Diffusion

Particles such as molecules and ions have kinetic energy, so they move randomly. If there are more particles in one region than in another, their movement tends to spread them from the more concentrated region into the less concentrated region.

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. The difference in concentration between the regions is called a concentration gradient. Diffusion occurs down this gradient.

Particles move in both directions, but more move from the higher-concentration region to the lower-concentration region than move in the reverse direction. The difference between these movements is the net movement. Diffusion requires no additional energy supply: it results from the random movement of particles that already have kinetic energy.

When the concentrations become equal, particles continue to move randomly in both directions, but there is no longer a net movement between the regions.

Diffusion in living organisms

Cells exchange substances with their surroundings across the cell membrane. Some substances cross by diffusion when their concentration differs on the two sides of the membrane and the membrane allows them to pass.

A difference in concentration is called a concentration gradient. Particles move randomly in both directions, but more move from the region of higher concentration to the region of lower concentration. This produces a net movement down the concentration gradient. The cell does not direct the particles, and diffusion does not require the cell to supply energy.

Diffusion helps cells obtain substances they need and remove substances they produce. For example, oxygen can diffuse into a cell when its concentration is higher outside. Carbon dioxide produced by the cell can diffuse out when its concentration is higher inside.

Dissolved solutes can also diffuse across cell membranes. For example, glucose can diffuse into some cells through membrane proteins when its concentration is higher outside. Not every substance can cross every membrane: some pass through the membrane directly, some require suitable membrane proteins, and others cannot pass through it.

Factors affecting diffusion rate

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. Its rate describes how quickly this net movement occurs.

• Surface area: A larger available surface area allows more particles to cross a boundary at the same time, increasing the diffusion rate.
• Temperature: At a higher temperature, particles have more kinetic energy and move randomly faster. This increases the diffusion rate.
• Concentration gradient: A steeper gradient means a greater difference in concentration over the same distance. It produces faster net movement from the higher-concentration region to the lower-concentration region.
• Diffusion distance: A shorter distance allows particles to cross from one region to the other more quickly, increasing the diffusion rate.

Investigating diffusion rate

To investigate one factor, change that factor and measure how much net diffusion occurs over the same time interval each time. Keep the other factors controlled so that a difference in the measured rate can be attributed to the factor being investigated. For example, when changing temperature, keep the surface area, concentration gradient and diffusion distance consistent.