LESSON 2b: CELL MEMBRANE TRANSPORT Flashcards
- is the boundary between
inside & outside…
– separates cell from its environment
Cell membrane
Comes in …..
Food:
-carbohydrates
-sugars,
-proteins
-amino acids
-lipids
-salts, O2,H2O
Comes out…..
Waste:
- ammonia
-salts
-CO2
-H2O products
Can cell membrane be an impenetrable boundary?
NO!
Cell membrane permeability is ___________
Selective
– Allows some materials to pass.
àWater, oxygen, carbon
dioxide
– Prevents others from passing.
àProteins, carbohydrates
Differentially (selectively)
Permeable
Factors that determine how a
substance may be transported
across a plasma membrane:
– Size
– Polar or Nonpolar
– charge
-2nd Law of Thermodynamics
governs biological systems
– universe tends towards disorder
(entropy)
PASSIVE DIFFUSION
Movement of molecules in passive diffusion
High —-> low concentration
Only________, _________
molecules are able to
diffuse across a
phospholipid bilayer
at significant rates by
using passive
diffusion
small and relatively hydrophobic molecules
In passive diffusion, Molecules have to
dissolve in _______
lipid interior
Molecules that can pass through plasma membrane by passive diffusion
- Gases (oxygen, carbon
dioxide) - Water molecules (rate
slow due to polarity) - Lipids (steroid
hormones) - Lipid soluble molecules
(hydrocarbons,
alcohols, some
vitamins) - Small noncharged
molecules (NH3)
Why is diffusion important to cells
and humans?
Important in:
* Cell respiration
* Alveoli of lungs
* Capillaries
* Red Blood Cells
* Medications: time-release capsules
- Diffusion through protein channels which do not
interact with hydrophobic interior
– For biological mol unable to dissolve in hydrophobic
interior
– no energy needed
Facilitated Diffusion
Facilitated =___________
Open Channel= ____________
-with help
-fast transport
Molecules that transport using facilitated diffusion
- Ions (Na+, K+, Cl-)
- Sugars (Glucose)
- Amino Acids
- Small water soluble molecules
- Water (faster rate)
The passage of material in facilitated diffusion is aided both by ____________ and ___________
concentration gradient and transport protein
Two kinds of protein in facilitated diffusion
-Carrier Protein
-Channel Protein
-bind specific molecules, undergo conformational
change to release molecule
- ex. Glucose transporters
Carrier Proteins
- form open pores for free diffusion
- found in gap junctions
Channel Proteins
Molecules will
randomly move
through the pores
in ____________
Channel Proteins.
Some Carrier proteins ____________
through the membrane
do not extend
Some carrier protein _____________ molecules
through the lipid bilayer and release
them on the opposite side.
bond and drag
Other carrier proteins
___________to move
materials across the cell
membrane
change shape
How do molecules move through the plasma
membrane by facilitated diffusion?
- Channel and Carrier proteins are specific:
- Channel Proteins allow ions, small solutes, and water to pass
- Carrier Proteins move glucose and amino acids
- Facilitated diffusion is rate limited, by the number of
proteins channels/carriers present in the membrane.
Most cells, including _____________, are exposed to extracellular glucose concentrations that are higher than those inside the cell, so facilitated diffusion results in the net inward transport of glucose.
erythrocytes,
- Cells obtain food for ______
- __________ communicate
- ____________ transport food to bloodstream
- Muscle cells _____
-cell respiration
-Neurons
-Small intestine cells
-contract
- Water Channels
- Protein pores used
during Osmosis - provide corridors allowing water
molecules to cross the membrane. - Allow for fast transport
- water channel proteins,
make possible
massive amounts of diffusion
Aquaporins
– is the diffusion of water across a differentially
permeable membrane.
OSMOSIS
the pressure that develops in a
system due to osmosis
Osmotic pressure
Concentrations of Water
- Hypertonic
- Hypotonic
- Isotonic
Direction of osmosis is determined by
comparing ___________
total solute concentrations
more solute, less water
Hypertonic
- less solute, more water
Hypotonic
equal solute, equal water
Isotonic
Cell survival depends on ___________
balancing water uptake & loss
Cell in Hypotonic Solution (Freshwater)
Animal Cell: Lysed
Plant Cell: Turgid (Normal)
Cell in Isotonic Solution (Balanced)
Animal Cell: Normal
Plant Cell: Flaccid
Cell in Hypertonic Solution (Saltwater)
Animal Cell: Shriveled
Plant Cell: Plasmolyzed
When membrane permeable to both solute molecules and water, both solution have same osmolarity: _____________, even if one solution have lower osmolarity and the other have greater osmolarity.
Volume Unchanged
When membrane is impermeable to solute molecules but permeable to water. both solution have same osmolarity: _____________,
but volume move to area with greater solute because water is free to move.
Isotonic: animal cell immersed in
______ solution
mild salt
Example: Blood cell in plasma membrane in isotonic solution. Thus:
- no net movement of water, it flows across membrane equally, in both directions
– volume of cell is stable
Example for hypotonic: Paramecium in Fresh water.
Problem?
Solution?
Problem: gains water, swell and can burst
Solution: Contractile Vacuole (pumps water out of the cell using ATP)
Example of Hypertonic: Shellfish in Saltwater
Problem?
Solution?
Problem: Lose water and Die
Solution: take up water or pump out salt
Non-lipid soluble substances
diffuse through__________
membrane
channels.
Why is osmosis important to cells and
humans?
- Cells remove water
produced by cell
respiration. - Large intestine cells
transport water to
bloodstream - Kidney cells form
urine
- Non lipid soluble passively diffuse ____________________ through channels
that cross the lipid bilayer;
some channels are open all
the time whereas others are
gated.
-down their electrochemical gradient
- The _____________ regulates the
opening/closing of the
channel
membrane potential
(voltage)
Example of a specific stimulus:
-Voltage (Voltage-gated channel)
-Ligand (ligand-gated channels)
-Specific Stress (Stress-activated channels)
______ permit the free passage
of ions and small polar
molecules through the outer
membranes of bacteria.
Porins
______ mediate the
passage of ions across plasma
membranes
Ion channels
__________ open in
response to the binding of
neurotransmitters or other
signaling molecules.
Ligand-gated channels
___________ open in
response to changes in electric
potential across the plasma
membrane.
Voltage-gated channels
- open or close depending
on the presence or absence
of a physical or chemical
stimulus. - Ex. neurotransmitters bind
to specific gated channels
on the receiving neuron,
these channels open. - This allows sodium ions
into a nerve cell. - When the
neurotransmitters are not
present, the channels are
closed
Gated Channels
example of voltage-gated channels
Membrane Potential
- K+ voltage-gated channels
-Na+ voltage-gated channels
regulates opening/closing
of the channel.
membrane potential
exist as either open or close
depending on the
membrane voltage. It has
only an activation gate.
K+ voltage-gated channels
-opens when the membrane
potential depolarizes (i.e.
becomes more positive).
- It has activation and
inactivation gates.
Na+ voltage gated channels
-a large family of cell surface proteins that respond to a variety of signals, including hormones, neurotransmitters, and light
-a type of cell surface receptor that interacts with G proteins to initiate intracellular signaling pathways
G protein-linked receptor
-Cells may need to move molecules against concentration
gradient
– shape change transports solute from
one side of membrane to other
– protein “pump”
– “costs” energy = ATP
Active Transport
The Importance of
Active Transport
- Bring in essential
molecules: ions,
amino acids, glucose,
nucleotides - Rid cell of unwanted
molecules (Ex.
sodium from urine
in kidneys) - Maintain internal
conditions different
from the
environment
*Regulate the volume of cells
by controlling osmotic
potential
*Control cellular pH
*Re-establish concentration
gradients to run facilitated
diffusion. (Ex. Sodium- Potassium pump and Proton
pumps)
Example of Active transport
Pumping Na+ (sodium ions) out and K+
(potassium ions) in against strong concentration
gradients
Process of Sodium-Potassium Pump Action
- binding of cytoplasmic Na+ to the protein stimulates phosphorylation by ATP
- Phosphorylation causes the protein to change its conformation
- The conformational change expels Na+ to the outside and extracellular K+ binds.
- K+ binding triggers release of a phosphate group.
- Loss of phosphate restores original conformation.
- K+ released and Na+ sites are receptive again., the cycle repeats.
____________ move out of
the cell and then _________ move into
the cell
-3 Sodium ions
-2 Potassium ions
Driven by the splitting of
ATP to provide energy and
conformational change to
proteins by adding and then
taking away a phosphate
group
Sodium-Potassium Pump (Active Transport)
Sodium Potassium pump is used to establish an _______________
across neuron cell
membranes
electrochemical gradient
Active transport can be classed into 2 groups
- PRIMARY ACTIVE TRANSPORT
- SECONDARY ACTIVE TRANSPORT
- _____________ is
directly used to move
substances across the
membrane against its
concentration gradient (i.e.
from an area of low
concentration .to an area of
high concentration)
Cellular energy (i.e. ATP)
A substance is actively transported using ATP through one channel. Then it diffuses back passively through a channel and its energy harnessed from transport is used to move another substance against it concentration gradient.
PRIMARY ACTIVE TRANSPORT
Active transport, a process in
which energy is provided by
another coupled reaction, is used
to drive the uphill transport of
molecules in the _______________
energetically unfavorable direction
Ion pumps responsible for
maintaining gradients of ions
across the plasma membrane,
provide important examples of
active transport driven directly by
______________.
ATP hydrolysis
Some molecules are transported against their concentration gradients using energy derived not from ATP hydrolysis, but from the ________________ in the energetically favorable direction.
coupled transport of a second molecule
The __________ of the intestine provide a good example of active transport drive by the Na+ gradient. (Na+ is coupled by glucose, where the energy harnessed by Na+ is used for transport)
epithelial cells lining
- Two substances are transported in the same direction across a
plasma membrane. - One of the substance moves passively down its concentration
gradient while the second substance uses ATP to move against
its concentration gradient across the plasma membrane.
SECONDARY ACTIVE TRANSPORT:SYMPORTER
-Two substances are transported across the membrane in opposite
directions across the plasma membrane.
- One of the substance moves passively down its concentration
gradient while the second substance uses ATP to move against its
concentration gradient across the plasma membrane.
SECONDARY ACTIVE TRANSPORT: ANTIPORTER
A ____________ can
transport only
a single
molecule
using the
facilitated
diffusion of
glucose.
Uniport
An _________
uses active
transport to
move two
molecules
in opposite
direction
antiport
- moving things out.
– Transport of large molecules
– Requires energy
– Keeps the macromolecule contained
Vesicle Formation
Vesicles form as a way to transport molecules out of a cell
Exocytosis
Exocytosis of Proteins
- Vesicle formation: Proteins destined for export are enclosed in membrane vesicles by the trans Golgi.
- Vesicle movement: The vesicles are transported to the plasma membrane.
- Vesicle fusion: The vesicle fuses with the plasma membrane.
- Content release: The contents of the vesicle are released into the extracellular space.
Vesicles
form as a way to
transport molecules
into a cell
Endocytosis
Types of Endocytosis
-Phagocytosis
-Pinocytosis
-Receptor-Mediated Transport
-Cell eating
- engulf Large, particulate
matter (Bacteria,
viruses, and aged or
dead cells).
-Example: Phagocytes
Phagocytosis
How insulin works in regulation of blood sugar level and how endocytosis take place when insulin level drops?
- Glucose transporters are stored within the cell in membrane vesicles,
- When insulin interacts with its receptor, this vesicles move to the surface to fuse with the plasma membrane, increasing the number of glucose transporters in the plasma membrane.
- When insulin level drops, glucose transporters are removed from the plasma membrane by endocytosis, forming small vesicles.
- The smaller vesicles fuse with larger endosomes.
- Patches of the endosomes enriched for the glucose transporters bud off to become small vesicle, ready to return to the surface when insulin levels rise again,
-Most common form of endocytosis
-* Cell forms an invagination
* Materials dissolve in water
to be brought into cell
* Called “Cell Drinking”
* Ex. Intestinal cells, Kidney
cells, Plant root cells
-Takes in dissolved molecules as vesicles
Pinocytosis
Best example for Pinocytosis
Plant Roots
Movement of very
specific molecules
into the cell with the
use of vesicles coated
with the ______________
Clathrin Protein
-a form of
pinocytosis, provides a
mechanism for the selective
uptake of specific
macromolecules.
-Coated pits are specific
locations coated with clathrin
and receptors. When specific
molecules (ligands) bind to
the receptors, then this
stimulates the molecules to
be engulfed into a coated
vesicle
- Ex. Uptake of cholesterol
(LDL) by animal cells
Receptor-Mediated Transport
Some integral
proteins have
receptors on their
surface to
_________________, hormones
cholesterol, etc
recognize & take in
are specialized regions of the
plasma membrane where
specific cell surface receptors
are found.
Clathrin-coated pits
a membrane associated GTP-binding
protein, assists in the
budding off of pits from the
plasma membrane.
Dynamin
Clathrin assembles into a _____________-that distorts the membrane, forming invaginated pits.
basketlike structure
Clathrin coated pits occupy about ____ of the surface area of plasma membranes.
1-2%
are vesicles with tubular
extensions, located at
the periphery of the
cell, that fuse with
clathrin-coated
vesicles which have
shed their coats.
Endosomes
An important
feature of
early
endosomes is
that they
maintain an
______________ as the
result of the
action of a
membrane H+
pump.
acidic internal pH
Process of endocytosis from clathrin-coated pits.
- Arrival of a nerve impulse at the terminus of a neuron triggers the fusion of synaptic vesicles with the plasma membrane, releasing neurotransmitters.
- The synaptic vesicle membrane is retrieved by endocytosis from clathrin-coated pits.
- The endocytosis vesicles fuse with early endosomes.
- Synaptic vesicles are regenerated by budding from the endosome and refilled by the uptake of neurotransmitters from the cytosol.
_________________ is the major
fate of membrane proteins taken up by receptor-mediated endocytosis.
Recycling to the plasma membrane
Ligands and membrane proteins destined for
_____________________ are transported from early endosomes to late endosomes, which are located near the nucleus
degradation in lysosomes
a phenomenon
where receptor-ligand complexes are removed
from the plasma membrane, thereby terminating
the response of the cell to growth factor
stimulation.
Receptor down-regulation
are phagosomes fused to lysosomes,
contain lysosomal acid hydrolases that digest the ingested
material
Phagolysosomes
The ______________ by phagocytosis plays distinct roles in different kinds of cells
ingestion of large particles
