Difference Between Bulk Flow and Diffusion

The key difference between bulk flow and diffusion is that bulk flow is the movement of a fluid or a mass due to a pressure gradient while diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration along the concentration gradient.

Bulk flow and diffusion are two methods by which molecules move from one place to another place. Bulk flow refers to the movement of a fluid driven by a pressure gradient. Diffusion is the movement of molecules down the concentration gradient. Both processes take place during human breathing. Diffusion and bulk flow are also ways of exchanging materials between blood and interstitial fluid. Moreover, both processes are very important in living things, especially in plants.

CONTENTS

1. Overview and Key Difference
2. What is Bulk Flow 
3. What is Diffusion
4. Similarities Between Bulk Flow and Diffusion
5. Side by Side Comparison – Bulk Flow vs Diffusion in Tabular Form
6. Summary

What is Bulk Flow?

Bulk flow is the movement of a mass of fluids down the pressure gradient. Bulk flow is an important process in plants. Water and solutes move along the tracheid and vessels elements of xylem and sieve tube elements of phloem due to bulk flow. Therefore, efficient long-distance transport of liquids in plants takes place with the help of bulk flow.

Bulk flow is analogues to the flow of water through a pipe or flow of blood through a blood vessel. Unlike diffusion, bulk flow moves the entire solution, not just water or solutes. Hence, substances move in a mass during the bulk flow. Moreover, bulk flow is fast than diffusion.

What is Diffusion?

Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. It occurs along the concentration gradient. Hence, it is a passive process which occurs without the use of energy. It happens on its own. It does not require stirring, shaking or wafting. Liquids and gasses move via diffusion. There are two types of diffusion. They are simple diffusion and facilitated diffusion. Simple diffusion takes place without the help of transporter proteins, while facilitated diffusion takes place with the help of carrier molecules. There are several factors affecting diffusion. They are temperature, size of the particle, steepness of the concentration gradient and area of interaction.

When you open a perfume bottle, the fragrance permeates still air due to diffusion.  When you walk to a coffee shop, you smell the coffee. This is also due to diffusion. Moreover, when you put an ink drop into a water glass, the colour spreads through the glass of water due to diffusion. Here, particles move randomly from place to place. In living cells, diffusion is an important process. Things go in and out from the cells via diffusion. Gas exchange in the alveoli also occurs via diffusion.

What are the Similarities Between Bulk Flow and Diffusion?

• Bulk flow and diffusion are two of three mechanisms that facilitate capillary exchange.
• Both bulk motion and diffusion occur in human breathing.
• These processes terminate when there is no gradient.
• They are passive transport methods.

What is the Difference Between Bulk Flow and Diffusion?

Bulk flow occurs due to a pressure gradient, while diffusion occurs due to a concentration gradient. Thus, this is the key difference between bulk flow and diffusion. Moreover, bulk flow moves the entire solution while in diffusion, solutes move from higher concentration to lower concentration. Further, bulk flow is a faster process than diffusion.

Below infographic shows a more detailed side by side comparison of both mechanisms to discern the difference between bulk flow and diffusion easily.

Summary – Bulk Flow vs Diffusion

Bulk flow is the movement of substances in bulk or in masses down a pressure gradient. On the other hand, diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration along the concentration gradient. Both processes are passive processes. However, bulk flow is a faster process, and it moves the entire solution. Diffusion is a slow process, and it moves only solutes. Bulk flow occurs as a result of a pressure gradient while diffusion occurs as a result of a concentration gradient. So, this is the summary of the difference between bulk flow and diffusion.

Reference:

1. “Diffusion.” Biology Dictionary, 4 Oct. 2019, Available here.
2. “18.8C: Bulk Flow: Filtration and Reabsorption.” Medicine LibreTexts, Libretexts, 14 Aug. 2020, Available here.

Image Courtesy:

1. “Flow and Exchange of Nutrients in the Phloem and Xylem of Plants” By Kl3580 (CC BY-SA 4.0) via Commons Wikimedia
2. “Blausen 0315 Diffusion” By BruceBlaus – Own work (CC BY 3.0) via Commons Wikimedia