Cell

In biology, the **cell** is the basic structure of organisms. All living things are made up of cells, from the organs in the human body to the parts of plants. All cells are made by the division of cells that already exist (pre-existing cells).

The environment outside the cell is separated from the cytoplasm inside the cell by the cell membrane. Inside some cells, parts of the cell stay separate from other parts. These separate parts are called organelles (like small organs). They each do different things in the cell. Examples are the nucleus (where DNA is), and mitochondria (where chemical energy is converted). Prokaryotes do not have membrane-bound organelles like Eukaryotes.

There are around 37 trillion (3.7 x 10<sup>13</sup>) cells in the human body. An old saying is that bacteria outnumber the number of cells in the body 10 times over, and there are 10 times more bacteria than cells (3.7 x 10<sup>14</sup>) but many are debating this.

## Kinds of cells

Cell can be divided into different types depending on the structure of the nucleus and the physiological function. In 1957 cells were divided into two types based on the structure of nucleus. These were the prokaryotic cell and the eukaryotic cell. Prokaryotes, such as bacteria and archaea, are simple cells that have no cell nucleus. They do have bacterial microcompartments.

Eukaryotes are complex cells with many organelles and other structures in the cell. They are larger than prokaryotic cells: they can be as much as 1000 times greater in volume. Eukaryotes store their genetic information (DNA) on chromosomes in the cell nucleus. Prokaryotes have all of their DNA on a single circular strand located in the nucloid region. Organisms (living things) which are made up of multiple cells are eukaryotes.

Most cells are small, the largest just being seen by the human eye (egg cell). They need to be small in order to work as they should. In things like cells and mechanical engineering (and other types of mechanic work), the smaller and more simple things are usually means they can survive better. This means that a lot of the thing things are as small as they can be.

## Regions of prokaryotic cell

The prokaryotic cell has three regions made up of the envelope, cytoplasmic region and flagella (or pili), called:

- **Cell envelope:** This consists of the plasma membrane covered by a cell wall. Some bacteria have a third layer called a capsule. Virions might have an envelope called the capsid. Prokaryotes such as Mycoplasma and Thermoplasma only have a cell membrane layer. The envelope gives protection to the cell and separates the inside of the cell from its environment. An example envelope is the nuclear envelope. The nuclear pores on the nuclear envelope allow for things to get in and out.
- **Cytoplasmic region:** This region is found inside the cell and contains the genome (DNA), ribosomes and other cell inclusions. Since prokaryotes do not have a nucleus, the genetic material is free in the cytoplasm.
- **Flagella/Pili:** These structures are not present in all prokaryotes. The flagella and pili extends from the surface of the cell, and they help in movement and communication between cells Flagella usually helps the cell move which cilia the cell move other cells (across its surface). Sperm (germ) has a flagella tail for movement and the throat uses cilia to move phloem (phlegm) down the throat.

### Unicellular (eukaryotic and prokaryotic)

Unicellular organisms are made up of one cell. Examples of unicellular organisms are:
- Amoeba
- Paramecium (pictured)

Prokaryotes are almost always unicellular, while eukaryotes can be unicellular or multicellular. The only kinds of prokaryotic organisms are bacteria and archaea. Prokaryotic organisms evolved before eukaryotic organisms, so at one point the world consisted of nothing but prokaryotic organisms. This has made humans think of some theories about how they came into existence.

Unicellular organisms need to:
eat, respire (most use oxygen to make sugar into energy), and get rid of waste (discard), like all other life. They need to do this throughout their lifespan to survive. Then they must perform sexual reproduction (make more of itself). Some cells reproduce using binary fission, which is a form of asexual reproduction that allows them to reproduce without a partner. Some unicellular organisms, like germ cells (including our own sperm) reproduce through mitosis, which is splitting in half to produce two germ cells, which are not always identical.

Some may do other things to help them survive, such as:
- move
- sense their environment
- get their energy from the sun (e.g., cyanobacteria), called photosynthesis
- ferment (e.g., yeasts)
- use anaerobic respiration (e.g. _Clostridium botulinum_)

Movement in response to the environment (the presence of food or dangers, like antibodies) is called chemotaxis. Movement as a response to the sun is called a photonastic response and some plants do this, like the californian poppy (Eschscholzia californica).

Some yeasts, like the yeast used to make beer, first use aerobic metabolism (to grow), then switch to anaerobic metabolism. Aerobic means "with air" and anaerobic is "without air".

### Viruses

There are also viruses, which are biological organisms. Viruses violate the rules that we have for cells (living things) so are not classed as living things. They _always_ reproduce inside the cells of other organisms, meaning they are cellular obligates. A lot of virus strains cannot survive outside of the cell, but some can for a short time, such as Hepatitis B and D (HBV & HDV). Hepatitis B virus can survive in the body for around 7 days.

They cause some important diseases. Viruses are made of RNA, or DNA, contained in a protein, and they reproduce themselves inside the cells of bacteria (bacteriophages) or eukaryotes. Sperm is a lot like a virus.

### Multicellular (eukaryotic)

Eukaryotic cells can be unicellular or multicellular. Multicellular organisms are made from many cells. They are complex organisms. This can be a small number of cells, or millions or trillions of cells. All plants and animals are multicellular organisms.

The cells of a multicellular organism are not all the same. They have different shapes and sizes, and do different work in the organism. The cells are specialised, this means they do only some kinds of work. By themselves they cannot do everything that the organism needs to live. They need other cells to do other work. They live together, but cannot live alone.

Alongside prokaryotes and eukaryotes, there were archea. These preferred (or could thrive in) extreme enviroments (like temperature or altitude.  It was thought these were always unicellular but there have been multicellular archea, and today mostly eukaryotes are focused on even when they prefer extreme environments.

One theory about these cells is that a prokaryotic cell was eaten by a larger cell, and this helped them to survive and reproduce as they evolved to have what we see today. An example of a prokaryotic cell (like germs on teeth or on surfaces) version of this is a group of cells living together on a biofilm for survival.

## Cell history

The cell theory includes these important ideas:
1. All living things are made of cells.
1. The cell is the basic unit of structure and function in all organisms.
1. Every cell comes from another cell that lived before it.
There is a 4th idea, because of advancements in biology: Cells carry genetic information in the form of deoxyribonucleic acid (DNA). This genetic material is passed on from parent to daughter cell.

Cells were discovered by Robert Hooke (1635–1703). He used a compound microscope with two lenses to look at the structure of cork, and to look at leaves and some insects. He did this from about 1660, and reported it in his book _Micrographica_ in 1665. He named cells after the Latin word _cella_, meaning room. He did this because he thought cells looked like small rooms in a cathedral.

Many other naturalists and philosophers tried out the new instrument. The structure of plants was investigated by Nehemiah Grew (1641–1712) and Marcello Malpighi (1628–1694). Grew's major work was _The anatomy of plants_ (1682). It is not clear who first saw animal cells, Malpighi, Jan Swammerdam (1637–1680) or Antonie van Leeuwenhoek (1632–1723).<sup>p17</sup>

Leeuwenhoek's discoveries and drawings of 'little animalcules' opened up a whole new world for naturalists. Protozoa, and microorganisms generally were discovered, and the discoveries about them are still going on today. Christian Gottfried Ehrenberg's book _Die Infusionsthierchen_ summarised what was known in 1838. Lorenz Oken (1779–1851) in 1805 wrote that infusoria (microscopic forms) were the basis of all life.

The idea that cells were the basis of the larger forms of life came in the 18th century. Finding out who did the work has taken some time:
- "The work of the Czech Jan Purkyně (1787–1869) and his student and collaborator Gabriel Valentin (1810–1883) was unjustly denigrated by the nationalistic Germans. They have a claim to some priority in the cell theory".<sup>Chapter 9</sup> Johannes Müller (1801–1858) also made great contributions. "It was, however, his student Theodor Schwann (1810–1882) and Matthias Schleiden (1804–1881) who got the credit for the _cell theory_, despite the fact that some of their observations were not correct, and their credits to previous workers were "a travesty".<sup>p97</sup>

## Cell reproduction

The body cells of animals divide by simple mitotic cell division. Sexual reproduction is ancestral in eukaryotes, and in animals it is carried out by specialised sex cells. They are produced by a process called meiosis.

Prokaryotic cells reproduce using binary fission, where the cell simply splits in half. For both mitosis and binary fission the cell must replicate (copy) all its genetic information (DNA) so that each new cell will have a copy.

## Related pages

- Cytology
- Plant cell