Transport Across Cell Membranes Flashcards
What is non-mediated transport
Transport that does not directly use a transport protein
What is mediated transport
Moves materisals with the help of a transport protein
What is passive transport
Moves substances down their concentration or electrochemical gradients with only kinetic energy
What is active transport
Transport that uses energy to drive substances against their concentration gradient or electrochemical gradient.
What is vesicular transport
Moves materials acros membranes in small vesicles by exocytosis or endocytosis.
What are the two main types of transport proteins and what is their function
Channels: which do not bind the slute and form pores acorss the membrane
Carriers: Bind the specific solute and undergo a conformational change in order to transfer the solute across the membrane.
What is non-mediated transport important for
The absorption of nutrients - excretion of wastes.
What kind of molecules use non-mediated transport
Non-polar hydrophobic molecules
- e.g oxygen , co2, N, fatty acids, steroids, small alcohols, ammonia and fat-soluble vitamins
What does the ion channle form
A water-filled pore that shields the ions from the hydrophobic core of the lipid bilayer.
What is the hydrophobic core of the ion channel due to
The selective permeability of the lipid bilayer. Iions can not get across bare lipid bilayer need transport protein and ion channels.
Why is transport very rapid in diffusion throug ion channels
Ions do not bind to channel pores. Have free diffusion of iosn through water pore. Freely diffuses down gradient across lipid bilayer cell membrane.
Label (3) of the ion channel
- water filled pore lined by hydrophilic amino acids
- hydrophobic core of bilayer
- hydrophobic amino acids
Ion channels exhibit ionic selectivity, what does this mean
Specific amino acids lining the pore determine the selectivity of the channel to ions
What happens as a result of channels exhibiting ionic selectivity
The channel can harness the energy stored in the different ion gradients.
Channel gating
Channels contain gates that control the opening and closing of the pore. Different stimuli control the opening and closing of the gate.
What are the types of stimuli that control the opening and closing of the the gate channel
Voltage, ligand binding, cell volume (stretch), pH, phosphorylation.
How is a current made in ion channel
The diffusion over 1 million iosn per second through a channel
What does using the patch clamp technique record
The current flowing through an individual channel
Current fluctuations represent what
The opening and closing of single ion channels and the conformation changes in channel structure that are associated with channel gating.
What happens in carrier mediated transport
The substrate to be transported directly interacts with the transporter protein
Why are the transport rates for carrier mediated transport slower compared to ion channels
The transporter in carrier mediated transport undergoes a conformational change.
What properties do carrier mediated transport proteins have
- specifity
- inhibition
- competition
- saturation
Carrier mediated transport: specifity
Specific for a particular clas ofion channels
Carrier mediated transport: inhibition
Can have molecule sit in binding pocket and wont move so its stuck. Molecules designed to fit in binding pocket but can’t transport across membrane
Carrier mediated transport: competition
2 competing inhibitors both can fit binding pocket, but compete for it, which causes a slower tranport rate.
Carrier mediated transport: saturation
When reach this maximum, can no longer transport molecule across cell membrane.
Do transport proteins catalyse chemical reactions
No do they not, they mediate transport across the cell membrane at a faster than normal rate.
Is mediated transport passive, active or both
Can be both
What does this saturation mean
Concentration increased, transport increased, and at certain concentration, the transporters blocked.
* Transporters display enzyme kinetics.
* Glucose transport occurs until all binding sites are saturated.
What is facilitated diffusion of glucose
1) glucose binds to transport protein (GLUT)
2) transport protein changes shape. Glucose moves across cell membrane (only down concentration gradient)
3) kinase enzyme reduces glucose concentration inside the cell by transforming glucose into glucose-6-phosphate
What does conversion of glucose materials do
Maintains concentration gradient for glucose entry.
What is active transport
An energy requiring process that moves molecules and ions against their concentration or electrochemical gradients
What are the two froms of active transport
- primary active transport
- secondary active transport
What is primary active transport
Energy is directly derived from the hydrolysis of ATP
A typical cell uses how much energy (ATP) on primary active transport
Uses ~30% of its energy
What is secondary active transport
Energy stored in an ionic concentration gradient and is used to drive the active transport of a molecule against its gradient.
What is Na/KATPase
- 3 Na+ ions are removed from cell as 2K+ brought into cell. Therefore the pump generates a net current and is electrogenic.
What is the function Na pump (primary active transporter)
The Na pump maintains a low concentration of Na+ and a high concentration of K+ in cytosol.
The difference in ion concentrations is important for what
- maintain resting membrane potential
- electrical excitability
- contraction of muscle
- maintenance of steady state cell volume
- uptake of nutrients via secondary active transporters
- maintenance of intracellular pH by secondary active transporters.
Why does the Na pump work continuosly
Because Na and K are continually leaking back into the cell down their respective gradients. This is known as the pump-leak hypothesis.
What do secondary active transporters as a result indrectly do
Use the energy obtained by the hydrolysis of ATP
What are examples of secondary active transporters
- Na antiporter or exchangers: Na+ ions rush inward, Ca2+ or H+ pushed out
- Na+ symporter or contransporters: Glucose or amino acids rush inward together with Na+ ions.
How to harness energy in ion gradients - Na+ pump
- Na+ pump uses energy of ATP hydrolysis to change concentration of Na pump.
- Actively excludes Na (have higher Na concentration outside cell than inside)
- Therefore Na diffuses down its electrochemical gradient and this energy used to accumulate glucose above its concentration gradient
How do secondary active transportes harness energy in ion gradients
Secondary active transporters use the energy stored in ion gradients created by primary active transporters to move other substances against their own concentration gradient. Thus these transporters directly use the energy obtained by hydrolysis of ATP.
What are cells of secondary active transporters powered by
By the Na+ gradient initially established by the Na/K pump.
