Photosynthesis

such cases, the person with an inability to do this develops a disease called Refsum disease. In the disease, a metabolite called phytanic acid builds up, leading to neurologic disease.

PHOTOSYNTHESIS

Photosynthesis is a necessary part of life on this planet. It is necessary for the plants and animals alike because it can capture sunlight energy in order to make food energy in the form of carbohydrates. Energized electrons are stored in covalent bonds inside sugar molecules. This energy can ultimately last many millions of years on earth.

Photosynthesis can occur in plants, algae, and in cyanobacteria. These organisms are known as photoautotrophs. Those that feed on other organisms are called heterotrophs. There are organisms known as chemoautotrophs, that extract energy from inorganic compounds. Photosynthesis helps to power 99 percent of the ecosystems on Earth.

Photosynthesis requires sunlight, water, and carbon dioxide. Carbon dioxide is low itself in energy and gets “raised” in energy by the sunlight. The end result is glyceraldehyde3-phosphate, which can get further converted into sucrose, glucose, or other sugar molecules. It takes six CO2 molecules and six water molecules in order to make a sixcarbon sugar and six oxygen molecules.

Photosynthesis takes place in the leaves of the plant. Leaves have several layers of cells with photosynthesis occurring in the mesophyll or middle layer. Gas exchange occurs through stomata, which are regulated openings in the leaves. The stomata are located on the underside of the leaf. Swelling and shrinking in response to osmotic changes will affect the size of the stomata.

The chloroplast is the place where photosynthesis takes place. These are doublemembraned structures that have an outer and inner membrane. Inside the chloroplast are flat stacks of discs called thylakoids. The thylakoid membrane has chlorophyll, which absorbs light. Inside the thylakoid membrane is the thylakoid lumen. One stack of thylakoids is called a granum. Figure 45 shows the structure of a chloroplast:

Figure 45.

There are two parts to photosynthesis: there is a light-dependent series of reactions and light-independent reactions. The light-dependent reactions take sunlight energy and convert it to stored chemical energy. In the light-independent reactions, there is assembly of the energy into sugars. These are interdependent reactions that depend on one another.

The light energy, of course, happens from the sun, which emits a great deal of electromagnetic radiation. Humans see part of this in the form of visible light. Light travels as waves and particles at the same time. One particle of light is called a photon. The electromagnetic spectrum is the range of all of the radiation emitted by the sun. Each type of electromagnetic radiation has a specific wavelength, with shorter wavelengths having the most energy. High-energy waves can damage living systems.

There is a narrow range of energy or wavelength that participates in photosynthesis. Retinal pigments can see between 400 and 700 nanometers of light, which is visible light. Plants absorb light in the range of 700 nanometers to 400 nanometers as well.

Pigments can absorb certain wavelengths of light and transmit the wavelengths that cannot be absorbed. Chlorophylls and carotenoids are the two major classifications of pigments in algae and plants. There are five major chlorophyll types and dozens of carotenoids. Green is reflected from chlorophyll a, leading to its green color.

Light-dependent reactions work by converting the sun’s energy into NADPH and ATP. This is the energy that supports the light-independent reactions. This occurs in a photosystem, of which there are two types in the thylakoid membrane. They differ in the redox reactions that occur within them.

These photosystems have antenna proteins that bind the chlorophyll molecules. Each photosystem has a light-harvesting complex in the thylakoids that contain chlorophyll a and b molecules, as well as carotenoids. One photon of light gets absorbed, pushing chlorophyll to its excited state. It gets transported to the reaction center. There is an electron transport chain that moves protons into the thylakoid lumen, lowering its pH. ATP synthase is tied to this in order to make ATP similar to that seen in mitochondria. The protons come from the splitting of water.

So, the light-dependent reactions make ATP and NADPH as fuel, which go on to the light-independent reactions in the stroma of the chloroplast, which make sugars that can survive for hundreds of millions of years. These sugars come from carbon dioxide, which is also a waste product of glucose metabolism. Carbon dioxide enters the stomata and participates in the light-independent reactions.

The light-independent reactions are also referred to as the Calvin cycle. Each turn of the cycle uses two ATP molecules and one NADPH molecule to make glyceraldehyde-3phosphate, which then goes on to make glucose outside of the cycle. Figure 46 depicts the Calvin cycle:

Figure 46.

There are three stages to the Calvin cycle. The first stage is “fixation” and involves a five-carbon structure called ribulose bisphosphate and the fixation of CO2. The second stage is called “reduction” in which the molecule gets reduced. The third stage involves “regeneration”, in which ribulose bisphosphate is regenerated to continue the cycle. It takes three cycles to make the three-carbon glyceraldehyde-6-phosphate and six cycles to make a molecule of glucose.