Sub Cellular Organelles Flashcards
Ribosome structure
Macromolecular structure 20x30 nm in size
Two subunits bound to strand of mRNA- core of the ribosomal subunit is a highly folded rRNA chain associated with more than 30 unique proteins.
Large subunit formed of 3 chains of rRNA and 50 other basic proteins.
Ribosome functions
Assemble polypeptides from amino acids using tRNA and mRNA. Provide structural support and position tRNAs in the correct reading frame.
Peripheral proteins seem to stabilise the catalytic core.
What is a polyribosome + function?
Many ribosomes bind to mRNA, increase the speed of transcription.
Polysomes bind to the endoplasmic reticulum translate mRNAs coding for membrane proteins of the ER.
Rough Endoplasmic reticulum structure
Network of membrane-limited channels called cistern that extends from the nucleus throughout most of the cytoplasm.
Covered in polyribosomes that synthesise proteins.
parallel stacks of flattened cisternae
Rough ER location
prominent in cells specialised for protein secretion. Pancreatic acing cells, fibroblasts and plasma cells.
Rough ER function
production of membrane associated proteins. Primary glycosylation and other postranslational modifications.
ER-associated degradation of proteins that cannot be assembled or folded using chaperones- unsalvageable proteins translocated back to cytosol, conjugated to ubiquitin and then degraded by proteasome.
Smooth endoplasmic reticulum structure
cistern are more tubular and sac like. Found continuous with the RER, no polyribosomes present. Interconnect channels
Smooth endoplasmic reticulum functions (3)
enzymes in SR perform synthesis of phospholipids and steroids
other SER enzymes, including those of the cytochrome P450 family, allow detoxification of potentially harmful exogenous materials such as alcohol
SER vesicles responsible for sequestration and controlled release of Ca2+- assumes a specialised form called sarcoplasmic reticulum
Golgi apparatus structure
Multi-layered membranous organelle near the nucleus. Consists of many smooth membranous saccules, some vesicular and others flattened. All contain enzymes and proteins being processed.
Contain different levels- cis, medial and trans
Golgi apparatus function
completes postranslation modifications that began in the endoplasmic reticulum. Packages and addresses the proteins to their proper destinations.
Different directions of vesicle transport
Vesicles moving towards the Golgi fuse with the cis face, vesicles leaving the Golgi come from the trans face
Lysosome structure
Membrane limited vesicles that contain about 40 different hydrolytic enzymes.
Lysosome function
Sites of intracellular digestion.
During digestion of macromolecules, released nutrients diffuse into the lysosome. Indigestible material is retained in a small vesicular body called a residual body.
Autophagy- degrade excess organelles
Endosome structure
membrane bound vesicle that originates at the trans Golgi surface- contains molecules on the way to lysosomal degradation or recycle back to golgi apparatus. Formed of multivesicular bodies, fuse with lysosomes.
endoscope function
provide an environment for material to be sorted before it reaches degradative lysosomes.
peroxisome structure
spherical organelles enclosed by a single membrane . Contain enzymes producing and degrading hydrogen peroxide.
Peroxisome function
contain oxidases that remove hydrogens and add to oxygen forming hydrogen peroxide
Contain catalases that break down hydrogen peroxide.
Mitochondria structure
two membranes- outer membrane and inner folded membrane called a cristae. Outer membrane contains many transmembrane proteins called porins that form channels that allow small molecules, such as pyruvate to move from the cytoplasm to the inter membrane space.
Matrix containing enzymes specialised for aerobic respiration
Mitochondria function
aerobic respiration producing ATP
Role in apoptosis- releases cytochrome C from intermembrane space which initiates apoptosis.
What is ubiquitin + function?
Small regulatory protein that tags proteins for breakdown
Cytoskeleton definition
Complex arrangement of microtubules, microfilaments and intermediate filaments that are present in the cytoplasm.
Microtubule structure
- 25nm in diameter
- long hollow cylinders made of alpha and beta tubulin
- rigid and straight
- contain microtubule organising centre (MTOC)
Microtubule functions
Stabilise cell shape due to rigid structure
form the tracks for kinesin based transport- away from cell nucleus and dynein transport towards the nucleus
How do microtubules grow?
Two ends, one plus and one minus end.
Growth is faster at the plus end. Energy for growth is derived from GTP bound to the incoming tubule subunits.
Explain dynamic instability
Continuous cycles of polymerisation and depolymerisation. Microtubules decrease in length if depolymerisation exceeds growth.
What are MAPs + function?
Microtubule association proteins that bind to and stabilise microtubules to enhance stability. Bind more strongly in adults, allowing neuronal stability- more loosely in embryonic development.
What are MTOCs and what occurs?
Microtubule Organisation Centres. Minus end embedded in the in the MTOC- plus end sticks out and grows- example of a centriole and the microtubules are the spindle fibres.
Microfilament structure + location
Composed of flexible G actin that assembles in the presence of potassium and magnesium ions into a double stranded helix of F actin.
Concentrated beneath plasma membrane
plus end- fast growth and shrinkage
minus end- slow growth and shrinkage
Microfilament functions
allow cell movement, allowing neutrophils to hunt bacteria- tread milling due to growing actin
protruding actin filament causes crawling cell, to engulf another cell
involved in muscle movement
myosin motors use F actin to transport cargo, usually towards + end
Intermediate filament structure + function
10-12 nm in diameter, stable.
subunits often coiled, rodlike dimers that form antiparallel tetramers which self assemble into cable like bundles
increase mechanical stability in cell structure
Examples of intermediate filaments, their location and function
Keratin- form large bundles called tonofibrils which attach to different cell junctions. Also present in stratum corneum preventing dehydration of the skin. keratin networks held by dilsulphide bridges
neurofilament- main intermediate filament in neurons, maintain axon shape. Form crosslinks to provide axon with tensile strength. Level of neurofilament expression controls the diameter of the axon, thus conductance velocity.
lamin- present in cell nucleus, form structural network called the nuclear lamina- provide anchoring point for chromosomes.
metabolic functions of peroxisomes
breakdown very long chain fatty acids through beta oxidation, before shuttling to mitochondria
2 main processes in peroxisome
enzymes that oxidise compounds, forming hydrogen peroxide
hydrogen peroxide is then converted by a catalase into water and oxygen, to prevent the formation of free radicals
