Organelles Working Together
- In order for cells to function and survive, their organelles work together to carry out specific tasks and perform specific roles. Each organelle has its own role that contributes to the survival of the cell. This is called the Division of Labour.
The Cytoskeleton of a cell consists of a network of protein fibres that give the cell stability and allow it to move. In Eukaryotic Cells, the proteins are arranged as microfilaments (small solid strands) and microtubules (very small protein cylinders).
The Cytoskeleton provides an internal framework for the cell, which preserves its shape and holds its organelles in position.
Part of the Cytoskeleton, fibres called Actin Filaments, are similar to the fibres found in muscle cells and they are able to move against each other. They are responsible for the movement of some of the organelles and also the movement and change in shape of White Blood Cells.
Undulipodia, Cilia and Flagella
- Another part of the Cytoskeleton, fibres called Microtubules, use energy in the form of ATP to drive movements to perform various functions. These are made of a protein called Tubulin. Proteins present on some Microtubules, called Microtubule Motors, move organelles, and other cellular content, along the fibres. This is the mechanism for the transport of vesicles from the Rough Endoplasmic Reticulum to the Golgi Apparatus and the movement of Chromosomes during Mitosis.
The Cytoskeleton is also responsible for the movement of the cell though its environment, or the movement (or “wafting”) of part of its environment past the cell. This is done by the organelles Undulipodia, Cilia, and Flagella.
In Eukaryotic Cells, Undulipodia (sometimes incorrectly called Flagella, which are actually only present in Prokaryotic Cells) and Cilia are responsible for this movement. They are both structurally similar; they hair-like structures composed of an outer membrane bounding a ring of nine pairs of Microtubules with a single pair in the centre.
Undulipodia are longer than Cilia and usually occur in ones and twos, whereas cilia are often found in large numbers. A sperm cell moves because it has a long tail. This is an Undulipodium. Cilia are responsible for the movement of mucus past ciliated epithelial cells.
Undulipodia and Cilia are able to move because the Microtubules within them use ATP to try to slide over one another (with the use of Dynein ‘arms’ - Motor Proteins). Because they are attached to the base, this causes the structure to bend.
- Some Prokaryotic Cells have Flagella, which look similar to Undulipodia, but have a very different internal structure. They are made of Flagellin, a spiral of protein that attaches to a disk at the base. When this rotates, using energy from ATP, the Flagellum spins, propelling the cell.
A good example of the Division of Labour is in Protein Synthesis.
The instructions to create a particular protein are found in a Gene in the DNA of a cell. So as to prevent damage to the DNA, when a protein is required to be synthesised, a copy of that particular gene is encoded onto an mRNA (messenger RNA) molecule. This is called Transcription. The molecule then leaves the nucleus through a Nuclear Pore.
The mRNA molecule then travels to a Ribosome, which may be located on the Rough Endoplasmic Reticulum, or in the Cytoplasm. There the sequence of Amino Acids is read and the instructions are Translated to a Polypeptide Chain.
The next stage is called Post Translational Modification. This is where the Polypeptide Chain is modified to create the final protein, for example, by adding Prosthetic Groups. This may done in the Rough Endoplasmic Reticulum or in the Golgi Apparatus.
When the protein moves from the Rough Endoplasmic Reticulum, it does so in Vesicles, which are ‘pinched off’ from the ER and then fuse with the Golgi Apparatus.
In the final stage of protein synthesis, the newly created protein is transported around the cell, again in Vesicles. Sometimes, the Vesicle may fuse with the Plasma Membrane and secrete the protein.