Membranes and Receptors 2 Flashcards
What is passive diffusion?
The simple diffusion of molecules across the hydrophobic domain of the lipid bilayer down their concentration gradient, a non-energy requiring process.
What is facilitated diffusion?
Passive diffusion of molecules utilising carrier or channel proteins.
What is active transport?
Transport of molecule across a membrane which required energy due transporting the molecule against a concentration and/or electrical potential gradient.
Define symport:
A co-transporter which transports different species of solute in the same direction across a membrane.
Define antiport:
A co-transporter which transports different species of solute in opposite directions across a membrane.
How is glucose transported into cells?
Secondary active transport (utilising a Na+ gradient set up by an Na pump on the basal membrane) of Na+ and glucose through a symport (SGLT-1) into lumenal cells. From there the glucose can diffuse through GLUT-2 (transport protein on basal membrane) to the bloodstream. This also occurs in the kidneys.
How are amino acids transported into cells?
Like glucose, amino acids use secondary active transport across a symport with Na+(using the Na+ gradient) to be transported into cells.
How are ions transported into cells?
Channels or carrier proteins = facilitated diffusion
How is cytoplasmic pH regulated?
It is held at a set point by the antagonistic effects of Na+-H+ exchange and Cl-HCO3- exchange proteins.
What properties of solutes affect their movement through membranes?
Charge, polarity, size
What are the general features of channel proteins?
a
Define a uniport transporter:
A transporter which transports a single solute species from one side of a membrane to another.
Define cotransport:
A transporter which transports more than one species across a membrane simultaneously or sequentially. These can be symporters (both in same direction) or antiporters (opposite directions).
What is the relationship between the rate of passive diffusion and the concentration gradient?
Linear - as the concentration increases the rate of passive diffusion increases.
What molecules passively diffuse into cells?
Non-polar molecules, water (osmosis)
In some cells the transport of water is facilitated by specific water channels. What is the name of this type of water channel that is found in the kidney proximal tubules?
Aquaporins
Why are many hydrophilic molecules and ions unable to passively diffuse across the lipid bilayer?
Tranversing the lipid bilayer requires a large free-energy change and therefore occurs very rarely. Consequently their permeability coefficients are very low. The exception to this is water which is able to passively diffuse and has a higher permeability coefficient.
List some of the roles that specific membrane transport systems have:
- Maintaining intracellular pH
- Maintaining ionic composition
- Regulating cell volume
- Regulating concentration of metabolic fuels and building blocks
- Extruding waste products and toxins
- Generation of ionic gradients necessary for the electrical excitability of nerves and muscle
What affects the permeability of a cell’s membrane to a polar molecule?
The concentration of proteins that are capable of transporting the polar molecule by facilitated transport.
What is the role of Band 3 protein in erythrocytes?
It is an anion exchanger in the erythrocyte membrane (amongst other cells). It exchanges Cl- for HCO3-.
What are the three theoretical models for facilitated transport?
- Protein pores (channels)
- Carrier molecules (ping-pong)
- Protein flip-flop (unlikely thermodynamically)
Describe three types of gated channels
- Ligand gated ion channels - open or close in response to a ligand binding to a receptor site.
- Voltage gated ion channels - open or close in response to a change in potential difference across a membrane.
- Gap junctions (connexin) - closed when the cellular calcium concentration goes above 10 micromolar or the cell becomes acidic.
What is the difference between active transport and passive transport?
Active transport of a molecule across a membrane requires energy whereas passive transport is spontaneous. The free energy change of a molecule across a membrane is determined by the transported species’ concentration gradient and (when the species is charged) the electrical potential across the membrane.
How is energetically unfavourable transport (against a concentration and/or electrical potential gradient) acheived?
By coupling to an energetically favourable process, either:
- Directly via use of energy from ATP, electron transport or light
- Indirectly via use of above sources of free energy
What is secondary active transport?
When the transport of one substance is linked to the concentration of another (which has been set up by active transport).
List gradients of substances that membrane transporters may utilise for secondary active transport
ATP, phosphenolpyruvate, Na+, H+, light and high-potential electrons.
What is the role of the Na+/K+ pump?
Sets up Na+ and K+ gradients.
What secondary active transport processes does the Na+/K+ pump enable?
- Ion homoeostasis
- Intracellular Ca2+ ion concentration
- Intracellular pH
- Cell volume
- Ion gradients underpinning resting membrane potential
- Nutrient uptake
Give examples of hydrophobic molecules that can diffuse across the lipid bilayer
oxygen, carbon dioxide, nitrogen, benzene and steroid hormones
Give examples of small uncharged polar molecules that can diffuse across the lipid bilayer
water, urea and glycerol
Give examples of large, uncharged polar molecules that cannot diffuse across the lipid bilayer
glucose
sucrose (galactose-glucose dimer)
Give examples of ions that cannot diffuse across the lipid bilayer
H+, Na+ etc… all of them!
What does passive transport depend upon?
Permeability and concentration gradient
How much of the basal metabolic rate of a cell is spent on the sodium pump?
Approximately 25%!
What is the main thing that is responsible for setting up the resting membrane potential?
K+ diffusion through channels NOT the Na pump which generates ~-5-10mV (cf to -70mV or resting membrane potential).
What transporter is affected in cystic fibrosis?
A chloride transporter named CFTR.
How does the toxin produced by cholera cause diarrhoea?
It binds to G proteins and prevents them catalysing GTP -> GDP and Pi, therefore they remain active for longer. This means that higher levels of cAMP are produced and PKA is over-activated. PKA phosphorylates CFTR and leads to ATP-mediated efflux of chloride ions into intestinal lumens, drawing out water and causing diarrhoea.
Why is maintaining the Ca2+ gradient so important?
High Ca2+ concentration are toxic to the cell. Small changes in intracellular Ca2+ concentration are used for cell signalling.
