Cell Biology-Chapter 14 notes

Mitochondria

Regulated transmembrane gradient generation transforms energy

Oxidative phosphorylation occurs in two stages

• electron transport

• proton gradient

Mitochondria and chloroplasts retain features of their bacterial ancestors.

Mitochondria cluster near cell regions with high ATP needs

Features of mitochondria (study these specifically)

• matrix (elaborate on this one)

• inner membrane (elaborate on this one)

• outer membrane (elaborate on this one)

• intermembrane space (elaborate on this one)

Catabolism produces activated carriers that deliver chemical energy to mitochondria

Transfer of e- releases sufficient energy to power proton gradient formation

Molecular traffic in and around mitochondria

Three complexes of the e- transport chain move protons with e- energy

1. Complex I (NADH: Ubiquinone Oxidoreductase):

   • Function: This complex accepts electrons from NADH, which are then transferred to ubiquinone (coenzyme Q). In the process, it pumps protons from the mitochondrial matrix into the intermembrane space, contributing to the proton gradient.

   • Components: It contains several subunits, including flavin mononucleotide (FMN) and iron-sulfur (Fe-S) clusters.

2. Complex II (Succinate: Ubiquinone Oxidoreductase):

   • Function: This complex accepts electrons from succinate (via the Krebs cycle) and transfers them to ubiquinone. Unlike Complex I, it does not pump protons across the membrane.

   • Components: It includes succinate dehydrogenase and iron-sulfur clusters.

3. Complex III (Ubiquinol: Cytochrome c Oxidoreductase):

   • Function: This complex transfers electrons from reduced ubiquinone (ubiquinol) to cytochrome c. During this process, it pumps protons from the matrix into the intermembrane space, further contributing to the proton gradient.

   • Components: It contains cytochromes b and c1, as well as an iron-sulfur protein.

4. Complex IV (Cytochrome c Oxidase):

   • Function: This complex transfers electrons from cytochrome c to oxygen, the final electron acceptor, forming water. It also pumps protons from the matrix into the intermembrane space, enhancing the proton gradient.

   • Components: It includes cytochromes and a3, and copper centers.

   • complexes 1,3, and 4 actually pump protons

Electrical and chemical gradients are aligned at the inner mitochondrial membrane

• the matric facing side of the inner mitochondrial membrane has a negative charge

• the electrical draw of the negative matrix leaflet works with the chemical gradient of protons (pH gradient)

• the electrochemical gradient creates a strong proton-motion force.

ATP synthase turns proton flow into mechanical energy into chemical energy

• the flow of protons through the synthase complex turns central stalk

• turning of the stalk causes cyclical conformational changes of the F1 ATPase head that presses inorganic phosphate and ADP together to form ATP

Proton flow at the inner mitochondrial membrane is reversible

• If ATP is high and proton concentration in the intermembrane space is low, ATP synthase can run backwards to re-establish the gradient

• in other cell/organelles, ATPases are highly similar to ATP synthase

The proton gradient also serves as a power source to coupled transport

• proton gradients between intermembrane space can be used to power active transport of intermediates needed for mitochondrial function

• pyruvate is transported to become a proton-pyruvate symporter

• inorganic phosphate is also brought into the matric via a proton-powered symporter

• ADP is brought into the mitochondrial matric via an ADP-ATP antiporter

During electron transfer, water can readily donate and accept protons

Orientation of electron transport chain complexes facilitates proton movement

free energy decreases and redox potential increased as e- are exchanged

Ubiquinone is a small molecule integral membrane carrier

Cytochrome c is a protein electron carrier that binds heme-associated iron

Cytochrome c oxidase transfers electrons to their final destination- oxygen

• cytochrome c oxidase is complex IV of the electron transport chain

• active site has heme and copper, which hold on to oxygen

• the energy of transferring electrons from cytochrome c to oxygen moves protons in intermembrane space

Uncoupling agents allow protons to bypass ATP synthase

• UCP-1 is a proton leak channel

• small molecule uncoupling agents are called ionopores

• protons passing across the membrane through UCP-1 release their energy as heat.

• Brown fat

Chloroplast and Photosynthesis

photosynthesis occurs in two stages- both in chloroplasts

• Stage 1: light reactions

• Stage 2: light-independent reactions

Chlorophylls absorb light in the violet/blue and red ranges

Each photosystem has a reaction center surrounded by antenna complexes

Resonance energy transfer moves energy between nearby molecules

The special pair transfers a high-energy electron to a carrier

• linked pair of chlorophyll molecules- special pair

• special pair can transfer an electron to another acceptor molecule

• the special pair can absorb light directly, but it frequently receives resonance energy

• electrons lost by the special pair are replaced by the electrons from water

Photosystem II feeds electrons to a proton pump that powers ATP synthesis

Photosystem I feeds electrons to an NADPH-producing enzyme

Serial movement and charging of e- powers ATP and NADPH production

Redox potentials vary over the course of the light reactions

Calvin cycle occurs in Stroma of chloroplast

Rubisco catalyzes carboxylation of ribulose, 1,5- biphosphate (step 1)

The Calvin cycle uses energy captured by the light reactions to produce sugars

Sugars and fats are made by and often stored inside chloroplasts

Plant cells rely on chloroplasts and mitochondria for metabolites and chemical energy

Water-splitting photosynthesis is the source of atmospheric oxygen