Plasma Membrane

An important component of a cell's function is the ability to control what can come into and out of the cell, but what separates the inside from the outside? This article will discuss the plasma membrane: its definition, structure, components, and function.

What is the Definition of the Plasma Membrane?

The plasma membrane -also known as the cell membrane- is a selectively permeable membrane that separates the cell's internal contents from its outside environment. Cells of plants, prokaryotes, and some bacteria and fungi, have a cell wall bound to the plasma membrane outside the cell.

Both prokaryotic and eukaryotic cells have a plasma membrane. The structure and components of the cell membrane are shown in Figure 1.

Fig. 1. The basic structure of the cell membrane. The core of the membrane is composed of a bilayer of phospholipids, which are the red balls with the two yellow tails.

What is the Structure of the Plasma Membrane?

The plasma membrane is organised into a fluid mosaic model composed of two layers of phospholipids into which proteins and carbohydrates are inserted.

Plasma Membrane Diagram: Fluid Mosaic Model

The fluid mosaic model is the most widely accepted model describing the structure and behaviour of the cell membrane. According to the fluid mosaic model, the cell membrane resembles a mosaic: it has many components, including lipids, proteins, and carbohydrates that make up the membrane plane. These components are fluid, meaning they move freely and constantly slide past one another. Figure 2 is a simple diagram showing the fluid mosaic model.

Fig. 2. The fluid mosaic model illustrates the cell membrane as a mosaic of protein molecules embedded and freely moving in a fluid bilayer of phospholipids.

What are the Components of the Plasma Membrane?

The plasma membrane is mainly composed of lipids (phospholipids and cholesterol), proteins, and carbohydrates. In this section, we will discuss each component.

Lipids (Phospholipids and Cholesterol)

Phospholipids are the most abundant lipids in the plasma membrane. A phospholipid is a lipid molecule made of glycerol, two fatty acid chains, and a phosphate-containing group.

Phospholipids are amphipathic molecules. Amphipathic molecules have both hydrophilic ("water-loving") and hydrophobic ("water-fearing") regions.

• The phosphate group makes up the hydrophilic head.
• The fatty acid chains make up the hydrophobic tails.

The cell membrane usually has two layers of phospholipids, with the hydrophobic tails facing inward and the hydrophilic heads facing outward. This arrangement is called a phospholipid bilayer. This arrangement is illustrated in Figure 3.

The phospholipid bilayer acts as a stable boundary between two water-based compartments. The hydrophobic tails attach to one another; they form the interior of the membrane. On the other end, the hydrophilic heads are exposed to aqueous fluids inside and outside the cell.

Fig. 3. This diagram illustrates the phospholipid bilayer.

Cholesterol is another lipid that is found in the membrane. It is composed of a hydrocarbon tail, four hydrocarbon rings, and a hydroxyl group. Cholesterol is embedded among the phospholipids of the membrane. It helps to maintain the fluidity of the membrane during temperature changes.

Phospholipids are the main component of the plasma membrane, but proteins determine most of the membrane's functions. Proteins are not randomly distributed in the membrane; instead, they are often grouped in patches that carry out similar functions.

Two main types of proteins are embedded in the cell membrane:

1. Integral proteins are integrated into the hydrophobic interior of the phospholipid bilayer. They can either 1) only partially go into the hydrophobic interior or 2) span across the entire membrane, known as transmembrane proteins. Transmembrane proteins are the most abundant proteins in the plasma membrane.

2. Peripheral membrane proteins are usually attached to integral proteins or phospholipids. They are found on surfaces inside and outside of the membrane. They do not extend into the hydrophobic interior of the membrane; instead, they are usually loosely attached to the surface of the membrane.

Carbohydrates

Carbohydrates (sugars and sugar chains) are attached to proteins or lipids to help cells recognize each other.

• When carbohydrate groups are attached to proteins, the molecules are called glycoproteins.

• When carbohydrate groups are attached to lipids, the molecules are called glycolipids.

Glycoproteins and glycolipids are usually found on the extracellular part of the cell membrane. These are different for each species, among individuals of the same species, and even among the various cells of an individual. The uniqueness of the glycoproteins and glycolipids and their position on the surface of the plasma membrane enables them to function as cellular markers that allow cells to recognize each other.

What is the Function of the Plasma Membrane?

The plasma membrane serves various functions depending on the type of the cell. These functions include structural support, protection, regulation of movement of substances into and out of the cell, and communication and cell signalling.

Structural Support and Protection

The cell membrane is a physical barrier separating the cytoplasm from the extracellular fluid. This allows activities (such as transcription and translation of genes or production of ATP) to occur inside the cell while minimizing the impact of the external environment. It also provides structural support by binding to the cytoskeleton.

Regulation of Substances Moving into and out of the Cell

The cell membrane controls the movement of molecules into and out of the cytoplasm. The semi-permeability of the cell 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 toxins are blocked from or expelled out of the cell.

Communication and Cell Signalling