Protein Synthesis
...r area. The nucleus is surrounded by a double-layered membrane called the nuclear envelope. Nuclear pores can be found along the circumference of the envelope, which serve the purpose of allowing certain molecules such as RNA to exit the cell, that are too large to escape a plasma membrane. When the ribosome enters the cytoplasm (the soluble part of the cell surrounding the organelles), by way of nuclear pores, it attaches to the endoplasmic reticulum. The endoplasmic reticulum is a system of membranous tunnels and sacs found in most eukaryotic cells and often lies near the nucleus. The endoplasmic reticulum comes in two forms: rough and smooth. Rough endoplasmic reticulum gets its name from its bumpy shape under a microscope. The bumps are actually ribosomes attached to the surface of the membrane assisting the protein in synthesis. Rough ER’s are usually more frequent in cells that produce proteins for export. As a protein is made, it is pushed into the interior of this sac, known as the lumen where substances are kept separate from the cytoplasm. Here, enzymes modify the protein and get it ready to move on to the next step of synthesis. Smooth endoplasmic reticulum is ER without ribosomes attached to it. They contain enzymes needed for the construction of molecules such as carbohydrates and lipids. Smooth ER’s are responsible for most of the phospholipid production in the cell, and they increase the surface area efficiently. Patches of ER membrane bud off to form membranous sacs called vesicles that transport the protein to the next site known as the Golgi complex. The Golgi complex is a stack of flattened, membranous sacs. Around the edges are round transport vesicles that carry molecules to and from this organelle. The main role of the Golgi complex is to modify, sort, and package the protein before it undergoes the final process of exocytosis. Proteins move from one large sac to another in a certain sequence, carried by transport vesicles. At each sac, the protein is getting closer and closer to the last sac which will finish and package the protein. From here the transport vesicle moves to the semi-permeable plasma membrane and fuses with it, discharging the contents of the vesicle out of the cell by a process known as exocytosis. The plasma membrane forms a barrier around a cell, controlling the passage of substances into or out of the cell, and recognizing and responding to stimuli from the environment. Plasma membranes are similar in structure and function to the membranes of organelles inside the cell. The main lipid in a membrane is a phospholipid. These molecules have a hydrophilic end and a hydrophobic end. When surrounded by water, the heads and tails form a bilayer. Lipid bilayers are fluid, which allows the membrane to stretch under stress and reseal itself during endocytosis, exocytosis or during any means of damage. Microfilaments, structural components of the cell, also play an important role in re...