International A Level Biology / Mammalian transport systems

Blood and haemoglobin: carrying oxygen and carbon dioxide revision notes

Edexcel International A Level Biology (YBI11), chapter 2: Mammalian transport systems, specification point 1.9. Free lesson.

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Blood carries oxygen and carbon dioxide in the plasma and red blood cells. Haemoglobin loads oxygen in the lungs, while most carbon dioxide is converted into hydrogencarbonate ions in red blood cells and carried in the plasma.

What plasma and red blood cells carry

Blood is a tissue made of cells and fragments carried in plasma, the liquid part, which is mostly water. Red blood cells are biconcave discs, dented on both sides, and have no nucleus. This leaves more room for haemoglobin. White blood cells are larger and have a nucleus; platelets are fragments that help blood clot. Plasma contains dissolved ions, glucose and proteins. Red blood cells contain haemoglobin and the enzyme carbonic anhydrase. Almost all oxygen travels in red blood cells as oxyhaemoglobin. Only a little dissolves in plasma. Most carbon dioxide travels in plasma as…

How haemoglobin is built

Haemoglobin is a compact, roughly round globular protein made from four polypeptide chains. Two are alpha chains and two are beta chains. Each chain holds a haem group, which contains an iron ion (Fe²⁺). One oxygen molecule binds to each iron ion, so one haemoglobin molecule can carry up to four oxygen molecules.

How haemoglobin loads and unloads oxygen

Oxygen binding is reversible, so haemoglobin can pick up oxygen in the lungs and release it in respiring tissues. The direction depends on the partial pressure of oxygen (pO₂), meaning how much oxygen is available. The equation is Hb + 4O₂ ⇌ HbO₈. Hb is haemoglobin, and HbO₈ is oxyhaemoglobin with all four oxygen-binding sites full. In the lungs, pO₂ is high, so haemoglobin loads oxygen. In respiring tissues, pO₂ is low, so haemoglobin unloads oxygen.

How red blood cells carry most carbon dioxide

At respiring tissues, carbon dioxide diffuses from the cells, through the plasma and into red blood cells. Carbonic anhydrase speeds up its reaction with water. The carbonic acid formed then dissociates, meaning it splits into ions. The reactions are CO₂ + H₂O → H₂CO₃, then H₂CO₃ → H⁺ + HCO₃⁻. HCO₃⁻ diffuses out of the red blood cell into the plasma. Chloride ions move in to balance the charge. This exchange is called the chloride shift. H⁺ binds to haemoglobin to form haemoglobinic acid (HHb), helping to stop the red blood cell becoming too acidic. This also encourages…

How some carbon dioxide binds to haemoglobin

Some carbon dioxide binds to amino groups on haemoglobin's polypeptide chains, forming carbaminohaemoglobin. It does not bind to the haem group. Carbon monoxide binds to haem and forms carboxyhaemoglobin, which is different. Carbon dioxide is carried in three forms: mostly as HCO₃⁻ in plasma, some bound to haemoglobin as carbaminohaemoglobin, and a little dissolved in plasma.