Golgi Apparatus- Definition, Structure, Functions and Diagram

The Golgi apparatus is one of the most fascinating and important organelles in eukaryotic cells. It is responsible for processing, sorting and delivering proteins and lipids to various destinations inside and outside the cell. It also plays a role in synthesizing carbohydrates, proteoglycans and other molecules that are essential for the structure and function of the cell.

The Golgi apparatus was named after its discoverer, Camillo Golgi, who observed it in 1898 using a staining technique that he developed. He described it as a "black reticulum" that was present in many types of cells. However, his findings were controversial and not widely accepted until the 1950s, when electron microscopy revealed the true structure and complexity of the Golgi apparatus.

The Golgi apparatus consists of a series of flattened membrane-bound sacs called cisternae, which are arranged in stacks called dictyosomes. The number and size of the stacks vary depending on the cell type and function. The cisternae are connected by tubules and vesicles that transport materials between them. The Golgi apparatus has two distinct faces: the cis-face, which faces the endoplasmic reticulum (ER) and receives newly synthesized proteins and lipids from it; and the trans-face, which faces the plasma membrane and releases modified proteins and lipids to their final destinations.

The Golgi apparatus performs different functions depending on the type of molecule that passes through it. Some molecules are modified by adding or removing sugar groups (glycosylation), phosphate groups (phosphorylation), sulfate groups (sulfation) or other chemical groups. Some molecules are sorted and packaged into different types of vesicles that bud off from the Golgi apparatus. Some molecules are transported to other organelles such as lysosomes, peroxisomes or mitochondria. Some molecules are secreted out of the cell by exocytosis.

The Golgi apparatus is essential for many cellular processes such as cell growth, differentiation, signaling, immunity, development and disease. It is also involved in the formation of some cellular structures such as the cell wall in plants, the acrosome in sperm cells and the cortical granules in egg cells. The Golgi apparatus is highly dynamic and responsive to changes in the cellular environment and demands. It can undergo structural and functional changes such as fusion, fission, fragmentation, relocation or degradation.

In this article, we will explore the definition, structure, functions and diagram of the Golgi apparatus in more detail. We will also discuss some of the discoveries and advances that have been made in understanding this remarkable organelle..

The Golgi apparatus is a cellular organelle that is found in most eukaryotic cells, which are cells that have a nucleus and other membrane-bound structures. The Golgi apparatus consists of a series of flattened membrane sacs called cisternae, which are arranged in stacks called dictyosomes. The Golgi apparatus also has a network of tubules and vesicles that connect the cisternae and transport materials within and outside the organelle.

The main function of the Golgi apparatus is to process, sort, modify and package proteins and lipids that are synthesized by the endoplasmic reticulum (ER), another cellular organelle. The Golgi apparatus acts as the "post office" of the cell, as it labels and sends the proteins and lipids to their correct destinations, either within the cell or outside the cell through secretion. The Golgi apparatus also synthesizes some carbohydrates and performs other chemical reactions, such as sulfation and phosphorylation.

The Golgi apparatus was discovered in 1898 by an Italian biologist named Camillo Golgi, who observed it under a light microscope in nerve cells. He named it the "internal reticular apparatus", but later it was renamed after him. The Golgi apparatus is also known as the Golgi body, the Golgi complex or simply Golgi. The structure and function of the Golgi apparatus were further elucidated by electron microscopy and biochemical techniques in the 20th century.

The Golgi apparatus is one of the most important and fascinating organelles in the cell, as it plays a vital role in many cellular processes, such as protein secretion, membrane trafficking, cell signaling, cell differentiation and cell death. In this article, we will explore the structure, functions and diagram of the Golgi apparatus in more detail.

One of the main functions of the Golgi apparatus is to modify, sort and package proteins that are synthesized in the endoplasmic reticulum (ER) and destined for different locations within or outside the cell. The proteins are transported from the ER to the Golgi in small vesicles called transitional vesicles or COPII-coated vesicles. These vesicles fuse with the cis-face of the Golgi and deliver their contents into the lumen of the first cisterna. The proteins then move through the cisternae from the cis-face to the trans-face, undergoing various modifications along the way. These modifications include:

Glycosylation: The addition of sugar groups to proteins to form glycoproteins. This process occurs in both the ER and the Golgi, but the Golgi is responsible for adding more complex and diverse sugar chains. Glycosylation helps to protect proteins from degradation, facilitate their folding and stability, and mediate their interactions with other molecules.
Sulfation: The addition of sulfate groups to certain amino acids or sugars in proteins. This process occurs mainly in the trans-Golgi network (TGN), which is a collection of tubules and vesicles at the trans-face of the Golgi. Sulfation enhances the activity and solubility of some proteins, such as hormones and growth factors.
Phosphorylation: The addition of phosphate groups to certain amino acids or sugars in proteins. This process also occurs mainly in the TGN and regulates the activity and localization of some proteins, such as enzymes and receptors.
Proteolysis: The cleavage of some proteins into smaller fragments by specific enzymes. This process occurs in both the ER and the Golgi, but the Golgi is responsible for activating some precursor proteins by removing their signal peptides or propeptides. Proteolysis is essential for generating functional proteins, such as hormones and enzymes.

The modified proteins are then sorted and packaged into different types of vesicles that bud off from the trans-face of the Golgi. These vesicles are coated with different proteins that help to direct them to their specific destinations. Some of these vesicles are:

Secretory vesicles: These vesicles contain proteins that are destined for secretion outside the cell by exocytosis. They fuse with the plasma membrane and release their contents into the extracellular space. Examples of secreted proteins include hormones, enzymes, antibodies and mucus.
Clathrin-coated vesicles: These vesicles contain proteins that are destined for endosomes, lysosomes or the plasma membrane. They are coated with a protein called clathrin that helps them to recognize and bind to specific receptors on their target membranes. Examples of proteins transported by these vesicles include lysosomal enzymes, membrane receptors and transporters.
COP-coated vesicles: These vesicles contain proteins that are destined for either the ER or another compartment of the Golgi. They are coated with a protein complex called coatomer or COP that helps them to maintain their shape and select their cargo. Examples of proteins transported by these vesicles include ER-resident proteins, Golgi enzymes and membrane lipids.

The role of Golgi in protein packaging is crucial for maintaining cellular homeostasis and function. By modifying, sorting and delivering proteins to their appropriate locations, the Golgi apparatus ensures that each protein can perform its specific role in the cell or in communication with other cells.

The Golgi apparatus was discovered by an Italian biologist named Camillo Golgi in 1898. Golgi was interested in the structure and function of the nervous system, and he developed a new staining technique that allowed him to visualize nerve cells with unprecedented clarity. His technique, which is now known as the Golgi staining or the Golgi method, involved hardening nervous tissue in potassium bichromate and then impregnating it with silver nitrate. This resulted in a random and selective staining of some nerve cells, revealing their entire morphology, including the cell body, the axon and the dendrites.

Golgi used his staining method to study various regions of the brain, such as the cerebellum, the hippocampus and the olfactory bulb. He also observed other types of cells, such as glial cells, blood cells and muscle cells. In April 1898, he reported his discovery of a novel intracellular structure that he called the "internal reticular apparatus". He described it as a network of fine tubules and vesicles that were distributed throughout the cytoplasm of some nerve cells. He speculated that this structure was involved in the secretion of substances from the cell.

Golgi`s discovery was initially met with skepticism and controversy by other scientists, who doubted the existence and significance of the internal reticular apparatus. Some argued that it was an artifact of his staining method, while others proposed alternative interpretations of its nature and function. For example, Santiago Ramón y Cajal, a Spanish neuroscientist who also used the Golgi method to study the nervous system, suggested that the internal reticular apparatus was actually a system of intracellular canals that transported nutrients and waste products within the cell.

It was not until the advent of electron microscopy in the 1950s that the true structure and function of the Golgi apparatus were confirmed and appreciated. Electron micrographs revealed that the Golgi apparatus consisted of stacks of flattened membrane-bound sacs called cisternae, surrounded by numerous vesicles and tubules. It was also shown that the Golgi apparatus played a crucial role in modifying, sorting and transporting proteins and lipids that were synthesized in the endoplasmic reticulum to various destinations within or outside the cell.

The Golgi apparatus is now recognized as one of the most important organelles in eukaryotic cells, and it is named after its discoverer Camillo Golgi, who shared the Nobel Prize in Physiology or Medicine in 1906 with Ramón y Cajal for their work on the structure of the nervous system.

The Golgi apparatus is a cellular organelle that consists of a series of flattened, stacked pouches called cisternae. The cisternae are surrounded by a complex network of tubules and vesicles that are involved in the transport and modification of proteins and lipids. The diagram below shows a simplified representation of the Golgi apparatus and its main components.

The Golgi apparatus has two main faces: the cis face and the trans face. The cis face is the side that receives vesicles containing newly synthesized proteins and lipids from the endoplasmic reticulum (ER). The trans face is the side that releases vesicles containing modified proteins and lipids to their final destinations, such as the plasma membrane, lysosomes, or other organelles. The cis and trans faces are also called the forming face and the maturing face, respectively.

Between the cis and trans faces, there are three main compartments: the cis cisternae, the medial cisternae, and the trans cisternae. These compartments contain different sets of enzymes that modify the proteins and lipids passing through them. For example, some proteins are glycosylated (addition of sugar chains), sulfated (addition of sulfate groups), or phosphorylated (addition of phosphate groups) in different cisternae.

The cis and trans cisternae are connected by tubules that allow the movement of proteins and lipids between them. Some proteins and lipids may also move by vesicular transport, which involves budding and fusion of membrane-bound vesicles. The vesicles can be coated with different proteins, such as clathrin or COP (coat protein), that help them to recognize and bind to specific receptors on the target membranes.

The cis and trans faces are also associated with two networks: the cis Golgi network (CGN) and the trans Golgi network (TGN). The CGN is a collection of tubules and vesicles that sort and recycle proteins and lipids from the ER or the Golgi apparatus. The TGN is a collection of tubules and vesicles that sort and package proteins and lipids for secretion or delivery to other organelles.

The structure and function of the Golgi apparatus are highly dynamic and adaptable to different cellular needs. For instance, the number, shape, and organization of the cisternae can vary depending on the cell type, metabolic state, or environmental conditions. The Golgi apparatus can also undergo fragmentation and reassembly during cell division or in response to stress.

The Golgi apparatus is composed of a series of flattened, stacked pouches called cisternae. Each cisterna is a membrane-bound compartment that contains enzymes and other molecules for modifying and sorting the products of the endoplasmic reticulum (ER). The number of cisternae in a Golgi stack varies depending on the cell type and function, but typically ranges from four to eight. Some single-celled organisms, such as algae and fungi, may have as many as 60 cisternae per stack.

The Golgi apparatus has a distinct polarity, with two different faces: the cis face and the trans face. The cis face is the side that faces the ER and receives vesicles containing newly synthesized proteins and lipids. The trans face is the side that faces the plasma membrane and releases vesicles containing modified products to their final destinations. Between the cis and trans faces, there are three main compartments: the cis Golgi network, the medial Golgi, and the trans Golgi network. These compartments have different sets of enzymes that modify the cargo molecules in different ways.

In addition to cisternae, the Golgi apparatus also contains an array of interconnected tubules and vesicles that surround and radiate from the stacks. The tubules are thin membrane-bound tubes that connect adjacent cisternae or form branching networks. The vesicles are small membrane-bound spheres that bud off from the cisternae or tubules and carry cargo molecules to or from the Golgi apparatus. There are three types of vesicles associated with the Golgi apparatus: transitional vesicles, secretory vesicles, and clathrin-coated vesicles.

Transitional vesicles are formed by budding from the ER and fuse with the cis Golgi network to deliver their cargo.
Secretory vesicles are formed by budding from the trans Golgi network and carry modified products to the plasma membrane or other organelles for secretion or incorporation.
Clathrin-coated vesicles are formed by budding from the trans Golgi network or the plasma membrane and have a distinctive coat of proteins called clathrin. They are involved in transporting membrane proteins and receptors between different compartments.

The structure of the Golgi apparatus is supported by a network of cytoskeletal proteins, such as microtubules and actin filaments. These proteins help maintain the shape and position of the Golgi stacks, as well as facilitate the movement of vesicles along the tubules.

The Golgi apparatus is composed of a series of flattened, stacked pouches called cisternae . These are membrane-bound vesicles that are arranged in a parallel fashion and form a stack called a dictyosome. Each cisterna is about 1 μm in diameter and has a smooth unit membrane that encloses a lumen. The lumen is the space inside the cisterna where proteins and lipids are modified and packaged for transport.

The number of cisternae in a Golgi stack varies depending on the cell type and organism. For example, animal cells typically have 5 to 6 cisternae per stack, while plant cells can have 20 or more. The cisternae are held together by matrix proteins and are supported by cytoplasmic microtubules. The microtubules also help to maintain the polarity and orientation of the Golgi apparatus within the cell.

The cisternae are biochemically and functionally distinct from each other. They can be classified into three main types: cis, medial, and trans . The cis cisternae are the ones closest to the endoplasmic reticulum (ER), where they receive proteins and lipids from transitional vesicles that bud off from the ER. The cis cisternae are also called the forming or entry face of the Golgi apparatus. The trans cisternae are the ones farthest from the ER, where they release proteins and lipids into secretory vesicles that bud off from the Golgi membrane. The trans cisternae are also called the maturing or exit face of the Golgi apparatus. The medial cisternae are the ones in between the cis and trans cisternae, where most of the modification and sorting of proteins and lipids take place.

Each type of cisterna contains different sets of enzymes that catalyze specific reactions on the proteins and lipids passing through them. These reactions include glycosylation, phosphorylation, sulfation, cleavage, and folding. The enzymes also act as markers that direct the proteins and lipids to their proper destinations within or outside the cell. For example, some proteins are tagged with mannose-6-phosphate groups in the cis Golgi network, which target them to lysosomes. Some lipids are modified with galactose or sialic acid residues in the trans Golgi network, which target them to the plasma membrane.

The cisternae are therefore the simplest unit of the Golgi apparatus, but they are also dynamic and complex structures that play a vital role in the processing and transport of cellular materials.

A complex array of associated vesicles and anastomosing tubules (30 to 50 nm diameter) surround the dictyosome and radiate from it. In fact, the peripheral area of the dictyosome is fenestrated (lace-like) in structure. Tubules clearly interconnect closely adjacent stacks of the Golgi apparatus and may represent a communication channel to synchronize stack function within the cell . A feasible hypothesis is that tubules may be a potentially static component of the Golgi apparatus in contrast to the stacked cisternal plates which may turn over continuously . The tubular network of the Golgi apparatus may also play a role in the transport of lipids and proteins between different cisternae and between the Golgi and other organelles.

Vesicles are small membrane-bound sacs that transport materials within or outside the cell. The Golgi apparatus produces different types of vesicles, each with a specific function and destination. The three main types of vesicles are:

Transport vesicles: These vesicles carry proteins and other molecules from the endoplasmic reticulum to the Golgi apparatus, or from one part of the Golgi apparatus to another. They also transport materials from the Golgi apparatus to other organelles, such as the plasma membrane, lysosomes, or endosomes .
Secretory vesicles: These vesicles contain materials that are destined to be released from the cell by exocytosis. They are formed at the trans face of the Golgi apparatus and move towards the plasma membrane. They may contain hormones, neurotransmitters, enzymes, antibodies, or other substances that are secreted by the cell .
Lysosomes: These vesicles contain digestive enzymes that break down various macromolecules, such as proteins, lipids, nucleic acids, and carbohydrates. They are formed at the trans Golgi network and fuse with endosomes or phagosomes that contain materials ingested by the cell. They also recycle worn-out organelles or damaged parts of the cell by autophagy .

The vesicles produced by the Golgi apparatus are essential for the proper functioning of the cell. They help in sorting, modifying, and delivering proteins and lipids to their target destinations. They also help in maintaining cellular homeostasis by regulating the metabolism and degradation of various molecules.

The Golgi apparatus is a membrane-bound organelle that plays a key role in transporting, modifying, and packaging proteins and lipids into vesicles for delivery to targeted destinations within or outside the cell. The Golgi apparatus also participates in various cellular processes such as cell wall formation, carbohydrate synthesis, and enzyme activation. Some of the main functions of the Golgi apparatus are:

Protein and lipid sorting: The Golgi apparatus receives proteins and lipids from the endoplasmic reticulum (ER) and sorts them according to their final destinations. The proteins and lipids are modified by enzymes in different compartments of the Golgi apparatus, called cis, medial, and trans cisternae. These modifications include glycosylation, sulfation, phosphorylation, and proteolysis. The modified proteins and lipids are then packaged into vesicles that bud off from the trans face of the Golgi apparatus. The vesicles are coated with specific molecules that act as labels for their recognition by target membranes or organelles .
Protein and lipid secretion: The Golgi apparatus is involved in the secretion of various proteins and lipids that are either released from the cell or incorporated into the plasma membrane. For example, the Golgi apparatus secretes digestive enzymes by pancreatic cells, mucus by goblet cells of the intestine, hormones by endocrine cells, antibodies by plasma cells, and collagen by fibroblasts. The secretory vesicles fuse with the plasma membrane and release their contents by exocytosis.
Lysosome formation: The Golgi apparatus is responsible for the formation of lysosomes, which are membrane-bound organelles that contain hydrolytic enzymes for breaking down macromolecules and cellular debris. The lysosomal enzymes are synthesized in the ER and transported to the Golgi apparatus, where they are modified by adding mannose-6-phosphate groups. These groups act as signals for sorting the enzymes into vesicles that bud off from the trans Golgi network. The vesicles then fuse with endosomes, which are membrane-bound compartments that receive materials from endocytosis or phagocytosis. The fusion results in the formation of lysosomes.
Cell wall synthesis: In plant cells, the Golgi apparatus is mainly involved in the synthesis and secretion of materials for the primary and secondary cell walls. The primary cell wall is a thin layer of polysaccharides that surrounds young plant cells and provides support and flexibility. The secondary cell wall is a thicker layer of polysaccharides and lignin that strengthens mature plant cells. The Golgi apparatus produces glycoproteins, lipids, pectins, and monomers for hemicellulose, cellulose, and lignin, which are then transported to the cell wall by secretory vesicles .
Carbohydrate synthesis: The Golgi apparatus is also a major site of carbohydrate synthesis in both animal and plant cells. The Golgi apparatus produces various types of carbohydrates, such as glycosaminoglycans (GAGs), which are long chains of sugars that attach to proteins to form proteoglycans. Proteoglycans are important components of the extracellular matrix and cell membranes. The Golgi apparatus also synthesizes other carbohydrates, such as glycogen in animal cells and starch in plant cells .
Enzyme activation: The Golgi apparatus plays a role in activating some enzymes by modifying them or removing inhibitory subunits. For example, the Golgi apparatus activates proinsulin by cleaving it into insulin and C-peptide. Insulin is a hormone that regulates blood glucose levels. Another example is the activation of proglucagon by cleaving it into glucagon and other peptides. Glucagon is a hormone that stimulates glycogen breakdown.

Golgi vesicles are small membrane-bound structures that bud off from the Golgi apparatus and carry various molecules to different destinations within or outside the cell. They are often referred to as the "traffic police" of the cell because they play a key role in sorting and directing many of the cell`s proteins and membrane constituents to their proper destinations.

To perform this function, Golgi vesicles contain different sets of enzymes and receptors that modify and recognize specific cargo molecules passing through the Golgi apparatus. These modifications and recognition signals determine the fate and destination of the cargo molecules, whether they are destined for the plasma membrane, lysosomes, endosomes, or other organelles .

There are three main types of Golgi vesicles that differ in their origin, content, and destination:

Transitional vesicles are formed by budding from the endoplasmic reticulum (ER) and fuse with the cis-face of the Golgi apparatus. They carry newly synthesized proteins and lipids from the ER to the Golgi for further processing and sorting.
Secretory vesicles are formed by budding from the trans-face of the Golgi apparatus and move towards the plasma membrane. They carry proteins and lipids that are either secreted by exocytosis or inserted into the plasma membrane.
Clathrin-coated vesicles are formed by budding from either the trans-face of the Golgi apparatus or from endosomes. They have a distinctive coat of clathrin proteins that helps them to select and transport specific cargo molecules. They are involved in intracellular traffic of membranes and secretory products between the ER, Golgi, endosomes, and lysosomes .

By using these different types of vesicles, the Golgi apparatus can efficiently sort and deliver a variety of molecules to their appropriate locations within or outside the cell. Thus, Golgi vesicles act as the "traffic police" of the cell, ensuring that each molecule reaches its correct destination.

One of the main functions of the Golgi apparatus in animal cells is to package and secrete various materials that are needed for the cell`s activities or interactions with other cells. These materials include hormones, enzymes, antibodies, neurotransmitters, growth factors, and extracellular matrix components.

The packaging and secretion process involves the following steps:

The proteins and lipids that are synthesized in the endoplasmic reticulum (ER) are transported to the Golgi apparatus in small vesicles called transitional vesicles. These vesicles fuse with the cis-face of the Golgi apparatus, which is the side that faces the ER.
The proteins and lipids are then modified by different enzymes in the cisternae of the Golgi apparatus. These modifications include glycosylation (adding sugar groups), sulfation (adding sulfate groups), phosphorylation (adding phosphate groups), and proteolysis (cutting proteins into smaller fragments).
The modified proteins and lipids are sorted and packaged into different types of vesicles that bud off from the trans-face of the Golgi apparatus, which is the side that faces the plasma membrane. These vesicles include:
- Secretory vesicles, which contain materials that are destined for secretion by exocytosis. Exocytosis is a process in which a vesicle fuses with the plasma membrane and releases its contents to the outside of the cell. Some secretory vesicles are stored in the cytoplasm until they receive a signal to be released, such as a hormone or a nerve impulse. These are called regulated secretory vesicles. Other secretory vesicles are continuously released without any signal. These are called constitutive secretory vesicles.
- Clathrin-coated vesicles, which contain materials that are destined for endocytosis. Endocytosis is a process in which a part of the plasma membrane invaginates and pinches off to form a vesicle that brings materials into the cell. Clathrin-coated vesicles have a coat of proteins called clathrin that helps them to form and to bind to specific receptors on the plasma membrane. Clathrin-coated vesicles can be involved in different types of endocytosis, such as phagocytosis (eating large particles), pinocytosis (drinking fluids), and receptor-mediated endocytosis (selectively taking up molecules that bind to receptors).
- Lysosomal vesicles, which contain materials that are destined for degradation by lysosomes. Lysosomes are organelles that contain digestive enzymes that break down various substances, such as bacteria, viruses, worn-out organelles, and macromolecules. Lysosomal vesicles fuse with lysosomes and deliver their contents for digestion.

The packaging and exocytosis of materials by the Golgi apparatus is essential for many cellular functions, such as communication, defense, metabolism, growth, and differentiation. For example, the Golgi apparatus is involved in:

The secretion of hormones by endocrine cells, such as insulin by pancreatic beta cells and thyroxine by thyroid cells.
The secretion of enzymes by exocrine cells, such as digestive enzymes by pancreatic acinar cells and salivary glands.
The secretion of antibodies by plasma cells, which are specialized immune cells that produce antibodies against foreign antigens.
The secretion of neurotransmitters by neurons, which are nerve cells that transmit electrical signals to other cells.
The secretion of growth factors by various cells, such as epidermal growth factor (EGF) by skin cells and nerve growth factor (NGF) by nerve cells.
The secretion of extracellular matrix components by fibroblasts, which are connective tissue cells that produce collagen, elastin, and other proteins that provide structural support and elasticity to tissues.

The Golgi apparatus is involved in the formation of certain cellular organelles that are derived from its vesicles or membranes. Some examples are:

Plasma membrane: The Golgi apparatus contributes to the growth and maintenance of the plasma membrane by delivering lipids and proteins to it. The secretory vesicles that bud off from the trans face of the Golgi fuse with the plasma membrane and release their contents to the extracellular space or incorporate them into the membrane. The clathrin-coated vesicles that bud off from the trans Golgi network also deliver membrane proteins and receptors to the plasma membrane or to endosomes.
Lysosomes: The Golgi apparatus is responsible for sorting and modifying the lysosomal enzymes that are synthesized in the endoplasmic reticulum. The lysosomal enzymes are tagged with mannose-6-phosphate groups in the cis and medial cisternae of the Golgi. The mannose-6-phosphate receptors in the trans Golgi network recognize and bind to these groups and package them into clathrin-coated vesicles that bud off and fuse with endosomes. The endosomes then mature into lysosomes, which contain the acidic environment and the digestive enzymes for breaking down various macromolecules .
Acrosome of spermatozoa: The acrosome is a cap-like structure that covers the head of a sperm cell and contains enzymes that help the sperm penetrate the egg during fertilization. The acrosome is formed by the fusion of several Golgi vesicles during the development of spermatids (immature sperm cells) in the testis. The acrosomal enzymes are synthesized in the endoplasmic reticulum and modified and packaged in the Golgi apparatus.
Cortical granules of oocytes: The cortical granules are secretory vesicles that are located under the plasma membrane of an oocyte (immature egg cell) and contain proteases, glycosidases, and other enzymes that modify the zona pellucida (the extracellular matrix that surrounds the oocyte) after fertilization. The cortical granules are formed by budding off from the trans Golgi network during oogenesis (the development of oocytes) in the ovary.

Transport of lipid molecules around the cell

The Golgi apparatus is not only involved in the modification and packaging of proteins, but also in the transport of lipid molecules around the cell. Lipids are essential components of cellular membranes and serve as signaling molecules, energy sources, and metabolic intermediates.

The Golgi apparatus synthesizes various types of lipids, such as phospholipids, glycolipids, and sphingolipids. These lipids are either incorporated into the Golgi membrane or packaged into vesicles for delivery to other organelles or the plasma membrane. The Golgi apparatus also receives lipids from the endoplasmic reticulum (ER) and modifies them further.

The transport of lipids between the ER and the Golgi apparatus is mediated by vesicles and tubules that bud from one membrane and fuse with another. The vesicles and tubules are coated with specific proteins that help them recognize and dock with their target membranes. For example, COPII-coated vesicles carry lipids and proteins from the ER to the cis face of the Golgi apparatus, while COPI-coated vesicles carry them back from the Golgi to the ER.

The transport of lipids within the Golgi apparatus is also regulated by vesicular and non-vesicular mechanisms. Vesicular transport involves the budding and fusion of membrane-bound carriers that move lipids from one cisterna to another. Non-vesicular transport involves the transfer of lipid molecules across adjacent cisternae without involving membrane carriers. This may be facilitated by lipid transfer proteins that bind and shuttle lipids between membranes.

The transport of lipids from the Golgi apparatus to other destinations is dependent on the type and function of the lipid. For instance, glycosylphosphatidylinositol (GPI)-anchored proteins are modified by the addition of a GPI lipid anchor in the Golgi apparatus and then transported to the plasma membrane. Similarly, glycosphingolipids are synthesized in the Golgi apparatus and then delivered to the plasma membrane or lysosomes. Phospholipids are also transported to various membranes by vesicular or non-vesicular mechanisms.

The Golgi apparatus thus plays a crucial role in maintaining the lipid composition and diversity of cellular membranes. By synthesizing, modifying, and transporting lipids, the Golgi apparatus ensures that each membrane has the appropriate lipid content and function.

Production of proteoglycans and synthesis of carbohydrates in Golgi

Proteoglycans are molecules that consist of a protein core with one or more covalently attached glycosaminoglycans (GAGs), which are long chains of repeating disaccharide units. Proteoglycans are important components of the extracellular matrix, where they provide structural support, regulate cell adhesion and migration, and modulate growth factor signaling.

The protein component of proteoglycans is synthesized by ribosomes and translocated into the lumen of the rough endoplasmic reticulum. Glycosylation of the proteoglycan occurs in the Golgi apparatus in multiple enzymatic steps. The first step is the addition of a common linker tetrasaccharide (xylose-galactose-galactose-glucuronic acid) to a specific serine residue on the protein core. The second step is the elongation of the GAG chain by alternating addition of uronic acid and hexosamine residues by glycosyltransferases. The third step is the modification of the GAG chain by sulfation, epimerization, or deacetylation by various enzymes. The degree and pattern of modification vary depending on the type and location of the GAG chain.

The Golgi apparatus is also a major site of synthesis of carbohydrates, especially those that are involved in glycoprotein and glycolipid formation. Carbohydrates are synthesized by sequential addition of monosaccharide units to lipid or protein acceptors by glycosyltransferases that reside in different cisternae of the Golgi apparatus. The carbohydrate structures that are generated depend on the availability and specificity of the glycosyltransferases, as well as the sorting and trafficking of the glycoconjugates within the Golgi apparatus. Some examples of carbohydrates that are synthesized in the Golgi apparatus are mannose-6-phosphate, N-acetylglucosamine, N-acetylgalactosamine, fucose, sialic acid, and galactose.

The Golgi apparatus thus plays a crucial role in the production of proteoglycans and synthesis of carbohydrates that are essential for various cellular functions and interactions.

Sulfation and phosphorylation processes involving the Golgi

The Golgi apparatus is not only involved in glycosylation, but also in other types of post-translational modifications of proteins, such as sulfation and phosphorylation. These modifications can affect the structure, function, stability, and interactions of proteins.

Sulfation is the addition of sulfate groups to specific amino acids, such as tyrosine, serine, or threonine. Sulfation is catalyzed by sulfotransferases, which use 3`-phosphoadenosine-5`-phosphosulfate (PAPS) as the sulfate donor. PAPS is synthesized in the cytosol and transported into the Golgi lumen by specific antiporters that exchange PAPS for adenosine monophosphate (AMP). Sulfation occurs mainly in the trans-Golgi network (TGN) and affects proteins such as hormones, growth factors, proteoglycans, and extracellular matrix components .

Phosphorylation is the addition of phosphate groups to specific amino acids, such as serine, threonine, or tyrosine. Phosphorylation is catalyzed by kinases, which use adenosine triphosphate (ATP) as the phosphate donor. ATP is synthesized in the cytosol and transported into the Golgi lumen by specific antiporters that exchange ATP for adenosine diphosphate (ADP). Phosphorylation occurs mainly in the cis- and medial-Golgi cisternae and affects proteins such as lysosomal enzymes, secretory proteins, and membrane proteins .

Both sulfation and phosphorylation processes require the import of nucleotide-sulfate or nucleotide-phosphate into the Golgi lumen by antiporters that maintain a balance between the luminal and cytosolic concentrations of these molecules. These antiporters are thought to be regulated by the membrane potential across the Golgi membrane and by the availability of substrates . The sulfation and phosphorylation reactions are also regulated by the localization and activity of the sulfotransferases and kinases within the Golgi compartments .

Sulfation and phosphorylation are important for modulating the biological activity of proteins that are destined for secretion or membrane localization. For example, sulfation enhances the binding affinity of hormones and growth factors to their receptors, while phosphorylation activates or inactivates lysosomal enzymes by exposing or masking their mannose-6-phosphate recognition signals . Therefore, these modifications are essential for proper cellular communication and function.