The Fundamental Unit of Life

The cell theory states that cell is the basic unit or the basic building block of life. It has three salient features:

• All organisms are made up of one or more cells, i.e., unicellularor multicellular organisms, respectively.
• It states that all cells carry out life activities.
• New cells arise only from other living cells by the process of cell division.

Basic Cell Organelles:

Plant cell and animal cell are composed of three main components.

• The cell membrane is a bi-lipid membranous layer. It separates the cell from its external environment.
• The cytoplasm is a jelly-like substance in which cell organelles are found.
• The nucleus is the largest cell organelle. It is dark and round and surrounded by a nuclear membrane. It is known as the control center.
• Some of the cell organelles present in both plant cells and animal cells are:

Endoplasmic Reticulum (ER):

• The large complex membranous network of tubular structures, enclosing a fluid-filled lumen which almost fills up the intracellular cavity inside a cellis called endoplasmic reticulum (ER).
• It is present within the cytoplasm of the eukaryotic cell.
• The membranes are continuous and join with the outer membrane of the nuclear membrane. They occur in almost every type of eukaryotic cell except red blood cells and sperm cells.
• Endoplasmic reticulum is the transportation system in the eukaryotic cell which is a membrane network within the cytoplasm of cells.
• It is involved in the synthesis, modification, and transport of cellular materials.
• It is of two types:
  Rough endoplasmic reticulum (RER): It has particles called ribosomes attached on its surface for synthesizing proteins. This type of endoplasmic reticulum is especially prominent in certain kinds of cells like hepatocytes where active protein synthesis occurs.

Smooth endoplasmic reticulum (SER) – These transpire with a smooth surface. Ribosomes are not present in it and are meant for secreting lipids. They also transport the products of the rough endoplasmic reticulum to other cell parts, notably the Golgi apparatus.

• The ER is absent in the red blood cells of mammals.
• The ER performs following functions.
  • It forms supporting skeletal framework of the cell.
  • ER provides a pathway for the distribution of nuclear material from one cell to the other.
  • Certain enzymes present in smooth ER synthesize fats (lipids), steroids and cholesterol.
  • Rough ER is concerned with the transport of proteins which are synthesized by ribosomes on their surface.