Introduction: Metabolism is the sum of all chemical reactions in an organism. These chemical reactions maintain cells, build new ones, and balances internal conditions (homeostasis). These reactions are fueled by a constant input of energy from the food we eat. Energy is not recycled. Thus, without a constant supply our metabolism would stop and life could not continue.
Where does the energy is food come from? Ultimately all energy comes from the sun. Plants store this radiant energy as chemical energy in the bonds of organic molecules like carbohydrates.

1. The Energy Currency of the Cell. (ATP)
   1. Energy is distributed inside cells by energy carrying molecules called adenosine triphosphate or ATP(ATP = 7 kilocalories of energy). ATP provides just the right amount of energy for a typical chemical reaction in the cell (3.5 to 7 kcal).
   2. Energy transfer is not efficient, losing much of the energy as heat or light. (7 kcal in ATP minus 3.5 kcal for a reaction leaves 3.5 kcal to be given off as heat.)
   3. The energy transferred by ATP is stored in high-energy phosphate bonds. ATP is made up of adenosine, ribose, and 3 phosphate groups.
      1. When the bonds between the last two phosphate groups break, energy is released. (-like using up a charged battery.)

              ATP => P + ADP + Energy

         (ADP = Adenosine diphosphate=2 phosphate groups)
         (P = free phosphate group)

      2. When the phosphate group is reattached, energy is stored in that bond. (Recharging the battery)

               ADP + Energy + P => ATP

      3. Two phosphate groups can be lost to form AMP (adenosine monophosphate), but this is not typical.
      4. Where does the energy come from to �recharge� the phosphate bond to create ATP? The energy comes from the bonds in glucose.

        

        

2. Transferring Energy from Glucose to ATP.
   Glucose is the main source of energy for many organisms. (Glucose = 650 kilocalories of energy). ATP is a more manageable form of energy for the cell, thus the energy is glucose must be transferred to ATP. Breaking down glucose all at once would burn up the cell; therefore glucose must be broken down in steps:
   1. Glycolysis: means �sugar breaking�
      When glucose first enters the cell, enzymes process and break glucose in half forming two molecules called pyruvic acids. This occurs in the cytoplasm and does not require oxygen.

           Glucose + 2 ATP => 2 pyruvic acids + 4 ATP

      (2 ATP provides the activation energy needed for glycolysis to begin. At the end of glycolysis, 4 ATP molecules are made. 2 of the 4 ATP will go back to start glycolysis, which leaves a net of 2 ATP available for the cell to use as energy.)

      1. To breakdown pyruvic acid fully and create many more ATP, it will enter the mitochondria where it undergoes cellular respiration. Cellular respiration occurs only if enough oxygen (from breathing) is available.
      2. If oxygen is not available pyruvic acid does not enter the mitochondria. Enzymes will transform the pyruvic acid using fermentation. Fermentation does not create anymore ATP.

   2. Cellular Respiration (also called aerobic respiration because it uses oxygen)
      In cellular respiration, the energy in the pyruvic acids is extracted and stored as ATP using two processes in the mitochondria: The Krebs Cycle and the Electron Transport Chain (ETS).
      1. Krebs Cycle
         Using oxygen, enzymes in the Krebs cycle break down pyruvic acid to form 3 carbon dioxide molecules, 2 ATP, and high-energy electron carriers called NADH and FADH2.

      2. Electron Transport Chain (ETC)
         ETC = Enzymes embedded in inner membranes of the mitochondria.
         1. NADH and FADH2 carry pairs of high-energy electrons from the Krebs cycle to the ETC. Theses carriers will pass their electrons down the ETC.
         2. As the high-energy electrons pass through the ETC, the energy in the electrons is used to form the high-energy phosphate bond in ATP.(32 ATP total from one glucose)
            
         3. The electrons, exhausted in the ETC, are picked up by oxygen to form water. (Oxygen is the best electron acceptor.)

      3. Overall chemical reaction for the complete breakdown of a glucose molecule using cellular respiration.

         
         Although cellular respiration is the most efficient way of getting energy out of glucose, it only recovers 40% of the energy. The rest of the energy is lost as heat.

   3. Anaerobic Respiration occurs in the absence of oxygen.
      1. 1. If oxygen were not available, then ATP production using cellular respiration would stop. (Life could stop.) However the first process mentioned, Glycolysis, could continue producing 2ATP because it does not require oxygen to work. The only problem is that glycolysis needs NAD+ molecules to pick up electrons that are produced.
         • If you remember, NADH gets rid of the electrons that it carries in the ETC of the mitochondria. Unfortunately, The ETC cannot work without oxygen. Without oxygen, NADH would build up in the cell and there would be no more NAD+ to continue the process of glycolysis.

      2. Fermentation is a primitive process of returning electrons (from NADH) back to pyruvic acid, freeing NAD+ molecules to continue glycolysis.
         There are two types:
         
         1. Lactic Acid Fermentation converts pyruvic acid into lactic acid. (toxic waste) It occurs in the cytoplasm of animal,bacterial, and fungal cells.
            1. When lactic acid builds up in muscle cells it causes burning and fatigue.
            2. This process in bacteria and fungi is used for the production of yogurt and cheeses.

         2. Alcoholic Fermentation converts pyruvic acid into alcohol and carbon dioxide. It occurs in the cytoplasm of yeast cells.
            1. This process is used in the production of alcoholic beverages and rising breads (Carbon dioxide bubbles make yeast breads rise).

        

        

3. Where does Glucose come from? Photosynthesis
   1. Photosynthesis is the process by which producers store light energy in carbohydrates and other molecules.
      
      (Photosynthesis reaction is the reverse of cellular respiration.)

      Requirements for Photosynthesis:

      1. Light -- Light is a form of radiant energy. White light or Visible light consists of different wavelengths seen as different colors. Each �color� carries a different amount of energy.
      2. Chloroplasts -- organelle in plants that changes light energy into the chemical energy stored in organic bonds.
      3. Pigments -- molecules that absorb light.
      4. Chlorophyll -- green pigments in the membranes of chloroplasts that reflect green light and absorb the energy is the blue and red regions.
      5. Water and Carbon Dioxide -- are both waste products of cellular respiration that are used as raw materials to create carbohydrates in photosynthesis.

   2. There are two types of reactions that occur in the chloroplasts: the Light Reactions (�photo� process) and the Calvin Cycle (�synthesis� process).

      1. The Light Reactions first convert light energy into two forms of chemical energy: ATP and NADPH.
         1. First, electrons in chlorophyll pigments (Photosystem II) absorb light energy, causing them to escape the pigment with high energy.
         2. Second, light splits water releasing low energy electrons to replace the ones that left the pigment. (Oxygen that we breathe is released in this process.)
         3. Then, the high-energy electrons that escaped move down an Electron transport chain (ETC), where the electron�s high energy is used to create ATP.
         4. The exhausted electrons are then transported to another chlorophyll molecule (Photosystem I), where it undergoes the same process. The only difference is instead of making ATP the electrons are captured in NADPH.

      2. In the Calvin Cycle enzymes combine the carbon from carbon dioxide with the H+ from NADPH to form simple sugars (using ATP energy to bond them).
         1. These simple sugars can then be used to build other carbohydrates (like starches, sucrose, and cellulose) or other molecules like lipids and proteins.

        

        

4. Comparing Cellular Respiration and Photosynthesis.
   1. Autotrophs / producers such as plants make their own food using photosynthesis to store energy in carbohydrates. Heterotrophs / consumers cannot make their own food and must �consume� carbohydrates to have a constant supply of energy.

   2. Photosynthesis and Cellular Respiration are interdependent on one another.
      1. Photosynthesis creates glucose and oxygen that is used in respiration.
      2. Respiration releases carbon dioxide and water that plants use in photosynthesis.

   3. Glucose is a major energy storage molecule for many organisms. Both plants and animals use cellular respiration to release the energy stored in glucose.