In humans the concentrations are equal inside and outside the cell. Therefore, water will neither enter or exit the cell. However, if the cells are removed from the body annd placed in a solution with a lower water content, the water will leave the cells thru osmosis. If the solution has a higher water concentration, the water will enter the cell causing swelling of the cell. Osmosis across plasma membranes is crucial to the functioning of many biological systems, including water uptake by plant roots, adsorbtion of dieting water from intestions and the readsorption of water and minerals in the kidneys.
Movement is along a concentration gradient, so no energy is required.
Active transport is yet another form of diffusion. This occurs when a cell uses it's own energy to push a molecule across a membrane against the concentration gradient. Active transport is a major factor in the ability of a cell to maintain internal concentrations of small molecules while being exposed to different concentrations in the surrounding environment. The work of active transport is performed by specific proteins that are inserted in the membranes. These proteins use cell energy to accomplish their work.