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CELL MEMBRANE & TRANSPORT ACROSS MEMBRANE (Membranes (FUNCTION…
CELL MEMBRANE & TRANSPORT ACROSS MEMBRANE
Membranes
Fluid Mosiac Model
phospholipids and proteins exhibit lateral movement
proteins scattered in a patchwork arrangement
FUNCTION
regulate passage of substances into and out of cells (selectively permeable)
fat soluble & small, non polar substances can pass through (hydrophobic core)
through SIMPLE DIFFUSION
net movement of particles from high concentration to low concentration (down a concentration gradient)
does not require energy
affected by:
concentration gradient
distance molecules diffuse across
area which molecules can diffuse over
size (smaller is faster) and nature (soluble is faster) of diffusing molecules
polrar and charged substances transported by protein carriers and channels - facilitated diffusion & active transport
facilitated diffusion
net movement of particles down a concentration gradient through channel or carrier proteins (since polar and charged particles are insoluble in lipids)
proteins shield them from hydrophobic core
by transport proteins
are specific (one substance/ group of structurally similar substances)
can be saturated (because of limited amount of transport proteins)
can be inhibited by competitive inhibition (ref. enzymes)
channel proteins
water filled pore (hydrophobic lining)
some channels are gated ion channels (only open with appropriate signal_
carrier proteins
binding site can face interior or exterior of cell
change in form of protein (from facing interior to exterior or vice versa) triggered by binding and release of transported molecule
active transport
movement of ions or molecules across a membrane from a region of lower concentration to higher concentration (against concentration gradient)
through: transport proteins (protein pumps - type of carrier protein)
condition: requires expenditure of ATP
use:
allows cells to take up nutrients (even when intracellular concentration is greater than extracellular)
allows cells to get rid of waste products
process: by protein pumps (type of carrier protein)
protein pumps bind to molecules on ONE SIDE of membrane
conformation change of carrier protein (ref. carrier protein) requires input of energy
group of molecules enclosed in a vesicle
bulk transport
transport of large or very hydrophilic particles by enclosing it within vesicle
vesicle: fluid-filled, membrane bound sac
condition: expenditure of energy
EXOCYTOSIS: released of material from cells
vesicles prinched off from the Golgi apparatus
vesicle moves to surface of cell and fuses with plasma membrane
vesicle opens to exterior and contents leave cell
ENDOCYTOSIS: taking in materials into cell
pinocytosis (liquid)
cell gulps droplets of extracellular fluid in tiny vesicles
all substances dissolved in fluid is taken in
not specific to what molecules
phagocytosis (solid)
specific particles come into contact with cell surface
cell membrane invaginate to form a flask-like depression
cell membrane extends outward, forming extensions around the particles
after invagination/ extension, the neck of the flask closes and seals off the invagination, forming a phagocytic vesicle
receptor mediated endocytosis (specific molecules through receptor interaction)
taking up large quantities of specific substances
substances bind to receptor protein in membrane (ref. proteins)
clathrin protein forms a coated piut in cytoplasmic side (intracellular), forming coated vesicles in cytoplasm
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IMPORTANCE OF SUBSTANCES TRAVELLING ACROSS
entry of glucose for respiration (to produce ATP)
excretion of waste products
secretion of extracellular enzymes
ionic gradient for ATP synthesis
buffer between intracellular and extracellular environment
ensures constant environment within cell
important for enzymes to function within cell
cell to cell communication (ref. glycoproteins and glycolipids)
maintains structural relationship with neighbouring cells
LIPIDS
Phospholipids
forms bilayer due to amphipathic nature (ref. lipids)
Affects membrane fluidity (fluidity affects permeability)
FUNCTION
bilayer: separates cell content from surrounding
bilayer: compartmentalisation within cells --> formation of organilles
barrier to water soluble molecules and ions (hydrophobic core) but allows small molecules and fat-soluble substances to diffuse across (eg. oxygen, CO2)
provides fluidity to membrane
orientation ensures membrane stability (polar phosphate heads face polar surroundings, non polar hydrophobic tail faces core)
Cholesterol
has a hydrophilic hydroxyl group and hydrophobic region
allowing it to fit between phospholipid molecules (hydrophilic region with phosphate head, hydrophobic region with fatty acid)
FUNCTION
mechanical stability to membrane
maintains membrane fluidity
buffer resisting changes in temperature
At low temp: strong IMF and not fluid membrane (rigid)
bad: too rigid membrane causes it to solidify and shatter easily (weak)
cholesterol increases fluidity by disrupting close packing of phospholipids
at high temp: IMF is largely broken and very fluid membrane -
BAD: disrupts membrane integrity as molecules can easily pass through (too permeable)
cholesterol decreases fluidity as forms hydrophobic interactions with hydrocarbon chains (ref. lipids)
hinders movement of phospholipids
reduces uncontrolled leaking of polar molecules by diffusion
ensures regulation as they move in and out via channels
CARBOHYDRATES
glycoproteins or glycolipids
formed by carbohydrates complexing with proteins or lipids
form an 'antennae', glycocalyx on external membrane surface
FUNCTION
Orientation of membrane structure
carbohydrates in glycoproteins/glycolipids are hydrophilic
orientate them to face exterior
stabilises membrane structure by forming H bonds with water
cell communication
eg: cell to cell recognition
PROTEINS
peripheral proteins (extrinsic)
loosely attached on the polar surface (ref. protein structure)
attached to surface of cells or exposed parts of integral proteins
FUNCTION
receptor molecule
binding to chemicals (hormones) and allowing cells to respond to external stimuli
eg: G protein linked receptor
integral proteins (intrinsic)
partially penetrate membrane
transmembrane: spans entire membrane
FUNCTION
allows water soluble
ions
to be transported into or out of the cell
charged and polar molecules can't move across hydrophobic core
eg: protein cells or protein pumps
energy transducer
BY: electron carriers and ATP synthase
Transport of electrons and protons during respiration and photosynthesis for ATP synthesis
enzymes: Adenylyl cyclase attaches to plasma membrane and catalyses conversion of ATP to cAMP
cAMP is involved in signal transduction pathways of cell
structural support
attach to cytoskeleton or extracellular matrix
stronger framework
strongly attached to membrane
FLUIDITY OF MEMBRANE
importance:
allows membrane to fuse
allows self repair
allows fusion of transport vesicles to organelles
ease of binding to surface receptors
activates membrane bound enzymes and their transport
Affected by
temperature
as temperature increases, KE increases, breaks IMF, causes increased fluidity
note: fluidity is dependent on the strength of the IMF, which affects how tightly packed molecules are
membrane fluidity is maintained despite changes in temperature due to presence of cholesterol (ref. cholesterol)
length of hydrocarbon chains/ phospholipid chains
increasing hydrocarbon chains increases hydrophobic interactions between molecules, decreasing membrane fluidity (ref. lipids)
saturation of hydrocarbon chains (ref. lipids)
more unsaturated, more fluid
more 'kinks' disrupts the close packing of molecules and disrupts hydrophobic interactions
