Golgi Complex: Structure and Function

Golgi Complex

• In secretory cells, the Golgi complex is well-developed and dispersed throughout the cell.
• Examples of cells with prominent Golgi complexes include mucus-secreting cells in the intestine (producing mucopolysaccharides) and digestive enzyme-secreting cells in the pancreas.
• The number of Golgi complexes varies depending on the cell function.
• Animal cells typically contain 10 to 20 Golgi complexes per cell.
• The endoplasmic reticulum (ER) sends proteins and lipids to the Golgi apparatus, where they are modified.
• The Golgi apparatus comprises flattened, membranous sacs called cisternae, resembling a stack of pita bread.
• It has two distinct poles or faces: the cis-face (entry face) and the trans-face (exit face).
• The cis-face is near the ER, and proteins from the ER enter the Golgi at this face for processing.
• Proteins exit the Golgi from the concave trans-face.
• After leaving the ER, many transport vesicles travel to the Golgi apparatus.
• The Golgi is a warehouse for molecules that are received, sorted, shipped, and processed further.
• Proteins are examples of molecules processed in the Golgi.
• The Golgi sorts and packages molecules into specific transport vesicles for destinations like the plasma membrane, lysosomes, endosomes, or secretory granules.
• Cis and trans faces are connected to specialized regions called the cis Golgi network (CGN) and trans Golgi network (TGN)
  • Together these structures form a complex with 5 functional regions.
• The Golgi complex possesses 5 functional regions: the cis-Golgi network, cis-Golgi, medial-Golgi, trans-Golgi, and trans-Golgi network.
• Each region contains different enzymes that modify proteins.
• Enzymes catalyzing early processing steps are located in the cis-face cisternae.
• Enzymes catalyzing later processing steps are located in the cisternae toward the trans-face.

Functions of the Golgi Complex

• Glycosylation, or the addition of carbohydrates to proteins, aids in protein folding and the transport of misfolded proteins to the cytosol for degradation.
• Glycosylation contributes to controlling the quality of proteins exiting the ER and protects against protease digestion, creating a glycocalyx
• Proteins gain different functions through modification of oligosaccharides in the Golgi.
• The Golgi complex is involved in the modification of carbohydrates.
• It is responsible for proteolytic processing of protein precursors like glycolipid and Sphingomyelin biosynthesis.
• The Golgi complex aids in the sorting of proteins.
• Golgi apparatus functions in lipid metabolism synthesis of glycolipids and Sphingomyelin occurs in Golgi, whereas, ceramide is synthesized in the ER.
• Sphingomyelin and glycolipids are then synthesized from ceramide in the Golgi

Sulfation and roles in protein-protein interaction

• Sulfation involves the addition of sulfo groups to some proteins.
• Sulfation is a biotransformation process involving a sulfotransferase catalyzing the transfer of a sulfo group from a donor co-substrate.
• Sulfation occurs on proteoglycans, such as mucopolysaccharides (secretory products).
• Sulfation plays a role in strengthening protein-protein interactions.
• Human Proteins undergoing tyrosine sulfation include
  • adhesion molecules
  • G-protein-coupled receptors
  • coagulation factors
  • extracellular matrix proteins and hormones
• Sulfation is important for animal growth, impacting body weight and postnatal viability.
• The Golgi apparatus is involved in the modifying, sorting, and packaging macromolecules for cell secretion (exocytosis) or for use within the cell.
• The Golgi primarily modifies proteins from the rough endoplasmic reticulum.
• Enzymes within the cisternae modify substances through the addition of carbohydrates (glycosylation) and phosphates (phosphorylation).
• It's involved in the transport of lipids around the cell, including phospholipids of the plasma membrane.
• Also responsible for the creation of lysosomes,
• It synthesises proteoglycans found in extracellular matrix of animals

Lysosomes

• Lysosomes are key sites for intracellular digestion.
• Lysosomes are membrane-enclosed organelles filled with hydrolytic enzymes that control the digestion of macromolecules.
• Lysosomes contain about 50 different hydrolytic enzymes, including proteases, nucleases, glycosidases, lipases, phosphatases, and sulphatases known as Lysosomal Acid Hydrolases-.
• They are active at acidic pH (4.5-5) but not at neutral pH (7.2) of the cytoplasm.
• Lysosomal enzymes aid in
  • Nucleic acid degradation (RNases, DNases)
  • Protein degradation (Catepsin, Collagenases)
  • Carbohydrate degradation (a & b Galactosidases, Lysosyme)
  • Mucopolysaccharide degradation, (a glucuronidase, Hyaluronidases),
  • Phosphatases (Acid Phosphatases, Acid phosphodiesterases),
  • and Sulphatases (Aryl Sulphatases).
• Hydrolytic enzymes and lysosomal membrane are made by the rough ER, then moved to Golgi Complex
• The Golgi Complex processes them for future use
• It is found the lysosomes use the trans face of the Golgi
• Lysosomes are active at acidic pH and need H+ ions to maintain that environment.
• These organelles use the energy of (ATP) hydrolysis to pump H+ ions into lysosomes.
• Acidic pH provides protection against the uncontrolled digestion of cells
• The membrane of lysosomes keeps the digestive enzymes out of the cytosol.
• If the lysosomal membrane breaks down and enzymes leak out, they would be inactive at the cytosolic pH of 7.2 (neutral pH).
• Lysosomes are dense, rounded bodies, 0.5 - 1 μm in diameter but their size/shape varies depending on the material inside.
• They are not found in plant cells but in all animal cells except for erythrocytes
• Plant cells contain vacuoles that are related to lysosomes and contain hydrolytic enzymes
• Typically, 300 Lysosomes are found in each cell and are abundant in macrophages/leucocytes which specialize in phagocytosis.
• Vesicles containing lysosomal enzymes are called primary lysosomes.
• Primary lysosomes fuse with vesicles that contain material to be digested; the end result is late endosomes and form secondary lysosomes
• Materials are delivered to lysosome via 3 pathways
  • Macromolecules from extracellular fluid enter lysosome via endocytosis
  • Materials enter lysosome via phagocytosis which defends against invading bacteria
  • Autophagy digests cell's worn out components

Cellular functions of Lysosomes include

• Endocytosis where extracellular material is engulfed & degraded from outside of the cell.
  • endocytosis includes pinocytosis (cell drinking)
  • and phagocytosis (cell eating) where pathogens & nutrients become engulfed.
  • Removal of old/damaged cells where they must be degraded via endocytosis to prevent harm.

Phagocytosis & cellular digestion

• Phagocytosis and cellular digestion occurs with Macrophages & Neutrophiles
• Foreign material (Microorganisms) are engulfed by cell, forming phagosome
• Phagosomes fuse with primary lysosome where, a phagolysosome is formed
• Any unneeded leftovers of digestion are released via exocytosis

Autophagy

• Autophagy degrades cell's own components + turnover of cell's own components to maintain cell strength
• Autophagy-degrades secretory granules
• Crinophagy- a type of autophagy

Autophagy steps

• A crescent membrane which will engulf the organelle (digested) creates
• An autophagosomal membrane forms+ double- membrane creates bounded compartment
• A new compartment combines with lysosome
• Degradation of the inner layer membrane + contents of the autophagosome

Peroxisome

• They have small vesicle-like organelles which are similar to lysosomes, and have single membrane
• Peroxisome proteins are created on free ribosomes + pass through to peroxisomes
• Note- euk cells have peroxisomes ex erythrocytes and 500 peroxisomes/ cell
• Peroxisomes have oxidative enzymes like Catalase (Converts H2O2 to H₂O) or Oxydases (e.g. Urate oxydase) and clear toxins esp alcohol

Mitochondria

• Largest of eukaryotic cells and occupy nearly 25% of cytoplasm
• Nearly same size as bacteria; considered evolution precursor to mitochondria
• Primary ATP sythesis site for energy by Mitochondria are found in all eukaryotic cells as energy converting organel
  • They are dynamic organelles and creates ATP during cell resp+ ox phosphorylation
  • living mitochondria- view vis phase-contrast microscope - vital dice
  • stain with flourescein dies and are generally 3 shapes(oval/sphere/ line) There are about 1.5-2.5 K liver cells and 1k single cell organisms known as s Cerevisia+ Chlorella fusca(moss) which contain one mitochondria,
  • Mitochondria are common where metabolic activity is high like kidney/muscle.
• These cells developed symbiotic relationship by living with larger cells and developing into mitochondria
• This is called as endosymbiosis which began 2B years ago

Cell Nucleus

• Found in all euk cells; DNA replication, trans/rna processing -Occurs here+ translated by cytoplasm
  • Nuclear envelope -1-2 lipid membraned covering+nuclear matrix (fibrous supporting struct) encloses it +chromatin and histone nonhistone proteins are found in it, and also nucleolus
  • Chromatin + 2 layers: inner and outer bound and also the cell's cytoplasm
  • Ribosomes connect to outer layer
• Nuclear pores pass cytoplasm/nucleus - mechanical reinforcement: laminins
• Lamina: reinforcement- imp in mitosis & DNA replication, +selective permeability

Nuclear envelope's functionality

• Molecules can pass thru w/ selectivity
• Biochemical differences maintained in the cell, transport + nuclear pores traverse/contact nucleo/cyto
• In nucleolus- RNA+ Ribosome proteins make non-coding DNA
• A cell may contain several, this can only be noticed when using intense proliferation activity/ cells creating protein+ malignant tumours during ribosome assembly/biogenesis
  • Nucleolus- produces what is left after ribonucleic extraction (fibres+ proteins, with segments which connect DNA) -Nuclear Matrix: A fibrous web is left and it comprises of proteins (connect DNA parts)