OCR AS Biology (F211) - Jan 2013.

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

Thanks

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.

Ok sure. I am assuming that you are talking about the Bohr effect,

What happens is that:

* carbon dioxide combines with water in the red blood cell forming carbonic acid. The enzyme that catalyses this reaction is carbonic anhydrase.

* Now the carbonic acid dissociated into H+ Ions and hydrogen carbonate ions. The hydrogen carbonate diffuse out of the red blood cell and in order to balance this charge difference, chloride diffuse in.

*Now, at the respiring cells of the tissues, oxygen is at a low partial pressure. But carbon dioxide is at a high partial pressure. H+ ions that were formed in the red blood cell compete with the oxygen in binding to the haemoglobin molecule.

* We know that at any particular oxygen tension, the oxyhaemoglobin realeases more oxygen, when more carbon dioxide is present. ( which is good for the respiring cells because they are in high demand of oxygen for respiration) Hence, the H+ ions are more likely to bind to the haemoglobin molecule than the oxygen is.

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.



Posted from TSR Mobile

Does anyone know what the evidence for and against translocation is? Thanks!

FOR:

* If you remove a ring of bark (containing phloem but not xylem) from a tree, a bulge forms above the ring. In the bulge is a higher concentration of sugars than below the ring, suggesting a downward flow of sugars.

* If we use aphids (they pierce the phloem and their bodies are removed, allowing the sap to flow out the mouthparts) we can see that the sap flows out quicker near the leaves than further down the stem suggesting a pressure gradient.

* If you put a metabolic inhibitor (stops ATP production) into phloem, translocation stops, so we are aware that active transport is involved

* We can use the experimental model for mass flow

AGAINST:

* Sugar travels in many different sinks, not just to the one with highest water potential as the model would suggest

* Sieve plates would create a barrier to mass flow. A lot of pressure would be needed for solutes to get through at a reasonable rate.

Hope this helps

Seems like a stupid question, but why do we never fully remove ALL air in our lungs when we exhale?

Anyone who wants to do a Q/A session?

Would someone mind explaining cell signalling as ive gone over it many times and it just is not making sense

yeah ok, you start.

What don't you get about it ? Maybe I can help

All of it really- i struggle with the protein channels and how they pass through

as the exams next wednesday, everyone just spit out questions to help everyone get the best possible mark!

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



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

Okay. So i guess your struggling with how the protein channels in the plasma membrane allow certain substances to pass through them...

or is it something else