Transcript RESPIRATION - BiologyMad

RESPIRATION
The stepwise breakdown of glucose
to carbon dioxide and water to release
energy
Aerobic respiration
• References: Rowland pages 120 - 132
Collins pages 11 - 17
• Aerobic respiration
glucose + oxygen
C6H12O6 + 6O2
carbon dioxide and water + energy
6CO2 + 6H20 + 38 mols ATP
• A great deal of energy is released when glucose is oxidised to carbon
dioxide and water in the presence of oxygen. If all this energy was
released in one go, as it is in combustion, it would damage the cell.
•
It is released in a stepwise catabolism of glucose to release usable
amounts of energy to produce ATP
ATP
• ATP - adenosine triphosphate = adenosine P - P - P
• ADP - adenosine diphosphate = adenosine P - P
ADP + P
ATP
ATP
ADP + P + energy for all cell activities
Oxidation and reduction (redox)
Three ways in which oxidation and reduction occurs in respiration
• Direct involvement of oxygen
A + O2 AO2
(oxidation = addition of oxygen)
AO2
A + O2 (reduction = removal of oxygen)
• Removal/addition of hydrogen
AH
A + H (oxidation = removal of hydrogen)
A + H AH
(reduction = addition of hydrogen)
• Removal/addition of electrons
AeA + e- (oxidation = removal of electrons)
A + e- Ae(reduction = addition of electrons)
Hydrogen carriers
• Oxidation of substrates in respiration often involves the removal of
hydrogen atoms (dehydrogenation) and is important in the synthesis
of ATP.
• With the help of a dehydrogenase enzyme, hydrogen atoms are
removed from a compound and taken up by a hydrogen carrier
(acceptor)
• There are two types of hydrogen carriers in respiration:
NAD
FAD
nicotinamide adenine dinucleotide
flavine adenine dinucleotide
AH2
NAD
BH2
A
NADH + H+
B
Aerobic respiration can be divide into 4 stages
•
•
•
•
Glycolysis
Link reaction
Krebs cycle
Electron transport chain
takes place in the cytoplasm
take place in the mitochondria
Total yield of ATP from one molecule of glucose = 38ATP
GLYCOLYSIS
•
•
•
•
Takes place in cytoplasm
Does not need oxygen
Each glucose molecule yields 2 ATP molecules
Common to both aerobic and anaerobic respiration
Glycolysis represents a series of reactions in which one molecule of
glucose is broken down into 2 molecules of pyruvate.
3 main steps:
• Phosphorylation of glucose(6C) to form fructose diphosphate (6C)
• Fructose diphosphate split into two molecules of 3C sugars
• Conversion of two 3C sugars into two molecules of pyruvate (3C)
• Net gain of 2 ATP and 2 NADH + H+
Glycolysis
Read textbook references
• Rowland pages122 – 125
• Collins page 11-12
• Make notes on glycolysis
What happens next?
• If no oxygen is available:
Pyruvate is converted (reduced) to lactate by accepting
hydrogen from NADH
There is no further production of ATP, so net gain of
anaerobic respiration is 2 ATP from 1 molecule of glucose
• If oxygen is available:
• Pyruvate enters the mitochondria and will be fully oxidised
to carbon dioxide and water and 38ATP produced
• The mitochondrion is the site of the Link Reaction, the
Krebs Cycle and the Electron Transport Chain
Into the mitochondrion
• Need to know the structure of a mitochondrion and to be able to
recognise electronmicrographs; see Core Principles
• The Link Reaction and the Krebs Cycle occur in the
matrix of the mitochondrion
• The Electron Transport Chain takes place on the
membranes of the cristae
• The cristae are highly folded to increase the surface area
for the reactions to take place
The Link Reaction
• The Link Reaction links glycolysis to the Krebs Cycle.
• Oxygen is needed for the process
• It occurs in the matrix of the mitochondrion
• The products are all x 2 as there are 2x pyruvates which
enter the Link Reaction
So what happens?
• Pyruvate enters the mitochondria (the inner mitochondrial
membrane is impermeable to glucose and other intermediates)
• The 3-carbon pyruvate combines with the carrier molecule
coenzyme A (CoA)
• It forms a 2-carbon compound called acetyl coenzyme A (acetylCoA)
• The extra carbon is lost as carbon dioxide.
• NAD+ removes hydrogen to form reduced NAD (NADH +H+)
• The process is therefore called oxidative decarboxylation
The Krebs Cycle
• Oxygen is needed for the process to occur
• It takes place in the matrix of the mitochondrion
• The cycle goes round twice for every molecule of glucose oxidised;
the products are, therefore, all x2
• It is where most of the oxidation takes place in aerobic respiration
• It results in the complete breakdown of pyruvate to carbon dioxide
and water
So what happens?
• Acetyl coenzyme A enters the cyclic series of reactions called the
Krebs Cycle
• The 2-carbon acetyl part of the molecule joins up with a 4-carbon
acceptor molecule to form a 6-carbon acid
• There are 2 decarboxylations to regenerate the 4-carbon acceptor
molecule and to release 2 molecules of carbon dioxide
• Hydrogens and electrons are removed at 4 points in the cycle;
involves NAD+ at three points and FAD at one point
• One molecule of ATP is produced for each turn of the cycle; called
substrate level phosphorylation
Result
At the end of glycolysis, the Link Reaction and Krebs Cycle for every
glucose molecule oxidised:
• Glycolysis
2NADH
2ATP
• Link Reaction
2NADH
2CO2
• Krebs Cycle
6NADH
2FADH2
2ATP
So where are the 38 ATP molecules?
Don’t miss next week’s exciting instalment!
4CO2
Link Reaction and Krebs Cycle
• Read textbooks and make notes on Link Reaction and
Krebs Cycle
– Rowland page 124 onwards
– Roberts page 101 onwards
– Collins page 12-13
Electron Transport Chain
• Oxygen is needed
• Takes place in the phospholipid bilayer of the inner
mitochondrial membranes - the cristae
• Reduced coenzymes NAD (NADH + H+) and FAD
(FADH2) are re-oxidised to release large amounts of ATP
• 34 molecules of ATP are produced from the oxidation of
the reduced coenzymes
So what happens?
• Reduced NAD and FAD are reoxidised by the removal of hydrogen
by dehydrogenase enzymes located on the cristae of the inner
membrane of the mitochondrion.
• Each hydrogen atom is split into a hydrogen ion (H+) and an
electron (e-)
• The electrons then pass from one electron carrier molecule to
another electron carrier molecule;
• Electron carrier molecules in the ETC are called cytochromes and are
proteins embedded in the cristae membranes
• The electron carrier molecules are at successively lower energy levels
And more ……
• As the electrons are transferred from one carrier to another some of
their energy is released and used to convert ADP into ATP
• One molecule of ATP is formed every time an electron is transferred
from one carrier to the next
• The removal of hydrogen ions and electrons is oxidation and so the
overall reactions of the ETC are known as oxidative phosphorylation
• Finally, each electron is reunited with a hydrogen ion (H+) which
immediately combines with oxygen to form water
ETC - even more detail!
• As the electrons are passed from one electron carrier to
another, the energy released is used to actively pump hydrogen ions
(H+ - protons) from the matrix into the space between the inner and
outer membranes of the mitochondrion
• Hydrogen ions accumulate and this sets up a large hydrogen ion
gradient with a higher concentration of H+ in the inter-membrane
space than in the matrix; this makes it more positively charged.
• Hydrogen ions can only diffuse back down the concentration and
electrical gradients through large protein carrier molecules which span
the membrane, called ATP synthases
• As H+ ions pass through the enzyme, it catalyses the synthesis of ATP
from ADP and Pi
And finally
…..
• The passage of a pair of electrons from one reduced NAD molecule
along the ETC provides enough energy to move sufficient hydrogen
ions to produce 3 molecules of ATP
• As the first electron carrier is FAD, the passage of electrons from
reduced FAD results in the production of only 2 ATP molecules
• The final carrier in the chain transfers the electrons to 2 oxygen atoms.
Each of these oxygen atoms picks up two protons (H+ ions) to produce
a molecule of water
2e- + 2H+ + O —>
2H2O
• The enzyme cytochrome oxidase catalyses this last reaction involving
oxidation by oxygen; it is inhibited by the poison cyanide
How much ATP is produced?
• Calculate the number of ATPs produced by these processes:
•
a) Substrate level phosphorylation
– glycolysis =
2
– Krebs Cycle =
• b)
2
Oxidative phosphorylation
• re-oxidation of 10 NADH along ETC @ 3ATPs a go
• re-oxidation of 2 FADH2 along ETC @ 2ATPs a go
• Total
38
30
4
Electron Transport Chain
• Read and make additional notes from
– Rowland pages 127 - 128
– Collins page 14
– Roberts pages 103 - 104