Eukaryotic cells boast a complex architecture, built upon a myriad of specialised organelles. Among these, ribosomes, the endoplasmic reticulum (ER), the Golgi apparatus, and vesicles are central to protein synthesis, modification, and transport.
Ribosomes
Ribosomes are instrumental for protein synthesis, converting genetic information from mRNA into functional proteins.
Structure
- Free Ribosomes: Dispersed within the cytosol, their primary function is to synthesise proteins needed within the cytosol.
- Bound Ribosomes: Adhered to the rough ER, they play a role in producing proteins destined for secretion or incorporation into cell membranes.
Function
- Protein Synthesis: Ribosomes decode the sequence of an mRNA strand to construct a specific sequence of amino acids, thus building proteins.
- Location-specific Tasks: The location of ribosomes (free or bound) often determines the type of proteins they produce, based on where the protein will function or be sent.
Endoplasmic Reticulum (ER)
The ER, a vast membranous network, facilitates a range of biochemical reactions. It exists in two primary forms, categorised by the presence or absence of ribosomes.
Rough ER (RER)
Structure
- Constituted by a branching series of sacs, with ribosomes coating the cytosolic side.
Function
- Protein Production: Bound ribosomes synthesise proteins that are secreted from the cell, integrated into the plasma membrane, or transported to lysosomes.
- Protein Modification: Proteins undergo their preliminary modifications within the RER. This might involve folding or the integration of other molecules.
Smooth ER (SER)
Structure
- It is devoid of ribosomes and manifests a more tubular appearance than its rough counterpart.
Function
- Lipid Manufacturing: The SER is pivotal in the synthesis of various lipids, from phospholipids essential for membrane formation to steroids.
- Detoxification Role: Within liver cells, the SER helps detoxify various drugs and poisons.
- Calcium Ion Reservoir: The SER also functions as a calcium storage unit, releasing ions to instigate several cellular processes.
Image courtesy of BruceBlaus.
Golgi Apparatus
A vital component in the modification, classification, and packaging of proteins and lipids for targeted delivery.
Structure
- Composed of a series of flattened membranous stacks, known as cisternae.
Function
- Modification: Upon arriving from the ER, proteins and lipids might undergo further modifications. These might include the addition or modification of carbohydrate chains, or changes to their molecular structure.
- Sorting and Packaging: Once modifications are complete, the Golgi apparatus categorises and packages these molecules into vesicles, which then bud off from the trans face of the Golgi.
- Targeted Dispatch: These vesicles are dispatched to their destined locations — this could be to the plasma membrane for secretion, or internally to other organelles or sites.
Image courtesy of Kelvinsong
Vesicles
Vesicles are the cellular answer to packaging and transportation of a myriad of materials within the cell.
Structure
- These are membrane-enclosed sacs differing in size and contents, based on their purpose.
Function
- Material Shuttle: Vesicles serve as the transport system, moving materials between the ER, Golgi apparatus, plasma membrane, and other destinations.
- Endocytosis and Exocytosis: They facilitate cellular processes for the intake (endocytosis) and expulsion (exocytosis) of materials. This is especially crucial for cells that engage in secretion or uptake of substances.
Image courtesy of PH-HY
Clathrin-coated Vesicles
A special type of vesicle that plays a primary role during the process of endocytosis.
Role of Clathrin
- Molecular Armour: Clathrin proteins form a structured coat, aiding in moulding the vesicle's membrane into its distinctive rounded shape.
- Selection of Cargo: These coated vesicles are selective. Their structure ensures that specific molecules destined for transport are incorporated within the vesicle.
- Vesicle Preparation: Once the vesicle is fully formed and budded off, the clathrin coat is shed. This requires energy, generally sourced from ATP. Once uncoated, the vesicle is primed for its subsequent roles.
FAQ
Proteins are directed to their appropriate destinations by molecular 'address labels' or tags. As proteins move through the Golgi apparatus, they undergo modifications, including the addition of specific molecular groups or sugar chains. These modifications serve as sorting signals, directing the proteins to their final destinations. For instance, mannose-6-phosphate is a tag for proteins destined for lysosomes. Vesicles budding off from the trans face of the Golgi recognise these tags and transport the proteins to their respective locations, whether it's the cell membrane, lysosomes, or elsewhere. This sorting and tagging system ensures precision in protein trafficking.
The number of ribosomes on the rough endoplasmic reticulum (RER) reflects a cell's protein synthesis requirements. Cells actively engaged in producing proteins for secretion, incorporation into membranes, or transport to lysosomes will have an RER densely packed with ribosomes. For instance, cells specialising in antibody production or secretion of digestive enzymes would have a higher density of ribosomes on their RER. In contrast, cells with lesser requirements for such protein synthesis will have fewer ribosomes bound to their RER. Essentially, the density of ribosomes on the RER can offer insights into the cell's functional role and its protein production demands.
The double membrane structure in organelles like the nucleus, mitochondria, and chloroplasts serves multiple functional purposes. In the nucleus, the double membrane forms the nuclear envelope, with nuclear pores allowing selective exchange of substances. This keeps the DNA isolated and protected from potentially harmful cytosolic molecules. In mitochondria, the double membrane allows compartmentalisation; the intermembrane space and matrix have distinct roles in cellular respiration. For chloroplasts, the double membrane is essential for the segregation of reactions during photosynthesis, with the thylakoid membrane inside housing the light-dependent reactions, while the stroma facilitates the Calvin cycle. Overall, this structure enhances the efficiency and specificity of the organelles' functions.
Vesicles with a clathrin coat are specially designed for selective uptake of specific molecules. Clathrin proteins aid in moulding the vesicle's membrane into its distinctive rounded shape during endocytosis. This unique structure ensures that specific molecules destined for transport, often receptors with their bound ligands, are selectively incorporated within the vesicle. After the vesicle is fully formed and has budded off, the clathrin coat is rapidly shed. This uncoating requires energy, typically sourced from ATP. Once the coat is removed, the vesicle becomes active and primed to fuse with target membranes, delivering its contained molecules.
The smooth endoplasmic reticulum (SER) is specially structured to support its roles. Unlike the RER, the SER lacks ribosomes, giving it a smoother appearance. This lack of ribosomes means there's more surface area available for enzyme activity associated with lipid synthesis. Its tubular form provides an increased membrane surface area equipped with specific enzymes to synthesise lipids. Moreover, in liver cells, the SER contains detoxifying enzymes that modify, and thus detoxify, potentially harmful substances like drugs or toxins. The SER’s expansive network allows for the distribution of these detoxifying enzymes, thus efficiently processing and neutralising harmful compounds.
Practice Questions
Free ribosomes are found dispersed within the cytosol, and they primarily synthesise proteins that function within the cytosol itself. These proteins can be enzymes or other proteins essential for cellular processes taking place in the cytosol. In contrast, bound ribosomes are attached to the rough endoplasmic reticulum (RER). They play a pivotal role in producing proteins that are either designated for secretion out of the cell, incorporated into the cell’s plasma membrane, or directed towards lysosomes. The location of the ribosome, whether free or bound, often determines the type and destiny of the protein it produces.
The Golgi apparatus acts as a processing centre for proteins and lipids synthesised in the endoplasmic reticulum. It comprises flattened membranous sacs called cisternae. As proteins and lipids move from the cis to the trans side of the Golgi, they undergo further modifications. These modifications can include the addition or alteration of carbohydrate chains, or molecular structural changes. Once these molecules are fully processed, the Golgi sorts and packages them into vesicles. These vesicles then bud off from the trans face of the Golgi and are dispatched to their destined locations, either towards the plasma membrane for secretion, internally to other organelles, or other specific sites within the cell.
