Bio lesson 6

molecular composition of cell membrane

-carbohydrates on outer surface of plasma membrane form a sugar coat (glycocalyx) which provides protection and facilitates cell to cell recognition

-phospholipids form the fundamental structure of membrane

-proteins provide membrane-specific functions

phospholipid structure

-amphipathic bc it has both hydrophillic/phobic regions

-polar hydrophilic head

-2 non polar hydrophobic tail

-C to C double bonds creates a kink in the chain

-held tg by non covalent interactions

phospholipid bilayer

-small nonpolar molecules can easily diffuse through bilyer

-small uncharged polar molecules diffuse, but slowly

-large uncharged polar molecules and ions are unable to penetrate through hydrophobic core of bilayer

-selective permeability leads to chemical differences btwn 2 sides of bilayer

fluid mosaic model

-extracellular faces at the top

-cytosolic face at the bottom

-cholesterol is a lipid molecule. since it has an amphipathic structure, it can inset among the phospholipids of the bilayer

cell membranes are 2D fluids

-phospholipids and proteins more laterally within membrane due to relatively weak hydrophibic interactions

-therefore, the membrane represents a 2D fluid

factors that reduce membrane fluidity

-lower temp

-longer fatty acid chains

-saturated fatty acid chains

integral membrane proteins

-embedded in phospholipid bilayer, must destroy bilayer to isolate

-transmembrane proteins

1. single pass and multipass proteisn

2. contain transmembrane domain= 20-25 hydrophobic amino acids that forms alpha helix

3. pass directly through the lipid bilayer and typically have a portion exposed on each side of membrane

-lipid anchored

1. instead of passing through lipid bilayer, covalently attahced to lipids or fatty acids that are part of the bilayer

2. only found on one side of bilayer

peripheral membrane proteins

-non covalently bound to transmembrane proteins/phospholipid heads

-can be removed from membrane w/o destroying bilayer

-present on one side or other

cell membrane are asymmetric

-2 faces are differesnt in composition and function

-each face has:

1. diff types of phospholipids

2. diff types of proteins

3. diff domains of transmembrane proteins

4. only outer face has carbohydrates

-ex of symmetry: peripheral protiens, no counterpart on the outside. both sides of membrane have diff functions, so they have diff structure

membrane proteins

-receptors

-enzymes

-cell identity

-cell adhesion

-transport

-cytoskeletal attachement

receptors

detects signal molecules and intiates cells response to signal

cell identity

give each cell an identity and allow one cell to recognize another (carbs covalently bonds w/ a protein/phospholipid)

enzyme

promote chem reactions

cell adhesion

allow one cell to attach to another or to the extracellular matrix

cytoskeletal attachement

allow cells to transmit changes in cytoskeleton to changes in plasma membrane, allowing cytoskeleton to control cell shape

transport

facilitates mvmt of small hydrophilic molecules across membrane

transport proteins

-cells use transport proteins to move ions and hydrophilic/larger molecules

-also present in organelle membranes

channel proteins

-type of transport protein

-create a pore in the membrane

-many involved in ion transport

-function by passive transport: molecules move down concentration gradient

-can be specific for certain molecules

-some are gated: these open/close in response to a signal

carrier proteins: passive transporters

-carry out facilitated diffusion

-work by a ping pong mechanism

-bind and transport specific molecules

carrier proteins: active transport

-use energy to create/maintain a concentration gradient

-molecules are transported or “pumped” up against their concentration gradient

-many active transporters are ATPases. these use energy from ATP hydrolysis to power molecules

-hydrolyze ATP, leads to ADP and inorganic phosphate, use energy released to power transport

-ex: sodium potassium pump

coupled transport

-ion gradient represent energy source for active transport

-antiporters: use the energy from ion moving down its concentration gradient to transport another molecule in the opposite direction (AKA exchangers) (diff direction transport)

-symporter: ion and other molecule being transported move in the same direction across membrane. both molecules are moving into the extracellular environment into the cell (same direction trasnport)

osmosis

-like solutes, water tends to move down its concentration gradient

-solute concentration may differ on 2 sides of cell membrane: this influences mvmt of water

-osmosis is the mvmt of water down its concentration gradient (toward a higher solute concentration)

-water diffusion through bilayer is limited so cells contain aquaporins to facilitate mvmt

osmotic pressure

force needed to stop osmosis

isotonic

solute concentration outside=solute concentration inside cell

water diffuses into/out of cell at same rate

hypertonic

the concentration of solutes is higher compared to inside the cell

hypotonic

solute concentration outside the cell is lower compared to inside the cell

endocytosis

-allows eukaryotic cells to ingest “larger” material from extracellular environment

-involves inward budding of plasma membrane to collect macromolecules or even cells. collected into vesticles, transported to lysosomes where its digested into smaller units

-3 types: pinocytosis, phagocytosis, receptor-mediated endocytosis

pinocytosis

cell drinking

constant inward budding of plasma membrane to form endocytic vesicles

receptor mediated endocytosis

-receptor protein binds specific molecule (target)

-receptor and target are collected are collected in clathrin-coated vesicles

-provides selective uptake of necessary molecules (ex: LDL)

phagocytosis

-selective englufment of another cell