activation energy

Examples of activation energy in the following topics:

• Activation Energy

  • This small amount of energy input necessary for all chemical reactions to occur is called the activation energy (or free energy of activation) and is abbreviated EA.
  • However, the measure of the activation energy is independent of the reaction's ΔG.
  • The source of the activation energy needed to push reactions forward is typically heat energy from the surroundings.
  • The higher the activation energy, the slower the chemical reaction will be.
  • This figure implies that the activation energy is in the form of heat energy.

• Enzyme Catalysis

  • Enzymes are proteins that accelerate biochemical transformations by lowering the activation energy of reactions.
  • Enzymes are proteins that are able to lower the activation energy for various biochemical reactions.
  • At the active site, the substrate(s) can form an activated complex at lower energy.
  • This change stabilizes the transition state complex, and thus lowers the activation energy.
  • Covalent catalysis: covalent bonding to side chains or cofactors can lower the energy of the transition state.

• ATP: Adenosine Triphosphate

  • ATP provides the energy for both energy-consuming endergonic reactions and energy-releasing exergonic reactions, which require a small input of activation energy.
  • Since ATP hydrolysis releases energy, ATP synthesis must require an input of free energy.
  • Exactly how much free energy (∆G) is released with the hydrolysis of ATP, and how is that free energy used to do cellular work?
  • To harness the energy within the bonds of ATP, cells use a strategy called energy coupling.
  • ATP is the primary energy currency of the cell.

• Humans: Work, Energy, and Power

  • By far the largest fraction goes to thermal energy, although the fraction varies depending on the type of physical activity.
  • The fraction going into each form depends both on how much we eat and on our level of physical activity.
  • The rate at which the body uses food energy to sustain life and to do different activities is called the metabolic rate.
  • We can measure the energy people use during various activities by measuring their oxygen use.
  • By far the largest fraction goes to thermal energy, although the fraction varies depending on the type of physical activity.

• Human Metabolism

  • The 1st law of thermodynamics explains human metabolism: the conversion of food into energy that is used by the body to perform activities.
  • Metabolism in humans is the conversion of food into energy, which is then used by the body to perform activities.
  • Considering the body as the system of interest, we can use the first law to examine heat transfer, doing work, and internal energy in activities ranging from sleep to heavy exercise.
  • For example, one major factor in such activities is body temperature—normally kept constant by heat transfer to the surroundings, meaning that Q is negative (i.e., our body loses heat).
  • Eating increases the internal energy of the body by adding chemical potential energy.

• The Role of Energy and Metabolism

  • All organisms require energy to complete tasks; metabolism is the set of the chemical reactions that release energy for cellular processes.
  • Plants convert light energy from the sun into chemical energy stored in molecules during the process of photosynthesis.
  • Some of these chemical reactions are spontaneous and release energy, whereas others require energy to proceed.
  • Energy is needed to perform heavy labor and exercise, but humans also use a great deal of energy while thinking and even while sleeping.
  • Just as energy is required to both build and demolish a building, energy is required for both the synthesis and breakdown of molecules.

• Secondary Active Transport

  • Unlike in primary active transport, in secondary active transport, ATP is not directly coupled to the molecule of interest.
  • While this process still consumes ATP to generate that gradient, the energy is not directly used to move the molecule across the membrane, hence it is known as secondary active transport.
  • Both antiporters and symporters are used in secondary active transport.
  • This secondary process is also used to store high-energy hydrogen ions in the mitochondria of plant and animal cells for the production of ATP.
  • The potential energy that accumulates in the stored hydrogen ions is translated into kinetic energy as the ions surge through the channel protein ATP synthase, and that energy is used to convert ADP into ATP.

• Life History Patterns and Energy Budgets

  • Plants, for example, acquire energy from the sun via photosynthesis, but must expend this energy to grow, maintain health, and produce energy-rich seeds to produce the next generation.
  • Thus, all species have an energy budget in which they must balance energy intake with their use of energy for metabolism, reproduction, parental care, and energy storage, as when bears build up body fat for winter hibernation.
  • Most of their energy budget is used to produce many tiny offspring.
  • Animal species that have few offspring during a reproductive event usually give extensive parental care, devoting much of their energy budget to these activities, sometimes at the expense of their own health.
  • It is a matter of where the energy is used: for large numbers of seeds or for fewer seeds with more energy.

• Transferring of Energy between Trophic Levels

  • Energy is lost as it is transferred between trophic levels; the efficiency of this energy transfer is measured by NPE and TLTE.
  • The fact is, after four to six energy transfers, there is not enough energy left to support another trophic level.
  • Assimilation is the biomass (energy content generated per unit area) of the present trophic level after accounting for the energy lost due to incomplete ingestion of food, energy used for respiration, and energy lost as waste.
  • The extra heat generated in endotherms, although an advantage in terms of the activity of these organisms in colder environments, is a major disadvantage in terms of NPE.
  • Because the NPE is low, much of the energy from animal feed is lost.

• Catabolic-Anabolic Steady State

  • Anabolic reactions require energy.
  • ATP, a high energy molecule, couples anabolism by the release of free energy.
  • Anabolism requires the input of energy, described as an energy intake ("uphill") process.
  • Catabolism is a "downhill" process where energy is released as the organism uses up energy.
  • The balance between anabolism and catabolism is also regulated by circadian rhythms, with processes such as glucose metabolism fluctuating to match an animal's normal periods of activity throughout the day.