Cell Structure and Function

The cell structure consists of individual elements with specific functions. These components include the cell membrane, cell wall, cytoplasm, nucleus, and cell organelles. Let’s study them in detail:

Cell Membrane

The cell membrane, also called the plasma membrane, is the membrane present in all cells that separate the cell interior from the outside environment. In bacterial and plant cells, the cell wall is attached to the outer surface of the cell membrane.

Structure of Cell Membrane

The main components of the plasma membrane are phospholipids, proteins, and carbohydrate groups related to certain lipids and proteins.

• Phospholipids: The membrane is made up in part of molecules called phospholipids, which spontaneously assemble into a bilayer with hydrophilic (water-loving) heads on the outside and hydrophobic (water-hating) tails are on the interior of the bilayer. These interactions with water allow for the formation of plasma membranes.
• Proteins: Proteins lie between the lipids that make up the membrane, and these transmembrane proteins allow the passage of molecules that otherwise would not be able to enter the cell, creating channels, pores, or gates. The cell thus controls the flow of these molecules as they enter and exit. Cell membrane proteins play a role in many other functions, such as cell signaling, cell recognition, and enzyme activity.
• Carbohydrates: Carbohydrates are also found in the plasma membrane. In particular, most membrane carbohydrates are part of the glycoproteins that are formed when a carbohydrate binds to a protein. Glycoproteins play a role in cell-cell interactions, including cell adhesion, the process by which cells stick together.

Fluid Mosaic Model

In 1972, Singer and Nicolson proposed a flowing mosaic pattern. According to this model, the near-liquid nature of lipids allows lateral movement of proteins across the bilayer. This ability to move within the membrane is measured by its fluidity. The plasma membrane consists of lipids arranged in bilayers and within the membrane, with the pole head pointing outward and the hydrophilic tails pointing inward. This protects the non-polar tail of the saturated hydrocarbons from the aquatic environment. The cell membrane is comprised of 52% proteins and 40% lipids. Peripheral proteins reside on the surface of the membrane while integral proteins are partially or fully buried within the membrane.

Functions of Cell Membrane

Some of the activities performed by the cell membrane are described below:

• Physical barrier: The plasma membrane separates the cytoplasm from the extracellular fluid outside the cell, protecting all cellular components from the external environment and allowing separate activities inside and outside the cell.
• Structural Support: Protects the cytoskeleton, a network of protein fibers within the cell that holds all parts of the cell together and gives the cell its shape.
• Selective Permeability: Plasma membranes are selectively permeable (or semipermeable), meaning that only molecules such as water, oxygen, and carbon dioxide can pass through them with ease. But the membrane can control how fast certain molecules, such as sodium, potassium, and other polar molecules, can move in and out of the cell.
• Endocytosis and exocytosis: Endocytosis allows a cell to take up large amounts of molecules or even whole bacteria from the extracellular fluid. Exocytosis occurs when the cell releases these materials. The cell membrane plays an important role in both processes.
• Cell signaling: Another important function of the membrane is to facilitate communication and signaling between cells. It does this by using different proteins and carbohydrates in the membrane.

What is Cell Wall?

Cell walls are the hard, inanimate objects that make up the plasma membrane’s outer covering in fungi and plants. The majority of plant cells, as well as those of fungi, bacteria, algae, and some archaea, have this outer layer right near the cell membrane. Animal cells, however, do not have a cell wall.

Class 11 Biology NCERT Notes for Chapter 8 Cell The Unit of Life: Cells are the basic units of life and serve as the building blocks of all living organisms. They differ in structure, composition, and function, but have similarities. The human body is made up of more than trillions of cells. They give structure to the body, extract nutrients from food, convert those nutrients into energy, and perform specific functions.

NCERT Notes for Chapter 8 Cell The Unit of Life further explains that cells also contain the body’s genetic material and can make copies of themselves. A bacterium or yeast is often a distinct, complete organism as it is single-celled or unicellular. Other cells acquire special functions as they mature. Understanding the structure and function of cells is crucial for comprehending the complexity of biological systems.