Here are the most common calculations you'll need to do.

To go from mm to μm you multiply by 1000
To go from mm to nm you multiply by 1000000
To go from μm to nm you multiply by 1000

To do the reverse conversions, just divide instead of multiply.

Reply 384

imthe12

11

ask me a biology question ?

Reply 385

otrivine

14

is anyone here doing f212 in jan

Reply 386

rachhhhhh

Original post by Priya_Biju

I just got the 2012 june F211 from my teacher. Anyone want it? Am i allowed to post it? Well, I desperately needed it, so I am sure other people need it too.
Please let me know if I have to take it off before u give me a warning or negative rating.

Is there a mark scheme available for this paper at all please?

Thanks

Reply 387

the A* guy

2

What is an organ system??

Reply 388

rachhhhhh

Original post by the A* guy

What is an organ system??

A group of organs working together to perform a particular function e.g. the digestive system, the respiratory system, the circulatory system.

Reply 389

the A* guy

2

Original post by rachhhhhh

A group of organs working together to perform a particular function e.g. the digestive system, the respiratory system, the circulatory system.

Thank you

Reply 390

notsure

11

To anyone: explain why a cell surface membrane is impermeable to most biology molecules? (4 marks)

This was posted from The Student Room's iPhone/iPad App

Reply 391

yousifalwandi

Can someone please explain to me why the curve for fetal haemoglobin is more to the left?

Reply 392

the A* guy

2

Original post by yousifalwandi

Can someone please explain to me why the curve for fetal haemoglobin is more to the left?

Fetal haemoglobin has a higher affinity for oxygen than of adult haemoglobin therefore Fetal haemoglobin is able to pick up oxygen at a lower partial pressure than of adult haemoglobin. This makes the Fetal haemoglobin curve on the graph to the left of Adult haemoglobin as it is able to pick up more oxygen at a lower partial pressure. (partial pressure is on the Y-axis)

Hope this Helps :biggrin:

predictions?!?!

Original post by amyAcurran

roles of components in membranes?..

I really hope so, they'res so much you can say xD

Original post by theworld

What does everyone think the 5 mark question is gonna be on?!

I reckon it'll either be on sources and sinks, Haemoglobin and CO2 or possibly the role of division of labour.

(edited 11 years ago)

Reply 395

ScienceGUYY

4

Hi does anyone have a definition list for this unit? :smile:

and in the exam if you bullet point your answers will you lose marks (on the 5 marks q's)

Reply 396

JamesNeedHelp2

5

Could anyone please explain how sucrose is loaded at the source and unloaded at the sink?

I get the loading part which i think is:

* The companion cells use atp to actively pump H+ ions out of their cytoplasm. This creates a hydrogen ion concentration gradient.

* Co-transporter ions move sucrose and H+ ions back into the companion cells by facilitated diffusion.

* This leads to sucrose building up in the companion cells causing it to move out of the companion cells into the sieve tube through the plasmodesmata.

So what happens after this?

Any help much appreciated!

Reply 397

tetspells

1

Original post by JamesNeedHelp2

Could anyone please explain how sucrose is loaded at the source and unloaded at the sink?

I get the loading part which i think is:

* The companion cells use atp to actively pump H+ ions out of their cytoplasm. This creates a hydrogen ion concentration gradient.

* Co-transporter ions move sucrose and H+ ions back into the companion cells by facilitated diffusion.

* This leads to sucrose building up in the companion cells causing it to move out of the companion cells into the sieve tube through the plasmodesmata.

So what happens after this?

Any help much appreciated!

Water moves into the sieve tube due to the sieve tube having a lower water potential. This increases hydrostatic pressure at source.

At the sink sucrose is removed from the sieve tube by diffusion or facilitated diffusion. This increases water potential of the sieve tube so water moves out decreasing hydrostatic pressure.

Sucrose moves from source to sink due to hydrostatic pressure gradient.

Reply 398

rachhhhhh

Please can someone explain to me the Haemoglobin and CO2 thing? I don't understand it at all!

Thanks

Reply 399

ErHi?

OP

7

Original post by rachhhhhh

Please can someone explain to me the Haemoglobin and CO2 thing? I don't understand it at all!

Thanks

CO2 diffuses into blood, some enters RBC, combines with water to form carbonic acid. Catalysed by enzyme carbonic anhydrase.
CO2 + H20 -> H2CO3
. Dissociates to form hydrogen ions and hydrogencarbonate ions
H2CO3 -> HCO3- + H+
. Hydrogencarbonate ions move out and chloride moves in to balance the charge = chloride shift.
. Hydrogen ions displace the oxygen to release oxygen and also prevent the RBC from becoming acidic.

. More CO2 present, more displacement of oxygen by hydrogen ions (product of dissociation of H2CO3 when carbon dioxide mixes with water) so more oxygen released.
. So more CO2 present, haemoglobin is less saturated with oxygen.
. Makes oxyhaemoglobin dissociation curve shift downwards and to the right.

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OCR AS Biology (F211) - Jan 2013.

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