Plants lack a digestive system that would enable them to eat the way humans and other animals eat. Instead, plants have the unique ability to create their own food. Plants use certain natural substances in order to make food through a process known as photosynthesis. During photosynthesis, plant cells undergo a two-stage process that creates food to sustain the plant. Photosynthesis occurs within specially designated parts of a plant cell.
Plants feature a part known as a chloroplast that floats within the cytoplasm in a plant cell. Chloroplasts contain chlorophyll, molecules that give plants their green colour. The chlorophyll within a plant cell's chloroplasts uses light energy and carbon dioxide to create food for the plant. Chloroplasts represent part of the plastid family of organelles that exist in plant cells. They contain copies of a plant's DNA. According to "Molecular Biology of the Cell," chloroplasts develop from proplastids, organelles within immature plant cells. In addition to being responsible for food production, chloroplasts and other plant plastids synthesise other materials for the plant.
Chloroplasts feature a structure similar to that of mitochondria. However, whereas mitochondria have two compartments, chloroplasts have three. Chloroplasts are often shaped like an elongated disc. The outer membrane of the chloroplast is permeable, while the inner membrane lacks the same level of permeation. The intermembrane space rests between the two membranes. A third membrane, the thylakoid membrane, contains systems to capture light for use in photosynthesis. The thylakod membranes surround flattened discs known as thylakoids, which hold the materials needed for a plant to conduct photosynthesis. Thylakoids stack within the chloroplast like pancakes, creating stacks known as grana. Stroma, a gelatanous material, surrounds the grana.
The first stage of photosynthesis, known as light reactions or light dependent reactions, occurs within the grana of the chloroplasts. Plants receive light from the sun or an artificial source, and this light is absorbed into the thylakoid membrane. The thylakoids store this light and take in water. As the electrons in the chlorophyll become active, the stroma creates ATP, or adenosine triphosphate. The last reaction that occurs during the light reaction phase of photosynthesis results in the creation of NADPH, or nicotine adenine dinucleotide phosphate. Oxygen results as a byproduct of light reactions and is given off by the plants.
Continuing the work of the chloroplasts in the light reactions, the dark reactions, also known as carbon-fixing reactions or light independent reactions, take the ATP and NADPH created and use them to make food within the stroma of the plants. This reaction uses the Calvin cycle to create food. Plants absorb carbon dioxide and combine it with ATP to create glucose, or sugar, and carbohydrates. For every molecule of carbon dioxide used, plants use three molecules of ATP and two molecules of NADPH. Plants survive by consuming this food created.