Haemoglobin Oxygen Dissociation Curve / Bohr Effect Exam Questions - How to understand the Sigmoid Curve

Pack of questions From OCR A, OCR B and Eduqas. 

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Y13 & Y12 OCR A and AQA weekly Masterclass information

The key to understanding dissociation curves is firstly to understand the concept of partial pressure and what would make it change. And to understand cooperative binding.

Partial pressure is the pressure exerted by one gas in a mixture.

You can increase the partial pressure of Oxygen by either having greater air pressure or a larger proportion of the air being Oxygen.

Therefore when you ascend Everest the percentage of Oxygen in the air does not fall - but the partial pressure in your alveoli does. 

Additionally, remember that Oxygen is the final electron acceptor in oxidative phosphorylation on the inner mitochondrial membrane, where it becomes water.

The harder a tissue works - the more ATP will be produced from aerobic respiration, therefore the lower the partial pressure of Oxygen in the tissues (as the Oxygen is becoming water !).

This lowered partial pressure lowers the affinity of Hameoglobin for Oxygen therefore more dissociates from the Haemoglobin.

Respiration produces Carbon Dioxide by decarboxylation of pyruvate in the link reaction and citrate (etc) in Krebs cycle. This diffuses (from mitochondrial matrix) into the plasma and hence into the cytoplasm of the red blood cell - where it is turned into carbonic acid (catalysed by carbonic anhydrase).

Greater respiration produces more Carbon Dioxide, hence more H ions, this lowers the affinity of Haemoglobin for Oxygen, so more Oxygen dissociates. This is the Bohr effect, whose consequence is that tissues with the most respiration receive more Oxygen.

Cooperative binding - when an Oxygen binds to a haemoglobin (remember 2 alpha chains and 2 beta chains and 4 Hame groups so can carry 4 Oxygen molecules in total), then the whole haemoglobin molecule changes in shape slightly and its affinity for Oxygen increases - hence the sigmoid shape of the curve - not a straight line.

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This from Wikipedia

A hemoglobin molecule can bind up to four oxygen molecules in a reversible way.

The shape of the curve results from the interaction of bound oxygen molecules with incoming molecules. The binding of the first molecule is difficult. However, this facilitates the binding of the second, third and fourth, this is due to the induced conformational change in the structure of the haemoglobin molecule induced by the binding of an oxygen molecule.

In its most simple form, the oxyhemoglobin dissociation curve describes the relation between the partial pressure of oxygen (x axis) and the oxygen saturation (y axis). Hemoglobin's affinity for oxygen increases as successive molecules of oxygen bind. More molecules bind as the oxygen partial pressure increases until the maximum amount that can be bound is reached. As this limit is approached, very little additional binding occurs and the curve levels out as the hemoglobin becomes saturated with oxygen. Hence the curve has a sigmoidal or S-shape.

Learning those A-level Biology definitions with Memrise - a terrific free tool

Frequently students fail to learn the vocabulary of Biology. To get a top grade it is essential to know what the words mean in a Biological context (niche, chromosome, chromatid...), and have the confidence to use those words in an exam answer. 

I recommend using www.memrise.com, it also has iOS and android apps, there are lots of glossaries to work your way through or you can write your own. It is cleverer than it looks.

Here are a couple Eduqas component 1, Eduqas component 2

How to Understand the Cardiac Cycle - Now with a Quiz and Questions

There are many concepts in A level Biology where a small adjustment in thought process and understanding can lead to sudden clarity. This is certainly true of the cardiac cycle.

Start with simple concepts and build upon them.

Don’t start with looking at the human heart, four chambers, four valves and a great deal of plumbing.

Begin with drawing a single flexible tube, filled with fluid, now imagine you squeeze the middle. The volume decreases, therefore the pressure increases and the fluid will move to where the pressure is lower. Remember this principle.

Now consider how we could increase the efficiency by preventing flow in one direction?

Introduce two flaps of flesh on the inside of the pipe. These are pushed open and pushed closed by the flow of blood, we call these valves.

The valves in your circulatory system are completely passive (they are pushed open by the flow of blood).

Now let’s go back to the valve we have added to create two regions of the pipe. On one side of the valve we have a chamber called the atrium, on the other we have a chamber called the ventricle.

As blood flows into the atria from the vein (vena cava or pulmonary vein), the atria fill with blood and the pressure increases (because there is more blood !!), when the pressure is higher in the atria than in the ventricle the blood pushes open the valve and falls into the ventricle.

After the valve has been pushed open by the flow of blood from atria into ventricle, then the atria contract to push the rest of the blood through the already open valve into the ventricle

Then the muscle (cardiac) surrounding the chamber (ventricle) contracts then volume of the chamber decreases, which increases the pressure (remember the principle) and the blood moves back toward the atrium which pushes the valve between the atrium and the ventricle closed.

Blood leaving the ventricle pushes open the valve between the ventricle and the aorta. The blood flowing through the valve into the aorta increases the pressure and pushes the walls of the aorta outwards. As the ventricle relaxes, the recoil of the aortic walls pushes blood back to the ventricle, which pushes the valve closed.

Think of the four-chambered human heart as two, two-chambered hearts joined back to back, as they pump blood round two completely separate systems (the systemic and the pulmonary).

Remember, as volume decreases, pressure increases. Blood pushes valves open, and pushes them closed. Blood flows to the lowest pressure. Stick to those principles and when you understand them, apply the knowledge to the human heart.