CONCEPTS IN BIOLOGY

PART II. CORNERSTONES: CHEMISTRY, CELLS, AND METABOLISM

 

7. Biochemical Pathways-Photosynthesis

 

7.2. An Overview of Photosynthesis

Ultimately, the energy to power all organisms comes from the sun. An important molecule in the process of harvesting sunlight is chlorophyll, a green pigment that absorbs light energy. Through photosynthesis, light energy is transformed to chemical-bond energy in the form of ATP. ATP is then used to produce complex organic molecules, such as glucose. It is from these organic molecules that organisms obtain energy through the process of cellular respiration. Recall from chapter 4 that, in algae and the leaves of green plants, photosynthesis occurs in cells that contain organelles called chloroplasts. Chloroplasts have two distinct regions within them: the grana and the stroma. Grana consist of stacks of individual membranous sacs, called thylakoids, that contain chlorophyll. The stroma are the spaces between membranes (figure 7.2).

FIGURE 7.2. The Structure of a Chloroplast, the Site of Photosynthesis

Plant cells contain chloroplasts that enable them to store light energy as chemical energy. It is the chloroplasts that contain chlorophyll and that are the site of photosynthesis. The chlorophyll molecules are actually located within membranous sacs called thylakoids. A stack of thylakoids is known as a granum.

The following equation summarizes the chemical reactions photosynthetic organisms use to make ATP and organic molecules:

There are three distinct events in the photosynthetic pathway:

1. Light-capturing events. In eukaryotic cells, photosynthesis takes place within chloroplasts. Each chloroplast is surrounded by membranes and contains chlorophyll, along with other photosynthetic pigments. Chlorophyll and the other pigments absorb specific wavelengths of light. When specific amounts of light are absorbed by the photosynthetic pigments, electrons become “excited.” With this added energy, these excited electrons can enter into the chemical reactions responsible for the production of ATP. These reactions take place within the grana of the chloroplast.

2. Light-dependent reactions. Light-dependent reactions use the excited electrons produced by the light-capturing

HOW SCIENCE WORKS 7.1

Solution to Global Energy Crisis Found in Photosynthesis?

The most important chemical reaction on Earth, photosynthesis, is thought to have been around about 3 billion years. There has been plenty of time for this metabolic process to evolve into a highly efficient method of capturing light energy. Terrestrial and aquatic plants and algae are little solar cells that convert light into usable energy. They use this energy to manufacture organic molecules from carbon dioxide and water.

Photosynthetic organisms capture an estimated 10 times the global energy used by humans annually. Scientists and inventors have long recognized the value in being able to develop materials that mimic the light-capturing events of photosynthesis. The overall efficiency of photosynthesis is between 3-6% of total solar radiation that reaches the earth. Recently the National Energy Renewable Laboratory (NREL) verified that new organic-based photovoltaic solar cells have demonstrated 6% efficiency. They are constructed of a new family of photo-active polymers—polycarbazoles. Developers see their achievement as a major breakthrough and are hoping to develop solar cells with efficiencies in excess of 10%.

These cells have the ability to capture light energy and, at the same time, be used in many a variety of situations. Flexible plastic, leaf-like sheets can be attached to cell phones, clothing, awnings, roofs, toys, and windows to provide power to many kinds of electronic devices.

events. Light-dependent reactions are also known as light reactions. During these reactions, excited electrons from the light-capturing events are used to produce ATP. As a by-product, hydrogen and oxygen are also produced. The oxygen from the water is released to the environment as O2 molecules. The hydrogens are transferred to the electron carrier coenzyme NADP+ to produce NADPH. (NADP+ is similar to NAD+, which was discussed in chapter 5.) These reactions also take place in the grana of the chloroplast. However, the NADPH and ATP leave the grana and enter the stroma, where the light- independent reactions take place.

3. Light-independent reactions. These reactions are also known as dark reactions, because light is not needed for them to occur. During these reactions, ATP and NADPH from the light-dependent reactions are used to attach CO2 to a 5-carbon molecule, already present in the cell, to manufacture new, larger organic molecules. Ultimately, glucose (C6H12O6) is produced. These light-independent reactions take place in the stroma in either the light or dark, as long as ATP and NADPH are available from the light-dependent stage. When the ATP and NADPH give up their energy and hydrogens, they turn back into ADP and NADP+. The ADP and the NADP+ are recycled back to the light-dependent reactions to be used over again.

The process of photosynthesis can be summarized as follows. During the lightcapturing events, light energy is captured by chlorophyll and other pigments, resulting in excited electrons. The energy of these excited electrons is used during the light-dependent reactions to disassociate water molecules into hydrogen and oxygen, and the oxygen is released. Also during the light-dependent reactions, ATP is produced and NADP+ picks up hydrogen released from water to form NADPH. During the light-independent reactions, ATP and NADPH are used to help combine carbon dioxide with a 5-carbon molecule, so that ultimately organic molecules, such as glucose, are produced (figure 7.3).

FIGURE 7.3. Photosynthesis: Overview

Photosynthesis is a complex biochemical pathway in plants, algae, and certain bacteria. This illustrates the three parts of the process: (a) the light-capturing events, (b) the light-dependent reactions, and (c) the light-independent reactions. The end products of the light-dependent reactions, NADPH and ATP, are necessary to run the light-independent reactions and are regenerated as NADP+, ADP, and P. Water and carbon dioxide are supplied from the environment. Oxygen is released to the environment and sugar is manufactured for use by the plant.

7.2. CONCEPT REVIEW

3. Photosynthesis is a biochemical pathway that involves three kinds of activities. Name these and explain how they are related to each other.

4. Which cellular organelle is involved in the process of photosynthesis?