Sample Answer
Prokaryotic and Eukaryotic Cells and the Evolution of Organelles
Introduction
Cells are the fundamental units of life and form the basis of all biological structure and function. Despite the enormous diversity of living organisms, cells can be broadly classified into two main types: prokaryotic and eukaryotic. Prokaryotic cells are generally simpler in structure and are found in organisms such as bacteria and archaea, while eukaryotic cells are more complex and are present in animals, plants, fungi and protists. Understanding the differences between these two cell types is essential for explaining how life evolved from simple to more complex forms.
This essay explores the key structural components of a prokaryotic cell and explains their functions. It then outlines four fundamental differences between prokaryotic and eukaryotic cells. Finally, the essay examines the endosymbiont theory of evolution and evaluates the evidence that supports this theory as an explanation for the origin of organelles such as mitochondria and chloroplasts.
Structures and Functions of a Prokaryotic Cell
Cell surface membrane
The cell surface membrane controls the movement of substances into and out of the cell. It is selectively permeable, allowing essential nutrients such as glucose and ions to enter while preventing harmful substances from passing freely. In prokaryotic cells, the membrane also plays a role in respiration, as it contains enzymes involved in energy production.
Cell wall
The cell wall provides structural support and maintains the shape of the cell. In most bacteria, it is composed of peptidoglycan, which gives strength and prevents the cell from bursting due to osmotic pressure. The cell wall also offers protection against physical damage and pathogens.
Ribosomes
Ribosomes are the site of protein synthesis. Prokaryotic ribosomes are smaller than those found in eukaryotic cells and are referred to as 70S ribosomes. They translate messenger RNA into proteins that are essential for cell growth, repair and metabolism.
DNA
Prokaryotic DNA is found in a region called the nucleoid and is not enclosed within a nuclear membrane. The DNA is usually circular and contains the genetic instructions needed for cell function and replication.
Cytoplasm
The cytoplasm is a jelly-like substance that fills the interior of the cell. It contains enzymes and other molecules involved in metabolic reactions. Many biochemical processes, such as glycolysis, take place within the cytoplasm.
Mesosome
Mesosomes are infoldings of the cell membrane that were once thought to increase surface area for respiration. Although modern research suggests they may be artefacts of cell preparation, they are still often discussed in basic biology as structures linked to cell division and respiration.
Plasmid
Plasmids are small, circular pieces of DNA that exist independently of the main chromosomal DNA. They often carry genes that provide advantages to the cell, such as antibiotic resistance, and can be transferred between bacteria through conjugation.
Mucilaginous layer
The mucilaginous layer, also known as a capsule or slime layer, surrounds some prokaryotic cells. It provides protection against desiccation and immune responses and helps the cell adhere to surfaces.
Pili or fimbriae
Pili and fimbriae are hair-like structures found on the surface of some bacteria. They help cells attach to surfaces or other cells. Some pili are involved in the transfer of genetic material between cells.
Photosynthetic membranes
In photosynthetic bacteria, such as cyanobacteria, photosynthetic membranes contain pigments like chlorophyll. These membranes allow the cell to carry out photosynthesis and produce energy from light.
Flagellum
The flagellum is a long, whip-like structure that enables the cell to move. It rotates to propel the bacterium through its environment, allowing it to move towards nutrients or away from harmful conditions.
