ch 3: lipids, membranes, and the first cells - bio 1801

lipids

carbon-containing compounds found in organisms → building phospholipids

• nonpolar and hydrophobic

• micelles and bilayers: hydrophillic heads interact with water, tails interact with each other

hydrocarbons

molecules that contain only carbon and hydrogen (both nonpolar)

fatty acid

hydrocarbon chain bonded to a carboxyl functional group (slightly acidic properties)

• 14-20 carbon atoms

• strong hydrophobic and nonpolar molecule

saturated fatty acid

hydrocarbon chains with only single bonds between carbons

• animal sources, solid form at RT, waxes have long tails

unsaturated fatty acid

hydrocarbon chains with one or more double bonds between carbons

• veg. sources, liquid at RT

• trans fats: synthetic process, linked to heart disease

hydrogenation

breaks double bonds and adds hydrogen bonds (but causes trans fatty acids) to saturate the double bonds

• double bonds switch to single bonds back and forth

  • leaves double bonds behind that were created → cys/trans fats

steroids

• 4 ring structure bulky lipids

• DIFFER by functional groups attached to carbons in the ring

• ex: estrogen and testosterone

  • cholesterol: component of PLASMA MEMBRANE, need it add to bilayer for homeostasis in fluidity)

fats

three acids linked to glycerol - triglycerides, formed by dehydration reactions

• role = ENERGY STORAGE

• crucial for body for insulation/cushion of organs and vitamins

• energy is more concentrated than proteins and carbs

• slow to digest and delays hunger

ester bond

holds polymer together

• hydrogen from hyroxyl group from glycerol and hydroxyl group from carboxyl group form the water

phospholids

glycerol linked to phosphate group bonded to a charged OR polar molecule

• two hydrocarbon chains, two fatty acids linked to glycerol

• role = FORM CELL MEMBRANES

• amphipathic: hydrophobic and hydrophillic regions

• hydrophillic polar heads face water, hydrophobic nonopolar tails face each other like an OREO

fats vs phospholipids in hydrophilic region

• fats: chains are HYDROPHOBIC and don’t interact with water

• phospholipids: phosphate containing group is HYDROPHILIC and interacts with water

phospholipid bilayer

• hydrophilic heads in each layer face out and form spontaneously 

• regulate and modify for different environmental conditions

• selective permeability: small/uncharged/nonpolar = QUICK, charged/large/polar/ionic past SLOW or don’t at all

  • ex: CO2 passes quickly

fatty acids and permeability

• SHORT and UNSATURATED hydrocarbon tails (bends) = higher permeability and fluidity

• LONG and SATURATED hydrocarbon tails = lower permeability and fluidity

• phospholipids are in constant lateral motion but rarely flip

cholesterol molecules and permeability

decrease membrane permeability, stabilize fluidity whether temperature goes up or down

temperature and permeability

• lower temp = lower permeability and fluidity

• higher temp = higher permeability and fluidity

concentration gradient

caused by difference in solutes from an area of high concentration to low concentration

diffusion

net movement occurs from HIGH concentration to LOW concentration regions

• increases entropy (disorder)

• spontaneous (high concentration frequently hitting bilayer)

• different rates across selectively permeable membrane = EQUILIBRIUM back and forth at equal rates

osmosis

diffusion of water across a membrane

• start with more solute on one side than other that cannot cross selectively permeable membrane

• water undergoes diffusion of LOW concentration to HIGH concentration due to pressure from gravity to equalize

why osmosis happens

cells have concentration gradients across their membranes due to selective permeability and transport

hypertonic

higher concentration compartment (where water moves towards)

hypotonic

lower concentration compartment

isotonic

solute concentrations are equal

• less solutes = more concentrated

• more solutes = less concentrated

osmosis in animal cells

• must maintain a balance between extracellular and intracellular solute concentrations

• no cell wall, harder to maintain size and shape

• osmosis can SHRINK (net flow of water out) or BURST (net flow of water into) membrane-bound vesicles 

• hypotonic = shrinking = crenation of vesicles

• hypertonic = burst = osmoticlysis of vesicles (caused by imbalance that causes excess water to diffuse in)

osmosis in plant cells

• a cell wall prevents major changes in cell size

• hypotonic = tugor pressure (pushes plasma membrane against cell wall to maintain size and shape)

• hypertonic = plasmolysis (membrane shrinks and pulls away from cell wall so water leaves plant cells INTO surrounding fluid)

passive transport

substances diffuse across a membrane with no energy

• simple: water goes right through bilayer or not depending on its own concentration gradient with no protein/carrier

• facilitated: requires a protein/carrier

• when moving through channels or carriers, it moves substances WITH their concentration gradient

active transport

substances diffuse across a membrane with external energy

• it moves substances AGAINST their concentration gradient

• requires protein

• ATP

why proteins are needed

• plasma membranes contain as much protein as phospholipids

• CAN insert into a membrane

• their side chains can be polar, charged, or nonpolar (amphipathic) = integrate into lipid bilayers

  • a hydrophobic region can be anchored into bilayer

  • polar and charged = hydrophilic ends stick out

  • nonpolar = hydrophobic

the fluid-mosaic model

the membrane structure is a fluid, dynamic mosaic of PHOSPHOLIPIDS AND PROTEINS

• transmembrane proteins

• peripheral membrane proteins

transmembrane proteins

integral(one side to the other) proteins span across membrane (bilayer) and are stuck in it and span the membrane, often using transport

• channels: pores (facilitated diffusion)

• carriers: change shape (facilitated or active)

• pumps: carriers using energy (active transport) (ex: sodium-potassium pump transports NA+ and K+ against their gradients)

peripheral membrane proteins

bind to the membrane without passing through it and locate on the interior or exterior of the cell

electrochemical gradients

when ions build up on one side of a plasma membrane

• making a concentration and charge gradient, ions diffuse their own gradients

• ion channels: proteins that allow diffusion of ions

• one side has high concentration and net positive charge and gradient goes towards low concentration and net negative charge

channel selectivity

• channel proteins’ residues facing inside the pore are HYDROPHILLIC

• each channel protein permits only a PARTICULAR type of ion/small molecule to pass

• aquaporins: permit water to cross the plasma membrane

• different channels = different things across membrane

gated channels

open or close in response to a signal

• closed channel: inside of membrane is negatively charged and gate blocks ions from entering

• open channel: inside is positively charged and filter allows only ions to pass

• binding of a particular molecule

• Change in electrical voltage across the membrane

• Outside (net positive), inside (net negative) = membrane is polarized

  • Positive charges (K+) coming out will reestablish polarization

carrier proteins

• facilitated diffuson

• changes shape to transport solutes across a membrane

• transport larger molecules like glucose

• lipid bilayer has limited permeability to glucose, so the GLUT-1 carrier increases permeability by changing shape

sodium-potassium pump

Pump is not energized yet -> loading NA+ -> sodium binding with ATP ->  phosphate group attaches with carrier -> carrier opened from inside NOW to outside -> sodium washes out -> K+ pulled from outside  -> phosphate group disassociates with K+ -> shape change -> pocket opens inside and K+ goes in

• how a pump produces an electrochemical gradient

secondary active transport

electrochemical gradients power and utilize movement of ANOTHER molecule against its gradient

• ATP not directly used

• pre-existing gradients represent potential energy

4 mechanisms of membrane transport

• passive: 1) simple diffusion, 2) facilitated diffusion through selective channels/carriers

• active: 3) primary active transport (ATP), 2) secondary active transport (electrochemical gradient)