The Endoplasmic Reticulum and Golgi Apparatus: Two Important Organelles in the Cell


The Endoplasmic Reticulum and Golgi Apparatus: Two Important Organelles in the Cell

The intracellular membrane, including the nuclear membrane, mitochondrial membrane, and endoplasmic reticulum (ER), is a complex membrane system that plays a crucial role in cell activity.

The endoplasmic reticulum has two main forms:

  • Rough ER: This consists of flattened sacs arranged in parallel, with ribosomes attached to its surface. The primary function of rough ER is to synthesize proteins, regenerate the nuclear membrane, and create secretory vesicles.
  • Smooth ER: This consists of circular tubes, away from the nucleus, with a surrounding membrane but no ribosomes attached to its surface. Smooth ER is involved in lipid synthesis, liver detoxification, and drug metabolism.

The lumen is the space inside the flattened sacs of the ER, where newly synthesized proteins, glycoproteins, and lipoproteins are stored. The lumen is connected to the perinuclear space and contains many active enzymes.

Within the lumen, polypeptide chains synthesized from ribosomes undergo chemical transformations:

  • Cleavage of peptide bonds: This shortens the molecule. For example, a hydrophobic oligopeptide after binding to the synthesis process may be cleaved off.
  • Addition of sugar residues (glycosylation): This occurs on the inner surface of the ER membrane, so proteins of the cytoplasm (outer surface) are never glycosylated.

The total volume of the ER lumen can occupy 1/10 of the cell’s volume.

The ER plays a crucial role in:

  • The synthesis, metabolism, and transport of proteins, lipids, phospholipids, glycoproteins, and glycolipids.
  • Rough ER synthesizes secretory proteins, membrane proteins, or lysosomal enzymes.

Newly synthesized proteins accumulate in the lumen of the rough ER and are transported to transitional regions, where they form tubeless loops without ribosomes. From here, membrane-bound transport vesicles will transport the products to the cis Golgi.

  • Structural proteins are synthesized in rough ER.
  • Soluble proteins are synthesized freely in the cytoplasm.

Smooth ER has the following functions:

  • Metabolism of hydrophobic substances.
  • Synthesis of structural membrane components like lipids (triglycerides), phospholipids, and glycolipids.

The synthesis of phospholipids occurs on the lipid layer facing the cytoplasm (outer surface) of smooth ER. Subsequently, new phospholipids are transferred to the inner lipid layer to balance the membrane structure.

When the amount of hydrophobic substances that the cell needs to metabolize increases, the amount of smooth ER also increases.

The cell membrane and membrane organelles like the Golgi apparatus are constantly replenished with new lipid material through the process of membrane fusion with transport vesicles and secretory vesicles.

Mitochondrial membranes contain a few specific lipids that must be synthesized in situ, not from the ER.

The ER membrane belongs to the system of intracellular membranes, with the lipid layer facing the lumen corresponding to the outer surface lipid layer of the cell membrane (which contains a glucid component).

Peroxisomes are single-membrane organelles that function in metabolism and detoxification for the cell.

Liver cells have a very developed smooth endoplasmic reticulum.

Camillo Golgi was the first to describe the Golgi apparatus in 1897.

The Golgi apparatus is a system of 5-7 flattened sacs that are not interconnected, performing the following functions:

  • Cis face (entry face) faces the nucleus.
  • Trans face (exit face) curves in the opposite direction of the nucleus.
  • Classification and packaging of products based on function and purpose.

There are two models of transport through the Golgi sacs:

1. Cisternal maturation model.

2. Vesicular transport model.

The level of organization of the Golgi:

  • Cytoplasmic vesicle → Golgi body → Golgi apparatus.

Transport vesicles (or sacs) are new vesicles that bulge, constrict, and separate, or are vesicles that separate from other lumens that have recently come near the edge. These vesicles always run in one direction from cis to trans.

From the outermost trans face, continuously larger vesicles than transport vesicles separate. These are secretory vesicles and primary lysosomes, containing processed and concentrated products.

Storage vesicles are secretory vesicles that merge with each other, are large in size, and only exocytose upon signal.

The Golgi has an asymmetric structure, with the Golgi membrane on the cis face being thinner than the trans face.

The pathway of synthesis of substances:

  • Endoplasmic reticulum → Golgi (metabolism, complexification of spatial configuration, creation of function, attachment of sugar residues) → Membrane.

In specific regions of the outermost trans face, vesicles that separate do not exocytose but form primary lysosomes, which contain acidic hydrolytic enzymes.

The Golgi plays a crucial role in sorting (directing) products to the correct location of use. To carry out this function, the Golgi must chemically modify the products it transports.

The pathway of transport of substances in the cell:

  • Synthesis in the ER → Maturation and in the Golgi → Distribution to the locations of use.

The Golgi causes changes to the glucid component of glycoproteins:

  • Removal of sugar residues, mainly mannose residues.
  • Addition of other sugar residues such as N-acetylglucosamine, sialic acid, fructose, galactose.
  • Attachment of phosphate residues to sugar residues.

The glucid component plays a crucial role in acting as signals for the Golgi to classify products.

Primary lysosomes, as well as secretory vesicles and storage vesicles, are no longer part of the structural organization of the Golgi apparatus.

The smallest unit of the Golgi is the membrane sac.

The Golgi transports material through three pathways:

1. Path 1: Exocytosis.

2. Path 2: Building of the cell membrane components.

3. Path 3: The Golgi will go to the lysosome organelle.



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