5.8 - 5.9 Flashcards
Active Transport
The movement of a substance across a biological membrane against its concentration gradient, aided by specific transport proteins and requiring an input of energy (often as ATP).
Note 1 —-»
In active transport, a cell must expend energy to move a solute against its concentration gradient—that is, across a membrane toward the side where the solute is more concentrated. The energy molecule ATP supplies the energy for most active transport.
Note 2 —-»
Active transport allows a cell to maintain internal concentrations of small molecules and ions that are different from concentrations in its surroundings. For example, the inside of an animal cell has a higher concentration of potassium ions (K+) and a lower concentration of sodium ions (Na+) than the solution outside the cell. The generation of nerve signals depends on these concentration differences, which a transport protein called the sodium-potassium pump maintains by actively moving Na+ out of the cell and K+ into the cell.
Note 3 —-»
Cell transport: -
- begins when solute molecules on the cytoplasmic side of the plasma membrane attach to specific binding sites on the transport protein.
- With energy provided by ATP, the transport protein changes shape in such a way that the solute is released on the other side of the membrane.
- The transport protein returns to its original shape, ready for its next passengers.
Cells actively transport Ca2+ out of the cell. Is calcium more concentrated inside or outside of the cell? Explain.
Active transport moves calcium against its concentration gradient.
Exocytosis
The movement of materials out of a cell by the fusion of vesicles with the plasma membrane.
Note 4 —-»
A cell uses the process of exocytosis to export bulky materials such as proteins or polysaccharides. A transport vesicle buds from the Golgi apparatus and moves to the edge of the cell, where it fuses with the plasma membrane. Its contents spill out of the cell as the vesicle membrane becomes part of the plasma membrane. For example, the cells in your pancreas that manufacture the hormone insulin secrete it into the extracellular fluid by exocytosis, where it is picked up by the bloodstream.
Endocytosis
Cellular uptake of molecules or particles via the formation of new vesicles from the plasma membrane.
Note 5 —-»
Endocytosis (endo, inside) is a transport process through which a cell takes in large molecules or droplets of fluid. A cell engulfs a particle by wrapping extensions called pseudopodia around it and packaging it within a membrane-enclosed sac called a vacuole. The vacuole then fuses with a lysosome, whose hydrolytic enzymes digest the contents of the vacuole.
Receptor-Mediated Endocytosis
The movement of specific molecules into a cell by the unfolding of vesicles containing protein with receptor sites specific to the molecules being taken in.
Note 6 —-»
Receptor proteins for specific molecules are embedded in regions of the membrane that are lined by a layer of coat proteins. The plasma membrane indents to form a coated pit, whose receptor proteins pick up particular molecules from the extracellular fluid. The coated pit pinches closed to form a vesicle, which then releases the molecules into the cytoplasm.
Note 7 —-»
Your cells use receptor-mediated endocytosis to take in cholesterol from the blood for the synthesis of membranes and as a precursor for other steroids. Cholesterol circulates in the blood in particles called low-density lipoproteins (LDLs). LDLs bind to receptor proteins and then enter cells by endocytosis. In humans with the inherited disease familial hypercholesterolemia, LDL receptor proteins are defective or missing. Cholesterol accumulates to high levels in the blood, leading to atherosclerosis, the buildup of fatty deposits in the walls of blood vessels.
As a cell grows, its plasma membrane expands. Does this involve endocytosis or exocytosis? Explain.
Exocytosis: When a transport vesicle fuses with the plasma membrane, its contents are released and the vesicle membrane adds to the plasma membrane.
