Classification of Cells
Inquiry question: What distinguishes one cell from another?
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investigate different cellular structures, including but not limited to:
 examining a variety of prokaryotic and eukaryotic cells
Introduction: Classification of Cells
Cells are the basic structural units of all living things - the building blocks of life. The cell is the basic unit of life and
the smallest unit capable of living independently. It is the first distinguishable level of the structural and functional
organisation of the biological world.
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There are two fundamentally different types of cells - prokaryotic and eukaryotic - however all cells regardless of
classification have the following in common:
Cell membrane (encloses the contents of a cell and controls movement of substances between the extracellular
fluid outside the cell and the intracellular fluid inside the cell)
Cytoplasm (everything inside a living cell but not including the nucleus)
Ribosomes (synthesises proteins)
DNA (the genetic material of the cell)
Organisms are classified according to the type of cell of which they are composed. Some organisms have only one
cell (unicellular) that must be capable of functioning on its own. Other organisms have many cells (multicellular)
each with its own specific structure and function, working together to ensure the organism functions.
Prokaryotic Cells
Prokaryotic cells have a cell membrane, however their organelles are not membrane-bound and most metabolic
reactions occur in the cytosol. Their DNA is found in the cytoplasm as they have no nucleus, and their chromosomes
are circular in shape. Prokaryotic cells are generally smaller and less complex than eukaryotic cells, ranging in size
from 01. to 5.0 µm.
Prokaryotes are usually unicellular organisms - some bacteria will clump together as a colony of cells working
together, but they are still unicellular organisms.
Many prokaryotes have fimbriae, short hair-like structures on the cell surface that help the organism 'stick' to other
cells or a substrate. They are often surrounded by a capsule, a sticky layer of protein or polysaccharide that can
help evade the host's immune system or help the organism stick to other cells or surfaces. They may also have
flagella, a tail-like structure used by motile bacteria for movement either towards or away from stimuli.
Prokaryotes can be classified into two main groups - bacteria and archaea.
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Bacteria
Bacteria are found in many different environments and can either be beneficial or harmful to other living organisms.
Bacteria that are harmful or cause disease are described as pathogenic.
There are many different types of bacteria, some examples include cyanobacteria and nitrogen fixing bacteria.
Cyanobacteria (blue-green bacteria) are photosynthetic; containing the pigments chlorophyll a (like plants) and
phycocyanin (making them 'blue-green') and phycoerythrin (red); they can also fix nitrogen from the air.
Nitrogen fixing bacteria include both bacteria and cyanobacteria - e.g. Rhizobia bacteria live in the roots of legumes
in a mutually beneficial relationship - the plants protects the Rhizobia in exchange for the compounds made by the
Rhizobia from the nitrogen in the air.
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Archaea
Archaea are prokaryotes found in harsh environments such as hydrothermal vents and hot springs. Some Archaea
are called extremophiles due to the extreme conditions in which they thrive. They can make a variety of protective
molecules and enzymes to help them survive in these conditions.
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Some archaea can produce defensive molecules on their cell surface to prevent the action of acids; others can use
a variety of materials as a source of energy, e.g. Hydrogen gas or carbon dioxide.
Methanogens produce methane gas as a waste product and can live in anaerobic conditions (without oxygen)
Halophiles live in salty environments
Thermophiles live at extremely hot temperatures
Psychrophiles live at unusually cold temperatures
Eukaryotic Cells
Eukaryotic cells can be classified as those which contain a membrane-bound nucleus that holds the genetic material
of the cell. Other organelles are also membrane-bound, e.g. vacuole and mitochondria, however not all eukaryotes
have these organelles - the nucleus is common to all eukaryotes. Eukaryotic cells are larger and more complex than
prokaryotic cells, ranging in size from 10 to 100 µm - generally ten times larger than the typical prokaryote. The
DNA found in the nucleus is linear in shape.
Eukaryotes are composed of eukaryotic cells and can be either unicellular or multicellular organisms. They include
protists, fungi, plants and animals. Eukaryotes have specialised cells that carry out biochemical processes, e.g.
Chloroplasts carry out photosynthesis; mitochondria carry out cellular respiration.
Endosymbiont theory
Endosymbiont theory proposes that organelles like chloroplasts and mitochondria were once small prokaryotes
that were engulfed by larger cells to form a symbiotic relationship with the host. Over time, the host (larger cell)
and the endosymbiont (smaller prokaryote) became inseparable and eukaryotes evolved from this.
One piece of evidence to support endosymbiont theory: the eukaryotic organelles chloroplasts and mitochondria
contain their own DNA (separate to the cellular DNA found in the nucleus), which scientists have identified as
being similar to the chromosomes found in bacteria.
A Venn diagram is an illustration of the relationships between and among 'sets' (groups of objects)
that share something in common.
- Properties that the objects share are shown in the intersection of circles.
- Properties that are unique to one object are shown in the part of the circle that does not
intersect with another circle.
ACTIVITY: Draw a Venn diagram showing the similarities and differences between prokaryotic and
eukaryotic cells.