IB Biology SL: Theme B - Form & Function

B3.1 - Gas Exchange

The Vital Exchange: Oxygen In, Carbon Dioxide Out

🫁 Introduction to Gas Exchange

Gas exchange is one of the most fundamental processes in biology—it's the mechanism by which organisms obtain oxygen for cellular respiration and remove carbon dioxide, a toxic waste product. Every living cell requires a constant supply of oxygen to produce ATP (energy), and must eliminate CO₂ to prevent cellular damage.

The basic equation for aerobic respiration highlights why gas exchange is essential:

Glucose + Oxygen → Carbon Dioxide + Water + ATP

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

In this topic, we'll explore how gas exchange occurs in two different systems: the mammalian lungs (focusing on human respiration) and plant leaves (focusing on photosynthesis and respiration). Despite their differences, both systems share common principles: large surface areas, thin exchange surfaces, and concentration gradients.

🫁 Gas Exchange in Mammalian Lungs

The Human Respiratory System

The human respiratory system is a complex network of organs designed to bring oxygen-rich air into contact with blood, allowing efficient gas exchange. Air travels through a series of structures before reaching the site of gas exchange:

Pathway of Air:

Nose/Mouth → Trachea → Bronchi → Bronchioles → Alveoli

Nose/Mouth: Entry point; air is warmed, humidified, and filtered
Trachea (windpipe): Reinforced with C-shaped cartilage rings; splits into two bronchi
Bronchi: Two main airways (left and right) leading to each lung
Bronchioles: Smaller branching airways that spread throughout the lungs
Alveoli: Tiny air sacs at the end of bronchioles—site of gas exchange

🔬 Alveoli: The Site of Gas Exchange

What are Alveoli?

Alveoli (singular: alveolus) are tiny, balloon-like air sacs found at the ends of the smallest bronchioles in the lungs. There are approximately 300-500 million alveoli in each human lung, creating an enormous surface area for gas exchange.

📊 Key Statistics:

Number of alveoli: ~300-500 million per lung
Diameter of each alveolus: ~200-300 μm (micrometers)
Total surface area: ~70 m² (about the size of a tennis court!)
Wall thickness: ~0.5 μm (extremely thin for rapid diffusion)

Structure of an Alveolus

Key Components:

Alveolar wall: Single layer of flattened epithelial cells (squamous epithelium)
Capillary network: Dense network of blood vessels surrounding each alveolus
Capillary wall: Also one cell thick (endothelium)
Thin layer of moisture: Lines the inside of alveoli; helps gases dissolve
Elastic fibers: Allow alveoli to expand and recoil during breathing
Type II cells: Produce surfactant to prevent alveoli from collapsing

⚡ Adaptations of Alveoli for Efficient Gas Exchange

Alveoli are perfectly designed to maximize gas exchange efficiency. Their structure incorporates several key adaptations:

1. Large Surface Area

Why it matters:

More surface area allows more gas molecules to be exchanged simultaneously, increasing the rate of gas exchange.

How it's achieved:

Millions of alveoli: 300-500 million per lung creates enormous total surface area (~70 m²)
Spherical shape: Maximizes surface area for the volume
Folded structure: Lungs contain many lobes and subdivisions

2. Thin Exchange Surface (Short Diffusion Distance)

Why it matters:

Fick's Law shows that rate of diffusion is inversely proportional to distance. Shorter distance = faster diffusion.

How it's achieved:

One-cell-thick alveolar wall: Flattened squamous epithelial cells
One-cell-thick capillary wall: Endothelial cells
Total barrier: Only ~0.5 μm thick (two cells + their basement membranes)
Gases only need to diffuse across two cell layers total

3. Extensive Blood Supply (Capillary Network)

Why it matters:

Maintains concentration gradients by constantly bringing deoxygenated blood (high CO₂, low O₂) and removing oxygenated blood (low CO₂, high O₂).

How it's achieved:

Dense capillary network: Each alveolus is surrounded by a mesh of capillaries
Continuous blood flow: Blood constantly circulates, maintaining gradients
Close contact: Capillaries wrap tightly around alveolar walls
Blood spends ~0.75 seconds in contact with each alveolus (sufficient for full gas exchange)

4. Steep Concentration Gradients

Why it matters:

Diffusion only occurs down concentration gradients. Steeper gradients = faster diffusion rates.

How it's maintained:

Ventilation (breathing): Constantly refreshes air in alveoli with high O₂, low CO₂
Blood circulation: Brings deoxygenated blood (low O₂, high CO₂) to alveoli
Metabolism: Cells constantly use O₂ and produce CO₂, maintaining demand

5. Moist Surface

Why it matters:

Gases must dissolve in moisture before they can diffuse across membranes. Dry surfaces prevent gas exchange.

How it's achieved:

Thin fluid layer: Lines the internal surface of each alveolus
Surfactant: Reduces surface tension and prevents alveoli from collapsing
O₂ and CO₂ dissolve in this moisture before diffusing through the membrane

6. Elastic Recoil

Why it matters:

Allows lungs to return to resting state after inhalation, making exhalation passive (requires no energy).

How it's achieved:

Elastic fibers: In alveolar walls allow stretch during inhalation
Natural recoil: Fibers spring back during exhalation
Helps expel air without muscular effort (during quiet breathing)

🔄 The Process of Gas Exchange in Alveoli

Oxygen Diffusion (Into Blood)

1. Inhaled air enters alveoli with high O₂ concentration (~21% or 104 mmHg)

2. Deoxygenated blood arrives from body with low O₂ concentration (~40 mmHg)

3. Concentration gradient exists: high in alveoli → low in blood

4. O₂ diffuses from alveoli → through moisture → through alveolar wall → through capillary wall → into blood plasma → into red blood cells

5. In red blood cells, O₂ binds to hemoglobin forming oxyhemoglobin

Carbon Dioxide Diffusion (Out of Blood)

1. Deoxygenated blood arrives from body with high CO₂ concentration (~46 mmHg)

2. Alveolar air has low CO₂ concentration (~40 mmHg)

3. Concentration gradient exists: high in blood → low in alveoli

4. CO₂ diffuses from blood → through capillary wall → through alveolar wall → into alveolar air

5. CO₂ is exhaled during expiration

🔑 Key Principle:

Both O₂ and CO₂ move by simple diffusion down their concentration gradients. This is a passive process requiring no ATP—the concentration gradients provide the driving force!

💨 Ventilation: The Mechanics of Breathing

What is Ventilation?

Ventilation (also called breathing) is the physical process of moving air into and out of the lungs. It has two phases:

Inhalation (Inspiration)

Moving air INTO the lungs

Exhalation (Expiration)

Moving air OUT of the lungs

Ventilation maintains concentration gradients necessary for gas exchange by constantly refreshing air in the alveoli!

⚙️ The Mechanism of Breathing

Breathing is based on pressure changes in the thoracic cavity (chest). Air flows from high pressure to low pressure.

Boyle's Law

P ∝ 1/V

Pressure is inversely proportional to Volume

↑ Volume → ↓ Pressure | ↓ Volume → ↑ Pressure

↓ INHALATION (Inspiration) - ACTIVE PROCESS

Step-by-Step Process:

1. External Intercostal Muscles Contract

External intercostals (between ribs) contract → ribs move upward and outward

2. Diaphragm Contracts

Diaphragm (dome-shaped muscle below lungs) contracts → flattens and moves downward

3. Thoracic Cavity Volume Increases

Chest cavity expands in all directions → volume increases

4. Lung Volume Increases

Lungs are pulled outward (attached to chest wall by pleural membranes) → lung volume increases

5. Pressure Decreases (Boyle's Law)

Increased volume → pressure inside lungs decreases below atmospheric pressure

6. Air Flows In

Air moves from high pressure (atmosphere) to low pressure (lungs) → AIR ENTERS LUNGS

⚡ Energy Requirement:

Inhalation is an ACTIVE process—it requires ATP for muscle contraction (diaphragm and intercostals)

↑ EXHALATION (Expiration) - PASSIVE PROCESS (Quiet Breathing)

Step-by-Step Process:

1. External Intercostal Muscles Relax

External intercostals relax → ribs move downward and inward

2. Diaphragm Relaxes

Diaphragm relaxes → returns to dome shape and moves upward

3. Thoracic Cavity Volume Decreases

Chest cavity gets smaller → volume decreases

4. Elastic Recoil of Lungs

Elastic fibers in lungs recoil inward → lung volume decreases

5. Pressure Increases (Boyle's Law)

Decreased volume → pressure inside lungs increases above atmospheric pressure

6. Air Flows Out

Air moves from high pressure (lungs) to low pressure (atmosphere) → AIR LEAVES LUNGS

⚡ Energy Requirement (Quiet Breathing):

Normal exhalation is PASSIVE—requires no ATP. Occurs due to muscle relaxation and elastic recoil.

💪 Forced Exhalation (Active):

During exercise or forceful breathing, internal intercostal muscles and abdominal muscles contract to force more air out. This is an ACTIVE process requiring ATP.

📊 Comparison: Inhalation vs. Exhalation

Feature	Inhalation (Inspiration)	Exhalation (Expiration)
Diaphragm	Contracts, flattens, moves DOWN	Relaxes, domes, moves UP
External Intercostals	Contract → ribs move up & out	Relax → ribs move down & in
Thoracic Volume	INCREASES	DECREASES
Lung Pressure	DECREASES (below atmospheric)	INCREASES (above atmospheric)
Air Movement	Air flows IN (atmosphere → lungs)	Air flows OUT (lungs → atmosphere)
Energy Required?	ACTIVE - requires ATP	PASSIVE (quiet) - no ATP needed

📊 Lung Volumes and Capacities

What are Lung Volumes?

Maximum volume that can be inhaled after a normal exhalation. Typical value: ~3500 mL

2. Functional Residual Capacity (FRC)

FRC = ERV + RV

Volume of air remaining in lungs after a normal exhalation. Typical value: ~2400 mL

3. Vital Capacity (VC)

VC = TV + IRV + ERV

Maximum volume that can be exhaled after maximum inhalation. Typical value: ~4700 mL (males), ~3100 mL (females)

4. Total Lung Capacity (TLC)

TLC = TV + IRV + ERV + RV

OR: TLC = VC + RV

Maximum volume of air the lungs can hold. Typical value: ~6000 mL (6 liters)

📊 Quick Summary:

TLC = VC + RV = (TV + IRV + ERV) + RV

🍃 Gas Exchange in Plant Leaves

Why Do Plants Need Gas Exchange?

Like animals, plants need gas exchange for two essential processes:

Photosynthesis

IN: CO₂ | OUT: O₂

Plants take in carbon dioxide for photosynthesis (making glucose) and release oxygen as a byproduct. Occurs during daylight.

Respiration

IN: O₂ | OUT: CO₂

Plants take in oxygen for cellular respiration (producing ATP) and release carbon dioxide as a waste product. Occurs 24/7.

⚡ Net Effect During Day:

During daylight, photosynthesis rate > respiration rate. Net result: plants take in MORE CO₂ than they produce, and release MORE O₂ than they use. This is why plants are called "producers" and "oxygen generators"!

🔬 Structure of a Leaf for Gas Exchange

Leaves are the primary organs for gas exchange in plants. Their structure is highly adapted for this purpose:

Cross-Section of a Leaf (Top to Bottom):

1. Waxy Cuticle (Upper Surface)

Function: Waterproof layer that prevents water loss by evaporation. BUT it also prevents gas exchange, which is why leaves need stomata!

2. Upper Epidermis

Function: Transparent protective layer. Allows light to pass through to photosynthetic cells below. Usually has no stomata (or very few).

3. Palisade Mesophyll Layer

Structure: Column-shaped cells packed with chloroplasts, arranged vertically just below upper surface. Function: Primary site of photosynthesis. Positioned to capture maximum light.

4. Spongy Mesophyll Layer

Structure: Irregularly shaped cells with large air spaces between them. Some chloroplasts present. Function: Air spaces allow gas circulation. Gases diffuse through these spaces to reach all cells.

5. Lower Epidermis

Function: Protective layer on underside of leaf. Contains many stomata for gas exchange. Vascular tissue (xylem and phloem) runs through this layer.

6. Stomata (Pores)

Structure: Tiny pores (openings) in lower epidermis, surrounded by two guard cells. Function: Allow CO₂ to enter and O₂/H₂O vapor to exit. Can open and close to regulate gas exchange and water loss.

7. Guard Cells

Structure: Kidney-shaped cells that surround each stoma. Contain chloroplasts. Function: Control opening/closing of stomata by changing shape (turgid = open, flaccid = closed).

⚡ Adaptations of Leaves for Gas Exchange

1. Large Surface Area

Flat, broad shape: Maximizes area available for light absorption and gas exchange. Many leaves per plant multiply this effect.

2. Thin Structure

Short diffusion distance: Leaves are typically only a few cells thick. This ensures gases can diffuse quickly to all cells. No cell is far from an air space.

3. Many Stomata

Numerous pores: Leaves have thousands to millions of stomata (mostly on lower surface). This creates many entry/exit points for gases, increasing exchange rate.

4. Air Spaces in Spongy Mesophyll

Internal gas circulation: Large air spaces allow gases to move freely within the leaf, reaching all cells efficiently. Moist cell walls allow gases to dissolve for diffusion.

5. Guard Cells Control Stomata

Regulated exchange: Guard cells open stomata when CO₂ is needed (daylight, photosynthesis) and close them to prevent water loss (night, drought, high temperature).

6. Moist Cell Surfaces

Facilitates diffusion: Cell walls in spongy mesophyll are covered with thin water film. CO₂ and O₂ must dissolve in this moisture before diffusing into/out of cells.

7. Stomata Mostly on Lower Surface

Reduces water loss: Lower surface is shaded, cooler, and less exposed to sun/wind. This minimizes evaporation while still allowing gas exchange. Upper surface is waxy to prevent water loss there.

🚪 How Stomata Work: Opening and Closing

Guard cells control stomatal opening through changes in turgor pressure (water pressure inside cells):

Stomata OPEN (Day)

Light detected: Guard cells receive light signal
Photosynthesis occurs: Guard cells (have chloroplasts) produce sugars
Solute concentration increases: K⁺ ions actively pumped in, sugars accumulate
Water enters by osmosis: Water moves into guard cells from surrounding cells
Guard cells become turgid: Fill with water, become swollen
Stoma opens: Turgid guard cells curve, creating opening

Stomata CLOSE (Night/Drought)

No light/low water: Signal to close
Photosynthesis stops: Sugars converted to starch (insoluble)
Solute concentration decreases: K⁺ ions leave, starch doesn't affect osmosis
Water leaves by osmosis: Water moves out of guard cells
Guard cells become flaccid: Lose water, become limp
Stoma closes: Flaccid guard cells collapse together, sealing pore

⚖️ The Compromise:

Plants face a challenge: open stomata allow CO₂ in (good for photosynthesis) but also allow water vapor out (transpiration, potentially leading to dehydration). Plants must balance the need for CO₂ with the need to conserve water. This is why stomata close at night (no photosynthesis anyway) and during drought (water conservation priority).

📊 Stomatal Density

What is Stomatal Density?

Stomatal density is the number of stomata per unit area of leaf surface (typically measured as stomata per mm²).

Stomatal Density = Number of Stomata / Leaf Area (mm²)

Stomatal density varies widely: from ~50-300 stomata/mm² in many plants, but can reach 1000/mm² in some species!

🌍 Factors Affecting Stomatal Density

Stomatal density is influenced by both genetic factors (species, cultivar) and environmental factors:

1. Light Intensity

Effect:

Higher light intensity → Higher stomatal density

Explanation:

Plants grown in bright sunlight develop more stomata to maximize CO₂ uptake for high photosynthesis rates. Shaded plants have fewer stomata because photosynthesis is limited by light, not CO₂ availability.

2. CO₂ Concentration

Effect:

Higher CO₂ concentration → Lower stomatal density

Explanation:

When atmospheric CO₂ is abundant, plants need fewer stomata to obtain sufficient CO₂. This is an evolutionary adaptation observed in fossil records and modern experiments with elevated CO₂.

🌍 Climate connection: Rising atmospheric CO₂ may lead to lower stomatal density in future plants, affecting water use and climate feedback loops.

3. Water Availability (Humidity)

Effect:

Lower water availability (drought) → Often lower stomatal density OR smaller stomata

Explanation:

Plants in dry environments (desert plants, xerophytes) often have fewer and/or smaller stomata to minimize water loss through transpiration. The relationship is complex because stomatal density must balance CO₂ uptake with water conservation.

4. Temperature

Effect:

Higher temperature → Variable effects (often decreased density or size)

Explanation:

High temperatures increase evaporation rate, so plants may reduce stomatal density to conserve water. However, higher temperature also increases photosynthesis rate, which may increase demand for CO₂. The net effect depends on water availability.

5. Leaf Position & Age

Effect:

Upper canopy leaves → Higher stomatal density
Shaded/lower leaves → Lower stomatal density
Younger leaves → Often higher density than older leaves

Explanation:

Leaves adjust stomatal density to their light environment. Upper leaves receive more light and need more CO₂, so develop higher density. Stomatal density is determined during leaf development and generally doesn't change after the leaf is fully expanded.

🔬 Why Stomatal Density Matters

🌱 Plant Productivity

Stomatal density affects the rate of CO₂ uptake and thus photosynthesis rate. Higher density (up to a point) can increase plant growth and crop yields.

💧 Water Use Efficiency

More stomata = more water loss. Plants must optimize stomatal density to balance CO₂ gain vs. water loss. This is especially important in agriculture and drought-prone regions.

🌍 Climate Change Indicator

Fossil leaf stomatal density is used to reconstruct past atmospheric CO₂ levels. Modern changes in stomatal density help us understand plant responses to rising CO₂ and climate change.

🧬 Evolutionary Adaptation

Different species have evolved different stomatal densities suited to their environments. Desert plants have low density, while plants in humid, high-light environments have high density.

🎯 Key Concepts Summary

✓ Gas Exchange is Essential

All organisms need gas exchange for aerobic respiration (O₂ in, CO₂ out). Plants additionally exchange gases for photosynthesis (CO₂ in, O₂ out during day).

✓ Alveoli Structure-Function

Alveoli are adapted with: large surface area (~70 m²), thin walls (~0.5 μm), extensive capillary network, moist surfaces, steep concentration gradients maintained by ventilation and circulation, and elastic recoil.

✓ Ventilation Mechanism

Inhalation (active): diaphragm/intercostals contract → volume ↑ → pressure ↓ → air in. Exhalation (passive): muscles relax → volume ↓ → pressure ↑ → air out. Based on Boyle's Law (P ∝ 1/V).

✓ Lung Volumes

Tidal Volume (~500 mL), Inspiratory Reserve (~3000 mL), Expiratory Reserve (~1200 mL), Residual Volume (~1200 mL). Vital Capacity = TV + IRV + ERV. Total Lung Capacity = VC + RV (~6 L).

✓ Gas Exchange in Leaves

Leaves are adapted with a large surface area, thin structure, and stomata (pores) controlled by guard cells. The spongy mesophyll's air spaces allow gases to circulate and reach all photosynthetic cells.

✓ Stomatal Function & Density

Guard cells open/close stomata to balance CO₂ uptake with water loss (transpiration). Stomatal density is influenced by light and CO₂ levels, reflecting a plant's adaptation to its environment.