Transport of oxygen and carbon dioxide - Biology (9700) - Cambridge A-Level

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The Body's Respiration Roadmap: Transport of Oxygen and Carbon Dioxide (9700 Syllabus 8.2)

Welcome to one of the most vital processes in your body! This chapter explains how your blood, particularly the red blood cells, manages the complex job of collecting oxygen from the lungs and delivering it to every active cell, while simultaneously picking up carbon dioxide waste and safely carrying it back for disposal.

Don't worry if the names seem tricky—we'll break down these processes, like the Chloride Shift and the Bohr Shift, into simple, understandable steps. This is crucial knowledge because it links gas exchange (Topic 9) directly to cellular respiration (Topic 12)!

I. The Role of Red Blood Cells in Gas Transport

1. The Oxygen Carrier: Haemoglobin (Hb)

Oxygen is not very soluble in plasma, so it relies heavily on the transport protein haemoglobin (Hb), found exclusively inside red blood cells.

Key Facts about Haemoglobin:

Hb is a large globular protein with a quaternary structure. (Remember globular proteins from Topic 2?)
It is made of four polypeptide chains (two alpha chains and two beta chains).
Each chain contains a non-protein haem group, which holds one iron ion (\(Fe^{2+}\)).
Since there are four haem groups per Hb molecule, one molecule can bind a maximum of four oxygen molecules (\(4O_2\)).

When oxygen binds to the iron in the haem group, the Hb molecule is called oxyhaemoglobin (\(HbO_8\)). This process is known as loading (or association).
When oxygen is released in the tissues, the process is called unloading (or dissociation), and the Hb reverts to its original form.

2. The Oxygen Dissociation Curve (ODC)

The relationship between the partial pressure of oxygen (\(P_{O_2}\)) and the percentage saturation of haemoglobin is shown graphically by the Oxygen Dissociation Curve (ODC).

Analogy: Think of the ODC as an oxygen taxi service. The curve shows how likely the taxi (Hb) is to pick up passengers (O₂) or drop them off, based on how crowded the street (\(P_{O_2}\)) is.

The ODC has a distinct S-shape (Sigmoidal Curve) because of cooperative binding:

Initial Binding (Low \(P_{O_2}\)): When the first \(O_2\) binds, it causes a conformational change (shape change) in the haemoglobin molecule. This makes it easier for the second and third \(O_2\) molecules to bind. The curve rises slowly at first.
Rapid Binding (Medium \(P_{O_2}\)): The easier binding leads to a steep rise in the curve, meaning small changes in oxygen pressure cause large changes in oxygen uptake.
Saturation (High \(P_{O_2}\)): Once three \(O_2\) molecules are bound, the binding site for the final \(O_2\) is slightly less accessible, and saturation plateaus.

Importance of the ODC at Different Partial Pressures

The shape of the ODC is vital for efficient gas transport:

In the Lungs: The \(P_{O_2}\) is very high (around 13.3 kPa). At this pressure, Hb is nearly 100% saturated. This ensures maximum oxygen is loaded efficiently, even if air pressure fluctuates slightly.
In Respiring Tissues (Resting): The \(P_{O_2}\) is much lower (around 5.3 kPa). The curve is still high, meaning Hb remains highly saturated, only releasing a small percentage of its oxygen (enough for resting cells).
In Highly Active/Respiring Tissues (e.g., muscle during exercise): The \(P_{O_2}\) drops very low (e.g., 2.0 kPa). The steep part of the curve means a small drop in \(P_{O_2}\) results in a large increase in oxygen unloading, satisfying the high demand of active cells.

Quick Review: O₂ Transport
Hb acts like an emergency reserve; it only releases large amounts of O₂ when the partial pressure drops significantly (when tissues are very active).

II. Carbon Dioxide Transport and Regulation

Unlike oxygen, which mainly uses haemoglobin, carbon dioxide (\(CO_2\)) is transported in three primary forms from the tissues back to the lungs.

1. Three Forms of \(CO_2\) Transport

Dissolved in Plasma (approx. 5%): A small amount of \(CO_2\) dissolves directly into the blood plasma. (Syllabus point 8.2.3)
Carbaminohaemoglobin (approx. 10%): \(CO_2\) binds directly to the amino groups on the haemoglobin polypeptide chains, forming carbaminohaemoglobin. (Syllabus point 8.2.1)
Hydrogencarbonate ions (\(HCO_3^-\)) (approx. 85%): This is the most important method, relying on enzyme action inside the red blood cell.

2. The Formation of Hydrogencarbonate Ions

This process is rapid because red blood cells contain the enzyme carbonic anhydrase (CA).

Step-by-Step in Tissues (CO₂ Pick-up):

\(CO_2\) diffuses from the high concentration in the respiring tissues into the blood and then into the red blood cell (RBC).
Inside the RBC, carbonic anhydrase catalyses the reversible reaction between \(CO_2\) and water, producing carbonic acid (\(H_2CO_3\)).
\begin{equation} \(CO_2 + H_2O \rightleftharpoons H_2CO_3\) \end{equation}
Carbonic acid immediately dissociates (breaks down) into a hydrogencarbonate ion (\(HCO_3^-\)) and a hydrogen ion (\(H^+\)).
\begin{equation} \(H_2CO_3 \rightleftharpoons HCO_3^- + H^+\) \end{equation}

3. The Role of Haemoglobin in Buffering (Forming Haemoglobinic Acid)

The production of \(H^+\) ions (step 3 above) makes the cytoplasm of the red blood cell acidic. If this were left unchecked, it would drastically lower the pH of the blood, disrupting protein structures (including enzymes).

This is where haemoglobin steps in again—it acts as a buffer (a chemical that resists pH change).

The free \(H^+\) ions immediately bind to the haemoglobin molecule, forming haemoglobinic acid (\(HHb\)). This buffering action is critical because it removes the acidic \(H^+\) ions, keeping the blood pH stable.

4. The Chloride Shift

As hydrogencarbonate ions (\(HCO_3^-\)) build up inside the red blood cell, they must be moved out into the plasma so that the transport process can continue.

The problem: If \(HCO_3^-\) ions just moved out, the inside of the cell would become positively charged, stopping further dissociation of \(H_2CO_3\).

The Solution: The Chloride Shift (Syllabus 8.2.2)

\(HCO_3^-\) ions diffuse out of the RBC and into the plasma.
To maintain electrical neutrality (balance the charges), chloride ions (\(Cl^-\)) diffuse from the plasma and move into the red blood cell.

Memory Aid: HCO₃⁻ goes out, Cl⁻ comes in. It's an ion trade-off to keep the charge balanced!

Importance of the Chloride Shift:

It maintains the electrical balance across the RBC membrane.
It allows the efficient removal of \(CO_2\) from the tissues (by keeping the concentration gradient for \(HCO_3^-\) steep).
It prevents excessive build-up of \(HCO_3^-\) inside the RBC, allowing the reaction \((CO_2 + H_2O \rightarrow HCO_3^- + H^+)\) to continue.

III. The Bohr Shift: Tailoring Oxygen Delivery

1. Definition and Mechanism

The Bohr shift (or Bohr effect) describes how an increase in the partial pressure of carbon dioxide (\(P_{CO_2}\)) or a decrease in pH shifts the Oxygen Dissociation Curve to the right.

A shift to the right means that for any given \(P_{O_2}\), haemoglobin is less saturated with oxygen. In simple terms, Hb's affinity for oxygen has decreased, meaning it releases \(O_2\) more readily.

Why does this happen?

Active tissues produce lots of \(CO_2\).
As explained in the previous section, \(CO_2\) leads to the production of \(H^+\) ions (acidity).
These \(H^+\) ions bind to haemoglobin (forming haemoglobinic acid, \(HHb\)).
When \(H^+\) ions bind to Hb, they change its three-dimensional shape, which reduces its affinity for oxygen.
Therefore, the presence of high \(CO_2\) (and low pH) forces Hb to dump its oxygen cargo exactly where it is needed: the active tissue.

2. Importance of the Bohr Shift

The Bohr shift ensures that oxygen is delivered precisely to the cells that are respiring most vigorously.

Active Tissues: High \(CO_2\) and low pH cause the ODC to shift right, boosting \(O_2\) unloading.
Lungs: Low \(CO_2\) and high pH cause the ODC to shift left (the opposite effect), boosting \(O_2\) loading.

Did you know? The Bohr shift is an incredible example of biological feedback. The very waste product produced by respiration (\(CO_2\)) is used as a signal to ensure more oxygen is delivered for more respiration!

Summary: The Complete Gas Exchange Loop

At the Tissues (Unloading and \(CO_2\) Pick-up):

Tissues have low \(P_{O_2}\) and high \(P_{CO_2}\) (and low pH).
High \(CO_2\) causes the Bohr shift, forcing Hb to unload \(O_2\) for cell use.
\(CO_2\) enters the RBC, is rapidly converted by carbonic anhydrase into \(H^+\) and \(HCO_3^-\).
\(H^+\) is buffered by Hb (forming haemoglobinic acid).
\(HCO_3^-\) leaves the RBC; \(Cl^-\) enters (Chloride Shift).

At the Lungs (Loading and \(CO_2\) Release):

Lungs have high \(P_{O_2}\) and low \(P_{CO_2}\) (and high pH).
High \(P_{O_2}\) causes \(O_2\) to load onto Hb.
This binding of \(O_2\) reduces Hb's affinity for \(H^+\), releasing the \(H^+\) ions (the reverse of the Bohr effect).
The released \(H^+\) combines with the \(HCO_3^-\) (which moves back into the RBC in exchange for \(Cl^-\)).
Carbonic acid reforms and is quickly broken down by carbonic anhydrase back into \(CO_2\) and \(H_2O\).
\(CO_2\) diffuses out of the RBC, into the plasma, and out of the blood into the alveoli for exhalation.

Key Takeaway: The transport system is finely tuned. The Chloride Shift ensures efficient \(CO_2\) transport by keeping the ions balanced, and the Bohr Shift ensures \(O_2\) is dropped off precisely where metabolic demand is highest.