The studies of Robert Hooke 1665 into a plant material would allow the determination of a pore like regular structure surrounded by a wall of which he called ‘cells’ this in itself unbeknownst to him, was the discovery of the fundamental unit of all living things.

In 1838 a botanist called Schleiden derived the theory ‘The basic unit of structure and function of all living organisms is the cell.’ Over 150 years later this can be regarded as one of the most familiar and important facts within the biological fields.

Drawing of cork cells published by Robert Hooke 1665

The Cell itself and use of Cytology:

The cell can be thought of as a bag in which the chemistry of life is allowed to occur, partially separated from the environment outside the cell, it exists within all living organisms as its basic structure.
The study of cells is made possible through the use of ‘cytology’ the preparation of materials for examination through microscopes as an average animal cell exists on a scale of 10 microns roughly one hundredths of a millimetres. Originally light microscopy was used in this field but with the advancement of knowledge scientists were restricted to 200nm magnification, or 2 tenths of a micron. Realising the existence of cell organelles within the cell structure, allowing the function of the cell itself to occur; It was necessary to increase magnification by utilising an alternate source radiation (alternate to light).The result was the electron microscope, whereby the short wavelength and negative charge of electrons when supplied with energy allowed for greater focusing with electromagnetism. This method bends the path of the beam in the manner of a lens to light.

Cell Organelles and the variation between Plant and Animal Cells:

We have already determined the cell to be the foundation to all organisms, however the term cell is associative and categorises a wide variation.
Every animal cell has a specified function whether it be the production of hair, mucus, or the process of other chemicals ( multiple reactions occur within a cell for other purposes i.e. creation of ATP, protein manufacture etc.) So from this we must examine the cell in more detail and determine what it is within the cell that creates it specialised function and separates it as an individual type.

Plant cells vary from animal through the existence of certain organelles.
Organelles are the substances that provide a cell with the ability to produce (a production line) and exist within the cells boundaries.

Typical Animal Cell.(Fig.1)
A plant cell requires a cell wall spanning the perimeter of the cells surface membrane and allocating a more defined form. This wall being rigid in nature embodies the pressure within the cell caused by the contained water (Large central Vacuole non existent within animal cells and surrounded by a Tonoplst membrane controlling the exchange between the vacuole and the cytoplasm.) This prevents the cell from bursting when more water enters through Osmosis. It is also recognised that Plasmodesmata links plant cells to neighbouring plant cells. These are fine strands of cytoplasm which pass through pore like structures in the walls of the neighbour.


Typical Plant Cell.(Fig.2)

Finally the plant cells required for photosynthesis contain chloroplasts these exist within the plastids family of organelles. Chloroplasts are relatively large green organelles that house chlorophyll necessary in collecting and processing sunlight.

Prokaryotes and Eukaryotes:
Eventually it was determined that cells could also be categorised into to two fundamental groups pro, and eukaryotes.
Organisms that lack nuclei are recognised as Prokaryotes ( Pro meaning before and karyote meaning nucleus). These cells all can be regarded as bacteria and exist at a magnification upto 10,000 times smaller than animal cells.
Eukaryote (Eu meaning true) these cells such as plant, animal and fungi all contain the DNA information stored within a nucleus and subsequently contain the ability to divide and replicate.

Organelles within Animal Cell The Nucleus:
The nucleus controls the cell’s activities and is the most noticeable organelle in a eukaryotic cell. Division of the nucleus precedes cell division the process in which cells multiply to create tissues, organs, and finally organisms (mitosis, meiosis).
Chromatin is contained within the nucleus this being the loosely coiled form of chromosomes (see later) and these exist within the nuclear plasma, which is contained via the nuclear membrane/envelope.
The Nucleus(Fig.3)
The nuclear plasma is the substance that acts as an atmosphere within the nucleus (similar to the cytoplasm within the cell.) This carries various materials whether it be for transportation to the exterior of the nuclei or just storage.
The nuclear membrane allows for the exchange of substances through pore like openings around its perimeter (nuclear pores) and grants access to these into the opposing cytoplasm.

Chromosomes:
Chromosomes are the carriers of DNA the substance which is eventually organised into genes and furtherly control the specialised function of the cell and its inheritance. DNA is a complex molecule carrier of the information determining cell processes it is associated with histone proteins and can resultantly be called chromatin.

The Nucleolus:
The large body central to the nuclei and used in the production of ribosome’s is known as the nucleolus.
The nucleolus is made up of closely formed loops of DNA.

Cytoplasm:
This is the aqueous material , varying in consistency from fluid to jelly-like. The cytoplasm is the unit of containment to all the organelles within the cell and makes up the major part of the cells form.

Ribosomes:
Produced within the nucleus via the nucleolus from ribosomal RNA and protein (65% RNA and 35% protein) The nuclear pores within its membrane allow passage of ribosomes into the cells liquid carrier cytoplasm where they either float freely or attach themselves to the endoplasmic reticulum.
They consist of two parts a smaller and larger sub-unit and function in order to synthesise various proteins through ‘translation’.
In combining with endoplasmic reticulum and resultantly creating rough ER the proteins produced are prevented from floating loosely` within the cytoplasm

Endoplasmic Reticulum:
A series of interconnecting flattened tubular funnels contained within all eukaryotic cells the endoplasmic reticulum (ER) exists at around a tenth of the cells total presence.
ER takes two forms Rough and small ER. Smooth ER serves for storage of key enzymes and the products of these enzymes. The large network of smooth ER increases the surface area of the cell for greater capacity.

Rough and smooth Endoplasmic Reticulum.(Fig.4)

Rough ER however functions as ribosomes transport carrier. Once linked to ER (becoming rough ER) ribosomes begin protein synthesis. Newly produced ribosomes are threaded through pores in the ER’s membrane where they accumulate within the cisternal space. (The membranes form a system of flattened sacs like sheets known as ‘Cisternae’.) Here they are able to fold into there normal three-dimensional shape.
Small Vesicles containing newly synthesised protein separate from the ends of the rough ER. Vesicles are the shipping containers within cells. They are used to package liquids containing a wide variety of substances and carry these materials to other parts of the cell or to the outside of the cell. Vesicles also form around material (liquid or solid) that are brought into the cell.
Once the proteins have been engulfed and separated into vesilcles they either pass directly into the cytoplasm for use within the cell or passage to its exterior , or they are collected via the Golgi Apparatus for further protein modification.

Golgi Apparatus:
A stack of membranous flattened sacs and directly associated with the endoplasmic reticulum.
The golgi apparatus collects proteins created via the ribosomes within rough ER and then transports them through pinched vesicles from the ER’s tip. These proteins are subsequently modified within the golgi apparatus. for example. Additional sugar molecules create glycoproteins. The Golgi Apparatus is responsible for Lysosome manufacture.


The above drawing shows an actual interface between the ER and the Golgi complex. The "Export complex" is seen at the top of the drawing. Note that the vesicle are moving to contribute to the cis-Golgi network of vesicles and cisternae. (Fig. 5)

Lysosomes:
Lysosomes more common in animal cells than plant contain hydrolyphic enzymes which are necessary for intercellular digestion for example white blood cells breaking down bacteria.
Lysosomes content are carefully released into the vacuole around the bacteria and serve to kill and digest. Uncontrolled release of hydrolytic enzymes into the cytoplasm can result in ‘necrosis (cell death)

Centrosomes/Microtubules:
The centrosome also known as ‘the microtubules organising center’ is an area within the cell responsible for the production of microtubules. They contain a pair of small organelles called centriolles which are arranged perpendicular to each other. Centriloes are made up of a ring of nine groups of microtubules, where by there are three fused microtubles in each.
Cell division showing the centrioles and the production of a spindle.(Fig.6)

During animal cell division the centrosomes divide allowing the centrioles to replicate. These then move to opposite ends of the nucleus where the microtubles grow into a spindle.
Microtubulars make up the spindle that separates chromosomes during mitosis (cell division). Theses are only present in cells dividing.

Peroxisomes
This organelle is responsible for protecting the cell from its own production of toxic hydrogen peroxide. As an example, white blood cells produce hydrogen peroxide to kill bacteria. The oxidative enzymes in peroxisomes break down the hydrogen peroxide into water and oxygen.

Mitochondria:
Mitochondria (singular: mitochondrion) are the sites of aerobic respiration, and generally are the major energy production centre in eukaryotes. They produce ATP the universal energy carrier of cells. Mitochondria have two membranes, an inner and an outer, clearly visible in this electron microscope photo of mitochondrion(fig .6). The ‘reticulations’, or many infoldings, of the inner membrane, serves to increase the surface area of membrane on which membrane-bound reactions can take place.



Mitochondrion as seen through an electron microscope. (Fig. 7)


Cell Membrane:
The thin membrane, which surrounds all cells, is essential in controlling exchange between the cell and its environment. It acts as a very efficient barrier, but also allows a controlled traffic of materials across it in both directions. The membrane is therefore considered as partially permeable, if this was not the case the cell chemicals would simply mix with external chemicals through diffusion and life would not exist.

The Cell and its various organelles. (Fig. 8)