Introduction to Cellular Respiration The majority of organisms on earth use glucose as their main energy source. Through a series of redox reactions glucose.

Introduction to Cellular Respiration

The majority of organisms on earth use glucose as their main energy source. Through a series of redox reactions glucose is broken down and free energy is released.

Aerobic Cellular Respiration is the most often used method of converting glucose to free energy.Aerobic means that oxygen is used in the

process. Respiration refers to the 20 or so reactions that

take place to free up the energy in glucose.

Introduction to Cellular Respiration

Overall chemical equation for cellular respiration:C6H12O6 + 6O2 6H2O + 6CO2 + nrg

Oxidation of GlucoseJust like in yesterday’s example with methane,

oxygen oxidizes the C-H in glucose in two ways:The 12 H are broken away to form 6 H2O,The 6 C to form 6 CO2.

In water and carbon dioxide the electrons are drawn closer to oxygen, therefore C and H are oxidized (LEO), oxygen is reduced (GER).

Oxidation of Glucose in the LabOxygen and glucose

are fairly stable molecules. They do not readily react.

Lots of activation energy is needed (flame)

2870 kJ/mol of energy is released as heat and light.

Oxidation of Glucose in the CellIn the cell enzymes catalyze each reaction

step, reducing the activation energy and making it easier for the cell to undergo aerobic cellular respiration.

3012 kJ/mol released. 34 % is trapped in the form of ATP. The rest is lost as heat and light.

Aerobes and AnaerobesOxygen is not the only oxidizing agent at

the end of the respiration process, other molecules such as NO

2, SO

4, CO

2, and Fe3+

are used in some forms of bacteria to help undergo respiration (obligate anaerobes)

Animals are obligate aerobes since they use and require oxygen as their final oxidizing agent.

Organisms that can tolerate the presence and absence of oxygen are called facultative aerobes (mostly bacteria).

Aerobic Cellular RespirationIn aerobic respiration there are three main goals:

break the bonds of glucose freeing the carbon to make CO2 (released as waste)

break the bonds of glucose freeing H to form water

trap as much free energy as possible in the form of ATP.

The entire process occurs in 4 main stages: Glycolysis, Pyruvate Oxidation, Krebs Cycle and the Electron Transport Chain (Figure 1 on page 94)

1. GLYCOLYSISGlycolysis is thought to have been the earliest

form of energy metabolism.It is the first 10 reactions of cellular respirationName means sugar-splittingIt occurs in the cytoplasm and is anaerobic.Each reaction is catalyzed by a specific

enzyme.(Figure 11, page 98, handouts).Glycolysis produces 2.1% of the entire free

energy of glucose in aerobic cellular respiration by:

Substrate Level PhosphorylationSubstrate-Level

Phosphorylation: the formation of ATP directly in an enzyme-catalyzed reaction. A phosphate containing compound transfers its phosphate group to ADP (forming ATP) directly on an enzyme.

Glycolysis Pathway

Steps in Glycolysis:Step 1: ATP is used and glucose is converted into glucose 6-

phosphate [hexokinase]. The phosphorylation primes glucose for Step 2.

Step 2: G6P is rearranged into Fructose 6-phosphate by [phosphoglucose isomerase].

Step 3: ATP is used to convert fructose 6-phosphate into fructose 1,6-bisphosphate [phosphofructokinase]. The phosphorylation primes glucose for cleavage.

Steps in Glycolysis:Step 4: Fructose 1, 6-biphosphate is split into

two trisoses, DHAP (dihydroxyacetone phosphate) and G3P (glyceraldehyde 3-phosphate). They are isomers. [aldolase]

Step 5: Immediately an enyzme changes DHAP into G3P [triose phosphate isomerase].

There are now 2 molecules of G3P.

Steps in Glycolysis:

Steps 6 through 10 happen twice (one for each molecule of G3P).

Step 6: A hydrogen and 2 electrons are removed from each G3P and transferred to NAD+ . G3P is oxidized and NAD+ is reduced to NADH + H+.

The energy released from this reaction results in the addition of an inorganic phosphate to G3P and 1-3 bisphosphoglycerate (BPG) is formed. [glyceraldehyde 3-phosphate dehydrogenase]

Steps in Glycolysis:NAD+ has a

nucleotide structure, it acts as a coenzyme and an electron carrier

Each NAD+ removes hydrogen and 2e- to form NADH.

NADH will transfer the electrons to another reaction later in cellular respiration (the Electron Transport Chain)

http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter25/animation__how_the_nad__works.html

Step 7: substrate level phosphorylation occurs - the high energy phosphate group is transferred to ADP as each 1,3-BPG is converted into 3-phosphoglycerate (3-PGA)[phosphoglycerate kinase]. 2 ATP are made.

Steps in Glycolysis:

3PGA will be converted into phosphoenolpyruvate , PEP:

(Step 8: each 3-PGA is converted into 2-PGA [phosphoglycerate mutase]. )

(Step 9: water is removed from each 2PGA to produce phosphoenolpyruvate (PEP) [enolase].)

Steps in Glycolysis:

Steps in Glycolysis:

Step 10: substrate level phosphorylation occurs – the final phosphate group is removed from each PEP producing pyruvate and 2 ATP [pyruvate kinase].

Summary2 ATP are used and 4 ATP are

made therefore a net production of 2 ATP (used for cell work)

2 NADH are produced (used later for production of more ATP)

2 pyruvate are left over (to next stage of cellular respiration)

SummaryNet reaction:

Glucose + 2ADP + 2P + 2NAD --> 2 pyruvate + 2ATP + 2NADH + 2H+

To SummarizeGlycolysis can be divided into TWO main

parts:Energy investment (steps 1-4) – using of ATPEnergy harvesting (steps 5-10) – production of

NADH and ATP

OR FOUR main parts:Activation/priming of glucose (steps 1-3)Sugar splitting (step 4 and 5)Reduction reactions (step 6)Substrate level phosphorylation (steps 7-10)