10. The cell membrane is made up of different layers of attractive force, polar and non-polar. Are there any types of molecules that can pass through both layers without assistance or using special passages? Give at least one specific example.

 10. The cell membrane is made up of different layers of attractive force, polar and non-polar. Are there any types of molecules that can pass through both layers without assistance or using special passages? Give at least one specific example.

answers:

Transport across the Cell Membrane

One of the great wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell. These substances include ions such as Ca++, Na+, K+, and Cl–; nutrients including sugars, fatty acids, and amino acids; and waste products, particularly carbon dioxide (CO2), which must leave the cell.

The membrane’s lipid bilayer structure provides the first level of control. The phospholipids are tightly packed together, and the membrane has a hydrophobic interior. This structure causes the membrane to be selectively permeable. A membrane that has selective permeability allows only substances meeting certain criteria to pass through it unaided. In the case of the cell membrane, only relatively small, nonpolar materials can move through the lipid bilayer (remember, the lipid tails of the membrane are nonpolar). Some examples of these are other lipids, oxygen and carbon dioxide gases, and alcohol. However, water-soluble materials—like glucose, amino acids, and electrolytes—need some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer. All substances that move through the membrane do so by one of two general methods, which are categorized based on whether or not energy is required. Passive transport is the movement of substances across the membrane using their own kinetic energy, without the expenditure of chemical energy. In contrast, active transport is the movement of substances across the membrane using energy from the hydrolysis of adenosine triphosphate (ATP).

What kind of molecules can pass through the membrane easily & without assistance?

Small nonpolar molecules, such as O₂ and CO₂, are soluble in the lipid bilayer and therefore can readily cross cell membranes. Small uncharged polar molecules, such as H₂O, also can diffuse through membranes, but larger uncharged polar molecules, such as glucose, cannot.

What types of molecules can move through the lipid bilayer without assistance?

Gases, hydrophobic molecules, and small polar uncharged molecules can diffuse through phospholipid bilayers. Larger polar molecules and charged molecules cannot.