7. Biochemical Pathways-Photosynthesis



Designer Bacteria-Future Source of Biofuels?

Genetically Modified to Generate Fuel.

A team of scientists has transferred cellulose-making genes from one kind of bacterium to another. The photosynthetic bacteria receiving the genes, cyanobacteria, are able to capture and use sunlight energy to grow and reproduce. The added genes give them a new trait (i.e., the ability to manufacture large amounts of cellulose, sucrose and glucose). Because cyanobacteria (formerly known as blue-green algae) can also capture atmospheric nitrogen (N2), they can be grown without costly, petroleum-based fertilizer.

The cellulose that is secreted is in a relatively pure, gellike form that is easily broken down to glucose that can be fermented to produce ethanol and other biofuels. The biggest expense in making biofuels from cellulose is in using enzymes and mechanical methods to break cellulose down to fermentable sugars.

Genetically modified cyanobacteria could have several advantages in the production of biofuels. They can be grown in sunlit industrial facilities on nonagricultural lands and can grow in salty water that is unsuitable for other uses. This could reduce the amount of agricultural land needed to grow corn that is being fermented to biofuels. There are social and financial pressures to use more corn, sugar cane, and other food crops for nonfood uses throughout the world, thus reducing the amount of food crops. For example, Brazil is being pressured to cut more of the Amazon rainforest in order to grow more sugarcane to meet growing world energy needs.

• How does photosynthesis trap light energy?

• What happens in photosynthetic organisms that results in the production of organic compounds?

• Should our government provide the same agricultural support payments to those who grow cyanobacteria as it pays to corn farmers?


ü  Background Check

Concepts you should already know to get the most out of this chapter:

• The energy levels and position of electrons encircling an atom (chapter 2)

• The basic structure and function of chloroplasts and the types of cell in which they are located (chapter 4)

• How enzymes work in conjunction with ATP, electron transport, and a proton pump (chapter 5)

•   The differences between autotrophs and heterotrophs (chapter 6)


7.1. Photosynthesis and Life


Although there are hundreds of different chemical reactions taking place within organisms, this chapter will focus on the reactions involved in the processes of photosynthesis. Recall from chapter 4 that, in photosynthesis, organisms such as green plants, algae, and certain bacteria trap radiant energy from sunlight. They are then able to convert it into the energy of chemical bonds in large molecules, such as carbohydrates. Organisms that are able to make energy-containing organic molecules from inorganic raw materials are called autotrophs. Those that use light as their energy source are more specifically called photosynthetic autotrophs or photoautotrophs.

Among prokaryotes, there are many bacteria capable of carrying out photosynthesis. For example, the cyanobacteria described in the opening article are all capable of manufacturing organic compounds using light energy.




Among the eukaryotes, a few protozoa and all algae and green plants are capable of photosynthesis. Photosynthesis captures energy for use by the organisms that carry out photosynthesis and provides energy to organisms that eat photosynthetic organisms. An estimated 99.9% of life on Earth relies on photosynthesis for its energy needs (figure 7.1). Photosynthesis is also the major supplier of organic compounds used in the synthesis of other compounds, such as carbohydrates and proteins. It has been estimated that over 100 billion metric tons of sugar are produced annually by photosynthesis. Photosynthesis also converts about 1,000 billion metric tons of carbon dioxide into organic matter each year, yielding about 700 billion metric tons of oxygen. It is for these reasons that a basic understanding of this biochemical pathway is important (How Science Works 7.1).



FIGURE 7.1. Our Green Planet

From space you can see that Earth is a green-blue planet. The green results from photosynthetic pigments found in countless organisms on land and in the blue waters. It is the pigments used in the process of photosynthesis that generate the organic molecules needed to sustain life. Should this biochemical process be disrupted for any reason (e.g., climate change), there would be a great reduction in the food supply to all living things.



1. What are photosynthetic autotrophs?

2. How do photosynthetic organisms benefit heterotrophs?