Biology Unit 3 Questions Megathread

[scocliffe09]

oh ok thanks for that shinny
just wanted to know to make my mind at ease
 
i have another question -  with homoeostasis, different books give you different types of homoeostasis examples. Like i know that nelson (schools textbook) only looks at thyroxine, glucagon, insulin and growth hormones whereas other books provide a lot more examples such as those to do with stomata regulation. Will i be at a disadvantage?

You only need to understand the principles behind each. Specific examples aren't required. However, it is an advantage to vaguely know the common ones beforehand (e.g. blood sugar regulation) to save time in the exam. With enough practice exams, you'll learn them anyhow. You shouldn't need to make a conscious effort to learn any specific examples.

You do need to know one specific example so you can fill in a feedback-response flow diagram if need be, with "stimulus", "receptor" etc... But apart from knowing one, I agree completely with Shinny. Familiarity is good, but if you do enough practice exams you should get the hang of it.

also from above; one of the main advantages of hormones travelling in the bloodstream is that they can act on many different parts of the body depending on where target cells are located. Also, they act in low concentrations and can provide an ongoing response. When you get to looking at the effects of adrenalin I think this will make more sense (especially compared to the advantages of nerves).

[HERculina]

what is threshold potential?

[Russ]

The membrane potential that needs to be reached to trigger an action potential.

[HERculina]

so resting potential --> threshold potential --> action potential --> repeat?
how is threshold potential initiated?

[Russ]

Yeah. Influx of ions changes the membrane potential, the threshold is just when a certain amount have entered - this triggers an action potential being sent

[HERculina]

so is it
first, you have normal resting potential.
Then a stimulus (?) causes the the ion channels for sodium to open.
Sodium ions flow in until enough has gone in (the threshold) to bring about a nerve impulse/action potential.
so is threshold potential more like a point in time right before the action potential rather than a period of time that ions flow in?

[shinny]

The diagram I made over here might help. The labels are quite abbreviated so if there's a step you don't get, I can elaborate further.

[HERculina]

thanks
does 'ligand' mean that the ion pump is a two-way pump - it pumps sodium out but also pumps potassium in?
and does that mean that threshold potential is when the sodium ion channels open?
 
just say that you was looking at a specific section on a neurone. the section before it depolarized and now this section does; so the action potential has passed from that section to this section. what exactly stimluated the second section to open its channels/become permeable to sodium. is it the electrical impulse itself?
and can someone explain the "all our nothing effect" to me

[shinny]

Ligand-gated refers to the fact that the ion channel opens when a ligand binds to the receptor on the channel. The ligand in this case would be the neurotransmitters sent from one neuron to another. So one neuron has an action potential going down it, it reaches the end of the nerve and the nerve then sends neurotransmitters across the synapse. These action potentials then bind to the start of the next neuron, activating these ligand-gated channels and allowing a small influx of sodium which brings the membrane potential above the threshold potential, allowing the voltage-gated channels to open and allowing a massive influx of sodium. This sodium then travels down the neuronal cell body and changes the membrane potential in the next section of nerve to be above threshold potential, thus perpetuating the cycle all along the nerve until it reaches the synapse.

So basically, the ligand-gated channels are only implicated in the very first section of nerve. From then on, that initial influx of sodium travels down the cell body to stimulate voltage-gated channels to perpetuate the rest of the neuron.

Regarding the 'all or nothing' effect, it refers to the fact that unless membrane potential reaches above the threshold, then nothing will happen. In addition, regardless of how much you reach above resting potential, there's no difference in the strength of the action potential. Basically, if you go above threshold, all of the voltage-gated channels will open to achieve a set membrane potential. If it doesn't go above threshold, no action potential is generated.

[HERculina]

Ohhhhhhhh i think ive got the gist of it now!  
so ligand-gated channels at the dendrites are completely different to the voltage-gated channels along the neurone.
i thought they were the same cause they both send sodium ions into the cell.

From then on, that initial influx of sodium travels down the cell body to stimulate voltage-gated channels to perpetuate the rest of the neuron.

so the initial influx of sodium is from the ligand-gated channels and as these same sodium ions travel down the neurone, more threshold potentials are reached and the voltage-gated channels opens as a response to enough charge being changed--> then voltage-gated channels makes inside of cell even more positive (this is action potential)

PS. does anyone know a good link to a nerve impulse transmission animation

[shinny]

Perhaps this one? Have a look around Youtube. There's heaps.

[HERculina]

found some moreeeeeeee:
http://outreach.mcb.harvard.edu/animations/actionpotential.swf
http://bcs.whfreeman.com/thelifewire/content/chp44/4402s.swf
http://www.biology4all.com/resources_library/source/63.swf
also this one for how neurotransmitters cross a synapse:
http://www.mind.ilstu.edu/curriculum/neurons_intro/neurons_intro.php (at the end of the page)

[Kaille]

How do antihistamines work? do they work by binding to the histamine receptor? or do they bind to mast cells to inhibit production of histamines???

[Lesliel1]

does a high concentration of auxin inhibit cell growth and low concentrations of auxin promote cell growth?

[shinny]

How do antihistamines work? do they work by binding to the histamine receptor? or do they bind to mast cells to inhibit production of histamines???

They bind to the histamine receptors found throughout the body.