Movement of Substances

Various terms are used to describe the movement of substances between cells and into and out of a cell. These terms differ in the following respects.

1. 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.

2. The substance whose movement is being described may be water (the solvent) or it may be the substance dissolved in the water (the solute).

3. Movement of substances may occur from higher to lower concentrations (down the concentration gradient) or the reverse (up or against the gradient).

4. Solute concentrations between two areas may be compared. A solute may be hypertonic (a higher concentration of solutes), hypotonic (a lower concentration of solutes), or isotonic (an equal concentration of solutes) relative to another region.

5. The movement of substances may be passive or active. Active movement requires the expenditure of energy and usually occurs up a gradient.

Bulk flow is the collective movement of substances in the same direction in response to a force or pressure. Blood moving through a blood vessel is bulk flow.

Passive transport processes describe the movement of substances from regions of higher to lower concentrations (down a concentration gradient) and do not require expenditure of energy.

1. 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), motion that 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 result of the movement. Eventually, a state of equilibrium is attained where molecules are uniformly distributed but continue to move randomly.

2. 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. Turgor pressure is the osmotic pressure that develops when water enters the cells of plants and microorganisms.

3. Dialysis is the diffusion of solutes across a selectively permeable membrane. The term dialysis is usually used when different solutes are separated by a selectively permeable membrane.

4. Plasmolysis is the movement of water out of a cell (osmosis) that results in the collapse of the cell (especially plant cells with central vacuoles).

5. Facilitated diffusion is the diffusion of solutes through channel proteins in the plasma membrane. Note that water can pass through the plasma membrane without the aid of specialized proteins.

6. Countercurrent exchange describes the diffusion of substances between two regions in which substances are moving by bulk flow in opposite directions. For example, the direction of water flow through the gills of a fish is opposite to the flow of blood in the blood vessels. Diffusion of oxygen from water to blood is maximized because the relative motion of the molecules between the two regions is increased and because the concentration gradients between the two regions remain constant along their area of contact.

Active transport is the movement of solutes against a gradient and requires the expenditure of energy (usually ATP). Transport proteins in the plasma membrane transfer solutes such as small ions (Na+, K+, Cl–, H+), amino acids, and monosaccharides across the membrane.

Vesicular transport uses vesicles or other bodies in the cytoplasm to move macromolecules or large particles across the plasma membrane. Types of vesicular transport are described below.

- Exocytosis describes the process of vesicles fusing with the plasma membrane and releasing their contents to the outside of the cell. This is common when a cell produces substances for export.

- Endocytosis 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 wraps around the solid material and engulfs it, forming a phagocytic vesicle. Phagocytic cells (such as certain white blood cells) attack and engulf bacteria in this manner.

• Pinocytosis (“cellular drinking”) occurs when dissolved substances enter the cell. The plasma membrane folds inward to form a channel allowing the liquid to enter. Subsequently, the plasma membrane closes off the channel, encircling the liquid inside a vesicle.

• Receptor-mediated endocytosis occurs when specific molecules in the fluid surrounding the cell bind to specialized receptors that concentrate in coated pits in the plasma membrane. The membrane pits, the receptors, and their specific molecules (called ligands) fold inward and the formation of a vesicle follows. Proteins that transport cholesterol in blood (low-density lipoproteins, or LDLs) and certain hormones target specific cells by receptor-mediated endocytosis. Cells