By Ariffi Zaher

Active Transport o Active transport is the movement of a substance towards its concentration gradient such as from low concentration to high concentration. o In other words, it describes what happens when a cell uses energy to transport something. It usually occurs across the cell membrane where there are thousands of proteins embedded in the cell’s lipid layer. o These proteins happen to be doing much of the work in the active transport process. They are placed in a manner where one part is found on the inside of the cell and one part on the outside.

o A cell must often expend energy to bring substances into the cell or out of the cell to control the internal environment. o When a cell uses up ATP directly during active transport, this is the process that is called Primary Active Transport and when using another energy source, e.g. The potential energy which is stored in an ion gradient, this process is called Secondary Active Transport.

Osmosis  The term ‘Osmosis’ describes the net movement of solvent through a semi-permeable membrane from a lower concentration of dissolved substances to a high concentration of dissolved substances.

 In biological term/system, Osmosis is extremely important since many biological membranes are semi-permeable and it causes many different psychological affects. For example, if an animal cell is exposed to a low water concentration surrounding, water will leave the cell and this causes the cell to shrink.  But if we do the exact opposite of that and place that cell in a high water concentration surrounding, the water molecules will move into the cell and cause the cell to swell.

 Suppose you were to place a hollow tube of a certain diameter into a beaker. Now while you are doing this, the water would rise inside this hollow tube and is going to reach the same level as the water that is on the outside. But suppose you sealed the bottom end of this tube using a semi-permeable membrane. And then filled half of the tube with salt water and once again inserted it into a beaker. You will notice that over a period of time, the relative levels of salt water and the regular water in the beaker will change, with the fresh water gradually rising.

DIFFUSION  Diffusion is the process/movement of molecules or particles from a higher concentration to a lower concentration area.  Unlike active transport, which involves chemical energy, diffusion does not. For example, H2S (g) will slowly diffuse into the air of a lab (when it’s placed in a test tube) until equilibrium is accessed.

 Lets make this easy to understand!

 Every organism needs to exchange substances such as food, wastes, gases and heat with their surroundings and these substances must diffuse between the two (organism and surrounding.)  Surface area x concentration difference ―――――――――――――― Distance

Facilitated Diffusion  Facilitated diffusion is a type of passive transport, allowing to cross membrane with ‘special transport proteins’ assisting or helping them along.  These substances can be transported into the cell via the use of ion channel proteins and carrier proteins (which are fixed and implanted in the cell membrane.)

 Charged ions and polar molecules can dissolve in water but they can not, under any circumstances, diffuse freely across the plasma membrane.  This happens because of the hydrophobic (water hating) nature of fatty acid tails of phospholipids. But on the other hand, small non-polar molecules, such as Oxygen can diffuse easily and freely across the membrane.