Chloroplasts and Energy Capture.

Chapter 14, pages 476-482; 484-486.

In the past couple of class periods, we have focused on the sorting and transport of proteins through the endoplasmic reticulum and Golgi apparatus. These organelles play a vital role in the processing of proteins and their delivery to the correct cellular destination.  We will now look at some extraordinary organelles, the chloroplasts and mitochondria, on whose activities life as we know it depends. One of the most remarkable things about living cells is the degree of order within them. Living cells create order with the help of energy input, primarily from solar radiation. Chloroplasts and mitochondria control the flux of energy from the sun through all living things and carry out the essential energy interconversions that drive life.

• Chloroplasts capture the energy in sunlight and use it to synthesize energy-rich carbohydrates. This food made by chloroplasts provides the chemical energy needed by all forms of life.
• Mitochondria help cells to extract the energy in food molecules efficiently to power cellular activities.

For each of these organelles in turn, we will first take a look at their structures, then at the mechanisms by which proteins are delivered into them, and finally at their metabolic roles in cells.

Mitochondria and chloroplasts are similar in the following ways:
1. Both are thought to have originally been free-living organisms, like bacteria, that were, during the course of evolution, engulfed by a larger cell in which they survived as intracellular symbionts (a.k.a., endosymbionts).
2. Both mitochondria and chloroplasts have their own genomes and their own protein synthesis machinery.
3. Both mitochondria and chloroplasts are sites of production of ATP, the energy currency of the cell.

The two organelles differ in many important ways, as well. We will note these differences as we go along. We will look at chloroplasts first.

What is the structure of a chloroplast like?
Chloroplasts, like mitochondria, are bounded by a double membrane structure. This double membrane structure is called the chloroplast envelope and it has an intermembrane space between the two membranes. The inner membrane of the chloroplast encloses the interior of the chloroplast which is called the stroma. Within the stroma is yet another membrane system called the thylakoids. This is a network of flattened disc like structures bounded by a third membrane, the thylakoid membrane. See Fig. 14.26






What is the function of the stroma?
The stroma contains the genome of the chloroplast as well as a variety of enzymes including the ones needed for the synthesis of sugar from carbon dioxide during photosynthesis.






What is the role of the thylakoids?
The thylakoid membrane houses the chlorophyll molecules and is the site of the electron transport chain in chloroplasts.






What is the chloroplast genome like?
The chloroplast genome is circular and present in several copies per chloroplast. It has over a hundred genes. These encode a variety of RNAs and proteins, including the thirty or so that are needed for photosynthesis.






If chloroplasts have their own genome and protein synthesis machinery are they independent of the cell they live in?
No, chloroplasts depend on the cell for about 90% of their proteins. These proteins are encoded in the nuclear DNA and synthesized on free, cytoplasmic ribosomes and then transported into the chloroplast.

How do proteins get into the chloroplast?
Chloroplast proteins have a characteristic sequence at their N-termini that serve as a signal that targets the proteins to the chloroplast.  This signal is recognized and bound by a guidance complex that brings proteins containing the signal to the chloroplast membrane. 

Chloroplasts have channels in their membranes called Toc (Translocase of the Outer Chloroplast membrane) and Tic (Translocase of the Inner Chloroplast membrane). As noted before, proteins targeted to the chloroplast are made on free ribosomes. As these proteins are made, they are bound by chaperones that keep them unfolded, so they can be threaded through the Toc and Tic protein channels in the chloroplast membranes.



What do chloroplasts do?
Chloroplasts are organelles that carry out photosynthesis.






What is photosynthesis?
Photosynthesis is the process by which energy from sunlight is captured by chlorophyll and used to drive the synthesis of sugar (carbohydrate) from carbon dioxide and water. The synthesis of carbohydrates is a reduction process that involves combining of the simple molecules carbon dioxide and water to produce (more complex) sugar.






Where is the chlorophyll in the chloroplast?
The chlorophyll molecules are found in the thylakoid membrane, in large complexes called photosystems.

How are the chlorophyll molecules organized in the photosystems?
Hundreds of chlorophyll molecules in the photosystem serve as antenna molecules that pick up the light energy and relay it to special chlorophyll molecules in a region of the photosystem called the reaction center. The chlorophylls that receive and pass on the light energy are called antenna chlorophylls, while the chlorophylls in the reaction center are called the special pair.

 See Figure 14.30





How can light energy be used to make sugar?
Light energy is not directly used to synthesize sugar. Instead, the energy from light is captured by chlorophyll molecules causing electrons in the chlorophyll to become "excited". These electrons are then passed along an electron transport chain in the thylakoid membrane. This process pumps protons into the lumen of the thylakoids, creating a proton gradient across the thylakoid membrane. As the protons flow back across the membrane, the energy released is used to synthesize ATP. The production of another molecule, NADPH, is also coupled to the electron transport chain in the chloroplast. The ATP made in this process provides the energy and the NADPH provides the reducing power needed to make sugar from carbon dioxide and water.

See Figure 14.28






Where does the NADPH come in?
The electrons that are being passed along the electron transport chain are finally passed on to molecules of NADP+ making NADPH. Thus, NADP+ is the terminal electron acceptor in the light reaction of photosynthesis.





What does ATP and NADPH synthesis have to do with making sugar?
The synthesis of ATP and NADPH are reactions that are light dependent. Once these two compounds are made, the rest of the business of making sugar can occur in the absence of light (these reactions are sometimes called the dark reactions of photosynthesis, not because they need darkness but because they don't need light).

In the dark reactions, which occur in the stroma of the chloroplast, carbon dioxide enters a series of reactions collectively called the Calvin cycle ( See Fig. 14.37) where it is ultimately reduced to produce the precursors for sugars. These reactions require:

• Energy in the form of ATP and
• Reducing power in the form of NADPH.

Overall, the reactions of photosynthesis use 6 molecules of water and 6 molecules of carbon dioxide to produce one molecule of glucose plus 6 molecules of oxygen. The importance of both the glucose and the oxygen to organisms like ourselves is obvious.



Copyright © 2011 Indira Rajagopal