Photosynthesis
Photosynthesis is the biochemical process through which plants convert light energy from the sun into chemical energy in the form of carbohydrates, fundamental for their growth and survival. This process involves the absorption of photons by pigments like chlorophyll, leading to the splitting of water molecules and the production of adenosine triphosphate (ATP) and oxygen. Essentially, photosynthesis not only provides food for all living organisms but also plays a vital role in the carbon cycle by utilizing carbon dioxide (CO₂) and releasing oxygen. Different plants have adapted their photosynthetic pathways based on climatic conditions, leading to three main types: Carbon 3 plants, which thrive in temperate regions; Carbon 4 plants, which are adapted to hot, sunny climates; and CAM plants, which are suited for arid desert environments. The process is sensitive to environmental changes, and climate change impacts the availability of water and CO₂, affecting photosynthetic efficiency. While rising CO₂ levels may initially boost photosynthesis, long-term effects can lead to plant acclimation and potential declines in some species. Understanding photosynthesis is crucial, especially in the context of climate change, as it influences ecosystem dynamics and food supply.
Photosynthesis
Definition
Photosynthesis is the process by which plants convert light energy from the Sun into chemical energy in the form of carbohydrates. The name carbohydrate literally means “carbon plus water,” a vivid descriptive term for the sugar formed from combining carbon dioxide (CO2) with water. The process of begins with the absorption of photons by plant pigments. As photons reach a reaction center made up of chlorophylls, light energy excites electrons from the splitting of water molecules. A portion of the energy released by the energized electrons is used to produce adenosine triphosphate (ATP). ATP is then used to produce carbohydrates from CO2 and water. In the process, water acts as the source of both electrons and oxygen gas. Photosynthesis produces food for all living organisms, directly or indirectly, in all ecosystems. In addition, it releases oxygen and utilizes CO2 and thus becomes an indispensable link in the carbon cycle.
Several pathways of photosynthesis are employed by different plants as a response to different climatic conditions, primarily temperature and water availability. In order to survive terrestrial environments, all land plants must cope with water deficits from time to time. When plants open their numerous microscopic pores, called stomata, to admit CO2 for photosynthesis, they risk losing water through these openings by evaporation. Plants will thus at times close stomata in order to conserve water for survival. The delicate balance between conserving water and admitting CO2 has led to the evolution of three major types of photosynthesis. The plants that employ each of these three different pathways not only display different anatomies but are adapted to different climates as well.
Some plants convert CO2 into a three-carbon sugar and are thus called carbon 3 plants. Carbon 3 plants are typically well suited to temperate regions, where the weather is not very dry or hot and water is generally not a limiting factor. Examples of carbon 3 plants include such crops as wheat, barley, and peas.
Other plants convert CO2 into a four-carbon sugar and are thus called carbon 4 plants. Carbon 4 plants are equipped with a CO2 pump that can concentrate CO2 inside their leaves. This enables these plants to perform photosynthesis even when their stomata have to partially or temporarily close in order to conserve water on a hot and sunny day. Thus, carbon 4 plants are well adapted to high daytime temperatures and intense sunlight with periodic water deficits. This is the reason the carbon 4 crabgrass often outcompetes carbon 3 lawn grass when water becomes a limiting factor in the summer. Other examples of carbon 4 plants include corn, sugarcane, and sorghum.
Carbon 4 plants generally do better than carbon 3 plants in hot and sunny habitats. Because operating their CO2 pumps requires energy, however, in a cooler and moist habitat where water is not a limiting factor, carbon 3 plants possess an advantage over carbon 4 plants. When water is plentiful, there is no need to close the stomata to conserve water. Thus, the energy expenditure of pumping CO2 by carbon 4 plants becomes a wasteful process.
Plants employing the third type of photosynthesis are uniquely adapted to desert habitats. Because of the scarcity of water, these plants cannot afford to open their stomata during the day at all lest they dehydrate. Desert plants thus engage in what is called crassulacean acid metabolism (CAM), in which they open their stomata to admit CO2 at night, convert it into acids, and store them inside their vacuoles. They release the CO2 stored inside the acids during the day for use in photosynthesis. CAM plants are well adapted to the high daytime temperatures, intense sunlight, and very low soil moisture of the desert biome. Some familiar CAM plants include cacti, pineapple, and sedums.
Significance for Climate Change
Both water and CO2 serve as essential raw materials in photosynthesis. Therefore, any climate change that affects the availability of either substance will affect photosynthesis. Given the various pathways different plants employ for photosynthesis, the same climate change will have different effects on different plants. Rising CO2 concentrations in the atmosphere would in theory increase the photosynthetic rates of all plants. This phenomenon is described as CO2 fertilization. However, the increase in photosynthesis from rising atmospheric CO2 concentrations is short-lived: The response decreases under long-term exposure because plants acclimate to elevated CO2 concentrations through a process known as down regulation.
Plants’ response to rising temperatures is complex, may be positive or negative, and is often compounded by other climatic factors. Moisture in the environment is particularly influential, as changes in temperature may correlate to changes in and evaporation rate, significantly affecting the availability of water. Depending upon the degree of increase, warming temperatures may drive some plants out of their natural habitats, causing declines or species extinction.
Carbon 3 plants tend to increase their photosynthetic rate as a result of CO2 fertilization to a point where the associated temperature increase may offset the positive effect of rising CO2. If the temperature increases above the level at which carbon 3 plants need to shut their stomata to conserve water, the positive effect of CO2 fertilization diminishes or disappears entirely. In fact, facing the prospect of dehydration and drought as a result of climate change, some carbon 3 plants may no longer be able to even survive in their natural habitat.
On the other hand, carbon 4 plants may benefit from a warmer climate simply because they reach their maximum photosynthetic rate at a much higher temperature. Within a certain range, the increases in both daily temperature and CO2 concentration may have short-term benefits for carbon 4 plants. The increases in atmospheric CO2 may have very little effect on CAM plants because the leading constraints for their photosynthesis lies with the capacity to store acids inside their vacuoles. Since CAM plants are already adapted to the desert biome, however, an appreciable temperature rise that renders deserts even more hostile could be detrimental to the survival of CAM plants in general.
Bibliography
Gregory, R.P. Photosynthesis. Springer, 2012. EBSCOhost, search.ebscohost.com/login.aspx?direct=true&db=nlebk&AN=2856803&site=ehost-live&scope=site.
Margaris, N. S., and H. A. Mooney, eds. Components of Productivity of Mediterranean-Climate Regions: Basic and Applied Aspects. W. Junk, 1981.
Morison, James I. L., and Michael D. Morecroft, eds. Plant Growth and Climate Change. Blackwell, 2006.
Morton, Oliver. Eating the Sun: How Plants Power the Planet. HarperCollins, 2008.
Owen, Ruth. Photosynthesis. SilverTip Books, 2024. EBSCOhost, search.ebscohost.com/login.aspx?direct=true&db=nlebk&AN=3620726&site=ehost-live&scope=site.
"Photosynthesis." National Geographic Society, 15 July 2022, education.nationalgeographic.org/resource/photosynthesis. Accessed 29 July 2024.