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Water soluble molecules

Water soluble molecules



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I believe I may have misunderstood water solubility this entire time.

I assumed that water soluble meant that it would literally dissolve in water. It would break down into atoms. A sugar molecule would break its bonds and become something else…

However I must be mistaken because I'm reading about why DNA is spiral shaped and it says that it is to keep the hydrophobic bases inside the spiral to protect them from the water, but keeping the hydrophilic phosphate and sugar molecules on the outside. I would assume that because these molecules are hydrophilic they would disassociate from each other but this has to be incorrect. What am I misunderstanding about water solubility and what would happen if the bases were exposed to water?


This question really belongs at Chemistry.SE, but I'll give you a quick answer. A substance is soluble in water when its solid form (such as a sugar cube) completely dissolves in water to become a sugar solution. The sugar molecules themselves are unaffected, essentially - instead of all being bound to one another in a crystal, they are now floating around in the water. The proof is in the taste - solid sugar and sugar solutions taste pretty much the same. If the sugar had been broken down into component hydrogen, oxygen, and carbon, it would taste much different!

On the other hand, hydrolysis is what happens when water actually participates in a chemical reaction to break one or more molecular bonds within a certain molecule, breaking it down to two new molecules (or one molecule and an atom, or whatever). While sugar can be hydrolyzed, it requires either very high heat, or the presence of a catalyst such as an enzyme.


Water soluble molecules - Biology

The solubility of organic molecules is often summarized by the phrase, "like dissolves like." This means that molecules with many polar groups are more soluble in polar solvents, and molecules with few or no polar groups (i.e., nonpolar molecules) are more soluble in nonpolar solvents. (You encountered these concepts in the "Membranes and Proteins" experiment and the related tutorial, "Maintaining the Body's Chemistry: Dialysis in the Kidneys".) Hence, vitamins are either water-soluble or fat-soluble (soluble in lipids and nonpolar compounds), depending on their molecular structures. Water-soluble vitamins have many polar groups and are hence soluble in polar solvents such as water. Fat-soluble vitamins are predominantly nonpolar and hence are soluble in nonpolar solvents such as the fatty (nonpolar) tissue of the body.

What makes polar vitamins soluble in polar solvents and nonpolar vitamins soluble in nonpolar solvents? The answer to this question lies in the types of interactions that occur between the molecules in a solution. Solubility is a complex phenomenon that depends on the change in free energy (ΔG) of the process. For a process (in this case, a vitamin dissolving in a solvent) to be spontaneous, the change in free energy must be negative (i.e., ΔG<0). The green box below describes the thermodynamic processes that govern solubility.

Thermodynamics of Dissolution (Solubilization)

The dissolution of a substance (solute) can be separated into three steps:

  1. The solute particles must separate from one another.
  2. The solvent particles must separate enough to make space for the solute molecules to come between them.
  3. The solute and solvent particles must interact to form the solution.

The free energy (G) describes both the energetics (i.e., the enthalpy H) and the randomness or probability (i.e., the entropy S) of a process ( ΔG=ΔH-TΔS, where T is the absolute temperature). The enthalpy and entropy changes that occur in the dissolution process are shown in Figure 2, below. In the dissolution process, steps 1 and 2 (listed above) require energy because interactions between the particles (solute or solvent) are being broken. Step 3 usually releases energy because solute-solvent interactions are being formed. Therefore, the change in enthalpy (ΔH) for the dissolution process (steps 1 through 3) can be either positive or negative, depending on the amount of energy released in step 3 relative to the amount of energy required in steps 1 and 2. In terms of the change in entropy (ΔS) of the dissolution process, most dissolution processes lead to a greater randomness (and therefore an increase in entropy). In fact, for a large number of dissolution reactions, the entropic effect (the change in randomness) is more important than the enthalpic effect (the change in energy) in determining the spontaneity of the process.

Figure 2

The figure on the left schematically shows the enthalpy changes accompanying the three processes that must occur in order for a solution to form: (1) separation of solute molecules, (2) separation of solvent molecules, and (3) interaction of solute and solvent molecules. The overall enthalpy change, ΔHsoln, is the sum of the enthalpy changes for each step. In the example shown, ΔHsoln is slightly positive, although it can be positive or negative in other cases.

In general, if the solute and solvent interactions are of similar strength (i.e., both polar or both nonpolar), then the energetics of steps 1 and 2 are similar to the energetics of step 3. Therefore, the increase in entropy determines spontaneity in the process. However, if the solute and solvent interactions are of differing strength (i.e., polar with nonpolar), then the energetics of steps 1 and 2 are much greater than the energetics of step 3. Hence, the increase in entropy that can occur is not enough to overcome the large increase in enthalpy thus, the dissolution process is nonspontaneous.

To illustrate the importance of ΔH and ΔS in determining the spontaneity of dissolution, let us consider three possible cases:

The dissolution of a polar solute in a polar solvent.

The polar solute molecules are held together by strong dipole-dipole interactions and hydrogen bonds between the polar groups. Hence, the enthalpy change to break these interactions (step 1) is large and positive (ΔH1>0). The polar solvent molecules are also held together by strong dipole-dipole interactions and hydrogen bonds, so the enthalpy change for step 2 is also large and positive (ΔH2>0). The polar groups of the solute molecules can interact favorably with the polar solvent molecules, resulting in a large, negative enthalpy change for step 3 (ΔH3<0). This negative enthalpy change is approximately as large as the sum of the positive enthalpy changes for steps 1 and 2 therefore, the overall enthalpy change (ΔH1+ΔH2+ΔH3) is small. The small enthalpy change (ΔH),together with the positive entropy change for the process (ΔS), result in a negative free energy change (ΔG=ΔH-TΔS) for the process hence, the dissolution occurs spontaneously.

The dissolution of a nonpolar solute in a polar solvent.

The nonpolar solute molecules are held together only by weak van der Waals interactions. Hence, the enthalpy change to break these interactions (step 1) is small. The polar solvent molecules are held together by strong dipole-dipole interactions and hydrogen bonds as in example (a), so the enthalpy change for step 2 is large and positive (ΔH2>0). The nonpolar solute molecules do not form strong interactions with the polar solvent molecules therefore, the negative enthalpy change for step 3 is small and cannot compensate for the large, positive enthalpy change of step 2. Hence, the overall enthalpy change (ΔH1+ΔH2+ΔH3) is large and positive. The entropy change for the process (ΔS) is not large enough to overcome the enthalpic effect, and so the overall free energy change (ΔG=ΔH-TΔS) is positive. Therefore, the dissolution does not occur spontaneously.

The nonpolar solute molecules are held together only by weak van der Waals interactions. Hence, the enthalpy change to break these interactions (step 1) is small. The nonpolar solvent molecules are also held together only by weak van der Waals interactions, so the enthalpy change for step 2 is also small. Even though the solute and solvent particles will also not form strong interactions with each other (only van der Waals interactions, so ΔH3 is also small), there is very little energy required for steps 1 and 2 that must be overcome in step 3. Hence, the overall enthalpy change (ΔH1+ΔH2+ΔH3) is small. The small enthalpy change (ΔH), together with the positive entropy change for the process (ΔS), result in a negative free energy change (ΔG=ΔH-TΔS) for the process hence, the dissolution occurs spontaneously.

The principles outlined in the green box above explain why the interactions between molecules favor solutions of polar vitamins in water and nonpolar vitamins in lipids. The polar vitamins, as well as the polar water molecules, have strong intermolecular forces that must be overcome in order for a solution to be formed, requiring energy. When these polar molecules interact with each other (i.e., when the polar vitamins are dissolved in water), strong interactions are formed, releasing energy. Hence, the overall enthalpy change (energetics) is small. The small enthalpy change, coupled with a significant increase in randomness (entropy change) when the solution is formed, allow this solution to form spontaneously. Nonpolar vitamins and nonpolar solvents both have weak intermolecular interactions, so the overall enthalpy change (energetics) is again small. Hence, in the case of nonpolar vitamins dissolving in nonpolar (lipid) solvents, the small enthalpy change, coupled with a significant increase in randomness (entropy change) when the solution is formed, allow this solution to form spontaneously as well. For a nonpolar vitamin to dissolve in water, or for a polar vitamin to dissolve in fat, the energy required to overcome the initial intermolecular forces (i.e., between the polar vitamin molecules or between the water molecules) is large and is not offset by the energy released when the molecules interact in solution (because there is no strong interaction between polar and nonpolar molecules). Hence, in these cases, the enthalpy change (energetics) is unfavorable to dissolution, and the magnitude of this unfavorable enthalpy change is too large to be offset by the increase in randomness of the solution. Therefore, these solutions will not form spontaneously. (There are exceptions to the principle "like dissolves like," e.g., when the entropy decreases when a solution is formed however, these exceptions will not be discussed in this tutorial.)

In general, it is possible to predict whether a vitamin is fat-soluble or water-soluble by examining its structure to determine whether polar groups or nonpolar groups predominate. In the structure of calciferol (Vitamin D2), shown in Figure 3 below, we find an –OH group attached to a bulky arrangement of hydrocarbon rings and chains. This one polar group is not enough to compensate for the much larger nonpolar region. Therefore, calciferol is classified as a fat-soluble vitamin.


What happens if a substance needs assistance to move across or through the plasma membrane? Facilitated diffusion is the diffusion of solutes through transport proteins in the plasma membrane. Facilitated diffusion is a type of passive transport. Even though facilitated diffusion involves transport proteins, it is still passive transport because the solute is moving down the concentration gradient.

Small nonpolar molecules can easily diffuse across the cell membrane. However, due to the hydrophobic nature of the lipids that make up cell membranes, polar molecules (such as water) and ions cannot do so. Instead, they diffuse across the membrane through transport proteins. A transport protein completely spans the membrane, and allows certain molecules or ions to diffuse across the membrane. Channel proteins, gated channel proteins, and carrier proteins are three types of transport proteins that are involved in facilitated diffusion.

A channel protein, a type of transport protein, acts like a pore in the membrane that lets water molecules or small ions through quickly. Water channel proteins (aquaporins) allow water to diffuse across the membrane at a very fast rate. Ion channel proteins allow ions to diffuse across the membrane.

A gated channel protein is a transport protein that opens a "gate," allowing a molecule to pass through the membrane. Gated channels have a binding site that is specific for a given molecule or ion. A stimulus causes the "gate" to open or shut. The stimulus may be chemical or electrical signals, temperature, or mechanical force, depending on the type of gated channel. For example, the sodium gated channels of a nerve cell are stimulated by a chemical signal which causes them to open and allow sodium ions into the cell. Glucose molecules are too big to diffuse through the plasma membrane easily, so they are moved across the membrane through gated channels. In this way glucose diffuses very quickly across a cell membrane, which is important because many cells depend on glucose for energy.

A carrier protein is a transport protein that is specific for an ion, molecule, or group of substances. Carrier proteins "carry" the ion or molecule across the membrane by changing shape after the binding of the ion or molecule. Carrier proteins are involved in passive and active transport. A model of a channel protein and carrier proteins is shown in Figure below.

Facilitated diffusion through the cell membrane. Channel proteins and carrier proteins are shown (but not a gated-channel protein). Water molecules and ions move through channel proteins. Other ions or molecules are also carried across the cell membrane by carrier proteins. The ion or molecule binds to the active site of a carrier protein. The carrier protein changes shape, and releases the ion or molecule on the other side of the membrane. The carrier protein then returns to its original shape.

An animation depicting facilitated diffusion can be viewed at http://www.youtube.com/watch?v=OV4PgZDRTQw (1:36).

Ion Channels

Ions such as sodium (Na + ), potassium (K + ), calcium (Ca 2+ ), and chloride (Cl - ), are important for many cell functions. Because they are charged (polar), these ions do not diffuse through the membrane. Instead they move through ion channel proteins where they are protected from the hydrophobic interior of the membrane. Ion channels allow the formation of a concentration gradient between the extracellular fluid and the cytosol. Ion channels are very specific, as they allow only certain ions through the cell membrane. Some ion channels are always open, others are "gated" and can be opened or closed. Gated ion channels can open or close in response to different types of stimuli, such as electrical or chemical signals.


Water soluble molecules - Biology

I. It Isn’t Easy Being Single

A. Because the concentration of ions and other substances outside a cell may rapidly become too high or low, a mechanism is needed to selectively permit substances to enter or leave the cell.

B. The plasma membrane - a surface of lipids, proteins, and some carbohydrate groups-regulates exchange of materials between cytoplasm and surroundings.

C. Within the cytoplasm, exchanges are made across internal membranes of the organelles.

A. The Fluid Mosaic Model of Membrane Structure 1. Within a bilayer, phospholipids show quite a bit of movement they diffuse sideways, spin, flex their tails to prevent close packing and promote fluidity, which also results from short-tailed lipids and unsaturated tails (kink at double bonds).

2. Their is an asymmetrical arrangement of molecules on each side of the membrane.

B. The Phospholipid Bilayer

1. The fluid portion of the cell membrane is made of phospholipids. a. A phospholipid molecule is composed of a polar hydrophilic head and 2 nonpolar hydrophobic tails.

b. If phospholipid molecules are surrounded by water, their hydrophobic fatty acid tails cluster and a bilayer results hydrophilic heads are at the outer faces of a two-layer sheet.

2. Bilayers of phospholipids are the structural foundation for all cell membranes.

a. Phospholipids differ in their hydrophilic heads and the length and saturation of their fatty acid tails.

b. Glycolipids have sugar monomers attached at the head end.

c. Cholesterol is abundant in animal membranes phytosterols occur in plants.

1. Transport proteins allow water-soluble substances to move through their interior, which opens on both sides of the bilayer. They regulate the movement of water-soluble molecules through the plasma membrane. a. A channel protein, whether it be perpetually open or gated, serves as a pore through which ions, water, and soluble substances can move.

b. A carrier protein binds specific substances and changes shape to shunt the materials across some work passively, while others require energy for pumping.

2. Receptor proteins have binding sites for hormones that can trigger changes in cell action.

3. Recognition proteins identify the cell as a certain type, help guide cells into becoming issues, and function in cell-to-cell recognition and coordination.

4. Adhesion proteins are glycoproteins that help cells stay connected to one another in a tissue.

Transport Across Membranes

A. Concentration Gradients and Diffusion

1. A fluid is any substance (liquid or gas) that can move or change shape in response to external forces without breaking apart.

2. Concentration refers to the number of molecules of a substance in a given volume of fluid.

3. A gradient is a physical difference between two regions of space, so that molecules tend to move from one region to another. Frequent gradients found in cells are concentration, pressure, and electrical charge.

4. Molecules constantly collide and tend to move down a concentration gradient (high to low).

5. Molecules cross the plasma membrane at different locations and at different rates, depending on the properties of the molecule in question. Therefore, plasma membranes are said to be differentially permeable. They allow some molecules to pass through, or permeate, more rapidly than others.

B. Routes Across Cell Membranes.

1. Passive transport includes simple diffusion, facilitated diffusion, and osmosis.

Passive transport - Movement of substances across a membrane, going down a gradient of concentration, pressure, or electrical charge. In passive transport, material passes through proteins without an energy boost.

2. Active transport - Movement of substances across a membrane, usually against a concentration gradient requiring the input of energy.

In active transport, proteins become activated to move a solute against its concentration gradient. Active transport includes endocytosis and exocytosis.

A. The net movement of like molecules down a concentration gradient from regions of high concentration to low concentration is called diffusion each substance diffuses independently of other substances present.

B. Small molecules (i.e. O2, CO2, and water) cross the lipid bilayer by simple diffusion These molecules easily diffuse accross the plasma membrane.

C. Factors Influencing the Rate and Direction of Diffusion

1. The rate of diffusion depends on concentration differences, temperature (higher = faster), and molecular size (smaller = faster).

2. When gradients no longer exist, there is no net movement (dynamic equilibrium).

1. Most water-soluble molecules, such as ions (Na+, K+, Ca++), amino acids, monosaccharides, cannot move through the plasma membrane. These molecules can diffuse across only with the aid of two types of transport proteins - channel proteins and carrier proteins. This process is called facilitated diffusion.

2. A carrier protein that functions in passive transport (also called "facilitated diffusion") tends to move molecules to the side of the membrane where they are less concentrated.

3. Passive transport will continue until solute concentrations are equal on both sides of the membrane or other factors intervene.

Most plasma membranes are highly permeable to water.

1. Osmosis is the passive movement of water across a differentially permeable membrane in response to solute concentration gradients, pressure gradients, or both.

For example, if a bag containing a sugar solution is placed in pure water, the water will diffuse inward (higher to lower).

2. Water moves across a membrane from a high concentration of free water molecules to a low concentration of free water molecules, or from high pressure to low pressure.

3. Dissolved substances, regardless of what they are, lower the concentration of free water molecules in a solution.

1. Tonicity denotes the relative concentration of solutes in two fluids.

2. Three conditions are possible:

a. An isotonic fluid has the same concentration of solutes as the fluid in the cell. Or, the concentration of water inside is the same as the concentration of water outside. Immersion in it causes no net movement of water.

b. A hypotonic fluid has a lower concentration of solutes than the fluid in the cell swelling. The concentration of a substance is lower outside the cell than inside a cell. Or the concentration of water is greater outside than inside. Therefore, water will want to enter the cell (high to low).

c. A hypertonic fluid has a greater concentration of solutes than the fluid in the cell shrivel. The concentration of a substance is greater outside the cell than inside a cell. Or the concentration of water is lower outside than inside. Therefore, water will want to leave the cell (high to low).

Energy-Requiring Transport Across Membranes

1. To move ions and large molecules across a membrane against a concentration gradient, special proteins are induced to change shape, but only with an energy boost from ATP.

2. Active transport is the movement of substances across a membrane, usually against a concentration gradient requiring the input of energy.

In active transport, proteins become activated to move a solute against its concentration gradient. Active transport includes endocytosis and exocytosis.

3. An example of active transport is the sodium-potassium pump of the neuron membrane.

Endocytosis and Exocytosis

A. Endocytosis encloses particles in small portions of plasma membrane to form vesicles that then move into the cytoplasm.

1. Amoebas are phagocytic (cell eater), as are white blood cells lysosomes fuse with the endocytic vesicles to digest the contents.

2. Droplets of liquid are also taken in (pinocytosis or "cell-drinking").

3. In receptor-mediated endocytosis, specific molecules are brought into the cell by specialized regions of the plasma membranes that form coated pits which sink into the cytoplasm.

B. In exocytosis, a cytoplasmic vesicle moves substances from cytoplasm to plasma membrane during secretion.

1. Most are carbohydrate frameworks for mechanical support in bacteria, protistans, fungi, and plants cell walls are not found in animals.

2. Plant cell walls are composed of cellulose and other polysaccharides while fungal cell walls are made of chitin.

Copyright 2000 by Steven Wormsley
Last Updated on September 19, 2000 by Steven Wormsley


Small hydrophobic ligands can directly diffuse through the plasma membrane and interact with internal receptors. Important members of this class of ligands are the steroid hormones. Steroids are lipids that have a hydrocarbon skeleton with four fused rings different steroids have different functional groups attached to the carbon skeleton. Steroid hormones include the female sex hormone, estradiol, which is a type of estrogen the male sex hormone, testosterone and cholesterol, which is an important structural component of biological membranes and a precursor of steriod hormones. Other hydrophobic hormones include thyroid hormones and vitamin D. In order to be soluble in blood, hydrophobic ligands must bind to carrier proteins while they are being transported through the bloodstream.

Figure (PageIndex<1>): Steroid Hormones: Steroid hormones have similar chemical structures to their precursor, cholesterol. Because these molecules are small and hydrophobic, they can diffuse directly across the plasma membrane into the cell, where they interact with internal receptors.


The main difference between fat soluble and water soluble vitamins

Water soluble vitamins

Water soluble vitamins can be directly absorbed from the intestine into the bloodstream.

The water soluble vitamins are vitamin C (ascorbic acid) and vitamin B group.

Because excess amounts of these vitamins are excreted in the urine, it is unlikely to overdose them and to reach toxic levels in the body.

Fat soluble vitamins

Fat soluble vitamins enter the body in the same manner as lipids and therefore a small amount of fat intake along with them is essential for their better absorption.

Fat soluble vitamins include A (retinol), D (calciferol), E (α-tocopherol), and K (phylloquinone). They readily pass through the plasma membranes of the gastrointestinal tract and other tissues.

The main difference between fat soluble and water soluble vitamins is that the excessive consumption of fat soluble vitamins can lead to the fact that they accumulate in the fatty tissues of the body and reach toxic levels. It is especially important to take them properly in supplements to prevent fat soluble vitamins overdose.

B3 (Niacin, Niacinamide, Nicotinic Acid)

B6 (Pyridoxine, Pyridoxal 5'-phosphate)

Cofactors and coenzymes


Hydrophilic

Hydrophilic is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water. It refers to having a strong affinity for water. Something that is hydrophilic is soluble in water and dissolves into water very easily. Hydrophilic is the opposite of hydrophobic.

Hydrophilic and hydrophobic surfaces both have important applications in all types of engineering, including:

Hydrophilic substances can cause corrosion to metal surfaces and alloys.

Hydrophilic Molecule

Water is a polar molecule. Polar molecules are molecules that have partial charges due to uneven bonding. The oxygen atom in a water molecule is highly electronegative, which means that it will pull the electrons in a bond closer to it. This, in turn, makes oxygen partially negative and hydrogen partially positive.

Hydrophilic molecule or portion of a molecule is one whose interactions with water and other polar substances are more thermodynamically favorable than their interactions with oil or other hydrophobic solvents. They are typically charge-polarized and capable of hydrogen bonding. This makes these molecules soluble not only in water but also in other polar solvents.

Hydrophilic molecules (and portions of molecules) can be contrasted with hydrophobic molecules (and portions of molecules). In some cases, both hydrophilic and hydrophobic properties occur in a single molecule. An example of these amphiphilic molecules is the lipids that comprise the cell membrane. Another example is soap, which has a hydrophilic head and a hydrophobic tail, allowing it to dissolve in both water and oil.

Since water has these partial charges, it can attract other chemicals that also have partial charges. Therefore, hydrophilic molecules must have a charged portion in order to dissolve in water. Hydrophilicity is an important quality of many essential materials in nature and in the human body.

Membrane Filtration of Hydrophilic

Hydrophilic membrane filtration is used in several industries to filter various liquids. These hydrophilic filters are used in the medical, industrial, and biochemical fields to filter such elements as bacteria, viruses, proteins, particulates, drugs, and other contaminates. Common hydrophilic molecules include colloids, cotton, and cellulose, which cotton consists of.

Unlike other membranes, hydrophilic membranes do not require pre-wetting: they can filter liquids in their dry state. Although most are used in low-heat filtration processes, many new hydrophilic membrane fabrics are used to filter hot liquids and fluids.

Hydrophilic Substances

Hydrophilic substances are those that have a chemical attraction to water. Even though water is a somewhat covalent molecule, the two electrons from the hydrogen atoms spend more time with the oxygen atom, which is quite electronegative. So up close, molecules or ions that are negatively charged, or that have one part that is negative and another part that is positive, tend to be attracted to the hydrogens in water. And positive ions tend to be attracted to the oxygen atom in water. Most salts are this way.

One such salt is calcium chloride, CaCl2, and is used as a dessicant. This means that if you have a substance that you want to remain dry in a humid room, you can put it into a container next to some calcium chloride, and the calcium chloride will attract most of the water, leaving your substance dry.

In general, hydrophilic substances have lots of oxygen or nitrogen atoms in their structure. For instance, Silica (quartz) is SiO2. Gypsum is CaSO4. Both of these compounds have lots of O atoms and asre strongly hydrophilic. Diamond (Carbon) has no O atoms and is Oleophilic.


Solute transport between blood and tissue

8.5 Classes of solute. I: Lipid-soluble molecules

The lipid-solubility of a molecule dramatic ally influences its permeation through the capillary wall. Capillary permeability to the lipid-soluble molecule oxygen, for example, is many thousand times greater than permeability to the lipid-insoluble molecule glucose. The second main factor affecting permeation is the solute's molecular size, and capillary permeability to glucose (Mw180) is nearly 1000 times greater than to albumin (Mw 69 000 see Table 8.1 ). Solutes thus fall into three main classes: lipid-soluble molecules, small lipid-insoluble molecules and large lipid-insoluble molecules (macromolecules).

Taking the lipophilic molecules first, capillary permeability increases in proportion to the solute's oil:water partition coefficient, indicating that these molecules traverse the capillary wall by dissolving in the lipid cell membrane. Virtually the entire capillary surface is therefore available for diffusion and this explains the high permeability. The oil:water partition coefficient is approximately 5 for oxygen and 1.6 for carbon dioxide, so capillaries are extremely permeable to respiratory gases: indeed, the permeability to oxygen is so high that some gas exchange even occurs in arterioles, and the haemoglobin saturation can fall to approximately 80% even before the blood enters the true capillaries. Anaesthetic agents and the flow-tracer xenon also fall into this class of solute.


Solubility

Solubility is a property referring to the ability for a given substance, the solute, to dissolve in a solvent.

It is measured in terms of the maximum amount of solute dissolved in a solvent at equilibrium.

The resulting solution is called a saturated solution.

Certain substances are soluble in all proportions with a given solvent, such as ethanol in water.

This property is known as miscibility.

Under various conditions, the equilibrium solubility can be exceeded to give a so-called supersaturated solution, which is metastable.

The solvent is often a solid, which can be a pure substance or a mixture.

The species that dissolves, the solute, can be a gas, another liquid, or a solid.

Solubilities range widely, from infinitely soluble such as ethanol in water, to poorly soluble, such as silver chloride in water.

The term insoluble is often applied to poorly soluble compounds, though strictly speaking there are very few cases where there is absolutely no material dissolved.

The process of dissolving, called dissolution, is relatively straightforward for covalent substances such as ethanol.

When ethanol dissolves in water, the ethanol molecules remain intact but form new hydrogen bonds with the water.

When, however, an ionic compound such as sodium chloride (NaCl) dissolves in water, the sodium chloride lattice dissociates into separate ions which are solvated (wrapped) with a coating of water molecules.

Nonetheless, NaCl is said to dissolve in water, because evaporation of the solvent returns crystalline NaCl.


Differences between Passive and Active Tranport

Active Transport vs Passive Transport

As minute as they are, cells in the body carry some very important processes deep within. These processes are all vital to the overall growth and development of every organism, may it be an animal or a plant. But every internal process must have some unique mechanisms done to make it successful. In this regard, nutrients, chemicals and other substances are flowing to and fro the cells with the use of certain transport systems. These transport mechanisms are classified into two, namely active and passive transport systems.

In the simplest terms, active transport is termed ‘active’ because of the inclusion of one vital component and that is the use of energy. This energy is being utilized by the cell, in the form of ATP (Adenosine Triphosphate) for it to be able to move most substances in and out of its cellular membranes. On the contrary, passive transport is regarded as such because it is just a plain old ‘passive’ mechanism. It does not use any energy (ATP) from the cell for it to carry out the said processes.

Another distinct characteristic that separates active from passive transport system is the difference in the concentration gradients. It must be made known that the concentration of substances that are partitioned by cell membranes are relatively different. For example, the inside of the cell has a concentration gradient that is higher (more concentrated) than the outside of the cell (less concentrated) or it can also be the other way around depending on various biological factors. Hence, in active transport, it tries to achieve a more difficult task of opposing the concentration gradient. If the cell wants to transport certain substances towards itself (in this situation, it so happens to be more concentrated) then it needs much energy for its protein or sodium pumps to operate and transfer the said substances.

In the case of passive transport, it is not against but along the concentration gradient. Because the cell sees that the same ions or molecules can be transferred to the other side immediately due to a ‘favorable’ concentration gradient, it no longer expends any energy. The word ‘favorable’ simply means that it follows the rules of normal diffusion. When the substances from the more concentrated internal environment of the cell are to be transported outside, that is for example the outside happens to be less concentrated, then the substances can easily flow out.

In brief, active and passive transport differ because:
1.Active transport makes use of energy in the form of ATP whereas passive transport does not utilize any.
2.Active transport involves the transfer of molecules or ions against a concentration gradient whereas passive transport is the transfer along a concentration gradient