24 - Membrane Transport Flashcards
Why are transporters needed in membranes (simple)
- all compartments of the cell are unique
- need a unique set of molecules
- some cells are polarized
- need different molecules on different sides
- membranes form the compartments
- impermeable to many molecules (e.g. proteins)
- so membranes must contain transport proteins for import/export of metabolites, ions, etc, in order to maintain function
what can and cant naturally cross lipid bilayers
Can:
- small hydrophobic molecules (O2, N2, CO2, benzene, steroid hormones)
- small uncharged polar molecules (ethanol, glycerol)
- water can pass some more permeable membranes - aquaporins in some less permeable membranes
Can’t:
- large uncharged polar molecules - glucose, sucrose
- charged ions - K+, Na+, H+, HCO3-, Cl-
- Charged polar molecules - amino acids,
-
transport across lipid bilayers by diffusion
- what affects rate of transport, direction (in/out membrane), etc.
- solute must be hydrophobic to dissolve in the membrane hydrophobic core
- rate of transport depends on size and hydrophibicity
- small, hydrophobic molecules move faster through (e.g. O2)
- rate and direction depends on the concentration gradient
- transport is down the gradient - from high conc. to low conc.
- transport continues to a dynamic equilibrium - same conc. either side of membrane
- Brownian motion - movement of substance in and out of membrane is equal (net zero movement in/out) - there is still constant movement of susbstances between spaces
two types of membrane protein transporters
- channels
- carriers
channel and carrier proteins - simple info
- active or passive transport
- speed of each transporter
- channels create a pore in membarne to pass through
- always passive transport/facilitated diffusion (uses a membrane protein - not move in on its own)
- faster than carrier
carriers - move molecule across membrane
- can be active or passive
- slower than channel - but can be used in active transport against conc. gradient
How selective are some membrane transporters
- some transport only one solute (Na+ transporter)
- some transport only a few closely related solutes (Na+ and K+)
- some transport several related solutes (Na+, K+, Ca2+, Mg2+ - all +ve charge - cations)
2 types of transporter and more info (diagram)
difference between active transport pumps and symports/antiports
- pumps directly use ATP - primary active transport against a gradient
- secondary active transport - indirect use of downhill gradients - symports and antiports use this
- energy indirectly comes from ATP
facilitated diffusion vs simple diffusion (graph)
- facilitated diffusion is faster
- fac. diffusion is saturable, as defined no. of channels are available
- fac. diffusion is specific
active transporters info
- use energy (usually metabolic energy) to transport solutes
- can transport compounds against a conc. gradient
- nutrients are often available at low conc. in external environment
- can establish conc. gradients
- ion gradients are used to generate acpots
- many active transporters involve ATP hydrolysis and use ~40% of ATP in resting humans to maintain ion gradients across cell membranes
3 main active transporter types
ATP-driven pumps - coupled to ATP hydrolysis
- Light-driven pumps - uses light as energy, e.g. bacteriorhodopsin
- Couple transporters - coupled to the pot. energy of downhill conc. gradients (electrochemical gradients)
Types of ATP-driven active transporters
- simple function of each
- Diagram included
- P (phosphorylate) type pumps - phosphorylate themeselves during transportation cycle - Ion gradients Na+, K+, H+, Ca2+
- F-type pumps - work in reverse using proton gradients to synthesise ATP from ADP and Pi - e.g. ATP synthases
- ABC transporters - pump small molecules as opposed to ions
- ATP hydrolysis can lead to conformational changes in tran membrane part of these transporters and transport happens
P-type pump example - H+-ATPases
…
Transport of ions via pumps to make stomach acid (simplified)
- pH of stomach lumen is 2.0
… more needed
P-type pump - example - Ca2+-ATPase
- used in muscle cells
