SQ #1-11
Define the terms "concentration gradient" and "simple diffusion". State the conditions under which simple diffusion across a membrane will occur and whether simple diffusion is a movement down a concentration gradient or against a concentration gradient. Explain the source of energy that drives diffusion and why diffusion occurs
Simple diffusion is the process of the movement of solute particles from an area of higher concentration to an area of lower concentration due to the particles' constant, spontaneous motion. It does not need to occur with a membrane present. Diffusion is vital for cells to receive nutrients and expel wastes. In order for simple diffusion to occur in cells, the substances must be lipid-soluble, neutral molecules to pass through the phospholipid bi-layer of the cell membrane. In addition, simple diffusion can only move substances in the downward direction of the concentration gradient. Concentration gradient is the condition that occurs when two or more substances are separated by a membrane and have different concentrations of a solute within each. The process of simple diffusion moves down, along with the gradient. To have a decreasing gradient, a high concentration of solutes within a substance will pass to a lower concentration substance. If the opposite would occur and the movement would be from the low concentration to the high concentration, that is called going up or against the gradient and would not occur during simple diffusion. The rate of diffusion can be affected by multiple factors such as temperature, molecular weight, steepness of concentration gradient, membrane surface area and membrane permeability; all of which impact the amount of kinetic energy within the substance, which is the driving force behind diffusion. Kinetic energy is involved during simple diffusion through the molecule's natural, rapid movement. This causes the molecules to bump into each other, sending the molecules into different directions which further drives diffusion.
State whether or not diffusion is effective over long distances in the body.
Diffusion is not effective over longer distances in the body. It can be a quicker, more effective process for shorter distances, but a very slow process over longer distances.
The bloodstream supplies cells with the molecules that they need to survive and function properly. Cells obtain these needed molecules by diffusion through the thin walls of microscopic blood vessels called capillaries. Relate your answer above to the fact that your body has an extensive network of over 60,000 miles of capillaries. Identify 2 chemical substances critical to cell function that cross cell membranes by diffusion and state whether or not cells must do work (expend energy) to make this happen.
The body's extensive network of capillaries is vital for the cells in our body to function. The body's smallest blood vessels are referred to as capillaries and function as a transportation system to reach interstitial spaces between cells. Through the membranes of the capillaries, there is an exchange of materials between the cells and the circulatory system. It transports the necessary nutrients in and carries the wastes away from the cells so our body is able to continue to function. Through the filtration process, the capillaries' membrane is able to keep the red blood cells and proteins inside the capillary and allow water and other nutrients to outflow into the extracellular fluid by passing through filtration pores or the epithelium. Water can cross the capillary membrane via gaps between the endothelial cells, filtration or diffusion, and specialized pores in the endothelial cells. Oxygen (O2) and carbon dioxide (CO2) are two molecules that the body is dependent upon to go through the diffusion process. Oxygen is needed for the cell to continue to function, where the cell uses the capillary transport to rid itself of unnecessary carbon dioxide. The cell membranes have selective permeability, which allows certain substances to pass through to the other side of the membrane. Oxygen and carbon dioxide are both small, nonpolar molecules that are able to pass through the lipid bilayers of the membrane through the simple diffusion process because the plasma membrane of the capillaries are permeable to these substances. This passive processes are dependent upon the kinetic energy, or the constant movement of the particles themselves to provide all the necessary energy. Another important molecule that is transported through the capillaries is glucose, which is a larger molecule that needs to bind to a transport protein so that the channel will conform its shape to allow the molecule to pass.
Describe how diffusing molecules can pass through a membrane including the role that membrane channels can play in diffusion.
The presence of a membrane can account for multiple factors in the diffusion process. If the membrane has a larger surface area, this can increase the rate of diffusion due to there being more area for the particles to diffuse through. The membrane's permeability also is an important factor. The phospholipid bi-layer allows nonpolar, hydrophobic substances that are lipid-soluble to pass easily. Substances that are polar and hydrophilic can pass through membrane channels through facilitated diffusion or active transport. A cell can take away or add more membrane channels, or open and close the gates to the channels, to accommodate for different particles, which is affected by size and charge of the particles. The adding/removal of channels regulates the cell's permeability, whereas the opening/closing of channel gates is how particles are passed through the membrane. Furthermore, each gated channel is specific for what types of molecules it lets pass through, which means that it doesn't open/close to accommodate for different particles because it's already specifically designed for certain particles. There are three types of gated membrane channels called ligand-gated channels, voltage-gated channels and mechanically gated channels. Ligand-gated channels respond to chemical stimuli, voltage-gated channels open/close to changes in electrical currents across the membrane, and mechanically gated channels respond to physical stress on a cell. There are also open and non-gated channels. With these channels, thousands of ions or substances can pass through regardless of selectivity. There is no binding site necessary for some of these channels to be open. It is open for the passage of intracellular substances or extracellular. Protein carriers can assist with facilitated diffusion by binding to one molecule and releasing it one the other side.
