Selective Permeability

The plasma membrane separates the internal contents of a cell from the extracellular space. Some molecules can pass through this membrane, while others cannot. What enables the plasma membrane to do this? In this article, we will discuss selective permeability: its definition, causes, and functions. We will also distinguish it from a related concept, semi-permeability.

What is the definition of "selectively permeable"?

A membrane is selectively permeable when only certain substances can move across it and not others. The plasma membrane is selectively permeable because only certain molecules can go through it. Due to this property, transport proteins and channels are needed so that, for example, ions can access or leave the cell.

Think of the cell as an exclusive event: some are invited in, while others are kept out. This is because the cell needs to take in substances that it needs to survive and to protect itself from harmful substances in its environment. The cell is able to regulate the entry of substances through its selectively permeable plasma membrane.

Substances that pass through the membrane can either do so passively or with the use of energy.

Going back to our scenario: the plasma membrane can be thought of as a gate that encloses the exclusive event. Some event-goers can easily pass through the gate because they have tickets to the event. Likewise, substances can pass through the plasma membrane when they fit certain criteria: for instance, small non-polar molecules like oxygen and carbon dioxide can easily pass through, and large polar molecules like glucose must be transported to enter the gate.

What causes the selective permeability of the plasma membrane?

The plasma membrane has selective permeability because of its composition and structure. It is composed of a phospholipid bilayer.

The phospholipids are arranged with the hydrophobic tails facing inward and the hydrophilic heads facing outward. This structure, called the phospholipid bilayer, is illustrated in Figure 1.

Fig. 1 - phospholipid bilayer

The phospholipid bilayer acts as a stable boundary between two water-based compartments. The hydrophobic tails attach, and together they form the interior of the membrane. On the other end, the hydrophilic heads face outward, so they are exposed to aqueous fluids inside and outside of the cell.

Some small, non-polar molecules such as oxygen and carbon dioxide can pass through the phospholipid bilayer because the tails that form the interior are non-polar. But other larger, polar molecules like glucose, electrolytes, and amino acids cannot pass through the membrane because they are repelled by the non-polar hydrophobic tails.

What are the two main types of diffusion across the membrane?

The movement of substances across a selectively permeable membrane can occur either actively or passively.

Passive transport

Some molecules do not require the use of energy for them to cross through a membrane. For example, carbon dioxide, produced as a by-product of respiration, can freely exit a cell through diffusion. Diffusion refers to a process where molecules move in the direction of the concentration gradient from an area of higher concentration to an area of lower concentration. This is one example of passive transport.

Another type of passive transport is called facilitated diffusion. The phospholipid bilayer is embedded with proteins that perform a variety of functions, transport proteins move molecules across the membrane through facilitated diffusion. Some transport proteins create hydrophilic channels for sodium, calcium, chloride, and potassium ions or other small molecules to pass through. Others, known as aquaporins, allow for the passage of water through the membrane. All of these are called channel proteins.

Active transport

There are times when energy is needed to move some molecules across the membrane. This typically involves the passage of larger molecules or a substance going against its concentration gradient. This is called active transport, a process by which substances are moved across a membrane using energy in the form of adenosine triphosphate (ATP). For example, kidney cells use energy to take in glucose, amino acids, and vitamins, even against the concentration gradient. There are several ways that active transport can take place.

Fig. 2 - In the sodium-potassium pump, sodium is pumped out of the cell, and potassium is pumped into the cell against the concentration gradient. This process draws energy from ATP hydrolysis.

Another way for active transport to occur is through the formation of a vesicle around the molecule, which can then combine with the plasma membrane to allow entry into or exit from the cell.

• When a molecule is allowed entry into the cell through a vesicle, the process is called endocytosis.
• When a molecule is excreted out of the cell through a vesicle, the process is called exocytosis.

These processes are illustrated in Figures 3 and 4 below.

Fig. 3 - This diagram shows how endocytosis takes place.

Fig. 4 - This diagram shows how endocytosis takes place.

What is the function of the selectively permeable plasma membrane?

The plasma membrane is a selectively permeable membrane that separates the cell’s internal contents from its outside environment. It controls the movement of substances into and out of the cytoplasm.

The selective permeability of the plasma membrane enables cells to block, allow, and expel different substances in specific amounts: nutrients, organic molecules, ions, water, and oxygen are allowed into the cell, while wastes and harmful substances are blocked from or expelled out of the cell.

The selective permeability of the plasma membrane is essential in maintaining homeostasis.

Examples of selectively permeable membranes

Besides separating the cell’s internal contents from its environment, a selectively permeable membrane is also important in maintaining the integrity of the organelles inside eukaryotic cells. Membrane-bound organelles include the nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and vacuoles. These organelles each have highly specialized functions, so selectively permeable membranes play an important role in keeping them compartmentalized and maintaining them in optimal condition.

What is the difference between a semi-permeable membrane and a selectively permeable membrane?

Semi-permeable and selectively permeable membranes both manage material movement by allowing some substances to pass through while blocking others. The terms “selectively permeable” and “semi-permeable” are often used interchangeably, but they have subtle differences.

• A semi-permeable membrane works like a sieve: it allows or prevents molecules from passing through based on their size, solubility, or other chemical or physical properties. It involves passive transport processes like osmosis and diffusion.