Introduction

Differential permeability is defined as a property of cellular membranes that allows only selective substances to enter or leave the cell. This feature of cellular membranes helps to maintain a constant internal environment regardless of the changes in the external environment. Many ions, water, glucose, and carbon dioxide are constantly imported and exported from the cell depending upon its requirements. Moreover, signaling molecules enter the cell, and carrier proteins cross the membrane to come into the extracellular space. The presence of a differential permeable membrane allows the cell to have control over the number of molecules that move into and out of the cell that is necessary to maintain a balanced cell environment.

Differential permeability is also known as selective permeability. The movement of substances across a selectively permeable membrane occurs by active transport and passive transport. It depends upon the different molecules that which process will be utilized for their movement across the membrane.

Differential Permeability and Cell Membrane

The cell membrane is the outermost covering in an animal cell. In the plant cell, it is covered by a cell wall (outermost covering in plant cell). Cell membrane or plasma membrane helps to maintain the shape of the cell. In addition to this, this membrane is differentially permeable due to its structure. It is composed of a phospholipid bilayer with proteins embedded in the lipid layer. This makes the plasma membrane selectively permeable means only specific substances will be able to cross the membrane. The phospholipid part of the plasma membrane gives it a hydrophobic character due to which polar molecules can not easily cross the membrane. The proteins embedded in the phospholipid layer serve as transporter and channel proteins for specific substances such as ions.

Read more about Cell Membranes

Structure of Differentially Permeable Membrane 

Selective permeability was first described by French Physicist Abbé Nollet. It is not easy to observe cell membranes using a light microscope. Different studies contributed to understanding the structure of the cell membrane that favors the permeability function. Initially, the membrane was considered a simple structure that separates the cytoplasm from the extracellular region. Later, studies showed semi-permeable gel-like regions surrounded by lipids. This explained the movement of water but not the charged particles. Then, the presence of pores was confirmed that allowed the movement of small molecules.

Fluid Mosaic Model 

This model was proposed to explain the structure of the cell membrane. The structure of the cell membrane explained by this model describes the permeability of the cell membrane. According to this model there are following three important components of cell membrane:

1. Phospholipid bilayer
2. Cholesterol
3. Proteins
4. Oligosaccharides

Phospholipid bilayer

The plasma membrane contains a phospholipid bilayer in which the phospholipids are packed close to each other throughout the membrane. The lipids consist of hydrophobic tails and hydrophilic heads. This phospholipid bilayer, sometimes also known as a lipid bilayer, makes the cell selectively permeable to substances allowing only selective substances to pass through it due to the hydrophobic character.

Cholesterol 

Cholesterol is often considered harmful for the body but in the case of the cell membrane, it is important. Cholesterol has a lot of rings in its structure that gives cholesterol a very stable structure. It inserts itself in the lipid layers and acts as a buffer. It maintains the fluidity of membranes according to the temperature changes. As the temperature becomes lower, it helps to increase the fluidity. Similarly, with temperature increase, the fluidity is decreased. So, cholesterol is essential for maintaining the fluidity of the cell membrane.

Proteins

In the phospholipid bilayers, different proteins are embedded. These membrane proteins have various important roles in the cell. They act as transporters and carriers of different substances. Moreover, enzymes are proteins that are required for catalyzing different reactions. The proteins present in the cell membrane are of two types:

1. Integral proteins
2. Peripheral proteins

Integral proteins are found completely embedded in the membrane and can’t be detached easily from the membrane. In contrast, peripheral proteins are located on the surface of the lipid bilayer and have weak interaction with the bilayer. These can be detached easily from the membrane.

Oligosaccharides

Oligosaccharides are polymers of sugars. In the plasma membrane, they can attach covalently to the lipids to form glycoproteins or can make covalent interactions with proteins to form glycoproteins. In the lipid bilayer, the sugar groups of the glycolipids are present on the outer surface of the membrane and can form hydrogen bonds. These glycolipids are responsible for a variety of functions that mainly includes communication, which includes cell recognition and cell-cell interaction (adhesion). The glycoproteins mainly act as integral proteins and have a role in immune response and protection of cells.

Transport Mechanisms for Differently Permeable Membrane

Different transport mechanisms are used for different substances to pass through the differential permeable membrane. These mechanisms include:

1. Passive Transport 
2. Active Transport 
3. Osmosis

Passive Transport 

Passive transport of substances across the plasma membrane does not require energy. The movement of substances is according to their concentration gradient (from higher concentration to lower concentration). Based on the chemical nature of substances, passive transport may occur by different processes. These processes include:

1. Diffusion
2. Facilitated transport

Diffusion

Diffusion is a passive transport process that does not require ATP. A substance moves from higher concentration to lower concentration until the number of molecules on both sides of space become equal. For example, if you open a bottle of ammonia in a room. Then the concentration of ammonia is higher in the bottle in comparison to the room. But ammonia starts to spread in the room slowly and it happens until the concentration becomes equal. Diffusion is responsible for the transport of substances from the extracellular fluid into the cell’s cytosol.

Facilitated transport

It is also called facilitated diffusion. In this process, the diffusion of molecules across the membrane occurs with the help of membrane proteins. The materials that are transported into and out of the cell by facilitated transport include ions or polar molecules.

Factors Affecting Diffusion

Following are some factors that can affect the Diffusion of substances, thus affecting the differential permeability of the membrane.

1. The difference in a concentration gradient
2. Shape and size of the molecules passing through the membrane.
3. Temperature
4. Solvent density
5. Solubility
6. Surface area and thickness of plasma membrane
7. Distance traveled by molecule

Active Transport

Active transport process for movement of molecules across the membrane requires the use of adenosine triphosphate (ATP). Energy is needed because the movement is against the concentration gradient and ATP provides the necessary energy required for the uphill movement.

Moving Against Gradient 

Energy is required for the movement of substances against the concentration gradient. The source of this energy is the Adenosine triphosphate (ATP) that is produced during the metabolism. The movement of the substances against the concentration gradient is done by active transport mechanisms, which are known as pumps. The movement of several small molecules occurs continuously through the cell membrane. Active transports mechanisms are actively involved in maintaining the concentration of ions and substances. As a result, these processes consume a major portion of the energy produced by the metabolic processes.

Carrier Proteins for Active Transport

For the transports of materials through the differentially permeable membrane, several carrier proteins and pumps play a significant role. Major types of the carrier proteins and transporter involved are below:

1. Uniporters
2. Symporters
3. Antiporters

The transport of one specific ion or molecule is done by uniporter proteins. For the movement of two or more different ions or molecules, a symporter plays its role. The transport by symporter is done in the same direction. Antiporters as the name indicate transport in the opposite direction. They serve as carrier proteins for two or more different ions. The transport of the small, uncharged molecules including glucose is done by antiporter proteins.

Types of Active Transport

There are two types of active transport mechanisms that are used to transport substances through the differential permeable membrane on the basis of the difference in a concentration gradient.

1. Primary Active Transport
2. Secondary Active Transport

Primary Active Transport

ATP is utilized directly in primary active transport.

The process is completed in the following steps:

1. In the beginning, the direction of the carrier enzyme pump is towards the inside of the cell. The binding affinity of sodium with this carrier enzyme is more and at a time three sodium ions can bind to it.
2. The protein is involved in catalyzing the hydrolysis reactions of ATP and binds a low-energy phosphate group to it. 
3. The shape of the carrier protein is modified due to phosphorylation and the direction is shifted to the outside. Due to this, the affinity for sodium ions lowers and three sodium ions move to the outside. 
4. As a result of this conformational change, the carrier protein allows the binding of two potassium ions due to increased binding affinity for it. Consequently, the low energy phosphate group detaches from the carrier protein.
5. The position of the carrier protein is shifted towards the interior of the cell after the removal of the phosphate group and binding of potassium ions.
6. Now due to this change in confirmation, two potassium ions are released into the interior space of the cell due to decreased affinity for potassium ions. Later, the protein again shifts to the original confirmation that has a high affinity for Sodium ions. 

By this process, the movement of ions and other substances occurs through the selectively permeable membrane. 

Secondary Active Transport

During secondary active transport, attachment of ATP to the carrier protein does not occur. In this process, the movement of the molecule or ion against the concentration gradient sets up a concentration gradient. The molecule is then moved down the concentration gradient. ATP is required for establishing a concentration gradient, but not for the movement of molecule. Due to indirect use of ATP in the process, it is known as secondary active transport. 

Osmosis

Osmosis is the process of movement of water across the semi-permeable membrane. The movement of water occurs according to the concentration gradient of water through the membrane and this concentration gradient is inversely correlated to the concentration of the solute.

Factors Affecting Permeability of Cell Membrane

The permeability of the cell membrane depends upon the following factors:

1. Polarity
2. Electric charge
3. The molecular mass of molecules that pass through it
4. Phospholipid bilayers

As mentioned above a cell membrane is made up of two phospholipid bilayers in which proteins are embedded. These layers have an electrically charged and hydrophobic head and an uncharged and hydrophilic tail too. The electrically charged heads of the two layers are pointed towards the water. The uncharged tails are oriented towards each other. Small and neutrally-charged molecules can easily pass through the membrane. While large and charged particles face difficulty in crossing the membrane. Moreover, the phospholipid bilayers don’t allow the passage of non-lipid soluble substances through the cell membrane.

The phospholipid bilayers are permeable to non-polar substances, such as oxygen, carbon dioxide, nitrogen, and steroids. To water, glycerol, urea, and ethanol, the membrane is less permeable as they are small and non-polar molecules. But, for larger polar molecules such as glucose and sucrose, the membrane has high impermeability.

The electric charge also affects the permeability of the cell membrane. Charged molecules and ions cannot pass through the cell membrane. Special carrier proteins are present for the transport of charged particles across the cell membrane. These transport proteins are also involved in the movement of glucose, water, and ethanol through the membrane. The permeability of the cell membrane helps the cell to have control over the number of molecules and the type of molecules that pass through it.

Importance of Differential Permeability of Membrane

Differential permeability of cells is required for the following reasons:

1. The cell can’t afford to allow every substance to come into its intracellular space. The selectively Permeability allows only those molecules to cross the membrane barrier that is required by the cell. Harmful substances are kept outside the cell.
2. It keeps the cell in an isotonic state that means the amount of water is the same inside and outside the cell. If the water level is not equal then the cell may burst or shrink. Thus, differential permeability of membrane maintains water balance.

Frequently Asked Questions

What is a differentially permeable membrane?

A membrane is said to be differentially permeable if it allows certain substances to diffuse through it while blocking the rest of other substances. Only selective substances can diffuse through a differentially permeable membrane. 

Why are cell membranes differentially permeable?

Plasma membranes or cell membranes are differentially permeable to control the movement of substances into and out of the cells. They allow free diffusion of water and other non-ionic small molecules while preventing the diffusion of charged and large sized particles.

What substances can pass through a differentially permeable membrane?

A differentially permeable membrane allows the free diffusion of solvents and non-ionic particles. Thus, water, non-ionic molecules and gases can diffuse freely across a differentially permeable membrane. 

Which 3 types of substances cannot pass easily through a differentially permeable membrane?

A differentially permeable membrane restricts the diffusion of large sized molecules, charged or polar substances and ions. Thus, these types of substances cannot diffuse freely across a differentially permeable membrane.

References

1.  Lodish, H; Berk, A; Kaiser, C; Krieger, M; Bretscher, A; Ploegh, H; Amon, A (2000). Molecular Cell Biology (7th ed.). New York, NY: W. H. Freeman and Company. p. 695.
2. Marieb, E. N., & Hoehn, K. (2014). Human anatomy & physiology. San Francisco, CA: Pearson Education Inc.
3. Biology Libre Text