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)