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A level Biology: Post your doubts here!

Karim Nabil

Karim Nabil

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I was checking june 2011 p13 until i just noticed some extraordinary answers
is this markscheme correct or cambridge has just faulted
for eg: q17 must be B but markscheme says it is C (m/j 2011 p13)
this makes me a little bit concerned, especially this was repeated last sessions only!
 
Alazne Diamond

Alazne Diamond

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Karim Nabil said:
I was checking june 2011 p13 until i just noticed some extraordinary answers
is this markscheme correct or cambridge has just faulted
for eg: q17 must be B but markscheme says it is C (m/j 2011 p13)
this makes me a little bit concerned, especially this was repeated last sessions only!
Click to expand...
trachea iz havin C shaped cartilage (incomplete) and bronchus has ring shaped cartilage
so anwer iz suppose to b C ryt?
 
Karim Nabil

Karim Nabil

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roxylara said:
Can someone explain to me why phagocytes act only against the bacteria and not human cells?
Click to expand...
They are having different types of ribosomes>>bacterial ribosome=18nm or 70s ; eukaryotic cell has a ribosome size = 22nm or 80s
therefore they are manufacturing different types of proteins, hence they have each separate antigen(receptor)
when bacteria invade a human cell it release histamine which plus any chemicals released by the pathogen attracts nearby phagocytes
this stimulates the phagocyte to engulf the bacteria, digest it or act as antigen representing cell for stimulating primary immune response

hope that helped you!
 
Karim Nabil

Karim Nabil

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♣♠ Magnanimous ♣♠ said:
View attachment 38955
can someone explain me the answer of ques -> explain how heart wala...
Click to expand...
  • SAN is located in the right atrium right besides the opening of vena cava
  • excitation originates in SAN, muscle tissue conducts the excitation to both atria so both atria contract
  • a non-conducting barrier (septum) prevents these excitation waves from spreading directly to the ventricles, thus delaying their contraction to allow them to be filled with blood.
  • at the base of the right atrium, AVN is located, passes the excitation wave to the ventricles via the purkinjie fibres (purkyne tissue) collectively called the bundle of His, which runs down the septum before spreading out into the walls of the ventricles so that papillary muscles that are attached to atrio-ventricular valves by tendons contract first to make valves close before ventricles contract, then ventricles contract while atria are relaxed.
  • after being stimulated there is a brief period when muscle is not responding to any further stimulation; the refractory period. Because of this cardiac muscle can beat without fatigue and without developing a permanently contracted state known as tetanus.
  • cardiac muscle continues to contract rhythmically even after the heart has been removed surgically from the body, only if it is maintained in suitable conditions of oxygen and nutrients, so the origin of the heartbeat is not a nerve impulse (not neurogenic) but rather is an inherent property of the cardiac muscle (myogenic)
 
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♣♠ Magnanimous ♣♠

♣♠ Magnanimous ♣♠

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Karim Nabil said:
  • SAN is located in the right atrium right besides the opening of vena cava
  • excitation originates in SAN, muscle tissue conducts the excitation to both atria so both atria contract
  • a non-conducting barrier (septum) prevents these excitation waves from spreading directly to the ventricles, thus delaying their contraction to allow them to be filled with blood.
  • at the base of the right atrium, AVN is located, passes the excitation wave to the ventricles via the purkinjie fibres (purkyne tissue) collectively called the bundle of His, which runs down the septum before spreading out into the walls of the ventricles so that papillary muscles that are attached to atrio-ventricular valves by tendons contract first to make valves close before ventricles contract, then ventricles contract while atria are relaxed.
  • after being stimulated there is a brief period when muscle is not responding to any further stimulation; the refractory period. Because of this cardiac muscle can beat without fatigue and without developing a permanently contracted state known as tetanus.
  • cardiac muscle continues to contract rhythmically even after the heart has been removed surgically from the body, only if it is maintained in suitable conditions of oxygen and nutrients, so the origin of the heartbeat is not a nerve impulse (not neurogenic) but rather is an inherent property of the cardiac muscle (myogenic)
Click to expand...
thanks bro.
 
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NinjaInPyjamas

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ZaqZainab said:
q15 it a cholesterol molecule and protein channel and i as i have stiuded they are made up both hydrophobic and hydrophilic
q16 double will increase the fluidity due to difficulty of the kinked acyl tail to interact . Polyunsaturated fatty acids are liquid at room temperature whereas saturated and hydrogenated fatty acids (trans fats) are solid at room temperature.
q23 A and T makes 2 hydrogen bonds while G and C makes 3
Click to expand...

thanks! you really helped! Jazakh'Allah :)
 
Alazne Diamond

Alazne Diamond

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NinjaInPyjamas said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Biology (9700)/9700_s08_qp_2.pdf

q5 (b) both parts help please!
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b) i) A primary response is brought by the immune system in which the T cell will get activated. The activation of T cell will cause clonal selection which detects the antigen and stimulates the B cell to divide by mitosis to produce plasma and memory cells. The memory cells retains in the body so when the antigen enters second time, memory cells divides to form clones and produce antibodies. Hence showing rapid response known as secondary response.

ii) Parasite is having many stages of life cycle inside mosquito and human body. Due to variation, the structure of the antigen changes. Hence the antibody doesn't work on it.
 
bubbles1997

bubbles1997

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I
@yesha said:
Help me ! I study well from books but i get confused while attempting questions..
Plz tell me how to attempt them
P.S Im an As student and i have time of 1 and half year to improve this prob
Click to expand...
In A levels, the theory is "Practise makes the man perfect". I am an AS student as well and I used to get confused. don't worry. just focus..
 
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