Plasma membrane in a cell – functions, structure, and diagram

The main function of the plasma membrane is to enclose the cell and protect it from dehydration or external harm. Other plasma membrane functions would be discussed below. It is also known as the cell membrane. This membrane covers or encloses the organelles from the rest of the cellular environment.

Table of Contents

Definition of plasma membrane in biology

A cell’s plasma membrane is defined as a network of lipids and proteins that forms the boundary between the contents of the cell and the external surrounding of the cell.

What is the plasma membrane?

The plasma membrane, also known as the cell membrane, is the membrane that separates the interior of the cell from the outside environment. It is found in all cells.

A cell wall is attached to the plasma membrane on the outside of bacterial and plant cells. The plasma membrane is made up of a phospholipid bilayer, proteins, lipids, carbohydrates, and other components and it controls the transport of materials into and out of the cell. The plasma membrane’s distinct components make it a selectively permeable barrier.

It is “semi-permeable” because it regulates what enters and exits the cell via cellular transport mechanisms. It also makes cellular signal transmission easier. It is extremely malleable, allowing certain cells, such as red and white blood cells, to change shape as they pass through narrow capillaries.

A plasma membrane is required for cellular homeostasis in both animal cells and plant cells. So, while they differ in some ways, both animal and plant cells have a cell wall that is fundamentally the same in terms of structure and function. Similarly, prokaryotes, like fungi, algae, protists, and other organisms, have a plasma membrane (also known as the prokaryotic cytoplasmic membrane).

Why is it called the plasma membrane?

It is so named because the semi-fluid living matter in the cell is called protoplasm (or simply plasm). This living matter, or plasm, is contained within the plasma membrane, a biological membrane.

Plasma membrane Vs cell membrane

As previously stated, the cell membrane is also the plasma membrane that surrounds the cell. The term “plasma membrane” also refers to the biological membranes that serve as the outer boundary of internal compartments (organelles). Thus, strictly speaking, the plasma membrane is a broader term because it includes the lipid bilayer membranes of the organelles in addition to the cell membrane, which in this case is the plasma membrane that defines the cell’s boundary with its external environment.

Cell membrane
Plasma membrane
The cell membrane encases all of the components of the cell area.
The plasma membrane envelops the cell and its internal compartments, known as cell organelles.
It is important in cytokinesis during cell division.
Not all of them play an important role in cytokinesis during cell division.
It serves as a gateway for molecules and other agents to enter the cell.
It controls the flow of molecules and ions into and out of the cell.
It promotes cell-to-cell communication.
A table showing plasma membrane vs cell membrane

Examples of plasma membrane

Plasma membranes can be seen in both plant and animal cells and some of the examples where the plasma membrane can be seen in action are during diffusion of gasses like oxygen and carbon dioxide and in maintaining the homeostasis environment of the cell through osmosis and active transport.

Plasma membrane functions

  1. Physical barrier
  2. Semi-permeability
  3. Endocytosis and exocytosis
  4. Cell signaling

The major function of the plasma membrane is to act as a barrier between the inside and outside of the body.

A physical barrier

This is the main function of the plasma membrane; it envelops all cells and physically separates the cytoplasm (the material that makes up the cell) from the extracellular fluid. This shields the cell’s components from the outside environment and allows separate activities to take place inside and outside the cell.

The plasma membrane serves as the cell’s structural support and it connects the cytoskeleton, which is a network of protein filaments inside the cell that holds all of the cell’s parts together. This is what gives the cell its shape.

Plants and fungi, for example, have a cell wall in addition to the membrane. The cell wall is made up of molecules like cellulose. It gives the cell extra support, which is why plant cells do not burst like animal cells when too much water diffuses into them.


Another plasma membrane function is the selection of the type of molecules allowed to go in and out of the cell. This is known as facilitated diffusion. Because plasma membranes are selectively permeable (or semi-permeable), only certain molecules can pass through them.

Like water, oxygen, and carbon dioxide can all pass through the membrane with ease. Ions (e.g., sodium, potassium) and polar molecules, in general, cannot pass through the membrane. Instead, they must pass through specific channels or pores in the membrane rather than freely diffusing through it.

As a result, the membrane can regulate the rate at which certain molecules enter and exit the cell.

Endocytosis and exocytosis

An important function of the cell membrane is Endocytosis, this occurs when a cell consumes contents that are larger than the single ions or molecules that pass through channels. Endocytosis allows a cell to take in large amounts of molecules or even whole bacteria from the extracellular fluid. Exocytosis is the process by which the cell releases these materials. Both of these processes rely heavily on the cell membrane.

To allow molecules to enter or exit the cell, the shape of the membrane itself changes. In order to transport materials to different parts of the cell, it also forms vacuoles, which are small bubbles of the membrane that can transport many molecules at once.

Cell signaling

Another essential plasma membrane function is the facilitation of communication and signaling between cells. This is accomplished through the use of various proteins and carbohydrates in the membrane. Proteins “mark” the cell so that other cells can detect it. The membrane also has receptors that allow it to perform specific tasks when molecules such as hormones bind to them.

The cell membrane’s function in an animal cell is to regulate what moves in and out of the cell. Substance transport through the membrane can either occur without the use of cell energy or cause the cell to consume energy while transporting it. As a result, the cell membrane serves as a narrow channel through which only a few substances can enter and exit the cell.

What is the plasma membrane made of?

The plasma membrane is made up of two back-to-back layers of phospholipids that form a bilayer. The essential structure is a phospholipid bilayer that forms a consistent significant barrier between the two fluids.

According to the cell layer, the components are located inside and outside of the cell. Membrane proteins, which are inserted within the lipid structure, are another basic component of the cell membrane.

To be more precise, the plasma membrane is composed of phospholipids, proteins, carbohydrates, and other lipids like sterol.

Diagram of plasma membrane

This is a diagram of the plasma membrane
A labeled diagram of plasma membrane

Biological processes or activities associated with plasma membrane

There are some cellular biological activities that involve the plasma membrane. Such activities aid in the transportation of substances to and from the cell. These biological processes include diffusion, osmosis, active transport, endocytosis, and exocytosis.

Why is the plasma membrane described as a fluid mosaic model?

The fluid mosaic model describes the plasma membrane as consisting of the fluid because of the numerous types of molecules that float along with the lipids.

The mosaic model of the plasma membrane states that the phospholipid bilayer is viscous and can shift the positions of individual phospholipids, cholesterol, and proteins. Also, the proteins encapsulate the phospholipid bilayer, resulting in a mosaic of components.

FAQ on plasma membrane

What is the plasma membrane composed of?

The plasma membrane is made up of glycerophospholipids, molecules composed of glycerol, a phosphate group, and two fatty acid chains.

What is the plasma membrane of skeletal muscle cells called?

The plasma membrane of skeletal muscle cells or fibers is called sarcolemma (it is described as highly elongated cells with a very elastic and resistant plasma membrane).

What is the plasma membrane in prokaryotes?

A prokaryotic cell’s plasma membrane is a lipid bilayer that surrounds the cytoplasm. It physically separates the cytoplasm from the rest of the body. Many metabolic reactions for the prokaryotic organism, such as respiration, fermentation, and photosynthesis, occur at the plasma membrane.

What is the plasma membrane permeable to?

Hydrophobic molecules and small polar molecules can pass through the plasma membrane, but ions and large polar molecules cannot. This means that oxygen crosses a plasma membrane by diffusion, or in other words, oxygen and carbon dioxide pass through the plasma membrane by diffusion.

Where is the plasma membrane located?

The plasma membrane is located in the outside environment of all cells because it serves to protect the interior of the cell. Also, a cell wall is attached to the plasma membrane on the outside of bacterial and plant cells.

What is the plasma membrane job?

The plasma membrane, also known as the cell membrane, protects the cell. It also maintains a consistent environment within the cell. The membrane serves several purposes and one function is to transport nutrients into the cell, while the other is to transport toxic substances out of the cell.

Another job is that the cell’s membrane, which is the plasma membrane, will have proteins on it that interact with other cells.

What is the plasma membrane structure?

The plasma membrane, like all other cellular membranes, is made up of both lipids and proteins. The phospholipid bilayer which is part of the plasma membrane forms a stable barrier between two aqueous compartments, is the membrane’s fundamental structure.

What is the plasma membrane of a neuron called?

The plasma membrane of a neuron is called an axonal plasma membrane or axolemma

Do plant cells have a plasma membrane?

Yes, the plasma membrane is surrounded by the cell wall which adds tensile strength and protection to the cell.

Do eukaryotes have a plasma membrane?

Yes, eukaryotes have a plasma membrane and the membrane encloses the organelles (both membrane bound organelles and non-membrane bound organelles) found in the cells.

A video briefly explaining the plasma membrane.