Video Transcript
Carbon dioxide is a small nonpolar
molecule. What is the most likely way for
this molecule to pass through membranes? (A) It diffuses through the
phospholipid bilayer. (B) It diffuses in using a carrier
protein. (C) It enters through a channel
protein. Or (D) it enters by
endocytosis.
Let’s start by taking a look at the
basic structure of cell membranes so we can work out how small, nonpolar molecules
like carbon dioxide might move across them. Cell membranes are made up of two
layers of phospholipids and so are often referred to as phospholipid bilayers. The fatty acid tails of
phospholipids point inwards toward each other, forming a nonpolar or hydrophobic
interior to the membrane.
The polar, or hydrophilic,
phosphate heads of the phospholipids point outward toward the aqueous environments
in the extracellular space and cell cytoplasm. The nonpolar interior of the
membrane makes it easy for nonpolar, especially small nonpolar, molecules to move
across it passively by diffusion. As we know, like dissolves
like. Remember, diffusion is the process
by which particles move from an area of high concentration to an area of low
concentration. It is described as a passive
process, as it does not require an input of energy from the organism itself.
Endocytosis is an example of bulk
transport. It involves large quantities of
molecules being actively transported into a cell using a vesicle. We can remember this as the prefix
endo- means into and the word part “cyto” means cell. Endocytosis is an active
process. This means that it requires an
input of energy. As we know that carbon dioxide is
nonpolar, it is likely to cross the cell membrane passively, not actively by
endocytosis.
Large, charged, or polar molecules
are unable to diffuse directly across membranes. Instead, they must use either a
carrier protein or a channel protein, embedded in the phospholipid bilayer. However, carbon dioxide is not
large, polar, or charged. So membrane proteins are unlikely
to be necessary for its transport. This molecule is stated to be both
small and nonpolar, suggesting that it can simply diffuse across the
phospholipids. Our correct answer is therefore
(A). It diffuses through the
phospholipid bilayer.
