5.1: Lab 5 Background

Learning Objectives

• Explain how molecules move across a concentration gradient.
• Describe the process of diffusion and osmosis across a membrane.
• Predict the movement of molecules in hypotonic, hypertonic, and isotonic solutions.

Introduction: Diffusion and Osmosis

Each atom and molecule is always moving, whether in air, in a liquid, in a solid like the classroom desks, or within living cells. This movement is largely random, but sometimes we can predict the direction that molecules will move. A concentration gradient describes a situation in which one area has more molecules (high concentration) than another area (low concentration). For example, if we spray an air freshener at the front of the classroom, we create a concentration gradient where there is a high concentration of scent molecules at the front and a low concentration of scent molecules at the back of the classroom. Diffusion explains that the scent molecules will move with the concentration gradient, from high concentration to low concentration. In this case, molecules would diffuse from the front to the back of the classroom. Diffusion is a passive process because it does not use energy - molecules diffuse on their own. It is similar to watching a ball roll downhill - we don’t need to push the ball, it just rolls on its own.

This same kind of movement occurs across cell membranes. However, membranes are considered selectively permeable because only certain substances can cross them. Small molecules, like O2 and CO2 , can freely cross the cell membrane on their own in a process called simple diffusion. Similarly, molecules with a similar hydrophobic and nonpolar chemical structure to the membrane’s phospholipid bilayer can also cross by simple diffusion. However, polar or charged molecules can only cross the membrane via facilitated diffusion, which requires different kinds of transport proteins.

Osmosis is the passive diffusion of water across a selectively permeable membrane. It happens in the same way as with diffusion, although we describe the direction of movement in a different way. To predict the direction in which water will move, we need to know the concentration of solutes (molecules dissolved in water, such as salt or sugar) in the water. Osmosis occurs from low solute concentration to high solute concentration (Figure 1). This may seem counterintuitive at first, but let’s consider the relationship between solute and water concentration. If a glass of water is 10% salt, then it’s 90% water (100-10=90). If another glass of water is 30% salt, then it’s 70% water. Moving from low to high solute concentration is the same thing as moving from high to low water concentration.

Figure 1. Osmosis across a semipermeable membrane. Water moves from low solute concentration to high solute concentration (from left to right in the figure). Source: OpenStax Concepts of Biology.

Let’s say we put a10% salt solution and 30% salt solution on opposite sides of a membrane. In which direction should the water move? It moves from low solute concentration to high solute concentration, so it should move toward the 30% salt solution.

Diffusion and osmosis occur until the system reaches equilibrium, meaning that the concentration of substances is equal everywhere (i.e., there no longer a concentration gradient). Remember that all molecules are always moving, so even though there is no net movement at equilibrium, molecules are still moving! They are just moving equally in all directions.

There are three different conditions that help to describe osmosis in and out of a cell. When a cell is in a hypotonic solution, there is a higher concentration of solutes inside the cell than outside the cell (hypo = below, like hypothermia). Therefore, water moves into the cell, making it swell up. In fact, animal cells may swell so much with water that they burst! Plant cells are protected by a cell wall, so they simply stop taking in water when they get too full. When a cell is in a hypertonic solution, there is a higher concentration of solutes outside the cell than inside the cell (hyper = too much, like a hyper pet with too much energy). In which direction should water move, and what effect will that have on the cell? The water gets sucked out of the cell, causing it to shrivel up like a raisin. Lastly, when a cell is in an isotonic solution, there is no concentration gradient, meaning that the solute concentration inside the cell equals the concentration outside the cell. In other words, cells in an isotonic solution are at equilibrium. Remember that water is still moving in and out of the cell, but it is moving into the cell at the same rate it is moving out of the cell.

Different conditions can affect the rate of diffusion and osmosis. For example, molecules move faster at higher temperatures. If diffusion is based on the movement of molecules, how do we think temperature affects the rate of diffusion (how quickly diffusion can occur)? Other factors that affect the rate of diffusion include the density of the medium through which molecules move and the size of the molecules.

In today’s lab, we are going to design experiments to explore the concepts of diffusion and osmosis. We will start by measuring the effects of different conditions on the rate of diffusion. Next, we will simulate diffusion across a membrane using dialysis tubing bags. Finally, we will use a potato to determine the effect of hypotonic, hypertonic, and isotonic solutions on osmosis in plant cells.