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The Law

Belief - II

Our Cellular Biology - I

Every living thing is made of cells, the fundamental building block of life. A cell is equipped with all capabilities to carry out the functions of life. It can eat, discharge waste, communicate with other cells, reproduce and replicate itself and it can respond to the environment and changes in the environment and adapt to such changes. 

Cells can live singly, as in bacteria, and can live in communes as in humans, animals and plants. It is estimated that a human body is made from 50 trillion or more cells. Most cells are too small for seeing with naked eye. The development of electron microscope has helped greatly in understanding cells.

Cells come in different shapes and sizes. A human nerve cell could be a meter long starting from the backbone and ending in the big toe that serves to transmit signals quickly. Some of the cells in the intestine have dozens of finger like projections which increases their surface area. Muscle cells have the elasticity to change their shape and size as we flex our muscles. The cellular biology, that we see today, has evolved over billions of year by progressively adapting itself to the environment.

Cells are three dimensional. Each cell is engulfed by a membrane known as cell membrane or plasma membrane. The membrane separates the interior of the cell from the outer environment. The membrane has a thickness of about 7 millionth of a millimeter. Cell membrane framework is made of fat based molecules called phospholipids. The phospholipids prevent water solvant substances from entering or exiting the cell interior. The membrane is studded with different substances like cholesterol and proteins. These proteins serve different purposes. Very simply speaking, proteins are chains of amino acid molecules and analogous with a chain containing a series of beads. 

We will need to understand the membrane in a greater detail. 
The plasma membrane that surrounds these cells has two layers of phospholids, which at body temperature are like vegetable oil (fluid). Each phospholipid molecule has a head that is attracted to water (hydrophilic) and a tail that repels 

water (hydrophobic). The phospholipids look like lollipops with two sticks. Both layers of the plasma membrane have the hydrophilic heads pointing toward the outside; the hydrophobic tails form the inside of the bilayer. See the diagrams.

Because cells reside in a watery solution (extracellular fluid - outside the cell), and they contain a watery solution inside (intracellular fluid - inside the cell), the plasma membrane forms a circle around each cell so that the water-loving heads are in contact with the fluid, and the water-fearing tails are protected on the inside.

Consider this analogy for the membrane construction. Visualise two layers of bread (hydrophilic), which are of course porous, with a slab of cheese (hydophobc) between them. The flaw in this analogy is bread is not fluid whereas the membrane is fluid.

Proteins and substances such as cholesterol become embedded in the bilayer, giving the membrane the look of a mosaic.

The molecules that are embedded in the plasma membrane also serve a purpose. For example, the cholesterol that is stuck in there makes the membrane more stable and prevents it from solidifying when your body temperature is low. (It keeps you from literally freezing when you’re “freezing.”) Carbohydrate chains attach to the outer surface of the plasma membrane on each cell. These carbohydrates are specific to every person, and they supply characteristics such as your blood type.

Some of these protein molecules act like gatekeepers in a casino who determine who can or can not cross the doors of the casino (membrane). Known as the transport proteins, these proteins make the membrane permeable to only certain types of molecules to enter and exit the interior of the cell. The mathcing colors in the diagram shows that each transport protein is specific to a certain type of molecule. Coming back to our analogy of a bread sandwich with cheese in between the two slices of bread, visualise that the sandwich is embedded with lots of hollow cylinders. Visualise that these cylinders have gates for permitting entry of other substances and exit of substances from the cell interior. The gates of some of these cylinders are always closed, whereas the gates of other cylinders open for permitting selective entry and exit from the cell interior. These hollow cylinders are analogous with the protein molecules embedded in the membrane.

Some of these protein molecules act as markers of cells belonging to the same organism and which are foreign cells. Some proteins bind cells together so that the bonded cells act as a team. Some proteins act as communicators, exchanging signals from other cells and the environment.

The interior of the cells, called cytoplasm, consists of 70% water and 30 % other cellular constituents, some of which are structural and some functional.

See you again

Namaste

Prabir