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Selective Permeability

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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 selective permeability?

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.

Selective permeability refers to the ability of the plasma membrane to allow some substances to pass through while blocking other substances.

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.

A phospholipid is a lipid molecule made of glycerol, two fatty acid chains, and a phosphate-containing group. The phosphate group makes up the hydrophilic (water-loving) head, and the fatty acid chains make up the hydrophobic (water-fearing) tails.

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.

Selective Permeability, Phospholipid bilayer diagram, Study Smarter

Figure 1. The phospholipid bilayer. Source: OpenStax, CC BY 4.0, via Wikimedia Commons.

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.

A concentration gradient is created when there is a difference in the amounts of a substance on the two sides of a membrane. One side will have a higher concentration of this substance than the other.

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.

One way active transport can take place is through the use of ATP-powered protein pumps to move molecules against their concentration gradient. An example of this is the sodium-potassium pump, which pumps sodium out of the cell and potassium into the cell, which is the opposite direction that they normally flow with diffusion. The sodium-potassium pump is important for maintaining the ionic gradients in neurons. This process is illustrated in Figure 2.

Selective Permeability, Extracellular space, StudySmarter

Figure 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. Source: LadyofHats, Public domain, via Wikimedia Commons.

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.

Selective Permeability, Endocytosis diagram, StudySmarter

Figure 3. This diagram shows how endocytosis takes place. Source: LadyofHats, Public domain, via Wikimedia Commons.

Selective Permeability, Exocytosis diagram, StudySmarter

Figure 4. This diagram shows how endocytosis takes place. Source: LadyofHats, Public domain, via Wikimedia Commons.

What is the function of the selectively permeable plasma membrane?

The plasma membrane is a selectively permeable membrane that separates the cells 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.

Homeostasis refers to the balance in the internal states of living organisms that allow them to survive. This means variables like body temperature and glucose levels are kept within certain limits.

Examples of selectively permeable membranes

Besides separating the cells 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.

For instance, the nucleus is enclosed by a double-membrane structure called the nuclear envelope. It is a double-membrane, meaning there is an inner and an outer membrane, both of which are composed of phospholipid bilayers. The nuclear envelope controls the passage of ions, molecules, and RNA between the nucleoplasm and the cytoplasm.

The mitochondrion is another membrane-bound organelle. It is responsible for cellular respiration. For this to be carried out effectively, proteins must be selectively imported into the mitochondrion while keeping the internal chemistry of the mitochondrion unaffected by other processes that take place in the cytoplasm.

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.

  • On the other hand, a selectively permeable membrane determines which molecules are permitted to cross using specific criteria (for instance, molecular structure and electrical charge). In addition to passive transport, it can use active transport, which requires energy.

Selective Permeability - Key takeaways

  • Selective permeability refers to the ability of the plasma membrane to allow some substances to pass through while blocking other substances.
  • The plasma membrane has selective permeability because of its structure. The phospholipid bilayer is composed of phospholipids arranged with the hydrophobic tails facing inward and the hydrophilic heads facing outward.
  • The movement of substances across a selectively permeable membrane can occur through active transport (requires energy) or passive transport (does not require energy).
  • The selective permeability of the plasma membrane is essential in maintaining homeostasis, the balance in the internal states of living organisms that allow them to survive.

Frequently Asked Questions about Selective Permeability

The selective permeability of the plasma membrane is caused by its composition and structure. It is composed of a phospholipid bilayer with the hydrophobic tails facing inward and the hydrophilic heads facing outward. This makes it easy for some substances to pass through and more difficult for others. The proteins embedded on the phospholipid bilayer also assist by creating channels or transporting molecules.

Selective permeability refers to the ability of the plasma membrane to allow some substances to pass through while blocking other substances.

The composition and structure of the cell membrane is responsible for its selective permeability. It is composed of a phospholipid bilayer with the hydrophobic tails facing inward and the hydrophilic heads facing outward. This makes it easy for some substances to pass through and more difficult for others. The proteins embedded on the phospholipid bilayer also assist by creating channels or transporting molecules.

The cell membrane is selectively permeable because of its composition and structure. It is composed of a phospholipid bilayer with the hydrophobic tails facing inward and the hydrophilic heads facing outward. This  makes it easy for some substances to pass through and more difficult for others. The proteins embedded on the phospholipid bilayer also assist by creating channels or transporting molecules.

The selective permeability of the plasma membrane enables cells to block, allow, and expel different substances in specific amounts. This ability is essential in maintaining homeostasis.

Final Selective Permeability Quiz

Question

What is the definition of selective permeability?

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Answer

Selective permeability refers to the ability of the plasma membrane to allow some substances to pass through while blocking other substances. 

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Question

What causes the selective permeability of the plasma membrane?

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Answer

The plasma membrane has selective permeability because of its composition and structure. The plasma membrane is composed of a phospholipid bilayer, with the hydrophobic tails facing inward and the hydrophilic heads facing outward. It also has proteins embedded that create channels or transport molecules through facilitated diffusion.

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Question

What part of the phospholipid faces outwards and is exposed to aqueous fluid?

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Answer

hydrophilic head

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Question

What part of the phospholipid faces inward and forms the interior of the membrane?

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Answer

hydrophobic tail

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Question

What proteins create hydrophilic passageways for sodium, calcium, chloride, and potassium ions or other small molecules to pass through?

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Answer

channel proteins

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Question

What proteins move molecules across the membrane through facilitated diffusion?

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Answer

transport proteins

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Question

What is passive transport?

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Answer

Passive transport is a process by which substances are moved across a membrane without the use of energy.

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Question

What is active transport?

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Answer

Active transport is a process by which substances are moved across a membrane using energy in the form of adenosine triphosphate (ATP).

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Question

What do you call the process where molecules move in the direction of the concentration gradient without the use of energy?

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Answer

Diffusion

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Question

What is homeostasis?

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Answer

Homeostasis is the balance in the internal states of living organisms that allow them to survive.

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Question

What parts of the cell are NOT bound by a selectively permeable membrane?

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Answer

ribosomes

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Question

Why is the selectively permeable membrane important in organelles?

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Answer

Organelles have highly specialized functions, so selectively permeable membranes play an important role in keeping them compartmentalized and maintaining them in optimal condition. 

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Question

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

Show answer

Answer

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.


On the other hand, a selectively permeable membrane determines which molecules are permitted to cross using specific criteria (for instance, molecular structure and electrical charge). In addition to passive transport, it is able to use active or facilitated transport, which requires energy. 

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Question

What process allows the entry of a molecule into the cell through a vesicle?

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Answer

endocytosis

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Question

How does the selectively permeable membrane maintain homeostasis?

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Answer

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. 

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