SCandium
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Why does haemoglobin need to be contained within red blood cells, and not just free in the plasma?
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Hajra Momoniat
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Good question

I think its because the red blood cells, being biconcave, have a large surface area allowing the haemoglobin to bind to oxygen more readily.
That's really all I can think of unless anyone else can up with a better answer.

What exam board do you follow?
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SCandium
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Thanks for the reply - makes sense as without rbc the haemoglobin in the middle of the capillary wouldn't be able to exchange the gases. I'm doing Edexcel IGCSE, but this isn't on the specification, I was just curious
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AortaStudyMore
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(Original post by SCandium)
Why does haemoglobin need to be contained within red blood cells, and not just free in the plasma?
There's probably loads of reasons, but the one I can think of is that haemoglobin is protein and a change in protein levels in the blood would change the water potential and disrupt the water balance in the body. The surface area idea doesn't quite work, free haemoglobin would have a larger surface area to volume ratio compared to RBCs
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Rana1997
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(Original post by AortaStudyMore)
There's probably loads of reasons, but the one I can think of is that haemoglobin is protein and a change in protein levels in the blood would change the water potential and disrupt the water balance in the body. The surface area idea doesn't quite work, free haemoglobin would have a larger surface area to volume ratio compared to RBCs
Water potential is equal between the plasma and the cytoplasm of erythrocytes.

I would say that because capillaries have pores and haemoglobin is a relatively small protein so will diffuse out of the blood vessels. That means haemoglobin will be left stationary around cells and absorbing O2 from them (and maybe other chemicals too such as H ions). Thus is has to be contained inside a plasma membrane (i.e erythrocytes)
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Hajra Momoniat
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Aorta and ranas answers are quite good too.
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Kvothe the Arcane
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Proteins within RBC allow it to regulate how haemoglobin works.

Free haemoglobin would likely be broken down by enzymes if free flowing and might clump together and inhibit the passage of blood.
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anosmianAcrimony
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(Original post by AortaStudyMore)
There's probably loads of reasons, but the one I can think of is that haemoglobin is protein and a change in protein levels in the blood would change the water potential and disrupt the water balance in the body. The surface area idea doesn't quite work, free haemoglobin would have a larger surface area to volume ratio compared to RBCs
Your username is close to perfect for answering this.
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Sam00
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The plasmas capacity for o2 is low compared to the capacity that haemoglobin can contain, a single haemoglobin can store 4 o2 diatomic molecules, and there are millions of haemoglobin porphyryn rings per red blood cell, therefore each RBC can carry around a billion oxygen molecules
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AortaStudyMore
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Just one more to add - free haemoglobin is toxic to the kidneys, so would possible cause renal failure. As I said, there'll be loads of reasons why haemoglobin is in RBCs
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