1. Protein molecules
2. Simple ions
3. Water molecules
4. Alcohol
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Answer: Simple ions (enter the cell by facilitated diffusion).
@Water molecules and Alcohol (enter the cell by simple diffusion).
@Proteins (enter the cells by active transport).
Explanation:
Before going to the complete answer, first, we learn what is diffusion and how many types of diffusion are there in biological systems?
Diffusion is a process by which biological molecules enter the cell from the region of their higher concentrations to their lower concentrations through the plasma membrane (downhill). Diffusion does not require energy; hence it is thermodynamically favorable.
Diffusion is mainly divided into two types based on the involvement of membrane proteins which include simple and facilitated diffusions.
1) In simple diffusion, molecules cross the membrane without membrane-bound proteins or channels. Simple diffusion examples: Small and non-polar molecules such as Water, Alcohol, Oxygen, Urea, Carbon dioxide, etc.
2) In facilitated diffusion, molecules reach the cytoplasm with the help of membrane proteins. Simple diffusion examples: Large and charged (polar) molecules like sugars and amino acids.
There is another process called active transport through which large polar and non-polar molecules will be entered the cell from their lower concentrations to their higher concentrations (uphill) through plasma membrane using membrane proteins (here we call them transporters instead of channels). Active transport requires energy to transporting any molecules against their gradient, hence it is thermodynamically unfavorable.