Introduction to the Organelles of the Eukaryotic Cell Flashcards
How much larger are eukaryotic cells than the average Escherichia coli?
1,000-10,000 fold
Why do eukaryotes develop adaptations?
cope with this increased volume
Give examples of eukaryotic adaptations
- internal membrane profusion
- organelles
Describe the adaptivity of internal membrane profusion
- increase SA:Vol
- increase rate of metabolic reaction
- facilitating membrane specialisation.
Describe organelles
- key feature of the eukaryotic cells
- supply greater membrane functions
- half of the cell volume
the organelles of a eukaryotic cell can be bisected into the
nucleus and the cytoplasm
Within the cytoplasm there exists
- the cytosol
- the suspended cytoplasmic organelles
What is the cytosol?
The aqueous element of the cytoplasm
Mitochondria
involved in metabolism of lipids, cofactors and energy
Endoplasmic reticulum and membrane-bound polyribosomes
protein modification and lipid synthesis
Peroxisomes
oxidative metabolism
Endosomes
a series of organelles endocytosed particles pass through
Lysosomes
digestive enzymes degrade defunct organelles, endocytosed particles and macromolecules
Organelles exhibit
topological relationships
Give examples of topological relationships between organelles
- difference
- equivalence
Describe topological equivalence
allow molecules to laterally transfer between compartments without crossing a membrane
What explains the inter-organelle topological relationships?
evolutionary origins
Give the two organelles that maintain topological difference
- mitochondria
- chloroplasts
Describe the topological difference of the mitochondria and the chloroplasts
- double-membraned
- isolated from inter-organelle traffic
- endosymbiotic generation
Describe the evolution of the mitochondrial matrix
evolved from the cytosol of its free living alphaproteobacterial ancestor post-engulfment by the host archeon
Describe the energetic metabolism of mitochondria
- maximised through extensive invagination of the internal membrane system
- optimising SA:Vol for rate of metabolic reaction
Describe the cristae
- very specific functional organisation. - between the crista and the intermembrane space there exists a junction formed by MICOS and optic atrophy-1
- membrane curvature is created by the angle formed by two ATP synthase dimers, which exist at the cristae terminals
Describe MICOS and optic atrophy-1
two conserved multiprotein complices
How is returning proton leakage across the crista membrane prevented?
the respirasome exists along the side of the crista.
Describe the respirasome
diverse ETC supercomplices
Describe the insulation of cristae
dual origin, from both:
- limited diffusion provided by the narrow cristae terminal junctions
- high density of ETC supercomplices
Describe evolution of the chloroplast stroma
evolved from the cytosol of its bacterial progenitor
Describe maximisation of energetic metabolism of chloroplast
extensive invagination of the internal membrane system, optimising SA:Vol for rate of metabolic reaction.
What is the chloroplast?
a differentiated plastid from its proplastid progenitor and latterly from the etioplast
- specialised to better fulfil its function.
List some differentiated plastids
- gerontoplasts
- dessicoplasts
- chromatoplasts
- leucoplasts
- elaioplasts
- amyloplasts
Describe gerontoplasts
senescing chloroplasts
Describe dessicoplasts
found in extremophilic, dessiccation-tolerant plants
Describe chromatoplasts
carotenoid synthesis and storage plastids for fruits, flowers and roots
Describe leucoplasts
synthesis and storage plastids
Describe elaioplasts
lipid storage plastids
Describe proteinoplasts
protein storage plastids
Describe amyloplasts
starch (in the form of both amylose and amylopectin) storage plastids for the roots and tubers
What necessitates inter-organelle transport between the topologically different and equivalent organelles?
- greatly reduced genome of both topologically different organelles
- still require >1000 varying proteins to fulfill their diverse functions
Describe the human mitochondrial genome
- 13 genes for ETC subunits
- 22 for tRNAs
- 2 for rRNAs
- totalling 37 genes
Describe the chloroplast genome
- 79 protein-coding genes
- 7 for rRNA
- 28 for tRNA
- totalling 114
Describe formation of mature mitochondrial proteins
- cytosolic protein unfolded by chaperone proteins, in order to pass through the translocase pore
- signal sequence of the new protein precursor binds to the import receptors which is then transported to the receptor protein in the TOM, which facilitates membrane insertion
- protein is then translocated into the matrix from the TOM complex to the TIM23 complex
- undergoes cleavage by a signal peptide to form a mature mitochondrial protein in the matrix space
TOM
translocase of the outer membrane
TIM23
translocase of the inner membrane-23
For thylakoid import, there exists … pathways
4
Describe the sec pathway
protein translocation is achieved by Sec-homologues
Sec-homologues
bacterial proteins that facilitate protein translocation across the bacterial plasmemembrane
Describe the SRP-like pathway
uses a chloroplast-homologue of the signal-recognition particle
Describe the TAT pathway
signal peptide has two critical arginine residues
TAT
the twin arginine translocation
Describe the spontaneous insertion pathway
does not require any protein translocator
Describe the chaperone-protein unwound thylakoid precursor protein
containing a thylakoid signal sequence
TOC
Translocator of the Outer Chloroplast Membrane
TIC
Translocase of the Inner Chloroplast Membrane
Describe the initial steps of any of the 4 mature chloroplast protein generation processes
- chaperone-protein unwound thylakoid precursor protein binding to a receptor complex in the TOC complex
- passed to the TIC complex
- undergoes GTP- or ATP- dependent translocation into the stroma
- cleavage of the chloroplast signal sequence creates an exposed thylakoid signal sequence.
Why are chloroplasts and mitochondria capable of fusion and fission?
In order to dynamically control their relative abundance inside a given cell at a given time.
How can chloroplasts and mitochondria capable of fusion and fission be visualised?
experimentally by differential red-green fluorescent tagging of a photo activated, protein-labelled mitochondria
Describe mitochondrial fission
- assembly-driven constriction
- controlled by dynamin-1 dimers
- constriction achieved through the targeted interaction of dynamin assemblies with the outer membrane proteins, forming a GTP-dynamin spiral
- hydrolysis-driven constriction
- results in fission
Describe dynamin-1 dimers
form larger oligometric structures in pairing with GTP hydrolysis
interaction of dynamin assemblies with the outer membrane proteins
through speiciric adaptor proteins
hydrolysis-driven constriction
a GTP-hydrolysis event in the dynamin subunits produces conformational changes
Describe mitochondrial fusion
must occur in two stages: both the outer and inner membrane.
Describe mitochondrial outer membrane fusion
formation of a complex of outer-membrane GTPase including subunits anchored in the two fusing membranes, where GTP is low
inner membrane fusion is achieved by
formation of an oligometric tethering complex by a dynamin-related protein, including subunits anchored in the two inner membranes fusing, where GTP is high
Describe the peroxisome basics
- highly diverse
- evolutionarily mystifying
- enzyme composition varies with both conditions and cell type
Describe the peroxisome specifics
- contain oxidative enzymes to such a degree that their crystalloid protein core is visible in electron micrographs
- function is essential to both respiration and photosynthesis
Describe the oxidative enzymes of peroxisomes
use diatomic oxygen to remove hydrogen atoms from substrate molecules, generating hydrogen peroxide in the equation RH2 + O2 R + H2O2
Describe peroxisome flexibility in methylotrophic yeasts
- under sugar nutrition, peroxisome number and size is small
- under methanol and fatty acid nutrition respectively peroxisome number and size is large
Explain peroxisome flexibility in methylotrophic yeasts
- methylotrophic oxidation
- β-oxidation forming acetyl CoA
In yeast and plants, peroxisomes are the
- sole site of β-oxidation
- essential for respiration
Describe the similarities of peroxisomes and mitochondria and chloroplasts
- peroxisomes must also utilise a traslocase pore for cytosolic protein import, due to their isolation from cellular vehicular traffic
-peroxisome abundance in the cell is controlled by dynamically-mediated fission
Describe the differences of peroxisomes and mitochondria and chloroplasts
- some cytosolic vehicular import is facilitated in peroxisomes by ER vesicle fusion, forming precursor vesicles
- peroxisomes do not have a local genome
- peroxisomes are only single-membrane bound, suggesting a lack of symbiogenetic origin
Describe an evolutionary theory for the innovation of peroxisomes
- vestige of an ancient endomembrane-derived organelle that evolved to use up O2 to keep levels low in the rest of the cell, thus avoiding oxygen toxicity
- since the oxygen-consuming function of the peroxisome was duplicated by mitochondrial acquisition, this may explain the sharing of oxidative metabolism between peroxisomes and mitochondria in modern eukaryotic cells
