Cellular Respiration Releases Energy from Organic Compounds Unit C: Section 7.1-7.2 Cellular Respiration Releases Energy from Organic Compounds

Overview of Cellular Respiration Cellular respiration releases energy by oxidizing glucose into carbon dioxide, which means that electrons are removed from glucose, releasing energy and producing carbon dioxide and water. Cellular Respiration occurs in the cytoplasm and the mitochondria in plant cells. Cellular respiration begins with a process called glycolysis or “the splitting of sugars.” Following glycolysis, cellular respiration can be carried out in both aerobic (with oxygen) and anaerobic conditions (without oxygen).

Glycolysis Glycolysis is an anaerobic process, which means that it can proceed without oxygen. It occurs in the cytoplasm, just outside the mitochondria. The role of glycolysis is to split glucose into two, three carbon compounds called pyruvate.

Glycolysis Glycolysis begins with a single glucose molecule. 2 ATP molecules are used to break down the glucose molecule into two intermediate 3 carbon compounds.

From the two intermediate 3 carbon compounds, a series of reactions occur to produce two identical 3 carbon compounds called pyruvate. During the conversion from the intermediate molecules to pyruvate, 4 ATP’s and 2 NADH’s are produced. Later the NADH’s are used to convert FAD into FADH2 (flavin adenine dinucleotide), another high energy molecule.

Final Products of Glycolysis 2 ATP’s ( 4 produced – 2 used = 2) 2 FADH2 2 pyruvate

The 3 Fates of Pyruvate Once pyruvate is produced, it can proceed in 3 different ways, depending on the presence or absence of oxygen. The Kreb’s Cycle (aerobic) Lactate Fermentation (anaerobic) Ethanol Fermentation (anaerobic)

Prep for The Kreb’s Cycle When sufficient oxygen is present in the cell, pyruvate is transported into the matrix of the mitochondria in preparation for the Kreb’s Cycle. Before pyruvate can enter The Kreb’s Cycle it must undergo one reaction. A molecule called Coenzyme A (Co A) reacts with pyruvate (3C) to produce acetyl coenzyme A (2 C) and carbon dioxide (1C). During this process one molecule of NAD+ is reduced to NADH.

The Kreb’s Cycle (Aerobic Cellular Respiration) Step 1: Acetyl CoA (2C) combines with a 4C starting compound to produce a 6C compound. Step 2: The 6C compound donates an electron to NAD+, reducing it to NADH. This produces a 5C molecule and a free molecule of CO2 .

Step 3: This 5C molecule donates another electron to NAD+, reducing it to NADH as well as adds a phosphate to ADP to form ATP. This produces a 4C compound and another free molecule ofCO2 .

Step 4: The 4C compound reduces FAD to FADH2 and NAD+ to NADH. This recreates the 4C starting molecule for the Kreb’s cycle, so the process can continue.

Final Products for The Kreb’s Cycle (including preparation) 4 NADH 8 NADH 1 FADH2 x 2 = 2 FADH2 1 ATP 2 ATP 2 CO2 4 CO2 ** Because glucose breaks into 2 pyruvate molecules, the Kreb’s cycle must happen twice to fully break down glucose.

The Electron Transport System and Chemiosmosis The NADH and FADH2 molecules produced during aerobic cellular respiration, transport hydrogen ions and their accompanying electrons to an electron transport system. Just like in photosynthesis the electrons are passed down the chain, losing energy with each transfer.

The energy released is used for pumping hydrogen ions out of the inner matrix into the intermembrane space of the mitochondria. The hydrogen ions re-enter the inner matrix through ATP synthase, which uses the energy in the concentration gradient to bind a phosphate group to ADP, forming ATP. This process is called chemiosmosis just like in photosynthesis

The final step on the electron transport system passes the electron to a oxygen atom. The oxygen accepts the electrons and hydrogen ions to form water.

Overall Production of ATP 1 NADH = 3 ATPS 1 FADH2 = 2 ATPS Glycolysis = 2 ATP  2 ATP 2 FADH2  4 ATP Prep for Kreb’s = 2 NADH  6 ATP (2 pyruvates)

Kreb’s Cycle = 6 NADH  18 ATP 2 FADH2  4 ATP 2 ATP  2 ATP = 36 ATP (2 cycles per one glucose molecule)