Cell Membrane: Components and Functions

Introduction: 

We know that all organisms consist of a basic unit of life called a cell. The body of lower organisms like bacteria, some amoeba is made up of a single cell, whereas the body of higher organisms like plants and animals are made up of many cells. Hence, they are multicellular organisms. 

 Cells vary in size and shape. Based on their function, the size and shape change. Cells are very tiny and can be observed by using a microscope. Let us discuss about the structure and function of cell organelles. 

Cell components: 

The major parts of the cell are cell membrane, cytoplasm and nucleus. 

Cell membrane: It is sometimes also called the plasma membrane. It is the outermost layer of an animal cell, and in plant cells it is an internal membrane to the cell wall. It is a very flexible membrane. It can be folded in and folded out. Plant and animal cell contains cell membrane. 

It is composed of proteins and lipids which are arranged in the bilipid layer. The upper and lower layer contains phospholipid and protein are embedded between the two layers. 

If you observe the cell membrane under microscope the structure looks like a network shown below

Functions of cell membrane (plasma membrane) 

All the cell organelles are enclosed inside the plasma membrane. 

Plasma membrane gives proper shape to the cell.  

It allows the transport of selective substances inside the cell and outside the cell. Hence, the plasma membrane is called a selectively permeable membrane. 

Plasma membrane helps to keep the adjacent cells in contact. 

The cell membrane allows transport of small molecules like water, mineral ions, glucose, amino acids, oxygen, carbon dioxide etc.). The movement of these small molecules across the plasma membrane is done by any one of the following methods: 

Osmosis:

In this method, the water molecules move from the area of higher concentration (where the solute is less and water is more in the solution) to the area of lower concentration (where the solute is more and water is less) through a semipermeable membrane. In this process, energy is not used  . In osmosis, the only movement of water molecules takes place by diffusion. Here, the movement of solute does not occur. 

Solutions are separated by semipermeable membranes that can be divided into 3 categories – hypertonic, hypotonic, or isotonic, depending on the relative concentration of the solute in each solution. 

The capacity of movement of water differs in plants and animal cells due to the presence of cell wall. In plant cell, the outermost layer is of cell wall which is very rigid and permeable to very small molecules. 

Hypertonic solution:

This solution is a concentrated solution where the amount of dissolved solute is more. When a cell is kept in a hypertonic solution, you will observe that the cell shrinks (plasmolysis), because the cell loses water into concentrated solution present out outside the cell. The water moves from a higher concentration (more water area) present in the cell to a lower concentration area (less water and more solute) present in hypertonic solution. In the process of plasmolysis, you will see that the cell membrane pulls away from the cell wall. 

HYPERTONIC SOLUTION 

Hypotonic solution: 

When a cell is placed in a hypotonic solution, the water enters the cell through the membrane, and the cell swells. 

HYPOTONIC SOLUTION 

Isotonic solution: 

When the cell is placed in an isotonic solution, no change will occur in the cell because the concentration of the solution is same inside and outside the cell. Hence, the amount of solute and solvent transported in and out of the cell is the same. 

ISOTONIC SOLUTION 

Diffusion:

In this method, the solute and solvent molecules move freely from the higher concentration area to the lower concentration area to reach an equilibrium. In this method, energy does not spend. For example, the diffusion of perfume spray in the air, diffusion of insecticide spray in the air. Diffusion and osmosis are passive transport mechanisms as they do not require energy for the process to take place. 

Active transport:

In this method, the direction of movement of molecules is in the opposite way, i.e., from lower concentration (more water less solute) to higher concentration (more solute, less water). Hence, active energy is required by the cell to complete the movement. This energy which is required for the movement is provided by ATP (Adenosine triphosphate). 

Cell wall:  

It is the outermost covering in plant cells and bacteria. It is present outside the cell membrane. The cell wall of bacteria is made up of peptidoglycan and the cell wall of the plant is made up of cellulose. 

The cell wall is rigid which gives shape to the cell. It protects the internal structure of the cell and also protects from mechanical stress. 

Cytoplasm: 

It is present between the cell membrane and the nuclear membrane. Cell organelles such as mitochondria, ribosomes, endoplasmic reticulum, nucleus etc. are present in the cytoplasm. Cytoplasm is a semi-liquid and jelly like substance. It is the place for many chemical reactions. Cytoplasm contains an aqueous solution called cytosol. 

Nucleus:  

It is a small, membrane bound (nuclear membrane) structure present in the cytoplasm of the cell. Nucleoplasm is the fluid which is present inside the nucleus. Nucleus contains the heredity information (DNA) which plays a major role in the cellular reproduction and cell growth. Nuclear membrane plays an important role in the transportation of materials in and out of the nucleus.