Entering a cell
For cell protection reasons, most molecules are unable to enter a cell on their own. This is why transport proteins, which live in the plasma membrane, are relied upon to decide which molecules can enter and exit. Various types of transport proteins are necessary to cater to the different molecules needed in the cell.
Choosing a direction
Molecules must be distributed evenly on both sides of the plasma membrane in order to support the cell. This is called equilibrium. When there in an uneven distribution, the molecules will go through a process called diffusion, where they travel from a highly concentrated area to a low concentrated area.
Crossing a membrane
When molecules go through the process of diffusion, they may need the help of transport proteins to cross the membrane. This may require a channel protein, which acts as a straw that allows the molecule to come through or, a carrier protein, which only transports specific molecules across.
It is also necessary for water to travel in and out of cells and thus, try to reach the equivalent of equilibrium. This process is called osmosis and happens when water moves from an area where it is most pure to an area where it is less pure.
Moving a molecule
It is often necessary for cells to move molecules to areas of high concentration rather than the usual areas of low concentration. This process is called active transport and necessitates energy from the cell. Carrier proteins will support the process by carrying the molecule that must be moved.
Creating an organism
When cells need to create an organism, they form groups called multicellular organisms. These cells must communicate in order to work together and they do this through cell-cell attachments, which form tissues. They also use signals to send messages to other cells at different distances.
Attaching to another cell
Cells will attach to each other for different reasons and these reasons will decide the type of attachment they have. They can be held together through tight junctions, which are held together by proteins and don’t let anything through, or through anchoring junctions, which are tight but do let things through.
Receiving a message
Hormones are the processes by which cells send signals to each other. Cells need receptors in order to respond, as receptors tell molecules to exit or enter the plasma membrane. Receptors targeting molecules that can cross the membrane live in the cell, while receptors targeting molecules that can’t cross live in the membrane
Acting as a loudspeaker
A signal transduction pathway is necessary when low-level signals are sent between cells. This is created when one molecule calls to lots of other molecules and they call to even more molecules. This action not only alerts the intended cell but acts as a loudspeaker.
Disabling a connection
In order to deactivate the signal transduction pathway, the first cell that started the signal must stop sending its message, allowing all the other cells to return to their tasks.