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Q11:
Define oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern?
Solution :
1. Definition of the Oxygen Dissociation Curve
The Oxygen Dissociation Curve (ODC) is a graphical representation illustrating the relationship between the partial pressure of oxygen ($pO_2$) and the percentage saturation of hemoglobin ($Hb$) with oxygen. On a standard Cartesian coordinate system, the x-axis represents the $pO_2$ (measured in $mm\,Hg$), while the y-axis represents the percentage saturation of hemoglobin (percent $HbO_2$).
2. Visual Representation of the Sigmoidal Curve
3. Analysis of the Sigmoidal Pattern
The sigmoidal (S-shaped) pattern of the oxygen-hemoglobin dissociation curve is a direct consequence of the molecular structure of hemoglobin and the phenomenon of cooperative binding. The mechanism is broken down as follows:
- Molecular Structure: Hemoglobin is a tetrameric protein consisting of four polypeptide subunits, each containing a heme group capable of binding one molecule of $O_2$ [Reference: Quaternary structure of protein].
- Positive Cooperativity: When the first $O_2$ molecule binds to one of the four heme groups, it induces a conformational change in the entire hemoglobin molecule (shifting from the 'T' or Tense state to the 'R' or Relaxed state).
- Increased Affinity: This structural transition significantly increases the affinity of the remaining heme groups for oxygen. Consequently, the second and third $O_2$ molecules bind with progressively higher ease.
- The "S" Shape Explained:
- Initial Lag Phase: At low $pO_2$, the binding of the first oxygen molecule is difficult, resulting in a slow initial rise.
- Steep Logarithmic Phase: Once one $O_2$ is bound, the cooperative effect causes a rapid increase in saturation, creating the steep middle section of the curve.
- Plateau Phase: As hemoglobin approaches 100% saturation, the probability of finding an unbound heme group decreases, causing the curve to level off as it approaches the maximum binding capacity [Per the Law of Mass Action].
4. Physiological Significance
The sigmoidal nature is physiologically advantageous because it allows hemoglobin to be highly sensitive to small changes in $pO_2$ within the tissues. In the steep portion of the curve, a small drop in $pO_2$ (such as in metabolically active tissues) results in a large release of oxygen, ensuring that oxygen delivery matches metabolic demand efficiently.
Final Solution: The oxygen dissociation curve is a sigmoidal graph plotting the percentage of hemoglobin saturation against the partial pressure of oxygen. Its sigmoidal pattern arises from positive cooperativity, where the binding of one oxygen molecule to a hemoglobin subunit facilitates the binding of subsequent molecules by inducing a conformational change in the tetrameric protein structure.
More Questions from Class 11 Biology Breathing and Exchange of Gases EXERCISES
- Q1: Define vital capacity. What is its significance?
- Q2: State the volume of air remaining in the lungs after a normal breathing.
- Q3: Diffusion of gases occurs in the alveolar region only and not in the other parts of respiratory system. Why?
- Q4: What are the major transport mechanisms for $CO_2$? Explain.
- Q5: What will be the $pO_2$ and $pCO_2$ in the atmospheric air compared to those in the alveolar air ?
- Q6: Explain the process of inspiration under normal conditions.
- Q7: How is respiration regulated?
- Q8: What is the effect of $pCO_2$ on oxygen transport?
- Q9: What happens to the respiratory process in a man going up a hill?
- Q10: What is the site of gaseous exchange in an insect?
- Q12: Have you heard about hypoxia? Try to gather information about it, and discuss with your friends.
- Q13(a): Distinguish between (a) IRV and ERV
- Q13(b): Distinguish between (b) Inspiratory capacity and Expiratory capacity.
- Q13(c): Distinguish between (c) Vital capacity and Total lung capacity.
- Q14: What is Tidal volume? Find out the Tidal volume (approximate value) for a healthy human in an hour.
CBSE Solutions for Class 11 Biology Breathing and Exchange of Gases
Chapters in CBSE - Class 11 Biology
- The Living World
- Biological Classification
- Plant Kingdom
- Animal Kingdom
- Morphology of Flowering Plants
- Anatomy of Flowering Plants
- Structural Organisation in Animals
- Cell – The Unit of Life
- Biomolecules
- Cell Cycle and Division
- Transport in Plants
- Mineral Nutrition
- Photosynthesis
- Respiration in Plants
- Plant Growth and Development
- Digestion and Absorption
- Body Fluids and Circulation
- Excretory Products and Elimination
- Locomotion and Movement
- Neural Control and Coordination
- Chemical Coordination and Integration
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