WEEK 2 (Transport systems) Flashcards
Define ‘Passive transport’
The movement of molecules from an area of a high concentration to an area of a lower concentration until equilibrium is reached
Define ‘Active transport’
The movement of molecules against a concentration gradient and requires energy from the cell
What is the cell membrane?
A selective permeability barrier that maintains distinct internal and external cellular environments
What are examples of transport across a biological membrane?
- Diffusion driven by a concentration gradient
- Diffusion of hydrophilic or charged particles driven by a voltage gradient
- Facilitated diffusion
- Active transport against a concentration gradient
What are aquaporins?
Membrane water channels that play critical roles in controlling the water contents of cells
Who discovered aquaporins?
Peter Agre & Roderick McKinnon
Frick’s Law of Diffusion is correct for ______, ____________ molecules
small, uncharged
Diffusion of large or charged molecules depends on ____________________
membrane potential
What are the properties of Aquaporins?
- Form tetramers (proteins with a quaternary structure of four subunits) in the cell membrane
- Facilitate the transport of H2O & some solutes across the membrane
- Completely impermeable to charged species
What type of process is water permeation through aquaporins?
A passive process that follows the direction of osmotic pressure across the membrane
What is an example of a sophisticated molecular mechanism that aquaporins have evolved to response to harsh conditions?
Aquaporins in plants close their channels in response to harsh conditions to the environments under which exchange of water can be harmful for the organism e.g drought & flooding
What does the osmotic pressure of a solution depend on?
Colligative property (the number of particles in solution)
What is the ‘Donnan-Gibbs effect’?
The Donnan-Gibbs effect describes the behaviour of charged particles across a semipermeable membrane
Donnan Effect states that when an ion on one side of a membrane cannot diffuse through, membrane distribution of other permeable ions across the cell membrane is affected in a permeable way due to the ions’ electrostatic presence
The glucose rate of diffusion is higher than calculated with which equation?
Frick’s equation
Concentration of glucose in blood plasma is much more higher than in ____________
erythrocytes
What is the Michaelis-Menten equation of enzyme activity?
V = Vmax/(1+Km/C)
Km is the measure of affinity to glucose
What is Einstein’s formula for Diffusion?
D = kT / 6πrn
k – Boltzmann constant
T – absolute temperature
r – molecular radius
n – viscmosity of the medium
What is Frick’s Law of Diffusion?
J = - DA dc/dx
J = - DA (Co – Ci )/(Xo – Xi)
J Net rate of diffusion
proportional of concentration difference and
inversely proportional of distance
D – diffusion coefficient
A – area of membrane
What is the equation that represents membrane as three compartment system?
J = - DAb (Co – Ci )/dx= - PA(Co – Ci )
P = Db/dx
b – partition coefficient
P - permeability
What is Van’t Hoff’s law of osmotic pressure?
π = i R T c
π – pressure
i - number of ions
R – ideal gas constant
T – absolute Temperature
c – molar concentration
this concludes that pressure will increase if
- no. of ions
- temperature
- molar concentration
increases
What do Voltage gated channels open specifically in response to?
A change in the electric field that exists across the plasma membrane of cells at rest
What are the different properties of Voltage gated channels?
- Responsible for the propagation of electrical impulses over long distances in nerve and muscle
- Open specifically in response to a change in the electric field that exists across the plasma membrane of cells at rest
What are the differences between Na+ and Ca2+ voltage-gated ion channels and the K+ voltage-gated ion channel?
Na+ and Ca2+ VG ion channels are constructed from a single large polypeptide chain and the interconnected domains are arranged as a tetramer surrounding the channel.
K+ VG ion channel consists of four identical subunits
What are the different Gated channels in the membrane?
- Trans membrane potential (ion channels)
- Ligand gated channel (needs an external mediator e.g neurotransmitter)
- Ligand gated channel (needs an internal mediator e.g ion or nucleotide)
- Mechanically gated channel (movement of cytoskeletal filaments is controlled by cellular deformation)
What is needed for active transport?
Membrane proteins and energy
Which molecule is hydrolysed to facilitate active transport?
ATP
What are the two types of active transport?
Primary active transport and Secondary active transport
What are the properties of ‘Passive transport’?
- Needs no energy
- Always goes down the concentration gradient
- Simple diffusion and facilitated diffusion
- A result of active transport
Why does active transport require energy?
Transport against a concentration gradient or there is an energy barrier for hydrophilic molecules in a hydrophobic environment
Why do cells need active transport?
- To create higher concentration of some substances
- To move certain molecules into and across the bilayer
What are the stages of Primary active transport?
- ATP and three Na+ ions binds with the channel
- Hydrolysis of ATP and phosphorylation of ATPase leads to conformational change which reduces the channel’s affinity for Na+ and increases the channel’s affinity for K+
- Na+ ions are released on the other side. Binding of two K+ ions induces dephosphorylation
- Reversion to original conformation so the channel once again has a reduced affinity for K+ and an increased affinity for Na+
What does the P domain (P ATPases) contain?
Canonical phosphorylated aspartic acid
What does Ca2+ ATPase do?
- transfers Calcium after muscle contraction
- Aids in nerve impulses, muscle relaxation, secretion and absorption in kidneys and absorption of nutrient in intestine
What does K+/H+ ATPase do?
-It is a proton pump in the stomach
- Exchanges potassium from the intestinal lumen with cytoplasmic H+
- Main source of acidification of stomach content
- Activation of digestive ferment pepsin
What does H+ ATPase do?
- Plasma membrane pump in plants and fungi
- Creation of electrochemical gradient
- Drives secondary active transport
What does the CPx ATPase do?
- A heavy metal transporter membrane protein
- Drives secondary active transport
Which ATPase transport phosphatidylserine from the outer surface of membrane to the inner?
Flippase
Secondary active transporters are always ___________ that are coupled with active transport
Co-transporters
What is the difference between secondary active transport of Na+ and Glucose?
Na+ is actively transported using ATPase whereas Glucose is actively transported using Permease
Diffusion coefficient depends on _________________
Size of particles
The Donnan-Gibbs effect tends to cause ____________________
Water to flow into the cell
Where does the energy come from for secondary active transport?
Concentration gradient
