Week 5 Flashcards
1
Q
What is a nicotinic
acetylcholine receptor?
A
a ligand
gated ion channel.
2
Q
What is the function of the a ligand
gated ion channel?
A
An ion channel which is activated that allows sodium ions to enter the cell.
An ion channel which is either activated or deactivated after it binds.
3
Q
What is Acetylcholine?
A
is a neurotransmitter at synapses between
motor neurones and muscle cells
4
Q
What happens when acetylcholine is binded to the nicotinic acetylcholine receptor (Na+ ion channel)?
A
results in channel opening and Na+ influx. This results in the generation of an action potential causing Ca2+ influx and muscle contraction. conformational change
5
Q
What is the nicotinic acetylcholine receptor is composed of?
A
5 subunits (2xα, 1xβ, 1xγ and 1xδ).
6
Q
What are the properties of the 5 subunits?
A
Each subunit is a single
polypeptide containing 4 transmembrane helices (M1-M4).
7
Q
What do the 5 subunits form?
A
The 5 subunits form a pentameric structure with a pore in the centre. The M2 helix from each subunit faces into and lines the pore. Together they form the channel “gate”.
8
Q
What lines the pore of the channel?
A
M2 helices
9
Q
What is a characteristic of each helix?
A
Each helix
has a characteristic kink that forms a constriction or
“gate” in the channel.
10
Q
What happens when acetylcholine binds to the α subunits?
A
the gate opens and Na+ enters the muscle cell.
11
Q
What also happens when acetylcholine binds to the α subunits?
A
causes all the subunits to rotate very slightly
12
Q
Why is rotating sufficient?
A
this is sufficient to
swing the kinked M2 helices outwards such that the gate
opens allowing Na+ entry
13
Q
plant _______ can be engineered to possibly benefit the human race.
A
metabolism
14
Q
what is found in the lower epidermis?
A
stomatal pores
15
Q
What are stomatal pores bounded by?
A
by 2 highly specialised cells which are known as guard cells
16
Q
What is the role of guard cells?
A
shrink and swell leads to pores opening and closing. therefore stomata on leaves behave as valves.
17
Q
What is evaporation responsible for?
A
for maintaining the uptake of water and mineral nutrients from the soil.
18
Q
What does drought lead to?
A
Pores closing, so allows the plant to perserve its existing supplies and withstand limited periods of drought.
19
Q
Why is Stomata control evapotranspiration
and CO2 uptake important?
A
➢ Water and mineral supply ➢ Leaf cooling ➢ Ability to withstand drought ➢ Photosynthesis and dry matter accumulation
20
Q
Stomata opens when…
A
- High Relative Humidity
- High light
- Blue light
- Low [CO2]
21
Q
Stomata closes when…
A
- ABA
- Darkness
- High [CO2]
- Low Relative Humidity
22
Q
what information do guard cells intergrate?
A
Guard cells integrate information from environmental signals to “set” the most appropriate stomatal aperture to suit the prevailing conditions.
23
Q
What does a signalling transmission pathway tell you?
A
This tells us there is an integration rule for these cells as they’re processing multiple signals
24
Q
What did Francis Darwin find?
A
Found that stomata close in the dry air of the room.
25
What does a reduction in atmospheric relative humidity (exposure to dry) air cause?
stomata to close.
26
How do we discover components involved in the guard cell reduced RH signalling pathway?
Carry out a forward genetic screen. Screen a mutagenized population of Arabidopsis (a large number of individual plants all of which carry
mutations). Identify the plants that DO not show stomatal closure when exposed to reduced RH
27
Why does the stomata not close when exposed to reduced RH?
The stomata in these plants do not close because they carry a lesion in a gene involved in the response.
28
Why is Isolating and identifying the mutated gene (and the protein it encodes) important?
results in the addition of a new component to the guard cell RH signalling pathway.
29
Why is it challenging to carry out a method of non-invasively measuring stomatal aperture in a large number of Arabidopsis plant?
This is a challenge because typically this is carried out
| under a microscope and we need to screen thousands of plants.
30
What else did Francis Darwin establish about plants exposed to RH.
If you took 2 plants and looked at the temp of the leaves. In 1st plant if the stomata were fully open and they are transpiring evapotranspiration, then leaf temp would be with respect to a plant in which system matter were closed. The leaf temp would be cooler. If we measure leaf temp this will act as a proxy measure for stomatal openness and closer. Able to conclude- that in response to a drop in RH the stomata of these plants did not close and therefore we were able to identity the genes involved.
31
what do we now use to measuring stomatal aperture?
infrared thermal imaging
32
What was concuded when Isolation of loci (genes) involved in responses to reduced atmospheric RH.
Mutants exhibit lower thermal profiles than WT consistent with stomata
that are impaired in reduced RH-induced closure. We identified the mutated gene responsible for the 30A phenotype and it encoded an enzyme called ABA2 that is involved in the synthesis of the plant hormone abscisic acid (ABA). This hormone builds up in drought and brings about stomatal closure.
The 34A phenotype was caused by a mutation in a gene called OST1. This encodes a protein kinase involved in the signalling pathway by which ABA brings about stomatal closure.
33
What do the experiments tell us about the effect of reduced RH-induced reductions in stomatal aperture?
These experiments told us that reduced RH-induced reductions in stomatal aperture involved ABA and at least one component of signalling pathway by which ABA induces stomatal closure.
34
Manipulating ______
| ______ to benefit humankind.
biochemical pathways
35
Why is vitamin A deficiency found in developing countries?
rice grain major source of food in
developing world is deficient in beta carotene
(yellow pigment) which is also known as provitamin A
36
Why does the lack of beta carotene provide a problem?
As provitamin A is converted to vitamin A in the human body the lack of beta
carotene in the diet is a major factor contributing to vitamin A deficiency
37
How can we solve the problem with rice that lack the yellow pigment?
Engineer rice grains to synthesise beta carotene and then cultivate the engineered rice in the affected regions. This avoids all the issues with transporting and distributing vitamin A.
38
What happens if Β Carotene synthesis is
| BLOCKED in rice grain?
In rice grains the biosynthesis pathway is blocked thus lucopene and beta-carotene never produced because the pathway is blocked.
39
How can we complete the pathway in rice?
Complete the pathway by
genetically engineering rice
plants such that they express 3 transgenes in the grain. This is“Golden Rice”
40
Why is golden rice yellow?
because it contains vitamin A precursor beta carotene
| provitamin A
41
What is the equation for Photosynthesis?
CO2 + 2H2O--------- [CH2O] + O2 + H2O
6CO2 + 6H2O → C6H12O6 + 6O2.
42
What are Photosynthetic organisms?
autotrophs
43
What are autotrophs?
they sustain themselves without eating anything
| derived from other living beings.
44
Other organisms such as animals are what?
heterotrophs
45
What are heterotrophs?
they are unable to make their own food, they live on compounds produced by other organisms.
46
Why is Photosynthesis is essential to life?
it provides all our food (directly or indirectly)
• without photosynthesis we would have no fossil fuels
(laid down by the decay of plant and marine organisms
in the Carboniferous era) – the reality is that until new
technology becomes widely available, and there is the
political will to move away from fossil fuels, they still
play an important role in the world’s economy.
• it provides biomass – 14% of the world’s total final
energy consumption (eg firewood in the third world)
• it was the advent of oxygenic photosynthesis that
changed the atmosphere of the earth from its primitive
reducing state containing virtually no free oxygen to
today’s breathable air
• it is the sole replenisher of oxygen in the atmosphere
47
What would happen if photosynthesis was stopped?
If photosynthesis stopped the atmosphere would return to its primordial reducing state on this timescale.
48
What is NADP+?
nicotine adenine dinulcleotide
phosphate (oxidised NADPH) – an
electron acceptor
49
What is NADPH?
reduced form of NADP+ - an electron
| donor (a source of reducing power)
50
Chloroplasts are ......
Chloroplasts are the site of photosynthesis
• 0.5 million chloroplasts / mm 2 leaf surface
• 30-40 chloroplasts/ mesophyll cell
• chloroplasts are about 5 x 10-6 m in length (5 µm)
51
What do chloroplasts contain?
chlorophylls
52
Where are the photosynthetic pigments located?
in the thylakoid membranes.
53
How are the thylakoid membranes organised?
These are organised into grana.
54
The remainder of the space inside a chloroplast is the _____
Stroma
55
What drives the synthesis of organic molecules
| during photosynthesis?
It is the solar energy absorbed by the chlorophylls
56
What are the two processes of photosynthesis?
the light
reactions and the light independent
reactions.
57
Outline the Light reaction
1) Pigments (chlorophyll) absorb light and this results in energy being passed
from chlorophyll to chlorophyll.
2) the electrons from chlorophyll are passed along an electron transport chain resulting in the generation of NADPH while the H+ are used to produce ATP
58
What happens when chlorophyll absorb light and energy is transferred?
This results in the splitting of water into H+ and oxygen and transfer of electrons along the electron transport chain
59
In the light independent reactions what is NADPH used as?
NADPH is used as a source of electrons to reduce CO2 and sugar is produced with the consumption of ATP
60
During the LIR what is produced?
During the light independent reactions sugar is produced with the generation of NADP+ and ADP which feed into the light reactions
61
What is the role of the Porphyrin ring for the LIR?
light absorbing head of molecule; note magnesium is in the centre.
62
What is the role of the hydrocarbon tail for the LIR?
interacts with hydrophobic regions of proteins inside tylakoid membranes of chloroplasts; H atoms not shown.
63
What do Isolated chlorophyll molecule absorb?
a photon
64
What happens when a photon is absorbed?
this causes an electron to be raised to a higher energy state (excited). This is unstable and the electron drops back to the ground state releasing energy – red/orange fluorescence
65
Where is chlorophyll not present in isolation?
in the thylakoid
66
How is are the chlorophyll organised in the thylakoid?
is organised with other pigments and proteins into a structure called a photosystem (PS).
67
What does a photosystem allow?
This allows the energy to be used
68
How many photosystems are there?
There are 2 PS and this is PSII. The PS are organised into light harvesting complexes (LHC) and the reaction centre.
69
What are photons absorbed by?
“antenna” pigments of the LHC
70
Once photons are absorbed where is the energy funnelled to?
energy is funnelled from pigment to pigment molecule until it reaches 2 special chlorophyll molecules (P680) in the reaction centre complex
71
How is energy transferred?
“resonance energy transfer” between pigment molecules, until it is passed to chlorophyll P680, which then loses an electron to the primary acceptor.
72
What does the light reactions result in the generation of?
oxygen
ATP (energy)
NADPH (reducing power)
Photosynthesis is important
73
Loss of electrons from P680 to the primary acceptor results in what?
results in the formation of a +ve charged P680. This allows it to accept electrons from water and this results in the oxidation of water producing OXYGEN and H+
74
Electrons are passed from the primary acceptor to .......
plastoquinone through the cytochrome complex (which results in the generation of ATP) and on to plastocyanin (the electron transport chain).
75
Where are Electrons from Pc and pigments (in PSI) passed?
) passed to chlorophyll P700 and then passed to the primary acceptor on PSI
76
what does Electrons passed from the primary acceptor of PSI to ferrodoxin and then to NADP+ reductase result in?
resulting in the generation of NADPH (from NADP+ and H+)
77
WHere is the concentration of H+ higher?
The concentration of H+ in the thylakoid space is greater than in the stroma (proton gradient).
78
Why is the conc of H+ greater in the thylakoid space?
1. the photolysis of water
2. the cytochrome complex in the electron transport chain acts as a H+ pump and 3. NADP+ reductase consumes stromal protons. H+ diffuse from the thylakoid space out into the stroma through ATP synthase which synthesises ATP from ADP and phosphate.
79
Outline the properties of the calvin cycle.
• reactions occur in the chloroplast stroma
• reactions are anabolic – building carbohydrate
from smaller molecules and consuming energy
• uses ATP as an energy source
• uses NADPH as reducing power for adding high
energy electrons to make the carbohydrate
• the immediate product is glyceraldehyde-3-
phosphate a 3 carbon compound – so process is
also called C3 photosynthesis
80
What are the three phases of the calvin cycle?
1. Carbon fixation
2. Reduction
3. Regeneration of RuBP
81
Why are three turns of the cycle needed?
3 turns of the cycle needed to generate 1 molecule of G3P (need 5 carbons to regenerate RuBP, so can only remove 1C per turn)
82
What is the net equation for the calvin cycle?
3CO2 + 9ATP + 6NADPH + 6H2O ---------1G3P + 9Pi + 9ADP + 6NADP+
83
Outline the calvin cycle
Co2 binds with rubisco to produce short lived 6 carbon compounf which is then converted into a 1,3 bisphoglyceride and then glyceraldehyde-3- phosphate -G3P can be further metabolized into glucose. Produces 3ADP, 6ADP and 6NADP+.
84
What is rubisco?
ribulose bisphosphate carboxylase / oxygenase.
| duel function enzyme- functions as carboxylase and oxygenase.
85
At 25 degrees how does the rate of the carboxylation reaction compare to the oxidatation reaction?
At 25° C the rate of the carboxylation reaction is 4x
| that of the oxidation.
86
What does the oxygenation reaction result in?
The oxygenation reaction results
in the loss of 20% of photosynthetically fixed carbon by
a process known as PHOTORESPIRATION
87
Ribulose-1,5-bisphosphate + CO2 results in what?
This reaction is catalyzed by rubisco and produces two molecules of the three carbon compound, three -phosphoglycerate
88
However this enzyme can also behave as an is oxygenase, in the presence of oxygen what will occur?
However this enzyme can also behave as an is oxygenase, in the presence of oxygen the reaction catalysed by rubisco produces 3-phosphoglycerate and 2-phosphoglycolate
89
What is the rate of photorespiration largely dependent on?
temperature ( more activity under higher temperature)
90
How does temperature effect the rate of photorespiration?
results in significant reductions in crop yield in the tropics
91
What does photorespiration generate and result in?
* Generates no ATP (in fact it consumes ATP)
* Decreases photosynthetic output by reducing the synthesis of PGA (produces phosphoglycolate) and by releasing CO2 which could otherwise be fixed.
* Photorespiration is wasteful
92
How have some plant species evolved?
Some plant species have evolved alternative
modes of carbon fixation that allow them to
live in hot and dry areas.
93
How have some plants evolved to live in hot climates?
They have evolved a system that allows them
to minimise photorespiration while optimising
photosynthetic carbon fixation. These are called C4 plants and they use the C4
photosynthetic carbon cycle
94
How do the C4 plants fix co2 in the mesophyll cells.
In the mesophyll cells, these plants fix CO2 into a 4 carbon compound – hence C4 photosynthesis
95
Where is this 4 carbon compound transferred?
This 4 carbon compound is then transferred to specialised cells called bundle sheath cells where it releases CO2 which then enters the Calvin cycle.
96
Where does photosynthesis occur in plants that use the C3 carbon photosynthetic cycle?
Photosynthesis occurs in the mesophyll cells and the sugar formed is passed into the vascular system
97
What is the anatomy in C4 species?
Photosynthetic cells of C4 plant leaf consisting of mesophyll and bundle-sheath cell. Vein(vascular tissue). Stoma.
98
What does the C4 anatomy allow?
```
This anatomy allows CO2 to
be kept high in the bundle
sheath cells and thereby
minimisers losses through
photorespiration
```
99
What is the benefit of having higher levels of CO2?
having higher levels of CO2 means we can derive rubisco in the carboxylase and function and that will ensure we generate products which are of use for synthetically.
100
Outline what occurs in the C4 pathway.
1) Malate (C4) is formed in the mesophyll from CO2 by a reaction catalysed by the enzyme PEP carboxylase.
2) The malate then enters the bundle sheath cells where the NADP + malic enzyme catalyses the conversion of malate to pyruvate and CO2. The CO2 then enters the Calvin cycle and the sugars produced enter the vascular tissue while the pyruvate moves into the mesophyll for the generation of PEP
101
What are the advantages of the C4 pathway
PEPcarboxylase has a much higher affinity for CO2 than Rubisco and no affinity for O2 – no oxygenase activity - PEPC can fix C efficiently even when it is hot.
- In effect mesophyll cells “pump” CO2 (as malate) into the bundle sheath cells. This keeps CO2 high in the bundle sheath cells and minimises losses through photorespiration.
- The C4 pathway does come at a cost as the conversion of pyruvate to PEP requires ATP – but it is worth it especially in hot conditions
102
Where are C4 pathways found?
C4 pathway found in maize, sugarcane, sorghum, sedges - about 1% of all known species have C4 metabolism
103
What is another way plants living in hot climates have evolved?
They have evolved a system that allows them to minimise water loss while maintaining photosynthetic carbon fixation. These are called CAM plants and they use Crassulacean Acid Metabolism
104
What species are CAM plants typically?
CAM plants are typically succulents such as cacti and
pineapples and live in hot dry environments such as
deserts.
105
Most plants open their stomata and take up CO2
| during the day. How does this present a problem for desert plants?
However, this presents problems for the
desert plant as leaving its stomata open during the day
will result in dehydration and of course loss of the
potential to fix carbon due to photorespiration.
106
Outline the function of the CAM plant?
1. CAM plants open their stomata at night.
2. CO2 is taken up, fixed by PEP carboxylase into oxaloacetate and stored asmalate during the night in the mesophyll cell vacuole.
3. In the morning the stomata close preventing water loss. Malate is released from the vacuole, transported to the chloroplast, decarboxylated by the NADP+ malic enzyme and the resulting CO2 enters the Calvin cycle.
107
What is CCK?
is a peptide secreted by the mucosal cells of the duodenum into the bloodstream
108
How is CCK secretion increased?
CCK secretion is increased when the mucosal cells encounter the products of the digestion of food
109
What happens when CCK reaches the pancreas?
when CCK reaches the pancreas it causes pancreatic acinar cells to secrete juices rich in digestive enzymes such as alpha amylase into the duodenum (via the bile duct)
110
What help in the digestion of food, in particular starchy foods?
alpha amylase
111
How does CCK stimulate α-amylase secretion?
Binding of CCK to its receptor on the acinar cell results in an increase in the concentration of free calcium ions in the cytosol ([Ca2+]cyt) of the acinar cell
- The increase in [Ca2+]cyt triggers the secretion of the alpha amylase into the common bile duct.
- In this example, calcium ions (Ca2+) act as intracellular second messengers.
Triggers a downstream response.
112
What is the first and second messenger in CCK α-amylase secretion?
Hormone is the first messenger and the calcium is the second messenger.
113
The CCK receptor is a member of a family of receptors called
G protein coupled receptors (GPCRs).
114
What are GPCRs?
Very large family – thousands known in mammals
involved in many responses – light, smell, adrenaline, acetylcholine, serotonin, CCK etc
half or all known drugs work through GPCRs
115
What is the CCK receptor?
The CCK receptor, like all GPCRs, is a single polypeptide. It has 7 transmembrane helices (H1-H7)
116
What does the 7 transmembrane helices enable?
enable it to criss-cross the plasma membrane 7 times.
117
What does the domain on the cytosolic side of the membrane enable?
that enables it to interact with another protein called a G protein.
Domain in this case means part of the protein.
118
Every gpcr has ____ transmembrane helices.
7
119
How many subunits is the G protein made up of?
three subunits (α,β and γ).
120
What do we call these protein subunits?
We call such proteins heterotrimeric (because they are made up of three different subunits).
121
Where do G proteins get their name from?
G proteins get their name because they bind GTP and GDP (guanosine triphosphate and guanosine diphosphate).
G protein and G protein couple receptor are completely different things.
122
Where does CCK bind?
to the inactive receptor.
123
What does the binding of the CCK to the GPCR allow?
allows the receptor to bind the G protein. This activates the G protein with the result that it disassociates from the receptor and migrates along the plasma membrane until it meets the enzyme phospholipase C (PLC).
124
The interaction of the G protein with PLC results in ______.
PLC activation.
125
What does the binding of the CCK to receptor induce?
induces a change in shape- referred to as a change in protein conformation.
Allows the receptor to bind to the G protein.
126
What does PLC do?
PLC- breaks down a specific phospholipid which allows the system to signal further into the cell
PLC hydrolyses a plasma membrane phosphoglyceride called PIP2. The hydrolysis products are IP3 and DAG.
127
What are IP3 and DAG molecules?
Both molecules are intracellular second messengers.
128
Outline what happens in IP3 action 1.
IP3 diffuses into the cytoplasm and binds to a receptor on the ER. The ER is a Ca2+ store- contain a much higher conc than in the cytosol. Binding of IP3 to its receptor results in the release of Ca2+ into the cytosol.
Diffusion of Ca+ from high to low down the concentration gradient.
129
Outline what happens in IP3 action 2
- The [Ca2+]cyt in an unstimulated (resting) cell is approximately 100nM (10-7M).
- -IP3-induced Ca2+ release causes [Ca2+]cyt to increase to approximately 1 μM (10-6M).
- --The increase [Ca2+]cyt causes an enzyme protein kinase C (PKC) to migrate to the plasma membrane where it is activated by DAG.
- ---Activated PKC participates in the reactions that control alpha amylase secretion
130
What does an increase in PLC activity lead to?
An increase in PLC activity leads to the activation of PKC and an increase in alpha amylase secretion
131
What does protein kinase do?
induces a change in structural shape.
132
How is the response stopped?
Removal of CCK will stop the activation of the GPCR-G protein-PLC signaling pathway. [Ca2+]cyt is then returned to its pre-stimulus (resting) level through the action of Ca2+-ATPases (enzymes that pump Ca2+ back into the ER or out of the cell).
You have to stop them as cells may start dividing in the wrong place – cancer etc
Against the concentration gradient and therefore moving the Ca+ from the cytosol to the ER- returning the concentration to the original state.
133
What is the action of adrenaline on skeletal muscle and the role of cAMP?
When an animal is frightened the adrenal gland secretes adrenaline into the blood stream.
In skeletal muscle adrenaline has two major roles: 1. it promotes the breakdown of glycogen to glucose–1–phosphate and 2. it inhibits the synthesis of glycogen.
We will concentrate on the first of these. The glucose–1–phosphate is oxidised through glycolysis to provide the ATP required for sustained muscle contraction.
This is why adrenaline is called the “fight or flight” hormone. The adrenaline receptor is a member of the GPCR family.
134
What does Synthesis of the intracellular second messenger cAMP lead to?
Synthesis: catalysed by adenylyl cyclase (AC) [also known as adenylate cyclase]
ATP --------------cAMP
135
What does degradation of the intracellular second messenger cAMP lead to?
Degradation catalysed by cAMP phosphodiesterase (cAMP-PDE)
cAMP ------------------ AMP
136
Outline the process of Adrenaline signalling in skeletal muscle
Protein kinase A is the target for cAMP
PKA phosphorylates and activates phosphorylase kinase, which phosphorylates and activates glycogen phosphorylase.
ATP used in muscle contraction
PKA- phosphorylates another enzyme
137
What are the consequences when a signalling pathway goes wrong?
Cholera
| Whooping cough
138
What is acetylcholine?
is a neurotransmitter at synapses between motor neurones and muscle cells.
139
Binding of acetylcholine to the nicotinic acetylcholine receptor (Na+ ion channel) results in what?
results in channel opening and Na+ influx. This results in the generation of an action potential causing Ca2+ influx and muscle contraction.
140
Why is it beneficial to have a pore in the centre of the 5 subunits?
Pore is important as acts as the gate for the Na+, beneficial for regulation
141
What happens when the acetylcholine binds to the acetylcholine receptor?
it causes a conformational change
142
What do extracellular signals bring about?
bring about changes in the cytosol. Regulate membrane trafficking, ion transport etc in order to bring about changes in the stomatal aperture.
143
Why is arabidopsis a model species?
Take a sample of thousands of seeds which have a range of mutations.
If you expose the plants to dry air the stomata will close.
Some plants will not, as they have a problem in the signalling pathway- one area of the SP is blocked.
144
Why is the leaf cooler when the stomata is open compared to the stomata that is closed?
due to evapotranspiration
