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Membrane • Different membranes – All have similar functions & structures – Plasma membrane separates inside of cell from outside of cell – Other membrane define organelles to form compartments of eukaryotic cells • Forms a selectively permeable layer – Lets some things in or out but not all – Like a window screen Membrane • Phospholipid bilayer – Phosphate group facing outward • hydrophillic – Fatty acid tails face each other • hydrophobic • Proteins also in membrane – Lots of functions • Signaling • Transport • Enzymatic – Peripheral- on inside or outside surface – Integral- proteins stick out on both sides Permeability in membranes • Selectively permeable – Some move easily through the lipid layer • Non polar molecules (O2, alcohols) • Small polar molecules (H2O, CO2) • Move with (or down) a concentration gradient – From areas of high to low concentration – Some need help to get through • Like ions and larger polar molecules (sugar, a.a.) • Transport proteins assist – Channel proteins - allow passage of charged particles – Carrier proteins- transport specific molecules Diffusion Defined as passive movement of molecules down a concentration gradient from areas of high to low conc. This does not require energy - (passive) Eventually the molecules will be equally distributed (equillibrium is reached) Osmosis • Movement of water molecules from high to low concentration across a semi-permeable membrane • Or diffusion of water • Hypotonic solution outside cell – Low solute concentration – High water concentration – Water moves into cell • Hypertonic solution outside cell – High solute concentration – Low water concentration – Water leaves cell • Isotonic solution – Concentration of water equal on both sides of membrane – No change in concentration will occur Water in Plants • Plant cells normally in hypotonic environment – Central vacuole full – Puts pressure on cell wall – Cell doesn’t burst because of cell wall • Plants in hypertonic solution – Lose water and usually kills cell Passive transport • Involves movement of molecules across a membrane • From areas of high concentration to areas of lower concentration across a membrane – Spilled perfume example • Without energy Facilitated transport • Like passive transport but requires a membrane protein to help (facilitate) the movement • Does not require energy • Moves with concentration gradient Active Transport • Moves molecules across a membrane – Via a membrane protein – Can move molecules against their concentration gradient • From low to high concentration • Not easy to put spilled perfume back into bottle – Requires energy (ATP) Exo or endocytosis • Exocytosis- molecules exiting cell – Ex. Transport vesicle releasing contents outside cell • Endocytosis- molecules entering cell – Phagocytosis - cell eating • Ex. White blood cell eating bacteria – Pinocytosis - cell drinking – Receptor mediated endocytosis • Molecule binds to protein receptor Ex. Cholesterol in liver cells Cell signaling • Signal transduction pathway – How cells talk to each other – Reception • Outside molecule binds to receptor • Ex. Adrenaline to muscle cells – Transduction • Message relayed inside cell via chemicals – Response • Chemical action • Ex. Breakdown glycogen to release glucose
