Step 5. Results of the Dialysis Experiment
Channel: Bad Idea
Date: 2021-02-06

Cells constantly exchange various molecules with their environment. Cells must import such nutrients as amino acids, sugars and eliminate waste such as CO2 and regulate intracellular concentrations of inorganic ions. Plasma membranes are selectively permeable, which means that only certain substances are allowed through the membrane. Plasma membrane is represented by a lipid bilayer that is hydrophobic inside. This chemical property of a membrane defines the ability of molecules to cross it. Membrane contains lipids that are non-polar (hydrophobic) compounds. This means that substances that are hydrophilic have difficulties crossing the membrane. On the contrary, molecules with chemical structure that is similar to the cell membrane can pass through much easier. In addition to solubility in the membrane, the ability of a molecule to cross the membrane depends on other factors such as its size and charge

Small hydrophobic molecules diffuse across the membrane very fast. The examples of such molecules are oxygen, carbon dioxide and nitrogen. Very small polar molecules such as water can also diffuse across the membrane. However, large polar molecules and charged ions are not able to move across the membrane without the help of special transporters that are embedded into a lipid bilayer

Substances can cross the membrane by passive and active transport. In the passive transport solutes move down its concentration gradient which means that the molecules spontaneously move from the region of high concentration to a region of low concentration. No energy required for the molecules to move across the membrane by passive transport. Passive transport includes simple and facilitated diffusion

Movement of molecules across the membrane is possible due to the presence of membrane transport proteins. The membrane transport proteins allow small hydrophilic molecules to cross the membrane. Simple diffusion does not require the help of membrane transport proteins. Facilitated diffusion requires either channel proteins or transporters. Active transport requires energy in the form of ATP because solutes move against its concentration gradient when moving across the membrane

The movement of a solute through a semi-permeable membrane is defined as a dialysis. In this experiment you will use dialysis tubing which is made of cellulose membrane. A dialysis membrane is a semi-permeable with the pores that are placed symmetrically throughout the membrane. Pores in the tubing allow the diffusion of molecules only according to their size. Molecules larger than the pores cannot pass through the membrane, but smaller molecules can do so freely. The pore size of a dialysis membrane determines the molecular weight cut-off (MWCO) of the molecules that can diffuse across it

In this laboratory you will use a sample that you will dialyze against a dialysate (water). The amount of dialysate should be 200-500 times the volume of the sample. A sample containing glucose, protein and starch will be placed inside the dialysis tube. You will study the movement of organic molecules through the pores of the semipermeable dialysis tubing by performing a dialysis. The movement of organic compounds will be confirmed by performing special tests that will allow detection of glucose, protein and starch