Final answer:
Lipids in a lipid bilayer diffuse within the plane of their own monolayer due to the fluid nature of cell membranes. Unsaturated fatty acids increase membrane fluidity, allowing such lateral movements, while transverse diffusion or flip-flop is less common. Small, uncharged substances can passively diffuse across the membrane, whereas polar substances need transport proteins.
Step-by-step explanation:
In a lipid bilayer, lipids rapidly diffuse within the plane of their own monolayer. This means that they move laterally or sideways within the same layer, which is a characteristic of the fluid nature of cell membranes. The fluidity is partly due to the presence of unsaturated fatty acids in the phosphate lipid tails, which prevent the lipids from packing too closely together. The bilayer consists of hydrophilic heads that are in contact with water and hydrophobic tails that face inward. While lipids move freely within their layer, flipping from one monolayer to another, known as transverse diffusion or flip-flop, occurs much less frequently because it requires more energy and can be facilitated by specific enzymes like flippases and scramblases.
The passive transport of small, uncharged, and hydrophobic molecules such as oxygen and carbon dioxide across the cell membrane occurs via simple diffusion, down their concentration gradient, without the need for energy input. However, polar or charged substances require assistance to cross the membrane due to repulsion by the hydrophobic interior of the lipid bilayer. This assistance can come from various transport proteins embedded in the membrane.