The cell membrane of a muscle cell allows nutrients to pass through.

Diffusion is a critical biological process in which molecules and compounds tend to spread out evenly in solutions (both liquid and gaseous). Diffusion is a passive way for molecules to enter or exit a cell, requiring no energy. Animal cells (such as our own) need to exchange materials with bodily fluids, such as blood, but the cell membrane stands in the way. Even in the presence of the barrier, cells can use two types of diffusion to accomplish membrane transport: active and facilitated.

Passive Transport

During active transport, a molecule is moving from a lower to higher concentration. Active transport requires a protein carrier and the use of cellular energy obtained from the breakdown of Adenosine Triphosphate (ATP). When ATP is broken down, energy is released. In this case, the energy is used to carry out active transport. Proteins involved in active transport are often called pumps. Just as a water pump uses energy to move water against the force of gravity, energy is used to move substances against their concentration gradients.

Facilitated Transport

During facilitated transport, a molecule is transported across the plasma membrane from the side of higher concentration to the side of lower concentration. This is a passive means of transport, because the cell does not need to expend energy to move a substance. Each protein carrier, sometimes called a transporter, binds only to a particle molecule, such as glucose.