Equilibrium
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Where does equilibrium happen in pharmacy ?
In the body eg drug ionisation, binding and metabolism In the bottle eg drug degradation, stability and formation chemo In the lab eg drug synthesis
What symbol is used to represent an irreversible reaction ?
Double arrow
What is the equation for weak acid dissociation?
HA = H+ + A-
What does the double arrow mean ?
It means both forward and backward reactions occur
Is the HA in the weak acid dissociation ionised or unionised?
It’s unionised
Is the A- in the weak acid dissociation ionised or unionised?
Ionised
Why is aspirin absorbed mainly in the stomach ?
Aspirin is made out of carboxylic acid so is a acid The acid dissociation HA is unionised whereas the A- is ionised The stomach has a low PH so aspirin remains mostly unionised which means it can pass through the phospholipid membrane for absorption due to fatty acids being uncharged Whereas in higher PHs like the intestines the molecule becomes ionised so less lipid soluble so cannot be absorbed
What are plasma proteins?
Proteins present in the blood plasma
What is the reversible reaction of a drug and a plasma protein?
Drug and plasma protein = ( double arrow ) drug protein complex
What do drugs bind to in the body ?
Plasma proteins
What does the reversible reaction of a drug protien complex show ?
Drug exists as bound or free molecules (not parts of one molecule) Only free (unbound) drug can cross membranes, bind receptors, be metabolised, or excreted Bound drug acts as a reservoir, releasing drug to maintain equilibrium
What does the forward reaction of the drug protien complex show ?
Shows the binding process
What dyes the reverse reaction of the drug protien complex show ?
The dissociation (release) of the drug
What does it mean when you have a lot of protein binding to the drug ?
• A large proportion of drug is bound to plasma proteins • Less free (active) drug is available initially • Slower distribution of drug into tissues • Reduced metabolism and excretion • Acts as a reservoir, resulting in a longer half-life
What do lab tests measure ?
They measure the total concentration of both bound and unbound drug and helps is understand the fraction which has a therapeutic effect
What does non selective nature / drug drug interactions mean ?
This is when more than 2 drugs eg the substrate competes the same protein which increases the free the concentration of the drug which increases the toxicity risk
What is warfun is replaced by another drug ?
Leeds to bleeding risk
What is the equation for Kc/K ?
Kc = [products] ^ number in front of balanced equations/ [ reactants ]^ number in front of balanced equation
What does a large K/Kc mean ?
You decrease the concentration of reaction so the reaction favours the products
What does a small K/Kc mean ?
You increase the concentration of products so the reaction favours the reactants so equilibrium shifts to the reactants
What is K/Kc dependent on ?
Temperature. K doesn’t change unless temperature changes
Why does aspirin smell of vinegar?
Overtime , Aspirin hydrolyses and degrades in the presence of moisture The reversible reaction is aspirin plus H20 = salicylic acid and acetic acid The acetic acid smells like vinegar
Why do patients with chronic obstructive pulmonary disorder struggle with O2 transport ?
COPD causes reduced O₂ in the alveoli (low alveolar PO₂) • Less O₂ diffuses into the blood • Haemoglobin binding is a reversible equilibrium: Hb + O₂ ⇌ HbO₂ • With reduced O₂, equilibrium shifts to the left, forming less oxyhaemoglobin • Result: reduced oxygen loading and transport
What are the 4 conditions that affect the position of equilibrium?
Pressure Temperature Catalyst Concentration