Cells were first discovered by Robert Hook in 1665. He observed the cells in a cork slice with the help of a primitive microscopic. Later in the year 1674 Leeuwenhoek discovered the free living cells in pond water for the first time. In the year 1831 Robert brown discovered the nucleus of the cell. In 1839 Purkinje coined the term protoplasm for the fluid sustance present inside the cell. Later two biologist Schleiden and Schwann presented a theory on cells known as cell theory which states that all the plants and animals are composed of cells and the cells are the basic unit of life. The cell theory was further explained by Virchow in 1855, He suggested that all the cells arise from pre-existing cells. With the discovery of the electron microscope it was possible to observe and understand the complex structure of cell and its various organelles.
“Cells are the structural as well as the fundamental unit of life”
If we study a cell under microscope we observe that the cell contains special components named as cell organelles. Below are some cell organelle which were present inside the cell:
Plasma Membrane: It is the outermost covering of the cell that separates the contents the cell from its external environment. It allows some substances to cross through its boundaries while it restricts other substances and that’s why its is also named as selectively permeable membrane.
Cell wall: In plant cell an extra covering is present outside the plasma membrane which is known as cell wall. It is mainly composed of cellulose. It provides structural strength to the plants.
Nucleus: It plays a central role in cellular reproduction, the process by which a single cell divides into two new cells. It also directs the chemical activities of the cell. Nucleus contains chromosomes which is visible only the cell is about to divide. Chromosomes are composed of DNA and protein. DNA molecules contains contains the information necessary for constructing and organizing cells. Functional segment of DNA are called genes. In a cell which is not dividing, this DNA is present as part of chromatin material which is visible as entangled mass of thread like structure. Nucleus has covered by a double layer covering called nuclear membrane. But in some organism this membrane was absent and this organisms are called as prokaryotes and the organism in which membrane is present are called as eukaryotes.
Cytoplasm: Cytoplasm is the fluid content present inside the plasma membrane which contains many specialized organelle.
Endoplasmic Reticulum(ER): ER is similar in structure to the plasma membrane. There are two types of ER: Rough Endoplasmic reticulum (RER) and Smooth Enplasmic reticulum (SER). RER looks rough because it contains particle on its surface, this particles are ribosomes which manufactures protein. The protein manufactured by ribosomes is then send to different parts of the cytoplasm with the help of ER. Whereas SER helps in the manufacture of fat molecules and lipids.
Golgi Apparatus: It consists of a system of membrane bound vesicles arranged approximately parallel to each other in stacks. These membrane often have connections with the membrane of endoplasmic reticulum. The material synthesized near the ER is packed and dispatched to various targets inside and outside the cell through the golgi apparatus.
Lysosomes: Lysosome is like a waste disposal system of the cell. It helps to clean the cell by digesting any foreign material as well as worn out cell organelle. Lysosomes are able to do this because it contain powerful digestive enzymes which is capable of breaking down all organic material During the disturbance in cellular metabolism, for example when the cells get damaged lysosome burst itself and the enzymes digest their own cell. Therefore lysosomes are also known as “suicide bag“.
Mitochondria: The requirement of energy for various chemical activities inside the cell is fulfilled by mitochondria. Mitochondria releases energy in the form of Adenosine triphosphate(ATP) molecules. ATP is known as the energy currency of cell and mitochondria is known as the power house of the cell.
Vacuoles: Vacuoles are storage bag for solid or liquid contents. Animal cells contains small size vacuole and plant cells have large size vacuoles. In plant cells vacuoles are full of cell sap and provided turgidity and rigidity of the cell. Vacuoles acts as a storage for various important substance such as amino acids, sugars and some protein.
Plastids: Plastids are only present in the plant cell. There are two types plastids- chromoplasts and leucoplasts.
Chloroplasts: Plastids containing the pigment chlorophyll are known as chloroplasts. It is important for photosynthesis in plants. It also contain various yellow or orange pigment in addition to chlorophyll.
Leucoplasts: This are primarily organelles in which material such as starch, oils and protein granules are stored. Its primary function is storage.