AQA BIOL5 ~ 17th June 2013 ~ A2 Biology

An generator potential is the form of energy that the receptor which acts as an transducer coverts from the stimuli. This is done to convert the energy of an stimulus to an form of energy which is able to be used and processed by the body and consequently leading to an response. An generator potential is an graded response so there isn't an threshold value which requires to be exceeded to create an generator potential and didn't receptors could combine their generator potentials to exceed the threshold value and create an action potential i.e the rod cells within the retina of the eye as there is an number of them attached to an single bipolar cell. Whereas an action potential is when the level of stimuli or generator potential exceeds the threshold value , due to it being an all or nothing response, which passes along the length of the neuron via saltory conduction if its myelinated and if not then it doesn't. Depolarization is the state of the neuron when action potential is reached; as it is when the cytoplasm if the axon of the neuron is more positive than the tissue fluid which surround the axon of the neuron. This due to it changing from -65mv to +40mv.
The process of an action potential occurs as the following:
Stage 1-When the axon of the sensory neuron is at resting potential (the axon of the neuron is polarized) (-65mv) all of the sodium voltage gated ion channels are closed along their plasma membrane although some of the potassium voltage gated ion channels are open even though it is mainly those which are permanently open.
Stage 2-The energy from an stimulus would cause the sodium voltage gated ion channel , along the plasma membrane to open, which would consequently result in an large influx of sodium ions diffusing in through these ion channels along the concentration gradient. As these sodium ions have an overall positive charge their influx causes the cytoplasm if the axon to become increasingly more positive.
Stage 3- The influx of sodium ions into the cytoplasm causes more sodium voltage gated ion channels to open along the axons plasma membrane and thus causes an larger influx of sodium ions into the axon of the neuron.
Stage 4- When the electrical potential difference across the cytoplasm is established at +40 mv (the neuron is depolarized) it causes the sodium voltage gate ion channels along the plasma membrane of the axon to close to prevent the further influx of sodium ions into the axon. In turn it allows the potassium voltage gated ions channels to open , which would reverse the electrical gradient which prevented the outward movement of potassium ions out of the axon to be reversed and in turn causes more potassium voltage gated ion channels to open further along the length of the axon.
Stage 5- This would in turn causes an large influx of potassium ions out of the axon of the neuron, as they would diffuse out of the axon of the neuron along the electrical gradient. Although there is an an slight overshoot and it causes hyper polarization to occur. This is when the inside of the axon becomes more negative than what it was at resting potential. At hyper polarization the sodium potassium pump is activated actively transport sodium ions out of the axon and potassium ions in to allow resting potential to be re-established.

When the axon is polarized it is said that the cytoplasm of the axon is more negative than the tissue fluid which surrounds the axon whereas when the axon is depolarized it is when the inside of the axon is more positive than the tissue fluid which surrounds.

Hope this helps you to understand x

A generator potential is when a stimulus causes a receptor to depolarise enough to create an action potential (e.g. in the pacinian corpuscle) so like the beginning of a nerve impulse (action potential mexican wave )

Depolarisation is just a term to describe a change in the membrane potential - and action potential is when the -55mv or whatever it is, is reached and this causes large depolarisation which then proceeds to pass down the neurone. The way I think of it is that you can get a little depolarisation but no impulse due to the all or nothing principle an action potential won't be created.

What you've wrote seems correct to me
I'm pretty sure you'd get away with using the terms interchangeably anyway as the text book seems to

Hope that helped a bit - someone correct me if I'm wrong it is 5am right now and my brain may not be working proper :P

So basically, a generator potential is always created in the presence of a stimulus because the receptor acts as a transducer to convert stimulus energy into generator potential. IF threshold is reached, Generator potential turns into action potential. Correct?

Step 2 is the generator potential and step 3 is the action potential.
If this is all right, then I now understand. Thanks for your help

Whilst all the cells in the plant contain the same genes in their nuclear DNA, not all of these genes are expressed (transcribed and made into proteins). This is controlled by transcription factors. In cells with more chloroplasts, there will be more activator transcription factors (which increase the rate of transcription) or fewer repressors (which decrease the rate of transcription) compared to cells with fewer chloroplasts.

TBH I don't know what the markscheme will say.

How are people revising units 1 and 2? My notes are a mess for those units, and I don't have the AS textbook. Currently relying on khan academy/youtube

What's khan academy?

I might just buy a CGP revision guide for AS and just read that loads


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I find using the CGP textbooks are really concise and good for revision, whereas the textbook is quite long winded, so i use the textbook, and then cross reference using the textbook

You mean 'I use the CGP book, and then cross reference using the textbook' or am I the stupid one here lol :')


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Looked at some essays today took a picture so you guys could look too first one was 15/25 so a not so good essay second was 24/25 15/25

24/25


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you absolute babe thanks x

I'm loving these DNA questions, brawhhhhh.

Seek help - immidiently. Revising Electrophoresis and restriction mapping

Once you learn it properly, it's a doddle.

The reason why I'm happy is because I went into the exam last year, having very limited knowledge about DNA technology. Now I feel much better about things.