Included in the Biological and Biochemical Foundations of Living Systems section of the MCAT, the growth and physiology of prokaryotic cells is a key topic you need to revise for the exam. This guide will provide a comprehensive overview of everything you need to know to help you achieve a good MCAT score.
You can also find more information about the classification and structure of prokaryotic cells in our blog.
Prokaryotes reproduce through binary fission rather than mitosis (seen in eukaryotes). It is a type of asexual reproduction that involves the single circular chromosome replicating, these two chromosomes then move to opposite poles of the cell as the cell elongates. Cytokinesis then divides the parent cell into two identical daughter cells. This means that prokaryotes experience no genetic diversity during reproduction.
There are many ways that a prokaryotic genome can be altered over time. The ability for the genome to change allows it to adapt to changing environmental conditions, something that can be very important, especially in times like the present where global warming is a massive threat to organisms that are not able to adapt well enough to the new conditions they find themselves in.
Mutation is a very common event that happens if genetic information has not been correctly replicated. There are many different types of mutations but the ones that are likely to lead to a change in an organism is called a missense mutation – this is a mutation that leads to a change in the amino acid sequence produced during protein synthesis. This change makes it more likely for a protein to be rendered inactive or to function differently, therefore, changing a potentially key aspect in the prokaryote’s life (for better or worse).
Prokaryotes can also obtain new genetic information through horizontal gene transfer. This is where genetic information can move between organisms without the need for replication. This process is how antibiotic resistance can be spread between bacteria.
There are three main processes that facilitate horizontal gene transfer:
Due to binary fission (one cell dividing into two cells), prokaryotic populations grow at an exponential rate which means that growth becomes increasingly quick. However, this can only happen if optimum conditions are maintained; in a closed system this optimum cannot be met for very long. As the population increases, access to food and space decreases and therefore growth slows until it eventually plateaus. You can see this occurring in figure 2 where the stationary phase is indicated.
There are three categories that prokaryotes come under when talking about whether they like to live in an aerobic (presence of free oxygen) or an anaerobic (without free oxygen) environment. Obligate aerobes are organisms that need oxygen in order to grow. Obligate anaerobes are organisms that die in the presence of oxygen. Finally, facultative anaerobes don’t need oxygen, but they grow better in the presence of it.
Relationships between prokaryotes and other organisms can be described as symbiotic relationships; a close interaction between two dissimilar organisms. There are many types of these relationships, including:
Chemotaxis is used as a signalling mechanism by prokaryotes as they are unable to visually survey their environment as bacteria do not have eyes. Therefore, organisms recognise chemical gradients and use this information to gather environmental cues that allow them to alter their behavior. This can include movement along the gradient, elongating their cell shape or polymerising actin (important in cell migration). This is a very useful adaptation that allows prokaryotes to escape danger as well as find food.
Hopefully this guide has helped to develop your understanding of the growth and physiology of prokaryotic cells in preparation for the MCAT. For guidance on how to prepare for the exam, visit our MCAT Guide and Checklist. Or for more revision materials, we have blogs covering a wide range of MCAT topics, from the principles of metabolic regulation, to the skeletal system and the skin system.
