Q1. Source: GATE 2016

When freshly isolated intact mitochondria were incubated with ADP and inorganic phosphate neither the oxygen consumption nor the ATP synthesis could be detected. Addition of succinate resulted in increased oxygen consumption as well as ATP synthesis with time. Subsequent addition of cyanide to this system will result in which of the following?

a. Both oxygen consumption and ATP synthesis are inhibited
b. Oxygen consumption continues but ATP synthesis is inhibited
c. Oxygen consumption is inhibited but ATP synthesis continues
d. Both oxygen consumption and ATP synthesis continues

Statement Analysis

Initially, mitochondria were supplied only with:

• ADP
• inorganic phosphate

But no electron donor was present. Therefore:

• the electron transport chain (ETC) could not function
• oxygen consumption did not occur
• ATP synthesis also did not occur

Then succinate was added. Succinate enters the ETC through Complex II (Succinate dehydrogenase). This donates electrons into the electron transport chain, leading to:

• oxygen consumption
• proton gradient formation
• ATP synthesis

Now Cyanide is added. Cyanide inhibits Complex IV (Cytochrome c oxidase). Complex IV is responsible for transferring electrons to oxygen, the final electron acceptor.

Once cyanide blocks Complex IV:

• electron transport stops
• oxygen consumption stops
• proton pumping stops
• ATP synthesis also stops

Therefore, Correct Answer: a. Both oxygen consumption and ATP synthesis are inhibited

Exam Insight

This question tests your conceptual understanding of:

• oxidative phosphorylation
• ETC inhibitors
• coupling between electron transport and ATP synthesis

Students often memorize “cyanide inhibits oxygen utilization” but fail to connect it to ATP synthesis. Remember:

Q2. Source: GATE 2017

Natural proteins are composed primarily of 20 alpha-amino acids. Which one of the following statements is true for any of these amino acids in a solution of pH 1.5?

a. Only the amino group is ionized
b. Only the carboxylic group is ionized
c. Both amino and carboxylic groups are ionized
d. Both amino and carboxylic groups are neutral

The key clue here is: pH 1.5. This is a highly acidic condition. Under acidic conditions:

• amino groups become protonated = $NH_3^+$
• carboxyl groups remain protonated = $COOH$

Therefore:

• the amino group carries a positive charge
• the carboxylic group remains neutral

So, correct option: a. Only the amino group is ionized.

Exam Insight

This is a simple question but exactly the kind students overthink during exams.

Under time pressure, many aspirants forget basic acid-base chemistry and get confused between:

• protonated
• deprotonated
• ionized
• neutral forms

A quick shortcut: