Question

Reflect and Apply Explain why a 50 S ribosomal subunit and a \(30 \mathrm{S}\) ribosomal subunit combine to form a 70 S subunit, instead of an \(80 S\) subunit.

Short answer

The 50S and 30S ribosomal subunits form a 70S subunit, not an 80S subunit, because Svedberg units are not additive and depend on the combined particle's size, shape, and density.

Step-by-step solution

Understand the Ribosomal Subunits

Ribosomes are made up of two subunits, a larger and a smaller one. In prokaryotic cells, the larger subunit is 50S and the smaller subunit is 30S.

Introduction to Svedberg Units

The S value stands for Svedberg units, which are a measure of how quickly particles sediment in a centrifuge. They are not additive because the rate of sedimentation depends on the size, shape, and density of the particles, not just their mass.

Combining Subunits

When the 50S and 30S subunits combine to form a functional ribosome, the resulting complex has a different shape and density than the individual subunits. Therefore, the combined sedimentation rate does not equal the sum of the individual rates.

Resulting Sedimentation Rate

For prokaryotic ribosomes, the combined sedimentation rate of the 50S and 30S subunits is 70S, not 80S, due to the aforementioned differences in the combined particle's size, shape, and density.

Key concepts

Svedberg Units

To understand why ribosomal subunits don't simply add up, let's delve into Svedberg units. A Svedberg unit (S) measures how fast a particle sediments when spun in a centrifuge. It essentially gauges how quickly a ribosomal subunit sinks to the bottom during the process.
Svedberg units are peculiar because they reflect a particle's size, shape, and density, not just its mass. Think of it like a parachute: even if two parachutes are the same weight, their shapes and built materials can change their fall speed.
For ribosomal subunits, even if the large subunit is 50 S and the smaller one is 30 S, when they combine, they do not just add up to 80 S. The way the two subunits fit together alters their shape and density, resulting in a sedimentation rate of 70 S, not 80 S.

Prokaryotic Ribosomes

Prokaryotic ribosomes are unique molecular machines vital for protein synthesis. They consist of two subunits: the larger 50 S and the smaller 30 S. Together, these form the functional 70 S ribosome.
The term 'prokaryotic' refers to cells without a distinct nucleus, like bacteria. These cells need ribosomes to read the genetic instructions and create proteins, their biological workhorses.
Despite having components named 50 S and 30 S, prokaryotic ribosomes don't combine arithmetically. Instead, the total sedimentation rate measures at 70 S because the combined size and shape of the two subunits differ from their individual states.

Sedimentation Rate

The sedimentation rate in centrifugation is essential for understanding biological particles like ribosomes. This rate tells us how fast particles settle at the bottom of a solution when spun.
The rate depends on three main factors:

• Size
• Shape
• Density

Unlike simple addition, combining two ribosomal subunits changes these factors. That's why the 50 S and 30 S subunits yield a 70 S ribosome—altering shape and density affects how they sediment together.
Imagine mixing different-sized marbles in a jar. The way they settle together is not just about their number, but also how they nestle against each other, altering their overall behavior in the spinning process.