Cells are structural and functional units of life. According to the cell theory all living things are composed of one or more cells. One-celled organisms are called unicellular organisms and those with more than one cell are called multi-cellular organisms. Virus particles do not have any cells and therefore, are termed as acellular. No matter what type of cell we are considering, all cells have certain features in common: cell membrane, nucleic acids, cytoplasm, and ribosomes. Cells are small 'sacks' composed mostly of water. The 'sacks' are made from a phospholipid bilayer. The membrane is semi-permeable, allowing some things to pass in or out of the cell and blocking others. Microscopes make it possible to magnify small objects such as cells in order to see the details of their structure. Both light and electron microscopes are used to study cells. Study of cells with a microscope is called cytology. There are some fundamental activities, which are common for most of cell types from bacteria to the nerve cells in humans. The study of these basic cellular processes is called cell biology.
Cells are 90% fluid (cytoplasm), which consists of free amino acids proteins, glucose, and numerous other molecules. The cell environment (i.e., the contents of the cytoplasm, and the nucleus, as well as the way the DNA is packed) affects the gene expression/ regulations, and thus is very important part of inheritance.
Cells basically fall into two groups: prokaryotic cells and eukaryotic cells.
Prokaryotes, Eukaryotes and Viruses: Cells are classified as prokaryotes or eukaryotes based on their basic structure and the way by which they obtain energy. Cells are also classified according to their need for energy. Autotrophs are "self feeders" that use light or chemical energy to make food. Plants are an example of autotrophs. In contrast, heterotrophs ("other feeders") obtain energy from other autotrophs or heterotrophs. Many bacteria an( animals are heterotrophs.
Prokaryotic Cells: Prokaryotes include bacteria and blue-green algae (cyanobacteria). Simply stated, prokaryotes are molecules surrounded by a membrane and cell wall. Prokaryotic cells lack characteristic eukaryotic subcellular membrane-enclosed "organelles," but may contain membrane systems inside a cell wall as an extension or infoldings of the cell membrane. The nucleus is not well-organized and is without any membrane.
Prokaryotic cells may have photosynthetic pigments, such as is found in cyanobacteria ("blue-green algae"). Some prokaryotic cells have external whiplike flagella for locomotion or hair-like pili for adhesion. Prokaryotic cells come in multiple shapes: cocci (round), baccilli (rods), and spirilla or spirochetes (helical cells). All prokaryotes are unicellular organisms and eukaryotes include both unicellular and multicellular organisms.
Eukaryotes : Basic Structure:
The basic eukaryotic cell contains the following: 1) A Plasma membrane2) Nucleus3) Cytoplasm (semifluid)
Viruses : Basic Characteristics of Viruses:
Simply stated, viruses are merely genetic information surrounded by a protein coat. They may contain external structures and a membrane. Viruses are obligate intracellular parasites meaning that they require host cells to reproduce. In the viral life cycle, a virus infects a cell, allowing the viral genetic information to direct the synthesis of new virus particles by the cell. There are many kinds of viruses. Those infecting humans include polio, influenza, herpes, smallpox, chickenpox, and human immunodeficiency virus (HIV) causing AIDS.
Tags: Bio Technology, Bio Genetics , Cell Structure
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