UCL MBBS Year 1 FHMP 100% PASSED

What limits cell size? - ANS Surface area to volume ratio! Larger cells are less efficient in moving nutrients and waste across the cell membrane. As cell size increases, distance for _________ to diffuse from the nucleus increases. - ANS As cell size increases, distance for TOXINS, IONS, etc. to diffuse from the nucleus increases. What can nano-devices be used for and why? - ANS They can interact with the receptors on the surface of cells and inside cells because they are 100-10,000 times smaller than human cells. How much of the body's lean mass is water? - ANS Around 2/3 What are hydrophilic substances? - ANS Ones that are polar (e.g. Na+, glucose, many proteins) and so are able to dissolve in water What are hydrophobic substances? - ANS Ones that are non-polar (e.g. fats and waxes) so cannot dissolve in water What are amphiphilic substances? - ANS Ones that contain polar and non-polar regions (e.g. phospholipids - have hydrophilic phosphate head and hydrophobic fatty acid tails) What is intracellular water? - ANS Water found inside the cells: makes up 2/3 of the body's water. Where is extracellular water found? - ANS Outside the cells: - Interstitial - outside blood vessels and bathes the cells - Plasma and transcellular - fluid in spaces such as the brain ventricles, peritoneal cavity and joints What is the purpose of the plasma membrane? - ANS Provides a hydrophobic barrier separating intracellular and extracellular components What sorts of molecules can freely pass the plasma membrane? - ANS Lipid-soluble molecules, because they dissolve in the phospholipid membrane. What is osmotic pressure? - ANS MRT (molarity x gas constant x temperature) plays an important role in transport of molecules across membranes depends on the number of particles present per unit volume of solvent (NOT CHEMICAL MAKE-UP) What effect do salts have on osmotic pressure? - ANS Exert double osmotic pressure as they have two constituent ions Define osmolarity - ANS The number of moles of solute particles per LITRE of solution (preferred by scientists) Define osmolality - ANS The number of moles of solute per KILOGRAM of solution (preferred by clinicians) What is the value of blood plasma osmolality? - ANS precisely 295 mOsmol/kg (around 300mOsmol/kg) What makes up blood plasma osmolality? - ANS Majority is Na+, Cl- and HCO3- Glucose contributes around 10 mOsmol/kg Plasma proteins contribute the least (around 1 mOsmol/kg) What is meant by isosmotic? - ANS Two solutions have the same osmolality. Why is intracellular and extracellular fluid normally isosmotic? - ANS Under normal conditions, the volume of cells is constant so intracellular fluid has the same osmolality as extracellular fluid. Fluids which are isotonic are always __________ but the reverse isn't necessarily true - ANS Fluids which are isotonic are always ISOSMOTIC but the reverse isn't necessarily true What type of cells constantly have to cope with the osmotic phenomenon? - ANS Erythrocytes (RBCs) What is used to dilute blood and why? - ANS 0.9% NaCl solution (rather than pure water) to ensure that cells do not lyse or crenate. What is meant by crenation? - ANS Water leaves the RBC by osmosis, so it shrinks and shrivels up. What happens to cells placed in hypotonic solutions? - ANS The solution is more dilute, so they lyse/burst. What happens to cells placed in hypertonic solutions? - ANS The solution is more concentrated so the cells crenate. What is meant by tonicity? - ANS A relative measure of the osmotic pressure of two solutions on either side of a semi-permeable membrane. How does homeostasis enable higher organisms to survive in and inhabit a range of conditions? - ANS Because, unlike unicellular organisms, they aren't forced to only live in places where conditions are correct for that cell as they have greater control over their body.