Anatomy & Physiology: Movement of Substances

The following terms are used to describe the movement of substances into, out of, and between cells:

The movement of substances may occur across a selectively permeable membrane (such as the plasma membrane). A selectively permeable membrane allows only specific substances to pass.
The substances, whose movements are being described, may be water (the solvent) or the substance dissolved in the water (the solute).
Movement of substances may occur from higher to lower concentrations (down the concentration gradient) or the opposite direction (up or against the gradient).
Solute concentrations vary. A solution may be hypertonic (a higher concentration of solutes), hypotonic (a lower concentration of solutes), or isotonic (an equal concentration of solutes) compared to another region.
The movement of substances may be passive or active. Usually occurring against the gradient, active movement requires the expenditure of energy.

Passive transport process

Passive transport describes the movement of substances down a concentration gradient and do not require energy consumption.

Bulk flow is the collective movement of substances in the same direction in response to a force, such as pressure. Blood moving through a blood vessel is an example of bulk flow.
Simple diffusion, or diffusion, is the net movement of substances from an area of higher concentration to an area of lower concentration. This movement occurs as a result of the random and constant motion characteristic of all molecules (atoms or ions) and is independent from the motion of other molecules. Since, at any one time, some molecules may be moving against the gradient and some molecules may be moving down the gradient (remember, the motion is random), the word “net” is used to indicate the overall, eventual end-result of the movement. If a concentration gradient exists, then the molecules (which are constantly moving) will eventually become evenly distributed (a state of equilibrium).
Osmosis is the diffusion of water molecules across a selectively permeable membrane. When water moves into a body by osmosis, hydrostatic pressure (osmotic pressure) may build up inside the body.
Dialysis is the diffusion of solutes across a selectively permeable membrane.
Facilitated diffusion is the diffusion of solutes through channel proteins in the plasma membrane. Note that water can pass freely through the plasma membrane without the aid of specialized proteins.

Active transport processes

Active transport is the movement of solutes against a gradient and requires the expenditure of energy (usually ATP). Active transport is achieved through one of the following two mechanisms:

Transport proteins in the plasma membrane transfer solute such as small ions (Na⁺, K⁺, Cl⁻, H⁺), amino acids, and monosaccharides.
Vesicles or other bodies in the cytoplasm move macromolecules or large particles across the plasma membrane. Types of vesicular transport include
- Exocytosis, which describes the process of vesicles fusing with the plasma membrane and releasing their contents to the outside of the cell. This process is common when a cell produces substances for export.
- Endocytosis, which describes the capture of a substance outside the cell when the plasma membrane merges to engulf it. The substance subsequently enters the cytoplasm enclosed in a vesicle. There are three kinds of endocytosis:
- Phagocytosis (“cellular eating”) occurs when undissolved material enters the cell. The plasma membrane engulfs the solid material, forming a phagocytic vesicle.
- Pinocytosis (“cellular drinking”) occurs when the plasma membrane folds inward to form a channel allowing dissolved substances to enter the cell. When the channel is closed, the liquid is encircling within a pinocytic vesicle.
- Receptor-mediated endocytosis occurs when specific molecules in the fluid surrounding the cell bind to specialized receptors in the plasma membrane. As in pinocytosis, the plasma membrane folds inward and the formation of a vesicle follows. Certain hormones are able to target specific cells by receptor-mediated endocytosis.