Transport in Animals: Haemoglobin, Oxygen Dissociation Curves, and the Bohr Effect

Haemoglobin, Oxygen Dissociation Curves, and the Bohr Effect

Red blood cells stuffed full of haemoglobin. Their bright red colour tells us their haemoglobin is in the form of oxyhaemoglobin, with bound oxygen.

This article includes an explanation of the topic followed by a short multiple-choice quiz with answers, and a collection of past-paper A-level exam questions for you to try.

Single-celled organisms absorb oxygen directly from their surroundings for use in aerobic respiration. But animals have cells buried deep within their bodies, far from the outside world. For this reason, the ability to transport essential substances like oxygen, and to remove waste products like carbon dioxide, is crucial for all animals.

This A-level Biology topic is challenging, but it’s really important to build an understanding of the mechanisms behind these processes. You’ll need to understand exchange and transport, and get to grips with the role of haemoglobin in transporting oxygen and carbon dioxide. And you really will need to understand it properly - as with most A-level topics, memorisation is not enough.

The really clever bit about oxygen transport in animals is the way that cooperative binding affects haemoglobin’s oxygen dissociation curve. Master that and you’ll be well on your way to getting full marks in this topic.

Vocabulary (for reference, don’t worry if you don’t know all these yet!):

1. Partial Pressure: The pressure exerted by one particular gas in a mixture.

2. Haemoglobin: A protein found in red blood cells. Able to bind oxygen, carbon dioxide, and other ligands.

3. Oxyhaemoglobin: Haemoglobin bound to oxygen, as occurs during oxygen transport.

4. Carbaminohemoglobin: Haemoglobin bound to carbon dioxide, as occurs during carbon dioxide transport. Yes it has a funny name! Be careful not to confuse it with carboxyhaemoglobin, which is hemoglobin bound with carbon monoxide.

5. Haemoglobinic Acid: Haemoglobin bound to a proton, as occurs during carbon dioxide transport.

6. Cooperative Binding: The phenomenon where the first oxygen molecule to bind to one of haemoglobin’s binding sites increases the affinity of the remaining binding sites for oxygen. And the first to release makes it easier for the others to release.

7. Carbonic Anhydrase: An enzyme that catalyses the reversible conversion of carbon dioxide and water into carbonic acid.

8. HCO3–: The bicarbonate ion, one of the ways in which carbon dioxide is transported in the blood.

9. Chloride Shift: The movement of chloride ions into red blood cells as bicarbonate ions move out, maintaining electrical neutrality during the transport of carbon dioxide.

10. Oxygen Dissociation Curve: A curve on a graph that shows how saturated with oxygen haemoglobin is at different partial pressures of oxygen. It curves because of the effect of cooperative binding. The position of the curve is affected by pH.

Partial Pressure:

None of this will make any sense if you don’t understand what partial pressure is, so that’s a good place to start. Partial pressure is the pressure exerted by one particular gas in a mixture. You can increase the partial pressure of oxygen molecules (O2) in air by either having greater overall air pressure, or by a larger proportion of the air being oxygen.

The percentage of Oxygen in the air is the same on Everest as at sea level.

• When you ascend Everest, the percentage of oxygen in the air does not fall, but the air pressure does. And so the partial pressure of oxygen in your alveoli falls.

• When you re-breathe the same air, the air pressure does not fall, but the percentage of oxygen in the air does. And so the partial pressure of oxygen in your alveoli falls.

Gases still have partial pressures when they are in solution. The partial pressure of carbon dioxide is really high in an unopened can of coke. And the partial pressure of oxygen in your body fluids must be carefully regulated if your cells are to survive.

Remember that oxygen is the final electron acceptor in oxidative phosphorylation on the inner mitochondrial membrane, where it becomes water (H2O). Respiration therefore removes oxygen molecules (O2) from the body, lowering its partial pressure. The harder a tissue works, the more ATP will be produced from aerobic respiration, and the more oxygen atoms will be moved from O2 into H2O.

Respiration therefore lowers the partial pressure of oxygen (O2) in the tissues. Especially in energy-hungry tissues like muscle.

The partial pressure of carbon dioxide (CO2) also changes in the body. Respiration produces carbon dioxide by decarboxylation of pyruvate in the link reaction and citrate (etc) in Krebs cycle. As the partial pressure of oxygen falls, that of carbon dioxide increases. It’s not the same oxygen atom (remember the one from O2 ended up in H20), but the two processes are closely connected.

Respiration therefore increases the partial pressure of carbon dioxide in the tissues. Especially in energy-hungry tissues like muscle.

This is why animals need a system to support gas transport - it is necessary to move oxygen in to the tissues from the outside world, and to move carbon dioxide out of the tissues to excrete it out of the body.

Haemoglobin’s role:

Haemoglobin plays a pivotal role in transporting oxygen and carbon dioxide in the bloodstream. Haemoglobin is a protein containing four polypeptide chains, each of which provides a binding site that can bind reversibly with oxygen or protons (and some other things too). The timing of its binding and release of its ligands depends on various factors:

Haemoglobin - there are four polypeptide chains, here two are shown in green, and two in red (apologies to anyone who is colour-blind, I didn’t make this graphic). Each polypeptide chain is folded to create a subunit of the protein, and each subunit provides a binding site which can bind one Oxygen molecule.

• Oxygen Binding: When oxygen levels are high, haemoglobin binds with oxygen molecules, forming oxyhaemoglobin. This happens in the lungs, where blood is brought close to the surface of the alveoli. Oxyhaemoglobin is bright red; this is what gives blood its red colour.

• Cooperative Binding: When the first oxygen molecule binds, haemoglobin changes shape in a way that increases the affinity of its remaining binding sites for oxygen. This results in a rapid increase in oxygen saturation once the first oxygen molecule binds to haemoglobin. There is a similar effect on release of oxygen - releasing one oxygen makes it easier for the others to be released.

• Carbon Dioxide Binding: Haemoglobin binds a small percentage of the carbon dioxide produced by the body tissues to help transport it back to the lungs. Haemoglobin bound to carbon dioxide is called carbaminohemoglobin, and is a dark maroon colour. It is not blue!
  
  
  

Carbon Dioxide transport:

About 30% of the carbon dioxide produced by body tissues gets directly bound by haemoglobin for transport. A lot more - about 70% - travels in the blood plasma. This is possible because of the action of the enzyme Carbonic Anhydrase.

• Carbonic Anhydrase: Inside the red blood cell, the enzyme carbonic anhydrase catalyses the reversible conversion of carbon dioxide and water into carbonic acid. This acid then dissociates into bicarbonate ions (HCO3-) and protons (H+).

• Haemoglobinic Acid: The positively-charged protons from the dissociated carbonic acid bind to the haemoglobin to form haemoglobinic acid. This keeps these positive charges inside the red blood cell.

• Chloride Shift: In contrast, the negatively-charged bicarbonate ions are free to diffuse out of the red blood cell into the blood plasma. To maintain electrical neutrality, chloride ions (Cl-) diffuse into the red blood cell. This is known as the chloride shift.
  
  
  

Cooperative Binding and the Oxygen Dissociation Curve:

The much-feared oxygen dissociation curve. The really interesting thing is that it is not a straight line. You need to know why not, and why this is crucial for oxygen delivery to the body tissues.

The Oxygen Dissociation Curve is a graph plotting the partial pressure of oxygen (how much oxygen there is in the environment) against how saturated the haemoglobin is with oxygen.

The graph is always drawn with the partial pressure of oxygen on the X-axis, showing low oxygen to the left, and high oxygen to the right. Oxygen binding is plotted against the Y-axis, with the curve plotted higher at partial pressures where the haemoglobin is more saturated with oxygen.

Take a little while to work out exactly what the graph is showing, as it can be a bit confusing at first.

As you might expect, when there is more oxygen around, more oxygen gets bound. But it’s not quite as simple as that.

The sigmoid shape of the oxygen dissociation curve shows why haemoglobin is so ideally suited to its role in oxygen transport. The shape of the curve is explained by haemoglobin’s remarkable property of cooperative binding.

Haemoglobin has four tertiary domains, each with its own binding site. So it can carry four oxygen molecules. The really clever bit is that when the first oxygen molecule binds, this causes a change of shape in the protein that increases the affinity of the other three binding sites for oxygen. This increased affinity increases their chances of binding oxygen too. This all has the effect that once just one oxygen has bound, the haemoglobin’s binding sites are quickly saturated. And the opposite happens when oxygen dissociates - the dissociation of the first oxygen reduces the affinity of the other binding sites.

The way in which one binding event encourages others is called Cooperative Binding. This ‘all or nothing’ tendency affects the shape of haemoglobin’s oxygen dissociation curve by squashing it down at the bottom and up at the top, creating its famous sigmoid shape.

The sigmoid shape of the oxygen dissociation curve is crucial for efficient oxygen delivery to tissues. At low oxygen concentrations (e.g., in tissues with high metabolic activity), haemoglobin exhibits low affinity for oxygen, allowing it to release oxygen to respiring cells. Conversely, at high oxygen concentrations (e.g., in the lungs), haemoglobin exhibits high affinity for oxygen, facilitating its uptake from the lungs.

The Bohr Effect: Carbon Dioxide and the Oxygen Dissociation Curve

Notice that the right-shifted, red curve is lower than the blue one. This tells us that at high CO2, haemoglobin has LESS affinity for oxygen. (The colours don’t mean anything.)

Haemoglobin’s oxygen dissociation curve isn’t fixed in place. It can move to different positions depending on the pH.

The pH in the red blood cell is affected by the partial pressure of carbon dioxide (remember how it behaves during transport?). High levels of carbon dioxide indicate that the body is active and needs more oxygen.

Active muscles also produce lactic acid through anaerobic respiration, which further reduces the pH.

A low (acidic) pH has the effect of moving the oxygen dissociation curve to the right. This means that any any given partial pressure of oxygen, this high-CO2, right-shifted curve is lower than before.

It’s not enormously intuitive, but you can see this clearly if you draw a line vertically up through the graph at a chosen partial pressure of oxygen. Look at where it meets each curve. It will hit the right-shifted curve before it hits the original curve, because the right-shifted, high-CO2 curve will have lower oxygen saturation at this (or any) patial pressure.

That means that at any partial pressure of oxygen, the high-CO2 haemoglobin has a lower affinity for oxygen than before, and is more likely to release its oxygen into the tissues.

Reducing haemoglobin’s affinity for oxygen at high partial pressures of carbon dioxide helps it release oxygen in the active tissues that need it most.

Fetal Haemoglobin and Myoglobin

Haemoglobin sometimes needs to pass its oxygen to other, similar oxygen-binding molecules. It needs to pass it to myoglobin for oxygen storage in muscles, and to fetal haemoglobin to pass oxygen to the growing foetus.

These similar-but-different molecules have their own oxygen dissociation curves. If oxygen is to be passed to them, they must have a higher affinity for oxygen than normal haemoglobin. And this is what is seen. When you look at their curves, they are left-shifted with respect to haemoglobin. Draw a line up from any chosen partial pressure of oxygen, and it will hit the curve for normal haemoglobin first, because the fetal haemoglobin or myoglobin will have higher oxygen saturation.

This means that at any partial pressure of oxygen, they have a higher affinity for oxygen, and are able to bind oxygen that has been released by the normal haemoglobin.

Multiple Choice Questions (answers below):

1. What is the primary function of haemoglobin in the bloodstream?
   a) Transporting nutrients
   b) Transporting oxygen
   c) Transporting waste products
   d) Transporting hormones
   

2. Which enzyme catalyzes the reversible conversion of carbon dioxide and water into carbonic acid?
   a) Carbon dioxide synthase
   b) Carbonic anhydrase
   c) Haemoglobinase
   d) Bicarbonate dehydrogenase
   

3. Which acid is formed when haemoglobin binds with a proton?
   a) Carbonic acid
   b) Haemoglobinic acid
   c) Hydrochloric acid
   d) Sulfuric acid
   

4. Which ions move into red blood cells when bicarbonate ions diffuse out?
   a) Sodium
   b) Potassium
   c) Chloride
   d) Hydrogen
   

5. What effect does an increase in carbon dioxide concentration have on the oxygen dissociation curve?
   a) It shifts the curve to the left
   b) It shifts the curve to the right
   c) It has no effect on the curve
   d) It decreases the steepness of the curve

Multiple Choice: answers

1. Answer: b) Transporting oxygen
2. Answer: b) Carbonic anhydrase
3. Answer: b) Haemoglobinic acid
4. Answer: c) Chloride
5. Answer: b) It shifts the curve to the right

How did you do?

These questions were just a quick test to see if you can remember some of the key points. If you struggled a bit then go back and review the content. But also check that you really understand what’s going on with this topic. The exam board will intentionally phrase the questions to make it as difficult as possible for anyone to answer by having just memorised key facts.

When you’re ready, try some real exam questions:

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This article was written by Dr Jenny Shipway in collaboration with Tom

Exploring Biological Molecules: Amino Acids, Protein Structure, and Function

Coronavirus Spike Protein

Proteins: Practical Polymers

Protein molecules are the workhorses of biological systems - they fulfill a dizzying number of functions. They can function as enzymes, signalling molecules, structural elements, and many more roles besides.

This is only possible because of their highly diverse structures, which are created by interactions between the R groups of different amino acid residues in their polypeptide chains.

Key Terms:

Before we delve into the intricacies of protein structure, let's define some key terms:

1. Amino Acid: The basic building block of proteins, consisting of a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a side chain (R group). The side chain is the bit that varies between different amino acids.

2. Peptide Bond: The covalent bond formed between the amino group of one amino acid and the carboxyl group of another amino acid during protein synthesis.

3. Residue: The amino acid after it has been joined to a chain by (a) peptide bond(s).

4. Protein / Peptide / Polymer: A peptide is a short chain of amino acids linked by peptide bonds. Because it is made from linking similar submits together, it is an example of a polymer. A polypeptide is a long peptide chain. A protein is a very long polypeptide, often with hundreds of amino acid residues.

5. Levels of Protein Structure: The hierarchical organisation of protein molecules, including primary, secondary, tertiary, and quaternary structures.

6. Electrostatic Interactions: Polar and charged groups can form electrostatic interactions with water molecules and each other. These are very important for protein structure, binding, and function.

7. Hydrogen Bond: A weak electrostatic attraction between a hydrogen atom bonded to an electronegative atom (e.g., oxygen or nitrogen) and another electronegative atom. Protein secondary structures are stabilised by hydrogen bonds.

8. Ionic Bonds: Electrostatic interactions between charged groups. Important for protein tertiary structures, binding and function.

9. Hydrophobic Interactions: Non-polar groups can’t form electrostatic interactions with water. Avoiding water leads them to interact with each other. These interactions are important for protein tertiary structures, binding, and for locating proteins within membranes.

10. Disulfide Bonds: Covalent bonds formed between the sulfur atoms of two cysteine residues within a protein molecule. These are important for protein tertiary and quaternary structures.
    
    

The amino acid Asparagine. The amino group is shown sticking up at the top. The carboxyl group is to the right, and the R group (CH2-CO-NH2) is sticking out to the left.

General Structure of an Amino Acid:

Amino acids are organic compounds composed of a central carbon atom (the alpha carbon) bonded to four groups: an amino group (NH2), a carboxyl group (COOH), a hydrogen atom (H), and a side chain known as the R group.

The R group varies among different amino acids. R groups can be hydrophobic, polar, or charged. The size, shape and potential for electrostatic and covalent interactions determine how each amino acid residue interacts with its environment.

Synthesis and Breakdown of Polypeptides:

During protein synthesis, amino acids are linked together through peptide bonds to form peptides and proteins.

Peptide bonds are formed through a condensation reaction between the amino group of one amino acid and the carboxyl group of another amino acid, resulting in the release of a water molecule.

Conversely, hydrolysis breaks peptide bonds by adding a water molecule, separating the amino acids.

Levels of Protein Structure:

Proteins exhibit four levels of structural organisation:

Lipocaine protein. Its secondary structures of alpha helix (yellow) and beta sheet (blue) are packed together to form a tertiary structure.

• Primary Structure: The linear sequence of amino acid residues in a polypeptide chain. The primary structure can be written down as a simple sequence of letters where each letter codes for a type of amino acid.

• Secondary Structure: Repeated patterns of folding involving the polypeptide backbone. The most common types of secondary structure are alpha helices and beta sheets. Secondary structures do not directly involve R groups; they are stabilised by hydrogen bonding beween atoms of the polypeptide backbone.

• Tertiary Structure: The overall three-dimensional shape of a single polypeptide chain. This usually involves packing elements of secondary structure together to form the overall structure. Tertiary structure is determined by interactions between amino acid side chains. These often include hydrophobic interactions, and can also include hydrogen bonding, disulfide bonds, and ionic bonds.

• Quaternary Structure: Arrangement of multiple polypeptide chains (subunits) in a protein complex, stabilized by the same types of interactions as tertiary structure.
  
  
  

Structure and Function of Globular Proteins:

Many proteins are compact, roughly-spherical proteins with hydrophilic surfaces and hydrophobic interiors. These are known as globular proteins. Examples of globular proteins include enzymes, transport proteins, and regulatory proteins.

• Haemoglobin: A conjugated protein with a quaternary structure of four globular protein subunits. Each subunit contains a heme prosthetic group, responsible for oxygen transport in red blood cells.

• Amylase: The enzyme in saliva that catalyses the breakdown of starch. This protein has one chain with three tertiary domains.

• Insulin: A peptide hormone consisting of two polypeptide chains connected by disulfide bonds. Insuling regulates blood sugar levels by promoting glucose uptake by cells. Insulin is synthesised as a single chain, and cut into two chains during processing.
  
  
  

Summary:

• Amino acids are the basic building blocks of proteins.

• They are composed of a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and an R group (side chain).

• Peptide bonds form between amino acids during protein synthesis, linking them together to form peptides and polypeptides.

• Proteins exhibit four levels of structural organisation: primary, secondary, tertiary, and quaternary structures.

• Protein struture is crucial for their function.

• Many proteins are globular proteins, with hydrophobic interiors and hydrophilic exteriors.

• Proteins can fulfill many functions, from structural roles to acting as enzymes, signalling molecules or membrane channels. And many other things besides.

So what type of molecule is made from amino acids and acts as a biological catalyst? Yes it’s a protein.

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