respiration1 Flashcards
1
Q
What is cellular respiration?
A
The breakdown of complex organic molecules linked to the synthesis of ATP • Glycolysis • Link reaction (or fermentation for anaerobic respiration) • Krebs cycle • Electron transport chain
2
Q
What is glycolysis?
A
• Anaerobic process • Glucose (6 carbon sugar), split into two smaller pyruvate molecules (3 carbon molecules) • ATP and reduced NAD (nicotinamide adenine dinucleotide) are also produced
3
Q
What are the main steps in
glycolysis?
A
1. Phosphorylation of glucose to hexose bisphosphate using ATP 2. Lysis - splitting each hexose bisphosphate molecule into into 2 triose phosphate molecules 3. Phosphorylation - adding another phosphate group to each triose phosphate forming triose bisphosphate molecules, using free inorganic ions present in the cytoplasm 4. Oxidation (aka dehydrogenation) of each triose bisphosphate to form 2 pyruvate molecules. NAD coenzymes (from dehydrogenase enzymes) accept the removed hydrogens, and they are reduced to form reduced NAD molecules. At the same time 4 ATP molecules are produced using phosphates from the triose bisphosphate molecules
4
Q
How is glycolysis an example
of substrate level
phosphorylation?
A
ATP is formed without the involvement of the electron transport chain • ATP is formed by the transfer of a phosphate group from the phosphorylated intermediate (triose bisphosphate) to ADP
5
Q
What is the overall net ATP
yield from glycolysis?
A
• 2 ATP molecules are used to prime the process at the beginning • 4 ATP molecules are produced • Overall net ATP yield is 2 molecules of ATP
6
Q
What is the reduced NAD used
for?
A
Used in a later stage to synthesise
more ATP
7
Q
Describe the structure of
mitochondria in eukaryotic
cells
A
• Outer mitochondrial membrane - separated the contents of the mitochondrion from the rest of the cell, creating a cellular compartment with ideal conditions for aerobic respiration • Inner mitochondrial membrane - contains electron transport chains and ATP synthase • Cristae - projections of the inner membrane which increase the surface area available for oxidative phosphorylation • Intermembrane space - proteins are pumped into this space by the electron transport chain. The space is small, so concentration builds up quickly • Matrix - contains enzymes for the Krebs cycle and the link reaction, also contains mitochondrial DNA
8
Q
What takes place in the
matrix?
A
The link reactions and the Krebs
cycle
9
Q
What does the matrix contain
to help with its function?
A
• Enzymes that catalyse the stages of the reactions • Molecules of the coenzymes NAD and FAD • Oxaloacetate - 4-carbon compound that accepts the acetyl group from the link reaction • Mitochondrial DNA • Mitochondrial ribosomes
10
Q
What does the outer
membrane contain to help with
its function?
A
• Made up of phospholipids • Contains proteins, some of which form channels or carriers that allow the passage of molecules e.g. pyruvate, into the mitochondrion
11
Q
What does the inner membrane
contain to help with its
function?
A
• Lipid bilayer • Less permeable to small ions e.g. H+, than the outer membrane • Folds into cristae to give large surface area for electron carrier and ATP synthase enzymes
12
Q
What does the intermembrane
space contain to help with its
function?
A
In close contact with the mitochondrial matrix, so the molecules of reduced NAD and FAD can easily deliver hydrogens to the ETC
13
Q
Electron transport chain
A
ATP Synthase Enzymes
14
Q
What is the link reaction also
known as?
A
Oxidative decarboxylation
15
Q
What happens in the link
reaction?
A
Pyruvate is decarboxylated and dehydrogenated, catalysed by pyruvate dehydrogenase • No ATP is produced during this reaction
16
Q
What are the steps in the link
reaction?
A
1. The carboxyl group is removed and is the origin of some of the CO2 produced during respiration 2. This decarboxylation of pyruvate, together with dehydrogenation, produces an acetyl group 3. The acetyl group combines with coenzyme A (CoA) to become acetylcoenzyme A (acetyl CoA) 4. The coenzyme NAD becomes reduced by accepting hydrogen • The overall process is known as oxidative decarboxylation (decarboxylation because CO2 is removed, and oxidation because hydrogen is removed) Reactants: 2 pyruvate + 2NAD + 2CoA Products: 2CO2 + 2 reduced NAD + 2 acetyl CoA
17
Q
How does pyruvate enter the
mitochondrial matrix?
A
By active transport via specific
carrier proteins
18
Q
What happens to the products
of the link reaction?
A
Acetyl CoA delivers the acetyl group to the next stage of aerobic respiration - the Krebs cycle • Reduced NAD is used in oxidative phosphorylation to synthesise ATP • CO2 with either diffuse away and be removed as a metabolic waste, or be used as a raw material in photosynthesis
19
Q
Where does the Krebs cycle
take place?
A
In the mitochondrial matrix. Each
complete cycle results in the
breakdown of an acetyl group
20
Q
What are the stages in the
Krebs cycle?
A
1. The acetyl group released from acetyl CoA combines with oxaloacetate (4-carbon compound) to form citrate (6- carbon compound) 2. Citrate is decarboxylated and dehydrogenated, producing a 5- carbon compound, 1 CO2 and 1 reduced NAD 3. This 5-carbon compound is further decarboxylated and dehydrogenated, producing a 4- carbon compound, 1 CO2 and 1 reduced NAD 4. 4-carbon compound temporarily combines with and is them released from coenzyme A. Substrate-level phosphorylation takes place, producing one molecules of ATP 5. 4-carbon compound is dehydrogenated, producing a different 4-carbon compound and 1 reduced FAD 6. Rearrangement of atoms in the 4-carbon molecule is catalysed by an isomerase enzyme, followed by further dehydrogenation. Oxaloacetate is regenerated so the cycle can continue
21
Q
What is the importance of
coenzymes in respiration?
A
They are required to transfer protons, electrons and function groups between many of the enzyme-catalysed reactions • Redox reactions are important in respiration, and without coenzymes transferring electrons and protons, many respiratory enzymes would be unable to function
22
Q
What are the similarities
between NAD and FAD?
A
Both coenzymes
• Both accept protons and electrons
released during the breakdown of
glucose in respiration
23
Q
What are the differences
between NAD and FAD?
A
• NAD takes part in all stages of respiration, but FAD only accepts hydrogens in the Krebs cycle • NAD accepts one hydrogen, and FAD accepts 2 hydrogens • Reduced NAD is oxidised at the start of the ETC releasing protons and electrons, while reduced FAD is oxidised further along the chain • Reduced NAD results in the synthesis of 3 ATP molecules, but reduced FAD only results in the synthesis of 2 ATP molecules
24
Q
Why are the link reaction and
Krebs cycle aerobic?
A
Although oxygen is not directly
used, these stages will not occur in
the absence of oxygen, so they are
aerobic
25
What other substrates besides
glucose can be respired
aerobically?
```
• Fatty acids can be broken down to
many molecules of acetate to
enter the Krebs cycle via acetyl
CoA
• Glycerol may be converted to
pyruvate and enter the Krebs cycle
via the link reaction
• Amino acids may be deaminated,
and the rest of the molecules can
either enter the Krebs cycle
directly, or be changed to
pyruvate, or acetyl CoA
```
26
What is oxidative
| phosphorylation?
The formation of ATP using energy
released in the electron transport
chain and in the presence of oxygen.
Last stage in aerobic respiration
27
What are the stages in
| oxidative phosphorylation?
```
1. Reduced NAD and reduced FAD
are deoxidised when they deliver
their hydrogen atoms to the ETC
2. The hydrogen atoms released
from the reduced coenzymes
split into protons and electrons
3. The protons go into solution in
the mitochondrial matrix
4. ATP is made by chemiosmosis
4H+ + 4e- + O2 → 2H2O
```
28
What is the total ATP
theoretical yield per molecule
of glucose in aerobic
respiration?
29
Why is the theoretical yield
| rarely achieved?
```
Some ATP is used to actively
transport pyruvate into the
mitochondria
• Some ATP is used in a shuttle
system that transports reduced
NAD, made during glycolysis, into
mitochondria
• Some protons may leak out
through the outer mitochondrial
membrane
```
30
What happens if oxygen is
| absent?
```
1. Oxygen can’t act as the final
electron acceptor at the end of
oxidative phosphorylation.
Protons diffusing through ATP
synthase associated channels
cannot combine with electrons
and oxygen to form water
2. Concentration of protons
increases in the matrix and
reduces proton gradient across
inner mitochondrial membrane
3. Oxidative phosphorylation stops
4. Reduced NAD and reduced FAD
can’t unload their hydrogen
atoms and can’t be deoxidised
5. The Krebs cycle and link reaction
stops
```
31
How are organisms
categorised by their
dependence on oxygen?
* Obligate anaerobes
* Facultative anaerobes
* Obligate aerobes
32
Describe obligate anaerobes
```
• Cannot survive in the presence of
oxygen
• Almost all are prokaryotes,
although there are some fungi as
well
```
33
Describe facultative anaerobes
```
• Synthesise ATP by aerobic
respiration if oxygen is present,
but can switch to anaerobic
respiration in the absence of
oxygen e.g. yeast
```
34
Describe obligate aerobes
```
Can only synthesise ATP in the
presence of oxygen e.g. mammals
• Individual cells of some organisms
e.g. muscle cells in mammals, can
be described as facultative
anaerobes because they can
supplement ATP supplies by using
both anaerobic and aerobic
respiration when oxygen
concentration in low
```
35
What is fermentation?
```
The process by which complex
organic compounds are broken
down into simpler inorganic
compounds without the use of
oxygen or an electron transport
chain
```
36
Why does fermentation
produce less ATP than aerobic
respiration?
```
• Organic compounds e.g. glucose,
are not fully broken down
• The small quantity of ATP
produced is synthesised by
substrate level phosphorylation
alone
```
37
What are the 2 types of
| fermentation?
• Alcoholic fermentation: yeast and
some plant root cells
• Lactate fermentation: animal cells
38
What are the steps in alcoholic
| fermentation?
```
1. Each molecule of pyruvate
produced during glycolysis is
decarboxylated and converted
into ethanal. Catalysed by
pyruvate decarboxylase
2. Ethanal accepts hydrogen atoms
from reduced NAD, and is
reduced to ethanol. Catalysed by
ethanol dehydrogenase
3. Reduced NAD is re-oxidised and
made available to accept more
hydrogen atoms from triose
phosphate, so glycolysis can
continue
```
39
What are the steps in the
| lactate fermentation pathway?
```
1. Pyruvate produced during
glycolysis, accepts hydrogen
atoms from the reduced NAD,
also made during glycolysis.
Catalysed by lactate
dehydrogenase
• Pyruvate is reduced to lactate
• The reduced NAD becomes
reoxidised
2. Reoxidised NAD can accept
more hydrogen atoms from
triose phosphate during
glycolysis, so glycolysis can
continue to produce enough ATP
to sustain muscle contraction for
a short period
```
40
What happens to the lactate
| produced?
```
Carried away from the muscles to
the liver. When more oxygen is
available, lactate may be either:
• Converted to pyruvate, which may
enter the Krebs cycle via the link
reactions
• Recycled to glucose and glycogen
```
41
What would happen if lactate
were not removed from the
muscle tissues?
The pH would be lowered, and this
would inhibit the action of many of
the enzymes involved in glycolysis
and muscle contraction
42
What is a respiratory
| substrate?
An organic substance that can be
oxidised by respiration, releasing
energy to make molecules of ATP
43
What are the 3 types of
| respiratory substrate?
* Carbohydrates
* Lipids
* Proteins
44
Why do fats produce more ATP
than an equivalent mass of
carbohydrate?
```
• Fatty acids are long-chain
hydrocarbons with a carboxylic
acid group
• In each molecule there are many
carbon and hydrogen atoms, and
very few oxygen atoms
• Source of many protons for
oxidative phosphorylation
```
45
How are lipids respired?
```
1. Using energy from the hydrolysis
of 1 ATP to 1 AMP, each fatty
acid is combined with CoA
2. The fatty acid-CoA complex is
transported into the
mitochondrial matrix and broken
down into 2-carbon acetyl
groups, each attached to CoA
3. This beta oxidation pathway
generated reduced NAD and
reduced FAD
4. The acetyl groups are released
from CoA, and enter the Krebs
cycle
```
46
When are proteins respired?
During fasting, starvation or
prolonged exercise when insufficient
glucose or lipid are available for
respiration
47
How are proteins respired?
```
• Excess amino acids are
deaminated in the liver
• The rest of the amino acid
molecule (a kept acid) enters the
respiratory pathway as pyruvate,
acetyl CoA, or a Krebs cycle acid
e.g. oxaloacetic acid
```
48
What is produced for every
acetyl group oxidised in the
Krebs cycle?
• 3 molecules of reduced NAD
• 1 molecule of reduced FAD
• One molecule of ATP (made by
substrate-level phosphorylation)
49
How is the respiratory quotient
of a substrate calculated and
measured?
```
RQ = CO2 produced / O2 consumed
Measured using a spirometer
• Carbohydrates = 1.0
• Protein = 0.9
• Lipids = 0.7
```
50
What does an RQ value greater
| than 1 show?
Some anaerobic respiration is taking
place because more CO2 is being
produced than O2 that is being
consumed
51
