Biological membranes are selectively permeable, allowing certain molecules to cross the membrane, but not others. Classify the molecules or ions depending on how they cross a biological membrane. Note that some of these examples may also utilize active transport to traverse a membrane. However, this question is limited to passive transport processes only.

a) Simple Diffusion
b) Facilitated Diffusion

1. fatty acids
2. Cl-
3. CO2
4. steroid hormones
5. amino acids

a. Simple difussion – 2, 3, 5

b. Facilitated difussion – 1, 4

Explanation:

Simple diffusion is an unassisted type of diffusion hence the name simple, in which a particle moves from higher to a lower concentration.

It occurs through the phospholipid bilayer Transports small, non-polar particles. It happens directly through the cell membrane.

The rate of simple diffusion is directly proportional to the concentration gradient and the membrane permeability of the solute molecule.

Examples of simple diffusion of gases is seen to happen across the respiratory membrane, diffusion of molecules from the blood to the cells through the interstitial fluid and occurs in nature a lot.

Facilitated diffusion is the transport of substances across a biological membrane through a concentration gradient by means of a carrier molecule.

It happens with the aid of transmembrane proteins.

Serves to transport large or polar particles. This occurs through specific facilitator molecules.

The rate of facilitated diffusion depends on the kinetics of carrier- mediated transport

Examples of where facilitated diffusion occurs are thus; in the counter-transport of chloride/bicarbonate in renal tubular cells, cotransport of sodium with sugars like glucose, galactose,etc