Biology 111-548- Chapter 5: Structure and Function of Plasma Membranes

Tonicity in Living Systems

In a hyptonic environment, water enters a cell, and the cell swells. In an isotonic condition, the relative solute and solvent concentrations are equal on both membrane sides. There is no net water movement; therefore, there is no change in the cell's size. In a hypertonic solution, water leaves a cell and the cell shrinks. If either the hypo- or hyper- condition goes to excess, the cell's functions become compromised, and the cell may be destroyed. A red blood cell will burst, or lyse, when it swells beyond the plasma membrane's capability to expand. If the cell swells, and the spaces between the lipids and proteins become too large, the cell will break apart.

Proteins

comprise the plasma membranes' second major component. Integral proteins as their name suggests, integrate completely into the membrane structure, and their hydrophobic membrane-spanning regions interact with the phospholipid bilayer's hydrophobic region.

Phospholipid Molecule

consists of a three-carbon gllycerol backbone with two fatty glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon. Has a head area, which has a polar character or negative charge, and a tail area, which has no charge. Head can form hydrogen bonds, but the tail cannot. This is vital to the plasma membrane's structure because, in water, phospholipids arrange themselves with their hydrophobic tails facing each other and their hydrophilic heads facing out. They form a lipid bilayer- a double layered phospholipid barrier that separates the water and other materials on the other side. When heated in an aqueous solution usually spontaneously form small spheres or droplets with their hydrophilic heads forming the exterior and their hydrophobic tails on the inside.

Isotonic Solultions

the extracellular fluid has the same osmolarity as the cell. If the cell's osmolarity matches that of the extracellular fluid, there will be no net movement of water into or out of the cell, although water will still move in and out. Blood cells and plant cells in hypertonic, isotonic, and hypotonic solutions take on characteristic appearances.

Factors that affect diffusion: Part 2

1. Extent of the concentration gradient: The greater the difference in concentration, the more rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower the diffusion rate. 2. Mass of the molecules diffusing: Heavier molecules move more slowly; therefore, they diffuse more slowly. The reverse is true for lighter molecules. 3. Temperature: Higher temperatures increase the energy and therefore the molecules' movement, increasing the diffusion. Lower temperatures decrease the molecules' energy, thus decreasing the diffusion rate.

Factors that affect diffusion: Part 3

4. Solvent Density: as density of solvent increases, the diffusion rate decreases. The molecules slow down because they have a more difficult time passing through the denser medium. If medium is less dense, diffusion increases. Any increase in the cytoplasm's density will inhibit the movement of the materials because cells primarily use diffusion to move materials within the cytoplasm. 5. Solubility: Non-polar or lipid-soluble materials pass through plasma membranes more easily than polar materials, allowing a faster diffusion rate. 6. Surface area and plasma membrane thickness: Increased surface area increases the diffusion rate; whereas, a thicker membrane reduces it. 7. Distance Travelled: the greater the distance that a substance must travel, the slower the diffusion rate. Places an upper limitation on cell size. Cells must either be small in size, as in the case of many prokaryotes, or be flattened, as with many single-celled eukaryotes.

Fluid- Lipids (phospholipids and Cholesterol)

A phospholipid is a molecule consisting of glycerol, two fatty acids and a phosphate-linked head group. Cholesterol, another lipids comprised of four fused carbon rings, is situated alongside the phospholipids in the membrane's core. The protein, lipid, and carbohydrate proportions in the plasma membrane vary with cell type, but for a typical cell, protein accounts for about 50 percent of the composition by mass, lipids account for about 40 percent, and carbohydrates comprise the remaining 10 percent. Carbohydrates are present only on the plasma membrane's exterior surface and are attached to proteins, forming glycoproteins, or attached to lipids, forming glycolipids. They form bilayers in aqueous environments.

Electrochemical Gradient

A substance's differential concentrations across a space or a membrane- but in living systems, gradients are more complex. Because ions move into and out of cells and because cells contain proteins that do not move across the membrane are mostly negatively charged, there is also and electrical gradient, a difference of charge, across the plasma membrane. The interior of living cells is electrically negative with respect to the extracellular fluid in which they are bathed, and at the same time, cells have higher concentrations of potassium and lower concentrations of sodium and than the extracellular fluid. Thus in a living cell, the concentration gradient of Na tends to drive into the cell, and its electrical gradient also drives it inward to the negatively charged interior. However, the situation is more complex for other elements such as potassium. The electrical gradient of K+, a positive ion, also drives it into the cell, but the concentration gradient of K+ drives K+ out of the cell. We call combines concentration gradient and electrical charge that effects an ion its electrochemical gradient.

Factors that affect diffusion: Part 4

A variation of diffusion is the process of filtration. In filtration, material moves according to its concentration gradient through a membrane. Sometimes pressure enhances the diffusion rate, causing the substances to filter more rapidly.

Carrier Proteins for Active Transport

An important membrane adaption for active transport is the presence of specific carrier proteins or pumps to facilitate movement: there are three protein types or transporters. A uniporter carries one specific ion or molecule. A symporter carries two different ions or molecules, both in the same direction. An anitporter also carries two different ions or molecules, but in different directions. All of these transporter can also transport small, uncharged organic molecules like glucose. These three types of carrier proteins are also in facilitated diffusion, but they do not require ATP to work in that process. Some examples of pumps for active transport are Na+-K+ ATPase, which carries sodium and potassium ions, and H+-K+ ATPase, which carries hydrogen and potassium ions. Both of these are antiporter carrier proteins. Two other carrier proteins are Ca2+ ATPase and H+ ATPase, which carry only calcium and only hydrogen ions, respectively. Both are pumps.

Carbohydrates: Part 2

Carbohydrate components of both glycoproteins and glycolipids- as the glycocalyx (meaning "sugar coating"). The glycocalyx is highly hydrophilic and attracts large amounts of water to the cell's surface. Aids in the cell's interaction with its watery environment and in the cell's ability to obtain substances dissolved in the water. It also is important for cell identification, self/non-self determination, and embryonic development, and is used in cell to cell attachments to form tissues.

Carrier Proteins: Part 2

Channel proteins transport much more quickly than carrier proteins.Channels proteins facilitate diffusion at a rate of tens of millions of molecules per second; whereas, carrier proteins work at a rate of a thousand to a million molecules per second.

Fluid Mosaic Model

Describes the plasma membrane as a mosaic of components- including phospholipids, cholesterol, proteins, and carbohydrates- that gives the membrane a fluid character.

Methods of Transport, Energy Requirements, and Types of Transported Material

Diffusion: Passive- small- molecular weight material Osmosis: Passive- water Facilitated transport/diffusion: Passive- sodium, potassium, calcium, glucose Primary Active Transport: Active- sodium, potassium, calcium Secondary Active Transport: Active- amino acids, lactose Phagocytosis: Active= large macromolecules, whole cells, or cellular structures Pinocytosis and Potocytosis: Active- small molecules (liquids/water) Receptor-mediated endocytosis: Active- large quantities of macromolecules

Osmoregulation in Freshwater fish

Freshwater fish live in an environment that is hypotonic to their cells. These fish actively take in salt through their gills and excrete diluted urine to rid themselves of excess water. Saltwater fish live in the reverse environment, which is hypertonic to their cells, and they secrete salt through their gills and excrete highly concentrated urine. In vertebrates, the kidneys regulate the water amount in the body. Osmoreceptors are specialized cells in the brain that monitor solute concentration in the blood. If the solute levels increase beyond a certain range, a hormone releases that slows water loss through the kidney and dilutes the blood to safer levels. Animals also have high albumin concentrations, which the liver produces, in their blood. This protein is too large to pass easily through plasma membrane and is a major factor in controlling the osmotic pressures applied to tissues.

Fluidity: Part 3

If decreasing temperatures compress saturated fatty acids with their straight tails, they press in on each other, making a dense and fairly rigid membrane. If unsaturated fatty acids are compressed, the "kinks" in their tails elbow adjacent phospholipid molecules away, maintaining some space between the phospholipid molecules. This "elbow room" helps to maintain fluidity in the membrane at temperatures at which membranes with saturated fatty acid tails in their phosphlipids would "freeze" or solidify. A cold envrironment usually compresses membranes comprised largely of saturated fatty acids, making them less fluid and more susceptible to rupturing. Many organisms are capable of adapting to cold environments by changing the proportion of unsaturated fatty acids in their membranes in response to lower temperature.

Plasma Membrane's Principal Components

Lipids (phospholipids and cholesterol) , proteins, and carbohydrates attached to some of the lipids and proteins.

Phospholipids

Membranes's main fabric comprises amphiphilic, phospholipid molecules. The hydrophilic or "water-loving" areas of these molecules are i contact with the aqueous fluid both inside and outside the cell. Hydrophobic, or water-hating molecules, tend to be non-polar. They interact with other non-polar molecules in chemical reactions, but generally do not interact with polar molecules. Hydrophobic molecules form a ball or cluster when in water. The phospholipids' hydrophilic regions form hydrogen bonds with water and other polar molecules on both the cell's exterior and interior. The membrane surfaces that face the cell's interior are exterior are hydrophilic. The cell membrane's interior is hydrophobic and will not interact with water. Phospholipids form an excellent two-layer cell membrane that separates fluid within the cell from the fluid outside the cell.

Factors that Affect Diffusion

Molecules move constantly in a random manner, at a rate that depends on their mass, their environment, and the amount of thermal energy they possess, which in turn is a function of temperature. This movement accounts for molecule diffusion through whatever medium in which they are localized. A substance moves into any space available to it until it evenly distributes itself throughout. After a substance has diffused completely through a space, removing its concentration gradient, molecules will still move around in the space, but there will be no net movement of the number of molecules from one area to another. We call this lack of a concentration gradient in which the substance has no net movement dynamic equilibrium. While diffusion will go forward in the presence of a substance's concentration gradient, several factors affect the diffusion rate.

Tonicity in Living Systems: Part 3

Paramecia and amoebas, which are protists that lack cell walls, have contractile vacuoles. This vesicle collects excess water from the cell and pumps it out, keeping the cell from lysing as it takes on water from its environment.

Plasma Membrane Components and Functions

Phospholipid: Location- main membrane fabric; Cholesterol: Location- attached between phospholipids and between the two phospholipid layers; Integral Proteins (for example, integrins): Location- embedded within the phospholipid layer(s); may or ma not penetrate through both layers; Peripheral Proteins: Location- on the phospholipid bilayer's inner or outer surface; not embedded within the phospholipids; Carbohydrates (components of glycoproteins and glycolipids): Location- generally attached to proteins on the outside membrane layer

Selective Permeability: Part 2

Plasma membranes are amphiphilic. Non polar and lipid-soluble material with a low molecular weight can easily slip through the membrane's hydrophobic lipid core. Fat-soluble drugs and hormones also gain easy entry into cells and readily transport themselves into the body's tissues and organs. Oxygen and carbon dioxide molecules have no charge and pass through membranes by simple diffusion.

Selective Permeability

Plasma membranes are asymmetric: the membrane's interior is not identical to its exterior. On the membrane's interior, some proteins serve to anchor the membrane to cytoskeleton's fibers. There are peripheral proteins on the membrane's exterior that bind extracellular matrix elements. Carbohydrates attached to lipids or proteins, are also on the plasma membrane's exterior surface. These carbohydrates complexes help the cell bind required substances in the extracellular fluid.

Selective Permeability: Part 3

Polar molecules connect easily with the cell's outside, but they cannot readily pass through the plasma membrane's lipid core. Additionally, while small ions could easily slip through the spaces in the membrane's mosaic, their change prevents them from doing so. Ions such as sodium, potassium, calcium, and chloride must have special means of penetrating plasma membranes. Simple sugars and amino acids also need the help of various transmembrane proteins (channels) to transport themselves across plasma membranes.

Mechanism: Part 2

Returning to the beaker example, recall that it has a solute mixture on either side of the membrane. A principle of diffusion is that the molecules move around and will spread evenly throughout the medium if they can. However, only the material capable of getting through the membrane will diffuse through it. In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system. Thus, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane-osmosis- will continue until the water's concentration gradient goes to zero or until the water's hydrostatic pressure balances the osmotic pressure.

Primary Active Transport: Part 2

Steps: 1. the carrier has a high affinity for sodium ions with the enzyme oriented towards the cell's interior. 3 ions bind to the protein 2. the protein carrier hydrolyzes ATP and a low-energy phosphate group attaches to it. 3. The carrier changes shape and reorients itself towards the membrane's exterior. The protein's affinity for sodium decreases and the three sodium ions leave the carrier 4. The shape change increases the carrier's affinity for potassium ions, and two such ions attach to the protein. Subsequently the lower- energy phosphate group detaches from the carrier. 5. With the phosphate group removed and potassium ions attached, the carrier protein itself towards the cell's interior 6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions moves into they cytoplasm. The protein now has a higher affinity for sodium ion, and the process starts again There are more sodium ions outside the cell than inside and more potassium ions inside than out. For every three sodium ions that move out, two potassium ions move in. The sodium-potassium pump is, therefore, an electrogenic pump, creating an electrical imbalance across the membrane and contributing to the membrane potential.

Plasma Membrane Function #1

The ability for complex, integral proteins, receptors to transmit signals. Proteins act both as extracellular input receivers and as intracellular processing activators. Membrane receptors provide extracellular attachment sites for effectors like hormones and growth factors, and they activate intracellular response cascades when their effectors are bound.

Plasma Membrane

The cell's plasma membrane defines the cell, outlines its borders, and determines the nature of its interaction with its environment. The plasma membrane must be very flexible to allow certain cells, such as red and white blood cells, to change shape as they pass through narrow capillaries. The surface of the plasma membrane carries markers that allow cells to recognize one another, which is vital for tissue and organ formation during early development, and which later plays a role in the immune response's "self" versus "non-self" distinction.

Membrane Fluidity

The integral proteins and lipids exist in the membrane as separate but loosely attached molecules. These resemble the separate, multicolored tiles of a mosaic picture, and they float, moving somewhat with respect to one another. The membrane is fairly rigid and can burst if penetrated or if a cell takes in too much water.

Exocytosis

The reverse process of moving material into a cell is the process of exocytosis. Exocytosis is the opposite of the processes we discussed above in that its purpose is to expel material from the cell into the extracellular fluid. Waste material is enveloped in a membrane and fuses with the plasma membrane's interior. This fusion opens the membranous envelope on the cell's exterior, and the waste material expels into the extracellular space.

Primary Active Transport

The second transport method is still active because it depends on using energy as does primary transport. One of the most important pumps in animal cells is the sodium-potassium pump, which maintains the electrochemical gradient in living cells. The sodium- potassium pump moves K+ into the cell while moving. Na+ out at the same time, at a ratio of three Na+ for every two K+ ions moved in. The Na+-K+ ATPase exists in two forms, depending on its orientation to the cell's interior or exterior and its affinity for either sodium or potassium ions.

Fluidity: Part 2

There are two other factors that help maintain this fluid characteristic. One factor is the nature of the phospholipids themselves. In their saturated form, the fatty acids in phospholipid tails are saturated with bound hydrogen atoms. There are no double bonds between adjacent carbon atoms. This results in tails that are relatively straight. Unsaturated fatty acids do not contain a maximal number of hydrogen atoms, but they do not contain some double bonds between adjacent carbon atoms. A double bond results in a bend in the carbon string of approximately 30 degrees.

Moving Against a Gradient

To move substances against a concentration or electrochemical gradient, the cell must use energy. This energy comes from ATP generated through the cell's metabolism. Active transport mechanisms, or pumps, work against electrochemical gradients. Small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances that living cells require in the face of these passive movements. A cell may spend much of its metabolic energy supply maintaining these processes. Because active transport mechanisms depend on a cell's metabolism for energy, they are sensitive to many metabolic poisons that interfere with the ATP supply.

Moving Against a Gradient: Part 2

Two mechanisms exist for transporting small-molecular weight material and small molecules. Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP. Secondary active transport does not directly require ATP: instead, it is the movement of material due to the electrochemical gradient established by primary active transport.

Mechanism

Water, like other substances, moves from an area of high concentration to one of low concentration. Imagine a beaker with a semipermeable membrane separating the two sides or halves. On both sides of the membrane the water level is the same, but there are different dissolved substance concentrations, or solute, that cannot cross the membrane. If the solution's volume on both sides of the membrane is the same, but the solute's concentrations are different, then there are different amounts of water, the solvent, on either side of the membrane.

Tonicity in Living Systems: Part 2

When excessive water amounts leave a red blood cell, the cell shrinks, or crenates. This has the effect of concentrating the solutes left in the cell, making the cytosol denser and interfering with diffusion within the cell. The cell's ability to function will be compromised and may also result in the cell's death. Some organisms, such as plants, fungi, bacteria, and some protists, have cell walls that surround the plasma membrane and prevent cell lysis in a hypotonic solution. The plasma membrane can only expand to the cell wall's limit, so the cell will not lyse. The cytoplasm in plants is always slightly hypertonic to the cellular environment, and water will always enter a cell if water is available. This water inflow produces turgor pressure, which stiffens the plant's cell walls. If you do not water the plant, the extracelllular fluid will become hypertonic, causing water to leave the cell. The cell does not shrink because the cell wall is not flexible. However, the cell membrane detaches from the wall and constricts the cytoplasm. We call this plasmolysis. Plants lose turgor pressure in this condition and wilt.

Carbohydrates

are always on the cells' exterior surface and are bound either to proteins (forming glycoproteins) or to lipids (forming glycolipids). These carbohydrate chains may consist of 2-60 monosaccharide units and can be either straight or branched. Carbohydrates form specialized sites on the cell surface that allow ells to recognize each other. Sites have unique patterns that allow for cell recognition, much the way that the facial features unique to each person allow individuals to recognize him or her.

Carrier Proteins

binds a substance and triggers a change of its own shape, moving the bond molecule from the cell's outside to its interior. Depending on gradient, the material may move in the opposite direction. Carrier proteins are typically specific for a single substance. This selectively adds to the plasma membrane's overall selectively. Proteins can change shape when their hydrogen bonds are affected, but this may not fully explain this mechanism. Each carrier protein is specific to one substance, and there are a finite number of these proteins in any membrane. This can cause problems in transporting enough material for the cell to function properly. When all of the proteins are bound to their ligands, they are saturated and the rate of transport is at its maximum. Increasing the concentration at this point will not result in an increased transport rate.

Secondary Active Transport (Co-transport)

brings sodium ions, and possibly other compounds, into the cell. As sodium ion concentrations build outside of the plasma membrane because of the primary active transport process, this creates an electrochemical gradient. If a channel protein exists and is open, the sodium ions will pull through the membrane. This movement transports other substances that can attach themselves to the transport protein through the membrane. Many amino acids, as well as glucose enter a cell this way. This secondary process also stores high-energy hydrogen ions in the mitochondria of plant and animal cells in order to produce ATP. The potential energy that accumulates in the stored hydrogen ions translates into kinetic energy as the ions surge through the channel protein ATP synthase, and that energy then converts ADP into ATP.

Bulk Transport

cells need to remove and take in larger molecules and particles. Some cells are even capable of engulfing entire unicellular microorganisms. You might have correctly hypothesized that when a cell uptakes and releases large particles, it requires energy. A large particle, however, cannot pass through the membrane, even with energy that the cell supplies.

Tonicity

describes how an extracellular solution can change a cell's volume by affecting osmosis. A solution's tonicity often directly correlates with the solution's osmolarity. Osmolarity describes the solution's total solute concentration. A solution with low osmolarity has a greater number of water molecules relative to the number of solute particles. A solution with high osmolarity has fewer water molecules with respect to solute particles. In a situation in which a membrane permeable to water, though not to the solute separates two different osmolarities, water will move from the membrane's side with lower osmolarity to the side with higher osmolarity. This effect makes sense if you remember that the solute cannot move across the membrane, and thus the only component in the system that can move- the water- moves along its own concentration gradient. An important distinction that concerns living systems is that osmolarity measures the number of particles in a solution. Therefore, a solution that is cloudy with cells may have a lower osmolarity than a solution that is clear, if the second solution contains more dissolved molecules than there are cells.

Receptor-mediated Endocytosis

employs a receptor proteins in the plasma membrane that have a specific binding affinity for certain substances. In receptor-mediated endocytosis, as in phagocytosis, clathrin attaches to the plasma membrane's cytoplasmic side. If a compound's uptake is dependent on receptor-mediated endocytosis and the process is ineffective, the material will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids and increase in concentration. The failure of receptor-mediated endocytosis causes some human diseases.

Cells

exclude some substances, take in others, and excrete still others, in all controlled quantities.

Hypotonic Solutions

hypotonic, isotonic, and hypertonic- relates the cell's osmolarity to the extracellular fluid's osmolarity that contains the cells. In a hyptonic situation, the extracellular fluid has lower osmolarity than the fluid inside the cell, and water enters the cell. It also means that the extracellular fluid has a higher water concentration in the solution than does the cell. In this situation, water will follow its concentration gradient and enter the cell.

Channels

integral proteins involved in facilitated transport are transport proteins, and they functions as either channels for the material or carriers. They are transmembrane proteins. Channel proteins have hydrophilic domains exposed to the intracellular and extracellular fluids. They have a hydrophobic channel through their core that provides a hydrated opening through the membrane layers. Passage through the channel allows polar compounds to avoid the plasma membrane's nonpolar central layer that would otherwise slow or prevent their entry into the cell. Aquaporins are channel proteins that allow water to pass through the membrane at a very high rate.

Diffusion

is a passive process of transport. A single substance moves from a high concentration to a low concentration area until the concentration is equal cross a space. Diffusion expends no energy. Concentration gradients are a form of potential energy, which dissipates as the gradient is eliminated. Each substance will diffuse according to that gradient.

Endocytosis

is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different endocytosis variations, but all share a common characteristic: the cell's plasma membrane invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle containing itself in a newly created intracellular vesicle formed from the plasma membrane.

Osmosis

is the movement of water through a semipermeable membrane according to the water's concentration gradient across the membrane, which is inversely proportional to the solutes' concentration. While diffusion transports material across membrane and within cells, osmosis transports only water across a membrane and the membrane limits the solutes' diffusion in the water. The aquaporins that facilitate water movement play a large roles in osmosis, most prominently in red blood cells and the membranes of kidney tubules.

Phagocytosis

is the process by which a cell takes in large particles, such as other cells or relatively large particles. In preparation for phagocytosis, a portion of the plasma membrane's inward-facing surface becomes coated with the protein clathrin, which stabilizes this membrane's section. The membrane's coated portion then extends from the cell's body and surrounds the particle, eventually enclosing it. Once the vesicle containing the particle is enclosed within the cell, the clathrin disengages from the membrane and the vesicle merges with a lysosome for breaking down the material in the newly formed compartment. When accessible nutrients from the vesicular contents' have been extracted, the newly formed endosome merges with the plasma membrane and releases its contents into the extracellular fluid. The endosomal membrane again becomes part of the plasma membrane.

Cholesterol

lies alongside the phospholipids in the membrane, tends to dampen temperature effects on the membrane. This lipid functions as a buffer, preventing lower temperatures from inhibiting fluidity and preventing increased temperatures from increasing fluidity too much. Cholesterol extends the temperature range in which the membrane is appropriately fluid and consequently functional. Cholesterol also serves other functions, such as organizing clusters of transmembrane proteins into lipid rafts.

Facilitated Transport

materials diffuse across the plasma membrane with the help of membrane proteins. A concentration gradient exists that would allow these materials to diffuse into the cell without expending cellular energy. These materials are polar molecule ions that the cell membrane's hydrophobic parts repel. This kind of transport shields these materials from the membrane's repulsive force, allowing them to diffuse into the cell. The transported material first attaches to protein or glycoprotein receptors on the plasma membrane's exterior surface. This allows removal of material from the extracellular fluid that the cell needs. The substances then pass to specific integral proteins that facilitate their passage. Some of these integral proteins are collections of beta-pleated sheets that form a pore or channel through the phospholipid bilayer. Others are carrier proteins which bind with the substance and aid its diffusion through the membrane.

Active Transport

mechanisms require the cell's energy, usually in the form of adenosine triphosphate (ATP). If a substance must move into the cell against its concentration gradient- that is, if the substance's concentration inside the cell is greater than its concentration in the extracellular fluid- the cell must energy to move the substance. Some active transport mechanism move small- molecular weight materials, such as ions, through the membrane. Other mechanisms transport much larger molecules.

Peripheral Proteins

on the membranes' exterior and interior surfaces, attached either to integral proteins or to phospholipids. They may serve as enzymes, as structural attachments for the cytoskeleton's fibers, or as part of the cell's recognition sites. The body recognizes its own proteins and attacks foreign proteins associated with invasive pathogens.

Passive Transport

plasma membranes must allow certain substances to enter and leave a cell, and prevent some harmful materials from entering and some essential materials from leaving. Plasma membranes are selectively permeable- they allow some substances to pass through, but not others. Some materials are so important to a cell that it spends some of its energy, hydrolyzing adenosine triphosphate (ATP), to obtain these materials. Most cells spend the majority of their energy to maintain an imbalance of sodium and potassium ions between the cell's interior and exterior, as well as on protein synthesis. Passive transport is a naturally occurring phenomenon and does not require the cell to exert any of its energy to accomplish the movement. Substances move from an area of higher concentration to an area of lower concentration. A physical space in which there is a single substance concentration range has a concentration gradient.

Hypertonic Solutions

the prefix hyper- refers to the extracellular fluid having a higher osmolarity than the cell's cytoplasm; therefore, the fluid contains less water than the cell does. Because the cell has a relatively higher water concentration, water will leave the cell.

Channel proteins: Part 2

they are either open at all times or they are "gated", which controls the channel's opening. When a particular ion attaches to the channel protein it may control the opening, or other mechanisms or substances may be involved. In some tissues, sodium and chloride ions pass freely through open channels; whereas, in other tissues a gate must open to allow passage. Cells involved in transmitting electrical impulses, such as nerve and muscle cells, have gated channels for sodium, potassium, and calcium in their membranes. Opening and closing these channels changes the relative concentrations on opposing sides of the membrane of these ions, resulting in facilitating electrical transmission along membranes or in muscle contraction.

Single-pass integral membrane proteins

usually have a hydrophobic transmembrane segment that consists of 20-25 amino acids. Up to 12 single protein segments comprise some complex proteins, which are extensively folded and embedded in the membrane. Has a hydrophobic region or regions, and one or several mildly hydrophobic regions.

Pinocytosis

variation of endocytosis is pinocytosis. This literally means "cell-drinking" This is a process that takes in molecules, including water, which the cell needs from the extracellular fluid. Pinocytosis results in a much smaller vesicle than does phagocytosis, and the vesicle does not need to merge with a lysosome. A variation of pinocytosis is potocytosis. This process uses coating protein, caveolin, on the plasma membrane's cytoplasmic side, which performs a similar function to clathrin. The cavities in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin. The vacuoles or vesicles formed in the caveolae are smaller than those in pinocytosis. Potocytosis brings small molecules into the cell and transports them through the cell for their release on the other side, a process we call transcytosis.