What is the Cell Membrane?

Despite differences in structure and function, all living cells in multicellular organisms are surrounded by a cell membrane. Just like the outer layer of the skin separates the body from its environment similarly, the cell membrane, also known as 'plasma membrane,' separates the inner content from its exterior environment.

Significance of the Cell Membrane

The cell membrane provides a protective barrier around the cell and regulates which materials can enter or exit the cell. The cell membrane is composed of certain macromolecules that maintain its rigidity and permeability. Various experiments were performed to study the cell membrane structure, but the "fluid mosaic model" is the widely accepted model among all. The plants have a cell wall surrounding the plasma membrane that provides additional rigidity to the cell. The cell has to perform various activities like cell communication, adhesion, signaling, and exchange. These vital activities are performed by various components of the cell membrane. The disruption of the cell membrane releases all the cytoplasmic content, resulting in cell death.

Structure of the Cell Membrane

The structure of the cell membrane can be explained through the fluid mosaic model.

Fluid Mosaic Model

Singer and Nicolson gave this model. They proved that with the two-dimensional liquid including lipid and proteins in the membrane, two lipid layers of lipidare formed, making the plasma membrane a phospholipid bilayer.

Lipid bilayer

The phospholipid layer is formed by self-assembly. The phospholipid in the membrane has lipid with a phosphate group. The amphipathic phospholipids are arranged so that the hydrophobic tail is away from the surrounding water, and the water surrounds the hydrophilic part. This structure makes the lipid bilayer. The hydrophobic interactions are the major forces in this membrane. Other non-covalent interactions like Vander Waal forces and hydrogen bonding are also present. This layer is impermeable to polar and ionic molecules such as amino acids, proteins, ions, nucleic acids. The cell membrane maintains the flow of molecules with the help of transmembrane protein complexes, including the channels and porins. 

Membrane structures

The cell membrane form different supramembrane structures, including podosome, focal adhesion, cell junctions, and caveola. These structures assist in cell adhesion, endocytosis, communication, and exocytosis. Certain membrane proteins like cadherins and integrins can be observed under electron microscopy.

Cytoskeleton

It is the underlying component of the cell membrane. It provides scaffolding to the components of the membrane that allow them to anchor to the organelles in the cytoplasm. The extensions like cilia and fimbriae are ensheathed in the membrane and rise from the same to help recognize the external environment. 

Intracellular membranes

The inside of the cell membrane has various other membranes that enclose the organelles like the nucleus, protecting it from the other components in the cytoplasm. These membrane-bound organelles contribute to the overall functionality of the cell. The chloroplast and mitochondria were studied to be the first membranous systems produced from the host's plasma membrane after a large cell engulfed an aerobic bacterium and cyanobacteria.

Variations

Certain cell membranes perform different work than the regular plasma membrane. The sarcolemma is the plasma membrane of a muscle cell, and it transmits the synaptic signals needed for muscle contraction. Oolemma is the membrane of the oocyte that does not contain lipid. The axolemma is the plasma membrane of the axons in the nerve cells that generate action potentials. Thus, there are exceptions to plasma membrane functionality.

Composition of the Cell Membrane

The plasma membrane has many types of biological molecules, especially lipids and proteins. These membrane components make the membrane true for the fluid mosaic model. The membrane proteins change their fluidity according to the environment. The change in the cholesterol amount in the plasma membrane helps determine the fluidity of the membrane. 

Lipids

Three types of amphipathic lipids are found in the membrane: phospholipids, sterols, and glycolipids. The type of cell determines the amount of each lipid. However, phospholipid usually has a higher concentration than the other two lipids. The glycolipids account for only 2%, phospholipids make more than 50%, and the sterols cover the rest. The fatty acid chains in the lipid bilayer and those attached to the carbohydrates usually contain an even number of carbon atoms. Some organisms can fluctuate the fluidity of the phospholipid membrane by altering the lipid composition, a process called home viscous adaptation. The entire cell membrane is held by covalent interactions. Moreover, the fluidity and flexibility of the membrane account for its liquid crystalline state that allows the lipid molecules to diffuse along with the layer.

Carbohydrates

The plasma membrane also has carbohydrates, especially glycolipids and glycoproteins, that are important for cell recognition. These carbohydrates are present on the cell's surface, from where they can locate the host cell and share information. The glycocalyx is important in the cells as it contributes to cell adhesion. The penultimate sugar present in the membrane is galactose.

Membrane proteins

Certain proteins like transmembrane proteins, lipid anchored proteins, and peripheral proteins present in the plasma membrane. The transmembrane proteins span the membrane. It has a cytosolic domain that interacts with hydrophobic and hydrophilic domains. Examples of transmembrane proteins are ion channels, and G-protein coupled receptors. The lipid anchored proteins are the membrane protein that is covalently bound to lipid molecules. The lipid portion is inserted into the plasma membrane and is attached to the proteins that are not in direct contact with the plasma membrane. The peripheral proteins are attached to the transmembrane proteins to interact with the associated biological membrane temporarily.

Functions of the Cell Membrane

• To regulate the movement of selected molecules to and from the cell.
• To aid in the synthesis of ATP (adenosine triphosphate) by chemiosmosis in the mitochondria.
• To facilitate endocytosis and exocytosis.
• To conduct osmosis and diffusion.

Common Mistakes

The cell wall and the cell membrane are different; the cell wall is found in plants, while the plasma membrane is found in plants and animals.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelor of Science in Biology
• Master of Science in Zoology

Related Concepts

1. Cell wall
2. Exocytosis
3. Phagocytosis
4. Nuclear membrane