Biology  5/e   Raven/Johnson
	Student   Online Learning Center

Respiration: Virtually all the work that a living cell or organism performs utilizes ATP. ATP is the universal energy currency. Cellular respiration is the oxidative process that results in the synthesis of ATP. All organisms carry out cellular respiration, either anaerobically or aerobically. Autotrophs obtain the energy-rich organic compounds to be oxidized in respiration by making them. Since heterotrophs cannot do this, heterotrophs obtain their fuels for respiration by eating autotrophs or other heterotrophs.

Nitrogen fixation: The triple bond that holds two nitrogen atoms together to form N₂ must be broken before atmospheric nitrogen can be incorporated into proteins, nucleic acids, and other biologically important molecules. This process is called nitrogen fixation. The development of the ability to fix nitrogen was a major event in the evolution of metabolism. Because oxygen prevents nitrogen fixation, today the process is restricted to anaerobic bacteria and to cyanobacteria with their specialized heterocysts. Without these nitrogen fixers there would be no utilization of atmospheric nitrogen or nitrogen cycle.

Populations: A population is made up of all the individuals of a particular species that occur together in a given place at a given time. Populations have specific characteristics, such as age, dispersion, growth rates, survivorship curves, and age distributions. The members of a population interact with each other and with members of other populations and are influenced by environmental factors. All the populations in a given area, plus the nonliving factors they interact with, make up an ecosystem.

Competition: Individuals compete with each other for limited resources. The competition may be intraspecific or interspecific and may occur over resources such as water, sunlight, food, or physical space. How successfully different organisms compete will influence the flow of materials and energy through ecosystems.

Interactions in communities: Communities are made up of all the different populations in a given area. All these populations of different species interact in numerous and complex ways. They may compete with each other, they may have predator-prey interactions, or they may be part of a symbiotic relationship. Over evolutionary time, many populations have coevolved, interacting with and adjusting to each other. This coevolution has led to remarkable adaptations. All the interactions that occur in a community help define the nature of the ecosystem and help determine the flow of materials and energy.

Copyright ©1998 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use and Privacy Policy.
McGraw-Hill Higher Education is one of the many fine businesses of The McGraw-Hill Companies.
For further information about this site contact mhhe_webmaster@mcgraw-hill.com.