Cell organelle and the cytoskeleton- Cell Biology
Why are cell membrane important
In order to difine the boundary of the cell and seperate the cytosol from the extracellular environment
Generation of electrical signals and ionic gradient for the movement of substances over the cell membrane
What does the cell membrane do
Delimit the semi autonomous fucntional unit
Control movement in and out the cell
Protection from external environment
Provide attachement sites
Function is cell Signalling
Why is the cell membrane always in motion
In order to allow the cell to take in food and excrete waste, to communicate with other cells, gather infomation about the environment, Repair damage
What is the Fluid Mosaic Model
Allows the membrane to develop fluidity and movement
It is mainly made up phospholipids that have a polar hydrophillic head that point outwards and then hydrophobic tails that point inwards in order to then create a pospholipid bilayer
There are also cholesterol and Proteins randomly located in the membrane as well as lipids
What is cholesterol needed for in the cell membrane
To regulate the rigidity of the cell membrane and to regulate communication between cells by the release and capturing of chemicals and proteins
How does cholesterol capture and release chemicals
It is able to capture chemicals by endocytosis where sections of the membrane engulf the chemicals and transport them to cells as vesicles
The Polar hydroxyl head of the cholesterol wil instert close to the polar head of the phospolipid
Only get cholesterol in animal cells
How does cholesterol regulate fluidity
At low temperature there is less Kinetic energy so then less movement in the phospholipids so they pack closer togetehr and form a crystaline state
At higher temperatures there is more Kinetic energy so then more phospholipid movement so they are packed less closely so then greater fluidity
Act as a buffer for fluidity at a changing temperature
Cis double bonds make it harder to pack together so hydrocarbons are spread out so a thinner bilayer if greater unsaturation
What do the proteins do in the cell membrane
Regulate what can enter and leave the cell
Non polar molecules can easily diffuse across the cell membrane by diffusion or active transport
Polar or charged molecules need transmembrane porteins in order to allow them to cross the membrane via a carrier or channel protein
What is the phospolipid structure in the cell membrane
In the hydrocarbon tails one of them is usually unsaturated so has a C=C bond to create a mink in the tail
The chain length and level of unsaturation will determine how closely the proteins canm pack together
What do the lipids in the cell membrane do
The lipids are self organsing in the membrane and it is energetically favourable to create a sealed environment so a bilayer is formed by them
Can also get glycolipids in the membrane
Can get formation of micelles when limited numbr of phospolipids have the heads exposed on the outside of the cell and tails are protected on the inside of the micelle in a circle formation
How do the phospolipids move in the cell membrane
Flip Flopping - There is movement of 2 phospholipids across the plane of the bilayer from one side to another
Flexion - When the phospholipid is able tom rotate by the movement of the hydrocarbon tail
Lateral diffusion- The swapping of places by 2 phospolipids that are next to each other
What are Lipid rafts
Not all domains of the bilayer have the same mobility level
Discrete membranes domain enriched in cholesterol and sphingolipids form rafts that can move laterally
Lipid rafts are associated with certain membrane proteins and are used for
Cell signalling and Uptake of extracellular molecules
What are the endomembrane organelles
These are organelle in the cell that have a membrane round them as well so are bilayered
Nuclear envelope, Golgi, ER, Vacuoles, Cell membranes, Vesicles and Lysosozomes
What is the nuclear envelope
It has a lipd bilayer and is contagious with the ER
Its role is too regulate movement in and out of the nucleus
It will break dowm during Prophase of mitosis
Has nuclear pores so is not a closed membrane
What is the nuclear pore
It is a complex of proteins that regukate movement in and out of the cell
It is highly selective of what can enter and leave the nucleus
What is the nuclear pore complex - NPC
Nuclear pore contain a NPC that is made up of 30NPC proteins to form octagonal symmetry
In each cell there are 3000-4000 NPC with each being able to move 500 macromolecules per second bilaterally
It can have bilaterlal directional as there are the movement of different things in different dircetion in the same pore
What will enter and leave the nucleus
DNA + RNA building blocks
Ribosomal proteins
Molecules used to provide energy
Ribsomal sub-units will move out the nucleus to form a working ribosome
What are the 2 signals used in the NPC
Nuclear localising signal - An amino acid that help to tag the a protein for entry into the nucleus
Nuclear export signal - An amino acid that tag a protein that is destined for leaving the nucleus
Why is the NPC important
Allow smaller molecules to move in and out of the cell
Allow the nucleus and cytoplasm to maintain there population of proteins in each area
Create the correct ribsome needed for protein synthesis
What is the endoplasmic reticulum ER
The rough ER have a ribosome coat so they are able to synthesize proteins but the smooth ER doesnt have this so can only synthesize lipids
The Rough ER used for transport and packaging of proteins to leave the cell
The smooth ER used for the synthesis of lipids as well as packingf them and transporting them out of the cell
What is co-translational protein import into the ER
The ribosome are able to bind to the rough ER and then the proteins are transported to the ER as they are being translated in the ribosome
What is postv translational protein import to the ER
The whole protein is translated before them being folded and being inserted to the rough ER membrane
How can a protein be imported to the ER
Transmembrane proteins are only able to partially translocate across the membrane to the ER
Water soluble proteins are able to fully translocate to the ER where they are then secreted or have a role in the ER lumen
What is cotransaltional transloaction steps into the ER
Require a signalling sequence from the ER and can only happen once ribsomke has bound to the tranlocator
An N-terminal signal peptide initastes the passage of the protein through the translocator
The single peptide is cleaved once the mature protein is fully synthesized
The mature protein now in the ER and then the ribsome unbind from the translocator
What is protein glycosylation
The addition of sugars to a protein in order to give certain proteins there function
This is done by precurose oligosaccharide tranferred from doliuchol lipid anchor that is a catalysed by oligosaccharide transferase enzyme
The trimming of the olgigosaccharide happens in the Golgi
What is the Golgi apparatus
Thye main site of carbohtydrate synthesis
Involved in the glycosylation of proteins and also in the sorting and dispatching of the proteins
There is an interconnected flattened cistae that is connected by a cis face on 1 side and a trans face on the other side of the Golgi
How do vesiccles move from the ER to the Golgi
Vesicles bud off from the ER at specialised points that have a COP 2 coat
The COP2 play a role in finding an exit or transport proteins that are needed for vesicle to form
If incorrect protein folding then they dont leave the ER
How do proteins get form the ER to the cis golgi face
COPII vesicles shed their coat and fuse to form the Vesicular tubulule cluster
This clustert then fuse with the cis Golgi
The KDEL receptor can retrive protein back to the ER via COPI ciat on the Golgi that is shed
How do proteins move through the Golgi
There is cisternal maturation model and the vesicle transport model
In cisternal they pass staright throught the flattened cistae area and in the vesicle they go in vesicles around the flattend cistae
What is glycosylation in the Golgi
Can get protein glycosylation where sugars for function are added or can bget removed and trimmed of compex sugar sidre chains added in the ER
What is the Cytosol made up of
Water, Vault Complexes, Protein Filament, cytoskeleton, Ribsome, Proteasomes
In the Cytoplasm there is Cytosol and the Cell organelle
What is the Mitochondria
They were orginally prokaryotic cells and they have a doyble membrane with highly folded inner membrane to increase surface area
They are mobile and associated with microtubules of the cytoskeleton due to this
They change shape and position
What is the structure of the mitochondria
Outer membrane - Include porins and membrane channels for very small molecules to enter
Intermembrane space
Inner membrane - Have cistae that are heavily folded and is the location of the electron transport chain and the creation of proton gradient to drive ATP synthase
Matrix - Enclosed space due to the cristae
Circular mitochondrial DNA
Granules
The intermembrane space is impermeable to electrons due to lipid cardiolipin so mitochomdria dont leak electron from ETC
What is mitochondrial fission
This is used to allow mitochondria to replicate and can also be used to remove unwanted waste or damage material from the mitochondria
How does the mitochondria produce ATP
Chemiosomosis - High energy electrons from the oxidation of food that are then moved along the ETC
Electrons lose energy doing this and are then released from the ETC and this causes H+ to be pumped across the membrane
Protion Gradient - H+ move back into the intermembrane space via channel protein coupled with ATP synthase so then ATP is produced
What is the electron transport chain
Can create a proton gradient by moving proton across tge inner mitochochondrial membrane
There is a 4 protein complex and at the very end 2 electron reduce oxygen to water
Oxygen is the final electron acceptor
What is the mitochondrial DNA
Evolve faster than genomic DNA but only code for 13 proteins in the human body with most being used to make ATP
Genes may have been moved from mitochondria to genomic DNA
It is usually maternally inherited so if mother has genetic defect in mitochondria 100% chance of being passed on
What are the Ribosomes
Prokaryotic ribosome is 70S made of a 50S unit and 30S unit
Eukaryotic ribosomes are 80S and made of 60S and 40S units
What are ribosomes made of
The larger and smaller sub units
Ribsomal proteins that make up the sub unit of the ribosome
Sub units also have rRNA in them
What do the Ribsomes do
Faciliate protein synthesis during translation of mRNA
This is done by binding to the codon of mRNA
There are 3 sites the E,P and A site in that order
a t-RNA that carry activasted from of amino acid bind to the A site and then move along all the sites
Ribsome can move along the mRNA chain until reach stop codon when ribosome will unbind and have a ful polypeptide made
What arev Ribsome inactivating proteins
Type 1 cant cross the cell membrane so can get into cell so cant inactivate the ribosomes
Type 2 can as they bond to B domain that allow them to cross into the cells so inactivate the ribosome
What are Perioxisomes
Have no DNA or ribosome in them
Proteins are imported from the cytosol
They contain oxiadative enzymes such as catalase and Urate oxidase, due to such large amount lead to cystalline core formation
How are perioxisomes and yeast linked together
Yeast grows on sugars that have small perioxisomes on them
Yeasy grown on methanol will have much larger perioxisomes that break down methanol
Yeast on fatty acids have larger perioxisomes on them that break fatty acids to acetly CoA
What reaction rely on perioxisomes to happen
RH2 + O2 --> R + H2O2 use oxygen to remove hydorgen atoms from organic substances
Important to make hydrogen peroxide for other reactions
Need to catalyse H2O2 due to toxic in the body to make 2H20 this is important for detoxification and breakdown of fatty acid to acetyl CoA
Perioxisomes are important for biogenesis and maturation
They are formed by the budding off from the ER
They then recruit key proteisn and enzymes needed thanks to PEX19 that is an import protein, by using a C terminal sequnce -SER-LYS-LEU
The sequence is too direct the proteins where they need to go like a postcode
What are the lysosomes
They contain many hydraulytic enzymes up to 40
They are involved in phagagcytosis and endocytosis where they engulf extracellular entities
Also carry our autophagy which is the digestion of self contents
What are endosomes
Intracellular sorting organelles, they are internalised from the extracellular space to form early endosomes
These then mature to late endosomes that then fuse with lysosomes
Golgi vesicles also fuse with endosomes to deliver material to lysosomes
Why wont the lysosome digest itself
A modified lipid membrane that have highly glycosylated proteins
Also membrane transporters that are able to remove recycle or excrete digestion products
Vacuolar H+ATPase to hydrolyse ATP and pump proton to lysosome
What are the Vault Complexes
There function is unknow but thought to be associated with NPC in the nuclear cytoplasmic transport system
mRNA localisation and cell signalling
Drug resistance
What is the structure of the Vault Complex
Large Ribonucleoprotein complexes
it is 3x larger than ribsomes
1 or several Vault RNA made of MVP, VPARP and TEP1
What are proteasomes used for
Allow the cell to produce correctly folded proteins, thsi is important as incorrect folding can lead to harmful proteins
They are able to destroy proteins that have been misfolded
Also important for rapid turnover of short lived proteins
What is the structure of proteasomes
They have a central hollow cylinder made of 4 stacked heptameric rings
Some of the proteins in the rings have proteasomal properties
Polyubiquitinylated proteins are targeted by them
What is the vacuole for
Fluid filled membrane bound organelle to maintian the internal pressure of the cell
Also for nutrient and waste storage
Can regulate the cell pH by pumping protons in and out the cell
What is the plant cell wall for
Give cell strenght to resist internal turgor pressure given out by the vacuole
What is the function of the cytoskeleton
Give the cell its shape and has the capacity to move or alter its shape
It organsisesthe the organelle and also transports them
It is used in cell divsion due to control chromosome organsisation and movement
What are the componenst of the cytoskeleton
Microfilamenst made up of actin
Intermediate filaments
Microtubules
The most abundant is the microfilament and the least is the microfilaments
What are the components of cytoskeleton made of
The microfilament and microtubule are made of sub-units that rapidly assemble and dissemble so they are very dynamic
Intermediate filaments are made of much more stable sub-units fibrous proteins
How are microfilamenst assembled
They are comprimised of linear assemblies of 43kDA actiin monomers and have 4 domain with a central cleft containing Ca2+ or MG2+
What is actin sub unit cycling
It needs ATP and ADP.
G actin, globular has a tighlty bound Ca2+ and non covalent ATP attached in its structure
G actin can polymersie to form F-actin fibrous, by the ATp being hydrolysed during polymerisation
F-actoin can then depolymeroise back to G actin where ADP is turned back to ATP
What is actin
It is a pear shaped molecule so has form of polarity so then in the polymer get a barbed end and a pointed end due to polarity
The barbed end is the + end as its easier for new monomer to be added at this end and so the pointed end is the - end due to need a conformationla change in the sub-unit before it can be added
What is the structure of the actin filmament
Has 2 strands of f-actin that are twisted together and the F actin has 1 Ca2+ ion and non covalent bonded ADP
Gte more elongation at barbed end due to easier to add sub-units
The sub-units added are G-actin due to being globular
Why is it good thgat cytoskeleton is dynamic
Helps in cells that need to move to or away from a stimuli
It can move by dissembling parts of the cytoskeleton and reassmble them at the opposite end of the cell so then push the cell membrane where it needs to go
What is microfilament synthesis caused by
ARP2 and ARP3 are similar to G actin but cant polymerise
They form an initiation complex and act as primer for actin polymerisation that can then recruit actin monomers and can elogngate the chain
ARP2/3 can bind to microfiliament to cause branching in the network as actin monomers will bind wherever the initiation complex is
How is the rate of elongation regulated
Controlled by thymosin that lead to less elomgation due to less actin monomers availble
Profilin lead to more elongation due to more G actin available
How is the microfilament regulated
Ca2+ dependent on the binding site of Gelsolin that cause cleave of the microfilament
This act as a primer for elongation
Gelsolin is freed by PIP2 that can make + end for rapid elongation
Gelsolin chop the microfilmanet in half so there are now 2 end that actin can be added to
How is microfilament association regulated
Filamin homodimers cross link microfilaments to form a gel like network
The crosslinking leads to bundling of microfilaments
What are the microtubules
There are 2 forms a-tubules and b-tubules the only difference is that a tubulue has bound GTP that wont hydrolyse and b-tubule may have GTP or GDP bound
They form a 110Kda heterodimer that act as the monomer to form the polymer
They are formed in the MTOC in centromere of the animal cells that have the mix of proteins being made like the y-tubulin ring that nucleate the microtubule growth, but in plant this can happen anywhere
How is microtubule assembly regulated
Polymerisation of free tubulin sub unit is energetically favourable
Stanthmim reduce the free pool of monomer so less elongation by bidning to the alpha or beta tubulin dimers so they cant be added to the polymer
How are microtubule stabilised
MAPs Protein allow crosslinking of Microtubule to stabilise them by packing them closer together
What are intermediate filmaments
There is no elomgation or shrinking so are non-dynamic
The omnly way to remove them is by digesting them and can only be made by transcribing the right gene
They are least soluble part of the cytoskeleton and are used for structural support in the cell
What are the intermediate filaments made up of
They are made up of fibrous polypeptides that assemble with side by side to give filament a high tensile strength
There are non-helical regions that stabilse the filament and intercat with other cytoplasm componets
Intermediate filamen are not active due to
No cellular pool for sub unit monomers that can be polymerised
No evdience for dynamic equilibrium between polymerised and soluble forms of intermediate filaments
The number and lenght if them are controlled by their degradation as they can be broken down by proteases which is controlled by cell
Why are the intermediate filament cell specific
Due to each IF will play a different role within the cell meaning overall function of IF will be cell specific
What does the cytoskeleton allow to move
Organelle, vesicles
Mitochondria - they move form stationary in a linear direction in short bursts so they move along the cytoskeleetal elemenst
Cellulose biosynthesis in plamnts the portien compex move in a linear fashion, this is controlled by CSA complex in a rosette structure
The complex move through plasma membrane in linear fashion
How does movement occur along the microtubule
Microtubule use ATP derived energt and are proteinsthat are able to move in a direction of fashion
There are 2 major groups of these proteins:= Kinesins that move to the + end of the microtubules and dyneins that move to the - end of the microtubule and so motor can move in both direction
The motor proteins can carry cargo from start to finish such as organelle and vesicles and used in positioning of daughter chromatid along mitotic spindles
What are the 2 confromations that the motor proteins do in walking
1 is where the motor protein is stabilised by binding to the ATP and the other one is where it is stabilised by binding to ADP
The microtubule provide a trackway for their to be movement of things from source to sink
What is the microtubule role in axons
Carry neurotransmitters from the cell body to the axon where they are released to trigger anterograde transport
Movement from the axon towards the cell body is called retergrade transport
What are the motor protein chrarcteristics
Globular head region - engege the filament and activly move along it
Tail region- Attatchement point of motor protein and cargo
Walking - Way to describe motor protein movement
Power stroke - ATP hydrolysis cause conformational change so the head is thrust back creating tension so lift the tail and move it along it forward with the cargo
Recovery stroke - Occur when protein loosely attached to the filament and the head slides along by thermal diffusion