Chapter 11, Problem 15STP

Summary Introduction

To explain:

The importance of functions performed by thylakoid membrane and mitochondrial membrane

Introduction:

In living organisms, chemical energy is stored in organic compounds and this energy is converted to other forms of energy. All living organisms need energy to carry out various biological functions. All the chemical reactions in a cell are together called metabolism. There are two metabolic pathways; anabolic pathway and the catabolic pathway.

Answer to Problem 15STP

Light reactions occur in the thylakoids during which light energy is absorbed by chlorophyll present in thylakoid membranes. The energy is stored in the energy storage molecules called NADPH and ATP.

In mitochondrial membrane ATP is produced as a result of oxidative phosphorylation.

Both these functions are important as they produce energy for the cells to carry out various cellular activities.

Explanation of Solution

Photosynthesis is an anabolic pathway in which light energy from the Sun is converted to chemical energy for use by the cell. Light energy is trapped by pigments called chlorophyll present in the chloroplasts and is converted to chemical energy during the process of photosynthesis. It occurs in two phases; Light dependent reaction and Light independent reaction. Light reactions occur in the thylakoids within the chloroplasts. During light reaction light energy is absorbed by chlorophyll present in thylakoid membranes. The energy is stored in two energy storage molecules- NADPH and ATP. ATP is produced in conjunction with electron transport by the process of chemiosmosis across thylakoid membrane. Thylakoid membranes have large surface area which provides the space needed to hold large number of electron transporting molecules and two types of photosystems.

Catabolic pathways release energy by breaking down larger molecules into smaller molecules. This occurs through cellular respiration in mitochondria. This includes Krebs cycle and electron transport. The final step in breakdown of glucose in aerobic respiration is electron transport during which high energy electrons and hydrogen ions from NADH and FADH₂ are used to convert ADP to ATP. This occurs along the mitochondrial membrane. 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 electron transport system. Protons and electrons are transferred to oxygen to form water.

Both thylakoid membranes and mitochondrial membranes help in metabolism. They help in generating energy for the cells in the form of NADPH and ATP.