how cells release stored energy cell respiration
TRANSCRIPT
How Cells Release Stored Energy
Cell respiration
When a molecule has hydrogen, it has more energy and removing them releases the energy. The energy produced from the "burning" of glucose is used to make ATP. In chemistry this process is called the oxidation of glucose.
The hydrogen carriers NAD and FAD are used to help release the energy in glucose by moving hydrogens and electrons around.
• A simple sugar
(C6H12O6)
• Atoms held together by covalent bonds
Glucose
In-text figurePage 136
Summary Equation for Aerobic Respiration
C6H1206 + 6O2 6CO2 + 6H20 glucose oxygen carbon water
dioxide
•Glycolysis -in cytoplasm-does not use oxygen
Occurs in Two Stages
• Energy-requiring steps
– ATP energy activates glucose and its six-carbon
derivatives
– Actually uses 2 ATP’s
• Energy-releasing steps
– The products of the first part are split into 2 three
carbon pyruvate molecules
– 4 ATP and 2NADH form
– 4 ATP’s form – 2 ATP’s used = 2 net ATP’s made
Glycolysis is often called anaerobic respriation because it does not need oxygen. This process occurs in the cytoplasm. Two net molecules of ATP are made for cell use. It involves glucose being converted to two molecules of pyruvic acid (or pyruvate).
This process is not very efficient at converting the energy of glucose into ATP as only 2 ADP are phosphorylated instead of 32 as in Krebs and chemiosmosis.
Energy-Requiring Steps 2 ATP invested
Energy-Requiring Steps of Glycolysis
glucose
PGAL PGALPP
ADP
P
ATP
glucose-6-phosphate
Pfructose-6-phosphate
ATP
fructose1,6-bisphosphateP P
ADP
Figure 8.4(2)Page 137
Energy-Releasing
Steps
ADPATP
pyruvate
ADPATP
pyruvate
H2OP
PEP
H2OP
PEP
P
2-phosphoglycerate
P
2-phosphoglycerate
ADPATP
P3-phosphoglycerate
ADPATP
P3-phosphoglycerate
NAD+
NADHPi
1,3-bisphosphoglycerateP P
NAD+
NADHPi
1,3-bisphosphoglycerateP P
PGALP
PGALP
Figure 8.4 Page 137
Phosphorylations
Glycolysis: Net Energy Yield
Energy requiring steps: 2 ATP invested
Energy releasing steps:2 NADH formed 4 ATP formed
Net yield is 2 ATP, 2 pyruvic acid and 2 NADH
After glycolysis:
-If oxygen is present, the pyruvic acid will go into the mitochondria where the Kreb’s cycle (or citric acid cycle) is performed followed by the ETC. This allows the rest of the energy stored in the hydrogen to be extracted.
-If no there is no oxygen, then the Kreb's cycle can not be completed (or started in some organisms). A cell can continue doing glycolysis in the absence of oxygen to produce some ATP, BUT it must regenerate NAD to keep glycolysis going.
This is called anaerobic respiration (or fermentation). Pyruvate will form either lactic acid (muscles) or ethanol (bacteria, yeast or plants). In either case NAD is regenerated so that glycolysis can continue.
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
C6H12O6
ATP
ATPNADH
2 lactate
electrons, hydrogen from NADH
2 NAD+
2
2 ADP
2 pyruvate
2
4
energy output
energy input
GLYCOLYSIS
LACTATE FORMATION
2 ATP net
Alcoholic Fermentation
C6H12O6
ATP
ATPNADH
2 acetaldehyde
electrons, hydrogen from NADH
2 NAD+
2
2 ADP
2 pyruvate
2
4
energy output
energy input
GLYCOLYSIS
ETHANOL FORMATION
2 ATP net
2 ethanol
2 H2O
2 CO2
Anaerobic Electron Transport• Carried out by certain bacteria
• Electron transfer chain is in bacterial plasma membrane
• Final electron acceptor is compound from environment (such as nitrate), not oxygen
• ATP yield is low
After glysolysis, if oxygen is present aerobic respiration proceeds in the mitochondria.
• Preparatory reactions– Pyruvate is decarboxylated
and oxidized (releasing carbon dioxide)
– NAD+ is reduced– Acetyl Co-A is formed– Ex. 1
PreparatorySteps
pyruvate
NAD+
NADH
coenzyme A (CoA)
O O carbon dioxide
acetyl-CoA
Ex. 2
The Krebs Cycle
Overall Products
• Coenzyme A
• 2 CO2
• 3 NADH
• FADH2
• ATP
Overall Reactants
• Acetyl-CoA• 3 NAD+
• FAD
• ADP and Pi
The enzymes for Kreb's is found in the inner compartment of the mitochondria.
Summary of Krebs- Occurs in Mitochondria
2X's
Pyruvate---> 3 CO2 6 CO2
1 ADP ---> 1 ATP 2 ATP
4 NAD ---> 4 NADH 8 NADH
1 FAD ---> 1 FADH2 2 FADH2
(also, oxaloacetate regenerates so cycle can continue)
Coenzyme Reductions during First Two Stages
• Glycolysis 2 NADH• Preparatory
reactions 2 NADH• Krebs cycle 2 FADH2 + 6 NADH
• Total 2 FADH2 + 10 NADH
• Occurs in the mitochondria
• Coenzymes deliver electrons to electron transfer chains
• Electron transfer sets up H+ ion gradients
• Flow of H+ down gradients powers ATP formation (chemiosmotic)
Electron Transfer Phosphorylation
The purpose of chemiosmosis is to extract the energy found in NADH and FADH2 to make more ATP. This involves the cristae. There are electron transport chains that are used.
The electrons from the NADH and FADH2 are used to move on the electron transport chain. As the electrons move down the electron transport chain, H+ ions are pumped across the membrane.
The electrons from one NADH can pump 6 H+ across the membrane, but the electrons from FADH2 can only pump 4 H+ across the membrane.
Creating an H+ Gradient
NADH
OUTER COMPARTMENT
INNER COMPARTMENT
The outer compartment of the mitochondria becomes positive and the inside becomes negative like a battery. This "battery" can do work. The hydrogen ions can cross an F1 particle and make ATP.
It takes 2 H+ to cross the F1 particle to provide enough energy to make ATP.
Making ATP: Chemiosmotic Model
ATP
ADP+Pi
INNER COMPARTMENT
Summary
8 NADH2 x 6 H = 48 H+
2 FADH2(Krebs)x 4 H = 8 H+
2 FADH2(glyc.) X 4 H = 8 H+
64 H+
64 H+ --> 32 ATP
Importance of Oxygen
• Electron transport phosphorylation requires the presence of oxygen
• Oxygen withdraws spent electrons from the electron transfer chain, 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
Overview of Aerobic Respiration
CYTOPLASM
Glycolysis
Electron Transfer
Phosphorylation
KrebsCycle ATP
ATP
2 CO2
4 CO2
2
32
water
2 NADH
8 NADH
2 FADH2
2 NADH 2 pyruvate
e- + H+
e- + oxygen
(2 ATP net)
glucose
Typical Energy Yield: 36 ATP
e-
e- + H+
e- + H+
ATP
H+
e- + H+
ATP2 4
Figure 8.3Page 135
• 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 (at
best)
• Most energy is lost as heat
Efficiency of Aerobic Respiration
**Fats/Lipids = Glycerol and 3 fatty acids
Glycerol is converted to PGAL and respired in glycolysis.
The fatty acids are chopped into 2 carbon acetyl groups and used in the Krebs or citric acid cycle.
**Proteins = amino acids
The amino acids are sent to the liver where the liver removes the amine group. The left over acid is then used at some point in the Krebs cycle.
Alternative Energy Sources
FOOD
complex carbohydrates
simple sugars
pyruvate
acetyl-CoA
glycogenfats proteins
amino acids
carbon backbones
fatty acids
glycerol
NH3
PGAL
glucose-6-phosphate
GLYCOLYSIS
KREBS CYCLE
urea
Figure 8.11Page 145
Other organic molecules can be involved in respiration.
• When life originated, atmosphere had little
(none) free 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 and were more
efficient
Evolution of Metabolic Pathways
Processes Are Linked
sunlight energy
water+
carbondioxide
PHOTOSYNTHESIS
AEROBICRESPIRATION
sugarmolecules oxygen
In-text figurePage 146
Linked Processes
Photosynthesis
• Energy-storing pathway
• Releases oxygen
• Requires carbon dioxide
• Makes carbs
Aerobic Respiration
• Energy-releasing pathway
• Requires oxygen
• Releases carbon dioxide
• Breaks carbs