There are a few concepts that need to be understood relating to the movement of substances.
- The movement of substances may occur across a semi‐permeable membrane (such as the plasma membrane). A semi‐permeable membrane allows some substances to pass through, but not others.
- 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 from 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. If movement is with the concentration or gradient, it is passive. If movement is against the gradient, it is active and requires energy.
Passive transport process
Passive transport describes the movement of substances down a concentration gradient and does not require energy consumption.
- 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 (due to kinetic energy) and is independent from the motion of other molecules. Since at any one time some molecules may be moving against the concentration gradient and some molecules may be moving down the concentration 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, the molecules (which are constantly moving) will eventually become evenly distributed (a state of equilibrium).
- Osmosis is the diffusion of water molecules across a semi‐permeable membrane. When water moves into a cell by osmosis, hydrostatic pressure (osmotic pressure) may build up inside the cell.
- Dialysis is the diffusion of solutes across a semi‐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, although special proteins called aquaporins can aid or speed‐up water transport.
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 solutes 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 the following:
- 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 enclosed 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.