Lecture 15
plasma membrane transport
Active transport
pp72-79
Active Processes
• Whenever a cell uses the bond energy of ATP
to move solutes across the membrane, the
process is referred to as active.
• Substances moved actively across the plasma
membrane are usually unable to pass in the
necessary direction by passive transport
processes. The substance may be too large to
pass through the channels, incapable of
dissolving in the lipid bilayer, or unable to
move down its concentration gradient.
Membrane Transport: Active Processes
• Two types of active processes:
– Active transport
– Vesicular transport
• Both use ATP to move solutes across a living
plasma membrane
Active transport
•
like carrier-mediated facilitated diffusion,
requires carrier proteins that combine
specifically and reversibly with the
transported substances. However, facilitated
diffusion always honors concentration
gradients because its driving force is kinetic
energy. In contrast, the active transporters or
solute pumps move solutes, most importantly
ions (such as Na+, K+, and Ca2+), “uphill”
against a concentration gradient. To do this
work, cells must expend the energy ofATP.
Lipid-insoluble
solutes (such as
sugars or amino
acids)
(b) Carrier-mediated facilitated diffusion via a protein
carrier specific for one chemical; binding of substrate
causes shape change in transport protein
Figure 3.7b
Active Transport
• Requires carrier proteins (solute pumps)
• Moves solutes against a concentration
gradient
• Types of active transport:
– Primary active transport
– Secondary active transport
Primary Active Transport
• Energy from hydrolysis of ATP causes shape
change in transport protein so that bound
solutes (ions) are “pumped” across the
membrane
Primary Active Transport
• Sodium-potassium pump (Na+-K+ ATPase)
– Located in all plasma membranes
– Involved in primary and secondary active transport of
nutrients and ions
– Maintains electrochemical gradients essential for
functions of muscle and nerve tissues
Extracellular fluid
Na+
Na+-K+ pump
Na+ bound
K+
ATP-binding site
Cytoplasm
1 Cytoplasmic Na+ binds to pump protein.
P
ATP
K+ released
ADP
6 K+ is released from the pump protein
and Na+ sites are ready to bind Na+ again.
The cycle repeats.
2 Binding of Na+ promotes
phosphorylation of the protein by ATP.
Na+ released
K+ bound
P
Pi
K+
5 K+ binding triggers release of the
phosphate. Pump protein returns to its
original conformation.
3 Phosphorylation causes the protein to
change shape, expelling Na+ to the outside.
P
4 Extracellular K+ binds to pump protein.
Figure 3.10
Extracellular fluid
Na+
Na+-K+ pump
ATP-binding site
K+
Cytoplasm
1 Cytoplasmic Na+ binds to pump protein.
Figure 3.10 step 1
Na+ bound
P
ATP
ADP
2 Binding of Na+ promotes
phosphorylation of the protein by ATP.
Figure 3.10 step 2
Na+ released
P
3 Phosphorylation causes the protein to
change shape, expelling Na+ to the outside.
Figure 3.10 step 3
K+
P
4 Extracellular K+ binds to pump protein.
Figure 3.10 step 4
K+ bound
Pi
5 K+ binding triggers release of the
phosphate. Pump protein returns to its
original conformation.
Figure 3.10 step 5
K+ released
6 K+ is released from the pump protein
and Na+ sites are ready to bind Na+ again.
The cycle repeats.
Figure 3.10 step 6
Extracellular fluid
Na+
Na+-K+ pump
Na+ bound
K+
ATP-binding site
Cytoplasm
1 Cytoplasmic Na+ binds to pump protein.
P
ATP
K+ released
ADP
6 K+ is released from the pump protein
and Na+ sites are ready to bind Na+ again.
The cycle repeats.
2 Binding of Na+ promotes
phosphorylation of the protein by ATP.
Na+ released
K+ bound
P
Pi
K+
5 K+ binding triggers release of the
phosphate. Pump protein returns to its
original conformation.
3 Phosphorylation causes the protein to
change shape, expelling Na+ to the outside.
P
4 Extracellular K+ binds to pump protein.
Figure 3.10
Secondary Active Transport
• Depends on an ion gradient created by
primary active transport
• Energy stored in ionic gradients is used
indirectly to drive transport of other solutes
Secondary Active Transport
• Cotransport—always transports more than one
substance at a time
– Symport system: Two substances transported in same
direction
– Antiport system: Two substances transported in opposite
directions
Symport
• carries 2 or more
solutes thru the
membrane
simultaneously in
the SAME direction
• Cotransport =
process
Antiport
• Carriers 2 or more
solutes in the
OPPOSITE
directions
• Counter transport
= process
Extracellular fluid
Glucose
Na+-K+
pump
Na+-glucose
symport
transporter
loading
glucose from
ECF
Na+-glucose
symport transporter
releasing glucose
into the cytoplasm
Cytoplasm
1 The ATP-driven Na+-K+ pump
2 As Na+ diffuses back across the
stores energy by creating a
steep concentration gradient for
Na+ entry into the cell.
membrane through a membrane
cotransporter protein, it drives glucose
against its concentration gradient
into the cell. (ECF = extracellular fluid)
Figure 3.11
Extracellular fluid
Na+-K+
pump
Cytoplasm
1 The ATP-driven Na+-K+ pump
stores energy by creating a
steep concentration gradient for
Na+ entry into the cell.
Figure 3.11 step 1
Extracellular fluid
Glucose
Na+-K+
pump
Na+-glucose
symport
transporter
loading
glucose from
ECF
Na+-glucose
symport transporter
releasing glucose
into the cytoplasm
Cytoplasm
1 The ATP-driven Na+-K+ pump
2 As Na+ diffuses back across the
stores energy by creating a
steep concentration gradient for
Na+ entry into the cell.
membrane through a membrane
cotransporter protein, it drives glucose
against its concentration gradient
into the cell. (ECF = extracellular fluid)
Figure 3.11 step 2
Vesicular Transport
• Transport of large particles, macromolecules,
and fluids across plasma membranes
• Requires cellular energy (e.g., ATP)
Vesicular Transport
• Functions:
– Exocytosis—transport out of cell
– Endocytosis—transport into cell
– Transcytosis—transport into, across, and then out
of cell
– Substance (vesicular) trafficking—transport from
one area or organelle in cell to another
Endocytosis and Transcytosis
• Involve formation of protein-coated vesicles
• Often receptor mediated, therefore very
selective
Types
• Endocytosis – vesicular process that brings
matter INTO the cell
• Exocytosis – vesicular process that releases
matter OUTSIDE the cell
Exocytosis
Figure 3.12a
Vesicular Transport
• Transcytosis – moving substances into, across,
and then out of a cell
• Vesicular trafficking – moving substances from
one area in the cell to another
• Phagocytosis – pseudopods engulf solids and
bring them into the cell’s interior
Vesicular Transport
• Fluid-phase endocytosis – the plasma
membrane infolds, bringing extracellular fluid
and solutes into the interior of the cell
• Receptor-mediated endocytosis – clathrincoated pits provide the main route for
endocytosis and transcytosis
• Non-clathrin-coated vesicles – caveolae that
are platforms for a variety of signaling
molecules
1 Coated pit ingests
substance.
Extracellular fluid
Protein coat
(typically
clathrin)
2 Proteincoated
vesicle
detaches.
Plasma
membrane
Cytoplasm
3 Coat proteins detach
and are recycled to
plasma membrane.
Transport
vesicle
Endosome
Uncoated
endocytic vesicle
4 Uncoated vesicle fuses
with a sorting vesicle
called an endosome.
Lysosome
5 Transport
vesicle containing
membrane components
moves to the plasma
membrane for recycling.
6 Fused vesicle may (a) fuse
(a)
with lysosome for digestion
of its contents, or (b) deliver
its contents to the plasma
membrane on the
opposite side of the cell
(transcytosis).
(b)
Figure 3.12
1 Coated pit ingests
substance.
Extracellular fluid
Protein coat
(typically
clathrin)
Plasma
membrane
Cytoplasm
Figure 3.12 step 1
1 Coated pit ingests
substance.
Extracellular fluid
Protein coat
(typically
clathrin)
2 Proteincoated
vesicle
detaches.
Plasma
membrane
Cytoplasm
Figure 3.12 step 2
1 Coated pit ingests
substance.
Extracellular fluid
Protein coat
(typically
clathrin)
2 Proteincoated
vesicle
detaches.
Plasma
membrane
Cytoplasm
3 Coat proteins detach
and are recycled to
plasma membrane.
Figure 3.12 step 3
1 Coated pit ingests
substance.
Extracellular fluid
Protein coat
(typically
clathrin)
2 Proteincoated
vesicle
detaches.
Plasma
membrane
Cytoplasm
3 Coat proteins detach
and are recycled to
plasma membrane.
Endosome
Uncoated
endocytic vesicle
4 Uncoated vesicle fuses
with a sorting vesicle
called an endosome.
Figure 3.12 step 4
1 Coated pit ingests
substance.
Extracellular fluid
Protein coat
(typically
clathrin)
2 Proteincoated
vesicle
detaches.
Plasma
membrane
Cytoplasm
3 Coat proteins detach
and are recycled to
plasma membrane.
Transport
vesicle
Endosome
Uncoated
endocytic vesicle
4 Uncoated vesicle fuses
with a sorting vesicle
called an endosome.
5 Transport
vesicle containing
membrane components
moves to the plasma
membrane for recycling.
Figure 3.12 step 5
1 Coated pit ingests
substance.
Extracellular fluid
Protein coat
(typically
clathrin)
2 Proteincoated
vesicle
detaches.
Plasma
membrane
Cytoplasm
3 Coat proteins detach
and are recycled to
plasma membrane.
Transport
vesicle
Endosome
Uncoated
endocytic vesicle
4 Uncoated vesicle fuses
with a sorting vesicle
called an endosome.
Lysosome
5 Transport
vesicle containing
membrane components
moves to the plasma
membrane for recycling.
6 Fused vesicle may (a) fuse
(a)
with lysosome for digestion
of its contents, or (b) deliver
its contents to the plasma
membrane on the
opposite side of the cell
(transcytosis).
(b)
Figure 3.12 step 6
Endocytosis
• Phagocytosis—pseudopods engulf solids and
bring them into cell’s interior
– Macrophages and some white blood cells
Phagosome
(a) Phagocytosis
The cell engulfs a large
particle by forming projecting pseudopods (“false
feet”) around it and enclosing it within a membrane
sac called a phagosome.
The phagosome is
combined with a lysosome.
Undigested contents remain
in the vesicle (now called a
residual body) or are ejected
by exocytosis. Vesicle may
or may not be proteincoated but has receptors
capable of binding to
microorganisms or solid
particles.
Figure 3.13a
Endocytosis
• Fluid-phase endocytosis (pinocytosis)—
plasma membrane infolds, bringing
extracellular fluid and solutes into interior of
the cell
– Nutrient absorption in the small intestine
(b) Pinocytosis
The cell “gulps” drops of
extracellular fluid containing
solutes into tiny vesicles. No
receptors are used, so the
process is nonspecific. Most
vesicles are protein-coated.
Vesicle
Figure 3.13b
Endocytosis
• Receptor-mediated endocytosis—clathrincoated pits provide main route for endocytosis
and transcytosis
– Uptake of enzymes low-density lipoproteins, iron,
and insulin
Vesicle
Receptor recycled
to plasma membrane
(c) Receptor-mediated
endocytosis
Extracellular substances
bind to specific receptor
proteins in regions of coated
pits, enabling the cell to
ingest and concentrate
specific substances
(ligands) in protein-coated
vesicles. Ligands may
simply be released inside
the cell, or combined with a
lysosome to digest contents.
Receptors are recycled to
the plasma membrane in
vesicles.
Figure 3.13c
Exocytosis
• Examples:
– Hormone secretion
– Neurotransmitter release
– Mucus secretion
– Ejection of wastes
Summary of Active Processes
Process
Energy Source
Example
Primary active transport
ATP
Pumping of ions across
membranes
Secondary active
transport
Ion gradient
Movement of polar or charged
solutes across membranes
Exocytosis
ATP
Secretion of hormones and
neurotransmitters
Phagocytosis
ATP
White blood cell phagocytosis
Pinocytosis
ATP
Absorption by intestinal cells
Receptor-mediated
endocytosis
ATP
Hormone and cholesterol uptake
• Also see Table 3.2
Thank you
Membrane Potential
• Separation of oppositely charged particles
(ions) across a membrane creates a
membrane potential (potential energy
measured as voltage)
• Resting membrane potential (RMP): Voltage
measured in resting state in all cells
– Ranges from –50 to –100 mV in different cells
– Results from diffusion and active transport of ions
(mainly K+)
Generation and Maintenance of RMP
1. The Na+ -K+ pump continuously ejects Na+
from cell and carries K+ back in
2. Some K+ continually diffuses down its
concentration gradient out of cell through K+
leakage channels
3. Membrane interior becomes negative
(relative to exterior) because of large anions
trapped inside cell
Generation and Maintenance of RMP
4. Electrochemical gradient begins to attract K+
back into cell
5. RMP is established at the point where the
electrical gradient balances the K+
concentration gradient
6. A steady state is maintained because the rate
of active transport is equal to and depends
on the rate of Na+ diffusion into cell
1 K+ diffuse down their steep
Extracellular fluid
concentration gradient (out of the cell)
via leakage channels. Loss of K+ results
in a negative charge on the inner
plasma membrane face.
2 K+ also move into the cell
because they are attracted to the
negative charge established on the
inner plasma membrane face.
3 A negative membrane potential
Potassium
leakage
channels
Cytoplasm
(–90 mV) is established when the
movement of K+ out of the cell equals
K+ movement into the cell. At this
point, the concentration gradient
promoting K+ exit exactly opposes the
electrical gradient for K+ entry.
Protein anion (unable to
follow K+ through the
membrane)
Figure 3.15
Cell-Environment Interactions
• Involves glycoproteins and proteins of
glycocalyx
– Cell adhesion molecules (CAMs)
– Membrane receptors
Roles of Cell Adhesion Molecules
• Anchor cells to extracellular matrix or to each
other
• Assist in movement of cells past one another
• CAMs of blood vessel lining attract white
blood cells to injured or infected areas
• Stimulate synthesis or degradation of adhesive
membrane junctions
• Transmit intracellular signals to direct cell
migration, proliferation, and specialization
Roles of Membrane Receptors
• Contact signaling—touching and recognition of cells; e.g.,
in normal development and immunity
• Chemical signaling—interaction between receptors and
ligands (neurotransmitters, hormones and paracrines) to
alter activity of cell proteins (e.g., enzymes or chemically
gated ion channels)
• G protein–linked receptors—ligand binding activates a G
protein, affecting an ion channel or enzyme or causing
the release of an internal second messenger, such as
cyclic AMP
1 Ligand (1st
messenger) binds
to the receptor.
2 The activated receptor
binds to a G protein and
activates it.
3 Activated G protein activates
(or inactivates) effector protein
(e.g., an enzyme) by causing its
shape to change.
Extracellular fluid
Effector protein
(e.g., an enzyme)
Ligand
Receptor
G protein
GDP
Inactive 2nd
messenger
Active 2nd
messenger
Activated
kinase
enzymes
Cascade of cellular responses
(metabolic and structural changes)
4 Activated effector
enzymes catalyze reactions
that produce 2nd
messengers in the cell
5 Second messengers
activate other enzymes
or ion channels
6 Kinase enzymes transfer
phosphate groups from ATP to
specific proteins and activate a
series of other enzymes that
trigger various cell responses.
Intracellular fluid
Figure 3.16
1 Ligand (1st
messenger) binds
to the receptor.
Extracellular fluid
Ligand
Receptor
Intracellular fluid
Figure 3.16 step 1
1 Ligand (1st
messenger) binds
to the receptor.
2 The activated receptor
binds to a G protein and
activates it.
Extracellular fluid
Ligand
Receptor
G protein
GDP
Intracellular fluid
Figure 3.16 step 2
1 Ligand (1st
messenger) binds
to the receptor.
2 The activated receptor
binds to a G protein and
activates it.
3 Activated G protein activates
(or inactivates) effector protein
(e.g., an enzyme) by causing its
shape to change.
Extracellular fluid
Effector protein
(e.g., an enzyme)
Ligand
Receptor
G protein
GDP
Intracellular fluid
Figure 3.16 step 3
1 Ligand (1st
messenger) binds
to the receptor.
2 The activated receptor
binds to a G protein and
activates it.
3 Activated G protein activates
(or inactivates) effector protein
(e.g., an enzyme) by causing its
shape to change.
Extracellular fluid
Effector protein
(e.g., an enzyme)
Ligand
Receptor
G protein
GDP
Inactive 2nd
messenger
Active 2nd
messenger
4 Activated effector
enzymes catalyze reactions
that produce 2nd
messengers in the cell
Intracellular fluid
Figure 3.16 step 4
1 Ligand (1st
messenger) binds
to the receptor.
2 The activated receptor
binds to a G protein and
activates it.
3 Activated G protein activates
(or inactivates) effector protein
(e.g., an enzyme) by causing its
shape to change.
Extracellular fluid
Effector protein
(e.g., an enzyme)
Ligand
Receptor
G protein
GDP
Inactive 2nd
messenger
Active 2nd
messenger
4 Activated effector
enzymes catalyze reactions
that produce 2nd
messengers in the cell
5 Second messengers
activate other enzymes
or ion channels
Activated
kinase
enzymes
Intracellular fluid
Figure 3.16 step 5
1 Ligand (1st
messenger) binds
to the receptor.
2 The activated receptor
binds to a G protein and
activates it.
3 Activated G protein activates
(or inactivates) effector protein
(e.g., an enzyme) by causing its
shape to change.
Extracellular fluid
Effector protein
(e.g., an enzyme)
Ligand
Receptor
G protein
GDP
Inactive 2nd
messenger
Active 2nd
messenger
Activated
kinase
enzymes
Cascade of cellular responses
(metabolic and structural changes)
4 Activated effector
enzymes catalyze reactions
that produce 2nd
messengers in the cell
5 Second messengers
activate other enzymes
or ion channels
6 Kinase enzymes transfer
phosphate groups from ATP to
specific proteins and activate a
series of other enzymes that
trigger various cell responses.
Intracellular fluid
Figure 3.16 step 6