecules with or without associated proteins that serve as attachment points for other biomolecules. These rafts tend to function as signalling molecules. Lipids can also move between the membrane layers, but this is energetically unfavorable due to the fact that the polar head group of the phospholipid must be forced through the nonpolar tail region in the interior of the membrane. Flippases are specialized enzymes that assist in the transition between layers. Cells involved in biosignaling processes, control the number of specific cellular receptors on their surface which enables them to meet their cellular requirements.
Lipids may be defined as hydrophobic or amphiphilic small molecules. An amphiphile is a chemical compound that contains both hydrophilic and lipophilic (fat-loving) characteristics. This amphiphilic nature allows a number of lipids to form structures such like vesicles. Most cells that contain mitochondria also have fat globules in them but fat is stored primarily in adipose tissue. Lipids can be categorized into three main groups: triglycerides, steroids and phospholipids. Triglycerides, or triacylglycerols provide insulation while protecting the internal organs with a layer of covering. When you don’t metabolise all the calories consumed, these fat products are converted to triglycerides and stored for future use. Triacylglycerols are stored as fat droplets in copious amounts in vertebrate fat cells. Triacylglycerols and free fatty acids behave as precursors for phospholipids and are normally found in low levels in the plasma membrane. Steroids are a type of lipid under which cholesterol is found. Cholesterol is produced by the body and it is also consumed through food, and it plays a part in the production of hormones. Cholesterol is needed by every cell in the body and it is the most abundant known steroid lipid found in the human body. It functions in cell repair and aids in the generation of new cells. The water insoluble nature of cholesterol causes it to be found in the middle of the membrane, in the hydrophobic region. It plays a role in stabilizing the membrane as well as keeping it flexible. Phospholipids are derivatives of triglycerides. They are very similar to them but slightly different on a molecular level. Half of a phospholipid molecule is water-soluble and the other half is not, which causes them to react differently than triglycerides. Phospholipids form double-layered membranes where the water-soluble molecules are located on the outside of the cell membrane while the water-insoluble molecules are found on the inside. These lipids also function to protect and insulate cells.
Phospholipids can be grouped into phosphoglycerides, phosphoinositides and phosphosphingosides. Waxes are a class of organic compounds that are hydrophobic solids at room temperature.1 A wax is a simple lipid, consisting of an ester of a long-chain alcohol with a fatty acid component. Up to 32 carbon atoms may be found in the alcohol chain of waxes. Waxes can be found in the cell membranes of some plants but rarely in the cell membranes of animals. The long-chain fatty acid as well as the long-chain alcohol play significant roles in the high melting point of waxes. Waxes can provide both stability and rigidity within the nonpolar tail region when they are present within the cell membrane. Most waxes serve the function of protection or waterproofing from the external environment. Sphingolipids are observed to protect the surface of a cell against detrimental environmental factors by forming a mechanically stable and chemically resistant outer leaflet of the plasma membrane lipid bilayer.
There are many different types of proteins associated with the membrane. Some of these proteins are found on the surface while others are partially submerged in the membrane. There are also some that traverse the membrane and come out from both surfaces of the membrane. The functions of these proteins vary and it includes helping to move molecules across the membrane and as an identity tag for most cells. Transmembrane proteins are the proteins of the cell membrane that pass completely through the lipid bilayer to protrude on both sides. Embedded proteins, however, are found either only on the interior (cytoplasmic) or exterior (extracellular) surface of the cell membrane. Both these type of proteins are considered integral proteins because of their association with the interior of the plasma membrane. Membrane-associated (peripheral) proteins may be bound through electrostatic interactions with the lipid bilayer, especially at lipid rafts, or to other transmembrane or embedded proteins, like the G proteins found in G protein-coupled receptors. Transporters, channels, and receptors are generally transmembrane proteins.
The fluid mosaic model basically describes the membrane as protein boats floating in a sea of lipids.
The cell membrane functions to control movement of substances into and out of the cell; however, it varies in its selectivity for different substances. Small nonpolar molecules traverse the cell membrane quickly by diffusion, while charged particles and larger molecules require more specialized transport processes. Substances that cannot readily move across the membrane are transported across the membrane either without energy (passive) or with energy (active). Membrane channels help ions to cross the membrane using ion channels. Depending on their thermodynamics, transport processes can be termed as active or passive. Spontaneous processes that do not require energy or have a negative ΔG proceed through passive transport, while those that are nonspontaneous and have energy requirements (positive ΔG) proceed through the active transport process. An increase in entropy (ΔS) is the main driving factor in most passive transport processes. Passive transport does not require energy stores but they instead make use of the concentration gradient to supply the energy for particles to move.
Diffusion is a passive process by which substrates move down their concentration gradient directly across the plasma membrane. Only chemicals and compounds that are freely permeable to the membrane are able to undergo simple diffusion. Osmosis is a specific kind of simple diffusion that deals with the movement of water. Water will move from a region of higher water concentration (more dilute solution) down its gradient to a region of lower water concentration (more concentrated solution). If the concentration of solutes inside the cell is higher than the surrounding solution, the solution is said to be hypotonic. This kind of solution will cause a cell to swell as water rushes in, sometimes to the point of bursting. A solution that is more concentrated than the cell is termed a hypertonic solution, and water will move out of the cell. If the solutions inside and outside are equimolar, they are said to be isotonic. A key point to isotonicity is that it prevents the net movement of particles. Water molecules will continue to move but the cell will neither gain nor lose water overall.
Facilitated diffusion is specialized form of simple diffusion for molecules that are impermeable to the membrane due to several factors such as they are too large, polar, or carry a charge. The energy barrier may also be too high for these molecules to freely cross without little assistance. Facilitated diffusion need integral membrane proteins to function as transporters or channels for these substrates to enter the cell. Active transport is the net movement of a solute against its concentration gradient which cannot occur naturally. Active transport always needs a source of energy, but the source can vary. Primary active transport is an active transport process that utilises ATP or another energy molecule to directly move molecules across a membrane. Primary active transport usually involves the use of a transmembrane ATPase to carry out its function. Secondary active transport also uses energy to traverse molecules across the membrane but unlike primary active transport, there is no direct attachment to ATP. Secondary active transport instead, utilises the energy expended by one particle going down its electrochemical gradient to move a different particle up its electrochemical gradient. Secondary active transport is thus also known as coupled transport. Symport is when both particles are flowing in the same direction across the membrane while when the particles flow in opposite directions, it is termed antiport. The membrane potential, which results from a difference in the number of positive and negative charges on either side of the membrane, is maintained primarily by the sodium–potassium pump, which moves three sodium ions out of the cell for every two potassium ions pumped in, and to a minor extent by leak channels that allow the passive transport of ions. Diffusion, facilitated diffusion, and osmosis increase in rate as temperature increases. Active transport may or may not be influenced by temperature and this is dependent on the enthalpy (ΔH) of the process.
Many eukaryotic cells take in food and liquids by protruding their plasma membranes towards food particles. The membrane encircles the particle and forming a vesicle, which is a membrane-bounded sac, around it. This process is called endocytosis. Endocytosis is the process of engulfing foreign material by folding the plasma membrane around it, forming a vesicle. When the material is composed of organic matter, the process is known as phagocytosis. When the engulfed material is a liquid, the process is referred to as pinocytosis. The reverse of endocytosis is exocytosis and it is when material is discharged from vesicles at the cell surface. The particles remain suspended in the vesicle as it fuses with the plasma membrane. The membrane that forms the vesicle is made of phospholipids, and as it comes in contact with the plasma membrane, the phospholipids of both membranes interact, forming a pore through which the contents leave the vesicle to the outside. Exocytosis is a necessary means of transporting the materials needed to construct the cell wall in plant cells through the plasma membrane. Exocytosis provides a means for secreting many hormones, enzymes, and other substances in animal cells. Receptor-mediated endocytosis often transport specific molecules into eukaryotic cells. Molecules are first transported into the cell by binding to specific receptors in the plasma membrane. The transport process is specific, as only molecules that have a complimentary shape will fit into the receptor. Receptor-mediated endocytosis does not bring substances directly into the cytoplasm of a cell as one would think, as the substance that have been engulfed are still separated from the cytoplasm by the membrane of the vesicle. It is other processes that act on the contents, to breakdown or release them.
Gap junctions enable cells to communicate directly with each other and usually found together as small bunches. Gap junctions are also known as connexions. They are formed by the alignment and interaction of pores that are made up of six molecules of connexion. They permit movement of water and some solutes directly between cells. Proteins do not generally move through gap junctions.
Tight junctions function to prevent solutes from leaking into the intracellular space between cells through a paracellular route. Tight junctions are found in epithelial cells, and are a physical link between the cells as they form a single layer of tissue. Tight junctions can hinder the permeability of the cell surface enough so that it generates a transepithelial voltage difference, based on the different concentrations of ions on either side of the epithelium. Tight junctions must form a continuous band around the cell to be effective as fluid could otherwise leak through spaces between tight junctions. The lining of renal tubules is one area where tight junctions can be found, where they restrict passage of solutes and water without cellular control. Desmosomes bind neighboring cells by attaching to their cytoskeletons. Desmosomes are formed by interactions between transmembrane proteins associated with intermediate filaments inside adjacent cells. Desmosomes are primarily found at the interface between two layers of epithelial tissue. Hemidesmosomes have a similar function, but their primary duty is to anchor epithelial cells to basement membranes.
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3) Hardin, J., Bertoni, G., & Kleinsmith, L. J. (2015). Becker’s World of the Cell (8th ed.). New York: Pearson.
4) Eurell, J. A., & al, e. (2006). Dellmann’s textbook of veterinary histology. Wiley-Blackwell.
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