Know that cell theory is a unifying concept stating that cells are a fundamental unit of structure,
function and organisation in all living organisms.
• Understand the ultrastructure and function of organelles in the following cells:
o prokaryote cells (bacterial cell) – nucleoid, plasmids, 70S ribosomes, capsule,
cell wall
o eukaryotic cells (plant and animal cells) – plasma membrane, cytoplasm, nucleus,
nucleolus, endoplasmic reticulum (smooth and rough), Golgi apparatus, vesicles,
lysosomes, 80S ribosomes, mitochondria, centriole
o eukaryotic cells (plant-cell specific) – cell wall, chloroplasts, vacuole, tonoplast,
amyloplasts, plasmodesmata, pits.
Prokaryotic and eukaryotic cells
At the most basic level, organisms can be divided into two classes, depending on the
fundamental nature of their cells. These classes are prokaryotes and eukaryotes.
Prokaryotes contain two groups: bacteria and archaea. They are always unicellular,
and contain no membrane-bound structures that are found in eukaryotes. On the
other hand, eukaryotes consist of plants, fungi and animals. They differ from
prokaryotes because they are much more complicated. They contain complex
substructures that are specialised for different functions, called organelles, which
work in concert to allow the cell to carry out its purpose
What do all cells have in common?
There are some key elements that a cell usually needs in order to function,
regardless of whether it is a prokaryote or a eukaryote. This includes the genetic
material (DNA), which is often referred to as the “instruction manual” for the cell. All
cells also have a plasma membrane, which is the outer layer of the cell that protects
the inner contents from the outer environment. Additionally, the cytoplasm, which
consists of all of the material inside the cell (excluding the highly specialised
substructures called the nucleus in eukaryotes), and ribosomes, which are another
important organelle that synthesise proteins based on the information contained
within DNA
Prokaryotic cells
Prokaryotes are much simpler than eukaryotes, and lack membrane-bound
organelles. One of the most specialised membrane-bound organelles that exists in
eukaryotes is the nucleus, where DNA is stored. Prokaryotes do not possess a
nucleus, and therefore the genetic material is not encased in a specialised
substructure, but is instead found in a cytoplasmic region called the nucleoid. The
DNA is also a much more simple structure than it often is in eukaryotes. In
eukaryotes, the DNA is organised into complex, protein-rich structures called
chromosomes. However, in in prokaryotes, the DNA is not bound to any proteins,
and forms circular structures called plasmids.
Prokaryotic cells also typically have a cell wall made out of peptidoglycan, a
polymer composed of sugar and amino acids that forms a protective mesh on the
other layer of the cell. This helps the cell protect its shape and regulate its water
content. Some bacteria also have an additional layer of carbohydrates called the
capsule. The capsule protects the cell from being engulfed by eukaryotic cells, such
as the cells of the immune system.
Some bacteria also possess flagella, which are outer whip-like structures that act as
sensors and help the bacteria to move. They may also have hair-like structures
known as pili, which are required for the exchange of information between cells
Eukaryotic cells
Eukaryotic cells have a variety of different compartments with specialized functions.
These structures are separated by their own membranes, meaning they can regulate
their own internal environment to diversify their functions.
Some key features of a eukaryotic cell include the presence of a nucleus, the
important structure that holds the genetic material. The genetic material is also
stored differently, it is packaged into multiple chromosomes, rather than in a circular
molecule. There are also several additional membrane bound organelles (such as
mitochondria, chloroplasts, Golgi bodies, vacuoles and an endoplasmic
reticuli).
Summary table: the differences between
prokaryotes and eukaryotes
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Until microscopes became powerful enough to view individual cells, no-one knew
for certain what living organisms were made from
A scientist called Robert Hooke is thought to be the first person to view cells
(including single-celled microorganisms) and Hooke also came up with the term
“cells” to describe these newly discovered structures
Matthias Schleiden and Theodor Schwann were two other scientists who
studied animal and plant cells
o In 1837, they came up with the idea that all living organisms are made
of cells
o This idea is known as ‘cell theory’
o The cell theory is a unifying concept in biology (it is universally
accepted)
The cell theory includes three main ideas:
o All living organisms are made up of one or more cells
o Cells are the basic functional unit (i.e. the basic unit of structure and
organisation) in living organisms
o New cells are produced from pre-existing cells