Palade's Discovery: Ribosomes and Protein Synthesis

Ribosomes, first studied by Palade, are some of the smallest structures suspended in the cytoplasm of a cell and can only be seen under an electron microscope. They are roughly spherical granular bodies that exist in two forms: (1) attached to the Rough Endoplasmic Reticulum (RER) or (2) freely dispersed in the cytoplasm. Ribosomes are composed of equal amounts of RNA and protein, making them ribonucleoproteins.

In eukaryotes, all ribosomes are made up of two subunits of different sizes, a larger one called the 60S particle and a smaller one called the 40S particle. When these two subunits attach to each other, they form an 80S particle. This attachment is controlled by the presence of magnesium ions or the formation of salt bonds between the phosphate group of RNA and the amino group of amino acids, or a combination of both involving magnesium ions and salt bonds.

When several ribosomes are attached to one mRNA strip, it is called a polysome, derived from "poly" meaning many and "soma" meaning body.

Ribosomes are formed in the nucleolus and then transported to the cytoplasm through the nuclear pores.

The main function of ribosomes is to serve as the sites of protein synthesis, where the information encoded in mRNA is used to synthesize proteins by linking amino acids together in a specific sequence according to the genetic code.

Endoplasmic Reticulum (ER) - an Overview

The endoplasmic reticulum (ER) is a complex network of membranous tubes, sacs, and flattened channels known as cisternae, which forms a distinct compartment within the cytoplasm of the cell. Composed of a double layer of lipids with various enzymes attached to its surface, the ER is continuous with the plasma membrane, nuclear membrane, and Golgi complex.

There are two types of ER based on their appearance and functions: Rough ER (RER) and Smooth ER (SER).

Rough ER (RER) is so named because it has ribosomes attached to its surface facing the cytoplasm, giving it a rough appearance when viewed under an electron microscope.

Smooth ER (SER) is a network of interconnected tubules that is continuous with the RER, but lacks ribosomes on its surface. Most cells contain both types of ER, although the relative proportions may vary among different cell types.

The functions of Rough ER (RER) include providing mechanical support to the cell and playing a role in protein synthesis and modification. Proteins synthesized by ribosomes on the RER surface enter the cisternal space where they are modified through processes such as folding and addition of lipids and carbohydrates. Eventually, the ER membrane encloses these modified proteins, forming sacs that have different destinations. These sacs may merge with the Smooth ER (SER) or enter the Golgi membrane system for further processing.

Smooth ER (SER) houses different enzymes depending on the type of cell. In the liver, for example, the enzymes in the smooth ER are involved in detoxification processes. Smooth ER also plays a role in lipid synthesis, including the production of steroids. In muscle cells, a specialized form of smooth ER called sarcoplasmic reticulum is responsible for storing and releasing calcium ions, which are critical for muscle contraction.

Smooth ER and Rough ER

The Cell Organelles

Cell organelles are specialized structures within a cell that perform specific functions necessary for the cell's survival and function. Here are some of the main cell organelles:

Nucleus: The nucleus is often referred to as the "control center" of the cell. It contains the cell's DNA, which carries the genetic information needed for cell growth, reproduction, and functioning. The nucleus also regulates gene expression and controls the synthesis of RNA.

Mitochondria: Mitochondria are known as the "powerhouses" of the cell. They generate energy for the cell through a process called cellular respiration, which produces ATP (adenosine triphosphate), the cell's main source of energy. Mitochondria also play a role in regulating cell metabolism and apoptosis (cell death).

Endoplasmic reticulum (ER): The endoplasmic reticulum is a network of interconnected membranes that are involved in protein synthesis and lipid metabolism. Rough ER, which has ribosomes attached to its surface, is responsible for protein synthesis and modification, while smooth ER is involved in lipid metabolism and detoxification.

Golgi apparatus: The Golgi apparatus, or Golgi complex, is responsible for processing, modifying, and packaging proteins and lipids that are synthesized in the ER. It consists of flattened membranous sacs called cisternae, and plays a crucial role in protein sorting and trafficking within the cell.

Lysosomes: Lysosomes are membrane-bound organelles that contain enzymes responsible for breaking down waste materials, cellular debris, and foreign substances. They are involved in cellular digestion and recycling, and play a role in maintaining cellular health and homeostasis.

Vacuoles: Vacuoles are membrane-bound sacs that are larger in plant cells compared to animal cells. They store water, nutrients, and waste materials, and play a role in maintaining cell turgor pressure, regulating cellular metabolism, and storing pigments or toxins.

Chloroplasts: Chloroplasts are found in plant cells and are responsible for photosynthesis, the process by which plants convert sunlight into energy. Chloroplasts contain chlorophyll, which gives plants their green color, and are involved in producing glucose, oxygen, and other organic compounds.

Ribosomes: Ribosomes are cellular structures responsible for protein synthesis. They can be found in the cytoplasm or attached to the rough ER, and read the information encoded in the mRNA to synthesize proteins.

Cytoskeleton: The cytoskeleton is a network of protein fibers that provide structural support, shape, and organization to the cell. It is involved in cell movement, cell division, and intracellular transport.

These are just a few examples of the many organelles found in eukaryotic cells, which are cells that have a true nucleus and membrane-bound organelles. Prokaryotic cells, such as bacteria, do not have membrane-bound organelles and have a simpler structure.