Cellular Respiration: Harvesting Chemical Energy
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Transcript of Cellular Respiration: Harvesting Chemical Energy
Cellular Respiration: Harvesting Chemical Energy
• Cellular respiration is an example of a metabolic pathway
– A series of chemical reactions in cells
• All of the reactions involved in cellular respiration can be grouped into three main stages
– Glycolysis
– The Krebs cycle
– Electron transport
The Metabolic Pathway of Cellular Respiration
A Road Map for Cellular Respiration
Cytosol
Mitochondrion
High-energyelectronscarriedby NADH
High-energyelectrons carriedmainly byNADH
Glycolysis
Glucose2
Pyruvicacid
KrebsCycle
ElectronTransport
Figure 6.7
Stage 1: Glycolysis
• A molecule of glucose is split into two molecules of pyruvic acid
– These molecules then donate high energy electrons to NAD+, forming NADH
– Glycolysis makes some ATP directly when enzymes transfer phosphate groups from fuel molecules to ADP
– Glycolysis occurs in the cytosol of the cell.
Figure 6.8
Glucose
2 Pyruvic acid
PGAL
If oxygen is available, pyruvic acid enters the pathways of aerobic respiration (respiration with oxygen)
Stage 2: The Krebs Cycle
• The Krebs cycle completes the breakdown of sugar
• Occurs inside of the mitochondria (mitochondrial matrix)
• In the Krebs cycle, pyruvic acid from glycolysis is first “prepped” into a usable form, Acetyl-CoA
2
Pyruvicacid
Aceticacid
Coenzyme A
Acetyl-CoA(acetyl-coenzyme A)
CO2
Mitochondria
• The Krebs cycle extracts the energy of sugar by breaking the acetic acid molecules all the way down to CO2
– The cycle uses some of this energy to make ATP
– The cycle also forms NADH and FADH2
Hans Krebs
2
3
4
6
One glucose molecule causes two turns of the Krebs cycle
The two turns produce 6 NADH, 2 FADH2, 2 ATP, and 4 CO2.
So now there have been 4 molecules of ATP created up to this point (remember the 2 created during glycolysis)
Things to Remember
Stage 3: Electron Transport
• Electron transport releases the energy your cells need to make the most of their ATP
• The molecules of electron transport chains are built into the inner membranes of mitochondria
– The chain functions as a chemical machine that uses energy released by the “fall” of electrons to pump hydrogen ions across the inner mitochondrial membrane
– These ions store potential energy
• When the hydrogen ions flow back through the membrane, they release energy
– The ions flow through ATP synthase
– ATP synthase takes the energy from this flow and synthesizes ATP (Chemiosmosis)
Figure 6.12
Proteincomplex
Electroncarrier
Innermitochondrialmembrane
Electronflow
Electron transport chain ATP synthase
The Versatility of Cellular Respiration
• Cellular respiration can “burn” other kinds of molecules besides glucose
– Diverse types of carbohydrates
– Fats
– Proteins
Figure 6.13
Food
Polysaccharides Fats Proteins
Sugars Glycerol Fatty acids Amino acids
Amino groups
Glycolysis Acetyl-CoA
KrebsCycle Electron
Transport
Adding Up the ATP from Cellular Respiration
Figure 6.14
Cytosol
Mitochondrion
Glycolysis
Glucose2
Pyruvicacid
2Acetyl-
CoA
KrebsCycle Electron
Transport
bydirectsynthesis
by directsynthesis
byATPsynthase
Maximumper
glucose:
FERMENTATION: ANAEROBIC HARVEST OF FOOD ENERGY
• Some of your cells can actually work for short periods without oxygen
– For example, muscle cells can produce ATP under anaerobic conditions
• Fermentation
– The anaerobic harvest of food energy
• Human muscle cells can make ATP with and without oxygen
– They have enough ATP to support activities such as quick sprinting for about 5 seconds
– A secondary supply of energy (creatine phosphate) can keep muscle cells going for another 10 seconds
– To keep running, your muscles must generate ATP by the anaerobic process of fermentation
Fermentation in Human Muscle Cells
• Glycolysis is the metabolic pathway that provides ATP during fermentation
– Pyruvic acid is reduced by NADH, producing NAD+, which keeps glycolysis going
– In human muscle cells, lactic acid is a by-product
Figure 6.15a
2 ADP+ 2
Glycolysis
Glucose
2 NAD
2 Pyruvicacid
+ 2 H
2 NAD
2 Lacticacid
(a) Lactic acid fermentation
Fermentation in Microorganisms
• Various types of microorganisms perform fermentation
– Yeast cells carry out a slightly different type of fermentation pathway
– This pathway produces CO2 and ethyl alcohol
• The food industry uses yeast to produce various food products
Figure 6.16
Two Types of Fermentation