Cell Transport Mechanisms
Diffusion, Osmosis, Active Transport
The cell as the fundamental unit of all life is a very dynamic entity that participates in a multitude of activities every second to maintain a living state. For the cell to remain alive it must separate and isolate its inner workings from the whims of the environment. The cell plasma membrane has this vital function. The plasma membrane is a barrier to the free flow of materials into and out of the cell. However this barrier can not be absolute or the cell would never get nourishment nor would it ever be able to dispose of waste products. So this barrier needs to be a 'smart barrier' that allows needed substances to be transported into the cell and unwanted substance to stay outside of the cell or be transported out of the cell. Because the plasma membrane controls the entry and exit of substances it is called a selectively or differentially permeable membrane. In today's lab you will examine some other processes involved in the movement of substances into and out of cells.
To learn more about the cell membrane and the fluid mosaic model of membrane structure view the video below.
Membrane transport mechanisms are classified as active or passive processes. Active processes require ATP, that is, they require the expenditure of energy. Active transport processes include protein/enzyme mediated processes and whole membrane processes, like endo- and exocytosis. Passive processes do not require the expenditure of ATP and may or may not require a protein channel. Passive processes include osmosis (movement of water), diffusion and filtration. The one essential requirement for passive transport is that a concentration gradient must exist between one side of the membrane and the other.
To understand passive processes you must first accept and understand that molecules are constantly in motion.The random movement of solutes in solution resulting from the bombardment of water molecules is called Brownian movement. We observe dye crystals under the microscope to see this motion. The dye crystals move in a jerky fashion. This movement of dye crystals results from random collision of water molecules with the crystals. The point of this exercise is to exemplify the fact that molecules, like water are constantly moving.
In this video, droplets of lipid are suspended in distilled water. Water molecules are always in motion and as they strike the droplets they move. The magnification of this slide is 400X.
Diffusion is a form of passive transport. For diffusion to occur a concentration gradient must exist. A concentration gradient occurs when a compound is more concentrated in one location than in a nearby location. Thermodynamics and the movement of molecules result in molecules in the area of high concentration moving towards areas of low concentration. We have all experienced a concentration gradient at some time in our lives. You arrive home and as you get out of your car you start to smell something really delicious. As you approach your door the smell is stronger and as you go inside the kitchen it smells overwhelmingly yummy. The savory food molecules originate in the food and are released by the cooking process. They diffuse from the area of the food (high concentration) to regions of lower concentration, outside by your car. As you move up the gradient, toward the food, the savory molecules are more concentrated and the odor is stronger. Diffusion always occurs from an area of higher concentration to an area of lower concentration. In cells, diffusion continues until the concentration of molecules is equal on both sides of the membrane. Diffusion processes do not allow the cell to concentrate molecules on one side or the other of the membrane.
The rate of diffusion is also influenced by several factors including temperature, concentration, lipid solubility and molecular size. In general, large molecules diffuse more slowly than small molecules. The more lipid soluble a compound is the faster it diffuses. The higher the temperature and the greater the concentration, the faster diffusion will occur.
Diffusion does occur across living membranes. Lipid soluble molecules freely diffuse across membranes. Small, uncharged molecules like oxygen (O2), carbon dioxide (CO2), water and small alcohols are also freely diffusible across all living membranes. They move in response to concentration gradients from an area of high concentration to an area of lower concentration. For example, blood entering the lungs has a higher concentration of carbon dioxide molecules and a lower concentration of oxygen molecules than the air in the alveoli (air sacs) of the lungs. Themodynamics dictates that carbon dioxide molecules diffuse down their concentration gradient from the blood stream into the air spaces of the lung. Similarly, the oxygen in the inspired air diffuses from the air spaces of the lung into the blood stream, that is, from an area of higher concentration to an area of lower concentration. All without the use of energy!! Diffusion across a membrane without the use of a protein carrier is called simple diffusion.
The video below is an animation exemplifying simple and facilitated diffusion.
Diffusion of large molecules or charged molecules can occur if a membrane channel protein is present. The gradient must still exist for movement to occur. This type of diffusion is called facilitated diffusion. Facilitate means to make easier, so diffusion or the movement along a concentration gradient occurs and is made easier by the presence of a membrane protein. Energy is NOT required.
A "prettier" video animation of facilitated diffusion. Note that energy is not required, a protein channel is required and movement can occur in both directions, into and out of the cell.
Osmosis is the movement of water across a selectively permeable membrane. Water is freely permeable across all living membranes. The only form of control for the movement of water is the control of solute concentration. Water will move from where water is more concentrated to where water is less concentrated. Or if you prefer, water will move from an area of lower solute concentration to an area of higher solute concentration. Or in the simplest of terms, water moves to the area of higher 'salt' concentration where 'salt' actually represents salts, sugars, proteins, etc.
Keep in mind that all cells are essentially a little, tiny baggy filled with salts, sugars, proteins and other biological molecules. Cells are not filled with water. The cytoplasm found in cells has so much stuff dissolved in it that it has the consistency of watery jello. Think about an uncooked egg; the egg white is very similar in consistency to the cytoplasm of a cell.
The videos below review the concept of osmosis.
Another You Tube video on osmosis.
The following video is a good summary of diffusion (simple and facilitated) and osmosis. Please review these basic concepts before proceeding on to the topic of tonicity.
Tonicity refers to the pressure on a membrane created by the flow of water. Tonicity reflects the environment in which a cell is found. The descriptive terms we will use, isotonic, hypotonic and hypertonic, all describe a cell's or organism's relationship to its environment. For example, it is incorrect to say a cell is hypertonic; a cell may be in a hypertonic environment, but the cell is not hypertonic.
Isotonic is the term used to describe a situation where the solute concentration inside the cell is the same as the solute concentration outside of the cell.
The prefix iso- literally means equal. If the solute concentrations are equal on either side of the membrane then water moves equally in both directions. For every molecule of water that moves into the cell a water molecule will move out of the cell. See image to the right. The short black arrows indicate the movement of water. Notice the movement of water as indicated by the arrows is equally into and out of the cell.
In a hypotonic environment or solution, the concentration of solutes in the environment is less than or under that found in the cell. Hypo- means under, think about a hypodermic injection which is an injection given under the skin. In this scenario, the cell cytoplasm is more concentrated than the environment and water will flow into the cell from the environment. The cell will swell as water accumulates in the cytoplasm and may lyse or break open if the external solution is very dilute. Keep in mind, only water is moving, not salts, sugars or proteins. These last compounds are not freely permeable across cell membranes. Refer to image to the left. The short black arrows indicate the movement of water. Notice the net movement of water is into the cell.
In a hypertonic environment the concentration of solutes in the environment is greater than the concentration of solutes in the cell. 
Water will flow from the cytoplasm of the cell into the environment. The plasma membrane will shrink in on itself. In red blood cells this is called crenation or crenellation. See image below to the right. The short black arrows indicate the movement of water. Notice the net movement of water is out of the cell.
The video link below will review these concepts and demonstrate tonicity using dialysis tubing.
Filtration is the forced movement of solutes and water through a membrane.The 'force' in question could be gravity or hydrostatic pressure. Filtration is an important process in the kidney. Blood enters the glomerulus via a wide diameter vessel. The exiting vessel has a smaller diameter. This change in vessel diameter creates a bottleneck and back pressure which squeezes fluid and solutes out of the capillaries. This filtrate is then modified as it passes through the nephron and ultimately yields urine.
Cells do have the ability to transport and concentrate molecules using active transport. Remember, active transport process require a protein transporter and the expenditure of energy. Using the energy from ATP.
Larger regions of the membrane are also involved in bringing things into the cell (endocytosis) or in secreting/excreting things out of the cell (exocytosis). Phagocytosis literally means cell eating and it is a form of vesicular transport in which the plasma membrane invaginates or dimples in around a large or solid substance on its surface. The membrane pinches off forming a phagosome on the inside of the cell which can then be processed in a number of ways.
Pinocytosis is a similar phenomenon but involves taking in water soluble compounds dissolved in a droplet versus large, solid objects. Pinocytosis is sometimes called cell drinking.
Receptor-mediated endocytosis occurs when a molecule or object (bacterium, virus) bind to specific receptors on the cell surface. Binding to the receptor triggers the membrane to invaginate and 'capture' in a phagosome the molecule or object.
Exocytosis is the excretion or secretion of vesicle contents to the outside of the cell. In the human body exocytosis is one of the mechanisms by which digestive enzymes are secreted from the pancreas.
The video below describes each of these processes very well.
