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Explain how solar energy is acquired and transferred by living organisms.

What is Primary Productivity?

a large leaf with the sun shining on it and photosynthesis in big letters CO2 is shown being absorbed from the air, water is shown being absorbed from the soil, glucose is used by the plant, and O2 is released into the air — ***Photosynthesis is known as primary productivity, where glucose is made from sunlight, water, and carbon dioxide (CO₂).***

Primary productivity is the rate at which photosynthetic or chemosynthetic organisms convert energy to organic substances.

We measure productivity in units of energy per unit area per unit time (e.g. kcal/m²/yr). Interestingly, plants only use about 1% of the energy from the sun that enters the atmosphere.

Primary producers form the basis of the food web, supplying energy for all life within an ecosystem. Herbivores eat producers, moving the sun's energy from plants to animals to decomposers.

Sunlight, nutrients, and climate affect primary productivity. Ecosystems with abundant sunlight and nutrients typically have higher rates of productivity.

In the dynamic map below (from NASA), you can see how primary productivity changes on short timescales, including seasonal and year-to-year variations in net primary productivity of terrestrial ecosystems across the globe.

Primary productivity affects the carbon cycle because plants absorb atmospheric carbon dioxide.

Essential Knowledge

Productivity is measured in units of energy per unit area per unit time (e.g. kcal/m²/yr).

Gross Primary Productivity

The diagram illustrates Gross Primary Productivity (GPP) and Net Primary Productivity (NPP). It features a blue cylinder with a green chloroplast at the top, labeled as GPP. An arrow points right, labeled ‘Respiration,’ and another arrow points down, labeled ‘NPP.’ NPP represents energy available for growth and reproduction after respiration. — ***GPP is the OVERALL rate of energy capture by photosynthetic or chemosynthetic producers.***

Gross primary production (GPP) is the sum of the energy that producers convert to biomass by photosynthesis AND the energy used by these producers for respiration (R).

In most ecosystems, we will be primarily concerned with photosynthetic organisms. GPP is influenced by factors such as light, temperature, water, nutrients, and carbon dioxide availability.

Essential Knowledge

Gross primary productivity is the total rate of photosynthesis in a given area.

Net Primary Productivity

Net primary productivity (NPP) is the rate of energy converted to biomass by producers when they photosynthesize.

This does NOT include the respiration energy used by autotrophs for their own maintenance. This energy is not transferred to the next tropic level. In fact, only about 10% of the energy an organism consumes transfers to the next trophic level.

Essential Knowledge

Net primary productivity is the rate of energy storage by photosynthesizers in a given area after subtracting the energy lost to respiration.

Calculating Primary Productivity

We can calculate Net Primary Productivity (NPP) and Gross Primary Productivity (GPP) using the equation below:

LaTeX: NPP=GPP-R

Watch the video below to learn how to calculate NPP, GPP, and R using the equation above:

Primary Productivity in Aquatic Biomes

When talking about marine biomes previously in this module, we discussed the photic zone (the depths to which the sun penetrates) and the aphotic zone (depths to which the sun does not penetrate). This is important when discussing primary productivity in aquatic biomes, especially biomes deeper than the photic zone.

Light is essential for photosynthesis, but it decreases as water depth increases. Red light has the longest wavelength and the lowest energy, while violet light has the shortest wavelength and the highest energy. White light is a combination of all the colors of the visible spectrum. When light enters water, it interacts with the water molecules and other substances in the water. Some of the light is reflected, some is scattered, and some is absorbed. Water absorbs more of the longer wavelengths of light, such as red, orange, and yellow, and less of the shorter wavelengths, such as blue and violet. This is why water appears blue or green, depending on the depth and clarity of the water.

an image of chlorophyll abundance in the ocean, showing the most chlorophyll is in the shallowest waters — ***Chlorophyll abundance in the ocean. Chlorophyll concentrations are highest in shallow, nutrient-rich waters.***

cyanobacteria image — ***Some cyanobacteria use phycobiliproteins and chlorophyll to absorb light.***

In aquatic ecosystems, photosynthesis is limited by the availability of light and carbon dioxide. Most of the photosynthesis occurs in the upper layers of the water, where there is enough light and carbon dioxide. As the depth increases, the amount of light decreases, and so does the rate of photosynthesis. Only blue and violet light can penetrate deeper than 100 m in the clearest water, but these wavelengths are not enough to support photosynthesis. Therefore, aquatic photosynthesizers need to be close to the water surface or have adaptations to capture and use light efficiently.

Other phytoplankton can adjust their buoyancy to move up and down in the water column to increase their exposure to available light. And some corals and sea anemones live in symbiosis with algae known as zooxanthellae. The algae receive a safe home and carbon dioxide from the corals, and the corals benefit from the oxygen and organic matter produced by the zooxanthellae.

Essential Knowledge

Most red light is absorbed in the upper 1m of water, and blue light only penetrates deeper than 100m in the clearest water. This affects photosynthesis in aquatic ecosystems, whose photosynthesizers have adapted mechanisms to address the lack of visible light.

AP Exam Tip

You will need to memorize the Primary Productivity formula. You cannot assume it will be provided to you on the AP exam.

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