Ferredoxin is not a transmembrane protein, making it the correct answer to this biology multiple-choice question on photosynthesis and cellular respiration components.

Correct Answer

4. Ferredoxin
Ferredoxin is the only soluble protein in the stroma of chloroplasts, not embedded in any membrane. It acts as a mobile electron carrier in photosynthetic electron transport, shuttling electrons from PSI to NADP⁺ reductase without transmembrane domains.

Option Breakdown

1. ATP Synthase

ATP synthase spans the inner mitochondrial membrane (or thylakoid membrane in chloroplasts) with its F₀ subunit forming a proton channel and transmembrane helices. The F₁ head protrudes into the matrix/stroma for ATP synthesis.

2. LHC (Light-Harvesting Complex)

LHC proteins, like LHCII, are integral membrane proteins in the thylakoid membrane. They contain transmembrane α-helices that bind chlorophyll and carotenoids to capture light energy for photosystems.​

3. PS II (Photosystem II)

PS II is a large transmembrane complex in the thylakoid membrane with core subunits (e.g., D1, D2) featuring multiple transmembrane helices for electron transport, water oxidation, and oxygen evolution.​

4. Ferredoxin

Ferredoxin is a small, soluble iron-sulfur protein located in the chloroplast stroma. It lacks transmembrane regions and freely diffuses to accept electrons from PSI’s stromal side.​