Line 1: Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palade (1953).
Explanation: This line introduces ribosomes and their key characteristics: They are granular structures (small, grain-like). They were first observed using an electron microscope, a powerful tool for viewing tiny structures within cells. They appeared as dense particles in these electron microscope images. George Palade is credited with their discovery in 1953.
Line 2: They are composed of ribonucleic acid (RNA) and proteins and are not surrounded by any membrane.
Explanation: This line describes the composition of ribosomes: They are made up of two main components: ribonucleic acid (RNA) and proteins. They lack a surrounding membrane, unlike some other cell structures.
Line 3: The eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S.
Explanation: This line introduces a key difference between ribosomes in eukaryotic and prokaryotic cells: Eukaryotic cells (animal and plant cells with a nucleus) have 80S ribosomes. Prokaryotic cells (bacteria) have smaller, 70S ribosomes. The “S” refers to Svedberg units, a measure of sedimentation rate that indirectly reflects size and density.
Line 4: Each ribosome has two subunits, larger and smaller subunits (Fig 8.9).
Explanation: This line describes the structure of a ribosome: Each ribosome consists of two subunits, a larger one and a smaller one. Figure 8.9 (not provided here) likely illustrates this structure.
Line 5: The two subunits of 80S ribosomes are 60S and 40S while that of 70S ribosomes are 50S and 30S. Here ‘S’ (Svedberg’s Unit) stands for the sedimentation coefficient; it is indirectly a measure of density and size.
Explanation: This line elaborates on the subunits: In eukaryotic ribosomes (80S), the larger subunit is 60S, and the smaller subunit is 40S. In prokaryotic ribosomes (70S), the larger subunit is 50S, and the smaller subunit is 30S. As mentioned earlier, “S” refers to Svedberg units, indicating their relative size and density.
Line 6: Both 70S and 80S ribosomes are composed of two subunits.
Explanation: This line emphasizes a key similarity between the two types of ribosomes: Regardless of their size (70S or 80S), all ribosomes have two subunits (large and small) for proper function.
Line 7: In prokaryotes, ribosomes are associated with the plasma membrane of the cell.
Explanation: This line describes the location of ribosomes in prokaryotic cells: In bacteria (prokaryotes), ribosomes are often attached to the plasma membrane, the cell’s outer boundary.
Line 8: They are about 15 nm by 20 nm in size and are made of two subunits – 50S and 30S units which when present together form 70S prokaryotic ribosomes.
Explanation: This line provides details about the size and structure of prokaryotic ribosomes: They are very small, measuring approximately 15 nanometers (nm) by 20 nm. They consist of two subunits, a larger 50S subunit and a smaller 30S subunit. When these subunits come together, they form a complete functional 70S ribosome.
Line 9: Ribosomes are the site of protein synthesis.
Explanation: This line highlights the most crucial function of ribosomes: They are the cellular factories where protein synthesis takes place. Proteins are essential molecules for various cellular functions and activities.
Line 10: Several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome. The ribosomes of a polysome translate the mRNA into proteins.
Explanation: This line describes how ribosomes work together for protein synthesis: Multiple ribosomes can attach to a single messenger RNA (mRNA) molecule. This arrangement is called a polyribosome or polysome. Each ribosome in the polysome “reads” the instructions on the mRNA molecule and and assembles amino acids into polypeptide or Protein. Essentially, each ribosome in the polysome acts like a tiny protein-building machine. By working together on the same mRNA molecule, multiple ribosomes can significantly increase the rate of protein synthesis in the cell. This is particularly important for cells that need to produce large quantities of specific proteins.