Membrane and Transport

The cellular membranes are there to isolate the cell from the outside environment. Nevertheless, cells are under constant conditions such as:

1. Cells live in dynamic environments, where conditions are constantly fluctuating;
2. Cells must maintain homoeostasis;
3. Cells must regulate the concentration of molecules inside them;
4. Cells must regulate the transport of molecules across membranes;

Therefore, cells must have a selective barrier.

This selectiveness is due to the very selective permeability of the phospholipid bilayer. It allows small uncharged molecules, small hydrophobic molecules to pass freely, but won't let any charged or large molecules to go through easily.

There are two main type of transport across cellular membranes: (1) diffusion and (2) active transport.

1. Diffusion is also known as passive transport. It does not require any source of energy because the molecules are moving down in their concentration gradient (high to low). It is divided into two types: simple diffusion and facilitated transport.
2. Active transport requires energy because the molecules are being transported against its concentration gradient. The source of energy comes from a reaction or process that is coupled with the transport so that there is a negative free energy overall, and therefore, is spontaneous.It is divided into primary transport and secondary transport.

Simple Diffusion

Occurs when molecules are able to pass through the membrane with no help from the any protein. Oone example would be the transportaion of oxygen. Oxygen is not polar or charged, so therefore can pass through. It is also found in greater quantities outside the cell, then inside of it. The membrane also lets the passage of water and carbon dioxide pass thorough, even though they are polar, but they are also very small.

Facilitated Diffusion

The diffusion of some molecules, as sodium ions requires a protein to facilitate the process. This is because these molecule can be charged or too big. Therefore they will need proteins to work as gates so they can pass through the hydrophobic membrane.

Water can also be transported through facilitated transport. Aquaporins are water channels that increase the rate of water movement!

Primary Active Transport

Some molecules don't have the chance to be pass through by passive transportantion, simply because they are already found in higher concetration inside the cell (or outside if the cell is trying to exit it).  Nevertheless, these molecules will need energy in order to be transported. In primary active transport the enrgy comes from other reactions such as the hydrolysis of ATP(so it basecally says that PAT uses ATP).

Secondary Active Transport

This mode of transport acquires energy from other transportation process, which must be passive since it provides energy for the other type.
There are two types: Symport (when the two transportations are in the same direction) and antiport (when both are going in opposite directions.)