BOTANY - THE CELL

PART 1

(UPSC,TNPSC,TRB Imporatant Notes)

The cell (from Latin cella, meaning "small room") is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life. Cells are often called the "building blocks of life". The study of cells is called cell biology or cellular biology.

Quick Facts:
Robert Hooke : Discovered and coined the term cell in 1665

Robert Brown: Discovered Cell Nucleus in 1831

Schleiden and Schwann: Presented The cell theory, that all the plants and animals are composed of cells and that the cell is the basic unit of life. Schleiden (1838) and Schwann (1839).
 
Note: With the discovery of the electron microscope in 1940, it was possible to observe and understand the complex structure of the cell and its various organelles.

CELL ORGANELLES :

1.    Plasma Membrane or Cell Membrane
•    Cell membrane is also called the plasma membrane.
•    It can be observed only through an electron microscope.
•    Plasma membrane is the outermost covering of the cell that separates the contents of the cell from its external environment.

2.    Endocytosis
•    The plasma membrane is flexible and is made up of organic molecules called lipids and proteins.
•    The flexibility of the cell membrane also enables the cell to engulf in food and other material from its external environment. Such processes are known as endocytosis (endo → internal; cyto → of a cell). Amoeba acquires its food through such processes.

3.    Diffusion
•    Plasma membrane is a selectively permeable membrane [The plasma membrane is porous and allows the movement of substances or materials both inward and outward].
•    Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion [spontaneous movement of a substance from a region of high concentration (hypertonic solution) to a region where its concentration is low (hypotonic solution)].
•    Thus, diffusion plays an important role in gaseous exchange between the cells as well as the cell and its external environment.

4.    Osmosis
•    Water also obeys the law of diffusion. The movement of water molecules through a selectively permeable membrane is called osmosis.
•    Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. Thus, osmosis is a special case of diffusion through a selectively permeable membrane.
•    Unicellular freshwater organisms and most plant cells tend to gain water through osmosis. Absorption of water by plant roots is also an example of osmosis.
•    Thus, diffusion is important in exchange of gases and water in the life of a cell. In additions to this, the cell also obtains nutrition from its environment.
•    Different molecules move in and out of the cell through a type of transport requiring use of energy in the form of ATP.

5.    Reverse Osmosis (RO)
•    Reverse osmosis (RO) is a water purification technology that uses a semipermeable membrane to remove larger particles from drinking water.
•    In reverse osmosis, an applied pressure is used to overcome osmotic pressure.
•    Reverse Osmosis is a phenomenon where pure water flows from a dilute solution [hypotonic] through a semi permeable membrane to a higher concentrated solution [hypertonic].
•    Semi permeable means that the membrane will allow small molecules and ions to pass through it but acts as a barrier to larger molecules or dissolved substances.