Chapter 11, Problem 11.1CC

Both mitochondria and chloroplasts are energy-generating organelles. Explain how the structure of their membrane systems is critical for their functions, emphasizing their similarities and differences.

Summary Introduction

To determine: The way in which the structure of the membrane systems of mitochondria and chloroplasts is critical for their functions and emphasizing their similarities and differences.

Introduction: Mitochondria and chloroplasts are cell organelles which provide energy to the cell. The cell uses energy in the form of ATP. They produce glucose by photosynthesis and then use this glucose to generate ATP, by cellular respiration.

Explanation of Solution

The first step by which energy is absorbed by a plant is by absorbing sunlight and converting it to chemical energy, which can be utilized by the plant. This process is conducted through photosynthesis. Various processes of photosynthesis are carried out on the membrane of chloroplasts. These membranes are the thylakoid membranes of chloroplasts. The thylakoid membrane system is critical for the function of chloroplasts as the electron transport system of the chloroplast is located on the thylakoid membrane.

Mitochondria are involved in the next step of energy-harnessing. In this step, energy is harnessed by mitochondria in the form of ATP. This process is conducted through respiration. It has a double-membrane structure in which the outer membrane has large pores but the inner membrane has a much more restricted permeability. Various processes of respiration are carried out on the inner membrane of mitochondria. The processes of electron transport and chemiosmotic generation of ATP occurs on the inner mitochondrial membrane.

The similarities in the membranes of chloroplasts and mitochondria are as follow:

• Chloroplasts and mitochondria, both are bound by a double membrane.
• Both the thylakoid membrane of chloroplasts and the inner membrane of mitochondria have functional similarity in the chemiosmotic generation of ATP.
• The mitochondrial matrix and the stroma of the chloroplast are similar in function. Both contain their own, individual genome and also several enzymes required for metabolism.
• Chloroplasts and mitochondria both have the enzyme ATP synthase. This enzyme helps in the utilization of the energy generated when protons pass through it, to convert ADP to ATP.
• Both these organelles have electron transport chains. It is embedded in the thylakoid membrane of chloroplasts and the inner membrane of mitochondria.

The differences in the membranes of chloroplasts and mitochondria are as follow:

No	Character	Chloroplast	Mitochondria
1	Membrane involved	The thylakoid membrane is involved in the process of energy generation in Chloroplasts.	The inner mitochondrial membrane is involved in the process of energy generation in Mitochondria.
2	The process of energy generation	Chloroplasts capture the energy of the light falling on the plant and convert this into chemical energy by photosynthesis.	Mitochondria help in the production of ATP by cellular respiration.
3	ATP synthase	In chloroplasts, ATP synthase points towards the stroma.	In mitochondria, ATP synthase points towards the matrix.
4	The direction of proton flow	Protons are pumped from the space between the thylakoid membranes to the stroma of the mitochondria.	Protons are pumped from the inter-membrane space to the matrix of the mitochondria.
5	Electron acceptors	Electron acceptors found in Chloroplasts are NADP.	Electron acceptors found in Mitochondria are NAD and FAD.
6	Source of energy	The energy required for ATP synthesis in Chloroplasts is derived from light, directly, by photophosphorylation.	The energy required for ATP synthesis in Mitochondria is derived from oxidation of glucose, by phosphorylation.

Thus, the thylakoid membranes of chloroplasts and the inner membrane of mitochondria play an important role in the generation of ATP. They are similar in function in fundamental ways but differ in their mode of ATP generation.