An Overview of Respiration
I. Metabolism and Cell Respiration
A. Metabolism, the chemical reactions in an organism, has two complementary parts:
1. Synthesis reactions- which combine small, simple organic molecules to form more complex compounds such as, proteins and nucleic acids, for cell growth and maintenance(these reactions consume energy)
2. Decomposition reactions- which release energy by breaking down organic food molecules.
a. Organisms use this energy to make ATP, the major energy carrier in metabolism
i. ATP molecules provide organisms w/ a ready supply of free energy in small, usable packets. Bc. ATP is made during decomposition and used during biosynthesis, it links the two together
Simple compounds produced by decomposition can also serve as carbon skeletons in biosynthesis
B. Cell respiration: a decomposition pathway that provides the energy cells need to function
1. It’s a series of reactions that release energy as they break down sugars and other substances to carbon dioxide and water
A. Each step in this decomposition is catalyzed by an enzyme
B. Respiration releases free energy by oxidizing sugars or other organic substrates. Some of this energy is conserved in ATP, which in turn provides the energy to power most life processes
2. Can occur in aerobic respiration- occurring in the presence of oxygen-oxygen is the oxidizing agent that receives electrons from the decomposed substrates
a. The raw materials for aerobic respiration are carbohydrates, fats, and proteins
b. the 6-carbon sugar glucose and glucose-phosphate are important substrates for respiration
i. animals produce them by digesting carbohydrates or by breaking down glycogen(animal starch) in the liver or muscles ii. glucose is also the main form in which animals transport carbohydrate through the bloodstream(sucrose fills this role in most plants) iii. plants produce glucose and glucose-phosphate by breaking down sucrose and starch
3. Anaerobic respiration-occurring without oxygen-the substrate may be only partly decomposed, releasing less energy, or a nitrogen or sulfur compound may substitute for oxygen
4. Cell respiration releases energy by oxidizing glucose in small steps, these steps release enough energy to produce one molecule of ATP
5. Provides both ATP and carbon skeletons needed for biosynthesis
II. The Stages of Aerobic Respiration- glycolysis, the Krebs Cycle, the electron transport system
A. Each stage is catalyzed by enzymes
1. Glycolysis- enzymes partially oxidize glucose and split it into two 3-carbon molecules
a. This oxidation provides enough energy to produce ATP
2. Krebs cycle- an enzyme releases a molecule of carbon dioxide from each 3-carbon molecule that was produces in glycolysis. Resulting 2-carbon molecules are oxidized completely to carbon dioxide
B. As glucose is oxidized in glycolysis and the Krebs cycle, it loses electrons and protons
Example: NAD+  NADH; NADP+  NADPH; FAD FADH+

C. NADH is oxidized as it donates protons and electrons to the electron transport system (this regenerates the supply of NAD+
1. The protons and electrons release energy as the ETS transfers them to oxygen, forming water
D. Bc oxygen must accept the electrons at the end of the ETS, stages 2 and 3 cannot occur w/o oxygen, hence aerobic respiration
III. Glycolysis
A. Glucose is an important raw material for glycolysis
B. Three important things happen during glycolysis-the glucose molecules breaks into two pieces, some ATP forms and some NAD+ id reduced to NADH
1. Glycolysis begins when an enzymes converts a molecule of glucose to glucose-6-phosphate
a. A molecule of ATP provides the phosphate and energy to power the reaction
2. Another enzyme rearranges the glucose-6-phosphate and a second ATP molecule donates another phosphate group
3. The molecule splits into two 3-carbon sugar-phosphates
4. Other enzymes catalyze the rearrangement and partial oxidation of these molecules to form