The journey into the human body is one of many surprises. The deeper we delve, the more intricate and complex the mechanisms start to look. It’s easy to get lost in these intricate details: either through being in awe of their beautiful perfection, or their complexity. In this blog we will look at the fundamentals of the cell, namely the organelles of the cell; what they do and how they function.
When thinking about the cell, it’s always helpful to think of it like a working machine, the organelles being the cogs in the wheel that all interlink and have a role to play. Through this analogy, we can see how all parts of the cell have a specific function. And this is true when we actually take a deeper look in all the processes. The nucleus contains DNA that is transcribed to mRNA which then travels to the ribosomes (or the rough endoplasmic reticulum) where translation occurs and proteins are formed. From here the proteins are packaged in vesicles and sent to the golgi apparatus where post translational modification of the protein occurs. This is followed by the protein either being sent outside the cell or being used elsewhere and this process can be repeated many, many times in a cell. Through this one process we can see how organelles work in conjunction with each other just like our analogy “cogs in the wheel”. However, the question still remains. What are the organelles and what do they do? We will look at all the organelles in detail below.
The nucleus is often described as being the control center of the cell. It coordinates the cell’s activities such as growth, metabolism, protein synthesis and cell division. It’s where all the DNA-that is the genetic material- of the cell is stored, tightly coiled around histone proteins in a complex called chromatin. Chromatin appears in 2 forms: a light form and a dark form, euchromatin and heterochromatin respectively. The only difference between the 2 types is that heterochromatin is more condensed than euchromatin (hence appearing darker) and thus has no active RNA synthesis whilst euchromatin does. Furthermore, within the nucleus there is a structure present called the nucleolus. It is here where rRNA synthesis takes place. The rRNA is then combined with proteins to form incomplete ribosomes which then mature when being exported from the nucleus through nuclear pores. The aforementioned nuclear pores is a protein-lined channel in the nuclear envelope that regulates the transportation of molecules between the nucleus and cytoplasm. The final component of the nucleus is the double layered membrane that is the nuclear envelope. Think of it as a barrier, controlling what enters and leaves the nucleus.
The rough endoplasmic reticulum:
The rough endoplasmic reticulum, usually continuous with the nuclear envelope, consists of a series of interconnecting membranes, vesicles and cisternae. What distinguishes it appearance-wise from it’s “smooth” counterpart is the ribosomes attached to the outer surface of the membrane giving it the “rough” part of its name. The ribosomes on the surface help it with its function to synthesise proteins.
The smooth endoplasmic reticulum:
The smooth endoplasmic reticulum similar to it’s rough counterpart, consists of a series of interconnecting membranes, vesicles and cisternae. The absence of ribosomes gives it the “smooth” part of its name. The smooth endoplasmic reticulum is responsible for lipid biosynthesis and intracellular transport.
Ribosomes are composed of 2 rRNA subunits (which are integrated with proteins) that wrap around mRNA to initiate the translation section for protein synthesis. This is a vital component in the cell and plays a massive role in the main dogma of biology.
The site of post translational modification. The golgi apparatus accepts vesicles from the rough endoplasmic reticulum and fuse with the convex face of the semi-circular organelle. Made up of cisternae, the golgi bodies have polarity, and the packaged bodies move from the convex to the concave section, being modified in the process. Throughout this process, the golgi sorts, modifies and packages the proteins which then bud off in vesicles from the concave face of the golgi apparatus.
These are membrane bound organelles that contain hydrolytic enzymes which break down excessive or worn out organelles. They can also ingest and digest bacteria and viruses that enter the cell.
Peroxisomes is another membrane bound organelle responsible for chemical detoxification and lipid metabolism. It contains enzymes that transfer hydrogens atoms from toxins and transfers them to oxygen to produce hydrogen peroxide. The toxins are then ingested and neutralised and the hydrogen peroxide produced can be neutralised to produce water and oxygen.
Often considered the ‘powerhouse of the cell’, mitochondria have quite an interesting backstory. Believed to be an organism in itself until through evolution it adapted to living in cells. This may explain why mitochondria has its own DNA (inherited from the mother) and ribosomes. These aren’t the only components of mitochondria. Mitochondria have a double membrane with an outer and inner lipid bilayer. The inner layer is significantly larger than the outer one hence it becomes highly folded into structures called cristae. This folding of the inner membrane is useful for the production of ATP (which is another topic in itself). Within the inner membrane lies the matrix which holds the mitochondrial: DNA, ribosomes and enzymes. The function of the mitochondria is for ATP synthesis for energy production
Cell surface membrane:
The cell surface membrane is composed of a phospholipid bilayer, usually with proteins embedded in the membrane. These proteins can be peripheral (only present on one side of the membrane) or integral (which span the width of the membrane).
Not necessarily an organelle in itself but still a major component of the cell. The cytoplasm is what surrounds all the organelles. It’s described as a thick jelly-like solution that fills each cell and is enclosed by the cell membrane. Consisting of mainly water, salts and proteins, it’s usually the medium for a lot of chemical reactions that happen in a cell.
Responsible for maintaining and changing cell shape. The cytoskeleton consists of microtubules, microfilaments and intermediate filaments. This organelle provides structural support for the plasma membrane and cell organelles. It also allows the movement of organelles and substances released from them around the cytosol and to different areas of the cell. This organelle allows contractility in cells such as those in the muscle, and also plays a key role in the cytokinesis stage of mitosis.
It’s absolutely fascinating how these sub-components work together in order to allow a cell to function – yet a cell is only the biological basic unit of life. If one of these components were to stop working, or even function partially, we would have major illnesses in our body; that which can even be fatal. This is another example of how complicated science is, and how even the smallest changes in basic units could cause major changes.