Cells represent the fundamental building blocks of all known life forms on Earth. These microscopic units are broadly categorized into two main types: prokaryotic and eukaryotic cells. Prokaryotic cells, such as bacteria and archaea, are generally simpler and lack a membrane-bound nucleus and other specialized compartments. In contrast, eukaryotic cells, which make up plants, animals, fungi, and protists, are typically larger and contain various membrane-enclosed organelles, including a true nucleus. Despite these structural differences, all cells share certain fundamental components necessary for their survival and function.
The Universal Cellular Machine
Among the various structures within cells, one organelle stands out for its universal presence across both prokaryotic and eukaryotic life: the ribosome. Ribosomes are tiny cellular structures responsible for manufacturing proteins. They are not enclosed by a membrane, which distinguishes them from many other organelles found in eukaryotic cells. Each ribosome is composed of two main parts, a large subunit and a small subunit. These subunits are intricate assemblies of ribosomal RNA (rRNA) and various proteins.
Why Ribosomes Are Essential for All Life
Ribosomes are essential for protein synthesis, a process also known as translation. Proteins are complex molecules that perform a vast array of functions within a cell, including providing structural support, catalyzing biochemical reactions as enzymes, transporting molecules, and transmitting signals. Without proteins, cells cannot grow, repair themselves, or carry out the metabolic processes necessary for life.
The ribosome acts as a cellular factory, reading genetic instructions carried by messenger RNA (mRNA) molecules. It then uses this information to link together specific amino acids, brought by transfer RNA (tRNA) molecules, into long chains that fold into functional proteins. This process ensures that the correct proteins are made, underpinning virtually every cellular activity. Every cell, regardless of its classification, must possess ribosomes to sustain itself.
Distinctions in Ribosomes
While ribosomes are found in all cells and perform the same fundamental task, there are notable differences between those in prokaryotic and eukaryotic cells. Prokaryotic ribosomes are generally smaller, classified as 70S ribosomes. They are composed of two subunits, a 30S small subunit and a 50S large subunit. Eukaryotic ribosomes, on the other hand, are larger, known as 80S ribosomes. These larger ribosomes are made up of a 40S small subunit and a 60S large subunit.
The ‘S’ in 70S and 80S refers to a Svedberg unit, which is a measure of a particle’s sedimentation rate during ultracentrifugation, indicating its size, shape, and density. It is not an additive unit, which is why the subunit Svedberg values do not sum directly to the total ribosome Svedberg value.
This structural difference allows certain antibiotics to specifically target and inhibit protein synthesis in bacteria without harming host eukaryotic cells. These antibiotics bind to the bacterial 70S ribosomes, preventing essential protein production and thus controlling bacterial infections.