mitochondria and plastids Flashcards
What do mitochondria do?
Site of respiration: occurs on the inner membrane, proton-motive force (Δp) develops in the intermembrane space (as a proton gradient) and is used to drive ATP biosynthesis by the
H+ transporting two-sector ATPase (EC 7.1.2.2, aka ATP phosphohydrolase.
Site of Krebs’ cycle: pyruvate produced in glycolytic pathways is translocated into the mitochondrial matrix by active transport.
Site of fermentation: if molecular oxygen is limiting, Krebs’ cycle halts and pyruvate is fermented into lactate to generate NADH (used to make NADPH for anabolism) – the only
ATP produced is substrate-level phosphorylation in glycolytic pathways.
Site of non-shivering thermogenesis: takes Δp and uncouples it from ATP biosynthesis, allowing protons to flow into matrix via thermogenin – the energy from the gradient is released as heat. Mainly occurs in brown adipose tissue.
Site of calcium homeostasis: Ca2+ is used widely in signal transduction – mitochondria take up and release Ca2+
ions, acting as ‘storage vessels’ for them, keeping them in the matrix.
Site of regulation of cell division: ATP and NADPH are needed to generate new biomass – when cells are dividing, mitochondrial production of these molecules regulates the speed of division.
Regulating apoptosis
Biosynthesis of steroids
Biosynthesis of heme cofactors
Hormonal signalling (particularly with estrogens)
What are matrix granules made of?
Typically phospholipids and calcium phosphates
What are the mitochondria membranes including sizes.
- Outer membrane is 60-75 Å
- Outer membrane contains many porins and gated ion channels.
-Outer membrane contains enzymes monoamine oxidase and long-chain fatty-acid-CoA ligase among others.
-Breaking the outer membrane causes proteins from the intermembrane space to initiate apoptosis - Mitochondria-associated endoplasmic reticulum-membrane,
transfers lipids to the mitochondrion. - Inner membrane is site of the membrane proteins of the respiratory chain and the H+ -transporting two-sector ATPase also contains transport proteins.
-Inner membrane is rich in cardiolipin (diphosphatidylglycerol with usually C18:1 and C18:2 fatty acids) – helps maintain Δp: protons in the intermembrane space bind to the head of cardiolipin. - Intermembrane space is the site in which Δp develops.
membrane-bound succinate dehydrogenase is on the inner membrane.
Mitochondria matrix
What content does it contain?
what ribosomes sizes are there?
How many genes and respiratory chain enzyme subunits are there etc in human mtDNA?
- 60 % of soluble protein content of the mitochondrion is herein.
- Contains enzymes of pyruvate oxidation and Krebs’ cycle, except for the membrane-bound succinate dehydrogenase which is on the inner membrane.
- Contains mitochondrial ribosomes: 70S, comprising 50S and 30S subunits
- Contains mtDNA (mitochondrial genome): c.16.5 kbp in most Mammalia
- Human mtDNA has maybe 37 genes, encoding 11 respiratory chain enzyme subunits, 2 H+ -transporting two-sector ATPase subunits, 2 ribosomal rRNAs, 22 tRNAs and the micropeptide humanin which is cytoprotective in some way.
- Human mtDNA has many genes that are associated with pathology if damaged
(e.g. MT-CYB and MT-RNR1, Parkinson’s).
What are etioplasts
Immature chloroplasts/rhodoplasts etc.. that have not yet been exposed to light.
What are Gerontoplasts
They are senescing chloroplasts/rhodoplasts (etc) – once the grana have been unstacked, thylakoid membranes ruptured and plastoglobi form, the gerontoplast has formed.
What are Leucoplasts and what do they differentiate into, include their roles?
Leucoplasts synthesise mononterpenes and they differentiate into:
* Amyloplasts = store starch and detect gravity (geotropism)
* Elaioplasts =store fats
* Proteinoplasts = store proteins and sometimes modify them.
* Tannosomes =synthesise polyphenols and condensed tannins.
What do chloroplasts do?
Site of phototrophic electron transport chain and thus where Δp is generated and used to drive ATP biosynthesis by the H+
-transporting two sector ATPase (EC 7.1.2.2) and where NADPH is synthesised by reverse electron transport.
* Plant immunity –Chloroplasts can trigger apoptosis, the hypersensitive response etc to prevent pathogen
colonisation. Chloroplasts also release salicylic acid, reactive oxygen species, jasmonate, methyl jasmonate etc that act as defence signals.
* Fatty-acid biosynthesis – all plant-cell fatty acids are made in chloroplasts.
* Sugar and starch biosynthesis – chloroplasts generate hexoses from glyceraldehyde 3-phosphate, which are then assembled into sucrose (in the cytoplasm) or starches (amylose, amylopectin)
Starch is the main carbon store in plants and algae – the more
CO2/DIC, the more starch.
Chloroplast membranes
- Outer membrane and inner membrane are each about 7 nm thick.
- Outer membrane contains mainly phospholipids (about half of total) and galactolipids (half) and a small fraction of sulfolipids.
- Inner membrane contains mostly galactolipids (80 %), then
phospholipids, then sulfolipids. Galactolipids interact with light harvesting systems. - Thylakoid membrane is similar to inner membrane with the addition of light harvesting pigments/photosystems etc.
- Protons are translocated from the stroma to the thylakoid lumen to generate Δp.
Chloroplast stroma
- Slightly alkaline – dissolved inorganic carbon, bicarbonate etc.
- Contains plastoglobuli – spherical lipid and protein ‘bubbles’
increase in number during stress and grow as the chloroplast ages into a gerontoplast.
Contain many lipids, vitamins and pigments – mainly “debris”. - Contains starch granules – composition varies.
- Contain enzymes of the Calvin-Benson-Bassham cycle, thus the site of CO2 fixation into 3-phosphoglyerate by ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO)
- Chloroplast ribosomes – less distantly evolved from the original “Cyanobacteria”.
- Contains cpDNA (chloroplast genome): 120-170 kbp in most
Viridiplantae. - Encodes chloroplast ribosomes, tRNAs, electron transport chain (and allied proteins) and core enzymes of the Calvin-Benson-Bassham cycle, as well as chloroplast envelope transport proteins.
Chromoplasts
- Formed from chloroplasts – Fruit ripening.
- Reversible!
- Mostly found in fruits and flowers.
- cpDNA is found in chromoplasts but with a high level
of methylation [cf. epigenetics] - Synthesise and store carotenoids, anthocyanins and
other flavonoids.
- Carotenoids containing oxygen
e.g. zeaxanthin, lutein are xanthophylls – yellow in colour.
- Carotenoids without oxygen
e.g. lycopene, β-carotene are carotenes – mainly red or orange.
- Anthocyanins
e.g. cyanidin 3-glycoside are blue to violet flavonoids
What is the role of cardiolipin in mitochondrial membranes?
The diphosphatidylglycerol ‘head’ can bind two protons, helping to maintain Δp by reducing proton leakage.
