Ribosomes are typically composed of two subunits, a large and a small subunit, forming a figure of a ‘snowman’ they are organelles that exist in a large amount, it is because they are responsible for the assembling of proteins which is a very important role within the cell, mRNA will be read by them and protein would be synthesis through translation by allowing tRNA to carry amino acids according to the codons on mRNA. Ribosomes exist within the cell as free ribosomes that free floats within the cytoplasm or fixed ribosomes that are attached to the rough endoplasmic reticulum.
The Rough Endoplasmic Reticulum (Rough ER) with ribosomes attached is responsible for the synthesis of proteins that would be released out of the cell; it is called ‘Rough’ because the attached ribosomes showed in electron microscopes a ‘rough’ surface on the membranes.
In contrast to the Rough Endoplasmic Reticulum, the Smooth Endoplasmic Reticulum (Smooth ER) that does not have any ribosomes attached to is responsible for lipid and carbohydrate synthesis.
Mitochondria are bounded by a double membrane in which the inner membrane is highly folded to increase its surface area (increase ‘productivity by increasing the amount of enzymes could be contained) to specialize for ATP production; the double membranes divide the mitochondrion into two distinct parts: the intermembrane space and the mitochondrial matrix. The intermembrane space is the narrow part between the two membranes while the mitochondrial matrix is the part enclosed by the innermost membrane. Several of the steps in cellular respiration (Krebs cycle and Oxidative Phosphorylation) occur in the matrix due to its high concentration of enzymes.
Golgi apparatus is the site where proteins produced by the ribosomes on rough endoplasmic reticulum further packaged and modified before releasing out of the cell through exocytosis.
Lysosomes are vesicles that contain various hydrolytic enzymes such as lysozymes, which are capable of digesting macromolecules such as fats and proteins. Lysosomes are responsible for the recycle of the cell’s organic material and are involved in apoptosis (programmed cell death).
The Cytoskeleton is made up by a three dimensional filamentous protein network, microtubule, microfilaments and intermediate filaments; it extends throughout the cell and is attached to the plasma membrane and internal organelles, hence providing a framework for cellular organization, giving support to the cell shape, organelle movement and cell motion.
The plasma membrane, cytoplasm and the centriole also play a very important role in cell compartmentalization.
The plasma membrane was a lipid bilayer made up by phosphorylated lipids which contains various proteins and lipids, such as glycoprotein, integral and peripheral protein and glycolipids. This bilayer act as a barrier selectively controls the entry and the release of nutrients and other molecules needed for cellular processes by exocytosis and endocytosis.
The cytoplasm the gel-like substances within the cell membrane which contains all organelles apart from the nucleus and most cellular activities occurs in the cytoplasm such as glycolysis which is involve in the production of ATP and process such as cell division.
Centrioles are paired cylinder-shaped cell structure, which are involved in the organization of mitotic spindle fibres in the metaphase of cell division. (They are very important for a correct chromosome division between the cells.)
Peroxisomes are micro bodies, they are bound by a single membrane and contain enzymes that will transfer hydrogen from a substrate to oxygen, forming H2O2 which is toxic to the cell, but peroxisomes also contain an enzyme that could convert H2O2 to water. Functions of peroxisomes include detoxification of alcohol, bile acid formation and using O2 to break down fats.
The compartmentalization of organelles largely increases surface area of organelles allowing more enzymes to be available for specific reactions to take place, and also provide a much higher concentration of reactants around organelles, such as mitochondria with a large surface area to hold up a lot more enzymes for ATP synthesis. This largely increases the efficiency of the reaction and allows the cell to undergo different reactions at the same time.
Last but not least, I would like to talk about protein synthesis and respiration in mammalian cells and how different organelles are involved in each process.
At the beginning of protein synthesis, mRNA would be synthesized within the nucleus by transcription, the mRNA produced carries a unique coding for the protein, it will then leave the nucleus through the nuclear pores and attach to ribosomes, allowing translation to take place, where tRNA carries amino acids to the ribosome according to the codons (triplets) on the mRNA, forming a long chain of polypeptide (protein).
If this protein was attached to a free-floating ribosome, this protein synthesized would then be an ‘own-use’ protein which is only used within the cell, however, if a ribosome which is attached to the rough endoplasmic reticulum was used, this protein synthesized would then be transported to the Golgi apparatus through a vesicle undergo further modification and packaging, at last the ‘ready’ protein would be transported to the membrane through a vesicle and be released through exocytosis.
The above diagram sums up the different stages in cell respiration (ATP synthesis).
ATP synthesis involves three stages, Glycolysis, Krebs cycle and Oxidative phosphorylation, in which Glycolysis take place in the cytoplasm and the other two stages take place in the matrix and the inner membrane of the mitochondria respectively.
The whole process was initiated by the breakdown of glucose into pyruvate (Glycolysis), it would then be converted into acetyl CoA which would then combine with oxaloacetate to form a six-carbon compound which would then be reduced (Krebs Cycle), releasing large amount of Hydrogen, reduced NAD and reduced FAD (which would release Hydrogen in the last stage) , the large amount of Hydrogen would pass through ATP synthase due to the electrochemical gradient between the matrix and the inner membrane of mitochondria, and this movement creates a large amount of ATP (oxidative phosphorylation).
To conclude, compartmentalization promotes a sense of division of labour, allowing different organelles maximizing their role and allowing them to work together to bring out important processes for the cell.