Gas Exchange and Surface Area

For efficient diffusion to meet the metabolic demands of complex multicellular organisms, respiratory surfaces must maximize their $SA:V$ ratio. Small, single-celled organisms can rely entirely on simple diffusion across their outer membrane, but as organisms grow larger, their volume (which dictates metabolic rate and thus oxygen demand) increases much faster than their surface area (which dictates the rate of gas exchange).

To compensate, evolutionary adaptations have resulted in specialized respiratory structures like gills in fish or alveoli in mammalian lungs. We can express the rate of diffusion using Fick's Law of Diffusion:

• Permeability: Surfaces must allow O₂ and CO₂ to pass through easily without significant resistance.
• Thin tissue layers: Reduces the diffusion distance, maximizing the rate of diffusion. E.g. alveolar walls are just one cell thick (squamous epithelium).
• Moisture: Gases must dissolve in a thin liquid film in order to diffuse across the phospholipid bilayers of cell membranes.
• Large Surface Area: Essential for absorbing and releasing sufficient volumes of gas to support a high metabolism.

Mammalian Lungs & Concentration Gradients

To maximize the numerator in Fick's Law, not only do we need a massive surface area (provided by millions of tiny air sacs called alveoli), but we also need a steep concentration gradient ($\Delta Concentration$).

Ventilation (breathing) and continuous, rapid blood flow ensure that concentration gradients of O₂ and CO₂ are maintained across the alveolar membrane. If air in the alveoli became stagnant, it would quickly reach equilibrium with the blood, and diffusion of oxygen into the blood would cease.

Ventilation of the Lungs

Breathing operates on the principles of Boyle's Law: $P \propto \frac{1}{V}$, which states that pressure is inversely proportional to volume in a closed system. By changing the volume of the sealed thoracic cavity, the body inversely changes the pressure inside the lungs, drawing air in (inspiration) or pushing it out (expiration).

During normal expiration, these muscles simply relax, and the elastic recoil of the lungs reduces the volume, pushing air out. Forced expiration (like blowing out candles) requires the contraction of the internal intercostal muscles and abdominal muscles.

Measurement of Lung Volumes

Spirometry is the standard method used to measure the volume of air an individual inhales and exhales. A spirometer produces a spirogram, a graph showing volume changes over time. This is a critical diagnostic tool for assessing lung function and diagnosing respiratory diseases like asthma or COPD.

• Tidal Volume ($V_T$): The volume of air breathed in and out during a normal, unforced resting breath (typically ~500 mL in adults).
• Vital Capacity (VC): The maximum volume of air that can be forcibly exhaled after taking the deepest possible breath.
• Inspiratory Reserve Volume (IRV): The extra volume of air that can be inhaled with maximal effort after a normal tidal inhalation.
• Expiratory Reserve Volume (ERV): The extra volume of air that can be forcibly exhaled after a normal tidal exhalation.
• Residual Volume (RV): The air remaining in the lungs even after a maximal exhalation, which keeps the alveoli inflated.

Leaf Adaptations for Gas Exchange

Unlike animals, plants do not have a dedicated active ventilation system (no breathing). Gas exchange in plants relies entirely on diffusion. CO₂ must enter the leaf for the Calvin cycle of photosynthesis, while O₂ produced during the light-dependent reactions must exit. This occurs primarily through microscopic pores on the underside of the leaf called stomata.

• Waxy Cuticle: A hydrophobic layer on the upper and lower epidermis that prevents excessive transpirational water loss.
• Spongy Mesophyll: Loosely packed cells in the lower half of the leaf containing extensive air spaces. This creates a huge internal surface area for the rapid diffusion of gases to the palisade mesophyll cells above.
• Stomatal Guard Cells: Pairs of specialized cells flanking each stoma. They change turgor pressure to open and close the stomatal pores, regulating gas exchange and carefully limiting water loss.

Transpiration and Stomatal Density

To acquire CO₂ for photosynthesis, plants must open their stomata. Because the internal air spaces of the spongy mesophyll are saturated with water vapor (100% relative humidity) and the outside air is usually drier, water vapor inevitably diffuses out of the leaf down its concentration gradient. This loss of water vapor is called transpiration.

While transpiration represents a loss of water that can lead to wilting, it is also the driving force (transpirational pull) that brings water and dissolved minerals up from the roots to the rest of the plant via the xylem.

Haemoglobin and the Bohr Shift

Haemoglobin (Hb) is a tetrameric globular protein found in erythrocytes (red blood cells). Each of its four polypeptide chains contains a haem group with an iron ion ($Fe^{2+}$) that can reversibly bind to one oxygen molecule, forming oxyhaemoglobin. The affinity (stickiness) for oxygen changes dynamically based on the local biochemical environment.

Oxygen Dissociation Curves

The curve plotting percentage saturation of haemoglobin against partial pressure of oxygen ($pO_2$) is not linear; it is sigmoidal (S-shaped). This is due to cooperative binding: as the first O₂ molecule binds to Hb, it induces a conformational change that makes it much easier for the second and third molecules to bind. The curve flattens at the top as Hb becomes saturated.

• Right Shift (The Bohr Effect): Caused by high $pCO_2$, low pH, or high temperature (conditions typical of exercising muscle). Hb has lower affinity and releases more O₂ to tissues.
• Left Shift: Caused by low $pCO_2$, high pH, or lower temperature. Also characterizes Foetal Haemoglobin and Myoglobin. Hb has a higher affinity and holds onto O₂ more tightly at lower partial pressures.

Check Your Understanding

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