Introduction to Parkinson's Science/PD Cells and Cytology

What is a cell?

In 1665 the British scientist and philosopher Robert Hooke was one of the first persons to carry out the microscopic observation of biological objects. In examining samples of cork he recorded that it comprised regular, discrete structures which he called “cells” likening them to the small rooms occupied by monks.

Further observations led to the publication in 1839 of a generalised cell theory propounded by Matthias Jakob Schleiden and Theodor Schwann, which states in summary that:-

• all living organisms are composed of one or more cells;

• all cells come from pre-existing cells;

• vital functions of an organism occur within cells;

• all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.

and this theory forms the foundation of modern biology. The branch of science relating to the study of cells is called Cytology.

What are a cell’s properties?

Some organisms consist of a single cell and are termed ‘unicellular’. Some organisms are made up of collections of cells with different functions e.g. liver cells and heart cells. These are termed ‘multicellular’ organisms.

All living organisms observed to date comprise two types of cell. Some cells have a relatively simple structure and are termed Prokaryotic. Others are more complex and are termed Eukaryotic.

Humans are eukaryotic creatures and each one comprises some 100,000,000,000,000,000 cells.

Geological evidence indicates that on Earth prokaryotic organisms appeared first some 3.5 billion years ago. Eukaryotic organisms appeared some 0.5 billion years ago and evolved rapidly into a wide variety of forms during a period known as the ‘Cambrian Explosion’. This evolutionary process is still ongoing.

How are organisms classified?

All (known) organisms are classified into one of three Kingdoms, each of which is subdivided into Families (phyla).

Archea are prokaryotic organisms. They were originally discovered in harsh environments such as hot springs and salt lakes and were for a long time thought to be a variety of bacteria, but significant differences emerged, and in 1977 they were designated as a Kingdom and may have evolved independently from bacteria. They have subsequently been found to inhabit all manner of environments including soils and oceans and the human gut. Five phyla have been identified, but study and classification is ongoing. No connections to human disease conditions have (yet) been identified and they have no known relevance to PD.

Bacteria are a prokaryotic Kingdom and constitute the major portion of the biomass. For example in a human being some 10% of cells are derived from the host but 90% are bacteria with the status of passengers. If the host dies the bacteria live on for some time and dine off their erstwhile host’s remains. Bacteria can be benign or beneficial or hostile.

The third Kingdom comprises all other organisms. They are eukaryotic and include mammals, birds, fish, invertebrates, fungi, plants, and complex unicellular organisms.

Eukaryotic cells are about 10 times the size of a prokaryote and can be as much as 1,000 times greater in volume. The major difference between prokaryotes and eukaryotes is that eukaryotic cells contain membrane-bound compartments in which specific metabolic activities take place. Most important among these is the presence of a nucleus, a membrane-delineated compartment that houses the eukaryotic cell’s hereditary material (DNA). It is this nucleus that gives the eukaryote—literally, ‘true nucleus’—its name.

Organisms which develop unique characteristics that are passed on unchanged between generations are termed ‘species’. . Sometimes organisms of different species form a working relationship to ensure mutual survival and are termed ‘symbiotic.

What are the features of prokaryotic cells?

PART	DESCRIPTION	FUNCTION
Membrane	Outer envelope	Determines what can enter the cell
Ribosome	Self-contained unit	Manufactures proteins
Cytoplasm	Jelly-like substance	Defines cell's size and shape
Flagella	Tail-like extrusion	Propels cell by undulation
Cilla	Oar-like extrusion	Propels cell by reciprocating action
DNA	Double helix of amino acids	'Recipes' for protein manufacture and the basis for inheritance
Pyli	Filament	Protects cell's surface and provides adhesion.

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What are the features (organelles)of eukaryotic cells?

Additional to those of a prokaryotic cell.

PART	DESCRIPTION	FUNCTION
Mitochondria	Resembles a prokaryotic cell	Power supply
Nucleus	Containment sac	Stores DNA
Endoplasmic reticulum		Surface for chemical activity
Lyosome		Digestion centre
Golgi body		Stores and releases chemicals
Nuclear membrane		Holds nucleus together
Plastid		Stores food or pigment
Vacuole		Contains water and dissolved minerals

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What are Proteins?

One of the functions of a cell is the manufacture of proteins, which are organic compounds carrying out a specific biological function.

The largest component of muscles, cells and other tissues is water. The second largest component is proteins built from amino acids.

An amino acid is a molecule made from amine(-NH2) and carboxylic acid (-COOH) Functional groups, along with a side-chain specific to each amino acid. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen. About 500 amino acids are known but humans make do with 20 of these.

Amino acids are joined together in chains to form peptides, which are joined together to form polypeptides.

Polypeptides then fold naturally into globular or fibrous shapes and the result is a protein. The shape of a protein normally determines its biological function.

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