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Cellular Structure


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More on Organelles
The History of Cell Discoveries

          What is an organelle?:

Organelles are compontents within a cell that performs specific functions for a cell. A simple real life analogy would be organelles are to a cell as organs are to a human.


Superficially there are two types of organelles; organelles that are bound by a membrane and those that are not.

                  Below are some examples of each:

Membrane Bound Organelles
  • Vacuoles
  • Lysosomes
  • Endoplasmic Reticulum
  • Golgi Apparatus
  • Mitochondria
  • Chloroplasts
Nonmembrane Bound Organelles
  • Ribosomes
  • Centrioles
  • Miscrotubules
There are several others that are so distinctive in their characteristics that they do not pertain to either of these groups. These include the nucleus, cytoplasm, and the plasma membrane and are essentially the building blocks of cells. We shall look at these first.

                    The Nucleus:

The nucleus is the cell 'brain'. It houses all the necessary cellular parts required to regulate intercellular functions. It does so through producing messages (messenger Ribonucleic cid or mRNA) that code for proteins. These messages travel through the rough endoplasmic reticulum which we will go into detail on later. Within the nucleolus ribosomes are produced. All of this interaction between the nucleus and other organelles are organized nuclear pores. These pores mantain the same function as the plasma membrane does for the cell. It distinguishes between the good and the bad. Note: The nucleus has a double membrane.

The most important function of the nucleus however, is its regulation of DNA. The nucleus contains chromosomes. These chromosomes store genes. Genes carry hereditary information. They also contain the instructions for the production of proteins, which make up the structure of cells and direct their activities. The nucleus also organizes the uncoiling of DNA to replicate key genes.


Cytoplasm is the aquatic environment within a cell. This fluid includes salts, organic molecules (including many enzymes that catalyze reactions), as well as water. All cells have cytoplasm though the consistency and content amount vary with different types of cells.
Cytoplasm has three main funtions: energy, storage, and manufacturing.

          Membrane Bound Organelles:


Chloroplasts are exclusive to plant cells. This is attributed to the fact that chloroplasts are the organelles that play a major role in photosynthesis, a function many of you are already familiarized with.


Plant and animal cells both contain vacuoles, although their sizes vary with the cell types. In plant cells vacuoles are excessive. This is due to their necessity for water and energy. Vacuoles are the cell's basic 'storage units'. They are used to transport and reserve water, energy, and waste products.


Are cells capable of suicide? Apparently, yes. But the causes aren't caused by a female lysosome. No, the cause of this 'suicide' are enzymes and the whole process is inborn. A lysosome is a membrane bag containing digestive enzymes. When a cell needs to digest food, the lysosome membrane actually fuses with the membrane of a food vacuole and squirts the enzymes it contains inside. The only thing that keeps the cell itself from being digested is the membrane surrounding the lysosome.

                    Golgi Apparatus:

The golgi apparatus consists of stacks of sacs. The sacs harbor small vesicles (minute versions of vacuoles) which are pinched off from the edges of the sacs. Some of these "sacs" are formed into lysosomes.

                    Endoplasmic Reticulum:

There are two types of reticulums; the smooth endoplasmic reticulum and the rough endoplasmic reticulum.

The rough endoplasmic reticulum is composed of only one membrane. Ribosomes are prevalent on the rough endoplasmic reticulum. Ribosomes serve to manufacture the proteins. These include digestive enzymes and protein hormones both of which serve their specific fuction.

The smooth endoplasmic reticulum consists of tubules and vesicles that branch forming a network. the smooth endoplasmic reticulum allows for an incresed surface area for the action or the storage of enzymes.


Mitrochondria are cells' power sources. Mitochondria retain the shape of a pill and unlike many other organelles, have a double membrane. The outer membrane provides a barrier from the outside. Then inner membrane however carries out the much more crucial function, producing the energy. The inner membrane consists of many folds (in order to increase its total surface area. Remember, the more surfacea area the more energy produced.). On these folds sugar is combined with oxygen to create ATP, energy for a cell.

          Nonmembrane Bound Organelles:


Ribosomes are prevalent on the rough endoplasmic reticulum. Ribosomes serve to manufacture the proteins. These include digestive enzymes and protein hormones both of which serve their specific fuction.


The centrosome is the area of the cells that create microtubules. The microbulules aid in replicating centrosomes as do centrioles. The centrioles and microtubules are arranged in such a way that replicating is easy.

The two centrosomes move to opposite ends of the nucleus, and from each centrosome, microtubules grow into a "spindle" which is responsible for separating replicated chromosomes into the two daughter cells.
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