In which case would facilitated diffusion be needed? A. With large polar molecules moving against the concentration
gradient
B. With very small non polar particles moving down the
concentration gradient.
C. With very small polar particles moving against the
concentration gradient.
D. With large non polar molecules moving down the concentration
gradient.

The case in which facilitated diffusion would be needed is in the transport of large non-polar molecules moving down the concentration gradient.

Explanation:

Facilitated diffusion (FD) involves a form of passive transport, that is, it does not require energy and mobilizes substances across the cell membrane in favor of a concentration gradient.

The passive transport mechanism of FD is necessary only in the case of transport of very large molecules or polar molecules that:

For FD to occur, it requires the use of a protein, which acts as a carrier of the molecules to be mobilized.

The other options are not correct because:

    A and C. Passive transport, as facilited diffusion,  implies movement in favor of a concentration gradient.

    B and C. Non-polar small or very small molecules can cross the membrane by simple diffusion.

hydroxyl -oh

alcohols, always polar, can form hydrogen bonds with h2o molecules

carboxyl -cooh  

cargoxylic acids, has acidic properties which is a source of hydrogen ions, found in cells in ionized for with a charge of oh-1

carbonyl > co

ketone if within skeleton, aldehyde if group is at the end of skeleton, k & a may be structural isomers, it determines 2 sugars (ketose & aldose)

amino -nh2

amines, acts as a base, it can pick up h+ ions from surrounding solution, ionized with charge of 1+

methyl ch2

addition to rna/dna affects and gene expression

phosphate -opo3 ²⁻

organic phosphates, contributes to negative charge to, molecule that it is a part of, and potential to react with h20

sulfhydryl -sh

thiols, cross-linking stabilize the protein, and it is structure

clumped distribution, because you'de have all of the population gathered in a relatively small are.

it's the most common because it provides protection and a mate for reproduction

example: humans, seeweed, cattles, etc.