OCR Biology F214 Communication, Homeostasis and Energy Wed 25 Jan 2012

OP

Original post by Princess Kawaii

Anyone want to have a go at this one? :smile:

Outline the roles of synapses in the nervous system

cholinergic synapses use the neurotransmitter acetyl choline.

action potential arrives at at the synoptick knob. This causes the voltage gated calcium ion channels to open. Ca2+ diffuse into the knob.They cause the vesicles containing the neurotransmitter acetyl choline to move and fuse with the presynaptic membrane and release Ach by exocytosis.

Ach diffuses across the synpatic cleft and binds to Na+ channels receptors on the postsynaptic membrane. Ach can bind to the Na ion channel receptors because it has a complementaty shape to the receptors.
This causes the Na+ ion channels to open allowing Na+ to diffuse into the cell.
if enough Na+ ion channel open and enough Na+ diffuse into the postsynaptic cell; and the generator potential reaches the threshold potential a new action potential is iniated in the postsynaptic neurone.

Next question: How do synpases ensure that the action potential only travels in one direction refering to acetylcholinesterase.

(edited 12 years ago)

Reply 121

dream_merchant

7

Original post by undertaker1

Next question: How do synpases ensure that the action potential only travels in one direction refering to acetylcholinesterase.

Here's my go (correct me if im wrong :smile:

)

-acetylcholine can only be released from the pre synaptic knob
-only the post synaptic membrane has complementary receptors to acetylcholine
-therefore the post synaptic knob has an action potenial triggered from the acetylcholine, and is therefore one way
-once the acetylcholine has been used it is released back into the cleft where acetlycholinesterase catalyses the breakdown of the neurotransmittor.
-the components then enter the pre synaptic knob via endocytosis to be reused
-therefore the action potential cant be retriggered from the acetyl choline until more is released from the pre synaptic knob (from another action potential)

there yah go, not sure about the bit in italics..

Reply 122

Original post by undertaker1

cholinergic synapses use the neurotransmitter acetyl choline.

action potential arrives at at the synoptick knob. This causes the voltage gated calcium ion channels to open. Ca2+ diffuse into the knob.They cause the vesicles containing the neurotransmitter acetyl choline to move and fuse with the presynaptic membrane and release Ach by exocytosis.

Ach diffuses across the synpatic cleft and binds to Na+ channels receptors on the postsynaptic membrane. Ach can bind to the Na ion channel receptors because it has a complementaty shape to the receptors.
This causes the Na+ ion channels to open allowing Na+ to diffuse into the cell.
if enough Na+ ion channel open and enough Na+ diffuse into the postsynaptic cell; and the generator potential reaches the threshold potential a new action potential is iniated in the postsynaptic neurone.

Next question: How do synpases ensure that the action potential only travels in one direction refering to acetylcholinesterase.

Your answer is absolutely correct but is more geared to the role of neurotransmitters in the transmission of action potentials rather than the role of synapses :wink:

try it again if you like.

My answer to your question:

Receptors for the neurotransmitter: (acetylcholine) are only on the post-synaptic knob.
Vesicles containing the neurotransmitter are only present in the pre-synaptic knob, making sure the neurotransmitter travels in one direction.
The enzyme acetlycholinesterase is only released from the post- synaptic knob once an action potential is generated in the post-synaptic axon which breaks acetylcholine into choline and ethanoic acid, they then diffuse back across the synaptic cleft and into the pre-synaptic knob, where they are combined once again and packaged into vesicles.

Does that sound ok? Does that even make sense lol :smile:

Next Question: Compare and contrast the structure and function of myelinated and non-myelinated neurones

(edited 12 years ago)

Reply 123

Original post by dream_merchant

Here's my go (correct me if im wrong :smile:

)

-acetylcholine can only be released from the pre synaptic knob
-only the post synaptic membrane has complementary receptors to acetylcholine
-therefore the post synaptic knob has an action potenial triggered from the acetylcholine, and is therefore one way
-once the acetylcholine has been used it is released back into the cleft where acetlycholinesterase catalyses the breakdown of the neurotransmittor.
-the components then enter the pre synaptic knob via endocytosis to be reused
-therefore the action potential cant be retriggered from the acetyl choline until more is released from the pre synaptic knob (from another action potential)

there yah go, not sure about the bit in italics..

The components re-enter the pre-synaptic knob by diffusion I think it would be better to say that than "endocytosis"
But other than that your answer's great :smile:

I've posted another question above, if you want to have go at that one too :tongue:

(edited 12 years ago)

Reply 124

Yasin-Ali

6

In response to a stimulus (e.g. touching of skin) could you guys tell me what haapens up to the point of the action potential being reached please?

It's the first bit of depolarisation im interested in, what ions flow in to begin with, through what pores etc. Up to the action potential being produced please.

Reply 125

Quick question:
Oxygen is the final hydrogen (and electron) acceptor in oxidative phosphorylation.So do some H+ go to reduce oxygen to H2O and some go through the channels associated with ATPase to make ATP?
I don't understand where O2 gets these hydrogen. Is it from the H+ that were in the matrix (where conc of H+ is lower to establish pH gradient)?

Reply 126

Original post by Yasin-Ali

In response to a stimulus (e.g. touching of skin) could you guys tell me what haapens up to the point of the action potential being reached please?

It's the first bit of depolarisation im interested in, what ions flow in to begin with, through what pores etc. Up to the action potential being produced please.

• A stimulus is detected causing Na⁺ channels to open
• Na+ enter the cell down a concentration gradient-at resting potential Na⁺ were being pumped out, so there was a high concentration of Na⁺ in the tissue fluid-from the surrounding tissue fluid causing the cell to become depolarised (axoplasm-cytoplasm of axon- is less negative relative to the tissue fluid)
• When generator potential > threshold value (+40mV), voltage gated Na⁺ open causing a large influx of Na⁺ into the cell. (Example of positive feedback)
• Na⁺ channels close, K⁺ ions open so K⁺ diffuses out of the cell down a concentration gradient. So the cell becomes repolarised.
• Potential difference overshoots slightly so that the cell becomes hyperpolarised (more negative than resting potential) due to excess K⁺ .
• Refractory period means another action potential cannot be stimulated as Na⁺ gates are closed. It also means that an action potential can only be transmitted in one direction.

(edited 12 years ago)

Reply 127

Original post by The Illuminati

Quick question:
Oxygen is the final hydrogen (and electron) acceptor in oxidative phosphorylation.So do some H+ go to reduce oxygen to H2O and some go through the channels associated with ATPase to make ATP?
I don't understand where O2 gets these hydrogen. Is it from the H+ that were in the matrix (where conc of H+ is lower to establish pH gradient)?

I would have thought that oxygen is the final electron acceptor only, no hydrogens involved because all hydrogens are needed for chemiosmosis to make as much ATP as possible. :s-smilie:

Reply 128

Original post by Princess Kawaii

I would have thought that oxygen is the final electron acceptor only, no hydrogens involved because all hydrogens are needed for chemiosmosis to make as much ATP as possible. :s-smilie:

You can't make O2 into H2O without the H part. The mark scheme says its a hydrogen acceptor even though technically it accepts H+ not H2.

Reply 129

Original post by The Illuminati

You can't make O2 into H2O without the H part. The mark scheme says its a hydrogen acceptor even though technically it accepts H+ not H2.

Oooooh sorry...I just read back on my notes...completely forgot

Basically the hydrogens come from the proton gradient after passing through the ATP synthase. They've done their job spinning the ATP synthase after which four of them get accepted by the oxygen to produce 2H2O.
It accepts four electrons as well. So Two (H+) + 2 electrons = H2

Reply 130

atman7

8

Original post by Princess Kawaii

Could you upload a couple of them with their mark schemes please?
It would be great. Thank u :smile:

yeah sure i'll try too today or tomorrow

Reply 131

sumsum123

2

Original post by The Illuminati

Quick question:
Oxygen is the final hydrogen (and electron) acceptor in oxidative phosphorylation.So do some H+ go to reduce oxygen to H2O and some go through the channels associated with ATPase to make ATP?
I don't understand where O2 gets these hydrogen. Is it from the H+ that were in the matrix (where conc of H+ is lower to establish pH gradient)?

Hey there =D , hydrogen atoms from reduced NAD are brought to the inner membrane where they are offloaded and split into hydrogen ions and electrons. Energy is given off when the electrons are being passed from one carrier to the next. Co-enzymes related to the enzyme carriers then pump hydrogen atoms into the intermembrane space , it is different from photosynthesis where the energy given off alone pumps the hydrogen ions into the thylakoid space. But don't forget FAD , reduced FAD also has hydrogen atoms which are split into electrons and hydrogen ions. But the hydrogen ions from the reduced FAD do not get pumped into the intermembrane space rather they join with oxygen and electrons to form water. Also hydrogen ions that are pumped into the mateix through atp synthase channel join the oxygen. The diagram in the book is suggesting two hydrogen ions from FAD and two from NAD plus 4 electrons to give you water.

Correct me on anything that is wrong & hope that helped :biggrin:

Reply 132

aquarius00

Hi can anyone answer this question please..suggest why there has been no selection of plants with rubisco-enzymes having redued oxygenase activity?...i know the answers prob simple but i cant think!!

Reply 133

OP

Original post by aquarius00

Hi can anyone answer this question please..suggest why there has been no selection of plants with rubisco-enzymes having redued oxygenase activity?...i know the answers prob simple but i cant think!!

the average temperature is not high enough for the selection pressure to occur. plants with reduce oxygenase activity may also have reduced carboxlayse activity which is not a selective advantage either.

plants evolved when there was very little free oxygen in the environment.

Reply 134

Original post by undertaker1

the average temperature is not high enough for the selection pressure to occur. plants with reduce oxygenase activity may also have reduced carboxlayse activity which is not a selective advantage either.

plants evolved when there was very little free oxygen in the environment.

can you explain the bits in bold?

Reply 135

OP

Original post by The Illuminati

can you explain the bits in bold?

imagine if there are plants which have reduced oxygenase activity. these plants have no advantage because the temperature which is acting as the selection pressure is low. in other words there is nothing which is doing the selecting. in reality it is temperature but because this is low on average therefore there is no pressure being applied so no selection of plants is taking place with reduced oxygenase activity because currently they have no advantage.

perhaps in the future the average temperature increases due to global warming, those plants with reduced oxygenase activity will be selected by the selection pressure which is temperature and those plants will survive long enough and reproduce passing on the favourable alleles for the reduced oxygenase activity. they will live longer because they will be able to produce organic molecules such as glucose via photosynthesis because there enzymes such as rubisco have more carboxylase activity.

to answer your other question if plants have reduced oxygenase activity on the rubisco enzyme there is a possiblity they may also have reduced carboxylase activity. having reduced carboxlase activity is not an advantage so these plants may not be selected when the environment changes. reduced carboxylase activity is not an advantage because less organic molecules such as glucose will be produced because less carbon dioxide will be fixed due to reduced carboxlase activity on rubisco

i think i have answered your questions. please let me know if there is anthing else you do not understand. dont forget this is probably most likely to be a suggestion questions so we can make certain assumptions.

(edited 12 years ago)

Reply 136

Original post by undertaker1

imagine if there are plants which have reduced oxygenase activity. these plants have no advantage because the temperature which is acting as the selection pressure is low. in other words there is nothing which is doing the selecting. in reality it is temperature but because this is low on average therefore there is no pressure being applied so no selection of plants is taking place with reduced oxygenase activity because currently they have no advantage.

perhaps in the future the average temperature increases due to global warming, those plants with reduced oxygenase activity will be selected by the selection pressure which is temperature and those plants will survive long enough and reproduce passing on the favourable alleles for the reduced oxygenase activity. they will live longer because they will be able to produce organic molecules such as glucose via photosynthesis because there enzymes such as rubisco have more carboxylase activity.

to answer your other question if plants have reduced oxygenase activity on the rubisco enzyme there is a possiblity they may also have reduced carboxylase activity. having reduced carboxlase activity is not an advantage so these plants may not be selected when the environment changes. reduced carboxylase activity is not an advantage because less organic molecules such as glucose will be produced because less carbon dioxide will be fixed due to reduced carboxlase activity on rubisco

i think i have answered your questions. please let me know if there is anthing else you do not understand. dont forget this is probably most likely to be a suggestion questions so we can make certain assumptions.

Wouldn't a temperature increase lead to increased oxygenate activity instead of reduced. I thought that at high temps oxygen competes for rubisco with co2. so wouldn't the plants with increased oxygenate activity survive and pass on genes.
saying this.... doesn't oxygen bind to rubisco during photorespiration. If O2 is binding with rubisco instead of CO2 that means that no organic molecules are being made. So photosynthesis stops (no CO2 fixation, no sugars made), if photosynthesis stops then wouldn't respiration stop because you aren't making sugars (glucose) to use as a substrate. So increased oxygenate activity is bad......
I am confusing myself

Reply 137

OP

Original post by The Illuminati

Wouldn't a temperature increase lead to increased oxygenate activity instead of reduced. I thought that at high temps oxygen competes for rubisco with co2. so wouldn't the plants with increased oxygenate activity survive and pass on genes.
saying this.... doesn't oxygen bind to rubisco during photorespiration. If O2 is binding with rubisco instead of CO2 that means that no organic molecules are being made. So photosynthesis stops (no CO2 fixation, no sugars made), if photosynthesis stops then wouldn't respiration stop because you aren't making sugars (glucose) to use as a substrate. So increased oxygenate activity is bad......
I am confusing myself

saying this.... doesn't oxygen bind to rubisco during photorespiration. If O2 is binding with rubisco instead of CO2 that means that no organic molecules are being made. So photosynthesis stops (no CO2 fixation, no sugars made), if photosynthesis stops then wouldn't respiration stop because you aren't making sugars (glucose) to use as a substrate. So increased oxygenate activity is bad......
I am confusing myself This is correct

Wouldn't a temperature increase lead to increased oxygenate activity instead of reduced. I thought that at high temps oxygen competes for rubisco with co2. so wouldn't the plants with increased oxygenate activity survive and pass on genes.

a temperature increase would lead to increased oxygenase activity--- this is bad so why would plants with increased oxygenase acitivity survive????--- they would die so wont pass on the favourable alleles because they wont be able to produce organic molecules

(edited 12 years ago)

Reply 138