Cell Biology & Macromolecules

Function of cholesterol as a bidirectional membrane fluidity buffer:

• At lower temperatures, cholesterol disrupts the tight regular close-packing of fatty acid hydrocarbon tails, hindering crystallization into a solid gel state, and thereby preventing phase transition to maintain membrane fluidity even during temperature drops.
• Cholesterol is not a desaturase enzyme and does not remove hydrogen atoms.
• Animals do not hydrogenate unsaturated fatty acyl chains via membrane cholesterol.
• Mechanical pressure tolerance is mediated by the extracellular matrix and cytoskeleton, not by cholesterol-mediated flexibility.

Primary functions of the Golgi apparatus:

• Photophosphorylation and trapping of light energy is the function of chloroplasts.
• Intracellular hydrolytic digestion of macromolecules is carried out by lysosomes.
• Cellular energy transformation (ATP generation) is performed by mitochondria.
• The Golgi apparatus serves as the central sorting and processing hub where secretory and membrane proteins undergo extensive post-translational modifications (such as O-linked glycosylation, terminal trimming of N-glycans, sulfation, and phosphorylation) and lipids are glycosylated to form glycolipids.

Cell size comparisons across biological domains:

• Typical bacterial and archaeal prokaryotic cells have a diameter/width in the range of $1\text{--}10\,\mu\text{m}$ (commonly $1\text{--}2\,\mu\text{m}$ for cocci/bacilli).
• $10\text{--}100\,\mu\text{m}$ is the characteristic size range of eukaryotic animal and plant cells.
• $1\text{--}10\text{ nm}$ represents molecular dimensions such as individual globular proteins, lipid bilayers ($4\text{ nm}$), or DNA diameter ($2\text{ nm}$).
• $10\text{--}100\text{ nm}$ represents viral particle diameters (e.g., poliovirus $30\text{ nm}$, influenza $80\text{--}120\text{ nm}$).

Nucleocytoplasmic trafficking across the nuclear envelope:

• Nuclear pores (specifically large multiprotein Nuclear Pore Complexes, NPCs) perforate the inner and outer nuclear membranes, regulating bidirectional macromolecular exchange: allowing passive diffusion of small solutes and regulated active export of nuclear products (such as mRNA, tRNA, and assembled ribosomal subunits) into the cytoplasm.
• G proteins are GTPase transducers in transmembrane cell signaling at the plasma membrane.
• MHC molecules are cell-surface glycoproteins involved in antigen presentation to T cells.
• Coated pits (clathrin-coated pits) mediate receptor-mediated endocytosis at the plasma membrane.