Endocytosis & Exocytosis Pooja Patel Elizabeth Pemberton

 Water and small molecules enter and exit cells through active transport and diffusion.  Water and small molecules enter and exit cells through active transport and diffusion.  Large molecules, however, (proteins and polysaccharides) as well as larger particles are transported in and out of the cell after being packaged in vesicles.  Large molecules, however, (proteins and polysaccharides) as well as larger particles are transported in and out of the cell after being packaged in vesicles.  Endo- and exo- cytosis require energy.

Exocytosis:  Cell secretes biological molecules by the fusion of vesicles with the plasma membrane.  Cell secretes biological molecules by the fusion of vesicles with the plasma membrane.  Steps:  Steps: 1. Transport vesicles that buds off Golgi body moves along microtubules of the cytoskeleton to the plasma membrane. 1. Transport vesicles that buds off Golgi body moves along microtubules of the cytoskeleton to the plasma membrane. 2. Lipid molecules of from the vesicles and the plasma membrane rearrange, forming a fused membrane. 2. Lipid molecules of from the vesicles and the plasma membrane rearrange, forming a fused membrane. 3. The contents spill outside of the cell. 3. The contents spill outside of the cell. 4. The vesicle membrane becomes part of the plasma membrane. 4. The vesicle membrane becomes part of the plasma membrane.

Exocytosis :  Examples:  Examples: Pancreas secretes insulin into extracellular fluid by exocytosis. Pancreas secretes insulin into extracellular fluid by exocytosis. Neurons release neurotransmitters that signal other neurons or muscle cells. Neurons release neurotransmitters that signal other neurons or muscle cells. Plant cells ( when making walls) deliver proteins and carbs from Golgi vesicles to outside of cell. Plant cells ( when making walls) deliver proteins and carbs from Golgi vesicles to outside of cell.

Endocytosis:  Cell takes in biological molecules by forming new vesicles from plasma membrane.  3 types:  3 types: Phagocytosis : (cell eats) Phagocytosis : (cell eats) Pinocytosis : (cell drinks) Pinocytosis : (cell drinks) Receptor-Mediated Endocytosis (RME) Receptor-Mediated Endocytosis (RME)

Endocytosis:  Steps:  Steps: 1. Small area of the plasma membrane sinks inward. 2. The pocket deepens and pinches into a vesicle containing materials from the outside of the cell. 2. The pocket deepens and pinches into a vesicle containing materials from the outside of the cell. 3. Becomes a vacuole (for food or water) 3. Becomes a vacuole (for food or water)

RME: Enables cell to acquire bulk quantities of specific substances, even though those substances may not be in high concentration outside the cell. Enables cell to acquire bulk quantities of specific substances, even though those substances may not be in high concentration outside the cell.  Steps:  Steps: 1. The ligands (molecule that binds to a specific receptor) attach to protein receptors that are on the outside of the cell. 1. The ligands (molecule that binds to a specific receptor) attach to protein receptors that are on the outside of the cell. 2. A “fuzzy” layer of coat proteins are on the opposite side of the receptors on the inside of the cell. 2. A “fuzzy” layer of coat proteins are on the opposite side of the receptors on the inside of the cell. 3. Once the ligands bind to the receptors pocket forms and pinches into the cell. (The vesicle is coated by the “fuzzy” coat proteins. 4. Once the ligands are released, the receptor proteins are recycled back to the plasma membrane. 4. Once the ligands are released, the receptor proteins are recycled back to the plasma membrane.

RME:  Examples :  Examples : 1. Cholesterols travel in blood in particles called low density lipoproteins (LDLs) 2. the LDLs act as ligands 3. When the LDLs are defective or missing, cholesterol builds up in blood vessels and can lead to arteriosclerosis.

 If pieces of the plasma membrane break off every time exocytosis occurs, how does the cell remain enclosed? Apparently, endo- and exo- cytosis offset each other. While one removes membrane, the other replenishes. Apparently, endo- and exo- cytosis offset each other. While one removes membrane, the other replenishes.

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