Biological Energy

Key concepts:
• A candle is made of long hydrocarbon molecules holding a lot of potential energy.
• Surrounding the candle are molecules of oxygen gas, also holding a lot of potential energy.
• The flame at the top of a burning candle is a chemical reaction taking place between these two types of molecules.
• In this reaction, the oxygen molecules are crashing violently into the molecules of wax (hydrocarbons).
  This causes the molecules to break apart and rearrange themselves into new molecular combinations.
• The new molecules formed at the end of the reaction are carbon dioxide and water.
• During the reaction, energy is given off into the environment in the form of light and heat.
• This reaction is obeying the second law of thermodynamics and is spontaneous.

Linking Spontaneous and Nonspontaneous Reactions

Key concepts:
• Living cells perform both spontaneous and nonspontaneous reactions every second.
• Food molecules are broken down by reacting them with oxygen.
• These are spontaneous reactions that give off energy into the environment. (Just like burning the candle in oxygen).
• Cells also carry out reactions that join together monomers (such as amino acids, sugars, nucleotides, etc.), to form long polymer chains.
• These synthetic, joining, reactions are nonspontaneous and will not take place unless extra energy is added from outside the reaction.
• Cells use the energy given off in spontaneous reactions to join together monomers and so create polymers (like proteins, polynucleotides, polysaccharides, etc.) the need.

The Amount of Energy in Food Molecules is a Problem

Key concepts:
• Polysaccharides (chains of sugars), proteins (chains of amino acids), polynucleotides (chains of nucleotides) and lipids (chains of CH2 units) are the major types of food molecules.
• These major food molecules are all polymers with lots of stored potential energy.
• For example, one gram molecule of a common type of lipid contains over 2 million calories of energy. This makes lipid molecules a good source of energy and a good molecule for storing energy.
• In a chemical reaction of oxygen with lipid, all this energy would be released into the environment.
• A sudden release of all this energy in an instant of time would be very explosive and dangerous. It would be like a bomb going off.
• Cells must therefore release the stored energy in food slowly, so they don't blow up or burst into flames.

ATP - Short Term Energy Currency

Key concepts:
• Adenosine Triphosphate is a nucleotide molecule with three phosphate groups joined in a row. (Look at the diagram of an ATP molecule on page 93 of the text book).
• About 7300 calories of energy are stored in the bond that joins the last phosphate group to the rest of the molecule.
• When cells join this third phosphate group onto the molecule it is a synthetic, joining, nonspontaneous reaction that requires the input of energy.
• This energy is then stored as part of the ATP molecule.
• When the third phosphate group is broken away from the ATP molecule, the stored energy is released.
• This, smaller amount of energy can then be safely used in other cellular reactions of life processes.
• ATP is the short term energy currency of the cell.

Aerobic Respiration

Key concepts:
• Respiration is a process for safely converting the large amounts of energy stored in food molecules into the smaller amounts of energy stored in ATP molecules.
• The breakdown and release of energy from a sugar molecule takes place in three phases.
• Glycolysis: The sugar molecule is broken into two smaller parts and some of the energy released is trapped as ATP and another energy carrying molecule called NADH.
• Krebs Cycle: The pieces of the glucose molecule left after glycolysis are broken down all the way to carbon dioxide.
  NADH and several other high-energy molecules like ATP are formed.
• NAD Regeneration: NADH molecules made in glycolysis and the Krebs cycle are converted back to NAD molecules (so they can be reused).
  During this complex regeneration energy is released and trapped in ATP molecules.
• Oxygen is combined with hydrogen to produce water.

The ATP-ADP Cycle

Key concepts:
• As food is broken down in aerobic respiration, the large stores of energy are gently converted to smaller stores in ATP molecules.
• Energy released from the food molecules is used to join together a phosphate group with the molecule adenine diphosphate (ADP), to create the ATP molecule and store energy.
• Elsewhere, the cell is carrying out nonspontaneous, synthetic, joining reactions (such as joining amino acids to form proteins), which require the input of energy.
• Cells use the energy in the ATP molecule to make sure these reactions can take place.
• As the energy in the ATP molecule is transferred to the nonspontaneous reaction, the bond between the last (third) phosphate group and the rest of the ATP molecule is broken, and the ADP molecule and a separate phosphate molecule are formed.
• The ADP molecule and the phosphate molecule are then recycled back to the area where aerobic respiration is taking place, and rejoined together again.
• In this way, ATP and ADP shuttle energy from the food breakdown processes to all the other processes in the cell that require an input of energy.