2.1.5 Biological membranes Flashcards
permeability definition
ability to let substances pass through
roles of membranes
partially permeable barrier between cell and environment or organelle and cytoplasm (to maintain conditions within membrane)
site of chemical reactions
site of cell communication
fluid mosaic model
fabric of membrane made up of phospholipid bilayer with protein randomly arranged (mosaic pattern)
lipid and protein molecules can change places with each other, giving fluidity
plasma membrane structure
phospholipid bilayer (7 nm) cholesterol spanning bilayer between phospholipids (regulates fluidity and gives stability, makes membrane less permeable to water molecules and ions - “plugs gaps”)
integral proteins: channel proteins (allows ions to pass through membrane) carrier proteins (changes shape to carry specific molecules across membrane)
peripheral proteins:
only on one side of lipid bilayer
lipid-bound protein (found enclosed within lipid bilayer)
glycocalyx (carbohydrate group attached to lipid/protein):
glycoproteins
glycolipids
how temperature affects fluidity of membrane
higher temperature = more fluidity
more heat energy in environment = more kinetic energy and movement of phospholipids = greater distance between between phospholipids = less IM forces
lower temperature = lower fluidity
less heat energy in environment = less kinetic energy and movement of phospholipids = closer together = more IM forces
factors affecting fluidity of membrane
temperature
cholesterol
ratio of saturated and unsaturated phospholipids
how cholesterol affects fluidity of membrane
higher temperatures = lowers fluidity
shape pulls phospholipids closer together = more IM forces
lower temperatures = higher fluidity
shape distances phospholipids from each other = less IM forces
also prevents water-soluble molecules diffusing through membrane
how ratio of saturated and unsaturated phospholipids affects fluidity
saturated = no double = straight chains = can get closer together = more IM forces = less fluidity unsaturated = double bonds = more awkward shape (not straight) = can’t get closer together = less IM forces = more fluidity
diffusion definition
passive net movement of molecules from an area of high concentration to an area of low concentration, down the concentration gradient
only gases and liquids
doesn’t need ATP (only kinetic energy)
facilitated diffusion definition
movement of molecules from an area of high concentration to an area of low concentration across a partially permeable membrane via protein channels or carriers
doesn’t involve metabolic energy (ATP)
how diffusion works
all molecules have kinetic energy and move freely
higher concentrations = molecules more likely to bump into each other
eventually spread further from each other (towards area of lower concentration) and become evenly dispersed (reach equilibrium)
still move randomly but remain evenly dispersed (no net diffusion)
factors affecting diffusion rate
stirring/moving/temperature diffusion distance surface area concentration gradient size of diffusing molecule
how stirring/moving/temperature affects diffusion rate
higher stirring/moving/temperature = higher KE in molecules = higher rate of random movement = higher rate of diffusion
how diffusion distance affects diffusion rate
thicker membrane = higher diffusion distance = lower rate of diffusion
how surface area affects diffusion rate
higher surface area = more area for more diffusion at one time = higher rate of diffusion
how conc. gradient affects diffusion rate
steeper gradient = higher rate of diffusion
how size of diffusing molecule affects diffusion rate
smaller molecules/ions diffuse faster than larger molecules
why water can still diffuse through plasma membrane
polar and insoluble (shouldn’t pass through phospholipid bilayer)
water can still pass through at high concentrations (less negative water potential) via direct diffusion (osmosis)
aquaporins via facilitated diffusion
active transport definition
movement of particles against concentration, using ATP and involves specific protein carriers
how active transport works
molecule/ion to be transported binds to receptors in channel of carrier protein outside of cell
ATP binds to carrier protein (inside of cell)
hydrolysed into ADP and phosphate ion
binding of phosphate molecule to carrier protein causes it to change shape (conformational change)
molecule/ion is released into insides of cell
phosphate molecule released from carrier protein and recombines with ADP to form ATP
carrier protein returns to its original shape
bulk transport definition
active transport of large molecules/cells
prefixes for bulk transport
exo-: out of a cell
endo-: into a cell
phago-: solids
pino-: liquids
where energy is required into endocytosis
to move the membrane (pseudopodium) around to form a vesicle in endocytosis
where energy is required into exocytosis
move vesicles to cell surface membrane to fuse in exocytosis
endocytosis method
molecule binds to receptor on cell surface membrane
causes cell surface membrane to fold in (invaginates)
cell surface membrane fuses with itself (forms vesicle, requires ATP)
vesicle moves through cytoplasm to where it is needed in cell
exocytosis method
vesicle moves towards cell surface membrane on microtubules (requires ATP)
vesicle fuses with cell surface membrane (requires ATP)
molecules ejected from cell
osmosis definition
passive net movement from region of high water potential to a region of water potential, down water potential gradient across a partially permeable membrane (continues until equilibrium reached, water potential on each side of membrane is equal)
measured in kPa
pure water = 0 kPa
water potential definition
measure of tendency of water molecules to diffuse from one region to another
water moves towards region with more negative water potential
animal cell / protoplast in hypertonic solution
more water moves out of cell than in
shrivels (crenation)
animal cell / protoplast in isotonic solution
no net movement of water
structure stays the same
animal cell / protoplast in hypotonic solution
more water moves in than out
bursts (cytolysis)
plant cell in hypertonic solution
more water moves out than in
plasma membrane pulls away from cell wall (plasmolysed)
plant cell in isotonic solution
no net movement of water
overall tissue becomes flaccid
plant cell in hypotonic solution
more water moves in than out
plasma membrane pushes against cell wall (turgid)
cell wall prevents cell from bursting
permeability and structure of plasma membrane as temperature increases
fluidity increases
permeability increases
membrane-embedded proteins reaction rates change (increases / may denature)
more fluidity may affect invagination of plasma membrane during endocytosis
may change cell signalling by releasing chemicals via exocytosis
cholesterol helps reduce increase in fluidity
structure of plasma membrane as temperature decreases
unsaturated fatty acids maintain membrane fluidity (kinks in tail push adjacent phospholipid molecules away)
cholesterol maintains fluidity (prevents phospholipid molecules from packing together too closely)
cell signalling definition
communication between cells in order to:
coordinate their actions
to trigger actions/responses in the cell
cell recognition
glycoproteins
interact with water molecules to stabilise membrane/cell
antigen for cellular recognition
binding site for cell aggregation
receptor in cell signalling
binding site for chemicals e.g. hormones/drugs
glycolipids
recognition site for cellular recognition
interact with water molecules to stabilise cell surface membrane
helps cell attach to other cells to form tissue (aggregation)
