LECTURE 2: MEMBRANES, CHANNELS & TRANSPORT Flashcards
- separates the cytoplasm from the external
environment - one of the most important cell organelles
- a highly selective permeable barrier that
surrounds all living cells - controls how molecules and compounds
move in and out of the cell - very important for proper nutrition,
maintenance of irritability of the cells, and
homeostasis - define boundaries and serve as permeability
barriers - surrounds all
animal cells - extraordinary thin
(6-23 nm) - lipid-based
structure that
encloses the cytosol
CELL MEMBRANE
Cell Membrane a _______ _______ _______barrier that surrounds all living cells
highly selective permeable
Cell Membrane is Important for
-proper nutrition,
-maintenance of irritability of the cells, and
-homeostasis
-signal detection
-cell to cell communication
-compartmentalization
Cell membrane is extraordinary thin, around _____
6-23 nm
Cell membrane sustains different concentrations of certain ions on
their two sides, leading to _____________
concentration gradient
__________ participates in the transport of substances
protein structures
- describes the organization of cell
membranes - phospholipids drift and move like a fluid
- bilayer is a mosaic mixture of
phospholipids, steroids, proteins, and
other molecules
The Fluid Mosaic Model
Cell membrane is composed of _________, ________, ____________ and are arranged in a _____ ______ structure.
-phospholipids, proteins, and
carbohydrates
-Fluid Mosaic
Cell Membrane is composed of _______ and ________ molecules kept together by non-covalent interactions and mostly “_______” in the plane of the bilayer.
- Lipid and Proteins
-Float
lipid and protein molecules kept
together by ______________________
non-covalent interactions
impermeable to the passage of
most water-soluble molecules
Lipid Bilayer
fundamental structure of the
membrane
Lipid Molecules
- inside surface maintain cell shape or cell
motility - enzymes - catalyzing reactions in the
cytoplasm. - act as receptors (specific binding site
where hormones or other chemicals can
bind; used for cell signaling and cell
recognition)
usually span from one side of the
phospholipid bilayer to the other (integral
proteins), but can also sit on one of the
surfaces (peripheral proteins) - can slide around the membrane very
quickly and collide with each other, but
seldom flip from one side to the other - responsible for most of the membrane’s
properties
Proteins
embedded in the lipid bilayer
provide a mechanism for trans-membrane
transport
Integral Proteins
Integral proteins embedded in the lipid bilayer provide a mechanism for trans-membrane
transport, includes:
-passive-transport pores and channels,
-active-transport pumps and carriers, membrane-linked enzymes,
-chemical signal receptors
-transducers
associated with the
surface of the membrane via electrostatic
interaction
Peripheral Proteins
Peripheral proteins are associated with the
surface of the membrane via_________
electrostatic
interaction
Proteins inside surface maintain ________ or ______
Cell shape or Cell Motility
Proteins acts as _______ catalyzing reactions in the cytoplasm.
Enzymes
Proteins acts as receptors that acts as the specific binding site where ___________ and other__________ can bind, used for __________ and ________
- hormones or other chemicals
-cell signaling and cell recognition
proteins in the plasma membrane may provide
a variety of major cell functions, this includes:
-Transport
-Enzymatic Activity
-Signal Transduction
- Intercellular Joining
-Cell-cell communication
-Attachment to the cytoskeleton and extracellular matrix
Each type of protein in a membrane has a
special function, including:
-Adhesion Protein
-Recognition Protein
-Receptor Protein
-Enzymes
-Transport Protein (active and passive)
-found on the outer surface and attached
to the proteins or sometimes to the
phospholipids
-form a cell coat or glycocalyx outside the
cell membrane
Carbohydrates
___________ is responsible for Maintaining the cell stability and cell recognition and is attached to the lipids
Glycolipids
bear oligosaccharide side chains
and are vital for cell recognition and
communication, attached to the protein
Glycoproteins
_______ is used for protection and cell
recognition
Glycocalyx
What are the primary types of lipids
- Phosphoglycerides
2.Spingholipids - Sterols
glycerol backbone
Phosphoglycerides
backbone
made of sphingosine
bases
Sphingolipids
cholesterol,
nonpolar and only slightly
soluble in water
Sterols
one end is hydrophilic –
water soluble; other end is
hydrophobic – water
insoluble
amphipathic
*amphipathic
(one end is hydrophilic –
water soluble; other end is
hydrophobic – water
insoluble)
* dual nature is crucial to the
organization of biological
membranes
* self-repairing
* differences in the lengths of
the two fatty acid tails and
their composition influence
fluidity
phosphoglycerides and sphingolipids
*lateral movement of lipids and proteins within a surface of the bilayer is very common
* depends on its composition and cholesterol
(binds weakly to phospholipids making the
membrane less fluid but stronger)
Membrane Fluidity
________________ of lipids and proteins within a surface of the bilayer is very common
lateral movement
binds weakly to phospholipids making the
membrane less fluid but stronger
Cholesterol
membrane molecules are held in place by
relatively _______________________
weak hydrophobic interactions
most lipids and some proteins can drift laterally in the plane of the membrane, but rarely ______ from one layer to the other
flip-flop
influenced by
temperature and
constituents
Membrane Fluidity
decreased temperature, membrane fluid state turns ________ where phospholipid are more closely packed.
solid state
- wedged between phospholipid molecules in
the plasma membrane of animals cells. - at warm temperatures, it restrains the
movement of phospholipids and reduces
fluidity - at cool temperatures, it maintains fluidity by
preventing tight packing
Steroid Cholesterol
At _________________, steroid cholesterol restrains the
movement of phospholipids and reduces
fluidity
Warm temp
At ___________________, steroid cholesterol maintains fluidity by
preventing tight packing
Cool Temp
- freeze-etch electron
micrographs of the
surface of a membrane
shows the progressive
removal of - proteins when subjected
to digestion with
proteolitic enzyme
Membrane Heterogeneity
- splits a membrane
along the middle of
the phospholid bilayer
prior to electron
microscopy. - shows protein
particles interspersed
with a smooth matrix,
supporting the fluid
mosaic model.
Freeze Fracture
- regulates the passage of materials (gases,
nutrients, wastes) in and out of the cell - hydrophobic interior makes membranes
highly impermeable to most polar
molecules
Selective permeability
- The rate at which a substance can
passively penetrate a cell membrane - Influenced by inherent properties of both
the membrane and the substance
Membrane permeability
What are the two transport systems?
-Passive Processes
-Active Transport
- no energy expenditure and move down
their normal gradient
Passive Transport
Types of Passive Transport
- simple diffusion/lipid diffusion
- osmosis
- facilitated diffusion/passive Transport
What are the three basic routes
- Dissolving in Lipid Phase
- Diffusion through labile or fixed aqueous
channels - Carrier mediated transport (facilitated or
active transport)
- molecules diffuses through the membrane
- diffusion and osmosis
*leaves the aqueous phase on one side of the
membrane - dissolves directly in the lipid bilayer
*diffuses across the thickness of the lipid or protein
layer
*enters the aqueous phase on the opposite side
Dissolving in lipid phase
- solute molecule remains in the aqueous phase
*diffuses through aqueous channels (water-filled
pores in the membrane)
Diffusion through labile or fixed aqueous
channels
- solute molecule combines with a carrier
molecule dissolved in membrane
*carrier “mediates” or “facilitates” the
movement of the solute molecule across the
membrane
Carrier mediated transport (facilitated or
active transport)
-random thermal motion of suspended
or dissolved molecules causes their dispersion
from regions of higher concentration to regions of
lower concentrations
-Net movement of particles (atoms, molecules,
or ions) along a concentration gradient from
an area of higher concentration to an area of
lower concentration
Diffusion
Diffusion is the random thermal motion of suspended
or dissolved molecules causes their dispersion
from______________________________________
regions of higher concentration to regions of
lower concentrations
Diffusion: Net movement of ________
Particles
movements of individual molecules are
________
random
In the absence of other forces, a substance will
diffuse from where it is_______________________________
more concentrated to
where it is less concentrated (down its
concentration gradient)
Each substance diffuses down its_____________________, independent of the
concentration gradients of other substances
own
concentration gradient
diffusion of a substance across a biological
membrane is ____________________-
passive transport (exergonic)
assist molecules with
limited permeability to diffuse through the lipid
bilayer
Transport Proteins
- Net diffusion rate across a fluid membrane is
proportional to the difference in partial
pressure, proportional to the area of the
membrane and inversely proportional to the
thickness of the membrane
Fick’s Law of Diffusion
Fick’s Law of Diffusion: Formula
J = - DA (∆C/∆ X)
______________of a membrane to a substance
is the rate at which that substance passively
penetrates the membrane under a specified
set of conditions
Permeability
INTRINSIC FACTORS GOVERNING DIFFUSION
ACROSS MEMBRANES:
- Size rule (Ex. water > urea)
- Polarity rule (Ex. Hexane - non polar > ethanol
polar) - Ionic rule (Ex. O2 > -OH)
- a few substances can diffuse directly through
the lipid bilayer part - lipid-soluble molecules such as steroids, or
very small molecules, such as H2O, O2 and CO2
Simple Diffusion/Lipid Diffusion
Rate of diffusion depends on five
factors:
◦ Size
◦ Temperature
◦ Steepness of the concentration gradient
◦ Charge
◦ Pressure
Factors that influence mobility of solute molecule
- Lipid Solubility
- Hydrogen Bond with water
- Molecular Weight
- Molecular Shape
- Partition coefficient
With increasing solubility the mobility increase
Lipid Solubility
Increase in hydrogen bond with water decreases mobility
Hydrogen bond with water
ratio of the distribution of a
substance between two different liquid phases
(e.g. oil and water)
Partition coefficient
Partition Coefficient: Formula
K = solute concentration in lipid/
solute concentration in water
Passive Diffusion: rate of influx increases in proportion to the
concentration of the solute in the extracellular
fluid (difference in the number of solute
molecules on the two sides of the plasma
membrane
Non- saturation Kinetics
diffusion of water across a selectively
permeable membrane is a special case of
passive transport called ________
Osmosis
Osmosis continues until the solutions are _________
isotonic
Osmosis: water moves from ______________________________
(down its concentration gradient)
higher to lower water potential
the movement of water can produce a ____________ (a fluid mechanical pressure), resulting in a
pressure gradient across a semipermeable
membrane
hydrostatic
pressure
Hydrostatic Pressure
A fluid mechanical pressure
Osmosis pertains to solvent particles; determined by __________
Osmotic Pressure
the difference in hydraulic
pressures of a solution and water (interfacing one another at either side of an SPM) which must be overcome to prevent the entry of water into the solution across the membrane
Osmotic Pressure
Osmotic Pressure is described by _________
Van t’Hoff equation
Van t’Hoff equation
π = Φn (C/M)R
when two aqueous solutions exert
the same osmotic pressure through a
membrane permeable only to water.
Isosmotic
if one solution exerts less
osmotic pressure than the other
Hypoosmotic
if one solution exerts greater
osmotic pressure than the other
Hyperosmotic
the concentration of a solution
expressed as the total number of solute particles
per liter
Osmolarity
response of cells or tissues
immersed in the solution
Tonicity
–osmotic pressure property of a solution
Osmoticity
no osmotic pressure
difference between the cell and interior and the
extracellular solution; no net water gain;
cell/tissue neither shrinks nor swell.
*Isotonic solution
If the tissue swells because it absorbs water,
the solution is _______ to the tissue.
hypotonic
If the tissue shrinks because it loses water, the
solution is ______ to the tissue
hypertonic
_______________ depends on the balancing water uptake and loss
Cell Survival
What happen to animal cell and plant cell in Hypotonic Solution?
Animal Cell: Lysed
Plant Cell: Turgid (Normal)
Why does plant cell don’t burst in hypotonic solution?
Because of the Cell Wall
What happen to animal cell and plant cell in Isotonic Solution?
Animal Cell: Normal
Plant Cell: Flaccid
What happen to animal cell and plant cell in Hypertonic Solution?
Animal Cell: Shriveled
Plant Cell: Plasmolyzed
- through transmembrane proteins
- transport proteins tend to be specific for one
molecule, so substances can only cross a
membrane if it contains the appropriate
protein
Facilitated Diffusion/ Passive Transport
Two kinds of transport protein
Channel and Carrier
*____________________ in the membrane
allows charged substances (usually ions) to
diffuse across membranes
water-filled pore or channel
most channels can be _____________, allowing the cell to control the entry
and exit of ions
gated (opened or
closed)
small organic compounds
that specifically transports ions across
the plasma membrane
Ionophores
some channel proteins, gated channels,
open or close depending on the ________________________
presence or
absence of a physical or chemical stimulus
transports a single solute from
one side of the membrane to the other
Uniporters
transfer one solute
and simultaneously or sequentially transfer a
second solute
Coupled transporters
-transfer solutes in the same
direction
-run on energy stored in
ion gradients
symporters
transfer solutes in opposite
directions
antiporters
Channel Mediated Transport: the rate of influx increases in proportion to the
concentration of the solute in the extracellular
fluid (difference in the number of solute
molecules on the two sides of the plasma
membrane
Saturation Kinetics
*binding site for a specific solute and
constantly flip between two states so that the
site is alternately open to opposite sides of
the membrane
*substance will bind on the side with higher
concentration and be released at the lower
concentration side
Diffusion through a carrier
binding site for a specific solute and
constantly flip between two states so that the
site is alternately open to opposite sides of
the membrane
Carrier
show saturation kinetics
Channel and carrier-mediated transport
*require metabolic energy
and moves substances against their gradients
* proteins are highly specific -
different protein pump for
each molecule to be
transported
* critical for a cell to maintain
its internal concentrations of
small molecules that would
otherwise diffuse across the
membrane.
* ATP supplies the energy for
most active transport
Active Transport
supplies the energy for
most active transport
ATP
actively maintains
the gradient of sodium (Na+) and potassium
ions (K+) across the membrane
Sodium-potassium pump
Important Features of Active Transport
- Transport can take place against substantial
concentration gradient - Exhibits high degree of selectivity
- ATP or other sources of energy are required
- Certain membrane pumps exchange one kind of molecule or ion from one side of the membrane for another kind of molecule or ion from the other side
- Some pumps perform electrical work by producing a net flux of charge
- Selectively inhibited inhibited by specific blocking agents
- Energy is released by the hydrolysis of ATP by enzymes (ATPases) present in the membrane
In _________, a membrane protein couples the transport of two solutes
Cotransport
A single ATP-powered pump that transports
one solute can indirectly drive the active
transport of several other solutes through
___________ via a different protein
cotransport
As the solute that has been actively
transported diffuses back passively through a
transport protein, its movement can be
______________________________ against its concentration gradient
coupled with the active transport of another
substance
Na+ /H+ antiporter in
the proximal tubule of the
mammalian kidney
- for each H+ expelled, one Na+ is
taken up into the cell
- advantages: avoiding the
expenditure of energy to perform
electrical work (two equivalent +
charges are exchanged)
- enables the kidney to reclaim
Na+ from urine and excess protons
Antiporters
No Energy, No Proteins, No Specific, Not controllable
Lipid Diffusion
No Energy, No Proteins, Yes Specific, Not Controllable
Osmosis
No energy, Yes Protein, Yes Specific, Controllable
Passive Transport (Facilitated)
Yes Energy, Yes Protein, Yes Specific, Contollable
Active Transport
Small molecules and water enter or leave the
cell through____________________
the lipid bilayer or by transport
proteins
Large molecules, such as polysaccharides and
proteins, cross the membrane via __________
vesicles
Fusion of separate regions of the lipid bilayer:
bilayers come into_________ and then
they ______
close apposition,
fuse
- transport large
molecules
Endocytosis and Exocytosis
Types of Endocytosis
*Pinocytosis
*Phagocytosis
*Receptor-mediated
endocytosis
cell brings in macromolecules and
particulate matter by forming new vesicles from the
plasma membrane
Endocytosis
*cell creates a
vesicle around a droplet of extracellular fluid
◦ non-specific process
Pinocytosis
Pinocytosis or also known as
Cell Drinking
Cell Eating
Phagocytosis
- Depends on the presence of receptor molecules
embedded in the cell membrane - triggered when extracellular substances bind to special receptors, ligands, on the membrane
surface, especially near coated pits - Upon binding of ligand, receptor-ligand complex
accumulate within coated pits (internalizes the
ligand) - Formation of vesicle that pinches off into the
cytoplasm
Receptor-mediated endocytosis
*transport vesicle that
budded from the Golgi
apparatus is moved by
the cytoskeleton to
the plasma membrane
*when the two
membranes come in
contact, the bilayers
fuse and spill the
contents to the
outside
Exocytosis
Three main types
of intercellular
links:
-tight junctions,
-adhering junctions (zonula adherens,
desmosomes),
- gap junction
➢membranes of
adjacent cells
are fused,
forming
continuous belts
around cells
➢prevent leakage of extracellular fluid
across a layer of epithelial cells
Tight junction
➢fasten cells together
into strong sheets,
much like rivets
➢reinforced by
intermediate filaments
of keratin
➢attach muscle cells to
each other in a muscle
Desmosome (anchoring junctions)
➢provide cytoplasmic channels
between adjacent cells
➢salt ions, sugar, amino
acids, and other small
molecules can pass
Gap junction (communicating junctions)
every cell maintains
concentrations of inorganic solutes inside the
cell that are different from those outside the cell
Ionic steady state
- animal cell do not have rigid
walls and cannot resist any buildup of large intracellular pressure - cells will change size when
placed in different concentrations
of impermeable substances
dissolved in water
Cell volume
osmotic movement of water
shrinkage or swelling
Two ways to prevent osmotic swelling
a. pump water out as fast as it leaks in
b. pump out solutes that leak into the cell – major mechanism for regulation of cell volume
membrane permeability to charged particles
depends both on the ________________ and on the ____________.
-membrane permeability constant
-electrical signal
Two Forces can act on charged atoms and
molecules to produce a net passive diffusion of
each species across a membrane:
Chemical Gradient and Electric Field
arising from differences
in the concentration of the substance on the
two sides of the membrane
chemical gradient
difference in electric potential
across the membrane
electric field
ion will move away from regions of ______________, and if that ion is positively charged it will also move toward ___________
-high concentration
-increasing negative potential
determined by the sum
of the combined forces of the concentration
gradient and electrical gradient
*Electrochemical gradient
potential at which
an ion is in electrochemical equilibrium
*Equilibrium potential
*Equilibrium potential
V or mV
ratio of the ion concentrations on opposite
sides of the membrane influence the________________
value of
the equilibrium potential
an ion species can passively diffuse against its
chemical concentration gradient if the electrical gradient (potential difference) across the membrane is in the___________ to and ________ the concentration gradient
- opposite direction
-exceeds
will not be
affected by electrical forces but by the
concentration gradient
uncharged molecules (sugar)
if diffusible solutes are separated by a
membrane that is freely permeable to water and
electrolytes but totally impermeable to one
species of ion, the diffusible solutes become
unequally distributed between the two
compartments.
Donnan Equilibrium
Cell is dead if _________________
Cell Membrane is not responsive
Diffusion is fast until equilibrium is reached
Dissolving in Lipid Phase
Diffusion/ Concentration slows down until equilibrium is reached
Diffusion through labile or fixed aqueous channel
Diffusion is very slow because the carrier is occupied, the diffusion will continue if the carrier is empty. This continues until equilibrium is reached
Carrier mediated transport (Facilitated or Active Transport)
Water pass through the membrane through ____________
Aquaporins
Process of Endocytosis
- Invagination
- Formation of Pouch
- Formation of Vesicle
- Lysosome Digest
- Undigested molecules undergo exocytosis
Na+
Exterior: 120 mM
Interior: 10 mM
K+
Exterior: 2.5
Interior: 140
Ca2+
Exterior: 2.0
Interior: <10-3
Cl-
Exterior: 120
Interior: 3-4
A-
Interior: 140
