Harvesting Energy: Glycolysis and Cellular Fermentation

Chapter 7

ATP Is Universal
Energy Source

•      Photosynthesizers get energy from the sun

 

•      Animals get energy second- or third-hand from plants or other organisms

 

•      Regardless, the energy is converted to the chemical bond energy of ATP

 Making ATP

•      Plants make ATP during photosynthesis

•      Cells of all organisms make ATP by breaking down carbohydrates, fats, and protein

Main Types of
Energy-Releasing Pathways

Anaerobic pathways

 

•      Evolved first

•      Don’t require oxygen

•      Start with glycolysis in cytoplasm

•      Completed in cytoplasm

 

Aerobic pathways

 

•      Evolved later

•      Require oxygen

•      Start with glycolysis in cytoplasm

•      Completed in mitochondria

Glucose 

•      A simple sugar

     (C6H12O6)

 

•      Atoms held together by covalent bonds

Main Pathways Start
with Glycolysis

•      Glycolysis occurs in cytoplasm

•      Reactions are catalyzed by enzymes

 

    Glucose                                    2 Pyruvate

     (six carbons)                     (three carbons)

 

 Glycolysis Occurs
in Two Stages

•      Energy-requiring steps

–  ATP energy activates glucose and its six-carbon derivatives

•      Energy-releasing steps

–  The products of the first part are split into three-carbon pyruvate molecules

–  ATP and NADH form

Glycolysis

Net Energy Yield
from Glycolysis

 Energy requiring steps:

                         2 ATP invested

 

Energy releasing steps:

                        2 NADH formed

                        4 ATP formed

 

Net yield is 2 ATP and 2 NADH

The Role of Electron Carriers

•      NAD+ and FAD accept electrons and hydrogen from intermediates during the first two stages

•      When reduced, they are NADH and FADH2

•      In the third stage, these carriers deliver the electrons and hydrogen to the transport system

Anaerobic Pathways 

•      Do not use oxygen

•      Produce less ATP than aerobic pathways

 

–  Fermentation pathways

 Fermentation Pathways

•      Begin with glycolysis

•      Do not break glucose down completely to carbon dioxide and water

•      Yield only the 2 ATP from glycolysis

•       Steps that follow glycolysis serve only to regenerate NAD+

Lactate Fermentation

Alcoholic Fermentation

Yeasts

•      Single-celled fungi

•      Carry out alcoholic fermentation

•      Saccharomyces cerevisiae

–  Baker’s yeast

–  Carbon dioxide makes bread dough rise 

•      Saccharomyces ellipsoideus

–  Used to make beer and wine

Overview of Aerobic Respiration

 

 

C6H1206 + 6O2          6CO2 + 6H20

     glucose       oxygen                        carbon        water

                                         dioxide

Overview of Aerobic Respiration

On to Aerobic Respiration 

•      Occurs in the mitochondria

•      Pyruvate is broken down to carbon dioxide

•      More ATP is formed

•      More electron carriers are formed

Preparatory Reactions

pyruvate + coenzyme A + NAD+

 

acetyl-CoA + NADH + CO2

 

•      One of the carbons from pyruvate is released in CO2

•      Two carbons are attached to coenzyme A and continue on to the Krebs cycle

•      One energy carrier is formed

What is Acetyl-CoA?

•      A two-carbon acetyl group linked to coenzyme A

                        CH3
 
                        C=O

 

                        Coenzyme A

Results of the Krebs cycle

•      All of the carbon molecules in pyruvate end up in carbon dioxide

•      Electron carriers are formed (they pick up electrons and hydrogen)

•      One molecule of ATP is formed

•      Four-carbon oxaloacetate is regenerated

Electron carriers produced thus far

 

•      Glycolysis                                      2 NADH

•      Preparatory

     reactions                                        2 NADH

•      Krebs cycle               2 FADH2  +  6 NADH

 

•      Total                           2 FADH2  + 10 NADH                                         

Electron Transport Phosphorylation 

•      Occurs in the mitochondria

•      Electron carriers deliver electrons to electron transport systems

•      Electron transport sets up H+ ion gradients

•      Flow of H+ down gradients powers ATP formation

 Electron Transport

•      Electron transport systems are embedded in inner mitochondrial compartment

•      NADH and FADH2 give up electrons that they picked up in earlier stages to electron transport system

•      Electrons are transported through the system

•       The final electron acceptor is oxygen

Creating an H+ Gradient

Making ATP:
Chemiosmotic Model

Importance of Oxygen

•      Electron transport phosphorylation requires the presence of oxygen

 

•      Oxygen withdraws spent electrons from the electron transport system, then combines with H+ to form water

Summary of Energy Harvest
(per molecule of glucose)

•      Glycolysis

–  2 ATP formed by substrate-level phosphorylation

•      Krebs cycle and preparatory reactions

–  2 ATP formed by substrate-level phosphorylation

•      Electron transport phosphorylation

–  32 ATP formed

Efficiency of
 Aerobic Respiration

•      686 kcal of energy are released 

•      7.5 kcal are conserved in each ATP

•      When 36 ATP form, 270 kcal (36 X 7.5) are captured in ATP

•      Efficiency is 270 / 686 X 100 = 39 percent

•      Most energy is lost as heat

Carbohydrate Breakdown
and Storage 

•      Glucose is absorbed into blood

•      Pancreas releases insulin

•      Insulin stimulates glucose uptake by cells

•      Cells convert glucose to glucose-6-phosphate

•      This traps glucose in cytoplasm where it can be used for glycolysis

 Making Glycogen

•      If glucose intake is high, ATP-making machinery goes into high gear

•      When ATP levels rise high enough, glucose-6-phosphate is diverted into glycogen synthesis (mainly in liver and muscle)

•       Glycogen is the main storage polysaccharide in animals

Using Glycogen

•      When blood levels of glucose decline, pancreas releases glucagon

•      Glucagon stimulates liver cells to convert glycogen back to glucose and to release it to the blood

•      (Muscle cells do not release their stored glycogen)

Energy Reserves

•      Glycogen makes up only about 1 percent of the body’s energy reserves

•      Proteins make up 21 percent of energy reserves

•      Fat makes up the bulk of reserves (78 percent)

Energy from Proteins

•      Proteins are broken down to amino acids

•      Amino acids are broken apart

•      Amino group is removed, ammonia forms, is converted to urea and excreted

•       Carbon backbones can enter the Krebs cycle or its preparatory reactions

Energy from Fats

•      Most stored fats are triglycerides

•      Triglycerides are broken down to glycerol and fatty acids

•      Glycerol is converted to PGAL, an intermediate of glycolysis

•      Fatty acids are broken down and converted to acetyl-CoA, which enters Krebs cycle

Evolution of Metabolic Pathways 

•      When life originated, atmosphere had little oxygen

•      Earliest organisms used anaerobic pathways

•      Later, noncyclic pathway of photosynthesis increased atmospheric oxygen

•      Cells arose that used oxygen as final acceptor in electron transport

Processes Are Linked

Aerobic Respiration

•      Reactants

–   Sugar

–   Oxygen

•      Products

–   Carbon dioxide

–   Water

Photosynthesis

•      Reactants

–   Carbon dioxide

–   Water

•      Products

–   Sugar

–   Oxygen

Life Is System
of Prolonging Order

•      Powered by energy inputs from sun, life continues onward through reproduction

•      Following instructions in DNA, energy and materials can be organized, generation after generation

•      With death, molecules are released and may be cycled as raw material for next generation

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