Verified answer

Substances can in general enter the cell one of three ways: simple diffusion, facilitated diffusion, and active transport.

Simple diffusion is used by relatively small polar molecules. This process requires no ATP because it moves from a high concentration outside the cell to a low concentration inside the cell.

Facilitated diffusion also requires no ATP and also moves with the concentration gradient. However this type of diffusion requires a carrier protein in the cell membrane.

Active transport uses ATP to actively move substances against the gradient.

Water is a small polar molecule and moves by simple diffusion.

Glucose is a large polar molecule that moves by facilitated diffusion.

Calcium ions use active transports to maintain gradients, as in muscle contractions.

Glucose.

It enters the cell by facilitated diffusion and no it doesn't need ATP to enter because it is moving from an area of higher solute concentration to an area of lower concentration. Once entered into the cell it is broken down into 2 pyruvate molecules which are used for aerobic respiration in the citric acid cycle. (not going to go into all the steps.)