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What does the biomembrane do?
Separates living cytoplasm from nonliving environment (essential for life)
Why is the biomembrane unique?
semi-permeable membrane - controls movement of materials across it
what is the fluid-mosaic model?
the current model for biomembrane structure
what are the critical features of the fluid-mosaic model?
membranes primarily composed of phospholipids (organized in a bilauer) with proteins and carbohydrates attached
lipids are in a fluid state (not a rigid barrier)
kinks in tail prevent tight packing of fatty acids (unsaturated)
proteins are dispersed in the membrane / free to float in the lipid fluid
tails are hydrophobic but heads are hydrophilic
what does the amount of fluidity in a membrane depend on?
temperature: as temp. decreases, so does fluidity
what is transition temperature?
Tm: the temperature at which the membrane undergoes fluid-to-solid phase change
Tm could be affected by changing the lipid structure of the membrane
What does a viscous (thick, sticky) membrane have?
saturated hydrocarbon tails (no C=C - no kinks)
What does a fluid membrane have?
unsaturated hydrocarbon tails (C=C - kinks)
What does increased number of C=C do to Tm?
Lower Tm - makes it easier to melt the membrane and increase fluidity
What is cholesterol made of?
it is a lipid so its made up of hydrocarbons, an OH group, and a hydrophobic tail.
What is a special property of cholestrol?
it is soluble in the hydrophobic core of the membrane bilayer (does have one polar region - OH group)
IS ABLE TO INTERACT with the hydrophobic tails and alters interactions between adjacent fatty acid chains to maintain membrane fluidity
How does cholesterol affect membrane fluidity?
reduces membrane fluidity at moderate temperatures; hinders solidification of membrane at lower temperatures
a fluidity buffer
what do variations in lipid compositions of cell membranes of different species mean?
means that lipid compositions have adapted to specific environmental conditions (in places where temp. varies, lipid compositions are able to change depending on the temperature)
what are integral membrane proteins?
proteins that are permanently attached to the cell membrane
tightly integrated into membrane structure
what is the most common type of integral membrane proteins?
transmembrane proteins: proteins that pass all the way through the membrane (outside to inside cell)
amphipathic: function on both sides of the membrane
what are anchored membrane proteins?
proteins covalently attached to lipid heads, they insert into membrane
not exposed to hydrophobic region
what are peripheral membrane proteins?
loosely bind to integral proteins or to lipids
removed without destroying the membrane
function on only one side of the membrane - not embedded in membrane
what are membrane carbohydrates?
short chains of monosaccharides added to protein or lipid (glycosylation)
attachment occurs in rough ER and golgi apparatus
are generally found on the outer membrane layer
what are the two types of membrane carbohydrates?
glycoprotein (bound to protein) or glycolipid (bound to lipids)
give one example of membrane protein function
HIV must bind to the immune cell surface protein CO4 and coreceptor CCR5 in order to infect a cell
HIV cannot enter the cells of resistant individuals that lack CCR5
What is selective permeability?
membrane lipids allow unlimited passage of some substances but limited or not passage of other substances
what is diffusion
movement of molecules from higher concentrations to lower concentrations
when does diffusion stop?
when it reaches equilibrium
what type of molecules pass through the membrane easily?
smaller and more hydrophobic molecules diffuse across the membrane more rapidly
hydrophobic molecules - easily diffuse
small, uncharged polar molecules - mostly diffuse
large, uncharged polar molecules - rarely diffuse
ions - never diffuse
what is the problem we encounter with selective permeability?
most biologically significant molecules are large and either polar/charged
how do we allow biologically significant molecules to pass through?
multi-pass, integral membrane (transmembrane) evolved to aid transport of polar or charged molecules
create a favorable interior for polar molecules to pass through hydrophobic membrane
what are channel proteins
integral membrane proteins that allow specific molecules to pass through rapidly
tend to have hydrophilic pores + have no specific binding to one molecule
what are carrier proteins?
integral membrane proteins that facilitate movement of specific molecules across the membrane by binding to them and undergoing conformational changes to shuffle them across (slower than channel proteins)
specific binding to solute; like an enzyme has a specific substrate
what is passive transport?
transport is “downhill” - driven by the direction of the electrochemical gradient
higher to lower
what is active transport?
transport where energy is added (ATP)
“uphill” - going against gradient
low to high
only carriers - not channels (channels are always passive)
carriers can be active or passive
what are solutes?
things that are dissolved in water
how do we balance different solutes’ concentration
sometimes, instead of moving the solutes, water moves across the membrane
what is osmosis?
passive movement of water across a membrane
what is a primary active transporter?
uses ATP to transport molecule against concentration gradient
what is a secondary active cotransporter?
uses stored potential energy of the electrochemical gradient of one molecule to transport another
what special channels does water use to move across the membrane?
aquaporins
if membrane is permeable to solute…
…then solute will go from high to low solute concentration
if membrane is permeable to water…
… then water will move from low to high solute concentration
what does isotonic mean?
solution around cell has the same solute concentration as inside the cell (no net movement of water)
what does hypotonic mean?
solution around cell has a lower solute concentration than inside the cell
water moves into the cell, causing it to swell - why we use saline in IV rather than H2O which could burst the cells.
what does hypertonic mean?
solution around cell has a higher solute concentration than inside the cell
net movement of water out of the cell
what is facilitated diffusion?
passive transport that is aided by proteins (channels and carriers)
what does a uniporter system mean?
a single thing is being moved across the membrane
what is a ligand gated channel mean?
proteins that open/close in response to ligands (a sort of stimulus molecule)
what is an electrically gated channel?
only a certain amount of charge is going to open up this channel
what is a mechanically gated channel?
channels that only open when stretched
what is cotransport?
some transport proteins can move more than one substance at a time
symport - two molecules the same direction
antiport - two molecules in opposite directions
can be passive or active
what is ATP hydrolysis?
ATP = ADP + Pi
what functions can ATP hydrolysis provide the energy for?
provides the energy to actively move two substances in two different directions - cotransport
what can be stored in an ion gradient?
energy supplied by ATP hydrolysis to transport one ion
what process can stored potential energy drive?
the active transport of another solute with a separate cotransport protein
what is bulk transport?
large proteins, such as proteins and polysaccharides, move by bulk transport
exocytosis: secretion (release of material)
endocytosis: mechanism for bringing material into the cell
what is phagocytosis (a type of endocytosis - bulk transport)
cellular eating
cell engulfs a particle into a vesicle
what is pinocytosis? (a type of endocytosis - bulk transport)
cellular drinking
“gulp” of fluid taken into vesicle
nonspecific uptake of solubilized material
what is receptor-mediated endocytosis?
used to bring in specific molecules
ligands bind to specific receptors
transferred to lysosome for digestions
a selective form of endocytosis that enables cells to internalize large quantities of specific substances.
what is cell signaling?
communication between cells in a multicellular organism
coordinates biochemical, mechanical, reproductive, and other essential activities
what is juxtacrine signaling?
communication between cells right next to each other
what is paracrine signaling?
communication between cells near each other
what is synaptic signaling?
electrical signal triggers release of neurotransmitter, which diffuses across synapse
what is endocrine signaling?
signaling between distant cells
what happens after a signal is sent?
all cells in range receive the signal
only cells with a receptor tuned to the specific signal can respond to the signal
what does responding to a signal mean?
responding means changing some cellular activity (ex: gene expression, enzyme activity, cell division)
what does interaction between a receptor and its signal (ligand) do?
when a signaling molecule (ligand) binds to a receptor, it causes the receptor to change shape - affects the receptor’s function
are most chemical signals hydrophilic or phobic?
hydrophilic: most need to help to pass through membrane (help from receptors, which are transmembrane proteins)
what kind of transport is NA+/K+?
primary active transport - antiporter
what are water soluble signals?
signaling molecules (hydrophilic) that dissolve in water - cannot pass through membrane, so they bind to receptors
what are lipid-soluble signals?
signaling molecules (hydrophobic) that dissolve in lipids - easily pass through membrane
what is intracellular signaling?
mechanism by which cells respond to their environment + communicate (signals are triggered by external stimuli)
intracellular receptors are receptors found in the cytoplasm or nucleus - signal must be very small and hydrophobic to reach them
what is a ligand-gated ion channel?
ligand has to bind to part of the channel to open it, allowing specific ions to pass through
what is a g-protein-coupled receptor?
receptors that activate G proteins when a ligand binds to them. this triggers a signaling pathway.
what is an enzyme-linked receptor?
an enzyme that happens to be a transmembrane protein (gets activated by a ligand)
what does the reception of a signal trigger and what is it called?
reception → transduction → response: signal transduction pathway (a series of biochemical reactions that lead to a cellular response.)
what does signal transduction usually include?
involves the activation or inactivation of enzymes
involves phosphorylation (adding a phosphate to activate the protein)
involves dephosphorylation (removing a phosphate to inactivate the protein)
what is protein kinase?
general name for an enzyme that helps catalyze the addition of a phosphate group from ATP to a protein (key part of phosphorylation)
can a single cell have more than one kinase?
yet, may have several hundred different kinases (each with different substrates and activation profiles)
how can phosphorylation be reversed?
phosphorylation can be reversed by dephosphorylation, which is done by phosphotases (removes the phosphate group from a protein, helping it return to its original conformation so the protein can be reused)
what are second messengers?
small nonprotein molecules + ions
activated by the initial signal (first messenger) and carry instructions through the cell
what is the most common second messenger?
cyclic AMP - hydrophilic.
helps cell pass on signals from the receptor to other parts of the cell, often by activating enzymes
ATP + Enzyme Adenyl Cyclase → cyclic AMP
how is cyclic AMP destroyed?
by phosphodiesterase (PDE) → breaks phosphodiester bond cyclic AMP has
what does protein kinase A do (PKa)
an enzyme activated by cyclic AMP that adds phosphate groups to proteins (phosphorylation), which changes their activity.
what type of responses of cellular signaling?
changes in gene expression, enzyme activity, motor activity, cytoskeletal structure, etc..
what can fine tuning of a cellular response lead to?
amplification, regulation, termination
what is amplification
a small signal leads to a large response
different cells can respond to the same signal…
in different ways - one signal is tied to different signaling pathways in different cells
what is termination
every response must be limited in duration
what are catabolic reactions accompanied by
the transfer of electrons between reactants
where exactly is energy stored
in chemical bonds (covalent > weak) in electrons (as potential energy)
if electrons are rearranged/released, potential energy can be reduced
what is combustion
rapid chemical combination with oxygen (oxidization - a reaction with O2)
exergonic
what is the formula for oxidation-reduction reaction
Ared + Box → Aox + Bred
Na + Cl → Na+ + Cl-
sodium is losing an electron (OIL) - oxidation
chlorine is gaining an election (RIG) - reduction
CH4 + 2O2 → CO2 + energy + 2H2O
CH4 loses bonds with hydrogen to become CO2 (oxidation/reducing agent)
2O2 gains bonds with hydrogen to become 2H20 (reduced/oxidizing agent)
what is an oxidizing agent?
substance that gains electrons
what is a reducing agent?
substance that donates electrons
can oxidation/reductions occur in isolation?
no - if one is oxidized, the other must be reduced
do electrons move alone
rarely - most of the time they combine with protons as they move
the molecule ____ is usually the more reduced one
______ is a really good oxidizing agent
1) hydrogen
2) oxygen