Chloroplasts are tiny factories inside the cells of plants. They are also found in the cells of other organisms that use photosynthesis. Chloroplasts take the energy from the sunlight and use it to make plant food. The food can be used immediately to give cells energy or it can be stored as sugar or starch. If stored, it can be used later when the plant needs to do work, like grow a new branch or make a flower. Inside chloroplasts are special stacks of pancake-shaped structures called thylakoids (Greek thylakos = sack or pouch). Thylakoids have an outer membrane that surrounds an inner area called the lumen. The light-dependent reactions happen inside the thylakoid. Our cells have mitochondria (Greek mitos = thread, and khondrion = little granule), our energy-producing structures. We don\’t have any chloroplasts. Plants have both mitochondria and chloroplasts. Both mitochondria and chloroplasts convert one form of energy into another form that cells can use.
How did plants get chloroplasts? Chloroplasts were once free-living bacteria! Chloroplasts entered a symbiotic (Greek syn = together, and bios = life) relationship with another cell, which eventually led to the plant cells we have today. Chlorophyll, a green pigment found in chloroplasts, is an important part of the light-dependent reactions. Chlorophyll soaks up the energy from sunlight. It is also the reason why plants are green. You may remember that colors are different wavelengths of light. Chlorophyll captures red and blue wavelengths of light and reflects the green wavelengths. Plants have different types of pigments besides chlorophyll. Some of them also assist in absorbing light energy. These different pigments are most noticeable during the fall. During that time, plants make less chlorophyll and the other colors are no longer hidden beneath green. But why don\’t plants have pigments that allow them to capture all wavelengths of light?
If you\’ve ever gotten a sunburn you know firsthand that sunlight can be damaging. Plants can also be damaged from excess light energy. Luckily, there are non-chlorophyll pigments in plants that provide a \’sunscreen\’. Additional images via Wikimedia Commons. Algae image by Leonardo RГ-Jorge.
Q. My tree has red leaves all year. How does a plant support itself without green chlorophyll? A. Some parasitic plants lack chlorophyll entirely and steal the products of photosynthesis from their green hosts, said Susan K. Pell, director of science at the Brooklyn Botanic Garden. Other plants, like a red-leafed tree, have plenty of chlorophyll, but the molecule is masked by another pigment. Chlorophyll absorbs red and blue light, Бreflecting, and thus appearing, green,Б Dr. Pell said. Chlorophyll uses this electromagnetic energy, along with carbon dioxide and water, to make glucose and oxygen. Most plants also have other pigments: carotenoids, which usually appear yellow to orange, and anthocyanins, which are red to purple.
One pigment usually dominates. So a plant with red leaves probably has higher than usual amounts of anthocyanins, Dr. Pell said. But chlorophyll is still present and at work. БWe used to think that all fall foliage color change resulted from the revealing of already-present carotenoids and anthocyanins when chlorophyll was broken down in preparation for dormancy,Б she said. We now know that leaves actually produce additional anthocyanins into old age, she said. The evolutionary advantages are not fully understood, Dr. Pell said. One theory is that extra anthocyanins provide shade under which chloroplasts (structures within cells) can break down their chlorophyll, helping the plant reabsorb its building blocks, especially valuable nitrogen. Another theory is that anthocyanins, which are powerful antioxidants, protect the plants in preparation for winter. question@nytimes. com