Eukaryotic cells make ATP by substrate level phosphorylation, oxidative phosphor

Question

Eukaryotic cells make ATP by substrate level phosphorylation, oxidative phosphorylation, and photophosphorylation. Even though the general outcome is the same in each case – i.e., ADP is phosphorylated to become ATP – various distinctions can be made among the three processes. Briefly address each of the following:

a) What enzyme activity is required for both oxidative phosphorylation and photophosphorylation, but not for substrate level phosphorylation?

b) In terms of phosphate source, how does substrate level phosphorylation differ from both oxidative phosphorylation and photophosphorylation?

c) What cellular compartments are associated with each of the three processes? Be as specific as possible.

d) What condition is required for oxidative phosphorylation but not for either substrate level phosphorylation or photophosphorylation? Why is this condition required?

e) What condition is required for photophosphorylation but not for either substrate level phosphorylation or oxidative phosphorylation? Why is this condition required?

Explanation / Answer

Both oxidative phosphorylation and photophosphorylation occur in specialized membrane systems called mitochondria and chloroplast respectively. The matrix of these membrane systems contains cytochromes.

However, the main enzyme that synthesizes ATP is ATP synthase during chemiosmosis,

Substrate level phosphorylation directly couples an inorganic phosphate to an ADP, whereas oxidative phosphorylation couples high energy phosphate ion to ADP

Substrate level phosphorylation occurs in cytoplasm. Oxidative phosphorylation occurs in the inner mitochondrial matrix. Photophosphorylation occurs in the thylakoid membrane (matrix)of chloroplasts

Oxidative phosphorylation occurs only in the presence of oxygen. Oxygen is required as terminal electron acceptor during the electron transport. The protons pumped during the electron transport generate a proton gradient, which is used to synthesize ATP

Photophosphorylation requires photolysis of water as a source of electrons. This is not required in oxidative phosphorylation since electrons are derived from oxidation of highly reduced metabolic compounds. Substrate level phosphorylation does not require any electrons

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