Biology Chapter on Mitochondria and Energy Metabolism

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Questions and Answers

What is the main source of metabolic energy in animal cells?

Photosynthesis in mitochondria

Oxidative breakdown of glucose and fatty acids (correct)

Oxidative breakdown of proteins

Reduction of NAD+ and FAD

Where does glycolysis occur in the cell?

Cell membrane

Mitochondrial matrix

Cytoplasm (correct)

Nucleus

What is produced from the oxidation of acetyl CoA during the citric acid cycle?

ATP

Fatty acids

NADH and CO2 (correct)

Lactic acid

What is the role of the proton gradient in oxidative phosphorylation?

It drives ATP synthesis Signup and view all the answers

Why is the inner mitochondrial membrane impermeable to most ions and small molecules?

To maintain the proton gradient Signup and view all the answers

What characteristic of the outer mitochondrial membrane distinguishes it from the inner membrane?

It contains porins for molecule passage Signup and view all the answers

What is believed about the evolutionary origin of mitochondria?

They evolved from a symbiotic relationship with bacteria Signup and view all the answers

Which molecule is a product of the electron transport chain during oxidative phosphorylation?

ATP Signup and view all the answers

What is the primary composition of the mitochondrial genome?

Circular DNA molecules Signup and view all the answers

How many proteins does the human mitochondrial genome encode for oxidative phosphorylation?

13 proteins Signup and view all the answers

Which components are synthesized in the cytosol for the biogenesis of the mitoribosome?

Mitoribosomal proteins Signup and view all the answers

Which proteins are involved in the import of mitochondrial ribosomal proteins?

TOM and TIM proteins Signup and view all the answers

What is the approximate size of the human mitochondrial genome?

16 Kb Signup and view all the answers

What role does cardiolipin play in the mitochondria?

It enhances the efficiency of oxidative phosphorylation. Signup and view all the answers

Where are peroxisome proteins primarily synthesized?

In free ribosomes in the cell cytoplasm. Signup and view all the answers

Which type of gene encoding is solely associated with the mitochondrial DNA?

Essential proteins for oxidative phosphorylation Signup and view all the answers

What role do nuclear genes play concerning mitochondrial functions?

They encode proteins necessary for transcription and translation. Signup and view all the answers

What type of membrane structure do peroxisomes possess?

Single membrane. Signup and view all the answers

What type of molecules can be transferred between the ER and mitochondria?

Phospholipid molecules. Signup and view all the answers

In the context of mitochondrial function, what do the terms mt-LSU and mt-SSU refer to?

They are subunits of the mitoribosome. Signup and view all the answers

How are most of the enzymes found in peroxisomes categorized?

Typical eukaryotic proteins. Signup and view all the answers

Which of the following statements about cardiolipin is true?

It is critical for maintaining the structure of the inner mitochondrial membrane. Signup and view all the answers

What is a byproduct of oxidative reactions carried out in peroxisomes?

Hydrogen peroxide. Signup and view all the answers

What determines the number of peroxisomes in human cells?

Cell activity levels. Signup and view all the answers

What is the primary role of Matrix processing peptidase (MPP) in mitochondrial protein processing?

To cleave the presequence of proteins. Signup and view all the answers

What is a primary characteristic of mitochondrial diseases?

They have no cure. Signup and view all the answers

How are multipass transmembrane proteins targeted to the inner membrane of mitochondria?

Using multiple internal signals. Signup and view all the answers

Which technique is employed to avoid transmission of mitochondrial diseases to offspring?

Mitochondrial replacement Signup and view all the answers

What facilitates the transport of proteins to the SAM complex for outer membrane insertion?

Tim9-Tim10 chaperones. Signup and view all the answers

What type of proteins are recognized by specific chaperones after exiting the Tom complex?

Proteins destined for intermembrane space only. Signup and view all the answers

What is the role of the Tom complex in mitochondrial protein internalization?

It directs protein translocation through the outer membrane. Signup and view all the answers

What is the function of the presequences in mitochondrial protein targeting?

They guide proteins to specific mitochondrial compartments. Signup and view all the answers

Where are most lipids in mitochondrial membranes synthesized before being imported?

In the Endoplasmic Reticulum (ER). Signup and view all the answers

How do mitochondrial matrix proteins cross the inner membrane?

Via the Tim23 channel, assisted by electrochemical potential Signup and view all the answers

Which translocase is involved in the insertion of mitochondrial membrane proteins synthesized in the matrix?

Oxa1 translocase. Signup and view all the answers

What characterizes the majority of outer membrane proteins that cross the Tom complex?

They are barrel-shaped β proteins. Signup and view all the answers

What type of proteins are synthesized by free ribosomes for mitochondria?

Nuclear encoded proteins Signup and view all the answers

What is the primary challenge in the import of proteins into mitochondria?

The existence of a double membrane system Signup and view all the answers

What unique feature does cardiolipin have that differentiates it from other phospholipids?

Contains four fatty acid chains. Signup and view all the answers

What happens to proteins that contain a transmembrane sequence during their mitochondrial import?

They exit laterally from the Tim23 channel into the inner membrane. Signup and view all the answers

What is the primary function of catalase found in peroxisomes?

To break down hydrogen peroxide Signup and view all the answers

Which fatty acids are primarily oxidized in peroxisomes?

Branched and very long-chain fatty acids Signup and view all the answers

Which protein complex first binds to the peroxisomal membrane when importing PTS1 proteins?

Pex5/protein complex Signup and view all the answers

What are the amino acid sequences that target internal peroxisomal proteins?

Ser-Lys-Leu for PTS1 and 9 aa sequence for PTS2 Signup and view all the answers

What role do peroxins play in the function of peroxisomes?

Acting as cytosolic receptors for matrix protein import Signup and view all the answers

What is the function of plasmalogens synthesized in peroxisomes?

Components in membranes of specific tissues Signup and view all the answers

How are some peroxisomal membrane proteins transported into peroxisomes?

Recognized by Pex19 and inserted into the membrane Signup and view all the answers

Which mechanisms are involved in the formation of new peroxisomes?

Budding from the ER and division Signup and view all the answers

Study Notes

Mitochondria and Peroxisomes

Mitochondria are responsible for generating most of the useful energy from the breakdown of lipids and carbohydrates.
Peroxisomes contain enzymes involved in various metabolic pathways such as fatty acid breakdown and photorespiration.
Proteins destined for mitochondria and peroxisomes are mainly synthesized on free ribosomes in the cytosol, except for membrane proteins of the peroxisome.
Proteins are imported into their target organelles in the form of complete polypeptide chains.
Mitochondria have their own genomes that contain genes which are transcribed and translated in the organelle itself.
Mitochondrial membranes are organized into inner and outer mitochondrial membranes separated by an intermembrane space. The inner membrane forms folds called cristae.
The mitochondrial matrix contains the mitochondrial genetic system and enzymes responsible for central reactions of oxidative metabolism.
Oxidative breakdown of glucose and fatty acids generates ATP through oxidative phosphorylation.
The oxidation of acetyl CoA to CO2 is coupled with the reduction of NAD+ and FAD to NADH and FADH2, respectively.
Most energy from oxidative metabolism is produced by oxidative phosphorylation in the inner mitochondrial membrane.
High-energy electrons from NADH and FADH2 are transferred to molecular oxygen through membrane transporters.
Mitochondria have a double membrane system.
Mitochondria fuse (fusion) with each other and divide (fission).
The inner membrane of the mitochondria is impermeable to most ions and small molecules.
Inner mitochondrial membrane is responsible for oxidative phosphorylation.
Outer mitochondrial membrane is permeable to small molecules.
The outer mitochondrial membrane is permeable to small molecules.

Mitochondria

Organization and function of mitochondria
Genetic system of mitochondria
Mitochondrial diseases
Protein internalization and mitochondria formation
Mitochondrial lipids

Genetic System of Mitochondria

Mitochondria are thought to have evolved from bacteria that developed a symbiotic relationship living inside larger cells (endosymbiosis).
The mitochondrial genome is made of circular DNA molecules in multiple copies.
Mitochondrial genomes vary considerably in size between different species.
Mitochondria contain their own genes, including those that produce the rRNAs and tRNAs necessary for oxidative phosphorylation.
Other proteins encoded by nuclear genes, which may have been transferred to the nucleus are also found in mitochondria.

Mitochondrial Diseases

Mitochondrial DNA (mtDNA) undergoes mutations that are often harmful to the organelle.
Almost all mitochondria in the fertilized egg are supplied by the oocyte, and germline mutations in mtDNA are generally transmitted by the mother.
Mitochondrial inheritance
Homoplasmy: Multiple identical copies
Heteroplasmy: a mix of wild type and altered mtDNA.
The proportion of mutated mitochondria needed for disease manifestation will vary depending on several factors like the mutation itself, the tissue affected and environmental factors.

Peroxisomes

Peroxisome functions
Assembly of peroxisomes
They are small organelles (0.1-1 µm) surrounded by a membrane.
Human cells contain between 100 and 1000, depending on cell activity
They contain enzymes involved in various metabolic reactions.
They do not have their own genome, all proteins are synthesized in free ribosomes in the cytoplasm.
They can be replicated by division or generated by the cell
Some peroxisome proteins resemble prokaryotic proteins, reflecting an endosymbiotic origin.
Oxidative reactions in peroxisomes produce hydrogen peroxide.
Peroxisomes contain catalase, to break down hydrogen peroxide.
Oxidation of uric acid, amino acids and some fatty acids occurs in peroxisomes.
Peroxisomes are involved in synthesis of plasmalogens.

Protein Internalization

Mammalian mitochondria contain around 1,500 proteins that are encoded by the nuclear genome of the cell.
The proteins encoded by these nuclear genes are synthesized by free ribosomes in the cytoplasm
Proteins needed for oxidative phosphorylation and mitochondrial metabolism are also encoded by the nuclear genome.
Mitochondrial proteins are transported as proteins or across double membrane systems through various protein complexes. Proteins targeted to specific regions within the mitochondria have different signals that direct them.
Mitochondrial matrix proteins cross the inner membrane through Tim23 and are directed by electrochemical potential established across this membrane.
Other proteins will have other sequences that allow them to be inserted into specific regions within the inner membrane.

Protein Targeting of Peroxisomes

Internal peroxisomal proteins originate from the cytosol's free ribosomes, whereas most transmembrane proteins derive from the endoplasmic reticulum (ER).
The membrane proteins of peroxisomes are called peroxins (Pex).
Most internal proteins are recognized and targeted to peroxisomes by short amino acid sequences (signals) located at the carboxyl or amino termini, referred to as PTS1 and PTS2 respectively.
These are recognized by cytosolic receptors (peroxins, such as Pex5 & Pex7) in the cytosol.
The receptor-protein complexes are transported to the peroxisomal membrane, where they are bound to specific translocons within the membrane to allow proteins to be inserted into the membrane itself, and are later recycled.
Some peroxisomal membrane proteins are also synthesized on free ribosomes, instead of being transported from the ER
These proteins have a different targeting signal (mPTS) that is recognized by the Pex19 protein and then guided into the lipid bilayer of the inner membrane of the peroxisome.

Lipid transfer between ER and mitochondria

Lipid transfer between the ER and mitochondria occurs at contact points mediated by phospholipid transfer proteins.
The proteins transfer lipids from ER membranes to the mitochondrial membranes and move through the aqueous cytosol.
Other components, for example, phospholipid transfer proteins, extract phospholipid molecules from the ER membrane to the mitochondrial outer membrane.

Formation of New Peroxisomes

Peroxisomes can be generated by budding, division or growth of pre-existing peroxisomes.
The development of new peroxisomes involves specific steps, for example, budding from the ER (endoplasmic reticulum).
The vesicles formed do not have a protein coat and do not first pass through the Golgi apparatus.
Vesicles that bud from the peroxisomal region of the ER contain proteins and peroxins, necessary for the assembly and function of the peroxisome.

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Test your knowledge on the function and structure of mitochondria, including processes like glycolysis and oxidative phosphorylation. The quiz covers key concepts such as the mitochondrial genome, proton gradients, and evolutionary origins. Perfect for biology students aiming to understand cellular energy production.