Electron Transport Chains

SNAB Biochemistry support topic 16

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Electron transport chains

Respiration and photosynthesis involve redox reactions

(where electrons are transferred from one molecule to another)

In cells, electrons are transferred indirectly via electron transport chains

In respiration and photosynthesis ATP (adenosine triphosphate) is synthesised

(using energy released when electrons are transferred from one electron carrier molecule to another)

These electron transport chains are found on the inner membrane of the mitochondria and on the thylakoid membrane inside chloroplasts

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Electron flow between metals

Zinc is oxidised by giving electrons to copper ions

Zinc + copper ions = zinc ions + copper

A Daniell cell uses this reaction to produce an electric current by keeping the zinc and copper ions apart, forcing electrons to flow along a wire

Respiration and Photosynthesis

In respiration and photosynthesis, the electron transfer between the reactants is carried out in a series of redox reactions

This flow of electrons can work in a similar way to the flow of electrons in an electrochemical cell

The electron flow in this case is used to synthesise ATP

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Electron Transport Chains

At the start of an electron transport chain, an electron is passed to the protein carrier in the chloroplast or mitochondria

The electron comes from chlorophyll in photosynthesis and a coenzyme in respiration

The electron is then passed along protein carriers in the membrane

Each carrier becomes reduced and oxidised as it passes the electron along

At the end of the chain, the electron reduces a final acceptor

Energy is released as it is passed along, and the electron loses energy

Energy from the electron transport chain is used to transport hydrogen ions across the membrane. Hydrogen ions are pumped across by proteins

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ATP Synthesis

Pumping positive hydrogen ions in one direction creates an electrochemical gradient across the membrane.

The charged ions can only diffuse back through special ion channels

These channels are associated with the enzyme ATPase

(which catalyses the synthesis of ATP)

When hydrogen ions pass through the ATPase channel, ADP is joined with a phosphate group to form ATP

Question:

What reaction involves the transfer of electrons from one molecule to another?

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