The plasma membrane surrounding a cell forms a barrier between the interior of the cell and the outside environment. Molecules can enter or exit the cell by diffusion or active transport.
Some small molecules, such as O2 and CO2, are able to pass through the membrane itself. Other molecules, such as glucose, are only able to cross the plasma membrane by passing through a protein tunnel. Protein tunnels are tube-shaped proteins embedded in the plasma membrane that selectively allow molecules to flow through them. Diffusion through the membrane and diffusion through a protein tunnel are both considered passive transport because no energy from the cell is required for transport to occur.
In Petri Dish, we do not distinguish between diffusion through the plasma membrane and diffusion through protein tunnels. Glucose, O2, PO32-, NH4+, and CO2 all diffuse into and out of the cell automatically. Most other molecules in the cell or in the environment are only able to cross the plasma membrane with the help of a transport protein.
Diffusion results in a net flow of molecules from areas of high concentration to areas of low concentration. If the O2 concentration is higher outside of the cell than inside, then there will be a net flow of O2 molecules from the outside to the inside of the cell. The rate of this net flow is proportional to the size of the concentration difference.
Notice how we describe this as a “net” flow of molecules. In this example, O2 molecules are actually flowing into and out of the cell at the same time; there are just more O2 molecules coming in than leaving. The statistical model of diffusion that we are using in Petri Dish accounts for this flow in both directions.
Cells can also transport molecules across plasma membranes using active transport. In active transport, a transport protein embedded in the membrane selectively binds with a molecule and either brings it into the cell or sends it out. Transport proteins are similar to tunnel proteins except they only work in one direction and are capable of transporting molecules against a concentration gradient. Transporting molecules from an area of low concentration to an area of high concentration is a bit like pumping water uphill. This is why active transport uses up energy from the cell each time a molecule is transported (even when the molecule is being transported “downhill”).
In order to operate, a transport protein needs two things: molecules to transport and energy. The rate at which a transport protein can transport molecules is limited by whichever of those two things is in short supply. If the molecules being transported are in short supply, then the rate of transport will be proportional to the concentration of those molecules. But if energy is in short supply, then the rate of transport will be proportional to the concentration of energy. Like diffusion, we use a statistical model to calculate the rate of active transport in Petri Dish.