Energy in the Cell (College Board AP Biology)

The Importance of Energy in Living Organisms

• In order to maintain life, all organisms require a constant input of energy
• This can take the forms of:
  • Regular chemical energy via feeding
  • A regular supply of light energy to photosynthetic organisms
  • A supply of thermal energy (heat) to ectotherms to enable them to begin functioning metabolically (see example below of the marine iguana)
  • A supply of gravitational potential energy to enable a gannet to dive into the ocean and hunt its prey

CC BY-SA 4.0, via Wikimedia Commons

The marine iguana (Amblyrhynchus cristatus) has to bask in the morning sun, to absorb heat energy in order to be able to move and begin its normal daytime activities

 CC BY-SA 2.0, via Wikimedia Commons

A gannet (Morus bassana) utilizes gravitational potential energy to accelerate into a streamlined dive.
Kinetic energy takes the gannet deep (up to 20 meters) into the ocean as it hunts its prey

Thermodynamics and Biology

• All living organisms are governed by the Laws of Thermodynamics
• The First Law states: the total energy of a system remains constant, even if it is converted from one form to another
  • Energy cannot be created or destroyed, merely converted from one form to another
• The Second Law states: any spontaneously occurring process will always lead to an escalation in the entropy (disorder) of the universe

The Laws of Thermodynamics apply to biology in these ways

• When an animal eats, the chemical energy stored in its food is converted to
  • Kinetic energy (to make the organism move)
  • Different chemical energy stores (as the animal creates new cells/tissues)
  • Thermal energy, as heat which is eventually dissipated into the atmosphere
    • This heat energy is not 'lost' - it remains in the universe - but is no longer of direct use to the organism that released it
• Life therefore requires a highly ordered system that does not break the second law of thermodynamics

• Energy input must exceed energy output to maintain order and to power cellular processes
  • The difference between an organism's energy input and output is generally dissipated as heat to the surroundings
• Cellular processes that release energy may be coupled with cellular processes that require energy
  • For example, respiration and photosynthesis
  • Respiration is exergonic, whereas photosynthesis is endergonic
• The loss of order or excess energy flow results in death
  
  • For example an animal that dies of hypothermia has suffered a deficit of energy in versus energy out

Energy and Metabolic Pathways

• Energy-related pathways in biological systems are sequential to allow for a more controlled and efficient transfer of energy
  • The energy on one molecule of glucose is not released in one step within cells; this would cause cellular shock and damage
  • Instead, the energy is carried by 38 individual ATP molecules that can release that chemical energy in small amounts for various differing cellular processes
• A product of a reaction in a metabolic pathway is generally the substrate for the subsequent step in the pathway
  • An example is pyruvate, the product of glycolysis, being a substrate for the Krebs cycle of aerobic respiration