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Why do cell membranes let small non-polar molecules through but won't let small polar molecules through?

Why do cell membranes let small non-polar molecules through but won't let small polar molecules through?


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If the hydrophobic hydrocarbon chain of the phospholipid prevents the movement of polar molecules through the membrane. Why does the hydrophilic phosphate head of the phospholipid not prevent the movement of non-polar molecules?


The plasma membrane consists of hydrophobic and hydrophillic characteristics. Towards the outsides, they are hydrophillic, so they can create bonds with water. The insides are hydrophobic, allowing no water inside and keeping them tight together due to the polar forces.

An non-polar particle (if small), can pass through this because it does not interfere with the hydrophobic/hydrophillic (polar) nature of the plasma membrane. However, polar particles would not have the opportunity to move in, because the insides (hydrophobic) are literally afraid of water, or charges, don't allow polar substances to pass through.

So only hydrophobic (nonpolar), gases, and small particles (nonpolar) can pass through. There are exceptions of $H_2O$ passing through the membrane in small amounts because their electric charge is very minor.


Describe the Kinds of Molecules That Cannot Easily Diffuse Through Cell Membranes

In order for a cell to function effectively, it needs to be able to control which substances can enter and exit through its membrane. The cell membrane's main trait is its selective permeability, which means that it allows some substances to cross it easily, but not others. Small molecules that are nonpolar (have no charge) can cross the membrane easily through diffusion, but ions (charged molecules) and larger molecules typically cannot.


Scientists use the term “fluid mosaic model” to describe the organization of phospholipids and proteins in the cell membrane. Individual proteins and phospholipids flow freely. Complexes of proteins form "rafts" that move through the membrane. Organelles stretch and bend and even flow through the cell. Fluid membranes allow cells to be dynamic and respond to their environment.

Phospholipid membranes form a barrier that most molecules cannot cross. But living things need to be able to interact with the outside world. At the very least, waste must be able to go out and raw materials need to come in. That's where membrane proteins come in. On average, proteins make up about half the mass of membranes.

What about the membrane proteins ? Scientists have shown that many proteins float in the lipid bilayer. Some are permanently attached while others are only attached temporarily. Some are only attached to the inner or outer layer of the membrane while the other proteins pass through the entire structure.


Illustrations of solubility concepts: metabolic intermediates, lipid bilayer membranes, soaps and detergents

Because water is the biological solvent, most biological organic molecules, in order to maintain water-solubility, contain one or more charged functional groups. These are most often phosphate, ammonium or carboxylate, all of which are charged when dissolved in an aqueous solution buffered to pH 7.

Sugars often lack charged groups, but as we discussed in our &lsquothought experiment&rsquo with glucose, they are quite water-soluble due to the presence of multiple hydroxyl groups.

Some biomolecules, in contrast, contain distinctly nonpolar, hydrophobic components. The &lsquolipid bilayer&rsquo membranes of cells and subcellular organelles serve to enclose volumes of water and myriad biomolecules in solution. The lipid (fat) molecules that make up membranes are amphipathic: they have a charged, hydrophilic &lsquohead&rsquo and a hydrophobic hydrocarbon &lsquotail&rsquo.

interactive 3D image of a membrane phospholipid (BioTopics)

Notice that the entire molecule is built on a &lsquobackbone&rsquo of glycerol, a simple 3-carbon molecule with three alcohol groups. In a biological membrane structure, lipid molecules are arranged in a spherical bilayer: hydrophobic tails point inward and bind together by London dispersion forces, while the hydrophilic head groups form the inner and outer surfaces in contact with water.

Interactive 3D Image of a lipid bilayer (BioTopics)

Because the interior of the bilayer is extremely hydrophobic, biomolecules (which as we know are generally charged species) are not able to diffuse through the membrane&ndash they are simply not soluble in the hydrophobic interior. The transport of molecules across the membrane of a cell or organelle can therefore be accomplished in a controlled and specific manner by special transmembrane transport proteins, a fascinating topic that you will learn more about if you take a class in biochemistry.

A similar principle is the basis for the action of soaps and detergents. Soaps are composed of fatty acids, which are long (typically 18-carbon), hydrophobic hydrocarbon chains with a (charged) carboxylate group on one end,

Fatty acids are derived from animal and vegetable fats and oils. In aqueous solution, the fatty acid molecules in soaps will spontaneously form micelles, a spherical structure that allows the hydrophobic tails to avoid contact with water and simultaneously form favorable London dispersion contacts.

Because the outside of the micelle is charged and hydrophilic, the structure as a whole is soluble in water. Micelles will form spontaneously around small particles of oil that normally would not dissolve in water (like that greasy spot on your shirt from the pepperoni slice that fell off your pizza), and will carry the particle away with it into solution. We will learn more about the chemistry of soap-making in a later chapter (section 12.4B).

Synthetic detergents are non-natural amphipathic molecules that work by the same principle as that described for soaps.


Why do cell membranes let small non-polar molecules through but won't let small polar molecules through? - Biology

All living cells, prokaryotic and eukaryotic, have a plasma membrane that encloses their contents and serves as a semi-porous barrier to the outside environment. The membrane acts as a boundary, holding the cell constituents together and keeping other substances from entering. The plasma membrane is permeable to specific molecules, however, and allows nutrients and other essential elements to enter the cell and waste materials to leave the cell. Small molecules, such as oxygen, carbon dioxide, and water, are able to pass freely across the membrane, but the passage of larger molecules, such as amino acids and sugars, is carefully regulated.

According to the accepted current theory, known as the fluid mosaic model , the plasma membrane is composed of a double layer ( bilayer ) of lipids, oily substances found in all cells (see Figure 1). Most of the lipids in the bilayer can be more precisely described as phospholipids , that is, lipids that feature a phosphate group at one end of each molecule. Phospholipids are characteristically hydrophilic ("water-loving") at their phosphate ends and hydrophobic ("water-fearing") along their lipid tail regions. In each layer of a plasma membrane, the hydrophobic lipid tails are oriented inwards and the hydrophilic phosphate groups are aligned so they face outwards, either toward the aqueous cytosol of the cell or the outside environment. Phospholipids tend to spontaneously aggregate by this mechanism whenever they are exposed to water.

Within the phospholipid bilayer of the plasma membrane, many diverse proteins are embedded, while other proteins simply adhere to the surfaces of the bilayer. Some of these proteins, primarily those that are at least partially exposed on the external side of the membrane, have carbohydrates attached to their outer surfaces and are, therefore, referred to as glycoproteins . The positioning of proteins along the plasma membrane is related in part to the organization of the filaments that comprise the cytoskeleton, which help anchor them in place. The arrangement of proteins also involves the hydrophobic and hydrophilic regions found on the surfaces of the proteins: hydrophobic regions associate with the hydrophobic interior of the plasma membrane and hydrophilic regions extend past the surface of the membrane into either the inside of the cell or the outer environment.

Plasma membrane proteins function in several different ways. Many of the proteins play a role in the selective transport of certain substances across the phospholipid bilayer, either acting as channels or active transport molecules. Others function as receptors, which bind information-providing molecules, such as hormones, and transmit corresponding signals based on the obtained information to the interior of the cell. Membrane proteins may also exhibit enzymatic activity, catalyzing various reactions related to the plasma membrane.

Since the 1970s, the plasma membrane has been frequently described as a fluid mosaic , which is reflective of the discovery that oftentimes the lipid molecules in the bilayer can move about in the plane of the membrane. However, depending upon a number of factors, including the exact composition of the bilayer and temperature, plasma membranes can undergo phase transitions which render their molecules less dynamic and produce a more gel-like or nearly solid state. Cells are able to regulate the fluidity of their plasma membranes to meet their particular needs by synthesizing more of certain types of molecules, such as those with specific kinds of bonds that keep them fluid at lower temperatures. The presence of cholesterol and glycolipids, which are found in most cell membranes, can also affect molecular dynamics and inhibit phase transitions.

In prokaryotes and plants, the plasma membrane is an inner layer of protection since a rigid cell wall forms the outside boundary for their cells. The cell wall has pores that allow materials to enter and leave the cell, but they are not very selective about what passes through. The plasma membrane, which lines the cell wall, provides the final filter between the cell interior and the environment.

Eukaryotic animal cells are generally thought to have descended from prokaryotes that lost their cell walls. With only the flexible plasma membrane left to enclose them, these primordial creatures would have been able to expand in size and complexity. Eukaryotic cells are generally ten times larger than prokaryotic cells and have membranes enclosing interior components, the organelles. Like the exterior plasma membrane, these membranes also regulate the flow of materials, allowing the cell to segregate its chemical functions into discrete internal compartments.


Illustrations of solubility concepts: metabolic intermediates, lipid bilayer membranes, soaps and detergents

Because water is the biological solvent, most biological organic molecules, in order to maintain water-solubility, contain one or more charged functional groups. These are most often phosphate, ammonium or carboxylate, all of which are charged when dissolved in an aqueous solution buffered to pH 7.

Sugars often lack charged groups, but as we discussed in our &lsquothought experiment&rsquo with glucose, they are quite water-soluble due to the presence of multiple hydroxyl groups.

Some biomolecules, in contrast, contain distinctly hydrophobic components. The &lsquolipid bilayer&rsquo membranes of cells and subcellular organelles serve to enclose volumes of water and myriad biomolecules in solution. The lipid (fat) molecules that make up membranes are amphipathic: they have a charged, hydrophilic &lsquohead&rsquo and a hydrophobic hydrocarbon &lsquotail&rsquo.

interactive 3D image of a membrane phospholipid (BioTopics)

Notice that the entire molecule is built on a &lsquobackbone&rsquo of glycerol, a simple 3-carbon molecule with three alcohol groups. In a biological membrane structure, lipid molecules are arranged in a spherical bilayer: hydrophobic tails point inward and bind together by van der Waals forces, while the hydrophilic head groups form the inner and outer surfaces in contact with water.

Interactive 3D Image of a lipid bilayer (BioTopics)

Because the interior of the bilayer is extremely hydrophobic, biomolecules (which as we know are generally charged species) are not able to diffuse through the membrane&ndash they are simply not soluble in the hydrophobic interior. The transport of molecules across the membrane of a cell or organelle can therefore be accomplished in a controlled and specific manner by special transmembrane transport proteins, a fascinating topic that you will learn more about if you take a class in biochemistry.

A similar principle is the basis for the action of soaps and detergents. Soaps are composed of fatty acids, which are long (typically 18-carbon), hydrophobic hydrocarbon chains with a (charged) carboxylate group on one end,

Fatty acids are derived from animal and vegetable fats and oils. In aqueous solution, the fatty acid molecules in soaps will spontaneously form micelles, a spherical structure that allows the hydrophobic tails to avoid contact with water and simultaneously form favorable van der Waals contacts.

Because the outside of the micelle is charged and hydrophilic, the structure as a whole is soluble in water. Micelles will form spontaneously around small particles of oil that normally would not dissolve in water (like that greasy spot on your shirt from the pepperoni slice that fell off your pizza), and will carry the particle away with it into solution. We will learn more about the chemistry of soap-making in a later chapter (section 12.4B).

Synthetic detergents are non-natural amphipathic molecules that work by the same principle as that described for soaps.


Small or Nonpolar

Water is a small molecule that easily diffuses through a cell membrane despite the lipid tails. Water diffusion is called osmosis. Oxygen is a small molecule and it’s nonpolar, so it easily passes through a cell membrane. Carbon dioxide, the byproduct of cell respiration, is small enough to readily diffuse out of a cell. Small uncharged lipid molecules can pass through the lipid innards of the membrane. Larger or charged molecules might be able to slowly diffuse across the membrane. The charge on a molecule might help or hinder its diffusion, based on the relative charges on either side of the membrane.


Difference Between Simple Diffusion and Facilitated Diffusion

Diffusion is said to be the movement of a particle from a higher concentration to a lower concentration. There are many types of diffusion such as gaseous diffusion, rotational diffusion, surface diffusion, atomic diffusion, electronic diffusion, and a lot more.

Two other types of diffusion that will be compared are simple diffusion and facilitated diffusion. Let us examine the differences.

Facilitated diffusion is also called facilitated transport or passive mediated transport. It is a process in which the type of transport is passive that is enabled by proteins. Diffusion by this type is also spontaneous, unstructured, or unplanned passage of molecules or ions in a membrane through the aid of proteins. Certain molecules cannot pass through a membrane such as certain polar and non-polar molecules. Small, non-polar molecules can diffuse easily. An example of this is oxygen. Large molecules are diffused through the aid of proteins.

Simple diffusion, on the other hand, is the passage of a molecule or ion in a membrane without the help or aid of another intermediary such as proteins. What drives the molecules and ions from a certain point to the other side of the membrane is through diffusion’s force. However, there are certain criteria before a molecule or ion can penetrate through the cell membrane. The molecule or ion must be able to pass through the hydrophobic wall of the membrane. A few types of molecules can pass with ease which are hydrophobic. These are oxygen, ethanol, and carbon dioxide. In simple diffusion there is no energy involved in the penetration of these molecules.

Simple and facilitated diffusion are two types of diffusion. With this phenomena we can understand which molecules can penetrate and which cannot. Thus, we can observe that there are other methods through facilitated diffusion.

1.In simple diffusion, the force made by diffusion pushes the molecule across the membrane, but in facilitated diffusion it is aided by proteins.
2.In simple diffusion, small hydrophobic molecules can pass, but those which are not small and hydrophobic pass through facilitated diffusion.


Why do cell membranes let small non-polar molecules through but won't let small polar molecules through? - Biology

In this experiment we will be observing the the movement of molecules through a semi permeable membrane. Students will be able to observe how some molecules (starch) are too large to pass through a membrane, while smaller molecules (iodine) can freely move.

Although this is a simple experiment it is effective in illustrating movement across membranes.

  • Dialyses tubing
  • Scissors
  • Beakers
  • Iodine
  • Starch
  • Paper clip

When doing this experiment, you can let the kids decide how to approach it. They may choose to place the iodine in the dialyses tubing and starch in the beaker or Vise Versa.

In this experiment I will be doing starch in the Dialyses tubing and Iodine in the beaker.

  1. Cut a piece of Dialyses tubing off
  2. Place the Dialyses tube in water and open it.
  3. Tie off one end of the tubing and poor starch solution into the tube.
  4. Tie off the top of the dialyses tube. The tube should not be leaking
  5. Place the tube in a Iodine and water solution. You can hold the tube up using the paper clips.
  6. Leave the beaker for a few minutes.
  7. The Iodine should being to diffuse into the starch, turning the solution from white to dark purple.

Science behind the experiment

The terms you need to understand are:

The Dialysis tubing provides a semi- permeable membrane. Only allowing smaller molecules to pass through it. Iodine molecules are small enough to pass freely through the membrane, however starch molecules are complex and too large to pass through the membrane.

Initially there was a higher concentration of iodine outside than inside the tube. Thus iodine diffused into the tube with the starch. This turned the starch a darker colour.

If we place the iodine in the tube and the starch in a beaker, the effect is much faster.

Notice the video to the left, almost immediately after we place the tube in the starch, the iodine diffuses out, and changes the colour of the starch.


Non-polar molecules

A non-polar molecule is one in which the electrons are distributed more symmetrically and thus does not have an abundance of charges at the opposite sides. The charges all cancel out each other.

The electrical charges in non-polar Carbon Dioxide are evenly distributed

Examples of non-polar liquids

Most hydrocarbon liquids are non-polar molecules. Examples include:

Alkynes are non-polar because they cannot be dissolved in water, as do polar molecules. However, alkynes but do dissolve in other non-polar substances. A rule is that like substances dissolve in like substances.

Examples of non-polar gases

Common examples of non-polar gases are the noble or inert gases, including:

Other non-polar gases include:

  • Hydrogen (H2)
  • Nitrogen (N2)
  • Oxygen (O2)
  • Carbon Dioxide (CO2)
  • Methane (CH4)
  • Ethylene (C2H4)

Since Chloroform is more soluble in fats than in water, it is also classified as non-polar.