Cell Processes - Lecture 2 Flashcards
what are the different types of ways substances cross the plasma membrane
- non-mediated transport
- mediated transport
- passive transport
- active transport
- vesicular transport (not discussed)
non-mediated transport: diffusion through the lipid bilayer
- important for absorption of nutrients –> excretion of wastes
- non-polar, hydrophobic molecules (oxygen, carbon dioxide, nitrogen, fatty acids, steroids, small alcohols, ammonia and fat-soluble vitamins (A, E, D and K)
Diffusion through ion channels
- the channel forms a water filled pore that shields the ions from the hydrophobic core of the lipid bilayer
- ions do not bind to channel pore, therefore transport is very rapid
- the ion channel has water filled pores lined by hydrophilic amino acids
- hydrophobic core of bilayer
- hydrophobic amino acids
properties of channels: ionic selectivity
- specific amino acids lining the pore determine the selectivity of the channel to ions
- by being selective to a particular ion the channel can harness the energy stored in the different ion gradients
properties of channels: gating
- channels contain gates that control opening and closing of the pore
- different stimuli can control (gate) channel opening and closing
- stimuli include: voltage, ligand binding, cell volume (stretch), pH, phosphorylation
what are the properties of ion channels: electrical current
- the diffusion of over 1 million ions per second through a channel generates a measurable current (~10^-12 amp)
- the current flowing through an individual channel can be recorded using patch clamp technique
- current fluctuations represent the opening and closing of single ion channels
- the current fluctuations represent the conformational changes in channel structure that are associated with channel gating
carrier mediated transport: mode of action
- the substrate to be transported directly interacts with the transporter protein
- because the transporter undergoes a conformational change transport rates are slower than those obtained for channels
Carrier mediated transport: properties
- these transport proteins have properties similar to enzymes
- They exhibit:
specificity
inhibition
competition
saturation (transport maximum) - transport proteins do not catalyse chemical reactions they mediate transport across the cell membrane at a faster than normal rate
- mediated transport can be passive (facilitated) or active
transporter properties: saturation
- transporters display enzyme kinetics
- glucose transport occurs until all binding sites are saturated
give the steps facilitated diffusion of glucose
1) Glucose binds to transport protein (GLUT)
2) Transport protein change shape. Glucose moves across cell membrane (but only down the concentration gradient)
3) Kinase enzyme reduces glucose concentration inside the cell by transforming glucose into glucose-6-phosphate
- conversion of glucose maintains concentration gradient for glucose entry
active transport
- an energy requiring process that moves molecules and ions against their concentration or electrochemical gradients
- Two forms:
–> Primary active transport
*energy is directly derived from the hydrolysis of ATP
*a typical cell uses 30% of its energy (ATP) on primary active transport
–> Secondary active transport
* energy stored in an ionic concentration gradient is used to drive the active transport of a molecule against its gradient
Primary active transporters: Na/KATPase (and give some other examples)
- 3 Na+ ions removed from cell as 2 K+ brought into cell
- pump generates a nett current and is electrogenic
other examples: Ca/KATPase (muscle SR), H/KATPase (stomach)
Primary active transporters: Na pump
The Na pump maintains low conc of Na+ and high conc of K+ in the cytosol
NOTE:
because Na & K are continually leaking back into the cell down their respective gradients the pump works continuously - known as pump-leak hypothesis
what is the difference in ion concentrations important for (primary active transporter Na pumps)
- 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
Secondary active transport
- uses energy stored in ion gradients created by primary active transporters to move other substances AGAINST their own concentration gradient
- secondary transporters indirectly use the energy obtained by hydrolysis of ATP
- cells have many secondary active transporters
- cells with these transporters are normally powered by the Na+ gradient initially established by the Na pump
