Mitochondria and peroxisomes Flashcards
What is the definition of mitochondria?
An organelle found in large numbers in most cells, in which the biochemical processes of respiration and energy production occur. It has a double membrane; the inner part being folded inwards to form layers (cristae).
Describe the structure of the mitochondrion
- Mitochondria may be rod shaped, thread-like or spherical, with diameters of 0.1 – 0.5 mm and lengths of 1-2 mm.
- All mitochondria have an inner and outer phospholipid membrane making up the envelope. The outer membrane is smooth, and the inner membrane is folded into cristae (Fold of inner mitochondrial membranes, increases surface area where reactions of the electron transfer chain can take place), giving it a large surface area. This increases the amount of ATP synthesis.
- Embedded in the inner membrane are proteins that transport electrons, and protein channels associated with ATP synthase enzyme that allow protons to diffuse through them.
- Between the inner and outer membranes of the envelope, is an intermembrane space.
- The mitochondrial matrix enclosed by the inner membrane, is semi-rigid and gel-like; it contains mitochondrial ribosomes, looped mitochondrial DNA and enzymes for the link reaction and Krebs cycle.
Describe how the structure of the mitochondrion enables it to carry out its function (matrix, outer and inner membrane and intermembrane space)
• The part of the mitochondria enclosed by the inner mitochondrial membrane which contains enzymes for the Krebs cycle and the link reaction.
• The matrix is where the link reaction and the Krebs cycle take place. It contains:
• Enzymes that catalyse the stages of these reactions.
• Molecules of coenzymes NAD (nicotinamide adenine dinucleotide) and FAD (flavine adenine dinucleotide).
• Oxaloacetate – the four-carbon compound that accepts the acetyl group from the link reaction.
• Mitochondrial DNA (DNA present within the matrix of mitochondria) – some of which codes for mitochondrial enzymes and other proteins.
• Mitochondrial ribosomes, structurally similar to prokaryotic ribosomes, where these proteins are assembled.
Other roles:
• Calcium homeostasis – muscle contraction.
• Apoptosis
• Site for antiviral signalling – involved in AIDS.
Mitochondria are transported on the cytoskeleton microtubules – binding to motor proteins to move around.
The outer membrane
• The membrane that separates the contents of the mitochondrion from the rest of the cell, creating a cellular compartment with ideal conditions for aerobic respiration.
• The phospholipid composition of the outer membrane is similar to that of membrane around other organelles in eukaryotic cells. It contains proteins, some of which form channels or carriers that allow the passage of molecules, such as pyruvate, into the mitochondrion.
The inner membrane
• The inner most of the two mitochondrial membranes. Seperates the mitochondrial matrix from the intermembrane space. It is the site where the electron transport chain takes place.
• The lipid composition of the inner membrane differs from that of the outer membrane. The lipid bilayer is less permeable to small ions such as hydrogen ions (protons) than is the outer membrane.
• The folds, cristae, in the inner membrane give a large surface area for the electron carriers and ATP synthase enzymes embedded in them.
• The electron carriers are protein complexes arranged in electron transport chains. Electron transport chains are involved in the final stage of aerobic respiration, oxidative phosphorylation.
The intermembrane space
• The intermembrane space between the outer and inner layers of the mitochondrial envelope is also involved in oxidative phosphorylation.
• The inner membrane is in close contact with the mitochondrial matrix, so the molecules of reduced NAD and FAD can easily deliver hydrogens to the electron transport chains.
Describe the 4 functions of mitochondria
- Haem synthesis
- Neurotransmitter synthesis, e.g. glutamate
- Nucleotide synthesis
- ATP synthesis
When does mitochondrial morphology change?
- Apoptosis
- Ca2+ transfer
- Cell cycle
- Nutrient starvation
Describe the production of mitochondrial proteins by two separate genetic systems
On image
Describe mitochondrial DNA
- Mitochondrial chromosomes are circular, as in bacteria.
- Mitochondrial genes are inherited cytoplasmically from the mother. Materially inherited. However, there is some evidence of some paternal inheritance.
- The mitochondrial DNA (‘mtDNA’) is located in the matrix. It has 13 Gene’s.
- Mitochondria also contain their own ribosomes.
- Human mtDNA contains 16,569 base pairs that encode 2 rRNAs, 22 tRNAs and 50 proteins.
- Other mitochondrial proteins are encoded in the nucleus, translated in the cytoplasm and imported into mitochondria.
- The mitochondrial genetic code differs from the standard nuclear code. For example, UGA is a stop codon in the nucleus but codes for tryptophan in mitochondria; conversely A GA and AGG usually code for arginine but are stop codons in the mitochondrial translation system.
- Mutations in mitochondrial give rise to heterogeneous diseases – covered more in the genomics module.
Describe how mitochondria are transported on the cytoskeleton microtubules
- Mitochondria are transported on microtubules by motor proteins Dynein and Kinesin.
- Dynein facilitates transport towards the minus end of microtubules and kinesin towards the plus end
- Milton and Miro are adaptor proteins on the outer surface of mitochondrial that facilitate binding to Dynein and Kinesin
- Mitochondria are transported on microtubules in cells. In neurons this is important for mitochondria to be delivered from the soma along axon projections to synapses where they are needed for e.g. energy in form of ATP or neurotransmitter synthesis
Describe endosymbiosis theory for the origin of mitochondria
On image
Describe localisation of mitochondria near sites of high ATP utilisation
- In a cardiac muscle cell, mitochondria are located close to the contractile apparatus, in which ATP hydrolysis provides the energy for contraction.
- In a sperm, mitochondria are located in the tail, wrapped around a portion of the motile flagellum that requires ATP for its movement.
Describe Beriberi
- A neurologic and cardiovascular disorder, is caused by a dietary deficiency of thiamine (also called vitamin B1).
- Thiamine pyrophosphate is a prosthetic group for two mitochondrial enzymes: pyruvate dehydrogenase and α-ketoglutarate dehydrogenase.
- Patients suffering from beriberi have higher than normal serum levels of these enzymes’ substrates (pyruvate and α-ketoglutarate).
- Damage to the peripheral nervous system.
- Pain in the limbs.
- Weakness of the musculature.
- Distorted skin sensation.
- The heart may be enlarged and the cardiac output inadequate.
Describe the structure and function of peroxisomes
- Small membrane-enclosed organelle that contains enzymes that degrades lipids and destroy toxins.
- Approximately 0.2 to 1 µm in diameter.
- Peroxisomes are surrounded by a single membrane – as they are not made by endosymbiosis.
- Contain no DNA or ribosomes.
- Most peroxisomal proteins are encoded in the nucleus, translated in the cytoplasm and then imported.
- At least some peroxisomal membrane proteins originate in the ER.
- Carry out oxidative processes.
- All peroxisomes contain enzymes that use molecular oxygen to oxidise various substrates.
- Reactions produce hydrogen peroxide (H2O2), which is broken down to water by the enzyme catalase. Stops damage to macromolecules’ in the cell
- Peroxisomes are important for the metabolism of long-chain fatty acids.
- Peroxisomes perform critical steps in the synthesis of certain lipids, e.g. cholesterol, plasmalogens, bile acids.
- Breakdown of excess purines (AMP, GMP) to uric acid.
- E.g. Zellweger syndrome, is caused by mutations that block peroxisomal protein import. Individuals with the disorder are born with severe abnormalities in their brain, liver, and kidneys. Most do not survive past the first six month of life.
