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:
144
