transport across the plasma membrane (active processes)
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What is active transport? What is used for it? Is there a transport maximum?
- A substance moving against its concentration gradient by using cellular energy, or ATP.
- Carriers are used for active transport, but can be used in passive transport as well.
- There is a transport maximum just like with facilitated diffusion.
What are the two sources of energy used to drive active transport?
- Primary active transport, ATP (hydrolysis of third phosphate)
- Secendary active transport, The movement of a solute (molecule) down its concentration gradient which provides energy (not ATP) to move a different solute against its concentration gradient using co-transport
What is primary active transport? What are pumps?
- ATP is hydrolyzed to provide energy for an integral protein (carrier) to change its shape to 'pump' a substance against its concentration gradient.
- Pumps are carrier proteins that use energy from ATP to pump substances against their concentration gradient.
- Typical body cell uses 40% of its ATP on primary active transport
What are sodium-potassium pumps? ( Na^+/K^+/ATPase ) What are the functions of them?
- Primary active transport
- Integral carrier proteins that maintain a cells electrical gradient by pumping three Na^+ out of the cell, then in its active position pumping two K^+ into the cell to return to its resting position.
- Na-K pumps use energy from ATP to change its shape to pump the Na out of the cell, then break its bond to the phosphate to pump K into the cell.
- Each cycle leaves a net charge of one in the ECF.
What is the purpose of sodium-potassium pumps?
- Maintain a cells electrical gradient and well as tonicity on either side of the plasma membrane to help cells keep their shape and volume.
- Allows some cells such as neurons to carry action potentials by maintaining electrical gradient.
Why are Na-K pumps also reffered to as Na^+/K^+/ATPase?
Because it catalyses the third phosphate off of ATP as well as pumping Na^+ and K^+.
What is secondary active transport?
- A carrier protein simultaneously binds to a Na^+ and another substance and uses the kinetic energy of the Na cation (moving down its concentration gradient) to move both solutes at the same time (other against its concentration gradient). (Uses kinetic energy of Na^+ to simultaneously move another solute against its concentration gradient)
- Can move the substances in the same direction (symporters) or in opposite direction (antiporters)
What are symporters and antiporters? What do they rely on to function?
- Integral carriers used in secondary active transport that move a solute down its concentration gradient, and another against its concentration gradient simultaneously
- Symporters move the solutes in the same direction
- Antiporters move the solutes in opposite directions
- They rely on Na^+ K^+ pumps so that solutes (Na) can move down their concentration gradients consistently
What are the three types of endocytosis?
- Receptor mediated endocytosis
- Bulk-phase endocytosis
What is receptor mediated endocytosis? What materials are moved by it?
- Highly selective type of exocytosis in which receptors and their ligands (specific moles that bind to receptors) in cells bind and a vesicle is formed around them allowing them into the cell
- Cells take in __ using receptor mediated endocytosis:
- Cholesterol containing LDLs (low density lipoproteins)
- Transferrin (iron transporting protein in blood) Antibodies
- Some hormones
- Certain vitamins
What are the 6 step of receptor mediated endocytosis of LDLs (and other ligands)?
- Vesicle formation
- Fusion with endosome
- Recycling of receptors to the PM
- Degradation in lysosomes
Describe binding (1st step) in receptor mediated endocytosis of LDLs (and other ligands).
LDLs bind to receptors (forming receptor-LDL complex) in clathrin-coated pits on the PM. Clathrin then binds to the portion of the receptors protruding into the cytosol and move together to form a basket like structure around the receptor-LDL complexes that cause the membrane to invaginate (fold inward).
Describe vesicle formation (2nd step) in receptor mediated endocytosis of LDLs (and other ligands).
Invaginated (folded in) edges in the clathrin-coated pits fuse together to form a clathrin-coated vesicle around the LDL-receptor complexes
Describe uncoating (3rd step) in receptor mediated endocytosis of LDLs (and other ligands).
- Clathrin molecules detach from the clathrin-coated vesicle almost immediately after it is formed and return back to the inner surface of the PM or form coats on other forming vesicles.
- The resulting vesicle is an uncoated vesicle.
Describe fusion with endosome (4th step) in receptor mediated endocytosis of LDL (and other ligands).
The uncoated vesicle quickly fuses with a vesicle called an endosome which separate the LDLs from their receptors.
Describe recycling of receptors to the PM (5th step) in receptor mediated endocytosis of LDLs (and other ligands).
- The now unbonded receptors in the endosome gather in elongated protrusions that pinch off into transport vesicles that move to the PM to reuse the receptors.
- An LDL receptor is returned to the PM about 10 mins after it enters.
Describe degradation in lysosomes (6th step) in receptor mediated endocytosis of LDLs (and other ligands).
- Transport vesicles that contain LDL molecules pinch off of the endosome and fuze with a lysosome.
- Lysosomes contain many digestive enzymes that break down the large protein and lipid portion of the LDL into amino acids, fatty acids, and cholesterol that leave the lysosome and are used by the cell.
- Cholesterol is used to rebuild parts of the membrane and synthesize other steroids while amino and fatty acids are used for production of ATP or in synthesis of other molecules needed by the cell.
What is phagocytosis?
- phago- to eat
- A form of endocytosis in which a certain type of cell (phagocyte) engulf large solid particles such as worn out cells, viruses, and bacteria.
What are the two main types of phagocytes?
- Macrophages (in many body tissues)
- Neutrophils (type of white blood cell)
What are pseudopods? How are they used in phagocytosis?
- pseudo=false, pods=feet
- Projections of a phagocytes plasma membrane and cytoplasm
- 1. A particle will attach to a phagocytes receptors causing it to extend pseudopods.
- 2. Membranes of the phagocytes pseudopods surround the particle then fuze to form a phagosome that enters the cytoplasm.
- 3. Phagosome fuzes with one or more lysosomes which breakdown the ingested material.
- 4. Undigested material in the phagosome remains sealed in the vesicle (residual body) and is either excreted from the cell (exocytosis) or stored as lipofuscin granules.
What is bulk-phase endocytosis? (pinocytosis)
- pino= to drink
- Most common form of endocytosis in which tiny droplets of EFC, that contain all solutes dissolved in it, are taken into cells without use of protein receptors.
Describe the steps in bulk-phase endocytosis.
- 1. Plasma membrane invaginates forming a vesicle around a droplet of EFC that is brought into the cell.
- 2. A lysosome fuzes with the vesicle and its enzymes degrade the solutes. The smaller molecules such as amino acids and fatty acids are then deposited into the cell to be used elsewhere.
What is exocytosis? In what two types of cells is it especially important?
- Releases materials from a cell instead of bringing them in (endocytosis).
- Secretory cells that liberate digestive enzymes, hormones, mucus, and other secretions
- Nerve cells that release substances call neurotransmitters
What is transcytosis? Where does it occur most often?
- Movement of a substance through a cell as a result of endocytosis on one side and exocytosis on the opposite side.
- Occurs most often across the endothelial cells that line blood vessels and allows for materials to move between blood plasma and interstitial fluid.
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