Mitochondria and Peroxisomes

Question 1

What are the features of the double membranes of a mitochondria?

Answer 1

Outer membrane: lipid bilayer is permeable to small molecules and contains proteins called porins that form channels which allow for free diffusion of small molecules.
Inner membrane: lipid bilayer not permeable to small molecules. It forms cristae which extend into the matrix.

Question 2

What are the spaces found in a mitochondria?

Answer 2

• Intermembrane space between outer and inner membrane

- Innermost space is the matrix

Question 3

What occurs in the matrix?

Answer 3

Most reactions take place at the matrix

Question 4

What are cristae and why are there so many?

Answer 4

Cristae are numerous folds that are formed by the inner membrane.
Cristae are extremely dense and convoluted.
There are so many because the membranes are packed with proteins which are used for the electron transport chain.

Question 5

Why do mitochondria look elongated in images produced by dimension electron tomograms?

Answer 5

Mitochondria can change shape dramatically (known as mitochondrial plasticity) and not alway a micrometer especially because they move along microtubules.

Question 6

Mitochondrial morphology changes during what processes?

Answer 6

• Apoptosis
• Ca2+ transfer
• Cell cycle
• Nutrient starvation

Question 7

What is nutrient starvation?

Answer 7

When cells are starved of nutrients and this causes mitochondria to spread out and fuse to become longer, but they are able to split up again.
*it is thought that elongation and fusion stops mitochondria from being broken down by autophagy as they are too long to be engulfed by macrophages.

Question 8

How many proteins does a mitochondria have?

Answer 8

1500-3000 different proteins.

Question 9

What is the pathway for most mitochondrial proteins?

Answer 9

• most of which are encoded by genes are found in nuclear DNA
• DNA will be transcribed forming RNA
• RNA will be translated into protein in cytosol
• Proteins often in precursor form having specific signal sequences will be imported into mitochondria

Question 10

How are mitochondria able to encode,translate and transcribe their own proteins?

Answer 10

Mitochondria contain their own DNA and ribosomes so they are able to do this. Their DNA is called mtDNA and is found in the matrix.

Question 11

What are the features of human mtDNA?

Answer 11

Human mtDNA contains a small number (16,569) of base pairs that encode 2 rRNAs, 22 tRNAs and 50 proteins (some used in the electron transport chain.
Mitochondrial chromosomes are circular.

Question 12

How are mitochondrial diseases passed through?

Answer 12

They are passed through the mother as sperm only contain little amount of mitochondria.
Mitochondrial genes are inherited cytoplasmically (transmissions of genes occurs outside the nucleus)
This is extracellular inheritance and refers to any genes that are passed on from structures that are not in the nucleus (mitochondria are found outside the nucleus and is passed through the mother).

Question 13

How does the mitochondrial genetic code differ from the standard nuclear code?

Answer 13

Different codons code for different amino acids e.g. UGA is recognised as a stop codon in the nucleus, but in mitochondria it codes for tryptophan.

Question 14

What is the endosymbiosis theory for the origin of mitochondria?

Answer 14

Mitochondria were evolved from the phagocytosis of a bacterium that evolved 1-2 billion years ago:
-Ancestral anaerobic eukaryote (can’t use molecular oxygen to produce ATP) had engulfed an anaerobic bacterium
-The bacterium carries out efficient metabolism and has bacterial genome unlike the eukaryote and started to increase the number of communities
-The engulfed bacterium has evolved to become mitochondria and forms either a non-photosynthetic eukaryote or photosynthetic eukaryote.
-The photosynthetic eukaryote must also have had to engulf a chloroplast also.
This all leads to the formation of multicellular organisms.
LOOK AT DIAGRAM

Question 15

What are the different functions of mitochondria?

Answer 15

• Breaks down fatty acids into acetyl CoA (to get energy from fatty acids).
• Decarboxylation of pyruvate to acetyl CoA, used in krebs cycle
• Citric acid cycle, an important metabolic pathway generating equivalents for ATP generation and metabolic intermediates for anabolic pathways (which lead to the production of more complicated molecules), uses energy.
• Oxidative phosphorylation, the formation of ATP, reducing equivalents derived from the breakdown of acetyl CoA and electrons combine to with O2 to form water.
• Thermogenesis
• Apoptosis

Question 16

What is thermogenesis?

Answer 16

This is where energy is stored in ATP is used to generate heat (depending on the species and tissue).
-important for infants and animals that have to survive
in extremely low temperatures (such as bears that
hibernate).
-Brown fat cells are brown because they contain many
mitochondria to generate heat, this could be used to
help people to lose weight and prevent the epidemic
of type 2 diabetes.

Question 17

How do mitochondria play a role in apoptosis?

Answer 17

Cells receive certain signal that tell them to die, causing outer membrane of mitochondria to become more permeable.
This allows cytochrome C from the intermembrane space to enter cytoplasm and activate enzymes need to degrade the cell.

Question 18

LOOK AT NOTES ON GLYCOLYSIS, LINK, CITRIC ACID CYCLE, ELECTRON TRANSFER CHAIN AND OXIDATIVE PHOSPHORYLATION

Answer 18

LOOK AT NOTES ON GLYCOLYSIS, LINK, CITRIC ACID CYCLE, ELECTRON TRANSFER CHAIN AND OXIDATIVE PHOSPHORYLATION

Question 19

How are mitochondria localised near the sites of high ATP utilisation? (SPERM and MUSCLE)

Answer 19

-During the development of the flagellum of the sperm
tail, microtubules wind helically around the axoneme,
which help localise the mitochondria near the tail.
The microtubules then disappear and the
mitochondria fuse.
-Muscles usually require a large source of ATP for the transport of Ca2+ and contraction. Normally these needs are met via glycolysis, but muscles that are frequently used have extremely extensive mitochondria which extend throughout the myofibrils- these a red in colour due to a large blood flow and myoglobin (stores oxygen) (FAST TWITCH muscles).

Question 20

What is Beriberi?

Answer 20

A neurologic and cardiovascular disorder, caused by dietary deficiency of thiamine (vitamin B1)
- Thiamine pyrophosphate is a prosthetic group (part of
a protein that is not encoded by a genome) for two
mitochondrial enzymes;
- Pyruvate dehydrogenase: important in conversion of
pyruvate
- α-ketoglutarate dehydrogenase: enzyme acts in citric acid cycle
- Patients suffering from beriberi have higher than
normal serum levels of these enzymes’ substrates
(pyruvate and α-ketoglutarate) – not broken down
because enzymes can’t function
- Damage to the peripheral nervous system
- Pain in the limbs – muscle weakness
- Weakness of the musculature
- Distorted skin sensation
-The heart may be enlarged and the cardiac output
inadequate

Question 21

What is the structure and function of peroxisomes?

Answer 21

• Often discussed with mitochondria because both responsible for breaking down of fatty acid.
• Mitochondria break down fatty acids that are short to medium in size up to 20 c atoms
• Peroxisome responsible for breakdown of very long fatty acids.
• Peroxisome have single membrane and have a urate oxidase crystalline core
• Approximately 0.2 to 1 µm in diameter
• Contain no DNA or ribosomes

Question 22

How does peroxisome biogenesis occur?

Answer 22

• Membrane of peroxisome is derived by budding of vesicle from ER.
• Vesicles will carry some membrane proteins found in membrane of peroxisome originating from ER
• Other proteins like the matrix proteins are imported from the cytosol.
• So encoded in nucleus, translated entirely by ribosomes in cytoplasm and imported in the peroxisomes.
• Peroxisome can grow by acquiring more membranes and can split by fission.
  LOOK AT DIAGRAM?

Question 23

What are the functions of peroxisomes?

Answer 23

• Reactions produce hydrogen peroxide (H2O2) as by product – can cause a lot of damage by reacting with lipids, DNA, proteins so it’s broken down to water and oxygen by the enzyme catalase found in peroxisome.
  NOTE: H2O2 can form in mitochondria as an accidental by-product.
• All peroxisomes contain enzymes that use molecular oxygen to oxidize various substrates
• Peroxisomes are important for the metabolism of long-chain fatty acids
• Peroxisomes perform critical steps in the synthesis of certain lipids, e.g. cholesterol and bile acids.
• Breakdown of excess purines (AMP, GMP) to uric acid