Where does the energy come from that power the recharging of the ATP molecule?

Cellular Respiration is a chemical process that occurs in all living cells, when trapped energy in the bonds of food molecules are converted to the stored energy in ATP molecules.

Cellular Respiration is a series of chemical reactions that frees the energy in food molecules, making it available to cells.

The chemical process of Cellular Respiration starts when a food molecule (i.e. glucose) enters the cell and is acted upon by the enzymes in the cytoplasm of the cell.

The steps of Cellular Respiration are controlled by ENZYMES.

This process takes place in the MITOCHONDRIA, in the presence of oxygen and is called AEROBIC RESPIRATION (oxygen requiring).

Respiration is generally defined as OXYGEN-REQUIRING; but respiration can also occur WITHOUT OXYGEN (ANAEROBIC RESPIRATION).

There are 2 steps to the Cellular Respiration Process:
1. GLYCOLYSIS (ANAEROBIC RESPIRATION)
– Glycolysis is the 1st series of chemical reactions in cellular respiration, in which glucose is converted to pyruvic acid.
– The entire process can take place whether or not oxygen is present. So Glycolysis is sometimes referred to as ANAEROBIC RESPIRATION.
– It is a series of reactions that take place in the CYTOPLASM of the cell.
– Each chemical reaction is catalyzed (to cause an action to begin) by an enzyme.
– The chemical reactions of glycolysis are anaerobic, because they occur without oxygen.
– All organisms can carry on glycolysis.
– Once GLUCOSE is present in the cell, its chemical bonds are broken down by glycolysis, with the help of enzymes, releasing free energy to make ATP.
– Glucose is not the only cellular fuel. ALL SIMPLE SUGARS in the diet are catabolized (process by which complex substances are converted to simpler compounds) and used in cellular respiration.
– To break down glucose, a small amount of energy is needed to get the reactions started, 2 ATP molecules supply this energy.
– In the initial reaction a glucose molecule is broken up into 2 new molecules of PYRUVIC ACID.
– Energy is released as the bonds of the glucose are broken to produce the 2 pyruvic acid molecules.
– During glycolysis one 6-carbon glucose molecule is broken down into two 3-carbon (pyruvic acid) molecules. Two ATP molecules are also produced.
– (1) 6-carbon glucose molecule -> (2) 3-carbon (pyruvic acid) molecules + (2) ATP molecules
– After the process of glycolysis has occurred, only about 10% of all available energy within the glucose molecule has been used.
– More energy can be extracted from the molecule, by using AEROBIC RESPIRATION.
– The KREB CYCLE is the aerobic process of cellular respiration.
– If oxygen is not present, the final product of glycolysis, PYRUVATE, is FERMENTED into ethanol and lactate.

2. There are 2 stages to AEROBIC RESPIRATION
1. KREBS CYCLE
2. ELECTRON TRANSPORT CHAIN

KREBS CYCLE
– The 2 pyruvic acid molecules enter the Krebs cycle.
– The Krebs cycle takes place in the MITOCHONDRIA of the cell, where the enzymes that run the reactions of the Krebs cycle are located.
– The Krebs cycle is a repeating cycle of aerobic reactions that breakdown the pyruvic acids produced by glycolysis.
– The Krebs cycle rotate twice, once for each of the pyruvic acid molecules and produces 2 ATP molecules, several CO2 (carbon dioxide) molecules, and HYDROGEN-CARRIER MOLECULES.
– The hydrogen-carrier molecules move to the ELECTRON TRANSPORT CHAIN in the mitochondria.

ELECTRON TRANSPORT CHAIN
– The molecules of the electron transport chain carry high-energy electrons from the hydrogen atoms down to lower and lower energy levels, releasing considerable amounts of energy along the way.
– A total of 34 more ATP molecules are produced from the electron transport process. The electron transport chain does not produce ATP directly, it produces a PROTON.
– The transported electrons, now deplete of most of their energy, are transferred to an OXYGEN ATOM.
– The oxygen atom combines with 2 hydrogen ions an creates WATER (H2O).
– C6 H12 O6 + 6 O2 -> 6 CO2 + 6 H2O + 36ATP
Glucose + Oxygen -> Carbon Dioxide + Water + Free Energy

FERMENTATION
When there is no oxygen available to complete the breakdown of glycolysis (the Krebs cycle requires oxygen) fermentation occurs.

Fermentation is the chemical release of energy from food without the use of oxygen.

The chemical process of fermentation does not release any more energy from the food (like the Krebs cycle and the electron transport chain) it only frees up molecules for continued respiration.

The process of converting pyruvate into ethanol and lactate is called fermentation.