Class Notes (836,153)
Canada (509,662)
Biology (2,253)
BIOL 130 (305)

Biological Membranes.docx

7 Pages
Unlock Document

BIOL 130
Heidi Engelhardt

Biological Membranes  membrane structure o membrane lipids (ref: Unit 2b Biomolecules Part 3, slides 19-36) o membrane proteins  functions, peripheral vs integral – transmembrane helices, pores o membrane carbohydrates (sugars)  glycocalyx  membrane transport o What drives movement of solutes? diffusion, osmosis, tonicity, turgor pressure o What determines what can cross a membrane? size, polarity, charge  for substances that can’t diffuse through bilayer:  membrane transporter proteins o carriers versus channels o passive versus active transport  ion channels – chemical & electrical gradients, selectivity, gating  carrier proteins  down gradient - facilitated diffusion  against gradient - active transport; Na+/K+-ATPase  coupled transport  membrane proteins and gene expression MEMBRANE STRUCTURE  all cells have a plasma membrane o encloses contents of entire cell  eukaryotic cells have membrane-bound organelles o nuclear ‘envelope’ o double membranes of mitochondria and chloroplasts o endoplasmic reticulum o Golgi apparatus o lysosomes / vacuoles o transport vesicles o et al.  Functions o scaffold for biochemical activities o provide a selectively permeable barrier • prevent unrestricted exchange of molecules o transport solutes • exchange of molecules across the membrane o respond to external signals - signal transduction • signals travelling from a distance or from nearby cells o energy transduction - conversion of one form of energy into another o compartmentalization (eukaryotes) • create separate environments for different activities  Phospholipids o Polar head group + phosphate group + glycerol + 2 fatty acid chains o Named by head group, not length of chain or saturation of chain (phosphatidyl choline (PC))  Membrane Fluidity o membrane fluidity : how easily lipid molecules move within a membrane leaflet o alignment of phospholipid tails • tightly packed tails  membrane more viscous, less fluid • freely moving tails  higher fluidity o influenced by: • length of fatty acids • from 14-24 carbons, 18-20 carbons most common • degree of saturation of fatty acids  # double bonds • typically one saturated fatty acid and one with one or more double bonds o all membrane lipids are amphipathic o cholesterol • under physiological conditions, cholesterol makes membrane stiffer – less fluid • cholesterol can make up to 50% of plasma membrane lipid in some animal cells o temperature • transition temp = temp at which membrane ‘gels • at and above ‘room temperature’ phospholipids in membranes are fluid, and move freely • as temperature drops, fluidity (and permeability) decreases • at very low temps, hydrophobic tails pack together and membrane ‘gels’ (solidifies) o fluid state must be maintained for normal cell function • change composition of membranes • alter phospholipids • desaturate fatty acids (to deal with cold) • change length of FA chains (yeast, bacteria) • adjust amounts of cholesterol (animals) • these mechanisms have been demonstrated in: • pond fish dealing with dramatic day / night temp differences • cold-resistant plants • extremophile bacteria living in hot springs • winter wheat preparing for autumn ↑ polyunsaturated FAs • sperm reduce their cholesterol just before fertilization …  Asymmetry of Membrane Lipids o Flipping is rare and controlled o the appearance of PS (lipid that is usually on cytosolic side) in outer ‘leaflet’ of membrane usually indicates that the red blood cell is going to die (through apoptosis) o Red Blood Cells and model organisms • best understood plasma membrane! • cells are inexpensive and available in large numbers • already present in single cell suspension • simple - no nucleus, no ER, no mitochondria, no lysosomes • very pure preps of plasma membranes • purified intact plasma membranes can be prepared by producing red blood cell ‘ghosts’ o Asymmetry is preserved during membrane transport (see diagram)  Membrane Proteins o Integral (transmembrane) • Go across whole membrane • Single pass transmembrane protein • Multi pass transmembrane protein o Peripheral • Lipid-linked • Protein-linked o Helices • Hydrophilic channels can be formed from several alpha-helices o Pores • Proteins folded into pleated sheets can form pores o Cells can restrict the movement of membrane proteins • Proteins holding membrane • Membrane proteins lock together (build tissues) • Tight junctions • Prevent different proteins from crossing (creates protein domains) • Creates seal between cells (prevents ‘leaking’)  Membrane carbohydrates o Eukaryotic cells are coated with sugars • Glycol calyx • Sugar on inside of organelles therefore sugars on outside of cell membrane MEMBRANE TRANSPORT  need to allow passage of certain substances in / out of cell o gases, ions, nutrients / waste products  lipid bilayers tend to block passage of polar (water-soluble) molecules  substances can enter a cell by … o passing directly through lipid bilayer o being transported across bilayer by membrane proteins acting as carriers or channels o being engulfed by the cell, avoiding passing through the membrane  What drives movement of solvents o Diffusion • dissolved solutes (molecules / ions in solution) are in constant, random motion • solutes will spontaneously ‘spread out’ (↑ entropy) until concentrations on all regions are equal • at that point – no NET flux (movement continues- don’t stop and stay still) • Dynamic equilibrium- random motion continues o Osmosis • diffusion of water across a semi-permeable membrane down its concentration gradient (toward a higher solute concentration) • once ‘water concentration’ equal on both sides, no net movement of water • water concentration depends on total concentration of osmotically active particles (solutes)- doesn’t matter what solutes they are (can be a mixture) • all ions, molecules dissolved in fluid • water is constantly moving through cell membrane in both directions • ideally, ‘osmotic tone’ (concentration of osmotically active substances) is equal inside and outside cell • intracellular and extracellular fluid are isotonic • if total solute concentration changes on either side, net movement of water will toward fluid with higher concentration of solutes
More Less

Related notes for BIOL 130

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.