Topic 4: Transport across Membranes Flashcards
The lipid bilayer is semipermeable, what does this mean?
Some molecules can pass in and out freely
-Small, hydrophobic molecules (oxygen, CO2, hydrocarbons)
What is a transport protein?
Integral membrane Protein that aids in the movement of some molecules across the membrane
-large, hydrophilic molecules, charged molecules can’t pass through on their own
What is Passive Transport?
Movement across a membrane that does not require an input of energy from the surroundings
-Favourable or Spontaneous process predicted by the 2nd law of thermodynamics (energy diffuses into available space; entropy increasing)
What are some types of passive transport?
Diffusion
Facilitated Diffusion
Osmosis
What is Active Transport?
Movement (Across a membrane) that Does require an input of energy from the surroundings
What is Diffusion?
Tendency of molecules to spread out into available space.
Dictated by the 2nd law of thermodynamics
*Movement of molecules from an area of high concentration to low concentration releases energy
What is Facilitated Diffusion?
Diffusion of substances aided by transport proteins.
*Passive
Carrier Proteins allow specific molecules to pass in/out of the cell based on concentration gradient.
-releases energy
What type of molecules would use Protein channels?
Carrier proteins?
- Small, polar, charged molecules (IONS) often use ion channels
- Larger molecules often use carrier proteins
How are ions transported across the membrane?
Ions can move passively via channels but may also be transported actively
*Ion Channels are specific for certain ions
What is the overall result of transporting ions in cells?
Results in voltage or Membrane Potential
- separation of charges across the membrane
- specific accumulation of ions on different sides of the membrane
What is the Electrochemical Gradient?
Combination of Chemical (concentration gradient) and electrical (membrane potential) forces that regulate the diffusion of ions across the membrane
What is Osmosis?
Diffusion of water across a selectively permeable membrane directed by solute concentration
What is Hypertonic solution relative to a hypotonic solution?
Hypertonic: contains higher concentration of solute
Hypotonic: Contains less solute concentration
How does water move through lipids?
Through Aquaporins
What happens to animal cells in a hypotonic solution?
Net movement of water is into the cell - cell swells and bursts (lysed)
What happens to an animal cell in a hypertonic solution?
Net movement of water out of the cell - cell loses water and shrivels
What happens to a cell that contains a cell wall in a hypotonic solution?
Net movement of water into the cell - cell wall provides back pressure that keeps cell from lysing
-Turgid (normal)
What happens to a cell that contains a cell wall in a hypertonic solution?
Net movement of water out of the cell - cell membrane separates from cell wall
-plasmolysis
What happens to a cell that contains a cell wall in an isotonic solution?
Net movement is equal - results in wilting
What is Active Transport?
- Used to build up concentration gradients (potential energy) and to establish membrane potentials
- Movement Against a concentration gradient
- Requires Energy*
What are the sources of energy for Active Transport?
ATP hydrolysis, energy stored in concentration gradients
What is a proton pump? Where does it get its energy?
Protein that transports protons across a membrane (such as to create acidity in lysosomes) sets up a concentration gradient and establishes a membrane potential
-Energy from ATP hydrolysis - phosphate from ATP binds to the proton pump - changes shape of pump
Mechanism of Sodium Potassium Pump (animal cells)
- 3Na+ bind to the transport protein on the inside face of the cell - binding stimulates ATP hydrolysis - released phosphate attaches to transport protein
- The transport protein changes shape, now facing the outside of the cell - the 3Na+ dissociate
- 2K+ bind - causes the transport protein to change shape - results in release/dissociation of the phosphate from the transport protein
- The transport protein undergoes another shape change, now facing the inside of the cell - the 2K+ dissociate
- Repeat
***Main reason for electrochemical gradient
Electrogenic Pump
-Contributes to the electrochemical gradient of the cell
(eg proton pumps and Sodium-Potassium pumps)
-Generates voltage across the membrane by separating charges
What is an example of coupling passive and active transport? Explain the process.
Sucrose-H+Cotransporter (sucrose proton cotransporter)
- Proton pump first establishes a membrane potetial and H+ gradient - uses the energy of ATP Hydrolysis
- Proton gradient is the energy source to drive sucrose through the cotransporter - High G, unstable, low entropy
- Protons diffuse back into the cell through a cotransporter protein - passive process: facilitated diffusion
- Diffusion of protons releases energy - changes the shape of the cotransporter protein and makes a site available for sucrose to bind
- Sucrose is actively transported into the cell - against concentration gradient - energy source directly from the proton concentration gradient
How are large molecules transported?
Uses ATP
-Cell surrounds molecules in membrane package (vesicles)
Exocytosis?
Molecules out - secretion via endomembrane system
Endocytosis?
Molecules in
Phagocytosis?
Cell engulfs material with pseudopodia - membrane remodeling
Pinocytosis?
Non-specific engulfing of liquids and dissolved solutes
