The Cell and Cell Theory
The word cell comes from the Latin word “cellula”, which means a small room. The cell is the smallest unit of a living organism, also known as “the building block of life”. Every living organism is consisted of a cell. There are bacteria (unicellular organisms), and multicellular organisms. There are different types of cells, prokaryotic and eukaryotic cells. All cells come from preexisting cells. “Every cell is self-maintaining; it takes in nutrients, converts them into energy, carries out special functions and reproduces itself if necessary.” The descriptive name for the smallest living biological structure was given by Robert Hooke in a book, which he published in 1665, while discovering “small units” in a cork. After that the research of the cell has progressed greatly.
Prokaryotic and Eukaryotic cells
There are two types of cells. While they are still cells which make them have a lot of similarities, they are also very different. The main difference between them is that a prokaryotic cell does not consist of a nucleus nor a nuclear membrane, while the eukaryotic on the other hand does. There are two types of prokaryotic cells, bacteria and archaea. The plasma membrane of the cell protects it from the outside environment. Most of them also have cell walls, but there are exceptions to that. Since the cells don’t have a nucleus the DNA is stored in the nucleoid. [pic]
The eukaryotic cell looks like this. They are around 10 times bigger than the prokaryotic cell. Unlike the prokaryotes they have a cell nucleus, where the cell’s DNA is stored, and a cell membrane. All of it’s chromosomal DNA is stored in the cell nucleus. The mitochondria in the eukaryotic cell also contain some of the DNA.
“There are several main functions that the cell has to carry out: molecule transport, reproduction, and energy conversion.” 1
There are two types of molecule transport: active transport and passive transport. The passive transport is used for molecules such as oxygen, ethanol, and carbon dioxide. To transport proteins, polynucleotides and polysaccharides, cells rely on active transport. [pic]
Active transport is divided into two basic types of it. Exocytosis and endocytosis. “Exocytosis involves sending macromolecules out of the cell, while the opposite applies to endocytosis.” 2
Every day the cell has to go through tens of thousands of chemical reactions. But to go through them, they need energy. We can divide cellular metabolism into two categories anabolism and catabolism. The purpose of catabolism is to break down large molecules, while anabolism on the other hand uses the energy to make parts of cells such as proteins and nucleic acids.
1 http://library.thinkquest.org/C004535/basic_cell_functions.html 2 http://library.thinkquest.org/C004535/molecule_transport.html
The main goal of all living organisms is to survive. This is why it is needed for cells to reproduce. Cells reproduce in two ways, mitosis and meiosis. “Mitosis is mostly used by somatic cells (cells of the body). Meiosis is a form of sexual reproduction and only occurs in gametes (reproductive cells).” 1
To continue the race of an organism, it must reproduce. The cell division starts with interphase, and then it goes through prophase, metaphase, anaphase, and telophase.
“Meiosis starts with diploid cells, or cells that have two sets of chromosomes from their parents.” In meiosis I the diploid cell changes into two. Then, in Meiosis II the two diploid cells are split into four haploid cells.” 2 The point of this type of reproduction is to increase the genetic variety of living organisms. [pic]
The cell theory
The main concept of the cell theory refers to that the cell is the basic unit of all living structures. Because of the advanced microscopy in the 17th century the theory was very well developed. A statement made in the theory is something like this: “new cells are formed from other existing cells and the cell is a fundamental unit of structure, physiology, and organization in all living organisms.”
What is the history of the cell theory?
Robert Hooke was working on a cork in 1665, when he discovered that the cells of the cork look something like small rooms. That is when he decided to give them the name cell. The discovery of cell walls was published in the magazine “Micrographia”. Robert Hooke was observing non- living cells. The first person to see a living cell under a microscope was Antonie van Leeuwenhoek. Ludolph Christian Treviranus and Johann Jacob Paul Moldenhawer proposed that cells were individual units. Henri Dutrochet said "The cell is the fundamental element of organization", which is included in the modern cell theory.
“The cell theory states:
• All living things are composed of cells.
• Cells are the basic unit of structure and function in living things. • All cells are produced from other cells.” 1
“Classical Cell Theory
1. All organisms are made up of one or more cells.
2. Cells are the fundamental and structural unit of life. 3. All cells come from pre-existing cells.” 2
“Modern Cell Theory
The generally accepted parts of modern cell theory include:
1. The cell is the fundamental unit of structure and function in living things. 2. All cells come from pre-existing cells by division.
3. Energy flow (metabolism and biochemistry) occurs within cells. 4. Cells contain hereditary information (DNA) which is passed from cell to cell during cell division 5. All cells are basically the same in chemical composition. 6. All known living things are made up of cells.
7. Some organisms are unicellular, made up of only one cell. 8. Others are multicellular, composed of countless number of cells. 9. The activity of an organism depends on the total activity of independent cells.”1
Exceptions to the Cell Theory
Virus comes from the Latin word meaning “poison” or “toxic”. Every virus has genes made from either DNA or RNA, long molecules, which carry out genetic information, protein coats. They are extremely small. Viruses can spread in many different ways. For example some viruses spread through the air, when people cough and sneeze. Some are spread by blood sucking insects, plant viruses are spread by plant insects. HIV is a virus which spreads during sexual contact. It is very confusing where viruses came from, but scientists think that some viruses originate from bacteria, and others from pieces of DNA (also known as plasmids). There are serious diseases which are caused by viruses, such as AIDS and SARS.
The Herpes Virus
All of the herpes viruses have the same structure. They are made up of a large double -stranded DNA linear. The herpes virus is nuclear-replicating, so when one cell is infected with this virus it passes the virus on while reproducing, because the DNA transcribes it to the RNA. That automatically infects the new cell.
The Bacteriophage T4 is a virus that infects E. coli bacteria. It has the longest DNA. The infection process is like this. “The T4 Phage initiates infection of an E. coli bacterium by recognizing cell surface receptors of the host with its long tail fibers (LTF). A recognition signal is sent through the LTFs to the baseplate. This unravels the short tail fibers (STF) that bind irreversibly to the E. coli cell surface. The baseplate changes conformation and the tail sheath contracts causing GP5 at the end of the tail tube to puncture the outer membrane of the cell. The lysozyme domain of GP5 is activated and degrades the periplasmic peptidogliycan layer. The remaining part of the membrane is degraded and DNA from the head of the Phage can travel through the tail tube and enter the E. coli.” 1 [pic]
Lambda phage is made up of a two stranded DNA linear and a tail. The injection into the bacteria goes through the tail. “The phage particle injects its DNA into its host through the tail, and the phage will then usually enter the lytic pathway where it replicates its DNA, degrades the host DNA and hijacks the cell's replication, transcription and translation mechanisms to produce as many phage particles as cell resources allow. When cell resources are depleted, the phage will lyse (break open) the host cell, releasing the new phage particles. However, under certain conditions, the phage DNA may integrate itself into the host cell chromosome in the lysogenic pathway.” 1
People behind the Cell Theory
He was born on July 18 1635 and lived until the 3rd of March 1703 in Freshwater on the Isle of Wight. When Robert was small he was always fascinated about observing and exploring things. In 1653 Hooke met his natural philosopher at the Christ Church, Oxford. He helped Robert Boyle with his invention – the air pump. Through the years 1655 to 1662 Hooke assisted Robert Boyle. These years were the most fascinating, because he started to get himself interested in other things to, such as astronomy, drawing and others. The Royal Society was founded in 1660, where Robert Hooke gave a fascinating explanation of the rising of water in slender glass pipes. That is when he started to get noticed. His explanation was published in “Micrographia”. Also in the same year he discovered the law of elasticity. In 1665 his piece on his microscopic observations of the cell was published in the same scientific magazine. He was also trying to discover the distance from earth to the star “Gamma Draconis”. He used the method parallax determination. Hooke was also an architect. After the Great Fire in London in 1666 Robert helped rebuild it, and he built a monument to the fire.
Antonie van Leeuwenhoek
He was born on October the 14th 1632 and died on August the 30th 1723. He is also known as “The Father of Microbiology”. In 1648 in Amsterdam he saw a magnifying glass connected to a handle. That was the first microscope he saw. In 1665 he read Robert Hooke’s book “Micrographia”, which got him interested in discovering living things with the use of his microscopes. After he developed a method of making very powerful microscopic lenses he was introduced to The Royal Society. “Amongst Van Leeuwenhoek's many discoveries are: • in 1674 he discovered infusoria (protists in modern zoological classification) • in 1676 he discovered bacteria, (e.g. large Selenomonads from the human mouth) • in 1677 he discovered spermatozoa
• in 1682 he discovered the banded pattern of muscular fibers” 1
He was born December the 7th, 1810 and died on January 11th, 1882. He was a German zoologist. He made a huge contribution to the cell theory, by basically developing it. "All living things are composed of cells and cell products." 1 This he said after discovering that new plant cells come from the nuclei of old ones.
Matthias Jakob Schleiden
He was born on April the 5th, 1804 and lived until June the 23rd, 1881. He was a German botanist. He founded the theory as well as Theodor Schwann and Rudolf Virchow. He was born in Hamburg, but he was going through education in Heidelberg. He studied plant structure under microscopes. He was also a professor at the University of Jena. He stated that the cell nucleus is very important and he was the first German biologist to accept the theory of evolution by Charles Darwin.
He was born on October the 13th, 1821 and lived until the 5th of September 1902. He was a German doctor and anthropologist, pathologist, biologist, and politician. He was born in the family of a farmer and studied cattle on a scholarship at the military academy of Prussia. He had a job at the Charité hospital in Berlin. Because of the political situation he moved to Würzburg where he studied and than taught anatomy. Then he returned to Berlin as a professor of anatomic pathology. He first recognized leukemia. He also contributed a lot to scientific discoveries. In 1869 he founded the Society of anthropology, ethnology and prehistory.
Table of Contents
The cell, cell structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Prokaryotic and eukaryotic cells . . . . . . . . . . . . . . . . . . . . . . . . 2
Cell Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4
The cell theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6
Viruses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-9
People behind the theory . . . . . . . . . . . . . . . . . . . . . . . . . . 10-14
Timeline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16