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A2 Biology | Post your doubts here

H

hassam

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well 52 was kinda normal...nt too easy nt too hard........i m expecting 24-25 marks
 
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Majid Mohahammad Qasim

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salaam hi guyzz hope all r fine, can anybody tell me from where i can get the soft copy of biology book "AS Level and A Level Biology" n thnx in advance :)
 
angelgirl:)

angelgirl:)

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ahh...iam missing the biology threads :(
good luck for all the new users of biology threads....
 
Ashique

Ashique

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Hey, can someone please help me with this question in p51- http://papers.xtremepapers.com/CIE/...nd AS Level/Biology (9700)/9700_w10_qp_51.pdf

The answer to the t test was 8.88. The degree of freedom was 58. However, there was no critical value for 58 in the table, so I assumed it to be 60. It was 2.02.

Now, the the value obtained from the t test was greater than the corresponding value on the table. Should that not mean that the difference between the two means is not significant, and was probably due to chance and not random error? Well, the ms says otherwise, could someone please explain why?
 
Ashique

Ashique

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Alice123 said:
AoA
Can any1 help me wid Q3 0f http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Biology (9700)/9700_s05_qp_4.pdf
i want the full answer please...thanks in advance
Click to expand...

Question 3-
a) The (pain) receptors present in the fingers detect a stimulus (change in environment). This causes the the sodium channels in the sensory neuron to open, thus Na+ ions flowing into the neuron down the concentration gradient. This increase in positive charge in the neuron causes the depolarization of the nerve to occur. The opening of the sodium channels, and thus the change in charges, triggers the opening of the sodium ions right next to the depolarized Na+ channels within the nerve. Hence, a wave of depolarization- or in other words, an action potential- is generated.

b) The motor neuron basically carries nerve impulses from the central nervous system to the effector. In this case (since it IS a reflex arc), the motor neuron forms a synapse with the relay neuron in the spinal cord, and this, again, generates an action potential in the motor neuron. The impulse is transmitted along the neuron into the effector, which then contracts to pull the finger away from the pin. (Hence showing the response to the stimulus).

c) This happens due to two reasons-
1) As mentioned above, the change in charge triggers the opening of Na channels on either side of it, but since the channels go through a refractory period (returning to the resting potential), only Na channels present in one side (which has not depolarized) is affected. Hence the transmission of the impulse in one direction
2) It travels in one direction mainly due to the synapses occurring in the synaptic cleft (small space between two nerves), The presynaptic nerve has a neurotransmitter substance called the acetyl coenzyme A, and the postsynaptic nerve has the receptor for acetyl coA. This, too, ensures the travelling of the impulse in one direction.
 
D

dumbledore.

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Which changes to the water potential and the volume of liquid in the phloem occur when
carbohydrate is taken out of a sink into a phloem sieve tube element?
There are different mcqs with different situations . Can anybody tell me the concept.
Q23.
 

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