Movement of Substances » Diffusion Processes

What you'll learn this session

Study time: 30 minutes

The process of diffusion and its importance in living organisms
How osmosis works and its role in plant and animal cells
Active transport mechanisms and when organisms need them
Factors affecting the rate of diffusion
Real-world examples of diffusion processes in biological systems

Introduction to Diffusion Processes

Living organisms need to move substances in and out of their cells to survive. Whether it's getting oxygen into your blood or helping plants absorb water, the movement of substances is essential for life. In this section, we'll explore the three main ways substances move: diffusion, osmosis and active transport.

Key Definitions:

Diffusion: The net movement of particles from an area of higher concentration to an area of lower concentration.
Osmosis: The diffusion of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.
Active Transport: The movement of particles from an area of lower concentration to an area of higher concentration, requiring energy from the cell.
Concentration Gradient: The difference in concentration of a substance between two areas.

🔍 Understanding Diffusion

Imagine you spray perfume in one corner of a room. At first, the smell is strongest near the spray, but eventually, you can smell it everywhere. This is diffusion in action! The perfume particles move from where there are lots of them (high concentration) to where there are fewer (low concentration).

In our bodies, oxygen diffuses from our lungs (where there's lots of it) into our bloodstream (where there's less). Carbon dioxide does the opposite, diffusing from our blood into our lungs to be breathed out.

🌱 Diffusion in Action

Diffusion happens without using any energy - it's a passive process. Particles naturally move and spread out until they're evenly distributed. This is why a cup of tea eventually cools down - heat energy diffuses from the hot tea to the cooler air around it.

For single-celled organisms like amoeba, diffusion is enough to get oxygen in and waste out. But larger organisms need special systems (like our circulatory system) to help diffusion work effectively over longer distances.

Osmosis: Special Diffusion of Water

Osmosis is a special type of diffusion that only involves water molecules. It's crucial for plants to stay upright and for our cells to maintain the right amount of water.

How Osmosis Works

Osmosis happens across a partially permeable membrane - a barrier that lets some molecules through but not others. Water molecules can pass through, but larger molecules like sugar can't. Water moves from where there's more water (and less dissolved stuff) to where there's less water (and more dissolved stuff).

💧 Hypotonic Solution

When a cell is placed in a solution with less dissolved substances than inside the cell, water moves INTO the cell by osmosis. Animal cells might burst, while plant cells become turgid (firm and swollen).

⚖ Isotonic Solution

When the concentration of dissolved substances is the same inside and outside the cell, there's no net movement of water. Animal cells maintain their normal shape and plant cells remain healthy but not fully turgid.

🌴 Hypertonic Solution

When there are more dissolved substances outside than inside the cell, water moves OUT of the cell by osmosis. Animal cells shrink (crenate), while plant cells become plasmolysed (the cell membrane pulls away from the cell wall).

Case Study: Plant Wilting

Have you ever forgotten to water a plant and found it drooping? This demonstrates osmosis in action! When soil becomes dry, it contains less water than the plant cells. Water moves out of the cells by osmosis, causing them to lose turgor pressure. The plant wilts as a result. When you water the plant, water moves back into the cells by osmosis and the plant stands upright again.

Active Transport: Moving Against the Gradient

Sometimes organisms need to move substances from areas of low concentration to areas of high concentration - the opposite direction to diffusion. This is like pushing a boulder uphill - it requires energy!

⚡ How Active Transport Works

Active transport uses special carrier proteins in cell membranes. These proteins change shape when they receive energy from ATP (adenosine triphosphate, the energy currency of cells). This shape change helps move specific molecules across the membrane, even against their concentration gradient.

The process requires energy because it's working against the natural tendency of particles to move from high to low concentration.

🌾 Examples in Living Organisms

Mineral absorption in plants: Plant roots use active transport to absorb mineral ions from the soil, where they're often in lower concentration than in the root cells.

Glucose reabsorption in kidneys: Your kidneys use active transport to reabsorb all the glucose from urine back into your bloodstream, preventing valuable glucose from being lost.

Sodium-potassium pump: Nerve cells use active transport to maintain different concentrations of sodium and potassium ions inside and outside the cell, which is essential for sending nerve impulses.

Factors Affecting Diffusion Rate

Several factors can speed up or slow down the rate of diffusion:

Concentration gradient: A bigger difference in concentration means faster diffusion.
Temperature: Higher temperatures give particles more energy, making them move faster and diffuse more quickly.
Surface area: A larger surface area allows more particles to diffuse at once. This is why your lungs have millions of tiny air sacs (alveoli) - they create a huge surface area for gas exchange.
Distance: Shorter diffusion distances mean faster overall movement. This is why capillaries run close to every cell in your body.
Size of particles: Smaller particles generally diffuse faster than larger ones.

Case Study: The Small Intestine

Your small intestine is specially adapted to maximise diffusion of nutrients into your bloodstream. It has millions of tiny finger-like projections called villi, which dramatically increase the surface area. Each villus contains a network of capillaries close to its surface, creating a short diffusion distance. The epithelial cells lining the intestine also have microvilli (even smaller projections), further increasing the surface area. This combination of features ensures efficient absorption of nutrients from your food.

Comparing the Three Transport Processes

Process	Requires Energy?	Direction	Examples
Diffusion	No (passive)	High to low concentration	Oxygen entering cells, carbon dioxide leaving cells
Osmosis	No (passive)	High to low water concentration	Water absorption by plant roots, water balance in cells
Active Transport	Yes (uses ATP)	Low to high concentration	Mineral uptake in plants, glucose reabsorption in kidneys

Practical Applications

Understanding diffusion processes helps us explain many biological phenomena and has practical applications:

Food preservation: Adding salt to meat draws water out through osmosis, preventing bacterial growth.
Kidney dialysis: Uses diffusion and osmosis to remove waste products from the blood of people with kidney failure.
Plant care: Understanding osmosis helps us know how and when to water plants properly.
Medicine delivery: Some medications use diffusion principles to release drugs gradually in the body.

Remember that all living organisms, from the simplest bacteria to complex humans, rely on these transport processes to survive. Without them, cells couldn't get the substances they need or remove waste products effectively.

🧠 Test Your Knowledge!