The Sun produces a tremendous amount of energy, and emits most of the energy into space in the form of light. Only 2.2 billionth of the sunlight that the Sun emits to all directions reaches Earth. 30% of it reflects back to space, and only 70% of it is absorbed into Earth. Still, the total amount of energy that the people of the world use for a year is equivalent to the solar energy that enters Earth only for one hour.
All living creatures need energy for survival, but the light energy cannot be used directly. It has to be transformed to the form of organic matter. Except some microorganisms, however, it is only plants that can store the light energy in the organic matters on Earth. All animals, including humans, get energy either by eating plants or through other animals that eat plants. In other words, the Sun is the source of energy that living creatures need, and all the energy that is necessary for living creatures is organic matters made through photosynthesis. Therefore, photosynthesis which takes place in the chloroplast, smaller than 10㎛, is the most marvelous and important activity of all vital phenomena on Earth.
Photosynthesis is a process to change light energy into electric energy by using water and to store it in an organic matter in the form of chemical energy. This extremely complicated and delicate process of photosynthesis was a matter of interest of many scientists, and there were three Nobel laureates in Chemistry for photosynthesis. Despite the history of its study that has been nearly 400 years long, we still do not fully understand photosynthesis. Among many facts about photosynthesis, the process how light energy turns into electric energy in chlorophyll still holds a lot of secrets.
When the sunlight reaches the chloroplast, which is a set of chlorophyll, it energizes the electrons within them, and the energized electrons move among the chlorophyll. This is how light energy changes into electric energy, which is the movement of electrons. The electric energy collected in the central chlorophyll is stored in the chemical substances and is used to synthesize glucose, an organic material, by using carbon dioxide.
Plants don’t seem to move in our eyes, but in fact, they busily move to look for the light, which is a material for photosynthesis, and to get carbon dioxide and water. Victoria water lily, which is the world’s biggest aquatic plant in South America, stretches out leaves toward the light from the stem under the water. As the huge leaves float above the surface, they quickly spread in a circular shape. Its leaves not only absorb light, but also accept vapor such as carbon dioxide that is needed for photosynthesis through stomata. This plant pushes its leaves out to the surface because vapor cannot enter the leaves if they are under water. Its leaves can grow up to 30 ㎝ [11.8 in] long a day, and once they are spread out completely, their diameter reaches over 2 m [6.6 ft]. Just as a shrunken balloon gets big as it is filled with water, the leaves spread out as the cells of each and every leaf absorb water and swell. The victoria water lily’s leaves that look like big pads floating on the water are strong enough to hold a person that weighs 45 ㎏ [99 lb].
The secret can be found on the back of the leaves. If you flip the victoria water lily leaves over, you can see veins that spread like spokes from a small dot in the center that is a few centimeters wide. Also, they have veins that look like thin planks in the shape of a concentric circle, which trap air in between and create buoyancy. The thick veins have an inside structure similar to that of a sponge, which allows them to float easily like a rubber tube.
Just as plants living in the water raise leaves up to the surface for photosynthesis, the trees that live on the ground grow upward toward the light. Redwoods, the world’s tallest trees, grow at an astonishing rate of 1.8 m [5.9 ft] per year, and the tallest one is 115.3 m [378.3 ft] long.
Photosynthesis requires water as well as light. Water always flows downhill. But plants grow in the opposite direction. How can these giant trees, which make us look like little people, pull up water to the top of such tall trees?
The root hair cells of plants are semi-permeable membranes that only allow solvents like water to pass through, not passing large solutes. Therefore, water moves to the roots that have relatively higher concentration than soil, and this is called osmosis. This is why when salting cabbages to make kimchi, the water inside the cabbages which have less concentration come outside.
Capillary action takes place to pull the water upward from the roots. If you insert a thin glass tube into a bowl of water, you can see water rises higher in the glass tube. Plants too have water tubes that start from the roots and pass through the stems to the leaves. They are too thin to be seen, and that’s how they have strength to push up water like glass tubes.
While the roots and stems push up the water, the leaves pull up the water. Water molecules, consisting of negatively charged oxygen and positively charged hydrogen, act as a chain, attracting each other like a magnet. Therefore, when the water evaporates through the stomata of the leaves, the water molecules connected to the bottom are pulled up. The power of roots, stems, and leaves is in work complexly to pull up water easily to the high parts of the plants.
Mankind has spent a long time trying to take advantage of the enormous amount of solar energy that is pouring down for free. Solar cells, which were originally developed to use in space, have been commercialized that we can see them on the roofs of some houses. Solar energy is considered as a clean energy alternative to fossil fuels in many countries and its related industries are growing.
The solar cell produced by mankind has a fairly long history of 130 years, but it still has a disadvantage that it takes up much space and its efficiency is only 8 to 15%. To compensate for the shortcomings, scientists have turned their eyes to plants. The efficiency of chloroplasts to convert light energy into electric energy is up to 95%. Through photosynthesis, plants can produce electricity for a home to use for a day with only two 1.5-liter bottles of water (less than one 1-gallon bottle) and enough sunlight. Scientists are developing more efficient solar cells by imitating plants’ photosynthesis systems.
Plants do not move even a step, but they get enough energy with a high strategy called photosynthesis. Every living thing on Earth gets energy from sunlight coming to Earth, through plants that contain green mysteries. In the green leaves that shine in the sunlight, we can feel the calm but dynamic breath of life. Even from one fresh leaf, we can see the great providence of life.
- Reference
- Lee Heung-wu, The Story of Photosynthesis Told by Engelmann (in Korean, 엥겔만이 들려주는 광합성 이야기), Jaeumgwa Moeum, 2010
- Hong Jun-eui and three others, Living Science Textbook 1 (in Korean, 살아 있는 과학 교과서 1), Humanist, 2011
- Newton Editorial Staff, Wonderful Plants – The Unknown World of Plants (in Korean, 경이로운 식물들-알려지지 않은 식물의 세계), October, 2013
- Lee Seong-gyu, Children of the Sun Paying Attention to Artificial Leaves (in Korean, ‘인공 잎’에 주목하는 태양의 아이들), Science Times, Jan. 24, 2014