Structure of cellular membranes Flashcards
What are cell membranes essential for?
Drug absorption
What is meningitis and what is it due to?
- An infection of the meninges of the brain
- Due to viral or bacterial infection
What are the meninges and what are they associated with?
- Membranes which cover the brain neural tissues
- Associated with a physiological/anatomical feature of the brain
Describe the blood brain barrier.
- A selectively permeable membrane
- Acts as a barrier between blood stream and the extracellular space of the brain; neurons
- Regulates the passage of molecules into the microenvironment of the neurons of the brain
Describe the gastrointestinal wall.
Comprised of layers of smooth muscle in longitudinal and circular orientation which helps with continued bowel movements and the movement of digested material out and along the gut.
State the steps of drug absorption.
- Ingestion
- Disintegration
- Dissolution - reasonably water soluble
- Absorption
For a drug to exhibit good water solubility what must it include?
- Several polar chemical groups i.e hydroxyl groups- hydrogen bonds
What can drugs that are soluble able to do?
Dissociate to create a proton and a negative ion.
Why are drugs described as poor electrolytes?
Because they do not dissociate rapidly - drug solubility is an issue
Describe modifications of the small intestine which make it good for absorption.
- The lining of the small intestine have finger-like projections called vili which increase SA
- They are supplied with blood which maintains conc. gradient
- Receive a lymph supply (lacteals) which is important with the absorption of fat soluble drugs
- Have a nerve supply
What do phospholipid units have?
- A water soluble polar head remains on the outside of the membrane
- The fatty acid chains remain on the inside of the membrane structure
How does lipid/water solubility affect the absorption of a drug?
Drug needs to be:
- Partly water soluble to penetrate the hydrophilic phosphate head
- Slightly lipophilic to pass through the fatty acid tails
How does molecular size affect drug absorption?
- Size of the drug molecule directly affects passive diffusion across a membrane
- Smaller molecules more likely to pass
How does the degree of ionisation affect drug absorption?
- Ionisation creates a chemical dipole which attracts water molecules
- Therefore, ionisation increases water solubility
- Ionised form of drug is water soluble in hydrophilic areas of membrane
- Ionisation reverses and drug becomes fat soluble when it enters fatty acid chain section
Which factors affect the passive diffusion of a drug?
- Surface area of the membrane
- Thickness of phospholipid bilayer; thinner membranes allow easier passage of a drug
- Concentration gradient i.e. more conc. solution of a drug will pass across a membrane to a lower conc.
