Structure and Function of the Cell Membrane

Section 1: Structure and Function of the Cell Membrane

A country has boundaries for its land, sea, and airspace; similarly, the skin and mucous membranes that separate the internal and external environments of the human body form its boundaries. System boundaries are crucial for system stability. As a fundamental life system, the boundary of a cell is the cell membrane (also known as the plasma membrane).

What role does the cell membrane play in the life activities of cells?

Function of the Cell Membrane The cell membrane serves as the boundary between the cell and the external environment. In the process of the origin of life, organic molecules in the primitive ocean (Figure 3-1) gradually aggregated and interacted, evolving into primitive life forms. In this "primordial soup" of the primitive ocean, the emergence of membranes was a crucial stage in the origin of life, separating life materials from the external environment, creating primitive cells, and forming relatively independent systems. The cell membrane ensures the relative stability of the internal environment of the cell.

Control of Substance Entry and Exit from the Cell The cell membrane acts like a customs or border checkpoint, rigorously "checking" substances entering and exiting the cell. Generally, nutrients needed by the cell can enter from the external environment, while substances not needed by the cell find it difficult to enter. The examples discussed above in the "problem exploration" illustrate the controlling role of the active cell membrane in substance entry into cells. Substances such as antibodies and hormones synthesized within the cell are secreted outside the cell, and waste produced by the cell is also discharged outside; however, useful components within the cell are not easily lost to the outside. Of course, the controlling effect of the cell membrane is relative; some harmful substances in the environment may enter, and some viruses and bacteria can invade cells, causing diseases.

Facilitation of Intercellular Communication In multicellular organisms, individual cells do not exist in isolation; they must maintain functional coordination to ensure the organism's healthy existence. This coordination depends not only on the exchange of substances and energy but also on the exchange of information. There are various ways for cells to communicate information with each other.

Multicellular organisms are busy and orderly cellular "societies." Without information exchange, an organism cannot function as a whole. Most intercellular communication is related to the structure of the cell membrane.

The functions of the cell membrane are determined by its composition and structure. However, the cell membrane is very thin, and even under high-power microscopes, its true appearance remains elusive. Understanding of the chemical composition and structure of the cell membrane has undergone a long process.

Exploration of Cell Membrane Structure Research into the composition of the cell membrane has found that it is mainly composed of lipids and proteins, with small amounts of carbohydrates. Lipids account for about 50% of the total mass of the cell membrane, proteins about 40%, and carbohydrates 2% to 10%. Among the lipids that make up the cell membrane, phospholipids are the most abundant, with small amounts of cholesterol. Proteins play an important role in the function of the cell membrane, so the more complex the function of the cell membrane, the greater the variety and quantity of proteins.

How are lipids and proteins and other components assembled into the cell membrane? In the 1940s, it was speculated that lipids were covered on both sides by proteins. In 1959, Robertson (J.D. Robertson) saw a clear three-layer structure of the cell membrane under an electron microscope (Figure 3-3). Combined with the work of other scientists, he boldly proposed a hypothesis about the model of the cell membrane: all cell membranes are composed of a protein-lipid-protein three-layer structure, with the middle bright layer seen under the electron microscope being lipid molecules, and the dark layers on both sides being protein molecules. He described the cell membrane as a static unified structure.

After the 1960s, there were increasing objections to this model. Many scientists questioned the idea that the cell membrane was static: if so, the complex functions of the cell membrane would be difficult to achieve, and even phenomena such as cell growth and amoeboid movement would be difficult to explain.

In 1970, scientists marked protein molecules on the surface of mouse cells with green fluorescent dye and protein molecules on the surface of human cells with red fluorescent dye, then fused mouse cells and human cells. When these two cells fused, half of the fused cells emitted green fluorescence and the other half emitted red fluorescence. After 40 minutes at 37°C, both colors of fluorescence were evenly distributed (Figure 3-4). This experiment and other related evidence indicate that the cell membrane has fluidity.

Based on new observations and experimental evidence, scholars have proposed several molecular structural models of the cell membrane. Among them, in 1972, Singer (S.J. Singer) and Nicolson (G. Nicolson) proposed the fluid mosaic model, which has been widely accepted by most people.

Basic Content of the Fluid Mosaic Model The fluid mosaic model suggests that the cell membrane is mainly composed of phospholipid and protein molecules. The phospholipid bilayer is the basic framework of the membrane, with hydrophobic ends of phospholipid molecules inside, preventing water-soluble molecules or ions from freely passing through, thus acting as a barrier. Protein molecules are embedded in the phospholipid bilayer in different ways: some are embedded on the surface of the phospholipid bilayer, some are partially or entirely embedded in the phospholipid bilayer, and some traverse the entire phospholipid bilayer (Figure 3-5). These protein molecules play important roles in substance transport and other aspects.

The cell membrane is not static but has fluidity, mainly manifested in the lateral movement of phospholipid molecules that make up the membrane and the mobility of most membrane proteins. The fluidity of the cell membrane is crucial for the cell to perform functions such as substance transport, growth, division, and movement.

Further research into the cell membrane has found that there are also carbohydrate molecules on the outer surface of the cell membrane. These carbohydrate molecules combine with protein molecules to form glycoproteins or with lipids to form glycolipids. These carbohydrate molecules are called glycocalyces. Glycocalyces play important roles in cellular activities, such as cell surface recognition and intercellular communication.