The first category of organelles are structural organelles. Structural organelles provide boundaries for the cell to protect it from the environment and regulate the passage of materials into and out of the cell. The most important organelle is the plasma membrane. All cells have a plasma membrane. The plasma membrane is made from a double layer of phospholipids. Embedded in the plasma membrane are proteins that allow substances into and out of the cell. Cholesterol molecules also help to make the plasma membrane more rigid. The cell wall is made from a special protein called chitin surrounds the plasma membrane and provides more protection and support for the cells of plants, many bacteria, certain protists and some fungi. No animal cell has a cell wall since it would make the cells too rigid to move well. The cytoskeleton is a series of tubes and filaments that act as a scaffold for cells and help them to keep their shape.
Certain organelles are used for controlling cell functions. The nucleus, found in all eukaryotic cells, is the storage space for the nuclear material and the place where the basic genetic functions take place. The genetic information is stored long strands of DNA or RNA called chromatin. All cells have chromatin since all cells need to have some form of nuclear material. Animal cells also contain an organelle that aids in the process of cellular division. The centrioles attach to the chromosomes (coiled chromatin) and help to pull the two identical copies of the DNA to opposite ends of the cell, helping to ensure that when the cell splits that the two new cells (called daughter cells) are exact copies.
Organelles that are used to produce the necessary proteins that keep a cell running include the nucleolus, ribosomes and the endoplasmic reticulum (ER). The nucleolus, found inside the nucleus, is responsible for producing the ribosomes from copies of rRNA. The ribosomes leave the nucleus to find the endoplasmic reticulum, a highly folded membrane just outside the nucleus used to increase the functional surface area of the inside of the cell. Once the ribosomes have attached to the ER, they await the mRNA and tRNA. The ribosome acts as am enzyme, connecting the amino acids that the tRNA bring with them according to the sequence coded into the mRNA. The lysosomes are organelles that break down structures that are old, damaged or simply worn out. The lysosome attaches to an organelle and injects a series of enzymes that break down the proteins that make it up. Any of the materials that can be used for other functions within the cell are kept and the other are shipped out of the cell as waste.
The cell has a need to be able to store materials in much the same way that a factory requires the storage of materials. The basic storage organelle is the vacuole. A vacuole is a membrane-bound void in the cell that can be filled with items like food, water, waste, amino acids, etc. The Golgi apparatus (it may also be called the Golgi body or the the Golgi complex) resembles pita pockets and is made from the same phospholipid material that makes up the plasma membrane. The Golgi body wraps proteins made on the ribosomes, checks their amino acid sequences to ensure that they are correct and sends them to the plasma membrane for export into the bloodstream.
The organelles that are responsible for energy transfers are discussed in greater detail in the section on photosynthesis, respiration and energy. The chloroplast is found in the cells of most producers. These organelles are responsible for creating glucose molecules to store the energy from the sun through the process of photosynthesis. The mitochondria are the organelles that undo the process of photosynthesis to release the energy of the glucose molecules and store it in ATP. Muscle tissue has a high concentration of mitochondrion due to the need for rapid energy production.
Certain cells require the addition of organelles that allow for movement. Cillia are short, hair-like fibers that surround cells in great numbers. Some single-celled organisms use cilia for movement while the cilia in lung tissue cells help to move the phlegm up to be coughed out of the respiratory tract. Cells like sperm cells and many single-celled organisms have one or two long, whip-like organelles called flagella. A flagellum, like cilia, are made from protein microtubules arranged in a circular pattern, but are much longer and thicker.
Plants cells and animals cells will never look exactly like this, but these are the key features that are required to run complex organisms. Note that the plant cells are square or rectangular while the animal cells are more rounded.
The development of specialized organelles is both support for the theory of evolution as well as a testament to the tenacity of living organisms on this planet. To prevent cellular damage from increased water flow into or out of the cell when certain protists like the paramecium find themselves in a hypotonic or hypertonic environment, these cells have developed small pumps called contractile vacuoles. The paramecia have also developed special waste removal organelles called anal pores. Paramecia have also developed two separate nuclei for handling cell functions, one larger than the other (a macronucleus and a micronucleus).
Other protists like the euglena have developed light sensing organelles called eyespots. Though the eyespot does not transmit images to a brain like our eyes, it does allow the euglena to react to sudden changes in light that may be caused by an approaching predator. One of the reasons that bacteria have been able to survive so long is their ability to attack and defend. Along with a plasma membrane and cell wall, most bacteria also have another exterior layer called a capsule. The capsule is designed to help protect the bacteria’s genetic material from attacks from things like the white blood cells (cells designed to ward off pathogens in humans). Bacteria also have an interesting way of attaching themselves to other cells. Surrounding the bacteria cell is a series of needle-like pilli. The pilli stick into the plasma membrane or cell wall of an organism that the bacteria may use as a food supply.
The organisms above, a bacterium (top), euglena (middle) and paramecium (bottom) all have specialized organelles allowing these organisms to survive in their environments more successfully. Single-celled organisms generally have to be very well adapted to their environment to ensure that changes do not cause problems.
Though not every plant or animal cell has all of the organelles listed above, the different cells have what is necessary to perform the functions that it needs. There are also dozens of other specialized organelles that can be found in very specialized [mainly single-celled] organisms. The theory of evolution concludes from the fossil record that the organelles in eukaryotic cells were once individual organisms that eventually found themselves in a very lucrative symbiotic relationship, but which organisms ended up where we do not know. The theory also shows that the complexity of life developed as the individual cells found ways to perform more and more complex tasks and developed into organisms with more than one cell. There are some single-celled organisms that are fairly complex, and some multi-celled organisms that are fairly simple. Likewise there are individual cells that very complex as well as others that are very simple within the same organism. Without more evidence, the answer to life’s great question, “Where did we come from?” may never have an answer outside of the variety of different religious explanations.