Passive and Active Transport, Exercises of Human Physiology

Download Passive and Active Transport and more Exercises Human Physiology in PDF only on Docsity! 1 Human Physiology Lab (Biol 236L) Passive and Active Transport Background: Substances are routinely transported (received and delivered) across the cell plasma membranes. These substances include compounds, liquids, nutrients, hormones and other signaling molecules, and waste products. Transport into and out of cells can be one of two ways: 1) Passive transport or 2) Active transport. Passive transport involves movement of substances that does not require energy (ATP). Typically in passive transport solutes move from an area of higher concentration to lower concentration (down or with their concentration gradient). Types of passive transport include simple diffusion, facilitated diffusion, osmosis, and filtration. In simple diffusion substances move (e.g. solutes like ions, molecules, gases) down their concentration gradient from higher to lower concentration without the aid of cell surface channels (Figure 1). This diffusion can continue until the solute concentration in the extracellular and intracellular space is the same or at equilibrium. In facilitated diffusion solutes move from higher to lower concentration with the help of cell surface channels. Whereas diffusion and facilitated diffusion involve movement of solutes, osmosis involves movement of water (or solvents). Figure 1. Example of a solute diffusing across a plasma membrane.At the start of diffusion the extracellular solute concentration is greater than within the intracellular space. After time passes the solute diffuses across the cell membrane until, at the end, solute concentration between extracellular and intracellular space has reached equilibrium. Osmosis is influenced by the concentration gradient of solutes found on the outside versus the inside of a cell. Water will move, by osmosis, from the side of a cell having a lower solute concentration to the side having a higher solute concentration. (Think of water as “wanting to dilute more concentrated solutions”.) Movement of water by osmosis depends on tonicity. Tonicity is a measure of the difference in concentrations (osmotic pressure gradient) of two solutions separated by a semipermeable membrane. You can observe the effects of tonicity on osmosis by examining what happens to red blood cells placed in solutions (Figure 2) that are either hypotonic (having lower solute concentration than inside the cell), hypertonic (having higher solute concentration than inside the cell), or isotonic (having the same solute concentration outside as inside the cell. Filtration 2 involves the movement of substances across an epithelial membrane, such as that found in capillaries. The substances are moved (or pushed) across the membrane by blood pressure within the capillaries. Active transport involves the movement of substances against their concentration gradient, usually with the aid of cell-surface pumps, from areas of lower to areas of higher concentration (Figure 3). This kind of transport is called active because it requires energy input. In the following laboratory you will observe the process of passive transport: osmosis and diffusion across cell membranes using two different experiments. Figure 3. Active transport of sodium ions across the cell membrane. Part 1: Osmosis across an egg cell membrane. To examine the effects of tonicity on osmosis across cell membranes you will examine the movement of fluid across the membrane of an egg that has been de-calcified by decalcification (24 hr soak in vinegar) to remove the calcium egg shell and expose the egg cell membrane. With the membrane exposed we will place the egg cell into hypertonic corn syrup solution or a hypotonic solution of tap water. [**Your instructor will have decalcified the egg ahead of time, taken initial egg volume measurements, and then soaked the egg in corn syrup about 2 hours ahead of lab. ] Methods: 1. Carefully take a decalcified egg and measure the circumference at its length and width using a tape measure. 2. Gently place the egg into a beaker and add enough corn syrup to cover the egg. 4. Allow the egg to sit in the corn syrup for 60 minutes. 5. After 60 minutes remove the egg and wipe off the corn syrup. Observe the egg for changes & record these in Table 1. Pour the used corn syrup back into the jar and clean out your beaker. 6. Carefully place egg into a new beaker and cover with water, and let egg sit for 1 hour. 7. After 1 hour check the egg and note any changes, and record these in Table 1. Table 1: Tonicity and Changes in Osmosis Across an Egg Cell Membrane Pre-Soak (original) VO Post-Syrup Soak (VS) Post-Water Soak (VW) Egg Diameter (Length)* Egg Diameter (Width)* Egg Volume*