WEEK 4 & 5 & 6 & 7 (Cytoplasmic organelles) Flashcards
What are some examples of non-membranous organelles?
Ribosomes & Proteasomes
What are the properties of Ribosomes?
- about 20 x 30 nm in size
- assemble polypeptides from amino acids on molecules of tRNA in a sequence specified by mRNA
- two subunits of different sizes bound to a strand of mRNA
- core of small ribosomal unit is a highly folded rRNA chain associated with more than 30 proteins
- core of large subunit has three other rRNA molecules and nearly 50 other proteins
What are the functions of the rRNA molecules?
- structural support
- position transfer RNA (tRNA) molecules bearing amino acids in the correct “reading frame”
- catalyse the formation of peptide bonds
What is the function of all the other proteins in the ribosome?
Stabilise the catalytic RNA core
Describe the synthesis of Ribosomes
- Synthesised in cytoplasmic ribosomes
- Imported to the nucleus where they associate with newly synthesised rRNA
- Ribosomal subunits move from the nucleus to the cytoplasm where they are reused many times
During protein synthesis many ribosomes typically bind to the same strand of mRNA to form larger complexes called _________________ or _______________
Polyribosomes or Polysomes
In stained preparations, why are polyribosomes intensely basophilic?
Numerous phosphate groups of the RNA molecules act as polyanions
What is the smooth endoplasmic reticulum and what is its function?
Extensive interconnected membrane network lacking ribosomes
- synthesises, transports and stores lipids
- metabolises carbohydrates
- detoxifies drugs, alcohol and poisons
- forms vesicles and peroxisomes
What is the endoplasmic reticulum?
a network that extends from the surface of the nucleus throughout most of the cytoplasm and encloses a series of interconnecting channels (Cisternae)
Which cells can Rough endoplasmic reticulum (RER) be found in?
Cells specialised for protein secretion
- Pancreatic acinar cells (making digestive enzymes)
- Fibroblasts (Collagen)
- Plasma cells (Immunoglobins)
What makes the RER have basophilic staining properties?
The presence of polyribosomes on the cytosolic surface of the RER
What is the Rough endoplasmic reticulum (RER) and what is its function?
An extensive interconnected membrane network that varies in shape with ribosomes attached on the cytoplasmic surface
- modifies, transports and stores proteins produced by attached ribosomes
- proteins are secreted, become components of the plasma membrane or serve as enzymes of lysosomes
Where does protein synthesis begin?
Polyribosomes in the cytosol
Describe the movement of polypeptides through the Rough endoplasmic reticulum (RER)
- The 5’ ends of mRNAs for proteins destined to be segregated in the ER encode an N-terminal signal sequence of 15-40 amino acids
- Newly translated signal sequence is bound by the signal-recognition particle (SRP) which inhibits further polypeptide elongation
- SRP binds to SRP receptors on ER membrane then SRP releases the signal sequence allowing translation to continue with the chain transferred to a translator complex
- Inside the RER lumen, the signal is removed by signal peptidase
- With ribosome docked at the ER surface, translation continues with growing polypeptide ‘pushing itself’ while chaperones and other proteins pull the polypeptide through the translator complex
- Upon release of the ribosome, post-translational modifications and proper folding of the polypeptide continue
What happens to new proteins that cannot be folded or assembled properly by chaperones?
They undergo ER-associated deflation (ERAD) in which unsalvageable proteins are
- translocated back into the cytosol
- conjugated to ubiquitin
- degraded by proteasomes
What differentiates the Smooth endoplasmic reticulum (SER) from the Rough endoplasmic reticulum (RER)?
- less abundant
- not basophilic (since lacks ribosomes)
- best seen with the TEM
- more tubular/saclike rather than flattened cisternae
What are the three major functions of SERs?
- Enzymes in the SER perform synthesis of phospholipids and steroids
- Other SER enzymes allow detoxification of potentially harmful exogenous molecules such as alcohol and drugs (in the liver, bile)
- Sequestration and controlled release of Ca2+
The sequestration and release of Ca2+ is well developed in muscle cells, where the SER has an important role in the contraction process and has a specialised form called the ____________________________
Sarcoplasmic reticulum
What is the Golgi apparatus and what is its function?
Series of several elongated, flattened saclike membranous structures
Modifies, packages, and sorts materials that arrive from the ER in transport vesicles; forms secretory vesicles and lysosomes
Where are the small Golgi complexes located in most cells?
near the nucleus
Describe the movement of materials through the Golgi apparatus
- Material moves from the RER cisternae to the Golgi apparatus in small, membrane-enclosed carriers (TRANSPORT VESICLES) that are transported along cytoskeletal polymers by motor proteins
- Transport vesicles merge with the Golgi-receiving region (cis-face)
- On the opposite ‘shipping’ region (trans-face), larger saccules or vacuoles accumulate, condense and generate other vesicles that carry completed protein products to organelles away from the Golgi
What is the function of various coat proteins including Clathrin?
- Form transport vesicles and secretory vesicles
- Regulate vesicular traffic to, through and beyond the Golgi apparatus
Forward movement of vesicles in the cis Golgi network of saccules is promoted by which protein?
coat protein COP-II
retrograde is COP-I
What are Secretory granules?
Granules surrounded by a membrane and contain a concentrated form of the secretory product
They are found in cells that store a product until the release by exocytosis is signaled by a metabolic, hormonal or neuronal message
What are Zymogen granules?
Granules with dense contents of digestive enzymes
What are lysosomes and what is its function?
Spherical-shaped membrane-bound organelles formed from the Golgi apparatus; contain digestive enzymes
Digest microbes or materials (e.g ingested by the cell, worn-out cellular components or the entire cell)
What is the size of the lysosome and what is it seen with?
0.05 to 0.5 micrometers
Seen with TEMs but is larger in macrophages and neutrophils so can be seen with a light microscope
How are cellular components protected from the lysosomes?
- membrane surrounding lysosomes
- enzymes have an optimal activity at an acidic pH (5.0) so practically harmless at cytosol pH (7.2)
Describe what happens to non-functional oligosaccharides brought to lysosomes
- The marker mannose-6-phosphate (M6P) is added by a phosphotransferase in the cis Golgi only to N-linked oligosaccharides destined for lysosomes
- Membrane receptors for M6P containing proteins in the trans Golgi network bind these proteins and divert them from the secretory pathway to lysosomes
- Material moves into cell by endocytosis and endocytosed material with lysosomal enzymes activates proton pumps that acidify the contents allowing digestion
What is the name for an active lysosome?
Heterolysosome
Heterolysosomes appear larger in the TEM due to material they are digesting
Indigestible material is retained within a small vacuolar remnant called a ___________________
Residual body
What is Autophagy?
Excess organelles or large aggregates of nonfunctional macromolecules in cytoplasm are degraded
What happens during Autophagy?
- A lipid bilayer membrane forms around and isolates the cytoplasmic portion forming an AUTOPHAGOSOME
- autophagosome fuses with a lysosome for digestion of enclosed material
- amino acids, nucleotides & other digestion products are released for reuse
What are Proteasomes and what is its function?
Large, barrel-shaped protein complexes located in both the cytosol and nucleus
Degrade and digest damaged or unneeded proteins; ensure quality of exported proteins
What is the difference between lysosomes and proteasomes?
Lysosomes digest organelles or membranes by autophagy whereas proteasomes deal primarily with free proteins as individual molecules
Describe the structure of a proteasome
- A cylindrical structure made of four stacked rings, each composed of seven proteins including proteases
- Each cylinder has a regulatory particle that contains ATPase and recognises proteins with attached molecules of Ubiquitin
Describe the stages of protein degradation in proteasomes
- Misfolded or denatured proteins are recognised by chaperones and targeted for destruction by enzyme complexes that conjugate UBIQUITIN to LYSINE residues of the protein followed by formation of a POLYUBIQUITIN CHAIN
- Ubiquinated proteins are recognised by regulatory particles of proteasomes, unfolded by ATPase using ATP and translocated into the cylindrical structure and degraded by endopeptidases
- Ubiquitin molecules are released for reuse and peptides produced can be broken down into amino acids or have other specialised destinations
What is the mitochondria and what is its function?
Double membrane-bound organelles containing a circular strand of DNA
Synthesise most ATP during aerobic cellular respiration by digestion of fuel molecules in the presence of oxygen
What is Glycolysis?
Converts glucose anaerobically to pyruvate which releases some energy.
More energy is captured when pyruvate is imported into mitochondria and oxidised to CO2 and H2O but not a lot of ATP is produced
Where does energy released in mitochondria go when not stored in ATP?
It is dissipated as heat that maintains body temperature
What are the properties of mitochondria?
- mitochondrial enzymes yield 15 times more ATP than is produced in glycolysis
- diameters of 0.5-1 micrometer
- rapidly changes shape and fuse with one another and dividing
- move through cytoplasm along microtubules
- number of mitochondria is related to the cell’s energy needs
What distinguishes mitochondrial membranes than other membranes?
- Higher density of protein molecules
- Reduced fluidity
Describe the structure of the mitochondria
- Two membranes with an inter membrane space and an innermost matrix
- Outer membrane contains PORINS (transmembrane proteins)
- Inner membrane has many long folds (CRISTAE) which increase surface area
What are the two ways that Peroxisomes are formed?
- Budding of precursor vesicles from the ER
- growth and division of preexisting peroxisomes
What are Peroxisomes and what is its function?
Smaller, spherical-shaped membrane-bound organelles formed from the ER or through fission; contain oxidative enzymes
Detoxify specific harmful substances either produced by the cell or taken into the cell; engage in beta oxidation of fatty acids to acetyl CoA
Which Peroxidase breaks down H2O2?
Catalase
What are microtubules and what is its function?
Hollow cylinders composed of tubulin protein
- Maintain cell shape and rigidity
- organise and move organelles
- support cilia and flagella
- participate in vesicular transport
- separate chromosomes during the process of cell division
Microtubules are also organised into larger, more stable arrays called _______________
Axonemes
What is the configuration of Microtubules?
- Two or more microtubules are linked side-by-side by protein arms or bridges
- protein subunit is a heterodimer of alpha and beta tubulin
- Heterodimers polymerise to form the microtubules with slight spiral organisation
- Tubulin subunits align lengthwise as PROTOFILAMENTS (13 parallel protofilaments form the circumference of each microtubule wall)
What is the polymerisation of tubules directed by?
Microtubule organising centres (MTOCs)
Microtubules are polarised structures with growth occurring more rapidly at the ________ end
Positive
Microtubules show dynamic instability with continuous cycles of polymerisation and depolymerisation which depend on what conditions?
- concentrations of tubulin
- Ca2+ and Mg2+
- various Microtubules associated proteins (MAPs)
Where is the energy needed for microtubule assembly derived from?
GTP bound to incoming tubulin subunits
The dominant Microtubule organising centres (MTOCs) in most cells is the ____________ which is organised around two cylindrical centrioles
What are Microfilaments and what is it’s function?
Actin protein monomers organised into two thin, intertwined protein filaments (actin filaments)
- Maintain cell shape
- Support microvilli
- Seperate two cells during cytokinesis
- Facilitate change in cell shape
- Participate in muscle contraction
What is the structure of Microfilaments?
- Thin
- Polarised polymers
- Shorter and more flexible than microtubules
- composed of globular G-actin monomers that assemble in the presence of K+ and Mg2+ into a double-stranded helix of filamentous F-actin
New proteins can be formed from from a pool of G-actin by the action of what proteins?
Nucleating
_______ binds to the side of pre-existing actin filaments and induces a new F-actin branch, a process which can lead to the formation of a Microfilament network
Arp2/3
Describe the assembly and disassembly of subunits from F-actin
Monomers are added rapidly at the (+) end with ATP hydrolysis at each addition; at the same time monomers dissociate at the (-) end.
This leads to a migration of subunits through the polymer in a process called TREADMILLING
What do Actin-binding proteins do to microfilaments?
Change dynamic physical properties such as length, interactions with other structures and viscosity of the local cytoplasm
What factors interacting with F actin influence cytoplasmic viscosity?
Cross-linking within networks of F-actin INCREASES cytoplasmic viscosity and severing the filaments DECREASES viscosity
________________ use ATP to transport cargo along F-actin
Myosin motors
Interactions between F-actin and myosin’s form the basis of which cell movements?
- transport of organelles, vesicles, and granules in the process of cytoplasmic streaming
- contractile rings of microfilaments with myosin II constricting to produce two cells by cytokinesis during mitosis
- membrane-associated molecules for myosin I whose movements along microfilaments produce the cell surface changes during endocytosis
What are Intermediate filaments and what is it’s function?
Various protein components
- Provide structural support
- Stabilise junctions between cells
What differentiates Intermediate filaments from microtubules and actin filaments?
- stable
- increased mechanical stability
- made up of different protein subunits in different cell types
Intermediate filaments can be localised by ___________________ in various cells
Immunohistochemistry
What are some examples of Intermediate filaments?
- Keratins
- Vimentin
- Neurofilament
- Lamins
What are inclusions and what is it’s function?
Inclusions contain accumulated metabolites of other substances but have no metabolic activity themselves
Function: Serve as temporary storage for these molecules
What is the different between secretory granules and lysosomes?
Secretory granule degenerate components outside of the cell whereas lysosomes degenerate specific cells
What is distinguishable about the mitochondria replication compared to other cells?
Mitochondria duplicate using its own mechanisms instead of using other organelles within the cell
What differentiates mitochondrial DNA from other DNA?
Mitochondrial DNA is circular and both strands are transcripted during transcription (in normal cells only one cell is transcripted)
