Download cell respiration

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.
Glucose
• A simple sugar
(C6H12O6)
• Atoms held
together by
covalent bonds
In-text figure
Page 136
Summary Equation for
Aerobic Respiration
C6H1206 + 6O2
6CO2 + 6H20
glucose
carbon
oxygen
dioxide
water
•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 of Glycolysis
2 ATP invested
Energy-Requiring
Steps
glucose
ATP
ADP
P
glucose-6-phosphate
P
fructose-6-phosphate
ATP
ADP
P
P
fructose1,6-bisphosphate
P
PGAL
P
PGAL
Figure 8.4(2)
Page 137
P
NAD+
Pi
P
PGAL
NADH
NAD+
Pi
PGAL
NADH
EnergyReleasing
Steps
P
P
P
P
1,3-bisphosphoglycerate
1,3-bisphosphoglycerate
ADP
ADP
ATP
ATP
Phosphorylations P
P
3-phosphoglycerate
3-phosphoglycerate
P
P
2-phosphoglycerate
H2
O
P
2-phosphoglycerate
PEP
PEP
P
ADP
ADP
ATP
ATP
pyruvate
H2
O
pyruvate
Figure 8.4
Page 137
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
GLYCOLYSIS
C6H12O6
2
ATP
energy input
2 NAD+
2 ADP
2
4
NADH
ATP
energy output
2 pyruvate
2 ATP net
LACTATE
FORMATION
electrons, hydrogen
from NADH
2
lactate
GLYCOLYSIS
Alcoholic
Fermentation
C6H12O6
2
ATP
energy input
2 NAD+
2 ADP
2
4
NADH
ATP
2 pyruvate
energy output
2 ATP net
ETHANOL
FORMATION
2 H2O
2 CO2
2 acetaldehyde
electrons, hydrogen
from NADH
2 ethanol
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.
PreparatorySteps
• Preparatory reactions
– Pyruvate is decarboxylated
and oxidized (releasing
carbon dioxide)
– NAD+ is reduced
– Acetyl Co-A is formed
– Ex. 1
pyruvate
coenzyme A (CoA)
NAD+
NADH
O
acetyl-CoA
O carbon dioxide
Ex. 2
The Krebs Cycle
Overall Reactants
Overall Products
•
•
•
•
•
•
•
•
•
Acetyl-CoA
3 NAD+
FAD
ADP and Pi
Coenzyme A
2 CO2
3 NADH
FADH2
ATP
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
• Preparatory
reactions
• Krebs cycle
2 NADH
2 FADH2 + 6 NADH
• Total
2 FADH2 + 10 NADH
2 NADH
Electron Transfer
Phosphorylation
• 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)
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
OUTER COMPARTMENT
NADH
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
INNER
COMPARTMENT
ADP
+
Pi
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
CYTOPLASM
2
glucose
ATP
4
Glycolysis
e- + H+
(2 ATP net)
2 pyruvate
2 NADH
e- + H +
2 CO2
Overview of Aerobic
Krebs
Respiration
Cycle
2 NADH
8 NADH
2 FADH2
e-
ATP
e - + H+
e-
+
4 CO2
H+
2
Electron
Transfer
Phosphorylation
H+
32
ATP
ATP
water
e- + oxygen
Typical Energy Yield: 36 ATP
Figure 8.3
Page 135
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 (at
best)
• Most energy is lost as heat
**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.
FOOD
fats
glycogen
complex
carbohydrates
proteins
simple sugars
amino acids
Alternative Energy Sources
fatty
acids
glycerol
glucose-6-phosphate
NH3
GLYCOLYSIS
PGAL
pyruvate
acetyl-CoA
Other organic
molecules can be
involved in
respiration.
Figure 8.11
Page 145
KREBS
CYCLE
urea
carbon
backbones
Evolution of Metabolic
Pathways
• 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
Processes Are Linked
sunlight energy
PHOTOSYNTHESIS
water
+
carbon
dioxide
sugar
molecules
oxygen
AEROBIC
RESPIRATION
In-text figure
Page 146
Linked Processes
Photosynthesis
Aerobic Respiration
• Energy-storing
pathway
• Energy-releasing
pathway
• Releases oxygen
• Requires oxygen
• Requires carbon
dioxide
• Releases carbon
dioxide
• Makes carbs
• Breaks carbs