Diffusion & the alveoli

Rate affected by 3 factors:

Distance	diffuses quicker if less to travel
Concentration difference	diffuse quicker if larger concentration difference from area diffusing from to area diffusing to
Surface area	more surface there is the faster molecules can diffuse

Key Factors Influencing Diffusion:

1. Concentration Gradient: The concentration gradient is the difference in the concentration of particles between two areas.

Effect: The steeper the concentration gradient, the faster the rate of diffusion. A greater difference in concentration leads to more particles moving from high to low concentration.

2. Temperature: Temperature affects the kinetic energy of the particles involved.

Effect: Higher temperatures increase the kinetic energy of particles, causing them to move faster. This results in an increased rate of diffusion. Conversely, lower temperatures slow down particle movement, reducing the rate of diffusion.

3. Surface Area: The surface area over which diffusion occurs plays a crucial role in determining how quickly substances can move.

Effect: Larger surface areas provide more space for particles to diffuse across, increasing the rate of diffusion. This is particularly important in biological systems, such as the alveoli in the lungs or the villi in the small intestine, where large surface areas facilitate efficient gas and nutrient exchange.

4. Distance: The distance that particles need to travel during diffusion also impacts the rate.

Effect: Shorter diffusion distances result in faster diffusion. In biological systems, cells and tissues are often thin to reduce the distance over which diffusion must occur, ensuring that substances are quickly delivered or removed.

5. Nature of the Diffusing Substance: The size and type of particles can influence how easily they diffuse.

Effect: Smaller molecules (e.g., oxygen and carbon dioxide) diffuse faster than larger molecules. Additionally, nonpolar molecules generally diffuse more easily through cell membranes than polar molecules due to their solubility in lipids.

Biological Importance:

• Efficient Gas Exchange: In the lungs, oxygen diffuses rapidly from the alveoli into the blood due to the large surface area, steep concentration gradient, and short diffusion distance.
• Nutrient Absorption: In the small intestine, nutrients diffuse into the bloodstream through the villi, which have a large surface area and thin walls, maximising the diffusion rate.
• Waste Removal: Diffusion is crucial for removing waste products like carbon dioxide from cells into the blood for exhalation.

Challenges in Multicellular Organisms:

• Distance: In large multicellular organisms, many cells are located far from the external environment. Diffusion over long distances is slow and inefficient.
• Surface Area to Volume Ratio: As organisms grow larger, their volume increases more rapidly than their surface area, reducing the relative surface area available for diffusion. This makes it difficult for direct diffusion across the outer surface to supply all cells adequately.

Adaptations to Enhance Diffusion:

Multicellular organisms have developed specialised structures like lungs, gills, and intestines to maximise diffusion efficiency. These surfaces typically have:

Large Surface Area: Structures such as the alveoli in the lungs or villi in the intestines provide a large surface area for diffusion.

Thin Membranes: Thin barriers reduce the distance over which diffusion occurs, speeding up the process.

Rich Blood Supply: Blood vessels are often close to these surfaces, helping to transport substances quickly away from or to the cells.

Ventilation Mechanisms: In the lungs, breathing helps maintain concentration gradients by continually bringing in fresh oxygen and removing carbon dioxide.

Examples of Diffusion in Multicellular Organisms:

• Gas Exchange in Lungs: Oxygen diffuses from the alveoli into the blood while carbon dioxide diffuses from the blood into the alveoli, facilitated by the large surface area and thin walls of the alveoli.
• Nutrient Absorption in the Small Intestine: The villi and microvilli increase the surface area for nutrients to diffuse into the bloodstream after digestion.
• Waste Removal in the Kidneys: Diffusion is involved in the filtration process where waste products like urea are removed from the blood.

Transport Systems:

• In addition to specialised exchange surfaces, multicellular organisms have developed transport systems (such as the circulatory system) to move substances more efficiently. The blood circulatory system, for example, transports oxygen, nutrients, and waste products to and from cells, complementing diffusion.

Importance of Diffusion:

Despite the complexity of multicellular organisms, diffusion remains a fundamental process for maintaining cellular function. It is integral to processes like respiration, nutrient uptake, and waste removal, ensuring that cells receive what they need to survive and function.

Summary:

In multicellular organisms, diffusion alone cannot meet all the demands of the organism due to limitations like distance and surface area. To overcome these challenges, these organisms have evolved specialised structures and transport systems that enhance diffusion efficiency and ensure that all cells receive the necessary substances for life.

Adaptations of Alveoli for Gas Exchange

Key adaptations of the alveoli:

Large Surface Area:

The lungs contain millions of alveoli, which provide a very large surface area for gas exchange. This large surface area allows more oxygen to diffuse into the blood and more carbon dioxide to diffuse out of the blood, increasing the efficiency of gas exchange.

Thin Walls:

The walls of the alveoli and the surrounding capillaries are one cell thick, minimising the diffusion distance. This short distance allows gases (oxygen and carbon dioxide) to move quickly and efficiently between the air in the alveoli and the blood in the capillaries.

Rich Blood Supply:

• Each alveolus is surrounded by a dense network of capillaries. The blood in the capillaries is constantly replenished with blood that is high in carbon dioxide and low in oxygen. This maintains a concentration gradient that favours the diffusion of oxygen into the blood and carbon dioxide out of the blood.

Ventilation:

Ventilation (breathing) ensures a continuous supply of fresh air into the alveoli. This keeps the oxygen concentration high and the carbon dioxide concentration low in the alveolar air space, maintaining a strong concentration gradient for the gases to diffuse efficiently.

Moist Lining:

The inner surface of the alveoli is coated with a thin layer of moisture. This moisture helps gases to dissolve and then diffuse across the alveolar membrane more easily. Oxygen dissolves in the moisture before diffusing into the blood, and carbon dioxide dissolves in the moisture before being expelled.

Elastic Fibers:

Elastic fibers in the alveolar walls allow the alveoli to stretch and recoil during breathing. This helps to expel carbon dioxide and allows fresh oxygen to enter, ensuring efficient gas exchange during each breath.

Flick's Law

Fick's Law of Diffusion:

The rate of diffusion is directly proportional to:

6. The surface area of the membrane.
7. The concentration gradient (the difference in concentration of a substance on either side of the membrane).
8. The diffusion coefficient (depends on the nature of the substance and the medium). And inversely proportional to: The thickness of the membrane (distance the substance has to diffuse through).

$$\text{Rate of diffusion} \propto \frac{\text{Surface Area} \times \text{Concentration Difference}}{\text{Thickness of Membrane}}$$

What this means:

• Larger surface area leads to faster diffusion.
• Greater concentration difference (gradient) leads to faster diffusion.
• Thinner membranes allow substances to diffuse more quickly.
• Diffusion coefficient depends on factors like the temperature and the properties of the diffusing substance.

Application:

In biology, Fick's Law is important for understanding gas exchange in the lungs:

• Alveoli have a large surface area, a thin membrane, and are surrounded by capillaries with a high concentration gradient of oxygen and carbon dioxide, all of which maximize the efficiency of gas exchange.

Blood from the lungs has come from body and contain lots of carbon dioxide and little oxygen. Enlarged concentration difference so diffusion is quicker