Diffusion is the movement of solute or solvent

• 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.

Differences Between Osmosis and Diffusion

They are similar in many ways, too

An example of osmosis: water traveling from an area of high water density through the gelatin to an area of low water density, swelling the candy.

Martin Leigh / Getty Images

Updated on February 06, 2020

Students are often asked to explain the similarities and differences between osmosis and diffusion or to compare and contrast the two forms of transport. To answer the question, you need to know the definitions of osmosis and diffusion and really understand what they mean.

Definitions

• Osmosis: Osmosis is the movement of solvent particles across a semipermeable membrane from a dilute solution into a concentrated solution. The solvent moves to dilute the concentrated solution and equalize the concentration on both sides of the membrane.
• Diffusion: Diffusion is the movement of particles from an area of higher concentration to lower concentration. The overall effect is to equalize concentration throughout the medium.

Examples

• Examples of Osmosis: Examples include red blood cells swelling up when exposed to freshwater and plant root hairs taking up water. To see an easy demonstration of osmosis, soak gummy candies in water. The gel of the candies acts as a semipermeable membrane.
• Examples of Diffusion: Examples of diffusion include the scent of perfume filling a whole room and the movement of small molecules across a cell membrane. One of the simplest demonstrations of diffusion is adding a drop of food coloring to water. Although other transport processes do occur, diffusion is the key player.

Similarities

Osmosis and diffusion are related processes that display similarities:

• Both osmosis and diffusion equalize the concentration of two solutions.
• Both diffusion and osmosis are passive transport processes, which means they do not require any input of extra energy to occur. In both diffusion and osmosis, particles move from an area of higher concentration to one of lower concentration.

Differences

Here's how they are different:

• Diffusion can occur in any mixture, including one that includes a semipermeable membrane, while osmosis always occurs across a semipermeable membrane.
• When people discuss osmosis in biology, it always refers to the movement of water. In chemistry, it's possible for other solvents to be involved. In biology, this is a difference between the two processes.
• One big difference between osmosis and diffusion is that both solvent and solute particles are free to move in diffusion, but in osmosis, only the solvent molecules [water molecules] cross the membrane. This can be confusing because while the solvent particles are moving from higher to lower solvent concentration across the membrane, they are moving from lower to higher solute concentration, or from a more dilute solution to a region of more concentrated solution. This occurs naturally because the system seeks balance or equilibrium. If the solute particles can't cross a barrier, the only way to equalize concentration on both sides of the membrane is for the solvent particles to move in. You can consider osmosis to be a special case of diffusion in which diffusion occurs across a semipermeable membrane and only the water or other solvent moves.

Key Points

Facts to remember about diffusion and osmosis:

• Diffusion and osmosis are both passive transport processes that act to equalize the concentration of a solution.
• In diffusion, particles move from an area of higher concentration to one of lower concentration until equilibrium is reached. In osmosis, a semipermeable membrane is present, so only the solvent molecules are free to move to equalize concentration.