Sugar molecules enter a cell against a concentration gradient by the process of

Osmosis is a type of passive transport that specifically involves the movement of water molecules across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. It does not directly involve the transport of sugar molecules or require energy input.

Facilitated diffusion is another form of passive transport, where substances move across cell membranes through specific transport proteins. However, this process moves substances from an area of higher concentration to an area of lower concentration, not against a concentration gradient as is required for sugar molecules entering a cell.

Passive diffusion is a process by which molecules migrate from areas of higher concentration to areas of lower concentration. This process does not require energy, hence the term "passive". Passive diffusion would not allow sugar molecules to enter a cell if the concentration of sugar is higher inside the cell, as there would be a natural tendency for the sugar to diffuse out of the cell, not into it.