Electronic micro-scoping has revealed the following structure in a cell.
Cell or Plasma membrane:
A membrane is a thin protoplasmic envelope. It encloses the cytoplasm and is selectively permeable which means it is permeable to certain substances while it prevents other substances from entering the cell. It consists of three layers.
Cell wall:
Nearly all plant cells have a cell wall made of cellulose and may contain waxes and suberin. The cell wall lies outside the plasma membrane and is non living. It is tough and firm and provides support for the plant body. It is a permeable membrane. Animals have no cell wall.
Cytoplasm:
Cytoplasm is a semi fluid substance formed in a living cell. The composition includes proteins, lipids, carbohydrates water and inorganic substances. It contains all the cell organelles like mitochondria, ribosome’s, nucleus, nucleolus, lysosomes, golgi bodies, endoplasmic reticulum, plastids etc. It exhibits the important characteristics of living things like growth, nutrition, respiration, excretion, reproduction and irritability.
Cell organelles:
Organelles are the persistent structures that have a specialized function within the protoplasm. In most of the cases, organelles are separated from rest of the cytoplasm by a selectively permeable membrane.
Nucleus:
The nucleus is a prominent structure found in the both animal and plant cell . The Nucleus is made of two components. The nuclear membrane has pores that facilitate exchange between the cytoplasm and the nucleolus. The nucleoplasm contains a chromatin network and the nucleolus. The chromatin threads contain genes that transmit hereditary traits from one generation to another.
Nucleolus:
There is a spherical body found in the nucleoplasm called the nucleolus. It disappears when the cell is about to divide but reappears after the cell division. It appears to be active centres of protein and RNA synthesis.
Endoplasmic Reticulum:
This is a very fine structure which can be seen only through an electron microscope. The endoplasmic reticulum is a network of tube like structures distributed extensively throughout the cytoplasm. It plays an important role in the synthesis and transportation of complex biological molecules.
Ribosomes:
Ribosomes are extremely minute granular or spherical structures found either in free state in the cytoplasm or attached to the surface of endoplasmic reticulum. The main function of ribosomes is to act as a platform or work place for the synthesis of proteins.
Mitochondria:
These are small rod shaped or spherical organelles found in large numbers. Each mitochondria is bound by two membranes; outer and inner. Their outer membrane is smooth and their inner membrane is projected with finger like structure called cristea. They have enzymes necessary for oxidation of carbohydrates and energy is released in the form Of ATP. They provide energy for the vital activities of living cells.
Golgi bodies:
This organelle is also known as golgiapparatus. It consists of few to many stacks of smooth Endoplasmic reticulum in the form of membranous flattened sacs, vesicles and vacuoles. Golgi apparatus condenses the protein synthesized by the ribosomes and also help in the formation of cell plate during cell division.
Lysosomes:
These are small enzyme containing, single membrane-bound vesicles. They are spherical and sac like organelles. Many digestive enzymes are present inside them. Lysosomes are formed by golgi complex. During starvation, lysosome act as their own cellular organelles and digest them. This results in cell death. Thus lysosome are also referred to colloquially as ‘suicidal bags’. They help in destruction of foreign bodies.
Plastids:
Plastids occur in plants cells and are absent in animal cells. Plastids have three major types.
Choloroplast:
These green plastids have owe their color to the presence of chlorophyll. These occur abundantly in green leaves, and also to some extent in the shoot of green plants. It synthesizes carbohydrates from carbon dioxide and water by the process of photosynthesis.
Amyloplast:
These plastids contains pigments other than green. These impart various color effects to flowers and fruits. These attract insects and other animals to ensure pollination and dispersal.
Leucoplasts:
They are colorless plastids occurring in seeds, meristematic cells, ground tissue of certain roots and stems. These store starch, protein, or lipids in cell.
Although there are some basic differences between a plant and an animal cell, their general structure remains the same.
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