VII. Membrane Function: Transport across membranes
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a. __structure—many molecules ordered into a higher leverl of organization
b. Particles move at different __
c. Nonpolar molecules (hydrocarbons, carbon dioxide, oxygen= hydrophobic) __
d. Hydrophobic interior impedes passage of__
e. __ play key role in regulating transport
- Can dissolve in the lipid bilayer of the membrane and cross easily
- ions and polar molecules, which are hydrophilic. They may pass slowly
Amount of specificity?
i. Hydrophilic channel used as tunnel through membrane (water via aquaporins)
i. Hold onto passengers and change shape allowing shuttling across membrane
3. Specific for the substance it translocates, allowing only a certain substance to pass by
i. Diffusion is the result of motion caused by __; movement of molecules of any substance so they spread out evenly into available space
1. Movement random, yet __of molecules is __ (dynamic equilibrium)
Rule of Passive Transport:
- 2. Rule: in the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated (down/ with concentration gradient, the region along which the density of a chemical substance increases or decreases (in this case, decreases)
- No work/ energy spontaneous
- Concentration gradietn represents potential energy
iii. For a dilute solution, __ do not affect the water concentration significantly
1. Instead, tight clustering of water molecules around hydrophilic solute molecules makes some water unavailable to move
a. This difference in free water concentration is what ___
- diffuses across the membrane from the region of lower solute concentration (higher free water concentration) to higher solute concentration (lower free water concentration) until the solute concentrations on both sides are equal
1. Depends on __ that can’t cross membrane relative to inside of cell
a. If higher concentration of nonpenetrating solutes outside= water __cell
- ability of a surrounding solution to cause a cell to gain or lose water
- concentration of solute
- 1. Isotonic environment: noo net movement of water across the plasma membrane
- 2. Hypertonic environment: cells lose water, shrivel, and die due to higher solute concentration outside
- 3. Hyptotonic solution: water enters faster than it leaves: cell will burst
Animals that lack cell walls have adaptations.
1. Osmoregulation: the control of solute concentrations and water balance
Water Balance of Cell Walls
- Hypotonic: water in to the point where wall exerts back pressure on cell (turgor pressure), preventing further uptake
- turgid: very firm/ flaccid: limp
- Hypertonic: plasma membrane pulls away from cellw all: plasmolysis
If they transport ions, they are called __
Why is it positive?
- 1. Provide corridors that allow specific molecules or ions to cross
- ion channels; gated: open and close in response to stimulus (electrical or substance)
- undergo subtle change in shape thta translocates across membrane
- no energy; solutes down concentration gradient
1) can do what?
2) requires what?
3) enables what?
- 1) move solutes against concentration gradients
- 2) energy (ATP), which transfers phosphate to transport protein, causing shape chagne and translocation
- -- sodium-potassium pump
- 3) enables cell to maintain internal concetrations of small solutes that differ from concentrations in environment
i. all cells have __across their plasma membranes
1. what is it? electrical potential energy—a separation of opposite charges
a. cytoplasmic side: negative in charge relative to the extracellular side because of unequal distribution of anions and cations on two sides
- electrical potential energy—a separation of opposite charges
- cyto: negative because unequal distribution of anions and cations
Voltage across a membrane: __
-- inside: negative/ outside posiive
-- favors what?
- membrane potential
- cations inside and anions outside
3. Membrane potential acts like __, an energy source that affects traffic of all charged substances across membrane
4. Two forces drive diffusion:
This is called __
- chemical force (ion's concentration gradiet)
- electrical force (effect of membrane potential on ion's movement
- electrochemical gradient
Explain Sodium potassium pump.
- 1. Pumpts three Na+ out for two K+ in
- a. One net transfer of positive charge from cytoplasm to extracellular fluid
i. A transport protein that generates voltage across a membrane is called an __.
Plants, fungi, bacteria:__
What do these pumps help do?
- electrogenic pump
- protom pump (transports protons out, transferring positive charge from cytoplasm to extracellular solution
- store energy that can be tapped for cellular work by generating a voltage
What is cotransport
- i. Single ATP-powered pump that transports a specific solute can indirectly drive the active transport of several other solutes
- 1. A substance that has been pumped across a membrane can do work as it moves back across the membrane by diffusion
- a. A cotransport separate from the pump can couple the downhill diffusion of this substance to the uphill transport of a second substance against its own electrochemical or concentration gradient
- Sucrose- H+ cotransport
- 1. Fusion of vesicles with plasma membrane to secrete certain biological molecules
- a. When vesicle membrane in contact with plasma membrane, proteins rearrange the lipid molecules of the two bilayers so they fuse and contents spill to outside
- i. Pancreas
- 1. Cell takes in biological molecules and particulate matter by forming new vesicles from the plasma membrane
- a. Plasma membrane sinks inward to form pocket, which forms vesicle
- Three types:
- phago, pino, receptor-mediated endocytosis
1. Cell engulfs particle by wrapping pseudopodia around it and packaging it within food vacuole; particle ingested after vacuole fuses with lysosome
1. “cell drinking” tiny vesicles
iii. Receptor-mediated endocytosis
- 1. Enables acquirement of bulk quantities of specific substances through proteins with specific receptor sites exposed to extracellular fluid to which ligands bind
- a. Receptor proteins cluster in regions of membrane called coated pits, which are lined on cytoplasmic side with coat proteins
- i. Each coated pit forms vesicles containing ligand molecules
Receptor Mediated endocytosis is used to take in __, which travels in the blood as __, which act as __ by binding to receptor sites.
Vesicles also provide mechanisms for __ or __ plasma membrane.
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