Cell Commonalities

• Whether one-celled, independent organisms or specialized components of a larger organism, cells are made up of the same basic chemicals. Each cell contains carbon, oxygen, nitrogen, hydrogen, phosphorus and sulfur. All cells require a source of energy to function and react to changes or stimuli in their environment.
  
  

Unicellular Organisms

• Unicellular organisms can be either prokaryotes or eukaryotes. Prokaryotes are the simplest of cell forms. They have a cell envelope, a cell wall and a plasma membrane. DNA is held within the cytoplasmic region. Eukaryotes are larger and more organized. They contain a defined nucleus that contains the cell's DNA. Other structures within the cell's outer membrane, called organelles, have distinct cellular functions. An example of an eukaryotic single-celled organism is the amoeba.
  
  

White Blood Cells

• White blood cells share characteristics with the eukaryote unicellular organism. They have a defined cell wall and a nucleus that contains DNA. However, they do not reproduce themselves by binary fusion, the dividing of one cell to create two new cells. White blood cells are created in the bone marrow. Their average lifespan is two days, and when they die, they are absorbed by surrounding white blood cells. Just as the amoeba, a white blood cell has a flexible outer membrane. This allows the cell to pass through the wall of a blood vessel into surrounding tissues. White blood cells are responsible for fighting infection in our bodies.
  
  

Red Blood Cells

• Red blood cells outnumber white blood cells astoundingly. In every drop of blood there are roughly 7,000 to 25,000 white blood cells, more if an infection is present. Red blood cells number in the millions. Though their structure is similar to one-celled eukaryotic organisms, they do not contain DNA. They also include iron-rich haemoglobin which helps them in carrying oxygen to all parts of the body. After their 120 day life cycle, the cells migrate back to the bone marrow where they were created. The iron is used by the bone marrow to create new red blood cells.
  
  

Protein Synthesis

• All cells, whether they are one-celled, independent organisms or specialized, contain ribosomes, which are responsible for protein synthesis. They either float in the cytoplasm or bind to the endoplasmic reticulum, an organelle found in eukaryotic cells. In red blood cells, these ribosomes are responsible for manufacturing hemoglobin. Unlike one-celled organisms, red blood cells eject their ribosomes at maturity just before they leave the bone marrow.