Hello Biologists! Getting to Grips with Gas Exchange and Transport (Unit 1: Diversity of Living Organisms)

Welcome to one of the most fundamental topics in Biology: how organisms take in the life-giving oxygen they need and get rid of waste carbon dioxide. This process links directly to respiration, metabolism, and the overall survival of every living thing! We’ll explore how tiny single cells manage this, and how giant mammals (like us!) rely on complex circulatory and breathing systems.

Don't worry if this seems like a lot of detail—we will break down the crucial adaptations that nature has developed to solve the oxygen delivery problem.

3.1.5.1 The Surface Area to Volume Ratio Problem

The need for specialised exchange systems is all about mathematics, specifically the relationship between an organism’s size and its surface area to volume ratio (SA:V).

Why size matters for gas exchange

  • Small Organisms (High SA:V): A single-celled organism, or a very small, flat multicellular one (like a simple flatworm), has a large surface area compared to its volume. This means the distance between the outside environment and the innermost cells is tiny.
  • Gas Exchange: Gases (O2 and CO2) can be exchanged efficiently simply by diffusion across the entire body surface. Diffusion alone is fast enough to supply all the metabolic needs.

Analogy: Imagine trying to cool a bowl of soup. If you pour it into a wide, flat saucer (high SA:V), it cools quickly. If you leave it in a deep mug (low SA:V), it takes much longer.

The Adaptation of Larger Organisms

  • Large Organisms (Low SA:V): As organisms get bigger, their volume increases much faster than their surface area. The internal cells are now too far from the surface for simple diffusion to be effective.
  • Solution 1: Specialised Exchange Surfaces: Large organisms develop structures like lungs, gills, or leaves, which dramatically increase the available surface area for diffusion (e.g., the alveoli in human lungs).
  • Solution 2: Mass Transport Systems: Diffusion is too slow to move oxygen from the lungs to the muscles. Therefore, large organisms need a system to actively carry substances around the body. This is called mass transport (e.g., the circulatory system in mammals).
Quick Review: Exchange Requirement

The greater the size and metabolic rate of an organism, the lower the SA:V ratio, meaning simple diffusion is insufficient, requiring specialised exchange surfaces and mass transport systems.

3.1.5.2 Diverse Gas Exchange Systems

Organisms have evolved various clever ways to achieve efficient gas exchange, each perfectly tailored to their environment and lifestyle.

1. Single-celled Organisms

As discussed, gas exchange occurs across the body surface via simple diffusion, down the concentration gradient. Their high SA:V ratio makes this possible.

2. The Tracheal System of an Insect

Insects are small, terrestrial (land-dwelling) creatures, but they still need specialised structures to overcome the low efficiency of diffusion through their tough exoskeleton.

  • Structure: Air enters through small holes called spiracles. These lead to a network of tubes: tracheae, which branch into smaller tracheoles.
  • Mechanism: Oxygen diffuses directly from the ends of the tracheoles into the respiring tissues and muscle cells.
  • Compromise: Terrestrial insects face the challenge of conserving water while exchanging gases. The spiracles can be opened and closed. If they are open for gas exchange, water vapour is lost. This represents a functional compromise between the need for efficient gas exchange and the limitation of water loss.

3. Gas Exchange in Dicotyledonous Plant Leaves

Plants exchange gases primarily through their leaves.

  • Structure: Leaves have a large surface area and contain pores called stomata (singular: stoma) usually on the lower epidermis.
  • Mechanism: Gases diffuse through the stomata into the air spaces inside the leaf, where they then diffuse across the moist surfaces of the mesophyll cells. CO2 is needed for photosynthesis, and O2 is a waste product (during the day).
  • Compromise (Xerophytic Plants): Plants adapted to dry environments (xerophytes) have evolved structural compromises to minimise water loss, such as:
    i) Thick waxy cuticle.
    ii) Rolling leaves to trap moist air.
    iii) Hairy leaves (to reduce air movement and trap moisture).
    These features reduce the rate of transpiration but also limit the rate of gas exchange.

3.1.5.2 The Human Gas Exchange System

The human respiratory system is a complex system designed to maintain a steep concentration gradient for rapid diffusion across the alveoli.

Gross Structure of the System

Air travels through a pathway of tubes to reach the lungs:

  1. Trachea (windpipe): Supported by C-shaped cartilage rings.
  2. Bronchi (singular: bronchus): Two branches leading to each lung.
  3. Bronchioles: Smaller, muscular tubes branching throughout the lungs.
  4. Alveoli (singular: alveolus): Tiny air sacs where gas exchange occurs.

The entire structure is contained within the lungs, which reside in the thoracic cavity (chest).

The Exchange Surface: Alveoli and Capillaries

The alveoli and the surrounding capillaries are perfectly adapted for rapid gas exchange (O2 into the blood, CO2 out). Key adaptations relate to the features that increase the rate of diffusion (based on Fick’s Law):

  • Large Surface Area: Millions of alveoli provide an enormous area for exchange (the size of a tennis court!).
  • Short Diffusion Distance (Thin Exchange Surface): The walls of the alveoli (alveolar epithelium) and the capillary walls are both only one cell thick, resulting in a diffusion distance of approximately 0.5 \(\mu\)m.
  • Steep Concentration Gradient:
    • Constant supply of oxygenated air (ventilation/breathing).
    • Constant removal of oxygenated blood (circulation).

The Mechanism of Breathing (Ventilation)

Breathing relies on changing the pressure and volume inside the thoracic cavity. This involves the diaphragm (a sheet of muscle below the lungs) and the intercostal muscles (muscles between the ribs).

Step-by-Step Inspiration (Inhaling):

  1. Diaphragm contracts and flattens (moves down).
  2. External intercostal muscles contract, pulling the rib cage up and out.
  3. The volume of the thoracic cavity increases.
  4. This increase in volume causes the pressure inside the lungs (pulmonary pressure) to decrease (Boyle's Law).
  5. Since the external pressure is now higher than the pulmonary pressure, air is forced into the lungs.

Step-by-Step Expiration (Exhaling):

  1. Diaphragm relaxes and moves up (domes).
  2. External intercostal muscles relax, causing the rib cage to move down and in.
  3. The volume of the thoracic cavity decreases.
  4. This decrease in volume causes the pulmonary pressure to increase.
  5. Since the pulmonary pressure is now higher than the external pressure, air is forced out of the lungs.
Did You Know?

Lung diseases (like emphysema or fibrosis) damage the exchange surface. If the alveolar walls thicken or are destroyed, the diffusion distance increases or the surface area decreases. This reduces the efficiency of gas exchange, which can be measured through a reduced ability to ventilate (move air) and lower oxygen saturation in the blood.

3.1.5.3 Haemoglobin and the Transport of Oxygen

Mammalian Circulation: The Transport System

Oxygen needs to be efficiently moved away from the lungs using a mass transport system—the circulatory system.

General Pattern:

  • Arteries: Carry blood away from the heart (usually oxygenated).
  • Capillaries: The smallest vessels, site of metabolic exchange with tissues.
  • Veins: Carry blood towards the heart (usually deoxygenated).

Key Named Vessels (Required):

  • Coronary Arteries: Vessels supplying oxygenated blood to the heart muscle itself. Blockage here causes a heart attack (myocardial infarction).
  • Vessels entering/leaving the Heart: Vena Cava (to heart), Pulmonary Vein (to heart), Aorta (from heart), Pulmonary Artery (from heart).
  • Vessels entering/leaving the Liver: Hepatic Artery (into liver), Hepatic Portal Vein (into liver from digestive system), Hepatic Vein (out of liver).

The Oxygen Carrier: Haemoglobin (Hb)

Oxygen is carried by red blood cells (RBCs), primarily bound to the protein haemoglobin (Hb).

  • Structure: Haemoglobin is a chemically similar group of molecules found in many organisms. It is a quaternary protein structure, consisting of four polypeptide (globin) chains.
  • Haem Group: Each chain is associated with an iron-containing haem group.
  • Binding: Each haem group contains an iron ion (\(Fe^{2+}\)) which can reversibly bind one oxygen molecule. Therefore, one haemoglobin molecule can carry a maximum of four oxygen molecules.
  • Process: The binding of the first oxygen molecule makes it easier for the second, third, and fourth to bind (a process called cooperative binding).

The Oxygen-Haemoglobin Dissociation Curve (OHDC)

This curve is critical for understanding how haemoglobin picks up oxygen in the lungs and releases it in the tissues. It plots the percentage saturation of haemoglobin (how much O2 is bound) against the partial pressure of oxygen (p\(\text{O}_2\)).

  • Shape: The curve is S-shaped (sigmoidal).
    • In the Lungs (High p\(\text{O}_2\)): The curve is steep initially, meaning Hb rapidly loads oxygen, becoming nearly 100% saturated.
    • In Resting Tissues (Lower p\(\text{O}_2\)): The curve is shallower, meaning only a small amount of O2 is released (around 25% of the capacity).
    • In Exercising Tissues (Very Low p\(\text{O}_2\)): The curve drops steeply again, meaning Hb rapidly unloads a large amount of oxygen where it is needed most.

The Effect of Carbon Dioxide (The Bohr Shift)

Metabolically active tissues produce large amounts of CO2, which is advantageous as it signals the need for more oxygen delivery.

  • Mechanism: When CO2 dissolves in the blood, it lowers the blood pH, making it more acidic.
  • Effect on Hb: A decrease in pH causes haemoglobin to change shape slightly, reducing its affinity (stickiness) for oxygen.
  • The Shift: This causes the entire oxygen-haemoglobin dissociation curve to shift to the right.
  • Result: At any given partial pressure of oxygen, the haemoglobin is less saturated, meaning it releases oxygen more easily to the tissues. This phenomenon is called the Bohr shift.
Memory Aid: Bohr is Right!

High CO2 means the tissues need more O2. This causes the curve to shift RIGHT, meaning oxygen is released.

Adaptations of Different Haemoglobins

Different organisms have different needs based on their environment. Haemoglobins are chemically similar but have varying oxygen affinities:

  • High Altitude/Low Oxygen Environments: Organisms living high up or in water with low O2 (e.g., Llama, lugworms) have haemoglobin with a higher affinity for oxygen. This means their OHDC shifts to the left. This allows them to load oxygen effectively even when the external p\(\text{O}_2\) is low.
  • Highly Active Animals: Organisms with high metabolic rates (rarely needed for syllabus, but good context) might have haemoglobin with a lower affinity, allowing them to unload O2 extremely easily when exercising.
  • Foetal Haemoglobin: Human foetuses must extract oxygen from the mother's blood, which already has a relatively low p\(\text{O}_2\). Therefore, foetal haemoglobin has a higher affinity (curve shifts left) than adult haemoglobin, ensuring efficient transfer across the placenta.

Key Takeaway for the Chapter: Gas exchange is a process driven by diffusion and enhanced by structural adaptations (high SA, short diffusion distance). In large organisms, efficiency is boosted by mass transport, relying on the sophisticated chemical properties of haemoglobin to load and unload oxygen precisely where it is needed.