Cell Membrane & Cell Transport

Selective permeability refers to the cell membrane's ability to actively choose which substances can enter or exit the cell, utilizing specific protein channels or carriers for transport. This process often requires energy, allowing the cell to maintain homeostasis by regulating its internal environment. In contrast, semi-permeability is a more passive mechanism where the membrane allows substances to pass based primarily on their size, without the need for energy or specific transport proteins. Thus, selective permeability is characterized by its active, regulated nature, whereas semi-permeability is fundamentally passive.

Gangliosides are complex glycosphingolipids that contain one or more sialic acid residues, which contribute to their negative charge. This charge is significant in cellular interactions and signaling. Additionally, the presence of sialic acid allows gangliosides to function as antigens, playing a crucial role in immune responses and cell recognition. In contrast, cerebrosides lack these sialic acid groups, making them neutral and less involved in antigenic properties. Thus, gangliosides are distinguished by their negative charge and role as antigens.

Cholesterol's structure includes a polar, hydrophilic hydroxyl group that interacts with the aqueous environment, while its carbon rings constitute the steroid backbone, providing rigidity to the membrane. The nonpolar tail of cholesterol is hydrophobic, allowing it to embed within the lipid bilayer, where it interacts with the hydrophobic tails of phospholipids. This arrangement helps maintain membrane fluidity and stability, preventing the fatty acid chains from packing too closely together, thus supporting overall membrane integrity and functionality.

At diffusion equilibrium, solute particles continue to move, but there is no net movement of particles from one side to the other because the concentrations are equal. This means that while individual particles are still diffusing, the overall concentration remains balanced, resulting in no change in concentration over time. Thus, it is incorrect to say that solute particles stop moving entirely; they are in constant motion even at equilibrium.

Amino acids, ATP, and glucose-6-phosphate are larger and polar molecules, making them less capable of crossing the hydrophobic lipid bilayer directly compared to smaller, nonpolar molecules like CO₂, O₂, and N₂. The lipid bilayer primarily allows the passage of small, nonpolar substances while restricting larger and charged molecules. Consequently, these substances require specific transport mechanisms, such as protein channels or carriers, to facilitate their movement across the membrane, highlighting their significant difficulty in permeating the lipid bilayer compared to other listed substances.

Channel proteins facilitate the rapid movement of solutes, such as ions, by providing a continuous passageway that does not require a change in shape. This allows for quicker transport compared to carrier proteins, which bind to specific solutes and undergo conformational changes to move them across the membrane. Carrier proteins are highly selective, ensuring that only certain molecules can pass through, while channel proteins allow for a more generalized and faster passage of substances. This distinction highlights the different mechanisms and efficiencies of solute transport in facilitated diffusion.

Fluidity of the phospholipid bilayer is influenced by several factors. Temperature affects kinetic energy, with higher temperatures increasing fluidity. The length of the fatty-acid chain tail plays a role, as shorter tails reduce interactions, enhancing fluidity. The degree of unsaturation introduces kinks in the tails, preventing tight packing and increasing fluidity. Lastly, the type of lipids present can vary in saturation and chain length, further affecting how closely they pack together. In contrast, the presence of aquaporins does not directly influence bilayer fluidity.