L17 Flashcards
Lysosomes
Only present in animal cells, contain digestive enzymes, degrade obsolete or worn out components of cell, destroy invading viruses and bacteria, involved in programmed cell death.
How are objects transported to the lysosomes?
Endocytosis, phagocytosis, autophagy
Endocytosis (lysosomes)
Invagination of the cell membrane forms a vesicle that buds off and transports objects through the cell to the lysosomes
Phagocytosis (lysosomes)
A form of Endocytosis, whole cells and other large insoluble particles move from the cell surface to the lysosomes.
Autophagy (lysosomes)
Worn-out organelles and bulk cytoplasm are surrounded by a membrane and delivered to the lysosomes
What happens in the lysosomes?
The lumen of the lysosomes is acidic and contains hydrolytic enzymes that degrade polymers into their monomeric subunits
What enzymes are in the lysosomes?
Nucleases degrade RNA and DNA into mononucleotides, proteases degrade proteins and peptides, phosphatases remove phosphate groups from mononucleotides, phospholipids, and other compounds. Other hydrolases degrade complex polysaccharides and glycolipids into smaller units
What are the types of lysosomes?
Primary and secondary
What are primary lysosomes?
Roughly spherical, do not contain obvious particulate or membrane debris. Formed by the fusion of Golgi vesicles with late endosomes.
What are secondary lysosomes?
Larger and irregular shaped, result from fusion of primary lysosomes with other membrane-bound organelles and vesicles
Further types of lysosome
Residual bodies - where indigestible materials pass outwardly and fuse with the plasma membrane. Auto-phagic vacuoles - a fusion of a primary lysosomes with a autophagosome
Vacuoles
Mostly in plants and fungi, most plant cells contain at least one membrane-limited internal vacuole.
Types of vacuole?
Storage and lytic
Storage Vacuoles
Vacoular membrane contains a variety of transport proteins that allow plant cells to accumulate and store water, ions, and nutrients
Lytic vacuoles
The lumen of a vacuole has a acidic pH and contains degradation enzyme
How does a vacuole get so big and keep its shape?
Vacuolar membrane is permeable to water, but poorly permeable to the small molecules within it. Solute conc is greater in vacuole lumen than in the cytosol or extracellular fluids. Water moves by osmotic flow into vacuoles, influx of water creates turgor inside the cell. Pressure is balanced by mechanical resistance of cellulose-containing cell walls surrounding the cell.
Peroxisomes
All animals cells (bar erthryocytes) and many plant cells contain peroxisomes. Play a key role in lipid metabolism and control of reactive oxygen species. In liver and kidney cells, degrades various toxic molecules that enter the bloodstream producing harmless products. The oxidise organic substances like fatty acids and amino acids.
Oxidation of fatty acids in peroxisomes
Yields no ATP, acetyl CoA generated during degradation of fatty acids cannot be oxidised further, it is transported into the cytosol for use in the synthesis of cholesterol and other metabolites. In the process of oxidation, hydrogen peroxide is formed. Hydrogen peroxide is removed by catalase in the peroxisomes.
Endoplasmic reticulum
Encoles by the largest membrane in the eukaryotic cells, contains an extensive network of closed, flattened membrane-bounded sacs called cisternas. Important for synthesis of lipids, membrane proteins, secreted proteins.
Two region of ER
Smooth - lacks ribosomes
Rough - cytosolic face is studded with ribosomes
They differ structurally and functionally but remain a continuous system.
Smooth ER
Synthesis of lipids, synthesis of fatty acids and phospholipids, steroid hormones, detoxification of harmful metabolic byproducts, storage and metabolism of calcium ions within the cell.
Where is smooth ER abundant?
Amount of smooth ER is usually small, however it is abundant in hepatocytes. Enzymes in the smooth ER of the liver modify or detoxify hydrophobic chemicals by chemically converting them into water-soluble products that can be excreted from the body. High doses of such chemicals results in large proliferation of the smooth ER in liver cells.
Rough ER
Ribosomes bound to the rough ER synthesise certain membrane and organelle proteins and also secreted proteins. Proteins to be secreted accumulate in the lumen of the ER. Proteins are transported from ER to their destination in membrane-bound vesicles via microtubules
Secretion of proteins from the ER to their destination
Membranes and secreted proteins have a signal sequences, signal sequences directs ribosomes to the ER membrane. Ribosomes dock on the translocon. Polypeptide synthesis starts, signal peptide is cleaved. Completed polypeptide leave the ribosome and folds into final conformation
Golgi complex
Main sorting hub of the secretory pathway. Receives proteins and membrane lipids first synthesised in the ER, complete the processing, package and transport to appropriate destinations.
Transport through the Golgi complex
Golgi complex consist of series of flattened membrane vesicles surrounded by a number of spherical vesicles. The stack of Golgi cisternae has three defined regions : the cis, the medial, the trans. Transport vesicles from the rough ER fuse with the cis membranes of the Golgi complex. Proteins in the vesicles move from the cis region to the medial region of the Golgi complex. Vesicles bud off the trans-golgi membranes, vesicles containing proteins move to the cell surface and others move to lysosomes.
What are the major protein-sorting pathways in eukaryotic cells
ER, Golgi complex, vesicles. At each site proteins are sorted into separate vesicles, depending on sorting signals. The proteins sorted in the ER have signals that directs the sorting system.
Secretory pathways
All nuclear-encoded mRNAs are translated on cytosolic ribosomes, ribosomes synthesising secreted nascent proteins are directed to the rough endoplasmic reticulum by an ER signal sequence. Translation is completed on the ER. Proteins can move via transport vesicles to the Golgi complex. Further sorting delivers proteins either to the plasma membrane or to lysosomes.
The secretory cell
Secreted proteins after synthesis on the ribosomes of the rough ER are moved to the lumen of the rough ER. Transport vesicles bud off and carry these proteins to the Golgi complex, proteins are concentrated and packaged into immature secretory vesicles. These vesicles then coalesce to form larger mature secretory vesicles. Vesicles accumulate under apical surface of the cell, fuse with plasma membrane releasing their contents. The release is a response to appropriate hormonal or nerve stimulation.
Non-secretory pathways
Synthesis of proteins lacking an ER signal sequence is completed on free ribosomes, proteins with no targeting sequence released into the cytosol and remain there. Proteins with an organelle-specific targeting sequence first released into the cytosol then imported into mitochondria, chloroplasts, peroxisomes or the nucleus. Majority of proteins located in mitochondria and chloroplasts are encoded by genes in the nucleus and imported into the organelles after their synthesis in the cytosol. But proteins encoded by mitochondrial or chloroplast DNA are synthesised on ribosomes within these organelles then directed to the correct sub-compartment.
