Predict whether or not each of the following substances can cross a biological membrane directly. Give a reason for each prediction
a. Nucleotides
b. Na+
c. Steroid hormones
d. Polar amino acids
e. Water
f. Oxygen
g. Cl?
What will be an ideal response?
Answer:
a. Nucleotides can't cross biological membranes directly because they are large, negatively charged molecules (students should recognize at least the large size, and probably that the functional groups are polar or charged).
b. Na+ can't cross biological membranes directly because it is charged.
c. Steroid hormones CAN cross biological membranes directly because they are nonpolar.
d. Polar amino acids can't cross biological membranes directly because they are large, polar molecules.
e. Water CAN cross biological membranes directly even though it is polar because of its small size.
f. Oxygen CAN cross biological membranes directly because it is nonpolar and small.
g. Cl− can't cross biological membranes directly because it is charged.
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