chapter 5
Fluidity of Membranes-
Membranes are not static sheets of molecules locked rigidly in place. A membrane held together primariy by hydrophobic interactions are much weaker than covalent bonds. Most of the lipids and some proteins can shoft about laterally-that is, in the plane of the membrane-like partygoers elbowing their way through a crowded room. The lateral movement of phospholipids within membrane is rapid. Protins are much larger than lipids and move more slowly, but some membrane proteins do drift. Some membrane proteins seem to move in a highly directed manner, perhaps driven along cytoskeletal fibers by motor proteins. However, many other membrane proteins seem to be helf immobile by their attatchment to the cytoskeleton or extracellular matrix. A membrane remains fluid as temperature decreases until the phopholipuds settle into a closely packed arrangement and the membrane solidifies, much as bacon greae forms lard when it cools. The temperature at which a membrane solidifies depends on the types of lipids it is made of. The membrane remains fluid to a lower temperature as if it is rich in phospholipids with unsaturated hydrocarbon tails. Because of kinks in the tails were double bonds are locaed, unsaturated hydrocarbon tails pack together as closely as saturated hydrocarbon tails, and this looseness makes the membrane more fluid. The steroid cholesterol, which is wedged between phospholipid molecles in the plasma membrane of animal cells, has different effects on membrane fluidity at different tempeatures. At relatively high tempeatures- a 37 degree Celsius, the body temperature of humans, for example-cholesterol makes the membranes less fluid by restraining phospholipid movement. Howeber, because cholesterol also hinders the close packing of phospholipids, it lowers the temperature reuiqrd for the membrane to solidify. Thus cholesterol helps membranes resits changes in fluidity when the temperature changes. When a membrane solidifies, it permeability changes, and enzymaitic proteins in the membrane may become inactive. However, membranes that are too fluid cannot support protein function either. Therefore, extreme environments pose a challenge for life, resulting I evolutionary adapatations that include differences in membrane lipid composition.
Synthesis and Sideness of Membranes-
Membranes have distinct inside and outside faces. The two lipid layers may differ in lipid composition, and each protein has directional oritentation in the membrane. The asymmetric arrangement of proteins, lipids, and their associated carohdyrates un the plasma membrane is determined as the membrane being build by the endolasmic reticumum and golgi apparatus. Steps 1. Membrane proteins and lipids are synthesized in association with endoplasmic reticulum. In the endoplasmic reticulum, carbohydrayes are added to the transmembrane proteins, making them glycoproteins. The carbohydrate portion may be modified. 2. Inside the golgi apparatus, the glycoproteins undergo further carbohydrate modification, and lipids acquire carbohyrates, becoming glycolipids 3. The glycoproteins, glycolipids, and secretory prproteins are transported in vesicles to the plasma membrane 4. Vesicles fuse with the plasma membrane, the outside face of the vesicle becomes contious with the isde(cytoplasmic) face of the plasmammebrane. This releases the secretory proteins from the cell, a process called exocytosis, and the positions the carbohydates of membrane glycoproteinsand glycolipids on the outside(extracellular) face of the plasma membrane.
Passive transport is diffusion of a substance across a membrane with no energy investment-
Molecules have a type of energy called thermal energy, due to their constant motion. One result of motion is diffusion, the movement of particules of any substance so that they tend to spread out into the available space. Each molecule moves randomly, yet diffusion of a population of molecules may be directional. A simple rule of diffusion states that in the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated. Put it this way, aby substance will diffuse down its concentration gradient, the region along which the density of a substance increases or decreases. No work must be done to make ths happen; diffusion is a spontaneous process, no input of energy. Each substance diffuses down its own concentration gradient, unaffected by the concentration gradients of other substance. Much of the traffic across cell membrnaes occur by diffusion. When a substance is more concentrated on one side of a membrane than on the other, there is a tendency for the substance to diffusion across the membrane down its concentration gradient.vThe diffusion of a substance across a biological membrane is called passive transport because cells do not have to spend energy to mamek it happen. The concentration gradient itself represents potential energy and drives diffusion.
How ligand gated channels work...
1. Here we show a ligand-gated ion channel receptor in which the gate remains closed until a ligand binds to the receptor 2. When the ligand binds to the receptor and the gate opens, specific ions can flow trough the channel and rapidly change the concentration of that particular ion insideThis change may directlu affect the activity of the cell in some way 3. When the ligand dissociates from this receptor, the gate closes, and ions no longer enter the cell
Three Stages of Cell Signaling:
1. Reception-it is the target cell's detection of a signaling moecule coming from the outside of the cell. A chemical signal is detected when the signaling mlecle binds to a receptor protein located at the cell;s surface, or in some cases inside the cell 2. Transduction-It is a step or series of steps that converts the signal to a form that can bring about specific cellular response. Transduction usually requires a sequence of changes in a series of different molecules-a signal transduction pathway. The moleules in the pathway are often called relay molecules. 3. Third stage-In this stage of cell signaling, the transduced sgnal finally triggers a specific cellular response. The response may be almost any imaginable cellular activity, such as catalysis by an enzyme, rearrangement of the cytoskeleton, or activation of specifc genes in the nucleus. The cell-signaling process helps ensure that crucial actiies like these occur in the right cells,a t the right time, ad in proper corrdnation wit the actities of other cells of the organism
Phagocytosis-
A cell engulfs a particle by entended pseupodia around it and packaging it within a membranous sac called a food vacuole. The particle will be digestd after the food vacuole fuses with a lysosome containing hydrolytic enzymes.
Cotransport-
A solute that exists in different concentrations across a membrane can dowork as it mves across that membrane by diffusion down its concentration gradient. In a mechanism called cotransprt, a transport protein(cotransporter) can couple the "downhill" diffusion of the solute to te "uphill" transport of a second substance ahainst its own concentration(or electrochemical) gradient. For example, a cotransporter couples the return of hydrogen ions to the transport of sucrose into the cell. This protein can translocate sucrose into te cell against its concentration gradient,but only if the sucrose molc\ecule tranels in te company of an hydrogen ion. The hydrogen ion uses the transport protein as an avenue to iffise down its own electrochemical gradient, wjich is maintained by the proton pump.
How Ion Pumps Maintain Membrane Potential-
All cells have voltages across their plasma membranes. Voltages is electrical potential energy-a separation of opposite charges. The cytoplasmic side of the membrane is negative in charge relative to the extracellulr side because of unqueal distribution of anions and cations on the two sides. The voltage across a membrane, called a membrane potential ranges from about -50 to -200 millivolts. The membrane potential acts like a battery, an enerfy source that affects the traffic of all charged substances across the membrane. Because the sindei of the cell is negative compared with the outside, the membrane potential favors the passive transport of cations into the cell and anions out of the cell. The two forces drive the diffusion of ions across a membrane: chemical force. This combination of fores acting on an ion is called the electrochemical gradient. In the case of ions, An ion diffuses not simply down its concentration gradient but more exactly down its electrochemical gradient. For example, the concentration of Na+ inside a resting nerve cell is much lower than outside it. When the cell is stimulated, gated channels opem that facilitate Na+ diffusion. Sodium ions then "fall" down their electrochemical gradient, driven by the concentration gradient of Na+ ans by the attraction of these cations to the negative side(inside) of the membrane. In this example, both electrical and chemical contribtions to the electrochemical gradient act in the same direction across the membrane bt this is not alwaus so. Some membrane proteins that actively transport ions contribute to the membrane potential . An example is te sodium potassium pump. The pump does not translocated Sodium and potassium one for one but pumpa three sodium ions out of the cell for ebry two potassium ions into th cell. With each "crank" of the pump, there is a net transfer of one positive charge from the cytoplasm to the extracellular fluid, a process that stores energy as voltage. A transport proten that generates voltage across a memnrane is called electrogenic pump. The sodium potassium pump appears to be the major electrogenic pump of animal cells. The main electrogenic pump of plants, fungi, and bacteria is a proton pump which actively tansports protons out of the cell. The pumping of hydrogen ions transfers poisitve charge frok the cytoplasm to the extracellular solution. By generating voltage across membranes, electrogenic pumps help store energy that can be tapped for cellular work. One important use of proton gradients in the cell is for ATP synthesis during cellular respiration. Another is a te of membrane traffic called cotransport.
Facilitated Diffusion: Passive transport Aided by Proteins-
As mentioned earlier many polar molecules and ions impended by the lipid bilayer of the membrane diffuse passively with the help of transport proteins that span the membrane. This phenomenon is called facility diffusion. Cell bioligits are still trying to learn exactly ow various transport proteins facilitate diffusion. Many transport proteins are very specific: They transport some substances but not others. As mentioned earlier the two types of transport proteins are channel proteins and carrier proteins Channel proteins simply provide that allow specific moleciles or ions to cross the membrane. The hydrophilic passageways privded by these proteins can allow water molecoels or small ions to diffuse quicjlu from one side of the membrane to the other. Aquaproins, the water channel proteins, facilitate the massive amounts of diffusion that occr in plant cells and in animal cells sich as red blood cell. Channel proteins that transport ions are called ion channels. Many ion channels function as gated channels which open or close in response to a stimulus. Carrier proteins such as glucose transported seem to undergo subtle cane in shape that somehow translocates the solute0binding site across the memhrane. Such a change in shape may be triggered by the binding and release of the transported molecule. Like ion channels, carrier proteins involved in facilitated diffusion result in the net movement of a substance down its concentration gradient. No energy input is reuired: This is passive transport.
The Role of Membrane carbohydrates in Cell-Cell Recognition-
Cell-cell recognition, a cell's ability to distinguish one type of neighboring cell from another, is crucial to the functioning of an organism. It is important, for example, in the sorting of the cells into tossues and organs in an aimal embryo. It is lalso the basis for the rejection of foreign cells by the immune system, an important line of defense in vertebrate animals. Cells recognize other cells by binding to moecles, often containing carbohydrates, on the trancellulr surface of the plasma membrane. Membrane carbohydrates are usually short, branched chains, of fewer than 15 sugar units. So,e are covalently bonded to lipids, forming molecules called glycolipids. However, most are covalently bonded to proteins, which are thereby called glycoproteins. The carbohydrtes on the exracellular side of the plasma membrane vary from species to species, among inviduals of the same species, and even from one cell type to another in a single individual. The diversity of molecules and their locations on the cell' surface enable membrane carbohydrates to function as markers that distinguish one cell from another. For example, the fournhuman blood types desiged fr A, B, AB, and O reflect variation in the carbohydrate part of glycoproteins on the surface of red blood cells. Recognizes the extracellular matrix of the other cell and thats how it binds. every different kind of cells have different extracellular matrix. all the components of the extracellular matrix are different.
Effects of Osmosis on Water Balance-
Imagine pores in s synthetic membrane are too small for sugar molecles to pass througb but large enough for water molecues to pass through. However, tight clustering of water molecules around the hydrophilic solute molecles make some of the water unavailable to cross the membrane. As a result, the solution with a higher solute concentration has a lower free water concentration. Water diffuses across the membrane from the region of higher free water concentration(lower solute concentration) to that of lower free water concentration(higher solute concentration) until the solute concentrations on both sides are more nearly equal. The diffusion of free water across a selectively permeable membrane, whether artificial or cellular is, called osmosis.
Endocytosis-
In endocytosis, the cell takes in molecles and particulate matter by forming new vesicles from the plasma membrane. Althougb proteins involved in the two processes are different, the events of endocytosis look like the reverse of exocytosis. First, a small area of the plasma membrane sinks inward to form a pocket. Then as the pocket deepends, it pinches in, forming a vesicle containing metarial that was outside the cell.
Fluid mosaic model-
In the fluid mosaic model, the membrane is a mosaic(individual) of protein molecules bobbing in a fluid bilayer of phospholipids. The proteins are not randomly distributed in the membrane, however. Groups of proteins are often assoiated in long-lasting, specialized patches, as are certain lipids. In some regions, the membrane may be much more packed with proteins. A model that describes the structure of cell membranes.
Intracellular Receptors-
Intracellular receptor proteins are found in either the cytoplasm or nucleus of target cells. T reach such a receptor, a signaling molecule passes through the target cell's plasma membrane. A number of signaling milecles can do this because they are hydrophobilc enough to cross th hydrophobic interior of the membrane. Thes ehydrophibic chemical messemgners inblude the steroid hormoens and thyroid hormoens of animals. In both animals ad plants, another chemical signaling molecule with an intracellular receptor Is nitric oxide, a gas its very small, hydrophobic molcles can easily pass between the membrane phospholipids. The behavior of aldosterone is rerpesentaitve of steroid hormones. This hormone is sereted by cells of the adrenal gland, a gland that lies over the kidney. It then trabesl through the blood and ecnters cells all over the body howeer, a response occurs only in kidme cells, which contain receptor molecules for aldonesterone. In these cells, the hormone bonds to the receptor protein, activating it. With the hormone e attatched, the active form of the receptor protein then enters the nucleus and turns on specific genes that control water and soldium flow in kieny cells, thus affecting blood volume. Special proteins called transcription factors control which genees are turned on that is which genes are transcribed into mRNA in a particular cell at a particular time.. when aldosterone receptor is activated, it acts as a transcription factor that turns of specific genes. By acting as a transcription factor, the aldosterone receptor itself carries out the transfuction part of the signaling pathway. Most of the intracellular receptor function in the same way, although many of them, sic as the d hormone receptor, are already in the nucleus efpre te signaling molecule reaches them. They pass through the menrane and bid to receptor in nucleus.estrogen Goes through membrane on its own because it is a lipid. Dimer of estrogen receptor complezes binds to DNA> transcription factors regulatr transcription genes.
Cellular membranes are fluid mosaics of lipids and proteins-
Like all biological membranes, the plasma membrane exihibits selective permeability, that is allows some substances to cross more easily than others. Lipids and proteins are staple ingredients of membranes, although carbohydrates are also important. The most abundant lipids in most membranes are phospholipids. The ability of phosphplipids to form membranes is inherent in their molecular structure. A phospholipid is an amphipathic molecule, mearning it has both a hydrophilic region and a hydrophobic region. A phospholipid bilayer can exist as a stable boundary between two aqueous compartments because the molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the hydrophilic heads to water. Like membrane lipids, most membrane proteins are amphipathic. Such proteins can reside in the phospholipid bilayer with their hydrophilic regions protruding. Their molecular orientation maximizes contact of the hydrophilic regions of a protein with water in the cytosol and extracellular fluid, while providing its hydrophobic parts with a nonaqueous environment.
Receptors in the plasma membrane-
Most water-soluble signaling molecules bind to specific sites on receptor proteins that span the cell's plasma membrane. Such a transmembrane receptor transmits information from the extracellukar environment to the inside of the cell by changing shape when a specific ligand binds to it. There are two major types o transmembrane receptors such as G protein-coupled receptors and ligand-gated ion channels. A G protein-coupled receptor(GPCR) is a cell surface transmembrane receptor that works with the help of a G protein, a protin that binds the energy-rich molecule GTP, which is similar to ATP.. Many signaling moleucles use GPCRs. These receptors vary in the binding sites for their signaling molecles(ligands) and for different types of G proteins insdei the cell. Nevertheless, GPCRs are all remarkably similar in strcture.A ligand-gated ion channel is a membrane receptor with a region that can act as a "gate" for ions when the receptor assumes a certain shape. When a signaling moecle binds as a ligand to the receptor protein, the gate opens or closes, allowing or blocking the diffusion of specifc ions such as sodium or calcium, through a channel in the protein. Like other membrane receptors, these proteins bund the ligand at a specific site on their extracelluka side. Ligand-gated ion channels are very important in the nervous system. Is in plants and animalcell.s ion channels open and flow ions through the membrane. Signals between the nerve and muscle cells or between teo nerve cells. How ligand gated channels work...
Permeability of the Lipid Bilayer-
Nonpolar molecules, such as hydrocarbons, CO2 and O2 are hydrophobic. They can therefore dissolve in the lipid bilayer of the membrane and cross it easily without the aid of membrane proteins. However, hydrophilic interior of te membrane impedes te direct passage through the membrane of ions and polar molecoles which are hydrophilic. Polar molecules such as gluxose and other sugars pass only slowly through a lipid bilayer, and even water are even less likely to penetrate the hydrophobic interior of the membrane. Further more thelipid bilayer is only aspect of the gatekeeper system responsilble for the cells selective permeability. Proteins build into the membrane play a key role in regulating transport.
Small Molecules and Ions as second Messengers-
Not all componens of signal transduction pathways are proteins. Many signaling pathways also involve small, nonprotein water soluble molecules or ions called second messengers. Becase they are small, second messengers can readily spread throughout the cell by diffusion. The two most common send messengers are cycluc AMP and calcium ions. In his research on epinerhine, Ear Sutherland discovered that the binding of epineprhhne to the plasma membrane of a liver cell elevates cytosolic concentration of cyclic AMP(cAMP; cylci adenose monophosphate). The binding of epinephrine to a G protein-coupled receptor leads, via a G protein, to activate of adenylyl cyclase, an enzyme embedded in the plasma membrane that converts ATP to cAMP. Each molecule of adenylyl cyclase can catalyze the synthesies of many molecules of cAMP. In this way, the normal cellular concentration of cAMP can be boosted in a matter of seconds. The cAMP broadcasts the signal to the cytoplasm. It does not persist for long in the absence of the hormone because a different enzyme converts cAMP to AMP. Another surge of epineprhoine is needed to boost the cytlasm cAMP concentration again. Research shows that epihephrine is only one of many hormoens and other signaling moelces that trigger the formation of cAM. The immediate effect of cAMP is siaully the activation of protein kinase A. The activated protein kinase A then phosphorylates varios other proteins.
Transport Proteins-
Specific ions and a vaiety of polar molecules cant move through cell membranes on their own. However, these hydrophilib substances can avoid contact with the lipid bilayer through transport proteins that span the membrane. Some transport proteins called channel proteins function by having hydrophilic cjhannel that certain molecules or atomic ions use as a tunnel through the membrane. For example, the passage of water molecues through the plasma membrane of certain cells is greatly facilitated by chanell proteins called aquaporins. Most aquaporin proteins consit of four dentical subunits.The polypeptide making up each subunit forms a channel that allows many water olecules to pass, more than would cross the membrane without aquaporin; ther transport proteins called carrier proteins hold onto their passengers and cnage the shape in a way that shuttles them across the membranes. A transport protein is specific for e substance it translocates, wllowing only a certainsustance to cross the membrane. Thus, the selective permeability of a membrane depends on both the discriminating arrier of the lipid bilaur ad the specifc transport proteons built into the membrane..
Water Balance of Cells with Cell Walls-
The cells of plants, prokaryotes, funi and some unicellular eukaryotes are surrounded by cell walls. When such a cell is immersed in a hypotonic solution, the cell wall helps maintain the cell's water balance. Like an animal cell, th plant cell swell as water is entered by osmosis. However, the relatively inelastic cell wall will expand onlu so mich before it exerts a back pressure on he cell wall called turgor pressure, that opposes forter water uptake. At this point, te cell is turgid, which is the health state for most plant cell. If a plant's cells and their surroundins are isotonic, tere is no net tendency for water to enter, and the cells become flaccid. Howveer, a cell wall is of no advantage if the cell is immersed in a hypertonic environment. In this case, a plant cell, like an animal cell, will lose water to its surroundings and shrink. As the plant cell shrivels, its plasma membrane pulls away from the cell wall at multiple places. This phenomenon called plasmolysis, causes the plant to wilt and can lead the plant to death.
Protein Phosphorylation and Dehophsoprylation-diagram on p.118
The phsoprylation of proteins and its reverse, dephospyrlation, are wideparead cellular mechanis for reglating protein activity. An enzyme that transfers phosphate groups from ATP to protein is known as protein kinase. Many of the relay molecules in signal transduction pathways are protein kinases, and they often act on other protein kinases in the pathway. A hypothetical pathway containing two different protein kinases that form a short phosphyrlation cascade. The sequence sjown is similar to many known patwhyas, although typically three protein kinases are involved. The signal is transmitted by a cascade of protein phosphyrlations each brnign with it a shape change. Eac such shape chane results from the interaction of the newly addedphosphate groups with charged or polar amino acids. The addition ofnphisphate group often changes the form of a protein from inactive to active. Together protein kinases regulae a large proption of thousands of prteins in a cell. Among those is those that regulate cell division. Equally important in phosphorylation cascade are the protein phosphatases, enzymes that can rapidly remove phosphate groups from proteins, a process called dephosphorylation. By dephosprylating and this inactiving protein kinases, phosphatases provide a mechanism for turning off signal transduction pathway when the initial signal is no longer rrpsent. Phosphatases also make the protin kinases avalible for reuse, enabling the cell to respond again to an extracellular signal. A phosphorylation-dephosphorylation system acts as a molecular switch in the cell, turning an activity on or off, up or down as required. At any given moment, the activaty of a protein is regulated by phospyrolation depends n the balance in the cell betwee the activie kinase molecles and active phosphate molecles. 1. Relay molecule activates protein kinase 1 2. Active protein kinase 1 activates protein kinase 2 3. Active protein kinase 2 phosphroylate a protein that brings about the cells response to the signal 4. Protein phosphatases catalyze the remove of the phosphate groups from proteins maming them inactive again
Long and Long-Distance Signaling-
The signaling moelces sent out from cells are targeted for other cells that may or may not be immediately adjacent. Eukaryotic cells may communicate by direct contact, a type of local signaling. Both animal and plans have cell junctions that, where present directly connect cytoplasms of adjacent cells; in animals there are gap junctions and in plants the plasmodesmata. In these cases, signaling substances dissolved in the cytosol can pass freely between adjacent cells. Also animal cells may communicate cia direct contact between memnrane-bound cell-surface molecules in cell-cel recognition. In many other cases of local signaling, the signaling cell secretes messenger molecles. Some of these travel onl short distances such as local regulator s infleucne cells in the vicinity. Ne class of loca regulators in animals, growth factors are compounds that stimulate nearby target cells to grow and vidie. Numberous cells cans simultaneously receive and respond to other molecules of growth favtor produced by a neaby cell. This type of local signaling is paracrine signaling. A more specialized type of local signaling called synaptic signaling occurs in the animal nervous sytem An electrical signal movin along a nerve cell triggers the secretion of neutrotransmitter molecles carrying a chemical signal. These oleces diffuses across the synapse, the narrow space between the nerve cell nd its target cell, triggering a response int eh target cell. Both animals and plants use hormoens for long distance signaling. In hormonal signaling anima,s also known as endocrine signaling, specialized cells release hormone oelcles which travel via the circulatory system to other parts of the body, where they reach target cells that can recognize and respond to hormones. Hormones vary widely in molecular size and type, as do local regulators.
Ligand-
The signaling molecule is complementary in shape to a specific site on the receptor and attatchers there, like a key in a lock. The signaling molecule acts as a ligand, a molecule that specific binds to another molecule, often a larger one. Ligand binding generally cases a receptor protein to undergo a change in shape. For many receptors. This shape change directly activates receptor enabiling, it to interact with other cellular molecules. Most signaling receptors are plasma membrane proteins. Their ligands are water-soluble and generallytoo large to pass freely through the plasma membrane. Other signal receptors, however, are located insdie the cell. They vary in structure and vary in type. What must be the chemical nature of the ligands for intracellular receptors? Polar molecules, small, hydrophobic , small and uncharged What must be the chemical nature of the ligands for membrane receptors? Large and charged
Membrane Proteins and their functions-
This is the mosaic aspect of the fluid mosaic mode. Somewhat like a tile mosaic, a membrane is a collage of different priteins embedded in the fluid matrix of the lipid bilayer. More than 50 kinds of proteins habe been found so far in the plasma membrane of red blood cells. Phospholipids form the main fabric of the mmerane, but proteins determine most of the membrane dunction. Different types of cells contain different set of membrane proteins, and the various memranes within a cell has a unique collection of proteins. There are two poluations of membrane proteins: integral proteins and peripheral proteins. Integral proteins penetrate the hydrophobic interior of the lipid bilayer. The majority are transmembrane proteins, which span the membrane; other integral proteins extend only partway into the hydrophobic interior. The hydrophobic regions of an integral proteome consists of one or more stretches of nonpolar amino acids usually coiled into a helics. The hydrophilic parts of the molecule are exposed to the aqueous solutions on either side of the membrane. Some proteins also have one or more hydrophilic channels that allow passage passage of hydrophilic substances. Peripheral proteins are not embedded in the lipid bilayer at all; they are loosely bound to the surface of the membrane, oftn to exposed parts of the integral proteins. On th cytoplasmic side of the plasma membrane, some membrane proteins are helf in place by attatchment of the cytoskeleton. And on the extracellular side, certain memnrane prteins are attatched fibers of the extracellular matrix. These attatchments combined to give animal cells a stronger framework than the plasma membrane alone could provide. A single cell may have membrane proteins carrying out several of these functions, and a single membrane protein may have multiple functions.
Water balance in cells without cell walls-
To explain the behavior of a cell in a solution, we must condiser both solute concentration and membrane permeability. Both factors are take into account in the concept of tonicity, the ability of a surrounding solutution to case a cell to gain or lose water. The tonocity of a solution depends in part on its concentration of solutes that cannot cross the membrane(nonpentration solutes) relatve to that inside the cell. If tere is a higher concentration of nonpenetrating solutes in the surrounding solution, water will tend to leave the cell and vise versa. If a cell without a cell wall, such as an animal cell, is immersed in an environment that is isotonic to the cell, there will be no net movement of water across the plasma membrane. Water diffuses across the membrane, but at the same rat in both directions. If a cell is in a hypertonic solution, the cell will lose water, shrivel and die. However, I we place the cell in a solution that is hypotonic to the cell, water will enter the cell faster than it leaves, ane the cell will swell and lyse. A cell without igid cell walls can tolerate neither excessive uptake nor excessive loss owater. This problem of water balance is automatically solved if such a cell lives in isotonic surroundings. Seawater is isotnonic to many marine invertebrates. The cells of most terrestrial(land-dwelling) animals are bathed in an extracellular fluid that is isotonic t cells. In hypertonic or hypotonic environments, howeber, organism that lack rigid cell walls must hage other adaptations such as osmoregulation, the control of solution concentrations and water balance.
Active transport uses energy to move solutes against their gradients-
To pump a solute across a membrane against its gradient requires work; the cell must expend energy. Therefore, this type of membrane traffic is called active transport. The transport proteins that move solutes against their concentration gradients are all carrier proteins rather than channel proteins. This makes snese because when channel proteins are open, they merely allow solutes to diffue down their concentration gradients rather than picking them up and transporting them against their concentration gradients. Active transport enables a cell to maintain intenral concentrations of small solutes that differ from concentrations in its environment. As in other types of cellular work, ATP supplies the energy for most active transport. One way ATP can power active transport us by transferring its terminal phosphate grou directly to the transport protein. This can induce the protein to change its shape in a manner than translocates a solute bound to the protein across the membrane. One transport system that works this wayis the soldium-potassium pump, which exchanges Na+ for K+ across the plasma membrane of animal cells.
Response: Regulation of Transcription or Cytoplasmic activities-
Ultimately, a signal transuction pathway leads to the regulation of one or more cellular activites. The response may occur in the nucleus of the cell or in the cytoplasm. Many signaling pathways ultimately regulate protein synthesis, usually by turning specific genes on or off in the nucleus. Like an activated steroid receptor, the final activated mole in a signaling pathway ma function as a transcription factor. The response to this growth factor signal is transcription, the synthesis in the cytoplasm into specific proteins. In other cases, the transcription factor might regulate a gene by turning it off; Often a transcription factor regulates several different genes. Sometimes a signaling pathway may regulate the activity of proteins rather than causing their synthesis by activating gene expression. This directlu affects proteins that function outside the nucleus. For example, a signal ma cause the opening or closing of an ion chanell in the plasma membrane or a change in the cell metabolism. The response of cells to hthe hormone epinephrine helps regulate cellular energy metabolism by affecting the activity of an enzyme: The final step in the signaling pathway that begins with epinephrine binding activates the enzyme that catalyzes the breakdown of glycogen.
Bulk transport acorss the plasma membrane occrs by exocytosis and endocytosis-
Water and small solutes enter and leave the cell b diffusing through the lipid bilayer of the plasma membrane or by being moved across the memrbaen by transport proteins. However, large molecules such as proteins and polysacchardies, as well as larger particles-generally cross the membrane in bulk packaged in vesicles. Like active transport, these ptocess require energy. Exocytosis-The cell secretes certain biological molecules by the fusion of vesicles with the plasma membrane; this process is called exocytosis. A transport vesicle that has budded from the golgi apparatus mves along microtubules of the cytoskeleton of the plasma membrane. When the vesicle membrane and plasma membrane come in contact, specific proteins rearrange the lipid molecukes of the teo bilayers so that the two membranes fuse. The contents of the vesicle then spill to the outside of the cell, and the vesicle membrane becomes part of the plasma membrane. Many secretory cells se exocytosis to export products.
Transduction by Cascades of Moleclar Interactions-
When receptorsfor signalignmolecles are plasma membrane proteins, the transduction state of cell signaling is usually a multistep pahway involving many molecules. Steps often include activation of proteins bynadditim or removal of phosphate groups or release of otjer small molcules or ions that act as messemgers. If each molecule in a pathway transmits the signal to numerous molecules at the next step in ther series, the result is a gemometric increase in the number of activated molecles by the end of the pathway. Moreever, multistep pathways provide more oppurtinities for coordination and control than do simpler systems. The binding of a specific signalin molecule to a receptor in the plasma memnrane triggers the first step in the chain of molecular interactions-the gianl transduction pathway-that leads to a particular response within the cell. Like falling dominos, the signal-acivated receptor activates anotjer molecule, which activates uet anotjer molecule and so on, until the protein that produces the final cellular response is activated. The molecles that relay molecules in this book are often proteins. The interaction of proteins is a major these of cell signaling. The original signaling molecule is not physicallu passed along a signaling pathway, in most cases it nenrs enters the cell. Whej we say that the sinal is related along a pathway, we mean certain information is passed onl, at each steo, the signaling is transudced into different form, commendlu via shape change in a protein. Very often, the shape change is brought about by phosphyrlaion, the addition of a phosphate groups to a protein.
Six functions performed by membrane
a. Transport-A protein that spans the memnrane may procide a hydrophilic channel across the membrane that is selective for a particular solte. Other transport proteins shittle a stance from one side to the other by changinc shapel some of the proteins hydrolyze ATP as an energy source to actively pump substances across the membrane b. Enzymatic activity-A protein build into the membrane may be an enzyme with its active site(where the target molecule binds) exposed to a substance in the adjacent solution. In some cases,several enzymes in a embrane are organized as a team that carries out several sequential steps of a metabolic pathway c. Signal transduction-A membrane pritein(receptor) may have a binding site with a specifc shape that fits the shape of a chemical messenger such as a hormone. The external emsseanger(signaling molecule) may cause the protein to change shape, allowing it to relay the message to the insode of te cell, usually binding to a cytoplasmic protein. d. Cell-cell recognition-some glycoproteins serve as identication tags that are specifically recognized by membrane proteins of other cells. Thus type of cell-cell binding is sually short-lived compared to that shown in e. e. Intracellular joining-Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions. This type of binding is more longlasting than show in d. make sure that things do not go into the lumen f. Attatchment to the cytoskeleton and extracellular matrix(ECM)-Microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins, a funtuon that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that can bind to ECM molecules can coordinate extracellular and intracellular changes.
Steps for intracellular hormone
1) The steroid horme aldonesteornr passes through the plasma membrane 2) Aldosterone binds to a receptor protein in the cytoplasm activating it 3) The hormone-receptor complex nters tne nucleus and binds to specific genes 4) The bound protein acts as a transcription factor, stimulating the transcription of the gene into mRNA 5) The mRNA is translated into a specifc protein
Receptor-
mediated endocytosis-is a specialized type of pinocytosis that enables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the plasma membrane are proteins with receptor sites exposed to the extracellular fluid. Specific solutes bind to the sites. The receptor proteins then cluster in the coat pits, and each coated pit forms a vesicle containing the bound molecules. Notice that there are realtively more bound molecles inside the vesicle but other molecles are also preset. After the ingested material is liberated from the vesicle, the emptied receptors are recycled to the plasma membrane shown by the same vesicle. Cell signaling-When functional, cell signaling include: 1)some way to reveive the message and is usually the intracellular protein 2)some way to relay or amplify the message which can be second messengers(transdunction), phoryolation realys, and binding to DNA as transcription factor 3) some cellular response to the message
Pinocyosis-
a cell continually "gulps" droplets of extracellular fluid into tiny vesicles, formed by infoldings of the plasma membrane. In this way, the cell obtins molecules dissolved in droplets. Because any or all solutes are taken into the cell, pinocytosis as shown here is nonspecific for the substances it transports. In many cases above, the parts of the lasma membrane that form vesciels are lined on theur cytoplasmic side by a fuzzy laer of coat protein.; the "pits" and resulting vesicles are said to be "coated".
