Endomembrane System -Definition, Function, and Components

Introduction

The endomembrane system is a collection of membrane-bound transport vesicles and tubes that operate in the cytoplasm. It comprises discrete organelles that control, regulate and facilitate different processes.

The endomembrane system is associated with nearly all cellular processes, for example, the synthesis of lipids and proteins. It also facilitates the transport of materials between cellular compartments and the degradation of waste products.

In plants, the Golgi apparatus is known as the Golgi body or secretory granule. In algae and plants, the peroxisomes have another organelle called the glyoxysome. This organelle is not present in eukaryotic cells.

Endomembrane System Components in Eukaryotic Cells

In eukaryotes, the endomembrane system consists of the following organelles and structures:

• Endoplasmic reticulum
• Golgi apparatus
• Ribosomes
• Lysosomes
• Peroxisomes
• Nuclear envelope
• Plasma membrane
• Vesicles

Endoplasmic Reticulum

The Endoplasmic Reticulum (ER) is a network of membranes found inside the cytoplasm of all eukaryotic cells. It is the site of synthesis and modification of proteins, lipids, and carbohydrates required by the cell and functions in the transport of molecules.

There are two types of endoplasmic reticulum, namely, rough ER and smooth ER.

Rough Endoplasmic Reticulum (RER)

All eukaryotic cells have the rough endoplasmic, which is involved in the synthesis of lipids and proteins. This organelle consists of a single, continuous network of membranes.

The rough endoplasmic reticulum has compartments made from a network of ribosomes that wrap around it, forming “rough endoplasmic reticulum cisternae.”

The functions of the rough ER are as follows:

• It synthesizes lipids and proteins.
• The ER synthesizes steroids and their precursors namely cholesterol, and steroid hormones.
• It packages and transports lipids.
• The ER synthesizes some amino acids
• It is a storage site for vitamins such as biotin, vitamin C (ascorbic acid), and fat-soluble vitamins A, D, E, and K.

Smooth Endoplasmic Reticulum (SER)

The Smooth ER forms a network in the cytoplasm of eukaryotic cells and is involved in synthesizing lipids and proteins.

The Smooth endoplasmic reticulum (SER) is found in the form of isolated flattened sacs called “smooth endoplasmic reticulum cisternae.” This endoplasmic reticulum does not have ribosomes wrapping the membranes.

The Golgi Apparatus

The Golgi apparatus is a series of flattened sacs (cisternae) interconnected by a series of membrane-bound tubes. These apparatus give eukaryotic cells their unique abilities to modify and sort proteins and lipids.

The Golgi is located at the end of the RER, and it receives modified proteins from the RER. The Golgi complex performs two main functions on proteins and other molecules:

• They package proteins by enclosing them into a protective coat called the “Golgi membrane.”
• They transport proteins to other cell compartments, such as endosomes, lysosomes, or plasma membranes.

The protein-processing activities of the Golgi apparatus are as follows:

Modification of proteins and lipids

Proteins that exit the ER have a lengthy hydrophobic amino-terminal end. The Golgi apparatus modifies these hydrophobic amino-termini by adding a hydrophilic sugar molecule, mannose. This process is known as glycosylation.

In addition to adding sugar groups, the Golgi modifies proteins by Adding a farnesyl group to the protein’s hydrophobic amino-terminal end. One of the sugar groups added to proteins is N-acetyl glucosamine.

Protein sorting

The Golgi apparatus sorts proteins according to their hydrophobic amino-termini, the method of modification (glycosylation), and the presence or absence of farnesyl groups. This organelle decides how to package a lipid-containing protein by determining whether the lipid is cholesterol-type or phospholipid-type.

Modification of lipids

The Golgi body adds a sugar group to the lipid-containing protein. The Golgi also adds a farnesyl group to the amino terminus of the protein. In addition, the Golgi apparatus adds a phospholipid group to the hydrophobic end of the protein.

Conjugation of proteins to lipids and other soluble molecules

The Golgi apparatus:

• Adds Phosphatidylserine group to the hydroxyl terminus of the protein to which it is conjoined with a lipid.
• Attaches a phosphorylcholine group to the N-terminal end of the protein adjacent to a sphingomyelin group.
• Adds Phosphatidylethanolamine to the N-terminal end of the protein adjacent to a ceramide group.
• Attaches a Phosphatidylcholine or phosphatidylinositol to the N terminus of the protein adjacent to a phosphatidylinositol group.
• Adds Cholesterol molecule to the N terminus to the protein adjacent to a sphingomyelin group.

Amino acid transport

The Golgi apparatus delivers:

• Proteins to other cellular compartments and return protein-containing vesicles of unmodified proteins to the ER for modification.
• Protein-containing vesicles from the ER to the lysosome, Endosome, and plasma membrane.

Ribosomes

Other organelles that are part of the endomembrane system are the ribosomes. These organelles are located in the cytoplasm of eukaryotic cells and are composed of two subunits: one small and one large. The subunits are called the small subunit (the 40S) and the large subunit (60S).

Ribosomes are some of the most important organelles in eukaryotic cells. They carry out various functions in living organisms as discussed below:

• Ribosomes add protein chains to a growing polypeptide chain (a protein) to form a complete polypeptide. The proteins produced by ribosomes are carried away to other organelles or out of the cell.
• They are also involved in RNA synthesis. During this process, the ribosomes themselves are modified to assemble the RNA molecules.
• They play a significant role in protein synthesis and degradation.
• Ribosomes facilitate the modification of membrane phospholipids and glycoproteins
• They help in the activation and inactivation of enzymes involved in lipid metabolism

The subunits are composed of rRNA, ribosomal proteins, and binding proteins. This complex forms a hollow shell containing the rRNA, proteins, and a pool of tRNAs.

The two subunits are held together by two molecules of GTP. There are several structural features found in the small subunit:

The central portion is composed of A- and P-site tRNAs and four ribosomal proteins. The B-site is where the first amino acid (phenylalanine) binds; this site is located at one end of a tunnel on the periphery of the subunit.

The 30S rRNA molecule is located at the bottom end of this tunnel, and it possesses a complex secondary structure containing stems joining helices that line the RNA tunnel.

The large subunit has two major regions:

• A cap segment -which includes six proteins (L7, L12, L20, L27, L37, and L38) – is located on the periphery of this subunit.
• Six proteins (L4, L5, L10, L11, L23, and L32) form a large central core that contains the “A site” that binds to mRNA via charged nucleotides. The core of the tRNA binds to the large subunit at one end and the small subunit via its amino acid chain at the other, opposite end.

The two regions are separated by a tunnel containing RNA molecules with parts of some of these secondary structures. The large subunit contains a second mRNA molecule, while the small one has only one.

The ribosomal proteins are involved in:

• Recognizing and binding to the anticodon of each tRNA
• Binding of primary tRNAs to the A-site and forming a ternary complex consisting of an aminoacyl-tRNA, a large subunit protein (L10), and a small subunit protein (S2) site.
• Positioning amino acids at the P and A sites
• Binding with GTP and GDP, polymerization of rRNA molecules
• Binding of tRNAs to the A- and P-sites, recognition of mRNA molecules
• The movements in which the large subunits of ribosomes are involved include:
• The conformational changes in the rRNA and proteins, the binding to mRNA by the small subunit, and interactions with GTP.
• The movements performed by the large subunit are related to peptide bond formation in polypeptide synthesis.

Lysosomes

Lysosomes are membrane-enclosed organelles in animal cells. These organelles contain hydrolytic enzymes such as proteases, lipases, phosphatases, nucleases, and phospholipases. These enzymes degrade substances in lysosomes into smaller components that can be recycled by the cell or used by the cell for other purposes.

Lysosomes are formed from portions of the endoplasmic reticulum and are surrounded by two membranes. The inner membrane contains enzymes that break down the substances to be digested. In contrast, the outer membrane contains other enzymes that protect the enzymes in the inner membrane from digesting other substances.

These organelles play essential roles in cell metabolism, cell growth and morphogenesis, cell physiology, and intracellular transport. For example:

Digestive enzymes in lysosomes break down the membrane proteins in a cell after it dies, which helps recycle the materials in that cell membrane.

Lysosomes also contain enzymes that digest and recycle macromolecules. These enzymes include proteases that break down proteins, lipases that break down fats, and nucleases that break down nucleic acids.

The lipid-digesting enzymes act similarly to detergents. These enzymes are responsible for breaking up large pieces of membrane and cell organelles.

Lysosomes participate in intracellular digestion in all animal cells and some plant and fungal cells. They contain digestive enzymes that break down polymers into monomers and macromolecules into small pieces.

The large surface area of the inner membrane of the lysosome allows for many macromolecules to be broken down into smaller pieces. Macromolecules are transported into lysosomes by vesicles via the endosomal pathway.

The enzymes in lysosomes are activated when they bind to macromolecules. These binding molecules include membranous organelles, polymers, or oligomers. The enzymes hydrolyze the bonds of these macromolecules to produce small monomers, which are typically recycled by the cell or used for energy.

Peroxisomes

Peroxisomes are membrane-enclosed organelles found in most eukaryotic cells and contain enzymes that perform essential functions for the cell, such as:

Metabolic pathways that peroxisomes play a part in include the β-oxidation of fatty acids and the breakdown of amino acids and derivatives of fatty acids. These pathways are anaerobic, meaning that they occur in the absence of oxygen.

Enzymes found in peroxisomes are also involved in the biosynthesis of lipids that can be used for membrane synthesis and steroid hormone production. Peroxisomes also produce hydrogen peroxide, which again is used for the oxidation of DNA and proteins.

Peroxisomal disorders are rare but severe, resulting in disorders that are often fatal before age 5. Although the specific genetic causes of these disorders have not been identified, they are believed to occur due to mutations in genes. The mutations occur in peroxisomal protein genes or enzymes involved in peroxisome biosynthesis.

These disorders include Zellweger syndrome, neonatal adrenoleukodystrophy (X-linked), and neonatal Refsum disease. Other possible disorders are infantile Refsum disease, peroxisomal biogenesis disorders 1-6, and adult Refsum disease.

Peroxisomes have a central role in fatty acid degradation. This process is essential for the production of cellular ATP (a nucleoside triphosphate).

Nuclear Envelope

The nuclear envelope is a double membrane that surrounds and separates the interior of the nucleus from the cytoplasm. Its structure consists of two lipid bilayers that are connected by integral and peripheral membrane proteins. A layer in between these two membranes is known as the nuclear lamina.

The nuclear envelope is continuous with the endoplasmic reticulum (ER) membrane at the nuclear envelope-endoplasmic reticulum junctions.

Unlike other membranes in the body, it is impermeable to most large molecules. It is semi-permeable, allowing small molecules such as water and oxygen to pass through, but keeping larger substances, such as proteins and certain sugars, out.

The envelope regulates what enters and exits the nucleus by selectively allowing transport between these two environments. This regulation ensures that the cell maintains the correct metabolism. Molecules that are hydrophilic or water-soluble can easily diffuse through the nuclear envelope.

Also, the nuclear envelope ensures that genetic material can only pass through DNA to mRNA, which is synthesized in the cytosol. Some small and hydrophobic molecules may be carried into the nucleus through some channels in the nuclear envelope.

Nuclear pores perforate the nuclear envelope to facilitate transport between the nucleus and cytoplasm. These channels are embedded in integral proteins of the envelope membrane, such as importins.

The nuclear pore complexes control the flow of material through these channels. The amount of pores that are open at one time depends on the cell type and developmental stage. The nuclear pore complexes contain proteins that determine which molecules can pass through. These complexes work with other proteins that bind RNA and DNA.

The nuclear envelope is also involved in cell cycle events such as mitosis and DNA replication. Near the end of mitosis, chromosomes are attached to the nuclear envelope.

In a chromosome condensation process, the condensed chromosomes then align along with the nuclear envelope until they reach a location called the kinetochore, where they are attached. The envelope then dissolves, and the chromosomes move into two aligned sets of chromatids. This dissolution allows for chromosomes to be duplicated and passed on to daughter cells during mitosis.

The nuclear envelope is also involved in DNA replication. After DNA is made, it leaves the nucleus as an uncoiled double-stranded molecule and enters a DNA unwinding process.

After DNA unwinding, the strands reform into a helix and are pulled to opposite ends of the cell. The nucleosome is then created when the DNA strands wrap around a protein complex called histone. The nucleosomes align with the nuclear envelope until they reach kinetochores, where the chromosomes are attached. At this point, the envelope dissolves, and DNA replication occurs.

The membrane is also involved in DNA repair. It is the site of action for enzymes. Here, enzymes can patch minor damage to nuclear DNA and move damaged bases from DNA to the nuclear envelope.

During apoptosis, activated proteins cause the breakdown of the nuclear envelope and release of its contents into the cytoplasm.

Vesicles

Vesicles are other components of the endomembrane system. They are small fluid-filled sacs of membranes that allow substances to be transported inside and outside a cell. Vesicles can have one or many membranes or a space filled with fluid, depending on their functions.

There are different vesicles, such as transport vesicles, which carry substances within a cell; contractile vesicles, which cause muscle contraction; and secretory vesicles, which secrete products like hormones or enzymes.

Vesicles can further be classified into COPI-coated, Clathrin-coated, and COPII-coated. COPI and COPII-coated vesicles facilitate the transport of materials between the Golgi apparatus and other cells.

COPI-coated vesicles are coated with COPI proteins, while COPII-coated vesicles have various coat proteins depending on the cargo inside.

Clathrin-coated vesicles are coated with clathrin and are found in endocytosis. They use the coat proteins clathrin and dynamin.

Vesicle transport is the movement of material from one place to another and is usually mediated by transport proteins in the vesicles. There are several different types of transport, including axonal transport and synaptic transmission, which is the transmission of a signal from one neuron to another.

These organelles are also involved in processes such as blood clotting and cell signalling.

Plasma Membrane

Also known as the cell membrane, the plasma membrane is both a structure and the material between two organelles. It is a phospholipid bilayer embedded with proteins that act as receptors or ion channels.

This membrane contains various enzymes called transmembrane protein kinases. These enzymes catalyze different cellular processes. The cell membrane is involved in processes such as transport, phagocytosis, and cell signalling.

The plasma membrane is a selectively permeable structure that separates the inside from the outside of a cell. The transport proteins in the membrane allow for the entrance and exit of substances.

For example, certain proteins can be transported across by specific transport systems, providing a mechanism for converting nutrients into a cell and exit waste.

The plasma membrane is also involved in phagocytosis or the process by which a cell engulfs particles using its plasma membrane. The cells that phagocytize are called “phagocytes,” and they come in many forms.

For example, neutrophils are phagocytes involved in the immune system by engulfing bacteria to help overcome infection. Phagocytosis is also used to break down pathogens or harmful material that enters the body.

The plasma membrane doesn’t have a rigid structure but rather a fluid structure that can deform as needed. The plasma membrane also has a lot of surface area, making it even more important in cell signalling.

As part of the endomembrane system, the cell membrane can have transport vesicles containing receptors that can bind with extracellular material. This binding leads to a signal transduction process that sends information throughout the cell or changes the shape of the plasma membrane.

You may also be interested in Meristematic Tissues

Bottom Line

The endomembrane system is one of the most complex systems in living organisms. It consists of the majority of cell organelles, and its functions are as diverse as the components. Understanding the endomembrane system requires you to read more materials. The information provided in this blog post will guide you as you study further.

Mary Gitonga

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