Question

1. What are the differences and similarities of “passive diffusion”, “facilitated diffusion” and “active transport”?
2. What is endocytosis? Explain. What are the different types of endocytosis?
3. What is exocytosis? Explain.

Answer 1

SIMILARITIES AND DIFFERENCES BETWEEN PASSIVE DIFFUSION, FACILITATED DIFFUSION AND ACTIVE TRANSPORT:

Passive diffusion, facilitated diffusion and active transport are all methods of transporting molecules across the phospholipid bilayer. They share some common characteristics which include the fact that both passive and facilitated diffusion are passive processes and therefore reuires no energy to take place as the movements takes place down the concentration gradient of the molecules, this means that some of the molecules may get diffuse into the cell, across the phospholipid bilayer due to there being a lower concentration of that molecule inside of the cell that in its external environment.

And on the other hand, facilitated diffusion and active transport cannot take place without extra mechanism such as transmembrane proteins which are required for facilitated diffusion. Active transport carries substances against the concentration gradient which needs ATP for their energy , also requires ATP for their specific carriers. The carriers undergo changes in shape to move the substance across the membrane. This use of protein carriers in active transport and facilitated diffusion confers selectivity of the phospholipid membrane as only specific carriers allows specific moleculesto pass into the cell through the phospholipid membrane.

ENDOCYTOSIS AND IT'S TYPES:

Endocytosis is the process of actively transporting molecules into the cell by engulfing it with its membrane. Endocytosis reuires energy from the cell. There are 3 types of endocytosis:

1. PINOCYTOSIS / CELLULAR DRINKING: Here the cell engulfs drops of fluid by pinching in and forming vesicles that are smaller than the phagosomes formed in phagocytosis. Pinocytosis is a non specific process in which the cell membrane folds onto itself creating a small pouch, the cell membrane closes around this pouch forming a vesicle. Any liquid or small molecules that were trapped in that pouch taken into the cell. The vesicle fuses with a lysosome , whose digestive enzymes break down the molecules so that its part can be recycled.

2. PHAGOCYTOSIS / CELL EATING: Phagocytosis is a process by which cell engulf and digest large molecules. The cell spends out projections of its cytoplasm called pseudopodia. The pseudopodia engulf the molecule and the cell membrane fuses around , trapping it inside the cellular vesicle. The vesicle joins with the lysosome again and the molecule is broken down.

3. RECEPTOR MEDIATED ENDOCYTOSIS: is a form of endocytosis in which receptor proteins on the cell surface are used to capture a specific target molecule. The receptor are the transmembrane proteins which cluster in the regions of the plasma membrane called coated pits. This name comes across from a layer of proteins called coat proteins which are found on the cytoplasmic side of the pit. When the receptor binds to their specific target molecule, endocytosis is triggered and the receptors and their attached molecules are taken into the cell via vesicle. The coat proteins participate in this process by giving the vesicle its round shape and helping it to bud off from the membrane. Receptor mediated endocytosis allows cells to take up large amounts of molecules that are relatively in low concentration in the extra cellular fluid.

EXOCYTOSIS:

The reverse process of moving materials into a cell is the process of exocytosis. Exocytosis is a process by which a cell transports secretory products through the cytoplasm to the plasma membrane, Secretory producta are packaged into transport vesicles.

Exocytosis can be either calcium dependent or calcium independent. In calcium dependent exocytosis, an influx of calcium into the cell will stimulate secretion. This happens when a protein is only secreted when needed. Transport vesicles with their cargo will travel from the golgi apparatus ( the protein packaging organelle) to the plasma membrane. The vesicle then docks at the plasma membrane and waits for secretion signal, this is also called regulated exocytosis because secretion from the vesicle is controlled.

Calcium independent exocytosis occurs constantly, just like with regulated secretion thetransport vesicle and its cellular productwill travel through the cell to the plasma membrane. Unlike regulated secretion, the vesicle does not wait at the plasma membrane, instead fusion with the plasma membraneand secretion of vesicle contents into the extracellular environment happens automatically. This constant flow of secretory product is also called constitutive exocytosis because secretion from the vesicle is not controlled.