Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration due to their inherent kinetic energy. This process continues until equilibrium is reached. It is a passive process, meaning it does not require cellular energy in the form of ATP.

Unlike plant cells, which possess a rigid cell wall, animal cells are bounded only by a plasma membrane (also known as the cell membrane). This thin, flexible lipid bilayer regulates the passage of substances into and out of the cell, maintaining internal homeostasis.

Tonicity refers to the ability of an extracellular solution to make water move into or out of a cell by osmosis. It is determined by the concentration of non-penetrating solutes in the solution relative to the cell's interior. Understanding tonicity is crucial in physiology, as it dictates whether cells will shrink, swell, or remain stable when placed in different environments, such as hypertonic, hypotonic, or isotonic solutions.

Cholesterol is a vital steroid lipid found in cell membranes. It serves as a precursor for the synthesis of steroid hormones, bile acids, and vitamin D, playing a critical role in maintaining membrane fluidity and cellular signaling.

Pinocytosis, often referred to as 'cell drinking,' is a mode of endocytosis where a cell engulfs extracellular fluid and dissolved solutes by forming small vesicles at the cell membrane. This process allows the cell to internalize nutrients and other molecules present in the surrounding medium, distinguishing it from phagocytosis, which involves the ingestion of larger solid particles.

A non-polar covalent bond occurs when electrons are shared equally between two atoms with similar electronegativity. This results in a symmetric distribution of charge, meaning no permanent dipole moment exists across the bond.

Active transport is the process of moving ions or molecules across a cell membrane into a region of higher concentration. This process requires energy, typically in the form of ATP, and the assistance of specialized transmembrane protein pumps to move substances against the electrochemical gradient.

According to the Arrhenius definition, a base is a substance that increases the concentration of hydroxide ions (OH-) in an aqueous solution. Bases typically have a pH greater than 7 and are characterized by their ability to neutralize acids.

Lipids are hydrophobic, non-polar molecules. Due to their chemical structure, they dissolve readily in non-polar organic solvents like ether, alcohol, and chloroform. However, they are insoluble in water, which is a highly polar solvent, because they cannot form hydrogen bonds with water molecules.

Phagocytosis, often referred to as 'cell eating,' is a form of endocytosis where a cell engulfs large particles, such as bacteria, food debris, or other cells, by extending its plasma membrane around them. This process forms a phagosome, which then fuses with a lysosome for digestion. It is a critical mechanism for immune defense and nutrient acquisition in many unicellular organisms.