Movement of Materials in Living Systems

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  • Low magnification micrograph of small intestinal mucosa. H&E stain. Digested nutrients (including sugars andamino acids) pass into the villi through diffusion.
  • Mansy, S. S. et al. Nature 454: 122–125 (3 July 2008).David W. Deamer, “How leaky were primitive cells?” Nature 454: 37-38 (3 July 2008).
  • Torri Tarelli, Grohovaz, Fesce and Ceccarelli (1985) J. Cell Biol. 101,1386


  • 1. Cell Membrane Function • Formation of a cell membrane that could allow some materials to pass while restricting the movement of others was a major step in the evolution of the cell. Such membranes can regulate the contents of the spaces they enclose and thereby maintain homeostasis.
  • 2. • The plasma membrane consists of a phospholipid bilayer with embedded proteins. o Some of the embedded proteins that span the entire membrane permit passage of substances across the membrane; they are called transport proteins. The shaded, globular objects are embedded proteins. Cell Membrane Structure
  • 3. Selective Permeability • All biological membranes are selectively permeable, or semipermeable.
  • 4. Diffusion • Some substances can pass into or out of the cell by diffusion. Diffusion is also the way most substances move within the cell. o Diffusion is the net movement of a substance from an area of higher concentration to an area of lower concentration. o Thus, a difference in concentration between two regions is required for diffusion to occur.
  • 5. • A gradient is a change in something as you move from one location to another. So a concentration gradient is a change in concentration of something from one place to another.
  • 6. If we picture each individual molecule as a little blue dot, a constant concentration of molecules (no gradient) would look like this: A concentration gradient, with a higher concentration of molecules on the left than on the right, would look like this:
  • 7. Why does diffusion happen? • The tiny particles (atoms, ions, and molecules) that make up all matter are in constant motion, called thermal motion, and the warmer they are, the faster they move. The energy of these moving particles is called thermal energy.
  • 8. • In liquids and gases, the overall movement of these particles is random and chaotic, each one moving in a different direction. o The direction of motion of any one molecule can be changed by collision with another molecule or with the side of the container, but once in motion the molecule will travel in a straight line until a collision occurs. o The probability of a molecule moving in one direction is the same as the probability of it moving in the opposite direction.
  • 9. • Now, imagine that a crystal of dye is added to a beaker full of water and begins to dissolve. Each dissolved dye molecule moves around randomly. o Initially, more dye molecules will be moving away from the crystal than toward it. However, the net movement will be away from the crystal. o When the dye molecules become evenly distributed, their random thermal motion in all directions will be equal, so their net movement will stop. This condition is called dynamic equilibrium.
  • 10. • A concentration gradient has potential energy because it has the capacity to do work. o If a substance has different concentrations on opposite sides of a membrane, its diffusion through the membrane could do work. o It’s like a dam that separates water at different elevations: the kinetic energy of the water as it passes through a tunnel in the dam could do work by spinning a water wheel or turbine.
  • 11. "Downhill" • Substances are said to diffuse in the "direction of their concentration gradient" or "down their gradient".
  • 12. • Diffusion of one substance is independent of the diffusion of all other substances. TIME
  • 13. • The rate of diffusion is affected by the size of the concentration gradient, the size of the particles, and the temperature. • Higher temperature → Diffuse fasterTemperature • Higher gradient → Diffuse faster Concentration gradient • Smaller particles → Diffuse fasterParticle size
  • 14. "Simple Diffusion" • The diffusion of solutes directly through the phospholipid bilayer of a cell membrane is called simple diffusion.
  • 15. • The size, charge, and polarity of substances determine whether they can pass through the phospholipid bilayer. Type of Substance Passes through a phospholipid bilayer? Examples Nonpolar molecule Yes O2, CO2, N2 Small polar molecule Yes H2O, ethanol Large polar molecule No Glucose, sucrose Ion No Cl−, K+, Na+, Ca2+
  • 16. • Substances diffuse into or out of the cell down their concentration gradients. o Since most cells use oxygen and produce carbon dioxide during respiration, oxygen is continually diffusing into the cell and carbon dioxide is continually diffusing out.
  • 17. Osmosis • The diffusion of water across a selectively permeable membrane is so important in biology that it’s given a special name: osmosis. o Since it is always true that the concentration of water in a solution decreases as the concentration of solute increases, water diffuses toward the side of the membrane with the higher solute concentration.
  • 18. o If the membrane is impermeable to the solute, then osmosis will cause an increase in the volume of the solution with the lower water concentration/higher solute concentration.
  • 19. o The solution with the lower water concentration/higher solute concentration is said to be hypertonic and the other hypotonic. Water diffuses from hypotonic to hypertonic solutions. o Two solutions with equal solute concentrations (and equal water concentrations) are said to be isotonic. No osmosis occurs.
  • 20. • Substances that cannot diffuse through the phospholipid bilayer may diffuse through transport proteins if they are available. This process is called facilitated diffusion. o Substances that cross by facilitated diffusion include glucose, Na+, K+, and Cl−. o Each transport protein is effective for one specific substance. "Facilitated Diffusion"
  • 21. • Some transport proteins use the kinetic energy of substances diffusing through them to do work. o For example, in the mitochondrion and chloroplast, the energy released by the diffusion of H+ through a transport protein (ATP synthase) is used to make ATP from ADP.
  • 22. Active Transport • Active transport occurs when a transport protein pumps a solute across a cell membrane from an area of lower concentration toward an area of higher concentration using energy from the breakdown of ATP. "Uphill"
  • 23. • For example, the sodium-potassium pump (Na+-K+ pump) is an active transport protein that simultaneously transports potassium ions (K+) into and sodium ions (Na+) out of the animal cell. About 30% of the energy used by animal cells is used just to run this pump (70% in their nerve cells).
  • 24. Endocytosis and Exocytosis • Moving substances into a cell or out of a cell by enclosing them in vesicles is called endocytosis and exocytosis respectively. o A vesicle is a small, spherical, membrane-bound sac within the cell used for transport or storage. o The processes are "uphill" because substances are being transported in the direction opposite to their concentration gradient. o The objects do not actually pass through a membrane on their way into or out of the cell.
  • 25. o In endocytosis, the vesicle is formed by the plasma membrane. Endocytosis is used by cells to import water, macromolecules, or larger objects, such as cell fragments or even whole cells. o In exocytosis, the vesicle is formed by an organelle within the cell. Exocytosis is used by secretory cells to export large groups of molecules such as hormones and other products.
  • 26. Summary of Cellular Transport Method Type of Process Source of Potential Energy Required Membrane Protein Simple diffusion "Downhill" Concentration gradient None Osmosis "Downhill" Concentration gradient None Facilitated diffusion "Downhill" Concentration gradient Transport Protein Active transport "Uphill" ATP Transport protein (pump) Endocytosis and Exocytosis "Uphill" ATP None