Welcome to topic 2.2, Prokarytoic Cells. This topic is about the smallest cells we know,
and also the simplest in structure. Prokaryotic cells are named after the Greek “pro”
meaning ‘before’ and “karyon” meaning ‘kernel’, in a reference to the absence
of a nuclear membrane.
- As you remember from topic 2.1, cells are
the smallest unit of life. “Small”, however, comes in different sizes and definitely different
shapes. Here you can see different types of bacteria, like Staphiloccocus (which are rod-shaped)
and Meningitidis (spherical). As you also remember from our Cell Theory chapter, these
cells are each around 1 micrometer in length. -
Some of the key characteristics of prokaryotic cells is that they have no internal membranes.
This means that all biochemical reactions happen dissolved in the cytoplasm. No internal
membranes also means no membrane-bound organelles, like mitochondria, chloroplasts or endoplasmic
reticulum.
The DNA is ‘naked’, meaning it’s not associated with proteins. The DNA, like everything
else, is dissolved in the cytoplasm, but found in a particular region of the cell called
the nucleoid.
The ribosomes are a common structure found in both prokaryotic cells (bacteria) and eukaryotic
cells. However, ribosomes in prokaryotic cells have a sedimentation rate of 70S, while 80S
ribosomes are found in eukaryotic cells.
Prokaryotic cells divide through binary fission, where the cell doubles it’s contents and
splits into two daughter cells. -
This is a diagram that shows the basic sequence of binary fission. You can see the DNA doubling
and separating in step 1, moving towards opposite ends of the cell in step 2 and the cell completely
separating in steps 3 and 4. -
One of the key structures of the prokaryotic cell is the pili, responsible for cell-to-cell
adhesion. Because there is power in numbers, unicellular organisms like bacteria usually
benefit from grouping to form colonies. The pili help them connect and attach to one another.
You can see the nucleiod, which we discussed a couple of slides back, as well as the ribosomes.
The plasma or cell membrane, delimits the cell, while the capsule and the cell wall
act as a defense mechanism for the bacteria. This cell wall, by the way, is often the target
of antibiotics, in an attempt to disrupt the structure of the bacteria causing harm to
other organisms.
Some bacteria have a flagella, which is used for movement.
Remember that you have to be able to draw the structure of an E. coli bacteria from
scratch. The simpler diagram on Allott page 6 is a better choice than the more complex,
three-dimensional diagrams on Campbell. Make the drawing simple and focus on clearly showing
the unique structures that are part of the prokaryotic cell, rather than on perspective
or shading. On your exam, you’ll absolutely have to draw something, and you want to make
sure not to spend more time than you absolutely have to on the drawing. In previous DP Biology
exams, for instance, a question asking you to draw and label a prokaryotic cell (without
annotations) is allocated 5 minutes. -
This is a great place to pause the video for a while and make sure you have recorded all
of the functions of these prokaryotic structures. -
This concludes topic 2.2. Make sure you have the diagram of the prokaryotic cell in your
drawing booklet and I’ll see you guys in class!
