The Cell theory was first formulated in the 19th century. According to the theory, all living organisms are made from basic units known as a cell. It is mentioned in the history of cell theory that all cells are formed from pre-existing cells, which form the basic structural and organisation unit. It also stated that the primitive cells were unicellular, which joined together to form a multicellular organism. 

Robert Hooke first discovered the cell in the year 1665. However, the cell theory was developed by Theodor Schwann and Matthias Jakob Schleiden and suggested that each structural part of the living organisms was made of cells. The topic discusses cell theory and how has history changed over time.

The Timeline for the History of the Cell Theory

Cell theory is one of the foundational principles of biology as it helps show the relationship between cells and living things. Here are the basic tenets of the cell theory:

• All living organisms are made of one or more cells. Single-celled organisms are called unicellular, whereas organisms with more than one cell are called multicellular.
• The cells arise from the pre-existing cells and are the basic structural and organisation unit in the living organisms.

The timeline of the history of the cell theory is as follows:

• The first compound microscope was invented in 1590 by Zacharias Janssen.
• Robert Cooke, in 1665, used a light microscope for the first time. He visualised thin slices of plant tissues and saw chambers within them. He coined the term “cell” for the chambers.
• In 1673 Anton Van Leeuwenhoek saw living unicellular and microscopic organisms in pond water.
• In 1838, Matthias Schleiden stated that “all the living plants are made of cells”.
• In 1839, Theodore Schwann found out that “all the living animals are made of cells.”
• In 1855, it was stated and concluded by Rudolph Virchow that all the cells exist from pre-existing cells.

History of Cell Membrane Theory

The cell theory is almost a decade old, but it was nearly just 200 years ago that the cell membrane theory was established. The cell membrane theory was established in the 19th century with the finding that the cell is separated from the outer environment through a semipermeable membrane. The existence of the semi-permeable membrane and its lipid bilayer was concluded only in 1925 after many experiments. It was found that proteins play a critical role in the cell membrane with time. The cell membrane is also called a fluid mosaic model with proteins floating in the lipid bilayer.

How Has History Changed Over Time for Cell Membrane 

Many experiments and early theories eventually led to the formulation of the fluid mosaic model for the cell membrane.

• For the first time, the lipid nature of the cell membranes was detected by Quincke. He stated that when broken into two halves, the cell forms two spheres and remains spherical in water. He also found a semi-permeable membrane, which is a thin oil layer.
• With experiments done by Hans Horst Meyer and Ernest Overton, it was found in the history of cell theory that the cell membrane consists of phosphatidylcholine and cholesterol.
• The presence of a semi-permeable membrane that acts as a solute barrier but is permeable to solvent was developed in the 18th century. Ernest Overton first proposed the presence of oil (lipid) in the plasma membrane in 1899.
• Harvey proposed it, and James Danielli in 1939 stated that the surface tension in the cell membrane could be measured due to the presence of protein on each side of the lipid bilayer.
• After going through a series of experiments and immense research, the history of the cell theory concludes that cell membranes exist in a fluid mosaic model. Singer and Nicholson, in 1972, were the first to develop the fluid mosaic model of the cell membrane.
• According to the model, the cell membrane consists of a lipid bilayer which consists of phospholipids. It consists of peripheral and integral proteins present along with the lipid bilayer. The fluidity of the membrane is maintained due to the presence of cholesterol.

Content of the Fluid Mosaic Model of Cell Membrane 

The history of cell membrane theory has changed over time with evolution and experiments. According to the fluid mosaic model, the cell membrane consists of the following:

• The phospholipid is the main component of the cell membrane. These are amphiphilic, which means it has both polar and nonpolar groups. It forms a lipid bilayer where the hydrophobic tail forms the core, and the hydrophilic heads interact with extracellular water.
• It consists of proteins where the integral proteins are embedded in the lipid bilayer matrix. The peripheral proteins are loosely spread across the membrane surface through weak interactions.
• Carbohydrate is present, which helps with cell signalling. The carbohydrate is attached to either lipid or protein, called glycolipid or glycoprotein.

Conclusion

The history of the cell theory begins in the 16th century stating that a cell is the basic unit of life in living organisms. Cells were considered unicellular, and each cell existed from a pre-existing cell due to the ability of the cells to divide. Only by the 19th century it was found that the cells were separated from their extracellular environment. The cell is separated by a cell membrane, which follows a fluid mosaic model which consists of a lipid bilayer made of phospholipids, proteins (integral and peripheral), and carbohydrates. Hence a complete evolution of the concept of cells and cell membrane has occurred over time.