There are estimated about 6x10¹³ cells in a human body, of about 320 different types. For instance, there are several types of skin cells, muscle cells, brain cells (neurons), among many others. The number of cell types is not well-defined, it depends on the similarity threshold (what level of detail we would like to use to distinguish between the cell types, e.g., it is unlikely that we would be able to find two identical cells in an organism if we count the number of their molecules). The cell sizes may vary depending on the cell type and circumstances. For instance, a human red blood cell is about 5µ (0.005 mm) in diameter, while some neurons are about 1 m long (from spinal cord to leg). Typically the diameter of animal and plant cells are between 10 and 100 microns.

Viruses are not quite living organisms, but when inside a living host cell they show some features of a living organism. Viruses are too small to be seen in an optical microscope, but are big enough to reveal their structure in an electron microscope (the characteristic size of the virus is about 0.05-0.1µ, while the wavelength of green light is about 0.5µ).

Before the invention of microscopes there was no way to observe the small-scale structure of living things. Starting in the 17th century technology began to develop to enable scientists to see the cells of microbes, and to see the cellular structure of larger animals. Robert Hooke (1665) coined the term "cell" while looking at the nonliving tissue known as cork. In 1675, Anton van Leeuwenhoek, a Dutch lens maker, first reported observations of living cells which he called "animicules". Robert Brown (1831) was the first to report the discovery the nucleus of the cell. The understanding that larger animals were multicellular was relatively slow in develop. It was not until the 19th century that the idea that all animals and plants were constructed of cells, and that cells have a kind of life-cycle of their own was thought of. Matthias Jakob Schleiden (1838), a German Botantist, after extensive studies reported that all plants were composed of fundamental units known as cells. In 1839, Theodore Schwann, a German Zoologist, after extensive studies reported that all animals were composed of fundamental units known as cells. Rudolf Virchow (1858) proposed that all cells came from pre-existing cells. The basic tenets of the cell theory:

• All organisms are composed of cells
• Cells are the basic unit of life
  
• Cells arise only from other pre-existing cell

There are two types of organisms - eukaryotes and prokaryotes, and two types of cells respectively. The term "eukaryote" means "true nucleus" while "prokaryote" means "before the nucleus". This emphasizes the central importance of the nucleus to the eukaryotic cell, and suggests that prokaryotes are more primitive organisms, and that eukaryotes evolved from them by, among other things, acquiring a true nucleus. The both do have DNA for genetic material, have a exterior membrane, have ribosomes, accomplish similar functions, and are very diverse. For instance, there are over 200 types of cells in the human body, that very greatly in size, shape, and function.

• Unicellular
• Nucleus absent
• Genetic material is not enclosed within a membrane
• Do not engulf solids
• Do not have centrioles or asters
• Do not have internal membrane-bound organelles
• Smaller than eukaryotes (diam 1µ)
• E.g. bacteria and cyanophyte

• Unicellular or multicellular
• Nucleus present
• Genetic material is surrounded by a membrane
• Engulf solids
• Have centrioles or asters
• Do not have internal membrane-bound organelles
• Larger than prokaryotes
• E.g. humans, algae, protazoa

Bacteria belong to the prokaryotes. However, most organisms which we can see, such as trees, grass, flowers, weeds, worms, flies, mice, cats, dogs, humans, mushrooms and yeast are eukaryotes. The distinction between eukaryotes and prokaryotes is rather important, because many of the cellular building blocks and life processes are quite different in these two organism types. This is believed to be the result of different evolutionary paths.

Prokaryotes are the simplest cells. Prokaryotic cells are smaller than eukaryotic cells (a typical size of a prokaryotic cell is about 1 micron in diameter) and have simpler structure (e.g., they do not have any inner cellular membranes that are always present in Eukaryotes). Prokaryotes are single cellular organisms, but note that being a single cell does not mean that an organism is a prokaryote. Being smaller than eukaryotes does not mean that prokaryotes are any less important – for instance it is quite likely that the number of bacteria living in the mouth and digestive tract of a human are larger than the number of eukaryotic cells in the same individual and many of these bacteria are necessary for a human being to live a normal life (these numbers are rather difficult to estimate, rather a hypothesis). Prokaryotes are sometimes also known as microbes.

• Prokaryotic cells have the simplest overall structure.
• The prokaryotic cell is bounded by a lipid bilayer membrane, but does not contain any internal membrane-bound organelles. It contains a region rich in DNA called a nucleoid.
• The nucleoid contains a circular molecule of DNA which is the cells genetic material, or genome.
• Surrounding the nucleoid is a region of cytoplasm rich in ribosomes, small protein-RNA structures which do the job of synthesizing proteins. Finally, surrounding the plasma membrane is a cell wall.
• Prokaryotes can have surface appendages which do particular jobs. Flagella are used for locomotion. Pili are used for sexual reproduction (mating)
• One of the more surprising aspects of cell structure is the way that DNA molecules are packaged into cells.
• The genome of a common bacterium like the gut bacterium Escherichia coli is about 1.5 millimeters in length, but the length of the bacterium is about 1000 times shorter.
• In the nucleoid, the DNA is both compacted so as to fit within the bacterium, and yet is still accessible to the machinery necessary both to read the genetic instructions (synthesize RNA) and to copy the DNA (replicate).

Eukaryotic cells are much more complex.

• By contrast to prokaryotic cells, eukaryotic cells are much more complicated. Much of the cell is taken up by subcellular organelles bound by their own membranes (analogous to the plasma membrane surrounding the cell)
• Most importantly, eukaryotic cells have a nucleus which contains all of the cell's DNA. In fact, .
• The problem of packaging DNA is greater in eukaryotes. A human genome would stretch about 1 meter, but must fit in a cell 50,000 times smaller.
• However, eukaryotic cells (which included plants, animals, and fungi) include many other subcellular structures with important roles in cellular metabolism

Cells are 90% fluid (cytoplasm) which consists of free amino acids, proteins, glucose, and numerous other molecules. The cell environment (ie. the contents of the cytoplasm, and the nucleus, as well as, they way the DNA is packed) affect the gene expression/regulations, and thus are very important parts of inheritance, below are approximations of other components:

Elements:

• 59% Hydrogen (H)
• 24% Oxygen (O)
• 11% Carbon (C)
• 4% Nitrogen (N)
• 2% Others - Phosphorus (P), Sulphur (S), etc.
  

As far as molecules that make up the cell:

• 50% protein
• 15% nucleic acid
• 15% carbohydrates
• 10% lipids
• 10% Other
  

What is inside the cell is the cytoplasm which is:

• Cytosol - a lot of water - all except the organelles.
  

Organelles (which also have membranes) in 'higher' eukaryote organisms:

• Nucleus (in eukaryotes) - where genetic material (DNA) is located, RNA is transcribed. Nucleolus is rich in RNA and site of ribosomal RNA synthesis and assembly.
  
• Endoplasmic Reticulum (ER) - Membranous structure important for protein synthesis. It is a transport network for molecules destined for specific modifications and locations. There are two types:
  
  • Rough ER - Has ribosomes, and tends to be more in 'sheets'. Synthesis of exported proteins.
    
  • Smooth ER - Does not have ribosomes and tends to be more of a tubular network. Synthesis of lipids.
    
• Ribosomes - Half are on the Endoplasmic Reticulum, the other half are 'free' in the cytosol, this is where the RNA goes for translation into proteins.
  
• Golgi Apparatus - Membranous structure important for glycosylation and secretion. Intermediate in protein export.
  
• Lysosomes - Digestive sacks - the main point of digestion, these are only found in animal cells.
  
• Peroxisomes - Use oxygen to carry out catabolic reactions, in both plant and animals.
  
• Microtubules - Made from tubulin, and make up centrioles,cilia,etc.
  
• Cytoskeleton - Comprises microtubules, actin and intermediate filaments; supports internal structures and gives shape to the cell.
  
• Mitochondria - convert foods into usable energy. (ATP production) A mitochondrion does this through aerobic respiration. They have 2 membranes, the inner membranes shapes differ between different types of cells, but they form projections called cristae. The mitochondrion is about the size of a bacteria, and it carries its own genetic material and ribosomes.
• Vacuoles - Storage organ for storage of nutrients & water. More commonly associated with plants which commonly have large vacuoles.

Found in Plants and not in animals:

• Plastids - Specialised organelles found only in Algae and Plants; include chloroplasts, chromoplasts and amyloplasts. Chloroplasts contains stacks of flattened membranes specialized in photosynthesis. Chromoplasts store pigments which color plant parts (e.g. petals). Plastids which store starch (e.g. in roots) are the amyloplasts, which are also called leukoplasts.
  
• Cell Wall - found in prokaryotic plants and it provides structural support and protection

• The most important difference between plant and animal cells is the fact that plant cells do not have the capacity for movement.
• Movement of animal cells is important in normal cell function (e.g., phagocytic cells). Movement is critical in development.
• Animal cells are also not able to trap energy from light since they lack chloroplasts. They are required to get energy necessary to life by eating other living things.

The rough endoplasmic reticulum is seen as stack of cisternae connected by bridges. It has an interior space or "lumen" which is continuous with the lumen of the nuclear envelope. The surface is covered by ribosomes, which impart the "rough" appearance. Proteins synthesised by these attached ribosomes pass into the lumen. Small transitional vesicles bud from the roughER which carry proteins destined for processing by the Golgi. As well as connecting to the nuclear envelope, the rough ER is continuous with the smooth endoplasmic reticulum.
Name(s): rough endoplasmic reticulum, rough ER
Location: throughout cytoplasm
Appearance: sacs
Size: about 100nm in depth
Function: used in protein synthesis.
The smooth endoplasmic reticulum is seen as a tangle of interconnected tubules. Unlike the rough ER, it lacks ribosomes. The smooth ER is continuous with the rough ER but has different functions.
Name(s): smooth endoplasmic reticulum, smooth ER, tubular endoplasmic reticulum, agranular endoplasmic reticulum
Location: in cytoplasm
Appearance: interconnected and ramifying tubules; no ribosomes
Size: tubules are about 150nm in diameter (variable)
Function: used in lipid synthesis, detoxification and other metabolic processes.

The cell cycle is an ordered set of events, culminating in cell growth and division into two daughter cells. Non-dividing cells not considered to be in the cell cycle. The stages are G1-S-G2-M. The G1 stage stands for "GAP 1". The S stage stands for "Synthesis". This is the stage when DNA replication occurs. The G2 stage stands for "GAP 2". The M stage stands for "mitosis", and is when nuclear (chromosomes separate) and cytoplasmic (cytokinesis) division occur.

Regulation of the cell cycle
How cell division (and thus tissue growth) is controlled is very complex. The following terms are some of the features that are important in regulation, and places where errors can lead to cancer. Cancer is a disease where regulation of the cell cycle goes awry and normal cell growth and behavior is lost.

• Cdk (cyclin dependent kinase, adds phosphate to a protein), along with cyclins, are major control switches for the cell cycle, causing the cell to move from G1 to S or G2 to M.
• MPF (Maturation Promoting Factor) includes the CdK and cyclins that triggers progression through the cell cycle.
• p53 is a protein that functions to block the cell cycle if the DNA is damaged. If the damage is severe this protein can cause apoptosis (cell death).
  • p53 levels are increased in damaged cells. This allows time to repair DNA by blocking the cell cycle.
  • A p53 mutation is the most frequent mutation leading to cancer. An extreme case of this is Li Fraumeni syndrome, where a genetic a defect in p53 leads to a high frequency of cancer in affected individuals.

p27 is a protein that binds to cyclin and CdK blocking entry into S phase. Recent research (Nat. Med.3, 152 (97)) suggests that breast cancer prognosis is determined by p27 levels. Reduced levels of p27 predict a poor outcome for breast cancer patients.

Mitosis is nuclear division plus cytokinesis, and produces two identical daughter cells during prophase, prometaphase, metaphase, anaphase, and telophase. Interphase is often included in discussions of mitosis, but interphase is technically not part of mitosis, but rather encompasses stages G1, S, and G2 of the cell cycle.

Interphase
The cell is engaged in metabolic activity and performing its prepare for mitosis (the next four phases that lead up to and include nuclear division). Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible. The cell may contain a pair of centrioles (or microtubule organizing centers in plants) both of which are organizational sites for microtubules.

Prophase
Chromatin in the nucleus begins to condense and becomes visible in the light microscope as chromosomes. The nucleolus disappears. Centrioles begin moving to opposite ends of the cell and fibers extend from the centromeres. Some fibers cross the cell to form the mitotic spindle.

Prometaphase
The nuclear membrane dissolves, marking the beginning of prometaphase. Proteins attach to the centromeres creating the kinetochores. Microtubules attach at the kinetochores and the chromosomes begin moving.

Metaphase
Spindle fibers align the chromosomes along the middle of the cell nucleus. This line is referred to as the metaphase plate. This organization helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome.

Anaphase
The paired chromosomes separate at the kinetochores and move to opposite sides of the cell. Motion results from a combination of kinetochore movement along the spindle microtubules and through the physical interaction of polar microtubules.

Telophase
Chromatids arrive at opposite poles of cell, and new membranes form around the daughter nuclei. The chromosomes disperse and are no longer visible under the light microscope. The spindle fibers disperse, and cytokinesis or the partitioning of the cell may also begin during this stage.

Cytokinesis
In animal cells, cytokinesis results when a fiber ring composed of a protein called actin around the center of the cell contracts pinching the cell into two daughter cells, each with one nucleus. In plant cells, the rigid wall requires that a cell plate be synthesized between the two daughter cells.