Mitochondria and Peroxisomes

Question 1

Structure of a mitochondrion

Answer 1

It has a double membrane (proteins embedded into the the membranes):
> outer membrane
> inner membrane- folded to maximise the surface. The larger the surface, the more enzymes in the mitochondria.
Cristae- the folded membrane
Matrix- innermost lumen
Inter membrane space- space between outer and inner membranes.

Question 2

Mitochondria Associate with Microtubules

Answer 2

Mitochondria is the Microtubules to move along them

Sometimes the mitochondria temporarily elongates along the Microtubules.

Question 3

Mitochondrial plasticity

Answer 3

It’s structure is not static as it undergoes many different changes in shape

Question 4

When does mitochondrial morphology changes?

Answer 4

Apoptosis
Ca2+ transfer- mitochondria stores a lot of Ca2+
Cell cycle
Nutrient starvation

Question 5

Nutrient starvation induced mitochondrial tabulation

Answer 5

When starved, the mitochondria fuse with each other and elongate. So They cannot be enveloped by the ER and targeted for degradation in the autophagosomes through the autophagy pathway.

Question 6

Production of mitochondrial proteins

Answer 6

Two separate genetic systems:
> mitochondria have their own genome- codes for a small no. of key proteins. The ribosomes in the mitochondria synthesise the proteins.
> vast majority of the proteins, that re specific to mitochondria, are encoded by the chromosomes in the nucleus. These proteins are synthesised in the cytoplasm and are then imported into the mitochondria.

Question 7

Mitochondrial DNA

Answer 7

Circular
Inherited cytoplasmically from the mother
mtDNA is located in the matrix

Question 8

The mitochondrial genetic code differs from the standard nuclear code

Answer 8

E.g. UGA is a stop codon in the nucleus but codes for tryptophan in mitochondria;
AGA and AGG usually code for arginine but are stop codons in the mitochondrial translation system.

Question 9

Function of mitochondria

Answer 9

> breakdown of fatty acids to acetyl CoA
Decarboxylation of pyruvate (3C) to acetyl CoA ( 2C)
Citric Acid Cycle ( in the matrix)- a key precursor to the pathway that generate ATP
oxidative phosphorylation ( by enzymes in the inner membrane)- formation of ATP and water from ADP, Pi, reducing equivalents and Oxygen
thermogenesis- generation of heat
cell apoptosis- when a cell receives a signal for cell death, mitochondria release specific proteins ( cytochrome C) which interacts with other components in the cell to keep the apoptosis process going.

Question 10

Localisaztion of mitochondria near sites of high ATP utilisation

Answer 10

Mitochondria are closely associated with muscle fibres as they deliver ATP to the machinery that helps catalyse muscle contraction.
Sperm tail- mitochondria elongated and wrapped around the flagellar axoneme- required for the movement of the sperm as a lot of ATP is required.

Question 11

Beriberi

Answer 11

A neurological and cardiovascular disease that effects mitochondria as it is cause by deficiency of Vitamin B1. Vitamin B1 gives rise to thiamine prophosphate which is a prosthetic group of two mitochondrial enzymes, pyruvate dehydrogenase and alpha- ketoneglutarate dehydrogenase. Beriberi pateints have higher than normal conc. of these enzymes. These cannot be metabolised so they end up outside the cell.

Question 12

Characteristics of Peroxisomes

Answer 12

Single membrane
Crystalline material (contains the enzyme catalase) characterise the Peroxisomes.

Question 13

Peroxisome biogenesis

Answer 13

• originate from the ER through a process called budding. Peroxisomal precursor vesicle formed that contains some of the membrane proteins that are typical of peroxisomes, e.g. specific proteins that catalyze protein import.
• peroxisomal proteins that are synthesised in the cytoplasm are imported into the peroxiome.
• they acquire additional membrane and through fission, can generate daughter peroxisomes. Growth by uptake of specific peroxisomal proteins and lipids from cytosol.

Question 14

Structure and function of Peroxisomes

Answer 14

• They metabolise very long- chain fatty acids
• They perform steps in the synthesis of certain lipids e.g. cholesterol, plasmalogens, bile acids.
• Excess purine broken down ( AMP, GMP) to Uric acid
• Peroxisomes contains enzymes that use oxygen to oxidise substrates.
• Catalase breakdown hydrogen peroxide into water
• 0.2 - 1 micrometer in diameter
• No DNA or ribosomes
• Most peroxisomal proteins are encoded in the nucleus and synthesised in the cytoplasm.
• Some peroxisomal proteins are from the ER.