diffusion

What is the definition of diffusion?

Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.

What are the three main factors that affect the rate of diffusion?

1. The difference in concentrations (the concentration gradient)
2. The temperature
3. The surface area of the membrane

How does temperature affect the rate of diffusion?

An increase in temperature increases the rate of diffusion because particles have more kinetic energy and move faster.

How does the concentration gradient affect the rate of diffusion?

The greater the difference in concentration between two areas (a steeper gradient), the faster the rate of diffusion occurs.

Give two examples of substances that move by diffusion in animal cells.

1. Oxygen and carbon dioxide during gas exchange.
2. The waste product urea from cells into the blood plasma for excretion in the kidney.

Why do single-celled organisms not need specialized gas exchange surfaces?

They have a relatively large surface area to volume ratio (SA:V), which allows sufficient transport of molecules into and out of the cell to meet its needs.

What is the relationship between the size of an organism and its surface area to volume ratio?

As an organism gets larger, its surface area to volume ratio decreases.

List four features that increase the effectiveness of an exchange surface in multicellular organisms.

1. A large surface area
2. A thin membrane (to provide a short diffusion path)
3. (In animals) An efficient blood supply
4. (In animals, for gaseous exchange) Being ventilated

How is the small intestine adapted for efficient diffusion?

It is lined with villi, which greatly increase the surface area; they also have a very thin wall and a good blood supply to maintain a steep concentration gradient.

How are fish gills adapted for gas exchange?

Gills have many gill filaments covered in lamellae to increase surface area, a thin surface layer of cells, and a counter-current system to maintain a large concentration gradient.