The Cell Section 2

Cell Theory

- A cell is the most basic unit of life. The cell was originally observed and discovered by Robert Hooke around 1665. Some orgainsms are as small as just one cell, while others have trillions of cells. Humans are made of approximately 100 trillion cells. - The cell theory is essentially one of the basic principles of biology and was first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann.

Cell

- All living organisms are made of cells. - When cells increase in size and when the cell numbers increase over a period of time, this is called growth. - An orderly growth in the number of cell and the development of functions in the cells results in the growth of an organism.

Virus- A cell?

- Another small entity that is often confused with a cell is a virus. Viruses are not capable of reproducing outside a host cell. They do not exhibit characteristics of living things. - Therefore, they do not meet the criteria of the definition of life and are not considered to be living. They do not fulfil the cell theory requirements. - Viruses are small infectious agents that are only able to replicate inside the living cells of organisms.

Eukaryotic Cell

- Eukaryotic cells are about fifteen times larger than prokaryotic cells and much more complex, with more organelles. - The biggest different between the two types of cells is that eukaryotic cells contain membrane-bound compartments and membrane-bound organelles. These are where the metabolic activities take place. - Eukaryotic cells, the DNA is organized into multiple, linear molecules called chromosomes, which are associated with histone proteins that hold them together. All the chromosomal DNA is stored in the cell nucleus and is separated from the cytoplasm by a nculear membrane. The eukaryotic organelle, called the mitochondria, also contains DNA, but it is circular not linear like the chromosomal DNA in the nucleus. Eukaryotic cells contain many specialized organelles.

Parokaryotic Cell

- Karyose comes from a Greek word that means kernel as in a kernel of grain. In biology, we use this root word to refer to the nucleus of a cell. - Pro means before and eu means true or god. So Prokaryotic means before a nucleus and Eukaryotic means possessing a true nucleus. - Prokaryotic cells have no nuclei, while eukaryotic cells do have true nuclei. - The nucleus is the central membrane-bound organelle in eukaryotic cells, and it holds the genetic material. - The prokaryotic cells are simple and smaller than the eukaryote cells. The prokaryoric cell does not have a membrane-bound nucleus nor the membrane-bound organelles that are found in eukaryotic cells. There are two prokaryotes: - Bacteria - Archaea

Eukaryotic Cell Diagram

- Nucleolus - Nucleus - Ribosmoe - Vesicle - Rough endoplasmic reticulum - Golgi apparatus - Cytoskeleton - Smooth endoplasmic reticulum - Mitochondrion - Vacuole - Cytosol - Lysosome - Centriole

What is a cell?

- The cell is a living entity. It can reproduce, respond to stimuli, comsume energy, and regulate its internal environment.

Cell theory states that:

- The cell is the basic unit of life - All organisms are composed of one or more cells (unicellular or multicellular). - Cells come from pre-existing cells - All cells have the same basic chemical composition - All cells contain the hereditary information necessary for regulating cell functions and transmitting information to the next generation of cells being created. - Energy flow occurs within cells.

Types of cells: Definition of a cell

- The word "cell" comes from the Latin word cellula, meaning a small room. Robert Hooke first observed cells by looking at thin slices of bottle cork under a microscope and likened what he saw to the small rooms monks lived, and thus, called them cells. There are two basic type of cells: - Prokaryotes - Eukaryotes

Virus

- Viruses are too small to be seen and can infect all types of living organisms. Viruses are much smaller than bacteria and have a very basic structure that consists of: Lipid envelope - This part is derived from the host cell membrane. Capsid - The capsid is made from proteins and is encoded by the vira genome. Genetic material - The genetic material is made from DNA or RNA and carries genetic information. Protein coat - Protects the genetic material.

Bacteriophage

A virus that infects bacteria

Comparision of Prokaryotic and Eukaryotic Cells

Eukaryotic Cells - Have a true nucleus, bound by a double membrane. - The purpose of the nucleus is to sequester the DNA-related functions of the big eukaryotic cell into a smaller chamber, for the purpose of increased efficiency. - DNA is linear. - DNA is complexed with proteins called histones, and is organized into chromosomes. - DNA is packaged into chromosomes. - The ribosomes of the eukaryotic cells are larger and more complex. - Composed of five kinds of RNA and about eighty kinds of proteins. - The cytoplasm of eukaryotic cells is filled with a large, complex collection of organelles, many of which are enclosed in their own membranes. Prokaryotic Cells - Have no nucleus. - DNA is condensed into a nucleoid and performs all DNA functions - DNA is circular - DNA is naked, meaning that it has no histones associated with it. - The DNA is not formed into chromosomes. - Small and simple. - Ribosomes are composed of only three kinds of RNA and about fifty kinds of protein. - Contains no membrane-bound organelles that are independent of the plasma membrane.

The parts of these cells are explained below from outermost to innermost structures.

Flagella and Pili: - These are structures that protrude from the cell's surface. - They are made from proteins and facilitate movement and communication between cells. Cell Envelope: - This is the outer structure of the cell. It comprises the cell wall, the cell membrane and the outer membrane. This part is rigid and separates the interior of the cell from the environment. Some bacteria also have a furthur outer covering called the capsule. The capsule is outside of the cell envelope when present. Cytoplasm: - This is the gel-like substance in the cell that holds the cell's DNA, ribosomes, and other cellular components in place. Nucleoid: - Prokaryotic cells do not have a nucleus, but their DNA is held in an irregularly-shaped region called a nucleoid. Plasmids: - Circular, double stranded extra-chromosomal DNA elements.