CELULAR / MISC VIVAS
2011-1, 2005-2, 2003-1
Please outline the different ways in which a substance can cross a cell membrane
Passive
- Diffusion
- Facilitated diffusion (via a carrier protein) w/ uniports for one substance, symports require two
together (Na + glucose), antiports exchange one for another (eg Na for K)
- Ion channels - ligand, voltage, mechanical gated
Active
- Exocytosis
- Endocytosis (phagocytosis, pinocytosis, caveolae dependant/independent)
- Primary active transport (e.g. the Na/K ATPase)
- Secondary active transport (e.g. Na+ coupled to other substances like glucose, or amino acids)
Can you please explain the process of secondary active transport?
- The movement of an ion down its electrochemical gradient provides energy to transport another
substance against its electrochemical gradient
- Example – Na/glucose, Na/aminoacids
- The glucose and amino acids are transported against their electrochemical gradient
- The energy is provided by the gradient of the Na which is maintained by the Na/K ATPase
2011-1
What is the difference between diffusion and osmosis?
Diffusion
- Net flux of solute particles down a concentration gradient
- From area of high to low concentration
- Time for equilibrium proportional to square of distance
- Rate proportional to cross-sectional area & gradient (Fick’s law)
Osmosis
- Net flux of solvent across a membrane to an area of higher concentration of solute to which
membrane impermeable
- Osmotic pressure given by nRT/V – property of solution (n = no. particles, R gas constant, T =
temp, V = volume… so related to number of particles in a volume = osmoles)
Define “Tonicity”
- Osmolality of a solution relative to plasma (0.9% saline is “isotonic”)
What is the genesis of the membrane potential?
- Differences in concentration gradient and electrical gradient of major cations (Na, K), across
the impermeable cell membrane
- Maintained by Na-K ATPase
- 3 Na out for each 2 K in so electrogenic
2003-1
Describe the synthesis and metabolism of cAMP
1. Formed inside the membrane
2. ATP is converted to cAMP via adenyl cyclase
3. Metabolised by phospho-diesterase
Discuss the function of cAMP.
1. Intracellular second messenger
2. Stimulate protein synthesis
3. Activate an intracellular enzyme system in the neurone
2007-1
How is water distributed through the body compartments?
-
TBW is 60% of body weight
ICF 2/3 of TBW = 40%
ECF 1/3 of TBW = 20%
Interstitial 3⁄4 of ECF = 15%
Plasma 1⁄4 of ECF = 5%
Total blood volume = 8%
How do age and gender affect total body water?
- Decreases with age
- Higher in males
2005-2
How do cells communicate one to the other?
- Cell to cell via gap junctions
- Chemical messengers in ECF
- Neural (neurotransmitters at synapses)
- Endocrine (hormones and growth factors)
- Paracrine (products of cells diffuse to neighbours)
- Autocrine = cell secretes messenger that acts on itself
- Juxtacrine = molecules attached to membrane that attaches to another cell.
- Same chemical can function in several ways
How do receptors respond to variations in messengers?
- Receptors change with physiological variations: messenger in excess -> decrease receptors
(down regulation, interalisation, desensitisation); deficient messenger -> increase receptors (up
regulation).
Hew do messengers act?
- Via ion channels (Ach, nicotinic, noradrenaline)
- Transcription of mRNAs (steroids, thyroid hormone)
- Activation of phospholipase C (angiotensin II, noradrenalin. vasopressin)
- Production of cAMP (noradrenalin)
- Production of cGMP (NO)
- Increased activity tyrosine kinase (insulin)
- Increased activity serine or threonine kinase (TGF, MAPK)
2005-2
What are the phases of protein synthesis
- Transcription of mRNA
- Post transcriptional modification of mRNA w/ translation of mRNA to AA chain along a ribosome
using tRNA
- Post translational modification of the protein in endoplasmic reticulum by hydroxylation,
carboxylation, glycosylation, phosphorylation, cleavage and folding
Describe the process of secretion of proteins from cells
- Polypeptide sequences are cleaved off, eg prohormones to hormones
- Some proteins have leader sequences that target endoplasmic reticulum and are secreted by
exocytosis
- Others are secreted from cytoplasm via ATP dependent membrane transporters