Which solutes were able

If they were to lose this selectivity, the cell would no longer be able to sustain itself, and it would be destroyed. Some cells require larger amounts of specific substances than do other cells; they must have a way of obtaining these materials from the extracellular fluids. This may happen passively, as certain materials move back and forth, or the cell may have special mechanisms that ensure transport. Most cells expend most of their energy, in the form of adenosine triphosphate ATP , to create and maintain an uneven distribution of ions on the opposite sides of their membranes.

The structure of the plasma membrane contributes to these functions, but it also presents some problems. The most direct forms of membrane transport are passive.

Passive transport is a naturally occurring phenomenon and does not require the cell to expend energy to accomplish the movement. In passive transport, substances move from an area of higher concentration to an area of lower concentration in a process called diffusion.

A physical space in which there is a different concentration of a single substance is said to have a concentration gradient. Plasma membranes are asymmetric, meaning that despite the mirror image formed by the phospholipids, the interior of the membrane is not identical to the exterior of the membrane. Integral proteins that act as channels or pumps work in one direction. Carbohydrates, attached to lipids or proteins, are also found on the exterior surface of the plasma membrane.

These carbohydrate complexes help the cell bind substances that the cell needs in the extracellular fluid. This adds considerably to the selective nature of plasma membranes. Recall that plasma membranes have hydrophilic and hydrophobic regions.

This characteristic helps the movement of certain materials through the membrane and hinders the movement of others.

Lipid-soluble material can easily slip through the hydrophobic lipid core of the membrane. Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma membranes in the digestive tract and other tissues. Molecules of oxygen and carbon dioxide have no charge and pass through by simple diffusion. Polar substances, with the exception of water, present problems for the membrane.

While some polar molecules connect easily with the outside of a cell, they cannot readily pass through the lipid core of the plasma membrane.

Additionally, whereas small ions could easily slip through the spaces in the mosaic of the membrane, their charge prevents them from doing so. Ions such as sodium, potassium, calcium, and chloride must have a special means of penetrating plasma membranes. Simple sugars and amino acids also need help with transport across plasma membranes. It is possible for large molecules to enter a cell by a process called endocytosis, where a small piece of the cell membrane wraps around the particle and is brought into the cell.

If the particle is solid, endocytosis is also called phagocytosis. If fluid droplets are taken in, the processes is called pinocytosis. Illustration of endocytosis. Note that the particle entered the cell surrounded by a piece of cell membrane. The opposite of endocytosis is exocytosis. Cells use exocytosis to secrete molecules too large to pass through the cell membrane by any other mechanism.

Click on the button above to open a problem solver to help you practice your understanding of membrane transport with the following examples:. A white blood cell engulfs a bacterium as you fight off an infection. Carbon dioxide a small uncharged gas molecule enters the lungs where it is less concentrated from the blood where it is more concentrated. Cells of the stomach wall transport hydrogen ions through a ATP-dependent membrane protein to the inside of the stomach, producing a pH of 1.

The pH of the cytosol fluid inside the cells of stomach wall cells is approximately 7. Recall that a low pH means high hydrogen ion concentrations. The lung cells of a victim who drowned in fresh water are swollen due to water entering the cells. Changes in Cell Shape Due to Dissolved Solutes : Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions.

Because the cell has a relatively higher concentration of water, water will leave the cell, and the cell will shrink. In an isotonic solution, the extracellular fluid has the same osmolarity as the cell. If the osmolarity of the cell 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. Cells in an isotonic solution retain their shape.

Cells in a hypotonic solution swell as water enters the cell, and may burst if the concentration gradient is large enough between the inside and outside of the cell. Cells in a hypertonic solution shrink as water exits the cell, becoming shriveled.

Osmoregulation is the process by which living things regulate the effects of osmosis in order to protect cellular integrity. Tonicity is the ability of a solution to exert an osmotic pressure upon a membrane. There are three types of tonicity: hypotonic, hypertonic, and isotonic.

In a hypotonic environment, water enters a cell, and the cell swells. In a hypertonic solution, water leaves a cell and the cell shrinks.

In an isotonic condition, the relative concentrations of solute and solvent are equal on both sides of the membrane. There is no net water movement; therefore, there is no change in the size of the cell. The membrane resembles a mosaic with discrete spaces between the molecules comprising it. If the cell swells and the spaces between the lipids and proteins become too large, the cell will break apart.

In contrast, when excessive amounts of water 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.

Turgor Pressure and Tonicity in a Plant Cell : The turgor pressure within a plant cell depends on the tonicity of the solution in which it is bathed. Various living things have ways of controlling the effects of osmosis —a mechanism called osmoregulation. 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 limit of the cell wall, so the cell will not lyse.

In fact, in plants, the cellular environment is always slightly hypotonic to the cytoplasm, and water will always enter a cell if water is available. This inflow of water produces turgor pressure, which stiffens the cell walls of the plant.

In nonwoody plants, turgor pressure supports the plant. Conversely, if the plant is not watered, the extracellular fluid will become hypertonic, causing water to leave the cell. In this condition, the cell does not shrink because the cell wall is not flexible.

However, the cell membrane detaches from the wall and constricts the cytoplasm. This is called plasmolysis. Plants lose turgor pressure in this condition and wilt.

Turgor Pressure and Plasmolysis : Without adequate water, the plant on the left has lost turgor pressure, visible in its wilting; the turgor pressure is restored by watering it right. Tonicity is a concern for all living things. For example, 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. Many marine invertebrates have internal salt levels matched to their environments, making them isotonic with the water in which they live.

Fish, however, must spend approximately five percent of their metabolic energy maintaining osmotic homeostasis. 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 amount of water in the body. Osmoreceptors are specialized cells in the brain that monitor the concentration of solutes in the blood.

If the levels of solutes increase beyond a certain range, a hormone is released that retards water loss through the kidney and dilutes the blood to safer levels. Animals also have high concentrations of albumin produced by the liver in their blood.

This protein is too large to pass easily through plasma membranes and is a major factor in controlling the osmotic pressures applied to tissues. Privacy Policy. Skip to main content. Structure and Function of Plasma Membranes. Search for:. Passive Transport. The Role of Passive Transport Passive transport, such as diffusion and osmosis, moves materials of small molecular weight across membranes.

Learning Objectives Indicate the manner in which various materials cross the cell membrane. Key Takeaways Key Points Plasma membranes are selectively permeable; if they were to lose this selectivity, the cell would no longer be able to sustain itself.

In passive transport, substances simply move from an area of higher concentration to an area of lower concentration, which does not require the input of energy. Concentration gradient, size of the particles that are diffusing, and temperature of the system affect the rate of diffusion. Some materials diffuse readily through the membrane, but others require specialized proteins, such as channels and transporters, to carry them into or out of the cell. Key Terms concentration gradient : A concentration gradient is present when a membrane separates two different concentrations of molecules.

Selective Permeability The hydrophobic and hydrophilic regions of plasma membranes aid the diffusion of some molecules and hinder the diffusion of others. Learning Objectives Describe how membrane permeability, concentration gradient, and molecular properties affect biological diffusion rates. Key Takeaways Key Points The interior and exterior surfaces of the plasma membrane are not identical, which adds to the selective permeability of the membrane. Fat soluble substances are able to pass easily to the hydrophobic interior of the plasma membrane and diffuse into the cell.

Polar molecules and charged molecules do not diffuse easily through the lipid core of the plasma membrane and must be transported across by proteins, sugars, or amino acids. A, center right Active transport bottom utilizes energy, often in the form of ATP, to drive solute uptake against its gradient resulting in a net accumulation of the solute. A well-studied example of a facilitated diffusion carrier is the glucose transporter, or GLUT [9]. From the activation energies for transmembrane simple passive diffusion of glycol, glycerol and erythritol presented in Table This large discrepancy is attributed to the presence of a glucose-facilitated diffusion carrier.

GLUTs occur in nearly all cells and are particularly abundant in cells lining the small intestine. GLUTs are but one example in a superfamily of transport facilitators.

GLUTs function through a typical membrane transport mechanism [10]. Glucose binds to the membrane outer surface site causing a conformational change associated with transport across the membrane. At the inner side of the membrane, glucose is released into the internal aqueous solution Fig. Glucose-facilitated diffusion transporter GLUT-1 [10]. In virtually all organisms there exists a wide variety of ion channels, the most widely distributed being potassium channels [11].

There are four basic classes of potassium channels, all of which provide essential membrane-associated functions including setting and shaping action potentials and hormone secretion:.

Potassium channels are composed of four protein subunits that can be the same homotetramer or closely related heterotetramer.

All potassium channel subunits have a distinctive pore-loop structure that sits at the top of the channel and is responsible for potassium selectivity [12]. This is often referred to as a selectivity or filter loop. The selectivity filter strips the waters of hydration from the potassium ion, allowing it into the channel. Elucidating the three-dimensional structure of this important integral membrane protein by X-ray crystallography Fig.

For this work from , Rod MacKinnon Fig. Until the potassium channel work, just obtaining the structure of non—water-soluble proteins was next to impossible. MacKinnon's work elucidated not only the structure of the potassium channel but also its molecular mechanism. It has served as a blueprint for determining the structure of other membrane proteins and has greatly stimulated interest in the field. Three-dimensional structure of the potassium channel [12].

They are both facilitated diffusion carriers that conduct the cation down the ion's electrochemical gradient. TTX is encountered primarily in puffer fish but also in porcupine fish, ocean sunfish, and triggerfish. TTX Fig. Puffer fish is the second most poisonous vertebrate in the world, trailing only the Golden Poison Frog that is endemic to the rain forests on the Pacific Coast of Colombia.

In some parts of the world puffer fish are considered to be a delicacy but must be prepared by chefs who really know their business, as a slight error can be fatal. Puffer poisoning usually results from consumption of incorrectly prepared puffer soup, and TTX has no known antidote! Saxitoxin STX, Fig. Saxitoxin is one of the most potent natural toxins, and it has been estimated that a single contaminated mussel has enough STX to kill 50 humans!

STX's toxicity has not escaped the keen eye of the United States military, which has weaponized the toxin and given it the designation TZ. The net movement of a solute is therefore determined by a combination of the solute's chemical gradient and an electrical gradient inherent to the cell. For decades it was assumed that water simply leaked through biological membranes by numerous processes described above Chapter 19, Section 2.

However, these methods of water permeability could not come close to explaining the rapid movement of water across some cells. Although it had been predicted that water pores must exist in very leaky cells, it was not until that Peter Agre Fig.

Aquaporins are usually specific for water permeability and exclude the passage of other solutes. A type of aquaporin known as aqua-glyceroporins can also conduct some very small uncharged solutes such as glycerol, CO 2 , ammonia, and urea across the membrane. However, all aquaporins are impermeable to charged solutes. Water molecules traverse the aquaporin channel in single file Fig. A characteristic of all living membranes is the formation and maintenance of transmembrane gradients of all solutes including salts, biochemicals, macromolecules, and even water.

Typical cell concentrations are:. The chemical and electrical gradients are maintained far from equilibrium by a multitude of active transport systems. Active transport requires a form of energy often ATP to drive the movement of solutes against their electrochemical gradient, resulting in a nonequilibrium distribution of the solute across the membrane. A number of nonexclusive and overlapping terms are commonly used to describe the different types of active transport. Some of these are depicted in Fig.

Basic types of active transport [18]. Primary active transport is also called direct active transport or uniport. It involves using energy usually ATP to directly pump a solute across a membrane against its electrochemical gradient. There are four basic types of ATP-utilizing primary active transport systems Table The enzyme was discovered in by Jens Skou Fig.

As is often the case in biochemistry, a serendipitous discovery of a natural product from the jungles of Africa has been instrumental in unraveling the enzyme's mechanism of action.

The compound is ouabain Fig. For decades after its discovery, ouabain was routinely used to treat atrial fibrillation and congestive heart failure in humans. More recently, ouabain has been replaced by digoxin, a structurally related, but more lipophilic cardiac glycoside. These include:.

Ouabain blocks the dephosphorylation step. Secondary active transport also known as cotransport systems are composed of two separate functions.

This ion gradient is coupled to the movement of a solute in either the same direction symport or in the opposite direction antiport, see Fig. Movement of the pumped ion down its electrochemical gradient is by facilitated diffusion. The purpose of both types of co-transport is to use the energy in an electrochemical gradient to drive the movement of another solute against its gradient. An example of symport is the SGLT1 sodium-glucose transport protein-1 in the intestinal epithelium [20].

The secondary active symport system for lactose uptake in Escherichia coli is shown in Fig. Lactose transport system in Escherichia coli [21]. This is an example of active transport, co-transport, and active. Note that the ability to take up lactose is a combination of the electrical gradient and the pH gradient.

Over 50 years ago, Peter Mitchell see Chapter 18, Fig. Water-soluble enzymes convert substrate to product without any directionality. Mitchell proposed that many enzymes are integral membrane proteins that have a specific transmembrane orientation. When these enzymes convert substrate to product they do so in one direction only.

For this revolutionary idea Mitchell was awarded the Nobel Prize in Chemistry. Vectorial metabolism has been used to describe the mechanism for several membrane transport systems. For example, it has been reported in some cases the uptake of glucose into a cell may be faster if the external source of glucose is sucrose rather than free glucose. Through a vectorial transmembrane reaction, membrane-bound sucrase may convert external sucrose into internal glucose plus fructose more rapidly than the direct transport of free glucose through its transport system.

Mitchell defined one type of vectorial transport as group translocation, the best example being the PTS phosphotransferase system discovered by Saul Roseman in PTS is a multicomponent active transport system that uses the energy of intracellular phosphoenol pyruvate PEP to take up extracellular sugars in bacteria.

Transported sugars include glucose, mannose, fructose, and cellobiose. Components of the system include both plasma membrane and cytosolic enzymes. The sequence is depicted in more detail in Fig. Although it is glucose that is being transported across the membrane, it never actually appears inside the cell as free glucose but rather as glucosephosphate.

Free glucose could leak back out of the cell via a glucose transporter, but glucosephosphate is trapped inside the cell where it can rapidly be metabolized through glycolysis. Group translocation is defined by a transported solute appearing in a different form immediately after crossing the membrane. The bacterial PTS system for glucose transport [27].

They are facilitated diffusion carriers that transport ions down their electrochemical gradient. Ionophores can be divided into two basic classes: channel formers and mobile carriers Fig.

Channel formers are long lasting, stationary structures that allow many ions at a time to rapidly flow across a membrane. Mobile carriers bind to an ion on one side of a membrane, dissolve in and cross the membrane bilayer and release the ion on the other side. They can only carry one ion at a time. Two basic types of ionophores: channel formers left and mobile carriers right [30]. Superficially valinomycin resembles a cyclic peptide Fig. However, upon closer examination the ionophore is actually a unit dodeca depsi peptide where amino acid peptide bonds are alternated with amino alcohol ester bonds.

Therefore the linkages that hold the molecule together alternate between nitrogen esters peptide bonds and oxygen esters. The outside surface of valinomycin is coated with nine hydrophobic side chains of d - and l -valine and l -hydroxyvaleric acid. Valinomycin was first recognized as a potassium ionophore by Bernard Pressman in the early 's [31] , [32].

Many studies showed that valinomycin dissipates essential transmembrane electrochemical gradients causing tremendous metabolic upheaval in many organisms including microorganisms. It is for this reason that valinomycin was recognized as an antibiotic long before it was identified as an ionophore. Currently several ionophores are added to animal feed as antibiotics and growth enhancing additives [33].

Recently valinomycin has been reported to be the most potent agent against SARS-CoV severe acute respiratory-syndrome coronavirus , a severe form of pneumonia first identified in [34]. It is a synthetic lipid-soluble proton ionophore that dissipates proton gradients across bioenergetic membranes mitochondrial inner, thylakoid, bacterial plasma. Elucidating the role of DNP in uncoupling oxidative phosphorylation was an essential component in support of Peter Mitchell's chemiosmotic hypothesis [25].

Electron movement from NADH or FADH 2 to O 2 via the mitochondrial electron transport system generates a considerable amount of electrical energy that is partially captured as a transmembrane pH gradient see Chapter Therefore, in the presence of DNP, electron transport continues, even at an accelerated rate, but ATP production is diminished.

The energy that should have been converted to chemical energy in the form of ATP is then released as excess heat. This combination of properties led to the medical application of DNP to treat obesity from to [35]. Upon addition of DNP:. DNP was indeed a successful weight loss drug. Two of the early proponents of DNP use as a diet drug, Cutting and Tainter at Stanford University, estimated that more than , people in the United States had tested the drug during its first year in use [35].

DNP, however, did have one disturbing side effect—death! Fatality was not caused by a lack of ATP, but rather by a dangerous increase in body temperature hyperthermia. Although general use of DNP in the United States was discontinued in , it is still employed in other countries and by bodybuilders to eliminate fat before competitions. Crown ethers are a family of synthetic ionophores that are generally similar in function to the natural product valinomycin [36].

The first crown ether was synthesized by Charles Pederson Fig. For this work Pedersen was co-awarded the Nobel Prize in Chemistry. Crown ethers are cyclic compounds composed of several ether groups. Crown ethers are given structural names, X -crown- Y , where X is the total number of atoms in the ring and Y is the number of these atoms that are oxygen. Crown refers to the crown-like shape the molecule takes.

Crown ether oxygens form complexes with specific cations that depend on the number of atoms in the ring.