Water (H2O), carbon dioxide (CO2), and oxygen (O2) are all quite small molecules, yet they move across cell membranes differently. Both CO2 and O2 passively diffuse across the cell membrane directly while H2O moves across the cell membrane by facilitated diffusion. What can be concluded about these three molecules based on the way they react to the cell membrane? (4 points)
A) The water molecules are attracted to the lipid tails of the cell membrane, but the carbon dioxide and oxygen molecules are not.
B) The water molecules are too small to pass between the lipid tails of the cell membrane, while the molecules of carbon dioxide and oxygen are not.
C) The water molecules are repelled by the lipid tails of the cell membrane, but the carbon dioxide and oxygen molecules are not.
D) The water molecules are too large to pass between the lipid tails of the cell membrane, while the molecules of carbon dioxide and oxygen are not.
Answers
Answer:
The phospholipids of plasma membranes are amphipathic: they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions. The hydrophobic core of the plasma membrane helps some materials move through the membrane, while it blocks the movement of others.
Structure of a phospholipid, showing hydrophobic fatty acid tails and hydrophilic hea. A bilayered membrane consisting of phospholipids arranged in two layers, with their heads pointing out and their tails sandwiched in the middle, is also shown.
Structure of a phospholipid, showing hydrophobic fatty acid tails and hydrophilic hea. A bilayered membrane consisting of phospholipids arranged in two layers, with their heads pointing out and their tails sandwiched in the middle, is also shown.
Image modified from OpenStax Biology.
Polar and charged molecules have much more trouble crossing the membrane. Polar molecules can easily interact with the outer face of the membrane, where the negatively charged head groups are found, but they have difficulty passing through its hydrophobic core. Water molecules, for instance, cannot cross the membrane rapidly (although thanks to their small size and lack of a full charge, they can cross at a slow rate).
Additionally, while small ions are the right size to slip through the membrane, their charge prevents them from doing so. This means that ions like sodium, potassium, calcium, and chloride cannot cross membranes to any significant degree by simple diffusion, and must instead be transported by specialized proteins (which we’ll discuss later). Larger charged and polar molecules, like sugars and amino acids, also need help from proteins to efficiently cross the membrane.