What is Cell in Human Body ? Functions and Components of Cell?

A cell is the basic structural and functional unit of all living organisms. All living organisms can be categorized into unicellular (e.g., protozoa, yeast, and bacteria) or multicellular organisms (e.g., plants and humans). A human body consists of about 100 trillion cells with a size and mass of 10µm and 1 nanogram respectively.

Cells are of two types: 

1) Prokaryotic Cells: 

These cells do not have a well-developed nucleus, though they have some other organelles present as in the eukaryotic cells. For example, archaebacteria, etc. 

2) Eukaryotic Cells: 

These cells have a well-developed nucleus and other organelles. For example, plants, animals, etc. A cell performs the following functions: 

1) Cell Growth and Metabolism: 

Cells grow either by increasing their number or their size. During the metabolism, various anabolic and catabolic processes occur. In catabolic processes, the breakdown of complex molecules occurs within the cells to produce energy; while during the anabolic processes, cells utilize energy to synthesize complex biomolecules and perform other biological functions. 

2) Cell Division: 

It involves the splitting of a single cell, i.e., the mother cell splits into two daughter cells. This results in the growth of multicellular organisms and the breeding (vegetative reproduction) of unicellular organisms. During the cell division, DNA is also synthesized and the process is known as replication. 

3) Protein Synthesis: 

It occurs within the cells, by two major steps, i.e., transcription and translation. 

4) Molecule Transport: 

The cell must be able to acquire nutrients and other molecules to survive.

Components of Cell 

A cell comprises the following three basic components: 

1) Cell membrane (or plasma membrane), 

2) Cytoplasm, and 

3) Nucleus. 

Cell Membrane/Plasma Membrane:-

The cell membrane is a selectively permeable biological membrane, allowing only certain substances to pass through it. It acts as a barrier between the inner and outer surface of the cell. It is also known as the plasma membrane or plasmalemma. It comprises mainly of proteins and lipids, along with other living molecules. These molecules are important for the normal functioning of the cells, e.g., in an ion channel conductance, cell adhesion, and cell signalling.

Properties of Cell Membrane:-

 1) It guards the cell against the external environment. 

2) It allows cellular transport between cells. 

3) It has transmembrane receptors (binding the signaling molecules) which helps in signal transduction and provide selective receptivity. 

4) It helps in recognising a cell. 

5) It has binding sites for the constituents of the cytoskeleton present in the external environment. 

6) It divides the cell into smaller sections having different structures and functions. 

7) It offers a stable site for the catalysis and binding of enzymes.

8) It allows the fusion of cell membrane with other membranes of cell organelle through special points of attachment. 

9) It regulates the transfer of certain specific molecules through the cell membrane, e.g., gap junctions. 

10) It enables motility of the cellular organelles. 

Structure (Fluid Mosaic Model):

To explain the nature and functions of cell membrane various models had been proposed in past, as mentioned below: 

1) Danielli and Davson model/Lamellar theory, 

2) Unit Membrane model, 

3) Robertson’s model, 

4) Lipid Bilayer model, and 

5) Fluid Mosaic model.

Structure of the Cell Membrane

However, out of the above given models, the Fluid Mosaic Model (figure 2.2) is the most accepted one. The fluid mosaic model of the plasma membrane was given by S.J. Singer (University of California) and G.L. Nicholson (Salk Institute) in the early 1970s. 

According to this model, proteins do not form a continuous layer extending from one side of the membrane to the other rather it is present in the form of globules, dotted randomly in a mosaic pattern. Some globules of the protein penetrate into the membrane to deeper extents; some of them remain attached to the membrane surface, while some globules extend through it and adhere to the other side. It explains the rigidity of the cell membrane. Experiments on viscosity of membrane show that it has a fluid-like consistency. This fluidity allows movement of lipid and protein molecules within it. Due to the fluidity and mosaic arrangement of the protein molecules, this model is known as the Fluid Mosaic Model.

Functions of Cell Membrane:

Cell membrane has the following functions: 

1) Protection of Cells: The receptors present on the cell membrane allow chemical messages to pass between the ells and systems, and thus provides protection to cells.

2) Providing Cytoskeleton and Mechanical Support: It provides a specific shape to the cell. It provides mechanical support and maintains the membrane potential. Attachment proteins present on cell membrane bind the extracellular and intracellular components. 

3) Separation from Surroundings: In an animal cell, it separates the internal environment from that of the external. Other organisms, including fungi, plants and bacteria, consist of an additional layer known as cell wall, forming the outermost protective layer. 

4) Permeability: It is a selectively permeable membrane as it allows the passage of solvent and several selected solutes while holding back the others. 

5) Tissue Formation: Groups of similar cells together form tissues. 

6) Particle Transportation: This occurs by various mechanisms like ion channel, carrier proteins, and ion pumps. Chemical exchange of solute between extracellular medium and cytosol is mediated by transport of channel and carrier proteins. 

7) Contribution to Enzyme Activity: It controls the activity of enzymes which further participate in metabolism and immune system. 

8) Control of Neurohormonal Flow: Receptor proteins on cell membrane mediate the flow of neurohormonal and other chemical information from extracellular medium to the cytosol.