Cell Membrane, Cytoplasm
The cell is surrounded by a delicate living cell membrane. It is a protective layer. It gives the cell a definite shape and firm ness. Its important function is, to allow the necessary substances to enter the cell and prevent the entry of unnecessary substances in to the cell. Inside the cell membrane is the cytoplasm. This is a color less fluid. It has 80% of water and 20% of many chemical substances.
The fluid between the cell membrane and the nucleus is called cytoplasm. There are a number of living Structures in the cytoplasm. These are called Organelles. They are : Mitochondria, Golgi Complex, Ribosomes, Endoplasmic Reticulum, Centrosome, Lysosome.
Mitochondria are minute rod like or oval shaped structures. They are the sites of respiration of the cell. During respiration, food (glucose) is oxidized and energy is released. Hence mitochondria are called the Power houses of the cell.
Golgicomplex: Golgi complex consists of several bag like structures. it is involved in the secretion of chemicals.
Ribosomes:Ribosomes are granular structures that help in protein synthesis. Hence they are called Protein Factories
Endoplasmic Reticulum:This forms a network in the cytoplasm and also connects plasma membrane and nuclear membrane. There are two types of endoplasmic reticulum.
(1) Rough endoplasmic reticulum with ribosomes
(2) Smooth endoplasmic reticulum without ribosomes. It helps in protein synthesis and transport of materials inside the cell and in between the cells.
Types of Nerve Cells
Based on the methods of transmission of messages, three types of nerve cells are recognized.
1.Sehsory Nerve Cells: Sensory nerve cells are those which carry information from the sense organs to the brain. These perceive the sense of heat, hearing, sight, taste, smell and touch.
2.Motor Nerve Cells: Nerves which transmit command messages from the brain to various parts of the body i.e., to muscles and glands.
3.Mixed Nerve Cells: These nerve cells are related to central nervous system. They bring in contact between the sensory and motor nerve cells.