Lecture #3 - Mitochondria and Peroxisomes Flashcards
Use of Organelles/cell compartments
Organelles/cell compartments provide :
1. Create specialization (Ex. Energy production)
- Example – Mitocondria is specialized for making ATP
2. Concentration of Activities in a cell (Ex. TCA cycle)
- Example - Mitochondria = concentrates TCA and Oxidative Phosphorylation
3. Sequestering of toxic metabolites (EX. ROS h2o2)
4. Make Microenvirnments (Ex. Membrane potential across membrane ; Ion or pH gradients across membrane)
Issues with organelles
Organelles pose probelms for the cell :
1. How to control organelle number and shape
2. How to transport small molecules in and out
- Need to transport small molecules in and out of organelles because they have membranes (need transporters)
3. How to assmble organelles with proteins and lipids
4. How to control location + movement + inheritance of organelles during cell division
Mitocondria membranes
Mitocondria = have 2 memebranes
- Outer membrane (Smooth)
- Inner membrane
- Inner membrane = folds into Cristae –> Cristae = increases the surface area to accommodate the many protein complexes (Ex. Oxidaive phosphorylation machinery + transporters)
Between the membranes = have inter membrane space
Insider the inner membrane = matrix (has mtDNA)
EM section of mitocondria
Image – EM image of mitocondria
- Shows inner and outter memebrane
- Small dots = ribosomes (Ribosomes = make proteins in the matrix of the mitocondria)
3D reconstruction of serial EM images (tomoagraphy) of mitocondria
Image – 3D view of mitocondria using EM tomography
- EM tomography = cuts serial sections of mitocondria and reconstitutes sections into 3D image
- Blue = Outer membrane ; Yellow = inner membrane
- See 3D of inner membrane –> shows the inner membrane is sheet structures NOT tubules
Number of mitocondria per cell
Each cell has hundreds of mitocondria
- Have 2 copies of nuclear genome in 1 cell BUT have >100 copies of mtDNA in 1 cell
Image:
- Red = Mitocondira ; Green = Nuclear DNA ; Yellow = mtDNA
- See many mitocondria in cell + see nuclear DNA + see many copies of mtDNA in cell cell
Functions of mitochondria
- Energy production (synthesize ATP)
- Major function of mitocondria = synthesize energy in the form of ATP
- Metabolsim
- Heat generation (Control body temperatureby generating heat)
- Involved in Signaling
- Intracellular Ca+ signaling (suing Ca) and ROS (ROS can be a signlaing molecule)
- Apoptosis (programmed cell death)
OVERALL – means mitocondria are essential organelles
Mitocondria metabolsism
- Some lipid syntehsis (made in mitochondria)
- Heme and Fe-S cluster synthesis
- Heme and Fe-S cluster = part of enzymes in cells
- Fatty acid metabolsim (making energy)
- Some Amino Acid metabolism
- Urea degredtion
How do you make ATP using mitochondria?
Answer – Mitocondira make energy by burning what you eat
- Get energy from burning (oxidizing) the substrates used for ATP synthesis –> make ATP
Mitocondria burns Sugars + proteins + fats AND using oxygen to extract energy from these molecules –> then make ATP
- In process of making ATP turn O2 –> CO2
Mitocondria ATP production (process)
- Sugars or fats or proteins undergo TCA cycle in the matrix
- TCA cycle produces high energy compounds (NADH and FADH2)
- NADH and FADH2 = made in the matrix with TCA cycle - NADH and FADH2 are used in ETC (proton pumps)
- ETC will make an electrochemical gradient across the inner mitochondrial memebrane
- F1/Fo ATPase (ATP synthase ; aka complex 5) will use the gradeint to make ATP
Use of NADH and FADH2
NADH and FADH2 = used to make membrane potential – creates an electrochemical gradient across the inner membrane using proton pumps
- Proton pumps - pump protons from the matrix to the intermembrane space
- When pumping the H+ - pumping H+ against their natural concetration of protons (pumping from low to high)
- Creating form of energy by mitocodnira
F1/Fo ATPase
F1/Fo ATPase (ATP synthase ; aka complex 5) will use the gradeint to make ATP
- H+ gradient = energy created by the mitocondria
After the gradient is made – H+ will flow down the gradient back into the matrix using ATP synthase
- When the proton goes into the matrix –> ATP synthase uses the energy to generate ATP
H+ potential across the inner membrane us used for many other things
Byproduct of electron transport
ROS is a bypoduct of electron transport
- Because using Oxygen to make ATP = make ROS as a byproduct
- ROS = toxic
ROS needs to be taken care of with enzymes that will detoxify ROS
Mitochondria and disease
Since mitocondria are essnetial – if have defects in mitocondrial function -> get many types of human disease
Genrally mitochondrial diseases often first/primarily affect tissues with high energy (ATP) demands:
1. Skelatal and msucle disorders
2. Cardiomyopathies (affcets heart)
3. Metabolic Disorders (Ex. Diabetes)
- Liver and adipocutes are affceted in metabolism disorders
4. Neurodegernative disorders (Ex. Parkinsones + many types of blindness + ataxia and dystonia)
- Nuerons = high energy demand = get neurological disorders
Mitocondria + Cancer
Mitochondria play a role in tumorgenisis
Some types of cancer are associated with defects in mitochondrial activity
Example – Mutations in complex 2 cause pheochromocytoma (neuroendocrine tumor)
Mitocondria + Aging
Mitochondria may be the primary cause of aging
Decline in mitochondrial function may contribute to the aging process in the body
Use of the proton gradient
Product gradinet can be used to make ATP BUT it can also be used to trasnport moecules into the matrix
Examples:
1. Can import ADP and phosphate into matrix
2. Once make ATP –> ATP can be transported out
Mitocondria have many transporters to control metabolism –> THE TRANSPORT IS DRIVEN BY THE H+ GRADIENT
Carriers in Inner membrane
Since the mitocondria Inner membrane is imperbale (even to H+) –> many special carriers are needed to transport small molecules in and out of the matrix
Many carriers use the inner membrane potential:
1. Voltage gradient drives ADP-ATP exchange
2. pH gradient drives pyruvate import
3. pH gradient drives phosphate import
H+ gradient
H+ gradients = form of energy the mitocondria makes using the ETC
Answer – ALL OF THE ABOVE (mitocondria like most organelles do many things for the cell and are thus essential)
Compartments of the mitochondria
Compartments of the mitochondria – Have 4 compartments and 2 membranes
- Outer membrane
- Inner membrane
- Intermembrane Space
- Matrix
General method for isolating mitocondira and other organelles
General method for isolating mitochondria and other organelles = subcellular fractionation
- Break up cells or tissue mechanically (homogenize) –> now have the cytosol + organelles
- Use differential centrifugation –> spin down organelles based on size + density
- Low speed centrifuging = get big structure (ex. Nucleus)
- Increase the speed = get pellet with mitocondria + lysosomes + peroxisomes (intermeduate size density structures
- Spin at higher speed = get pellete with smaller structures (Ex. ER)
- If use ultra centrifugation = get pellete with protein complexes (Ex. Get ribosomes)
Using this people have isolated the mitocondria to study structure and function
Subceullar Fractionation on Mitocondria
Mitochondria = can be isolated by subcellular fractionation (aka differential centrifugation)
Mitochondria can be further fraction into outer membrane or inner membrane or inter membrane space or matrix
- Can study the compartments by separating them from isolated mitochondria
Mitocondrial Outer membrane
Outer membrane = has big pores = makes membrane permeable to small molecules and ions
- Phosphates + ATP + ADP = can go through the outer membrane BUT need to be transported across the inner membrane with transporters because the inner membrane is impermeable to these molecules
Pore in outer membrane
Porin creates pores in the outer membrane
Pore = allows small molecules (Ex. ATP + ADP + phosphates + sugars) to freely diffuse in/out across the outer membrane (don’t need a specific transporter)
- EXCEPTION = need trasnpoorters for proteins
Porin = type of beta barel protein
- Beta barel protein = also found in bacterial membrane (found in bacteria membrane + mitochondria membrane = links mitocondria and bacteria in orgin)
Mitocondria Intermembrane Space
Intermebrane space = between the the outer and inner membrane
Intermembrane space has very few proteins but does have Cytochrome C
Cytochrome C
Cytochrome C –> protein in the ETC machinery (moves between complexes 3 and 4)
Cyt C = controls life AND death of cells
- Life - Cyc C normally functions as a protein SU of ETC complex 4 –> part of the Oxydative Phosphorylation machinery that creates the H+ gradient and actually creates ATP = importnt for cells to live
- Death – when cells decide to die through apoptosis –> Cyt C is released from complex 4 and goes to the cytosol -> in the Cytosol Cyt C binds to regulator of apoptosis and induces apoptosis = cell dies
Depending on the condition of the cells Cyt C will function in ATP production or cell death
Mitocondrial Inner Membrane
Inner membrane has Oxidative Phosphorylation machinery
- Inner membrane Cardiolipin –> important for mitocondria function
Inner membrane = has big membrane area because of Cristae –> Area allows for a lot of transporters + oxidative phsophorylation machinery
Where is the ETC found
ETC complex (Oxidatative phosphorylation machinery) - found on the inner membrane
Mitocondria inner membrane carries the machinery for oxidative phosphorylation (ATP production)
5 complexes (made of of many proteins)
ATP synthase
Complex 5 = ATP synthase –> uses the gradient to create ATP from ADP and phosphate
- Once ATP is made (it will be in the matrix) –> ATP is transported to the intermembrane spoace and to the cytosol (goes to the cytosol through porins in the outer membrane)
Complex 1-4 = used to generate H+ gradient by pumping protons from the matrix to the intermembrabe space (Pumps H+ using NADH and FADH2 as teh energy source (because pumping against the gradient))
- Transporters transport ATP and ADP through the inner membrane using the H+ gradient
Mitocondria matrix
Matrix = inside inner membrane
Matrix has mtDNA
TCA cycle = happens in the matrix (has the enzymes for the TCA cycle)
- TCA = uses pyruvate as primary sugar source to create higher energy compounds (high energy compouds = makes NADH and FADH2) –> NADH and FADH2 = used to create proton gradient in ETC
- Pyruvate = sugar produced in glycolysis
mtDNA vs. nuclear DNA
mtDNA is different from nuclear DNA
- mtDNA uses different codons
- Example – Nuclear DNA uses AUA as isoleucine BUT mtDNA uses AUA for methionin
- Patterns are similar to the pattern of codons found in bacteria (Again suggest the link between mitochondria and and bacteria)
- Structure of mtDNA is different from nuclear DNA (Nuclear DNA = linear ; mtDNA is circular (similar to bacteria) )
What does mtDNA code for
mtDNA has 13 protein coding genes
- ALL genes code for SU for different ETC complexes
- Different species have different numbers of genes in mtDNA
MtDNA also codes for rRNA and rTNA for protein syntehsis for genes encoded in mtDNA (Have translation in matrix)
Mutation rate of mtDNA
mtDNA tends to have more mutations at an increased rate compared to nuclear DNA
Have more mutations because mtDNA have more ROS that can damage the DNA in the mitocondria AND because mitochondria have less efficient repair mechanisms for DNA damage
Diseases + mtDNA
Many diseases are caused by mutations in mtDNA (many diseases affect the mitochondria because of mutations in mtDNA)
Example:
1 . 1 – Leber’s hereditary optic neuropathy (LHON)
- Mutations in genes ND1,4, and 6 (SU in complex 1)
2. Myoclonic epilepsy with Ragged Red fiberes (affects muscles)
- Mutate the tRNA = creates defects in protein synthesis in the mitocondria
3. 3 – Neuropathy, ataxia, and retinitus pigmentosa (affects nervous system)
- Mutations in compelx 5 (ATP synthase)
These are types of disease caused by mtDNA defects BUT many mitochondria proteins are ALSO encoded in the nuclear genome (mutations in nuclear genoms encoding mitocondrial proteins can also lead to mitocondrial diseases)
mtDNA inheritnace
Have specific pattern of inheritance of mtDNA because mtDNA is only inherited in the oocyte (inherited from the mom)
- Get from mom because mitocondria and mtDNA are degraded once fertiization and fusion between the oocyte and sperm happens –> only maintain the mtDNA and mitocondira derived from the oocyte
- MEANS mutations in oocyte can be inherited BUT mutation in mtDNA in sperm is not = have a maternal inheritance pattern
Pedigre – shows 4 chidlren ate affected –> Son – does not pass to offspring BUT the daughteer can pass to offspring
2nd factor for mtDNA associated disease
2nd factor for mtDNA associated disease = mtDNA associated diseases are not 100% penetrance
- Not 100% penetrance because have lots of mtDNA (100s) in each cell –> depending on the ratio of mutant DNA to WT mtDNA the symptoms/outcomes can be different
Ex – even of the mom is affected –> the ratio between mutant mtDNA and WT mtDNA can chnage in each individual
- Each tissue can have diffrent ratio + the ratio can chnage over time
- MEANs not all offspring will have the disease even though the mom has a mutation (depending on the amount of mutant mtDNA you may or may not have the symptims)
Mouse model of mtDNA inheritance
Found that need over 60% of the mitocdonria to be mutated to see defected –> shows that the WT copy of mtDNA can be dominant
- Explains why there is heterogeneity in inheritance of mitocondrial diseases
Answer – False –> Cells often contain more than one type of mtDNA (called heteroplasmy). So, the ratio of mutated mtDNA to wild-type dictates whether you will have disease or not. Note, the ratio can change in different individuals, in different tissues of the same individual, and over time!
How do you treat a mitochondrial disease caused by mtDNA mutation
Done by replacement of diseased mtDNA and mitocondira with WT mitocondria
Do mitocondrial transfer in human occytes
–> Take nucleus from affected oocyte and transport the nucleus to a healthy donor that has had the nuclear DNA removed = replace the affected with mitochondria with healthy mitochondria and still have the parents nuclear DNA = remove the disease causing mutation
The successful replacement of mtDNA in human oocytes and genration of blastocytes and embryonic stem cell line have been reported
Answer – Yes –> human cells can lose mtDNA
WHY - mtDNA encodes only some OXPHOS proteins (need mtDNA for Oxphos) –> BUT As long as cells have another way to make ATP, they don’t need mtDNA.
- Can make ATP in other ways (ex. glcyolysis) to bypass the requirement for oxidative phosphorylation
- IF you culture human cells and add glucose and other metabolites –> you can force the mitocondria to lose mitocondrial DNA (cells can live in culture)
Answer – 100 lb
NOTE – glycolysis occurs in the cytosol ; In cancer – ATP is mainly made by glycolysis
What to mitocondria subunit often contain
Multi-subunit complexes often contain SU encoded in the mtDNA and other SU coded in the nucleus (oxphox machinery uses mtDNA and nuclear DNA)
Example – Complex 1 –> 7 mtDNA encoded SU ; 39 nuclear encoded SU
- Some proteins are generated in the matrix and inserted into the inner membrane ; some proteins are made in the cytosol and transported to the mitochondria and inserted into the inner membrane –> Biogenesis depends upon the import of nuclear encoded SU and their coordinated assembly with mitocondria encoded SU
- Import and assmbly = highly regulated process
Nuclear-encoded proteins are translated in the cytosol –> after synthesis they must
- Precurosr proteins need to find the mitochondria (taregting)
- Have a targeting signal that allows the precursor protein to go ti the mitochondria outer membrane
- Once the precursor protein reach the mitocondria it crosss one or both mitochondria membranes (translocation)
- Have pores and transporters to help the proteins cross
- Once the proteins get in they must sort themselves into the outer membrane vs. Inner membrane vs. Inter membrane space Vs. Matrix
- Assemble with other SU (some encoded by mtDNA) (find protein complex)
Targeting sequence on precursor protein
Targeting sequences of precursor protein = called the pre-sequnece
- In the preseqeucne the Amino Acids are not important BUT it needs to form a unique 3D structure
3D structure = alpha helix (on side is hydrophobic and one side is positively charged) –> pattern is important for being recognized by receptor proteins
All presequences contain:
1. Several basic residues
2. No Acidic resideues
3. No long stretches of hydrophobic aa’s
4. Common 3D structure
Translocation of proteins to the matrix
Uses TOM and TIM machinery to corss the membranes
- TOM and TIM = have pores for proteins to go through
TOM = translocase outer membarne (Pink in image)
TIM = translocase inner membrane (yellow in image)
Chanels do NOT function alone –> Have many proteins that help (example – receptor proteins)
- Once a precursor is recognized by a receptor they can go through the pore (different receptor recognizes different seq)
What do TOM and TIM form
The TOM and TIM complexes form large aqueous pores in the outer and inner membrane
Image – negative stain of purified TOM complexed by EM
- Have 3 holes in TOM
- Size of pore = small (Diamter of holes = 2-4 nm)
Inhibition and promoting of protein folding
Cytoplasmic chaperones (ex. Hsp70) = play a role in import + preventing premature folding (keep precorsor protein unfolded after translation) + prevent aggregation of proteins
- Unfolded proteins tend to agregate because they have hydrophobic parts that would bind together BUT the chaparones bind to protein to prevent the agregation of proteins
During tranlocation across the outer and inner memrbarens through TOM and TIM – chaperones keep the precursor straight so they go through unfolded
Matrix chaparones
Matrix chaperones play an essential role in helping imported proteins fold and assemble in the matrix
- Help protein form functional structure
BOTH cytoplasmic and matrix chaperones use ATP
Protein Sorting
Once the protein is in the mitocondria it can go to different places
- Mitocondria has different mechanisms to send proteins to the matrix vs. Inner membrane vs. Outter membrane
Where the protein goes depends on the transporters in the outer and inner membrane
- Matrix proteins = use TOM and TIM
- Outer membrane = uses TOM and SAM
- Inner membrane – use TIM22 and TOM
Lipid Transport to the mitocondria
Lipid transport = happens at the contact site between the RER and the mitochondria
Lipids = get to the mitochondria at the physical connections between the ER and the mitochondria (called the MAMs –> mitocondiral associated ER membranes)
- Have contact between the RER and the mitochondria (red circle in image)
Mitocondiral associated ER membranes
IF want to connect 2 memebranes –> THEN want protein in mitochondria outer membrane and protein in ER –> the two proteins will interact
- Image – protein complex in green – have 1 Su that is embded in the ER membrane and 2 SU that are embedded in the mitochondria outer membrane + have a lipid soluble protein between –> ALL form a complex = can physically connect ER and mitochondria membranes
Connection between the 2 membranes allows lipid transport between the ER and the mitocondria membrane
Generation of Cardiolipin
Cradiolipin = generated in several steps controlled by enzymes in the mitochondria inner membrane
Precursor of cardiolipin = phosphatidic acid (PA = phospholipid made in the ER) –> Phsphatidic acid = transported to the outer membrane then the inner mebrane using the contact between the ER and the outer membrane
Once in the inner membrane –> PA is modified to make Cardiolipin –> most cradiolipin will stay in the inner membrane but some of it is transported to the outer membrane
Uniquness of cardiolipin
Phospholipids usually have 1 heads and 2 tails (tails = hydrophobic)
Cardiolipin = dimer of phospholipids –> have 4 tails with 1 head (big hydrophibic phospholipid)
Shape of mitocondria
Mitocondria are in different shapes and different locations in different cell types
Liver = round structure
Pancratic acinar cells = elongated mitochondria
Heart muscles = mitochondria sit next to the myofibrils (provides ATP for effective muscle contraction)
Location of Mitocondria
Sperm –> mitochondria are enriched at the base of flagella = give ATP for the sperm to swim
Epithelial cells –> mitondira are enriched at the apical surface = used for taking up molecules from the apical surface
Movement of Mitocondria
Mitochondria are often tethered to and move along the cytoskeleton
Video – axons of nuerons –> see things moving
- Longer things = mitochondria
- Mitochondria can go in both directions – can go towards the synapse at the end of the axon or towards the cell body
Types of dynamics of mitochondria
Types of dynamics of mitochondria = fusion and division
Regulated fusion and fission = controls the number + shape + size of mitocondria in the cell
Block fusion in mitocondria
IF you block fusion or division you can see changes in mitocondria
Example (image):
- WT mitochondria = have a tubular strctures
- IF block fusion = mitochondria will continue to divide and make small structures = the mitocondria are not happy because they lose mtDNA because the cell is making so many mitochondria
Make mitocondira beyond the copy number of mtDNA = make empty mitochondria = defective because mtDNA is important
Block mitochondria division
If block mitochondria division = keep fusing the mitochondria together and get a big structure –> not good because now the mitochondria are hard to move in the cell because they are all connected
What mediates mitochondrial division and fusion
Fusion and division process = controlled by GTPases associated with the mitochondria
Division - When mitocondira divide the Drp1 GTPase assembles into the surface of the mitocondria and wraps around –> Drp1 GTPase uses GTP hydrolysis –>the ring will squeeze and cut the mitocondria
Fusion – Daughter mitochondria can fuse back together by fusing the oter membrane then the inner membrane (using another set of GTPases)
- Uses Opa1 and Min1/2
Why do fusion and division
Fusion helps mix mitochondrial contents and exchange information of material among mitocondria to maintain homeostasis
Division = quality control for mitochondria (ex. mitochondria damaged by ROS)
- Division = needed to remove damaged mitocondria –> part of the damaged mitocondria can be separated by mitocondrial division and the isolated mitocondria ca be targeted for degradation by autophagy
- Mitocondria import makes mitocondria bigger = once the mitochondria get to be a certain size the mitochondria need to divide to maintain size and number in the cell (esocially during cell division)
- Division is important for maintaining of function and the maintain of mitochondria number and size in each cell
Origin of mitochondria
Have similarity between mitocondria and bacteria (suggests that bacteria may be the origin of mitocondria during evolution)
Similarities:
1. DNA is circular and shared codons + Prokayotic like tranlation system
2. Have porins in beta barel in both bacteria and mitochondrial membrane
3. Cardiolipin in mitocondira and also found in bacteria
4. 2 memebranes (1 memebrane is from bacteria and 1 memebrane is from the host cell plasma membrane)
Idea – many yeas ago cells did not have organelles –> cells engulfed bacteria –> instead of killing the bacteria the cells used the bacteria to make ATP
Peroxisome appearance and structure
Peroxisomes – surrounded by a single membrane and do NOT contain DNA
Have many peroxisomes in the cell (Image – green dots = peroxisoms )
EM image – shows that peroxisimes have 1 membrane
What do peroxisomes contain
Peroxisomes contain a number of oxidases
Peroxisomes = produce H2O2 (Type of toxic ROS) during lipid metabolsim BUT peroxisomes ALSO make catalase to degrade H2O2
- Beta oxidation and plamalogen producton = produces ROS
Catalase = converts toxic ROS H2O2 into water and O2 – peroxisome can protects against the toxic ROS generated by the peroxisome itself
What else does peroxisome Catalase protect against
Sysem can ALSO help mitocondria
Mitocondria and peroxisomes can interact –> peroxisomes will help detoxify the mitochondria generated ROS (turns H2O2 into H2O and O2)
- Mitocondria ROS can be transported to the perosoxisomes through contact site and then catalse will make water and O2
IF you seperate the peroxisome and the mitochondria THEN the mitochondria accumulate more ROS
Peroxisome import machinery
Peroxisome precursor proteins have their own targeting signal (signal = PTS)
- Signal has its own sequence and structure (3 Amino Acids is sufficient to target the protein to the peroxisome and the peroxisome receptor)
Signals:
1. PTS1 - Most peroxisomal matrix proteins carry a C-terminal import signal (called PTS1), consisting of ser-lys-leu (SKL)
2. PTS2 - located at the N-terminus.
- Only a few proteins have PTS2.
Peroxisome import machinery - Signal Recognition
The PTS1 signal is recognized by a cytosolic protein, called Pex5, that targets the protein to the peroxisomal surface
PTS1 is recognized by a receptor –> Receptor is in the Cytosol (NOT the peroxisome surface) –> receptor complexes with precursor protein –> complex can target protein to a protein to the peroxisome surface –> then will hand the protein to the translocase transporter in the membrane –> translocase will trasnport the protein
Peroxisome assmeble
Peroxisome membranes also come from ER
Peroxisome proteins are inserted into teh ER memebrane –> ER can create small precurosrs vesicles/immature peroxisomes (make mini version of peroxisome coming from ER) –> THEN can import other components into the percursor form of peroxisome –> then make mature peroxisomes with all peroxisome proteins
Once you reach a certain size – need to divide to maintain the number (increase the number) and size for cells to divide in the future
MEANS that peroxisomes can be generated de novo
De novo biogensis
De novo means that when cells don’t have peroxisomes you can still make peroxisomes becasue peroxisomes can be made by ER
Can’t make mitocondria de novo because there is no way to make mitocondria from the ER = always need pre-existing mitochondria to make more mitochondria by importing lipids and proteins
Mitocondria arise ONLY from growth and division of pre-existing organelles
Growth and division
Other proteins are added to the vesicles –> forming mature organelles
Further import + growth and division of peroxisomes generate additional organelles
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