**Title: Vitamin Diffusion and Cellular Entry****Question:**Which of the following vitamins do you expect to enter the cell via diffusion (i.e., NOT need a transporter)?**Image Description:**The image contains chemical structures labeled A, B, C, and D, representing different vitamins:- **A:** This chemical structure displays a long, nonpolar, hydrocarbon chain with a ring structure at the end. It appears to be a lipid-soluble vitamin due to its extended hydrophobic chain.- **B:** This structure contains multiple hydroxyl (OH) groups, suggesting a polar and possibly water-soluble vitamin. It has a ring structure with an additional carboxylic acid group.- **C:** Displays a ring structure including nitrogen atoms, with a hydrogen-bonding capable group attached which indicates some degree of polarity.- **D:** This structure includes multiple nitrogen atoms in a heterocyclic ring and potentially polar functional groups.**Explanation:**Understanding whether a vitamin can enter the cell via simple diffusion depends largely on its polarity and solubility:- **Lipid-Soluble Vitamins:** Likely to diffuse across the lipid bilayer of the membrane. These are typically nonpolar. - **Water-Soluble Vitamins:** Usually require transporters to cross cell membranes due to their polarity.From the structures provided, structure **A** would most likely enter the cell via diffusion because it appears to be nonpolar and lipid-soluble. Structures **B, C, and D** are more polar, suggesting they may require transporters to cross the cell membrane.

Diffusion refers to the movement of a substance from a region of higher concentration to a region of lower concentration. There are three types of diffusion: simple diffusion facilitated diffusion and osmosis.Simple diffusion is when ions or molecules diffuse from a region of higher concentration to a region of lower concentration. Facilitated diffusion is when movement requires passing through a channel or transporter protein. Osmosis is the diffusion of water across a semipermeable membrane driven by the difference in osmotic pressureThe structure of molecule A resembles the structure of vitamin D due to the characteristic tocopherol ring motif. The structure of molecule B resembles the structure of vitamin C because of the hexauronic acid ring structure.The structure of molecule C resembles the structure of vitamin B3 or niacin. The structure of molecule D resembles the structure of vitamin B6 thiamin. Vitamin B3, B6 and C are water-soluble proteins since they have polar groups on them. Being water soluble also means that they cannot cross the membrane via diffusion because not only the cell membrane is impermeable to polar or charged solutes, their large and complex structure prevents them from freely diffusing across the membrane. So vitamin B3, B6 and C require a transport protein to enter the cell. Vitamin D precursor is 4-dehydrocholesterol, a lipid that is found in the cell membranes of keratinocytes. Photochemical reactions driven by UV light in the sunlight turn 7-dehydrocholesterol to the cholecalciferol- the active form of vitamin D and regulate mechanisms involved in calcium uptake. So vitamin D can freely enter the cell without the need for a transport protein. This is also the reason why vitamin D is sometimes considered more of a hormone or signalling molecule than a vitamin. The correct choice is option A.