Endomembrane System in Eukaryotic Cells
Introduction
Eukaryotic cells, unlike prokaryotic cells, possess a complex network of internal membranes called the endomembrane system. This system includes various organelles such as the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes, and vacuoles. These organelles play crucial roles in cellular function.
Endoplasmic Reticulum (ER)
The ER is a network of interconnected tubules and sacs extending throughout the cytoplasm. It exists in two forms: rough ER (RER) and smooth ER (SER).
Rough Endoplasmic Reticulum (RER)
The RER is studded with ribosomes on its cytosolic surface. It consists of interconnected tubules and flattened sacs. RER is absent in mammalian red blood cells.
Functions:
- Synthesizes and distributes proteins.
- Glycosylates proteins.
Smooth Endoplasmic Reticulum (SER)
The SER lacks ribosomes and is primarily composed of a network of tubules. It is abundant in striated muscle cells.
Functions:
- Synthesizes most cellular lipids.
- Stores and transports lipids.
- Detoxifies lipid-soluble toxins.
- Facilitates muscle contraction.
- Supports mineral metabolism and pigment production.
- Mediates glycogen degradation in hepatocytes.
- Synthesizes bile acids from cholesterol.
Golgi Apparatus
The Golgi apparatus consists of stacks of flattened sacs called cisternae, along with associated vesicles. It is found in all eukaryotic cells except mammalian red blood cells and is typically located near the nucleus.
Golgi Apparatus Structure
The Golgi apparatus has two distinct faces:
- Cis face: Located near the ER, it receives molecules from the ER via transitional vesicles.
- Trans face: Situated near the plasma membrane, it releases secretory vesicles containing processed molecules.
Functions:
- Modifies, packages, transports, distributes, and secretes molecules synthesized in the ER.
- Regenerates the plasma membrane.
- Completes glycosylation of proteins and lipids.
- Synthesizes polysaccharides in plant cells.
- Forms primary lysosomes.
Lysosomes
Lysosomes are membrane-bound vesicles containing about 50 different hydrolytic enzymes capable of breaking down biomolecules. Their inner membrane is coated with glycoproteins to protect it from enzymatic degradation.
Types of Lysosomes
- Primary lysosomes: Newly formed lysosomes containing only hydrolytic enzymes.
- Secondary lysosomes: Formed by the fusion of primary lysosomes with vacuoles containing material to be digested (phagolysosomes or autophagolysosomes).
Peroxisomes
Peroxisomes are small, membrane-bound organelles containing oxidative enzymes (peroxidases and catalases). They resemble lysosomes but contain oxidative enzymes instead of hydrolases.
Functions:
- Oxidize molecules using molecular oxygen, producing hydrogen peroxide.
- Decompose hydrogen peroxide into water and oxygen using catalase.
Vacuoles
Vacuoles are membrane-bound compartments found in the cytoplasm of eukaryotic cells. Animal cells typically have small and few vacuoles, while plant cells often have a large central vacuole.
Functions:
- Store reserve substances (sugars, proteins, fatty acids), waste products, pigments, and toxic substances.
- Maintain turgor pressure in plant cells by storing water.