Session 2.1 - Role Of Membrane As Permeability Barriers Flashcards
What is required to move a small hydrophillic molecule or ion across a semi permeable membrane?
A large free energy change
What does the difficulty involved in movement of small hydrophillic molecules across a membrane mean?
The transverse movement of hydrophilic molecules across a membrane is a rare event
How is the movement of ions and hydrophillic molecules mediated and regulated?
By specific membrane transport systems.
Give six roles of transport processes in cell membrane
PIE Can’t Make Gravy
pH, Ionic, Extrusion, Cell volume, metabolic fuels + building blocks, Generation of ionic gradients
- Maintenance of pH
- Maintenance of ionic composition
- Regulation of cell volume
- Concentration of metabolic fuels and building blocks
- Extrusion of waste products of metabolism and toxic substances
- Generation of ionic gradients necessary for the electrical excitability of nerve and muscle
How do hydrophobic molecules move through cell membrane?
easily diffuse through the lipid bilayer – including O2 and CO2
How do small, uncharged polar molecules move through membrane?
Diffuse
How do large, uncharged polar molecules move?
e.g. glucose and sucrose, are not able to diffuse through the lipid membrane – transport proteins required
How do ions move through membrane?
Not able to diffuse, require transport proteins
Complete the sentence
The smaller the membrane coefficient…
… the larger the net rate of transport
What is the rate of diffusion of molecules proportional to?
Concentration gradient
What is the term for the diffusion of water across a membrane up concentration of solute?
Osmosis
What is different about osmosis in cells such as those that line the kidney proximal tubule?
Facillitated by water channels - aquaporins
Define diffusion
Passive (i.e. spontaneous), non-energy requiring, movement of molecules across the lipid bilayer
Describe facilitated transport
Give an example of facilitated transport
Specific proteins increase the permeability for a polar substance e.g Band 3 and Cl-
Why can water move the lipid bilayer easily?
Because it is a very small molecule
What does the band 3 protein do in cells?
HCO3- out, Cl- in anion exchange. Example of facilitated diffusion
Why is facilitated diffusion described as a saturatable process?
Because each carrier can interact with a few ions at any moment, and there are a finite number of transporters in membrane.
What is the Vmax for facilitated diffusion?
When all transporters are saturated
What is not altered by facilitated transport and why?
Equilibrium - Speeds up the rate at which equilibrium is reached, does not modify it
Name three types of pores in facilitated diffusion
Ligand gated ion channels
Voltage - gated ion channels
Gap junction
What is a ligan gated ion channel?
Opens or closes in response to ligand binding to a receptor site
What is a voltage fated ion channel?
Open/close in response to potential difference across the membrane
What is a gap junction (connexin)
Closed when cellular calcium concentration rises above 10 micromoles or cell becomes acid
What is the difference between passive and active transport?
If the transport of an ion or molecule can occur spontaneously or requires energy (active transport)
What is the free energy change in the transport of a molecule across a membrane determined by?
The concentration gradient of the transported species and by the electrical potential across the membrane bilayer
How do ions overcome unfavourable chemical or electrical gradients?
Movement of transported ion coupled to a thermodynamically favourable reaction
What provides the energy for primary and secondary active transport?
The hydrolysis of ATP
What proportion of a cells ATP is used on active transport?
30-50%
Give an example of an active transport system
Na+-K+-ATPase pumps 3 Na+ out and 2 K+ in at the expense of one ATP.
How can active transport be used to generate ATP? Give an example
If active transport pump runs in reverse
In mitochondria, a gradient of H+ ions is employed to drive ATP synthesis via an ATP-dependent proton transporter
What is a cotransporter?
Co-transporter: transfer of one species depends on the simultaneous/sequential transfer of a second species
Why is co-transport knownn as a secondary active transport process?
Hydrolysis of ATP used indirectly. Membrane transport may be drivin by many different gradients, such as sodium, phosphophenolpyruvate and protons
Give an example of three cotransport systems
Na+ - Glucose transport
Na+ - Ca2+ exchange
Na+ - H+ exchange
What is symport?
Transfer of second solute in same direction as first
What in uniport?
One solute molecule species is transported from one side of the membrane to the other
What is antiport?
Transfer of second solute in opposite direction to the first
Describe the Na+ - Glucose transport cotransport system and give what type of cotransport it is
Co-transport system of the small intestine and kidney. Entry of sodium provides the energy for the entry of glucose to the cell.
Symport
Describe the Na+ - Ca2+ exchange cotransport system and give what type of cotransport it is
Inward flow of sodium down its concentration gradient drives outward flow of Ca2+ up its concentration gradient.
ANTIPORT
Describe the Na+ - H+ exchange exchange cotransport system and give what type of cotransport it is
Inward flow of sodium down its concentration gradient leads to cell alkalization by removing H+
What are the two categories of co-transport?
Symport and Antiport
