What+Are+Cells?

[|Cells Alive]Video: [|What Are Cells?] == [|__**Cells Alive**_==] _ Eucaryotic Cell Interactive Animation: Discover the Interactive Cell. Take a few moments and learn the Cell Organelles: Click on the different organelles and learn their functions. Compare Animal Cell to a Plant cell. Explore more interactive animations: CellCam, Mitosis, Meiosis, The Cell Cycle, Pumping Myocytes, Quiz your knowledge on Cell Biology.

= What is a cell? = The word cell comes from the Latin word "cella", meaning "small room", and it was first coined by a microscopist observing the structure of cork. The cell is the basic unit of all living things, and all organisms are composed of one or more cells. Cells are so basic and critical to the study of life, in fact, that they are often referred to as "the building blocks of life". Organisms - bacteria, amoebae and yeasts, for example - may consist of as few as one cell, while a typical human body contains about a trillion cells. According to //Cell Theory,// first proposed by Schleiden and Schwann in 1839, all life consists of cells. The theory also states that all cells come from previously living cells, all vital functions (chemical reactions) of organisms are carried out inside of cells, and that cells contain necessary hereditary information to carry out necessary functions and replicate themselves.

All cells contain:
 * Lipid bilayer boundary ([|plasma membrane])
 * [|Cytoplasm]
 * [|DNA] (hereditary information)
 * [|Ribosomes] for protein synthesis

Eukaryotic cells also contain:
 * At least one [|nucleus]
 * [|Mitochondria] for cell respiration and energy

Cells may also contain: Science has so far been unable to create any living cell from non-living materials. 
 * [|Lysosomes]
 * [|Peroxisomes]
 * [|Vacuoles]
 * [|Cell walls]

History of cell knowledge
The optical microscope was first invented in 17th century. Shortly thereafter scientists began to examine living and dead biological tissues in order to better understand the science of life. Some of the most relevant discovery milestones of the time period include:
 * The invention of the [|microscope], which allowed scientists for the first time to see biological cells
 * [|Robert Hooke] in 1665 looked at cork under a microscope and described what he called cork "cells"
 * [|Anton van Leeuwenhoek] called the single-celled organisms that he saw under the microscope "animalcules"
 * [|Matthias Jakob Schleiden], a botanist, in 1838 determined that all //plants// consist of cells
 * [|Theodor Schwann], a zoologist, in 1839 determined that all //animals// consist of cells
 * [|Rudolf Virchow] proposed the theory that all cells arise from previously existing cells

In 1838, the botanist Matthias Jakob Schleiden and the physiologist Theodor Schwann discovered that both plant cells and animal cells had nuclei. Based on their observations, the two scientists conceived of the hypothesis that all living things were composed of cells. In 1839, Schwann published 'Microscopic Investigations on the Accordance in the Structure and Growth of Plants and Animals', which contained the first statement of their joint cell theory. 

Cell Theory
Schleiden and Schwann proposed [|spontaneous generation] as the method for cell origination, but spontaneous generation (also called [|abiogenesis]) was later disproven. Rudolf Virchow famously stated "Omnis cellula e cellula"... "All cells only arise from pre-existing cells." The parts of the theory that did not have to do with the origin of cells, however, held up to scientific scrutiny and are widely agreed upon by the scientific community today. The generally accepted portions of the modern Cell Theory are as follows: (1) The cell is the fundamental unit of structure and function in living things. (2) All organisms are made up of one or more cells. (3) Cells arise from other cells through cellular division. (4) Cells carry genetic material passed to daughter cells during cellular division. (5) All cells are essentially the same in chemical composition. (6) Energy flow (metabolism and biochemistry) occurs within cells. 

Microscopes

 * Allow greater resolution, can see finer detail
 * Eye: resolution of ~ 100 μm
 * [|Light microscope]: resolution of ~ 200 nm
 * Limited to cells are larger organelles within cells
 * Confocal [|microscopy]: 2 dimension view
 * [|Electron microscope]: resolution of ~0.2 nm
 * Laser tweezers: move cell contents

Cell size
One may wonder why all cells are so small. If being able to store nutrients is beneficial to the cell, how come there are no animals existing in nature with huge cells? Physical limitations prevent this from occurring. A cell must be able to diffuse gases and nutrients in and out of the cell. A cell's volume does not increase as quickly as its surface area, and as a result a large cell may require more input of a substance or output of a substance than it is reasonably able to perform. Worse, the distance between two points within the cell can be large enough that regions of the cell would have trouble communicating, and it takes a relatively long time for substances to travel across the cell. That is not to say large cells don't exist. They are, once again, less efficient at exchanging materials within themselves and with their environment, but they are still functional. These cells typically have more than one copy of their genetic information, so they can manufacture proteins locally within different parts of the cell.
 * Key concepts:** Cell size:
 * Is limited by need for regions of cell to communicate
 * Diffuse oxygen and other gases
 * Transport of [|mRNA] and [|proteins]
 * Surface area to volume ratio limited
 * Larger cells typically:
 * Have extra copies of genetic information
 * Have slower communication between parts of cell