Chapter 8, Problem 43A

Summary Introduction

To compare and contrast:

Electron transport in cellular respiration and in photosynthesis.

Introduction:

Photosynthesis is an anabolic pathway in which light energy from the Sun is converted to chemical energy for use by the cell. It occurs in the chloroplasts of cells.

Organisms obtain energy through a process called cellular respiration. This process occurs in two stages; glycolysis and aerobic respiration. Glycolysis is an anaerobic process while aerobic respiration includes the Krebs cycle and electron transport. It occurs in the mitochondria of cells.

Answer to Problem 43A

Electron transport and chemiosmosis in cellular respiration are similar to these processes in photosynthesis as both produce chemical energy in the form of ATP, both use ATP synthase enzyme and both depend on the movement of electrons.

However, there are some differences also. These are tabulated as given below:

	Electron transport in cellular respiration	Electron transport in photosynthesis
Location	Membranes of mitochondria	Membranes of thylakoids
Source of electrons	Hydrogen carriers (NADH and FADH2) release electrons	Water is split to release oxygen and electrons
Final products	ATP and water	ATP and NADPH
Final electron acceptor	Oxygen	NADPH
Where H+ ions are pumped into	Mitochondrial matrix	Lumen of thylakoids

Explanation of Solution

Electron transport in photosynthesis:

Thylakoid membranes have a large surface area that provides space to hold a large number of the electron transporting molecules and two types of protein complexes called photosystems. Light energy is absorbed by photosystem II. It is used to split the water molecule. When water splits, oxygen is released from the cell, protons (H+ ions) stay in thylakoid space and an activated electron enters the electron transport chain. As electrons move through the membrane, protons are pumped into thylakoid space. At photosystem I, electrons are re-energized and NADPH is formed.

During light reactions, ATP is produced in conjunction with electron transport by the process of chemiosmosis. The H+ ions produced by the splitting of water molecules accumulate in the interior of the thylakoid. Due to the difference in concentration of H+ ions in the interior of thylakoid and stroma, the H+ ions diffuse down the concentration gradient through ion channels. ATP synthases help in diffusing of H+ ions. ATP synthase is an enzyme used during the light reaction of photosynthesis to generate ATP. As a result of this movement, ATP is formed in the stroma.

Electron transport in cellular respiration:

The final step in the breakdown of glucose is electron transport during which high energy electrons and hydrogen ions from NADH and FADH₂ are used to convert ADP to ATP. As NADH and FADH₂ release electrons, the energy carriers are converted to NAD+ and FAD. H+ ions are pumped into the mitochondrial matrix. These ions diffuse down their concentration gradient back across the membrane and into the matrix through ATP synthase molecules in chemiosmosis. Oxygen is the final electron acceptor in the electron transport system. Protons and electrons are transferred to oxygen to form water.