Cells Case

Although a cell may be the smallest functional, living unit known to man; it is certainly not the least important. In fact, all life is based on cells. All functions of an organism are dependent on individual and collective cell functions.

Almost all cells have three basic regions which include: the plasma membrane, cytoplasm, and the nucleus; all of which play an active role in cell functions. The plasma membrane acts as an outer boundary for the cell. This boundary is flexible, semi-permeable, and is made to control the passing of molecules in and out of the cell. It also contains the cell's cytoplasm and protects it from the outer environment. The cytoplasm is located inside the cell and is basically a watery fluid that contains all the cell organelles. The nucleus is like the control center of the cell. It contains all the DNA and information of the cell and it directs what the rest of the organelles need to do.

The cytosol of the cell is located inside the cytoplasm and is the intercellular liquid in which organelles and cells are suspended. However, the mitochondrion is a cell organelle located inside the cytosol. It is double membrane bound and supplies most of the cell's ATP and has its own RNA and DNA. Ribosomes are also a cell organelle and are granules containing protein and RNA. Ribosomes are the sight of protein synthesis and these proteins can either be incorporated into the cell membrane or exported from the cell. There are two types of endoplasmic reticulum located in the cell. The rough endoplasmic reticulum is covered with ribosomes and creates lipid bound proteins and also synthesizes proteins and phospholipids. The smooth endoplasmic reticulum does not have any ribosomes on it and makes mostly fats and cholesterols. Mitochondria, ribosomes, and endoplasmic reticulum are all membrane bound organelles.

Lysosomes and peroxisomes are also extremely valuable and necessary for cell functions. Lysosomes are spherical membranous bags containing digestive enzymes. The lysosome takes in bacteria, viruses, and toxins and digests them. Peroxisomes contain the reducing enzyme catalase and sometimes some oxidases.

The plasma membrane is composed of mainly proteins, which make about 50% of the membrane. These proteins are designed to transport molecules, are receptors for signal transduction, attach the plasma membrane to the cytoskeleton and extracellular matrix, they produce enzymatic activity, intercellular joining, and cell-cell recognition. Cholesterol, which increases membrane stability and fluid, along with glycolipids, which are lipids with polar sugars on their outer surfaces, also compose the plasma membrane. The lipid bilayer of the membrane also has phospholipids in it which have phosphate heads that attract water (polar and hydrophilic), and fatty acid tails which repel water (nonpolar and hydrophobic).

There are different types of structures that the plasma membrane can have. One of these is tight junctions. Tight junctions are bound strongly together and prevent most things from passing through the structure such as molecules and fluids, meaning they are impermeable. Desmosome is another type of junction which holds cells together with "spot-welds" and "rivets". The last type of junction is the gap junction. In this junction the membrane is held together with transmembrane proteins which form small, pore like openings that allow smaller molecules to pass through.

To get molecules and fluids through the plasma membranes, there are different