1a. What is photosynthesis and why is it important?

1b. Which wavelengths of solar radiation are used for photosynthesis?

1c. Why do leaves appear green to the human eye?


2a. What is respiration and what is its role in living organisms?

2b. Is light required for respiration to take place?

2c. Describe how energy "moves" through the food chain.


3a. Why are plants called "primary producers"?

3b. What is the difference between gross primary production and net primary production?

3c. Are NPP rates fairly constant around the globe? Explain.


4a. What is the role of vegetation in the hydrologic cycle?

4b. How does the elimination of vegetation affect the hydrologic cycle?


5a. What is the role of vegetation in the carbon cycle?

5b. What is the role of soil in the carbon cycle?


6a. Do human interactions alter natural energy flow and biogeochemical cycles?

6b. Contrast agricultural systems with natural ecosystems.

6c. What effect does deforestation and biomass burning have on the biosphere?

6d. Do humans use a disproportionate amount of net primary productivity? Explain.



1a. Photosynthesis is the process in which plants use solar energy to convert carbon dioxide and water into carbohydrates. Photosynthesis is important because it creates food and oxygen, necessary resources for the continuation of life on Earth. See Section 1.1.

1b. Light in the visible wavelengths, from about 0.4-0.7 µm, is the solar radiation used for photosynthesis and is termed photosynthetically active radiation or PAR. See Section 1.1.

1c. Leaves appear green to the human eye because leaf surfaces reflect more green light than other visible wavelengths. See Section 1.1.

2a. Respiration is the reverse of photosynthesis. During respiration, stored energy is converted into available energy necessary for the maintenance and formation of cell material. This is necessary for all metabolic processes. See Sections 1.1 and 2.3.1.

2b. No, respiration can occur with or without light.

2c. Food produced by photosynthesis forms the base of most ecosystem food chains. Herbivores eat plant material and carnivores eat herbivores, gaining some energy from the foods they consume. Only a fraction of the energy consumed at a lower level is carried to the next level. See Section 2.1 and Figure 3.01.

3a. Plants are called primary producers because they produce the material and store the energy at the bottom of the food chain. In other words, all animals are dependent, either directly or indirectly on the food materials stored in plants. See Section 2.1 and Figure 3.01.

3b. Gross primary production (GPP) is the total rate at which material is produced and net primary production (NPP) is the rate at which material is accumulated in excess of respiration. In other words, NPP is GPP minus respiration. See Section 1.2.

3c. No. Net primary production is directly related to ecosystem conditions. Ecosystems where the environment most favors plant growth and lower respiration rates will have the highest rates of NPP . See Section 1.2.

4a. Plants are important components of the hydrologic cycle. They act as physical buffers by slowing down the flow and impact of hard rain, while their roots help to bind soil. This helps the soil to absorb more water and inhibits erosion. Vegetation uptakes, stores, and eventually releases water back into the atmosphere by the process of transpiration (also known as evapotranspiration). See Section 2.2 and Figure 3.02.

4b. As vegetation is removed, the rate of runoff tends to increase. Less water is absorbed and retained in the ecosystem, and soil erosion is accelerated. See Section 2.2 and Figure 3.02.

5a. Carbon is the most abundant element in the nonwater portions of plants (about 50%). As vegetation grows, carbon is taken from the atmosphere and stored in plants. When a plant decays or is burned, this carbon is released back to the atmosphere and the soil. See Section 2.3.1 and Figure 3.03.

5b. Soil is a large sink for carbon. Organisms in the soil transform dead organic material into carbon-rich humus. Carbon may remain in soils for long periods of time. See Section 2.3.1 and Figure 3.03.

6a. Yes. Human activities can significantly modify the natural pathways of energy flow and biogeochemical cycles. This includes changing amounts of elements stored and released to various Earth realms, alteration of nutrient and energy flow pathways, depletion and removal of soils, disruption and destruction of habitats, and decreased diversity in plant and animal communities. See Section 3.1.

6b. Production is maximized in agricultural systems, but requires more energy and water input than natural ecosystems to sustain yields. Monocultural crops are generally more susceptible to disease and insect attack than natural ecosystems. See Section 3.2.

6c. Deforestation reduces biological diversity and increases water runoff and soil erosion. Deforestation also changes local and sometimes regional climate patterns by altering surface albedos, wind speeds, and latent heat cooling. Biomass burning releases carbon dioxide, trace gases and particulates into the atmosphere. See Section 3.3

6d. Yes. When indirect usage of NPP is considered, the total of global terrestrial NPP coopted by humans and domestic animals ranges from roughly 40-58%. This indirect appropriation of NPP by humans is substantially disproportionate to direct use (3% of global NPP) by humans. The more NPP humans coopt, the less available for natural ecosystems. See Section 3.1.