A level Biology: Post your doubts here!

[Shada Mohammed]

Could anyone explain for me the loading and the unloading of sucrose?

 

[samia khalid]

Asalam o alaikum plzzz can anyone tell me how to make serial dilutions Q1 (b) 9700/53/o/n/12.
from 25mmoldm-3 sodium nitrate to 20,15,10 and 5 mmoldm-3.
molar mass of sodium nitrate is 85g mol-1...
plz help someone!!

 

[joanna omar]

Could anyone explain for me the loading and the unloading of sucrose?

LOADING OF SUCROSE:
1)sucrose moves from the mesophyll cells to phloem by symplast or apoplast pathways
2)The hydrogen ions inside the companion cells are pumped out by active transport using energy from ATP produced by mitochondria
3)The hydrogen ions concentration increases outside the companion cells,so they diffuse back into the companion cell passively down their conc gradient through a carrier protein for both hydrogen ions and sucrose at the same time.sucrose molecules are carriend into the companion cell actively aganist their concentration gradient.
4)The sucrose molecules can then move from companion cell into the sieve tube through plasmodesmata which connect them
5)The increased solute , decreases the phloem water potential and water enter by osmosis, the increased water movement increase the pressure of the phloem near the source. Hydro-static pressure builds up in the sieve tube,but its is the greatest in the source .

 

[Shada Mohammed]

I need any notes for practical which is really useful and helpful.
I don't know I feel I can't easily solve p3 any tips.

 

[sentry]

Hi everyone, can someone kindly tell me how to get above 75/100 in paper 4 (A2 structured), and also tell me how to correctly use the mark schemes for paper 4, cause some of the answers in the mark schemes don't make sense sometimes...

 

[joanna omar]

Hi everyone, can someone kindly tell me how to get above 75/100 in paper 4 (A2 structured), and also tell me how to correctly use the mark schemes for paper 4, cause some of the answers in the mark schemes don't make sense sometimes...

True that I have the Same issue !

 

[joanna omar]

Anyone Tell me the model answer of this Question when its about ( 7 marks )

Explain the Roles of synapses in the nervous system ?

 

[joanna omar]

u can look at the standard booklet if there is one.

HOW ? and where is that ?

 

[6Astarstudent]

Anyone Tell me the model answer of this Question when its about ( 7 marks )
Explain the Roles of synapses in the nervous system ?

I remember this question, one of the toughest to explain. Here is what I probably would have put.. just jumbling up my own knowledge and the mark scheme.
Synapses ensure one way transmission of impulses as neurotrasmitter is released only on the presynaptic neurone and receptors proteins are only on the post synaptic neurone. There is thus no way impulses can travel in the opposite direction. Synapses also filters out weak impulses to prevent brain to be overloaded with sensory information because if the depolarisation of the post synaptic membrane does not reach the threshold no impulse is transmitted.

Synapses enables a wide range of behaviour which is particularly useful in situations where multiple effectors are needed, ie. stress / dangerous situations. This is because synapses allows the interconnection of many different nervous pathways. One neurone can spread throughout the whole body to reach many intermediate neurones/effectors for immediate action.

Lastly, synapses are involved in memory/learning. New synapses are formed between effectors of particular pathways; for example, the brain automatically pictures a strawberrry after smelling one if the brain has frequently received these 2 information together before.

 

[joanna omar]

PLEASSEEEEE ! Explain, using examples, how the environment may affect the phenotype of an
organism. [8] (I need brief detailed explanation) its a pastpaper Q's

I Know this Question is so confusing !! 6Astarstudent

 

[joanna omar]

I remember this question, one of the toughest to explain. Here is what I probably would have put.. just jumbling up my own knowledge and the mark scheme.
Synapses ensure one way transmission of impulses as neurotrasmitter is released only on the presynaptic neurone and receptors proteins are only on the post synaptic neurone. There is thus no way impulses can travel in the opposite direction. Synapses also filters out weak impulses to prevent brain to be overloaded with sensory information because if the depolarisation of the post synaptic membrane does not reach the threshold no impulse is transmitted.

Synapses enables a wide range of behaviour which is particularly useful in situations where multiple effectors are needed, ie. stress / dangerous situations. This is because synapses allows the interconnection of many different nervous pathways. One neurone can spread throughout the whole body to reach many intermediate neurones/effectors for immediate action.

Lastly, synapses are involved in memory/learning. New synapses are formed between effectors of particular pathways; for example, the brain automatically pictures a strawberrry after smelling one if the brain has frequently received these 2 information together before.

Thankss a Bunch !!!

 

[joanna omar]

I Know this Question is so confusing !! 6Astarstudent

how the environment may affect the phenotype of an
organism. [8]

 

[6Astarstudent]

how the environment may affect the phenotype of an
organism. [8]

Phenotypic variation results from the interaction between the genotype and the environment. Environment may modify the expression of genes such as tallness; this means that even if you inherit "tallness alleles" from your parents but lacked the sufficient nutrients, there is a good chance that you will not grow very tall. This also applies to other phenotypes such as size and mass.

Another example of how the environment can affect the phenotype would be the colour of hair of animals such as the Himalayan rabbits. They develop dark instead of white hair in the coldest body parts, often the paws, ears and nose to increase amount of heat absorbed. Temperature also determines the sex of crocodile while they're in eggs. Eggs incubated in high temperature are often male while egss incubated at lower temperatures often hatch as females.

Lastly, change in the environment means a change in the most successful genes. This implies that environments may ultimately induce mutations thus affecting phenotype. A good example would be the beak of birds such as wood peckers where the beaks continue to grow longer to remain competitive in terms of feeding on worms in tree trunks.

 

[joanna omar]

Phenotypic variation results from the interaction between the genotype and the environment. Environment may modify the expression of genes such as tallness; this means that even if you inherit "tallness alleles" from your parents but lacked the sufficient nutrients, there is a good chance that you will not grow very tall. This also applies to other phenotypes such as size and mass.

Another example of how the environment can affect the phenotype would be the colour of hair of animals such as the Himalayan rabbits. They develop dark instead of white hair in the coldest body parts, often the paws, ears and nose to increase amount of heat absorbed. Temperature also determines the sex of crocodile while they're in eggs. Eggs incubated in high temperature are often male while egss incubated at lower temperatures often hatch as females.

Lastly, change in the environment means a change in the most successful genes. This implies that environments may ultimately induce mutations thus affecting phenotype. A good example would be the beak of birds such as wood peckers where the beaks continue to grow longer to remain competitive in terms of feeding on worms in tree trunks.

Thanks ALOOOOT !!!

 

[6Astarstudent]

Thanks ALOOOOT !!!

No problem, keep the questions coming. I'm pretty bored until university starts in September.

 

[joanna omar]

No problem, keep the questions coming. I'm pretty bored until university starts in September.

Ouh okaay dont worry
May God make ur dreams true

 

[roxylara]

There's a question in October November 2012. It is:
Which of the bonds will be the last to break as the temperature of an enzyme is increased?
A. Covalent
B. Hydrogen
C. Hydrophobic interactions
D. Ionic

The answer was covalent but I don't understand why? Aren't ionic and hydrogen bonds much stronger then simple covalent bonds? Or is it the fact that these bonds are most sensitive to change in pH and temperature so they break first leaving covalent bonds remaining?

 

[ZaqZainab]

There's a question in October November 2012. It is:
Which of the bonds will be the last to break as the temperature of an enzyme is increased?
A. Covalent
B. Hydrogen
C. Hydrophobic interactions
D. Ionic

The answer was covalent but I don't understand why? Aren't ionic and hydrogen bonds much stronger then simple covalent bonds? Or is it the fact that these bonds are most sensitive to change in pH and temperature so they break first leaving covalent bonds remaining?

The intermolecular bonds (e.g. hydrogen) holding the structure of the enzyme (a protein) in place are easily broken by heat. So its totally not hydrogen bonds..... You know that disulphide bonds are the strongest? well they are the covalent bonds so basically A is the answer

 

[roxylara]

The intermolecular bonds (e.g. hydrogen) holding the structure of the enzyme (a protein) in place are easily broken by heat. So its totally not hydrogen bonds..... You know that disulphide bonds are the strongest? well they are the covalent bonds so basically A is the answer

Ohh I get it. Thankyou so much

 

[Snowysangel]

Explain, interms of protein structure, how it is possible for each T cell receptor to bind specifically to one type of antigen. [3]
Please help. Thanks

It is related to the arrangement of the amino acids in the receptor site. That needs to be complementary