Chapter 5 Summary

CONCEPT 5.1 Organisms obtain energy from sunlight, from inorganic chemical compounds, or through the consumption of organic compounds.

• Autotrophs convert energy from sunlight (photosynthesis) or inorganic chemicals (chemosynthesis) into energy stored in the carbon–carbon bonds of carbohydrates and fats.
• Heterotrophs acquire energy by consuming organic compounds from other organisms, living or dead.

CONCEPT 5.2 Radiant and chemical energy captured by autotrophs is converted into stored energy in carbon–carbon bonds.

• During chemosynthesis, bacteria and archaea oxidize inorganic substrates to obtain energy, which they use to fix CO₂ and synthesize compounds for energy storage.
• Photosynthesis has two main steps, the absorption of sunlight by pigments to produce energy in the form of ATP and NADPH (the light reaction), and the use of that energy in fixation of CO₂ by the Calvin cycle, and subsequent synthesis of carbohydrates (the dark reactions).
• Photosynthetic responses to variation in light levels, water availability, and nutrient availability include both short-term acclimatization and long-term adaptation.

CONCEPT 5.3 Environmental constraints resulted in the evolution of biochemical pathways that improve the efficiency of photosynthesis.

• Photorespiration operates in opposition to photosynthesis, lowering the rate of energy gain, particularly at high temperatures and low atmospheric CO₂ concentrations.
• The C₄ photosynthetic pathway concentrates CO₂ at the site of the Calvin cycle, minimizing the amount of photorespiration.
• CAM plants reduce transpirational water loss by opening the stomates at night to take up CO₂ and releasing it to the Calvin cycle during the day, when the stomates are closed.

CONCEPT 5.4 Heterotrophs have evolved mechanisms to acquire and assimilate energy efficiently from a variety of organic sources.

• Variations in the chemistry and availability of food determine how much energy heterotrophs gain from different food sources.
• Heterotrophs display tremendous diversity in morphological and physiological adaptations that enhance their efficiency of energy acquisition and assimilation.
• Optimal foraging theory predicts that animals will obtain the maximum amount of energy per unit of time and per unit of energy invested in seeking, capturing, and extracting food resources.
• The marginal value theorem suggests that an animal foraging in a heterogeneous environment should remain in a food patch until it depletes the food to the level found in the habitat as a whole.