Golgi Apparatus; Structure and Functions

Golgi apparatus, also known as the Golgi complex or Golgi body, is a membrane-bound organelle in eukaryotic cells, consisting of cisternae, tubules, vesicles and vacuoles, mainly responsible for transporting, modifying, and packaging proteins and lipids.

It is located in the cytoplasm near the endoplasmic reticulum and close to the nucleus.

Golgi apparatus plays a key role in transporting, modifying, and packaging proteins and lipids into vesicles for targeted delivery. While most cells have only one or a few Golgi apparatus, plant cells may have hundreds of them.

Table of Contents

Functioning like a factory, the Golgi apparatus processes and sorts proteins from the endoplasmic reticulum (ER) for transport to their destinations, such as lysosomes, the plasma membrane, or secretion outside the cell.

Additionally, it synthesizes glycolipids and sphingomyelin. In plant cells, the Golgi is also responsible for synthesizing complex polysaccharides that make up the cell wall.

In 1898, Camillo Golgi discovered the Golgi apparatus in the nerve cells of barn owls and cats using a metallic impregnation technique.

Named after its discoverer, the Golgi apparatus has also been referred to as the Golgisome, Golgi material, Golgi membranes, Golgi body, and other similar names.

Structure of Golgi Apparatus

The size and shape of Golgi bodies differ among various cell types, yet they share a similar organization across all cells. For instance, they are highly developed in secretory and nerve cells but relatively small in muscle cells.

Structurally, Golgi bodies consist of a central stack of flattened sacs, known as cisternae, along with numerous tubules and vesicles on the periphery.

Fig: Golgi Apparatus Structure [Image Courtesy: National Human Genome Research Institute]

Cisternae

Cisternae are flat or slightly curved, displaying a specific orientation: the convex side faces the cell membrane, while the concave side faces the nucleus.
These structures lack ribosomes and feature swollen ends, resembling the smooth endoplasmic reticulum and occasionally connecting to it, suggesting that the Golgi apparatus may originate from the smooth ER.
A single cisterna is approximately 0.5-1 µm in diameter with a cavity about 100 Å wide.
Fenestrations at the edges of each cisterna transition into tubules, and all cisternae have a continuous lumen filled with fluid.
In animal cells, cisternae usually number between 3 and 7, while in plant cells, they range from 10 to 24.
Arranged in stacks, they are spaced in parallel layers with 200-300 Å wide inter-cisternal spaces containing parallel fibers, known as inter-cisternal elements, that help support the cisternae and maintain consistent spacing.

Tubules

Tubules are the structures create a complex network, particularly near the periphery and maturing face of the apparatus.
Tubules originate from the fenestrations in the cisternae and measure 30-50 nm (or 300-500 Å) in diameter.
They serve to link the various cisternae together.

Vesicles

Vesicles are small, droplet-shaped sacs that attach to the tubules along the edges of the cisternae.

There are two types of vesicles i.e smooth vesicles and coated vesicles.

Smooth vesicles: These are small sacs with diameters ranging from 20 to 80 nm and have a smooth surface. Known as secretory vesicles, they contain secretory materials and bud off from the ends of cisternal tubules within the network.

Coated vesicles: These spherical structures, roughly 50 nm in diameter, have a rough surface. Located at the periphery of the organelle, usually at the ends of individual tubules, they are visually distinct from secretory vesicles and are involved in producing membrane proteins.

Golgian Vacuoles

These are large, rounded sacs located on the maturing face of the Golgi apparatus.
They form either from the expansion of cisternae or through the fusion of secretory vesicles, becoming modified into vacuoles.
These vacuoles develop from the concave, or maturing, side and contain either amorphous or granular material.
Some Golgian vacuoles also serve as lysosomes.

The Golgi complex consists of three functional regions: the cis region, located closest to the ER; the medial region, positioned in the center; and the trans region, which includes the trans Golgi reticulum near the plasma membrane.

Each of these regions contains distinct enzymes that perform specific modifications to secretory and membrane proteins passing through.

The primary modification is glycosylation, where sugars are added to proteins to form glycoproteins—a process that begins in the ER and is completed in the Golgi complex.

Further changes in the Golgi involve the addition of lipids to form lipoproteins (liposylation), along with the attachment of other molecular groups.

Functions of Golgi Apparatus

Formation of Secretory Vesicles: The Golgi complex processes and packages proteins and lipids from the ER for transport to various parts of the cell or secretion outside the cell. This packaging involves enclosing the materials in a membrane to form secretory vesicles. These vesicles can contain substances such as zymogen in pancreatic cells, mucus in goblet cells, lactoprotein in mammary gland cells, pigment granules in pigment cells, collagen in connective tissue cells, and hormones in endocrine cells.
Synthesis of Carbohydrates: The Golgi apparatus synthesizes certain mucopolysaccharides from simple sugars.
Formation of Glycoproteins: The Golgi apparatus attaches sugars to proteins from the rough ER, forming glycoproteins.
Formation of Lipoproteins: Lipids and proteins from the ER are combined into lipoproteins in the Golgi apparatus.
Addition to Cell Membrane: The Golgi apparatus supplies membrane material to the plasma membrane, especially when it needs to expand for processes like forming pinocytic and phagocytic vesicles or the cleavage furrow during animal cell division. The movement of membrane material from the ER through transition vesicles, the Golgi complex, and secretory vesicles to the plasma membrane is known as membrane flow.
Membrane Transformation: The Golgi apparatus converts one type of membrane into another. As membranes move through the Golgi complex, they gradually change from the ER type to one resembling the plasma membrane.
Cell Wall Formation: In certain algae, the Golgi complex synthesizes cellulose plates for the cell wall. In higher plants, it produces pectin and other carbohydrates essential for cell wall formation, and generates secretions like mucilage and gums.
Lysosome Formation: The Golgi complex creates primary lysosomes through budding. Lysosomes can also originate from the ER.
Acrosome Formation: The Golgi complex forms the acrosome in sperm cells.
Yolk and Cortical Granules Formation: The Golgi complex is responsible for producing yolk and cortical granules in eggs. The process of yolk formation is known as vitellogenesis.
Nematocyst and Trichocyst Formation: The Golgi apparatus produces nematocysts in Hydra and possibly other coelenterates, as well as trichocysts in ciliates like Paramecium.
Storage of Secretions: The Golgi complex stores cellular secretions, including protein and lipid.
Absorption of Materials: The Golgi apparatus helps absorb materials from the environment. For instance, cells in the intestinal lining utilize the Golgi complex to absorb lipids from the intestine.

Summary

In 1898, Camillo Golgi discovered the Golgi apparatus in the nerve cells of barn owls and cats using a metallic impregnation technique. Named after its discoverer, the Golgi apparatus has also been referred to as the Golgisome, Golgi material, Golgi membranes, Golgi body, and other similar names.

Structurally, Golgi bodies consist of a central stack of flattened sacs, known as cisternae, along with numerous tubules and vesicles (smooth and coated) on the periphery. Along with cisternae, tubules and vesicles it also consist of golgian vacuoles which are large, rounded sacs located on the maturing face.

The Golgi complex consists of three functional regions: the cis region, located closest to the endoplasmic reticulum; the medial region, positioned in the center; and the trans region, near the plasma membrane.

Golgi apparatus several functions like formation of secretory vesicles, synthesis of carbohydrates, formation of glycoproteins and lipoproteins, membrane transformation, formation of cell wall, formation of lysosome, yolk and acrosome formation, absorption of lipids etc.