Comprehensive theory, key formulas, diagrams, and memory aids for The Fundamental Unit of Life.
While examining a thin slice of cork in 1665, Robert Hooke saw that the cork resembled the structure of a honeycomb consisting of many little compartments. He called these boxes "cells." Cell is a Latin word for "a little room." This might seem like a small observation, but it was a monumental discovery in the history of science. It was the first time someone noticed that living things consist of separate units.
In this chapter, we will look inside this "little room" and understand why the cell is called the fundamental structural and functional unit of life.
If you peel a piece of onion and observe it under a microscope, you will see a network of similar structures regardless of the size of the onion you used. These small structures that we see are the basic building units of the onion bulb. These structures are called cells.
Not all organisms are made of many cells. * Unicellular Organisms: Organisms constituted by a single cell. Examples: Amoeba, Chlamydomonas, Paramecium, and bacteria. In these organisms, a single cell performs all basic functions of life. * Multicellular Organisms: Organisms where many cells group together in a single body and assume different functions in it to form various body parts. Examples: fungi, plants, and animals.
Every multicellular organism has come from a single cell (the fertilized egg). Cells divide to produce cells of their own kind. Thus, all cells come from pre-existing cells (as proposed by Virchow in the Cell Theory).
If we study a cell under a microscope, we would come across three features in almost every cell: the plasma membrane, nucleus, and cytoplasm. All activities inside the cell and interactions of the cell with its environment are possible due to these features.
This is the outermost covering of the cell that separates the contents of the cell from its external environment. * Selective Permeability: The plasma membrane allows or permits the entry and exit of some materials in and out of the cell. It also prevents movement of some other materials. Therefore, it is called a selectively permeable membrane. * Diffusion and Osmosis: Substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion (movement from a region of high concentration to a region of low concentration). Water also obeys the law of diffusion. The movement of water molecules through such a selectively permeable membrane is called osmosis.
Plant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall. The cell wall lies outside the plasma membrane. * The plant cell wall is mainly composed of cellulose, a complex substance providing structural strength to plants. * When a living plant cell loses water through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis.
The nucleus has a double-layered covering called the nuclear membrane. The nuclear membrane has pores which allow the transfer of material from inside the nucleus to its outside (to the cytoplasm). * Chromosomes: The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide. Chromosomes contain information for inheritance of features from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules. * Chromatin: In a cell which is not dividing, this DNA is present as part of chromatin material (entangled mass of thread-like structures). * The nucleus plays a central role in cellular reproduction and determines the way the cell will develop and what form it will exhibit at maturity.
In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the absence of a nuclear membrane. Such an undefined nuclear region containing only nucleic acids is called a nucleoid. * Prokaryotic Cells: Cells lacking a nuclear membrane (pro = primitive; karyote = nucleus). They also lack most other cytoplasmic organelles. * Eukaryotic Cells: Cells having a well-defined nuclear membrane and membrane-bound organelles.
The cytoplasm is the fluid content inside the plasma membrane. It also contains many specialized cell organelles. Each of these organelles performs a specific function for the cell.
To keep their complex activities separate from each other, eukaryotic cells use membrane-bound little structures (organelles) within themselves.
New cells are formed in organisms in order to grow, to replace old, dead and injured cells, and to form gametes required for reproduction. * Mitosis: The process of cell division by which most of the cells divide for growth. A mother cell divides to form two identical daughter cells with the same number of chromosomes. * Meiosis: Specific cells of reproductive organs divide to form gametes. It involves two consecutive divisions resulting in four new cells. The new cells have only half the number of chromosomes than that of the mother cells.
The cell is a microscopic, highly organized factory. The plasma membrane acts as a selective gatekeeper, while the nucleus functions as the control center housing the DNA blueprint. Suspended in the cytoplasm are specialized organelles like mitochondria (power), ribosomes (protein synthesis), and lysosomes (waste disposal), each working in harmony to sustain life. Understanding the cell is the first step to understanding all biological processes in plants, animals, and humans.