AQA BIOL2~3rd June 2013~AS Biology (Now Closed)

does anyone have any guesses to topics which are likely to come up?

Movement of water through a plant!

Was wondering if anyone could help me out;

I'm revising Mitosis but I'm so confused on cell cycle.

I know Mitosis is split into Interphase, Prophase, Metaphase, Anaphase and Telophase but when does the cell cycle start and end?

It's confusing and when there are questions asking "how long were the cells in Mitosis", how on earth do you go about working them out?

Any help at all is appreciated, please quote

Could anyone explain quickly the appoplast and symplast pathway, does water in both travel by osmosis?


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does anyone have any guesses to topics which are likely to come up?

is that like transpiration pull and cohesion tension?

Basically water wants to get from the soil into the xylem vessels. It can do this by 2 methods or 'pathways' called the SYMPLAST PATHWAY or APOPLAST PATHWAY. (Might be useful if you get a labelled diagram showing xylem/root hair cell etc whilst reading this).

Symplast Pathway

In the Symplast pathway, water moves through the CYTOPLASMS of the cells.
Water diffuses along a water potential gradient through PORES between the cells of the Cortex called plasmodesmata. The Cortex is simply all the cells between the Root hair cell and the Xylem vessel.

Apoplast Pathway

In the Apoplast pathway, water moves through the CELL WALLS of the cells.
Water stops at the ENDODERMIS as there is a CASPARIAN STRIP which prevents the water from passing further, making the water pass through a membrane by osmosis down a water potential gradient INTO THE SYMPLAST PATHWAY, which is useful as it prevents harmful substances such as toxins or viruses from entering the xylem vessels as the membrane would not let them past.

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EXTRA

- Water potential in the root hair cell is lowered by the active transport of mineral ions from the soil, hence water can move from the soil into them down a water potential gradient.

Watch this, if you need more help:

http://www.youtube.com/watch?v=TWNtXw-MWtE

Could anyone explain quickly the appoplast and symplast pathway, does water in both travel by osmosis?


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Could somebody briefly describe DNA hybridisation?Thank you

Q1D JAN13 -

How do you work out magnification guys?

Thank you.

Anyone have model answer for advantages of counter flow current rather than parrallel flow in fish? Thanks!

I don't know about 'model answer' but I'll explain it a bit...

Countercurrent flow is more efficient than parallel flow for diffusion in the gill lamellae of fish.

The water, which has its maximum concentration of oxygen, meets blood which has a high concentration of oxygen. Therefore, there is a concentration gradient which favours the diffusion of oxygen from the water into the blood. As the water moves along the gill lamella, it consistently meets blood with a lower concentration of oxygen, therefore oxygen continues to diffuse from the water into the blood. As the water leaves the gill lamella, it has a low concentration of oxygen, but this concentration is still higher than that of the blood, therefore diffusion still occurs. 80% of the oxygen in the water can enter the blood.

If it were parallel flow, only 50% maximum could be achieved because the blood and water would reach an isotonic point half way along the lamella and therefore the last half of the lamella would be a pointless to diffusion.

So, countercurrent flow achieves more diffusion of oxygen into the blood. Parallel flow achieves diffusion, but not as much as countercurrent flow.

OK so, it's a method of finding out how closely related two different species are.

You go to a lab and this is what you do...

1. Samples of DNA are taken from the two different species. They are heated separately. The heating breaks the hydrogen bonds holding the two polynucleotide strands together and therefore, the strands separate.

2. The two samples of DNA (from the two different species) are mixed together and allowed to cool. The strands of DNA will try to bond with hydrogen bonds to get back into the helical structure...they don't like being on their own...This means that the strands of the different species will try to bond = hybridise.

3. The hybridised strands are heated until they separate. The precise temperature at which they separate is recorded.

So, if I've got a chimp-human hybrid strand and it separates at 45.23 C.
and I have a gorilla-human hybrid strand that separates at 23.65 C.
Then I can see that the chimp-human strand has formed more hydrogen bonds because it needed more heat to break them. On the other hand, the gorilla-human strand did not need as much heat because less bonds were formed.

So, I can infer that chimps and humans are more closely related than gorillas and humans...because chimp and human DNA form more bonds than chimp and gorilla.

Basically, if one of the strands of the human read as AGTCGC for example,
and the chimp strand read as TCAGAT...then the first four bases would pair because they are complementary...the last two wouldn't. So, chimps and humans would have a lot of similar base sequences.

Does this make sense? I hope so

Yeah but you just don't know what you're gonna get with how science works questions

With immunological comparisons do you add a protein from another species into another which produces serum as they see it as a non self, and then you add this to the protein you added and the amount of precipitate determines how closely related the species is?


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