Biology SL/HL help

[HJ:)]

Thanks Maha, you're a lifesaver ;P

oh um.. another question, what do i do if it is a linked ??

my exams very soon, next week

the teacher told us that she is going to put a question on every chapter we took so it's putting a stress on me.

by the way, maha thanks

Edited June 4, 2009 by Mahuta
HJ, no multiple posts ;). It takes longer to 'merge' now..lol.

[Mahuta ♥]

If its linked, they normally tell you which two alleles are linked. For example, if they told you T and r are linked, that AKLL your gametes are going to have Tr, because they are on the same chromosome, so if you think about it, if the chromosome went to a pole of the cell, both alleles will go with it, so whatever gamete you're going to have, its going to have Tr. so you just do the cross using the gamete.

[new2ib]

I'm working on post-lab questions about pig dissection and just read about various structures in the abdominal cavity. The text mentions that the parietal peritoneum lines the cavity, and the visceral peritoneum covers the organs. I was wondering about the contents of the space between these two epithelial membranes and its function- any ideas? Thanks so much!

[sweetnsimple786]

Is there a way to remember the hormones--where they're made, where they go to, what they do? I guess it's not so bad with posterior pituitary because they are only two, but what's the best way to know everything about the hormones? I always get confused by what ADH actually does.

Andd how do you remember everything to know with the female menstruation cycle, where hormones and their effects are concerned?

[Mahuta ♥]

*TOPIC 6 (6.6.2/.3) --Menstrual Cycle and Hormones.

I remeber hormones by their functions. ADH just somehow reminds me of water, so I know that it's the hormone that is reponsible for Water regulation in the body.

The only way I can possibly think of to know the menstrual cycle very well is by remembering the graph given in almost all biology books. Just keep repeating the explanation, as if you are explaining to someone, keep repeating it and it will just stick until you can recite it like your name, lol. Seriously, you can also make connections, when hormone 'X' reaches a peak the hormone 'Y' immediatly starts going up. It's all about understanding the graph.

You can also relate it to the time of the month, for example, between the 12th-16th day, the body would be preparing for a fertilized egg, what is the hormone that is going to prepare this? Progesterone, you'll notice that progesterone level would start increasing.

Although Oestrogene is also in charge of the wall of the uterus, the way to think about it, is that from the 14th day, Oestrogene levels would start falling slowly. How to remember that? Just think about it as if it went down so that Progesteron could do its job, and then it starts increasing again to maintain the endometrium.

Similarly, LH (Leutanizing Hormone)'s job is during ovulation, so it's going to reach its peak around that time.

FSH would increase around that time as well, but not as high as LH and the others. One way to remember this point is that FSH is a stimulating hormone, it doesn't have to do a HUGE JOB...just stimulation, so the curve is small, as if it's a background effect, you know?

Just analyze the graph slowly and deduce points like the ones I did.

If it's still unclear, please ask again I may be able to explain it better now.

[x___x]

Hi everyone,

how can anyone possibly study photosynthesis and cellular respiration for IB biology HL ?

no matter how much i try, i don't get them as fully as one is supposed to.

any help?

thanks!

[flsweetheart422]

I had problems memorizing and understanding those processes too. There are a bunch of online animations which are very helpful.

Here is an example of a great animation for respiration. This website also has animations for different processes within the cell (including photosynthesis).

Good Luck!

[Hedron123]

Okay. First we'll talk about photosynthesis.

As you know, Photosynthesis is the process by which plants (algae and some bacteria too) convert light energy from the sun into chemical energy (stored as bonds in the organic compounds made --> mainly glucose) It is divided into two main types of reaction: light-dependent and light-independent reactions. The former requires a continual supply of light while the latter can be carried out in darkness (this clarifies the myth that, at night, plants are not undergoing photosynthesis; they just will not carry out the light-dependent reaction)

Having said that, let's begin with the light-dependent reaction.

This reaction is also divided into two main processes but the difference is that they take place almost simultaneously. These are: photolysis of H₂O and photophosphorylation.

Chlorophyll is located in the thylakoid membranes of the chloroplast in groups of molecules called photosystems (there are two types of photosystem: 1 and 2). When chlorophyll absorbs sunlight energy, an electron within the molecule is raised to a higher energy level (the electron is said to be excited). Once this happens, chlorophyll is photoactivated. Excited electrons pass from molecule to molecule until they reach an e^- acceptor at the reaction centre of photosystem II. Once here, the electron will pass along a chain of carriers in the thylakoid membrane. The flow of the electron will cause the pumping of protons into the thylakoid. Soon, a proton gradient is created (there is more protons in the fluid space inside the thylakoid than on the outside). ATP synthase (located in the thylakoid membranes) will then allow the protons to flow across the concentration gradient, using the energy released to synthethize ATP. The production of ATP by means of an excited electron from Photosytem II is non-cyclic photophosphorylation.

The light-dependent reaction is not over yet

Once the electron has released energy to make ATP, it passes onto Photosystem I (from Photosystem II). Photosystem I can now be photoactivated by absorbing light. When this happens, the excited electron will also travel through a short chain of carriers until it reaches NADP⁺ in the stroma. NADP⁺ accepts two high-energy electrons from the electron transport chain and one H⁺ ion from the stroma, forming NADPH + H⁺. This is good but as you may have noticed, Photosystem II has given away electrons to supply Photosystem I. Hence, PII will need to replace these electrons by means of the photolysis of water molecules. The clorophyll molecule at the reaction centre is positively charged. By means of an enzyme (at the reaction centre), water molecules in the thylakoid space are split (photolysis) into H+ and oxygen ions and electrons(the electrons are given to the positively charged chlorophyll). Both oxygen and H+ are by-products of this breaking down of water molecules. Oxygen is a waste product and is excreted (think of the equation for photosynthesis).

For the light-independent reaction you must have a diagram in order for me to explain. Tell me if you do have one or I'll try to find one to complete my explanation. I hope all of what I've said is useful!

G'luck

Edited February 20, 2010 by Hedron123

[x___x]

yaay, thank you. now it seems easier with this explanation.

and about the diagram, would this work?

and i have a question, you said that:

"When this happens, the excited electron will also travel through a short chain of carriers until it reaches NADP+ in the stroma"

hmm, i didn't quite get how this happens.

sorry to be annoying, but i really need to understand these topics.

and thanks for the wonderful explanation.

[Hedron123]

That diagram does not depict the Calvin Cycle; it just shows you that the outputs of the light-dependent reaction are the inputs of the light-independent. As regards your doubt, the concept of the electron travelling across the short chain of carriers until it reaches NADP+ is equivalent to the starting point of the light-dependent reaction. Try re-reading the first part of my explanation and relate it, NADP+ is an electron acceptor. I hope this clarifies your doubt.

Let's start with the light-independent reaction.

The first difference is that this reaction takes place in the stroma of the chloroplast. The process is cyclical because the first substance used in it is RuBP (ribulose biphosphate, 5 carbon sugar) and RuBP is also a product of this independent reaction. Having said that, I recommend you to visualise Wikipedia's diagram of the calvin cycle while you read my explanation.

The first step is pointed with the blue arrow that says Rubisco. Rubisco is an enzyme which is in charge of the process of Carbon Fixation. Carbon dioxide enters the stroma by diffusion and combines with RuBP by the action of rubisco. The product is an unstable six carbon coumpound which will break down immediately into two 3 carbon compounds: glycerate 3-phosphate which can be abbreviated as 3PGA.

The second step involves these two carbon 3 compounds (2 x 3PGA). Both 3PGA are reduced while 2 NADPH+H (product of light-dependent) is oxidized into 2NADP+. 2ATP are used as energy leaving as a result 2 ADP + P. The product of the reduction of the PGAs is triose phosphate (a 3 carbon compound; notice that no carbon is lost , PGA is only reduced).

Now this part is important. The 2 triose phosphates will now follow two separate ways. 1/6 of the TP is used to produce gluocose phosphate (which will then be polymerised into starch). As regards the other 5/6 of the TP, this will be used to regenerate RuBP to continue with the cycle. Five molecules of TP are converted by a series of reactions into three molecules of RuBP (using ATP in the process of phosphorylation). For every six molecules of TP formed in the cycle, five must be converted to RuBP.

Okay, if you have any doubt about this ask me. I'll continue with Cellular Respiration once you fully understand Photosynthesis.

Regards.

Edited February 22, 2010 by Hedron123

[x___x]

That was very clear.

i have a general question:

what kind of question would the IB bring in the exam ?

i understood your explanation fully, but i'm not sure how to target the question.

and thank you for all the time you have taken to reply to my question, that was very helpful

x___x

[Hedron123]

That was very clear.

i have a general question:

what kind of question would the IB bring in the exam ?

i understood your explanation fully, but i'm not sure how to target the question.

and thank you for all the time you have taken to reply to my question, that was very helpful

x___x

Well, remember that photosynthesis does not end up here; you must know the action spectrum, the information about the pigments and the limiting factors (with the graphs to see how they affect the rate of photosynthesis). There are many multiple choice questions regarding the stuff that I have mentioned previously. The explanation of the whole processes as I have detailed in the former posts generally come in the structured questions (called Essay questions) which appear on Paper 2 (they are worth like 20% of the total IB mark so they are very important).

An example could be:

Explain the reactions involving the use of light energy that occur in the thylakoids of the chloroplast. (8 marks)

Always remember that the value of the question refers to the number of concepts you have to explain. Moreover, these type of questions require a proper organization of the ideas so that the examiner can have a fluent lecture. Each essay question has 3 parts (a,b,c) sometimes (d) and they add up to 20 points. 18 points for the content of the answer and 2 points if you have respected this of the organization.

The answer of the former question could be:

Plants contain a pigment called clorophyll which is capable of absorbing light. Once light is absorbed in the photosystem (part of the clorophyll molecule), an electron is excited and will consequently rise to a higher energy level (photoactivation). The excited electron will then pass between chlorophyll molecules in the photosystem until they reach an electron acceptor at the reaction centre of photosystem II.

At photosystem II, the electron will pass along a chain of carriers in the thylakoid membrane. The electron flow causes protons to be pumped across the thylakoid membrane into the fluid space inside the thylakoid. Soon enough, a proton gradient is created (there is more protons in the fluid space inside the thylakoid than on the outside). ATP synthase (located in the thylakoid membranes) will then allow the protons to flow across the concentration gradient, using the energy released to synthethize ATP. The production of ATP by means of an excited electron from Photosytem II is non-cyclic photophosphorylation.

Once the electron has released energy to make ATP, it passes onto Photosystem I (from Photosystem II). Photosystem I can now be photoactivated by absorbing light. When this happens, the excited electron will also travel through a short chain of carriers until it reaches NADP+ in the stroma. NADP+ accepts two high-energy electrons from the electron transport chain and one H+ ion from the stroma, forming NADPH + H+. This is good but as you may have noticed, Photosystem II has given away electrons to supply Photosystem I. Hence, photosustem II will need to replace these electrons by means of the photolysis of water molecules. The clorophyll molecule at the reaction centre is positively charged. By means of an enzyme (at the reaction centre), water molecules in the thylakoid space are split (photolysis) into H+ and oxygen ions and electrons(the electrons are given to the positively charged chlorophyll). Both oxygen and H+ are by-products of this breaking down of water molecules. Oxygen is a waste product and is excreted:

6CO₂ + 6H₂O ---= C₆H₁₂O₆ + 6O₂ + Energy

[bLub]

on this site you can find many great animations: http://www.yteach.co.uk/

(just type in your keyword, i.e. "photosynthesis" or "respiration" into the upper left box)

For example, look at this photosynthesis animation: http://www.yteach.co.uk/page.php/resources/view_all?id=Calvin_cycle_photolysis_of_water_Rubisco_photosystems_membranes_thylakoid_grana_stroma_ribosomes_photophosphorylation_ATP_NADPH_ADP_electrons_molecules_reduction_regeneration_of_RuBP&from=search

or this one for respiration:

http://www.yteach.co.uk/page.php/resources/view_all?id=acetyl_group_acyl_group_coenzyme_oxidases_proton_pump_thioester_bond_vitamin_glycolysis_respiration_pyruvic_acid_acetyl_CoA_NADH_FADH2_electrons&from=search

hope that helps!!

[bLub]

Well, I don't really understand "what" the nucleosome is or where it is... please help, thanks!!!

Edited March 7, 2010 by bLub

[pomoni]

DNA is really long.. around 2 meters. So to organize the DNA and for this long DNA to actually fit in the nucleus, it coils around 8 histone molecules with and additional histone. This not only organizes the DNA but also prevents transcription. (when an mRNA strand is produced using DNA as a template). This structure of having the DNA coil around the 8 histones with the additional histone is called "nucleosome".

Check this out:

http://www.molecularstation.com/molecular-biology-images/data/502/Nucleosome-1.jpg

If there's still anything unclear you can just ask me

Edited March 7, 2010 by pomoni

[bLub]

stupid me, I think I got it now. The DNA, which consists of two strands of nucleotides wound together into a double helix, is wrapped (twice) around eight histone proteins (protein molecules) and held together by another histone protein. This forms a chromosome.

Is that right?

Edited March 7, 2010 by bLub

[pomoni]

Yeah, the chromosone consists of DNA. But this specific structure of histones etc is called nucleosome.

Good luck!

[bLub]

other - a bit related question - does a deoxyribonucleoside triphosphates look like this:

? (couldn't really find a pic no the internet, so "made it up"...)

or are the phosphates all linked to the sugar (rather than to one another)?

[pomoni]

Yeah I think it looks like this. The second and third phosphate groups are removed later when the deoxyribonucleocide triphosphate attaches to the growing strand. This is when "Deoxyribonucleocide triphosphate" becomes a "deoxyribonucleotide".

And as far as I know the phosphates are all linked to one another, and not to the sugar.

Edited March 7, 2010 by pomoni

[bLub]

Ok, I've put together the following info:

The 'gene coding region' (about 1.5 % of our DNA) codes for a polypeptide (around 25, 000 proteins).´Around 3% of the human genome is regulatory coding for genetic switches which control development.

The non-coding region function remains unclear but can be as much as 5-45% of the total genome. These regions are often made of highly repetitive sequences of bases each some 5-300 bases long. These are referred to as satellite regions. Due to the combination of bases in the repeating regions they tend to create dense and less dense DNA regions. These are the parts of DNA used in finger print technologies.

and the following:

Many eukaryotic genes contain introns. These are non-coding sequences that are transcribed but not translated. Exons are transcribed and translated.

So are the above "gene coding regions" exons!? and the "non-coding regions" introns!?

And introns are also hihgly repetitive sequences, i.e. satellite regions (and someone I read they're also called "junk DNA"??)

THANKS!!

Edited March 10, 2010 by sweetnsimple786
Please don't use lime green... it's hard & irritating to read.