Difference Between Cell Membrane and Plasma Membrane

In this post we discuss the differences between cell membranes and plasma membranes, what are cell membranes and plasma membranes and the function of cell membranes and plasma membranes.

Plasma membrane and cell membrane are often confused because they are considered the same term. However, they are very different from each other. The plasma membrane encloses the cell organelles, while the cell membrane encloses all cell components.

What is Cell Membrane?

The cell membrane is a type of plasma membrane that encloses the entire contents of the cell, including the cytoplasm and all organelles. The cell membrane is not always the outermost layer of the cell because plant cells also have a cell wall which further encloses the cell membrane.

However animal cells do not have a cell wall and the cell membrane is a barrier between the inner contents of the cell and the external environment. The cell membrane consists of a phospholipid bilayer which also contains integral proteins. Integral proteins often span the membrane and provide a pathway for molecules to move through the membrane.

The membrane is selectively permeable, due to the presence of hydrophobic (water repellent) lipids. The phosphate head is hydrophilic (do not repel water) compared to the lipid tail. The lipid tail is a glycerol molecule. Other lipids in the membrane include cholesterol which helps provide rigidity and support to the membrane.

The cell membrane is important in cytokinesis during cell division. In some organisms the membrane is modified with cilia for feeding and locomotion. Knowledge of the structure of microbial cell membranes has enabled researchers to develop antimicrobial agents to attack pathogenic bacteria and fungi.

The cell membrane has several important functions in the cell. These functions include:

• Protects cells from outside invaders such as viruses and bacteria.
• Maintains correct tonicity of cell contents by controlling the movement of water and dissolving in and out of cells.
• Provides a selectively permeable barrier through which some molecules can easily move in or out of the cell while others cannot.
• Molecules can move across membranes by passive transport that does not require energy, such as simple diffusion, or by active transport that does require energy, such as the sodium-potassium pump.
• Plays a role in signal transduction mechanisms at the cell surface.

What is a Plasma Membrane?

The plasma membrane is the membrane that surrounds the individual organelles or contents of a cell. The cell membrane is a type of plasma membrane that encloses the cell. There is a plasma membrane found surrounding the cytoplasm and the contents of the cell as well as surrounding individual organelles such as chloroplasts and mitochondria.

This means that plasma membranes have several different functions depending on where they are located. The structure of the membrane can vary depending on what surrounds it, whether it is the whole cell or the organelles within the cell.

The plasma membrane always provides a degree of protection regardless of what structure is around it, and many also play a role in the phosphorylation reaction. The plasma membrane surrounding the organelle will have slightly different functions depending on the organelle it surrounds.

The plasma membranes of organelles still have selective permeability and carefully control what substances enter and leave the organelle, but the types of molecules required by the organelle will be different. The molecules that need to enter and leave the mitochondria compared to chloroplasts for example, will differ at times because the organelles have different functions.

For example, pyruvate needs to enter the mitochondria for aerobic cellular respiration to occur, while carbon dioxide needs to enter the chloroplast for photosynthesis to occur.

Difference between Cell Membrane and Plasma Membrane

Location

The cell membrane is only found surrounding the contents of the cell while the plasma membrane can also be found surrounding certain organelles.

Tonicity
While the cell membrane is always involved in helping to regulate tonicity, this is not the case for all plasma membranes.

Eating and moving
Cell membranes can be modified in organisms to make it easier to eat and move, for example Paracemium has cilia to eat and move. Other plasma membranes are not modified in this way.

Protection
The cell membrane helps protect the cell from bacterial and viral intrusion, while other plasma membranes within the cell do not have this function.

Signal transduction
The cell membrane contains the initial receptors for signal transduction mechanisms, and thus is the first step in which messages are received from other cells during signal transduction. The plasma membrane surrounding the organelle is not the first step for signal transduction, although it can be involved in the process.

Cytokinesis
Phospholipids in cell membranes play a role in cytokinesis during cell division. This is not the case with plasma membranes which are not cell membranes.

Targets for antimicrobials
Bacterial and fungal cell membranes can be targeted by antimicrobial agents. The plasma membrane is generally not targeted unless it is a cell membrane.