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Summary Lecture 9 - mitochondria and chloroplasts
- Instelling
- Rijksuniversiteit Groningen (RuG)
Summary of lecture and book about mitochondria and chloroplasts by professor Thunnissen.
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Lecture 9 (BOOK) – Mitochondriaand chloroplasts (Thunnissen)
Chapter 14 page 753-762, 774-785 and 806-809
CH14 Energy Conversion: Mitochondria and
Chloroplasts
In eukaryotic cells, most ATP is produced by mitochondria (burn food molecules to produce ATP by
oxidative phosphorylation) or chloroplasts (in plant and green algae only, harness solar energy to
produce ATP by photosynthesis).
The common evolutionary origin of the energy-converting machinery in mitochondria, chloroplasts
and prokaryotes is reflected in the fundamental mechanism that they share for harnessing energy:
chemiosmotic coupling. Chemiosmotic coupling is signifying a link between the chemical bond-
forming reactions that generate ATP and membrane transport processes. The chemiosmotic
processes occurs in two linked stages:
1. High-energy electrons are transferred along a series of electron-transport protein complexes
that form an ETC. Each electron transfer releases a small amount of energy that is used to
pump protons, generating a large electrochemical gradient.
2. The protons flow back down their electrochemical gradient through ATP synthase, which
catalyses the production of ATP from ADP and Pi.
For mitochondria, the first electron carrier is NAD+ which becomes NADH. NADH transfers these
electrons to the inner mitochondrial membrane. There, the electrons enter the ETC. The final step of
the ETC is where the electrons combine with O2 to produce water. As mentioned, during each step in
the ETC, energy is released which causes the drive of H+.
The energy-conversion systems of mitochondria and chloroplasts can be described in similar terms.
Among the crucial constituents that are unique to photosynthetic organisms are two photosystems
that use chlorophyll to capture light energy and power the transfer of electrons. Chloroplasts drive
electron transfer in the direction opposite to that in mitochondria: electrons are taken up from water
to produce O2 and these electrons are used to produce CO2 and water.
The mitochondrion
Mitochondria occupy up to 20% of the cytoplasmic volume of a eukaryotic cell. Mitochondria are
done associated with the microtubular cytoskeleton which determines their orientation an
distribution in different cell types.
, Mitochondria also interact with other membrane systems such as the ER. Contacts between
mitochondria and ER define specialised domains thought to facilitate the exchange of lipids between
the two membrane systems.
The outer and inner mitochondrial membrane
The inner membrane which surrounds the internal mitochondrial matrix compartment, is highly
folded to form invaginations called cristae, whose membranes contain the proteins of the ETC.
Where the inner membrane runs parallel to the outer membrane between cristae, it is called the
inner boundary membrane, and the gap between the inner boundary membrane and the outer
membrane is called the intermembrane space.
The crista membrane is continuous with the inner boundary membrane, and where their membranes
join, tubes or slits arise called crista junctions.
The outer mitochondrial membrane is freely permeable to ions and small molecules (up to 5000
daltons), because it contains many porin molecules which are a special class of beta-barrel-type
membrane proteins. As a consequence the intermembrane space has the same pH and ionic
composition as the cytoplasm.
Unlike the outer mitochondrial membrane, the inner mitochondrial membrane is a diffusion barrier
to ions and small molecules. However, selected ions can pass through it by means of special
transport proteins.
In the inner mitochondrial membrane, the boundary membrane region is thought t ocotillo the
machinery for protein import, new membrane insertion and ensemble of the respiratory chain
complexes. The membranes of the cristae contain the ATP synthase and they also contain the large
protein complexes of the respiratory chain = mitochondrion’s ETC.
The folding of the inner membrane into cristae greatly increases the
membrane area available for oxidative phosphorylation.
Citric acid cycle
Mitochondria can use both pyruvate and fatty acids as fuel. Both of
these fuel molecules are transported across the inner mitochondrial
membrane by specialised transport proteins, after which they are
converted to acetyl CoA by enzymes in the mitochondrial matrix. The
acetyl groups in acetyl CoA are then oxidised in the matrix via the CAC
cycle.
The matrix contains the genetic system of the mitochondrion, including
the mitochondrial DNA and the ribosomes. Mitochondrial DNA is
organised in compact bodies called nucleoids.
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