No living organism exists in a vacuum. To stay alive, every cell must continuously take in useful substances from its surroundings and get rid of waste materials. In this topic, we will explore why size matters when it comes to exchange, how multicellular organisms have evolved specialized exchange surfaces, and how we calculate the efficiency of gas exchange.

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What you'll learn

• Why organisms need to exchange substances with their environment
• How to calculate and compare surface area to volume ratios (SA:V)
• How the human alveoli are adapted for efficient gas exchange
• The factors affecting the rate of diffusion and how to use Fick's Law (Separate Biology / Higher Tier only)

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1. Why do organisms need to transport substances?

Cells are constantly busy carrying out metabolic reactions (such as aerobic respiration) to release energy. To support these reactions, substances must move into and out of cells across the cell membrane.

Substances that must be taken in:

• Oxygen: Required for aerobic respiration by almost all living cells.
• Dissolved food molecules: For example, glucose (for respiration) and amino acids (to build proteins).
• Water: The medium in which all metabolic reactions take place.
• Mineral ions: Such as sodium, potassium, and calcium in animals, or magnesium and nitrates in plants.

Substances that must be removed (excreted):

• Carbon dioxide: A waste product of aerobic respiration that becomes toxic if it builds up.
• Urea: A poisonous waste product formed in the liver from the breakdown of excess amino acids, which must be transported to the kidneys for excretion.

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2. Surface Area to Volume Ratio (SA:V)

Why can an amoeba (a single-celled organism) survive simply by letting gases diffuse across its cell membrane, while a human needs a complex pair of lungs and a circulatory system? The answer lies in the surface area to volume ratio (SA:V).

Single-Celled Organisms

A single-celled organism has a very large surface area compared to its tiny volume. Because its body is so small, the distance from the outer membrane to the very center of the cell is tiny.

• Substances can diffuse directly into and out of the cell quickly enough to meet its needs.
• No specialized exchange surfaces or transport systems (like blood vessels) are needed.

Multicellular Organisms

As an organism gets larger, both its surface area and its volume increase—but they do not increase at the same rate. Volume increases much faster than surface area.

As a result:

• The SA:V ratio decreases as size increases.
• The diffusion distance becomes too great. If a multicellular organism relied on simple diffusion across its skin, oxygen would take days to reach the cells in its core, and those cells would die.
• Therefore, multicellular organisms require specialized exchange surfaces (like lungs or gills) with huge surface areas, and transport systems (like the circulatory system) to deliver substances directly to cells.

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3. Alveoli: Gas Exchange in the Lungs

In humans, the specialized exchange surfaces responsible for gas exchange are the alveoli (singular: alveolus). These are tiny, microscopic air sacs located at the very ends of the bronchioles inside your lungs.

Oxygen diffuses from the air inside the alveoli into the blood capillary network that wraps around them. At the same time, carbon dioxide diffuses from the blood plasma into the alveoli to be exhaled.

Key Adaptations of the Alveoli

To make diffusion as rapid and efficient as possible, alveoli have several key structural adaptations:

1. Huge Surface Area: There are millions of alveoli in each lung, creating a massive total surface area (about the size of a tennis court!) for diffusion to occur.
2. Extremely Thin Walls: Both the alveolar wall and the capillary wall are only one cell thick. This means the diffusion distance is incredibly short (less than a micrometer), allowing gases to cross almost instantly.
3. Moist Lining: The inside of the alveolus is coated with a thin layer of moisture. Gases must dissolve in this liquid layer before they can diffuse across the cell membranes.
4. Rich Blood Supply: Each alveolus is surrounded by a dense network of capillaries. Blood is constantly moving, carrying oxygenated blood away and bringing deoxygenated blood back. This maintains a steep concentration gradient.
5. Ventilation (Breathing): By constantly breathing in (bringing in high $O_2$ air) and breathing out (removing high $C{O}_2$ air), we maintain a high concentration of oxygen inside the alveolus compared to the blood.

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4. Factors Affecting the Rate of Diffusion (Separate Biology)

The rate at which a substance diffuses across a membrane is determined by physical properties. There are three primary factors:

• Surface Area: The larger the area available for molecules to pass through, the more molecules can diffuse at once. (Rate of diffusion is directly proportional to surface area).
• Concentration Difference: The greater the difference in concentration between the two sides of the membrane, the faster the particles will net-migrate down their gradient. (Rate of diffusion is directly proportional to concentration difference).
• Diffusion Distance: The thicker the membrane, the longer it takes for particles to travel across it. (Rate of diffusion is inversely proportional to membrane thickness).

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5. Fick's Law of Diffusion (Separate Biology & Higher Tier)

We can bring all of these variables together into a single mathematical relationship known as Fick's Law.

$$Rate\; of\; Diffusion\propto \frac{Surface\; Area\times Concentration\; Difference}{Thickness\; of\; Membrane}$$

The symbol $\propto$ means "is proportional to". This formula is incredibly useful because it allows us to calculate how changing one or more of these variables will scale the rate of diffusion.

• If you double the surface area, the rate of diffusion will double.
• If you double the concentration difference, the rate of diffusion will double.
• If you double the thickness of the membrane, the rate of diffusion will be halved.

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