Describe how photosynthesis and cellular respiration power ecosystems of all kinds on this planet

What is photosynthesis?

Photosynthesis is the chemical process by which plants, algae, and some bacteria use the energy from sunlight to transform carbon dioxide (a greenhouse gas) from the atmosphere, and water, into organic compounds such as sugars. These sugars are then used to make complex carbohydrates, lipids, and proteins, as well as the wood, leaves, and roots of plants.  The amount of organic matter made by photosynthesizing organisms in an ecosystem is defined as the productivity of that ecosystem.  Energy flows through the biosphere as organisms (including some animals) eat photosynthesizing organisms (called herbivores), and as organisms then eat those herbivores (carnivores), etc., to get their energy for growth, reproduction, and other functions.  This energy is acquired through the process of cellular respiration, which usually requires oxygen.  Oxygen is a byproduct of photosynthesis. About 70% of the oxygen in the atmosphere that we breathe comes from algae in the ocean. Atmospheric oxygen from photosynthesis also forms the ozone layer, which protects organisms from harmful high-energy ultraviolet (UV) radiation from the Sun. Because photosynthesis also requires water, the availability of water affects the productivity and biomass of the ecosystem, which in turn affects how much and how rapidly water cycles through the ecosystem.

Fossil fuels are derived from the burial of photosynthetic organisms, including plants on land (which primarily form coal) and plankton in the oceans (which primarily form oil and natural gas). While buried, the carbon in the organic material is removed from the carbon cycle for thousands of years to hundreds of millions of years. The burning of fossil fuels has dramatically increased the exchange of carbon from the ground back into the atmosphere and oceans. This return of carbon back into atmosphere as carbon dioxide is occurring at a rate that is hundreds to thousands of times faster than it took to bury it, and much faster than it can be removed by photosynthesis or weathering. Thus, the carbon dioxide released from the burning of fossil fuels is accumulating in the atmosphere, increasing average temperatures and causing ocean acidification.

A simplified diagram showing the overall inputs – carbon dioxide, water, and sunlight, and products
– oxygen and sugar (glucose), of photosynthesis.

The rate of photosynthesis in ecosystems is affected by various environmental conditions, including:

• Climatic conditions, such as the amount of sunlight available at different latitudes, temperature, and precipitation For example, ecosystems at low latitudes, such as tropical rainforests, have higher productivity and biomass than ecosystems near the poles because of they receive more sunlight and rainfall than regions at higher latitudes.
• Nutrients, especially nitrogen and phosphorus, which when limited can decrease productivity, but when abundant can increase productivity and biomass. Photosynthesizing organisms extract nutrients from the environment, and return them to the soil when they die and decay.
• Numerous other abiotic environmental factors, including soil quality (often related to nutrient levels), wildfires, water acidity, and oxygen levels.
• Species interactions, including the resources species provide for each other, and how they compete for resources such as water, light, and/or space. Species that reduce or increase the success of other species alter population sizes, thus affecting productivity and biomass.
• Evolutionary processes that can change the growth and reproduction rates of photosynthesizing organisms over time, as well as the growth and reproduction of rates of the organisms that eat them.

Humans have altered the rate of photosynthesis, and in turn productivity, in ecosystems through a variety of activities, including:

• Deforestation, habitat destruction, and urbanization, which remove plants and trees from the environment and disrupt ecosystems.
• Agricultural activities that increase the amount of crops available to feed the growing global human population.
• The use of fertilizers for agricultural activities that increase the amount of nutrients, especially nitrogen and phosphorous, in soil or water. These nutrients increase plant and algae growth, including growth of species that are toxic to other organisms. Increased nutrients is not always a good thing. For example, in aquatic environments, nutrient-rich runoff can cause large amounts of algae to grow – when these algae die, they are consumed by bacteria which can reduce oxygen levels in the water, killing fish and other species. This process is known as eutrophication.
• Human freshwater use, which can limit the amount of water available for plants and trees in an ecosystem.
• The release of pollutants and waste, which can reduce growth and reproduction or kill plants.
• Activities that release carbon dioxide and other greenhouse gases that cause global warming, such as the burning of fossil fuels, agricultural activities, and deforestation. Increasing carbon dioxide levels may increase photosynthesis rates in some plants, but this can also make plants less nutritious. Increasing average global land and ocean temperatures and changes in precipitation patterns also affect plant and algae growth, and can make certain species more susceptible to disease.
• Activities such as the burning of fossil fuels, agricultural activities, and deforestation that release carbon dioxide into the atmosphere, which is absorbed by the ocean causing acidification. The decreasing pH of ocean waters (along with ocean warming) causes physiological stress for many plant and algae species, which can decrease growth, reproduction, species population sizes, and biomass.
• Introducing invasive species that compete with native plant or algae species for nutrients, water, light, or other resources, reducing native species populations.

Earth system model about photosynthesis

The Earth system model below includes some of the processes and phenomena related to photosynthesis.  These processes operate at various rates and on different spatial and temporal scales. For example, carbon dioxide is transferred among plants and animals over relatively short time periods (hours-weeks), but the deforestation alters ecosystems over decades to centuries, or longer.  Can you think of additional cause and effect relationships between photosynthesis and other processes in the Earth system?

Explore the Earth System

Click the bolded terms (e.g. respiration, productivity and biomass, and burning of fossil fuels) on this page to learn more about these process and phenomena. Alternatively, explore the Understanding Global Change Infographic and find new topics that are of interest and/or locally relevant to you.

Investigate

Learn more in these real-world examples, and challenge yourself to construct a model that explains the Earth system relationships.

• The bacteria that changed the world

Links to Learn More

• New York Times: ‘Global Greening’ Sounds Good. In the Long Run It’s Terrible
• USGCRP: Climate and Health Assessment, Food Safety, Nutrition, and Distribution
• HHMI BioInteractive: Photosynthesis

How does photosynthesis and cellular respiration power ecosystems of all kinds on this planet?

How are photosynthesis and cellular respiration connected in an ecosystem?

How does cellular respiration power ecosystems?