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A2 Biology OCR June 2015 Revision Thread

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Reply 2140

BioAgent

Its the final countdown..... dun dunanana.

Reply 2141

Maham88

Original post by BioAgent

Its the final countdown..... dun dunanana.

what time is your exam?

Reply 2142

BioAgent

Original post by Maham88

what time is your exam?

lol 1:15 same for everyone.

Reply 2143

Star Light

What is the photosynthesis experiment being mentioned? Is it just the one with pondweed in a tube to measure the volume of O2 produced?

Reply 2144

Molly_xox

Original post by mybutton jammed

ascending and descending limb? what do i need to know about them

At the top of the ascending limb, Cl- and Na+ are actively transported from the tubule fluid to the medulla, therefore the medulla has a very, very low water potential. Its wall is impermeable to water to be able to maintain this low water potential. At the bottom of the ascending limb these same ions move out of the tubule fluid via diffusion.

Descending limb = permeable to water. Due to the low water potential caused by the active pumping in the ascending limb, water moves out of the tubule via osmosis, and is then reabsobed into the capillary network. Whilst this happens, Na+ and Cl- diffuse back into the tubule at the descending limb.

The longer the Loop of Henle, the more water conservation can happen, so you'll find animals that live in arid conditions (such as camels) will have long Loops of Henle.

Reply 2145

BioAgent

Original post by Star Light

What is the photosynthesis experiment being mentioned? Is it just the one with pondweed in a tube to measure the volume of O2 produced?

Yes!! do you know what we need to know?

Reply 2146

Maham88

Original post by BioAgent

lol 1:15 same for everyone.

mines is at 1:30, some people have it 2 or even 1

Reply 2147

BioAgent

Original post by Maham88

mines is at 1:30, some people have it 2 or even 1

Oh never knew that. The exam officer at my sixthform takes atleast half an hour to go through his speech though.

Reply 2148

Star Light

Original post by BioAgent

Yes!! do you know what we need to know?

My book says a bit about it, but normally in the exam you're 'given' a diagram of it already set up and asked to interpret results, maybe think about control variables, reliability, etc.

The most commonly used plant is Canadian Pondweed, Elodea (no idea why). You put it in a tube full of water with a source of CO2 (bubbled through/hydrogencarbonate in solution/dry ice), and have a tube going through the bung to collect bubbles of oxygen gas produced. After a certain time, you use a syringe at the end of the collecting tube to move the bubble so it's alongside your scale/ruler so you can measure its length, you use the length + the cross-sectional area of the tube to calculate volume of oxygen produced.

You can investigate light intensity by using a lamp at different distances from the plant, temperature by changing the temperature, CO2 by giving it different levels of CO2 I suppose, and keeping everything else constant.

You might get less oxygen than the actual value produced because some is used for aerobic respiration, some will dissolve in the water, some might get trapped on leaves/air spaces.

The other photosynthesis experiment uses leaf discs - they're sunk in water by using a syringe to remove the air from them, then you shine light on them and see how long it takes for them to float again due to producing oxygen gas which lowers their density.

(edited 8 years ago)

Reply 2149

Nadiabafekr

Original post by mybutton jammed

can someone also show me the their notes of the structure of a cholorplast? with all freatures labelled

thanks x

Hope this helps!

Reply 2150

Nadiabafekr

Original post by mybutton jammed

can someone also show me the their notes of the structure of a cholorplast? with all freatures labelled

thanks x

This is the right pic sorry

Reply 2151

Meesh14

whats the difference between voltage gated sodium ion channels and sodium ion channels? are they the same thing?

Reply 2152

Nadiabafekr

I realllllllly hope the control of insulin secretion comes up as a 5 market

Reply 2153

Nadiabafekr

Original post by Meesh14

whats the difference between voltage gated sodium ion channels and sodium ion channels? are they the same thing?

You're always better off putting "voltage-gated" before the ion used as you're more likely to get a mark as I've seen mark schemes underline the words voltage gated for a mark so it's worth writing, especially when talking about communication with synapses definitely

Reply 2154

Nadiabafekr

Respirometers aren't on the spec nor in the OCR textbook so if they do decide to put it in the paper, that's just cheeky. Photosynthometers are definitely looking a posibilty tho

Reply 2155

Molly_xox

Original post by Meesh14

whats the difference between voltage gated sodium ion channels and sodium ion channels? are they the same thing?

Voltage-gated only open once the threshold potential of -50 mV has been reached

Reply 2156

EspDe97

Could anyone explain the difference between endocrine and exocrine please? Thank you :smile:

Also, are we allowed to write in bullet points for questions other than the QWC ones?

Reply 2157

Star Light

Original post by Meesh14

whats the difference between voltage gated sodium ion channels and sodium ion channels? are they the same thing?

'Normal', non-voltage-gated sodium ion channels are the ones which are opened by things other than voltage. Receptor cells have sodium ion channels which are opened when the cell is stimulated/excited by a stimulus - eg light, pressure, blood pH. These allow some sodium ions into the neurone. If that causes a generator potential above the threshold potential (about -55mV), then that causes voltage-gated sodium ion channels to open.

Neurotransmitter binding to receptors on the postsynaptic membrane causes sodium ion channels to open, this can lead to voltage-gated sodium ion channels opening if the membrane is depolarised enough.

Saying voltage-gated sodium ion channels are opened by the stimulus/neurotransmitter will lose you that mark.