1. Chapter 3 Cells: The Living Unit Part B Shilla Chakrabarty, Ph.D.

2. Membrane Transport: Active Processes
Active processes use energy (ATP) to move solutes across a living plasma membrane
Two types of active processes:
Active transport
Vesicular transport

3. Active Transport Requires carrier proteins (solute pumps)
Moves solutes against a concentration gradient
Types of active transport:
Primary active transport
Secondary active transport

4. Primary Active Transport
Figure 3.10
Extracellular fluid
K+ is released from the pump protein and Na+ sites are ready to bind Na+ again. The cycle repeats.
Binding of Na+ promotes phosphorylation of the protein by ATP.
Cytoplasmic Na+ binds to pump protein.
Na+
Na+-K+ pump
K+ released
ATP-binding site
Na+ bound
Cytoplasm
ATP
ADP P K+
K+ binding triggers release of the phosphate. Pump protein returns to its original conformation.
Phosphorylation causes the protein to change shape, expelling Na+ to the outside.
Extracellular K+ binds to pump protein.
Na+ released
K+ bound Pi 1 2 3 4 5 6
Energy from hydrolysis of ATP causes shape change in transport protein so that bound solutes (ions) are “pumped” across the membrane
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

5. 1 Extracellular fluid Na+ Na+-K+ pump K+ ATP-binding site Cytoplasm
Cytoplasmic Na+ binds to pump protein.
Figure 3.10 step 1

6. Na+ bound P
ATP
ADP 2
Binding of Na+ promotes phosphorylation of the protein by ATP.
Figure 3.10 step 2

7. Na+ released P
Phosphorylation causes the protein to change shape, expelling Na+ to the outside. 3
Figure 3.10 step 3

8. K+ P 4
Extracellular K+ binds to pump protein.
Figure 3.10 step 4

9. K+ bound Pi
K+ binding triggers release of the phosphate. Pump protein returns to its original conformation. 5
Figure 3.10 step 5

10. 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

11. Stages of Primary Active Transport
Extracellular fluid
Na+
Na+-K+ pump
ATP-binding site K+
Na+ bound
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.
Phosphorylation causes the protein to change shape, expelling Na+ to the outside. 3 P 4
Extracellular K+ binds to pump protein.
Figure 3.10

12. 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
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

13. 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

14. Secondary Active Transport Symport System Transporting Sodium Ions And Glucose Into Cells
Figure 3.11
The ATP-driven Na+-K+ pump
stores energy by creating a
steep concentration gradient for
Na+ entry into the cell.
As Na+ diffuses back across the
membrane through a membrane
cotransporter protein, it drives glucose
against its concentration gradient
into the cell. (ECF = extracellular fluid)
Na+-glucose
symport
transporter
loading
glucose from
ECF
symport transporter
releasing glucose
into the cytoplasm
Glucose
Na+-K+
pump
Cytoplasm
Extracellular fluid 1 2

15. Vesicular Transport
Transport of large particles, macromolecules, and fluids across plasma membranes
Requires cellular energy (e.g., ATP)
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

16. Exocytosis Examples: Hormone secretion Neurotransmitter release
Figure 3.14a 1
The membrane-
bound vesicle
migrates to the
plasma membrane. 2
There, proteins
at the vesicle
surface (v-SNAREs)
bind with t-SNAREs
(plasma membrane
proteins).
Extracellular
fluid
Plasma membrane
SNARE (t-SNARE)
Secretory
vesicle
Vesicle
SNARE
(v-SNARE)
Molecule to
be secreted
Cytoplasm
Fused
v- and
t-SNAREs 3
The vesicle
and plasma
membrane fuse
and a pore
opens up. 4
contents are
released to the
cell exterior.
Fusion pore formed
Examples:
Hormone secretion
Neurotransmitter release
Mucus secretion
Ejection of wastes

17. Endocytosis
Phagocytosis — pseudopods engulf solids and bring them into cell’s interior
Example: Macrophages and some white blood cells
Pinocytosis — Fluid-phase endocytosis. Plasma membrane infolds, bringing extracellular fluid and solutes into interior of the cell
Example: Nutrient absorption in the small intestine

18. Endocytosis: Phagocytosis
The cell engulfs a large
particle by forming pro-
jecting pseudopods (“false
feet”) around it and en-
closing 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 protein-
coated but has receptors
capable of binding to
microorganisms or solid
particles.
Phagosome
Figure 3.13a

19. Endocytosis: Pinocytosis
(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

20. Endocytosis: Receptor-mediated Endocytosis
Clathrin-coated pits provide main route for endocytosis and transcytosis
Example: Uptake of enzymes low-density lipoproteins, iron, and insulin

21. Endocytosis: Receptor-mediated Endocytosis
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

22. Endocytosis and Transcytosis
Coated pit ingests substance.
Protein- coated vesicle detaches.
Coat proteins detach and are recycled to plasma membrane.
Uncoated vesicle fuses with a sorting vesicle called an endosome.
Transport vesicle containing membrane components moves to the plasma membrane for recycling.
Fused vesicle may (a) fuse with lysosome for digestion of its contents, or (b) deliver its contents to the plasma membrane on the opposite side of the cell (transcytosis).
Protein coat (typically clathrin)
Extracellular fluid
Plasma membrane
Endosome
Lysosome
Transport vesicle
(b)
(a)
Uncoated endocytic vesicle
Cytoplasm 1 2 3 4 5 6
Involve formation of protein-coated vesicles
Often receptor mediated, therefore very selective
Figure 3.12

23. Coated pit ingests substance. Extracellular fluid Plasma membrane1
Extracellular fluid
Plasma membrane
Protein coat (typically clathrin)
Cytoplasm
Figure 3.12 step 1

24. Coated pit ingests substance. Extracellular fluid Plasma membrane1
Extracellular fluid
Plasma membrane
Protein coat (typically clathrin)
Cytoplasm 2
Protein- coated vesicle detaches.
Figure 3.12 step 2

25. Coated pit ingests substance. Extracellular fluid Plasma membrane1
Extracellular fluid
Plasma membrane
Protein coat (typically clathrin)
Cytoplasm 2
Protein- coated vesicle detaches.
Coat proteins detach and are recycled to plasma membrane. 3
Figure 3.12 step 3

26. Coated pit ingests substance. Extracellular fluid Plasma membrane1
Coated pit ingests substance.
Extracellular fluid
Plasma membrane
Protein coat (typically clathrin)
Cytoplasm 2
Protein- coated vesicle detaches. 3
Coat proteins detach and are recycled to plasma membrane.
Endosome
Uncoated endocytic vesicle
Uncoated vesicle fuses with a sorting vesicle called an endosome. 4
Figure 3.12 step 4

27. Coated pit ingests substance. Extracellular fluid Plasma membrane1
Coated pit ingests substance.
Extracellular fluid
Plasma membrane
Protein coat (typically clathrin)
Cytoplasm 2
Protein- coated vesicle detaches.
Coat proteins detach and are recycled to plasma membrane. 3
Transport vesicle
Endosome
Uncoated endocytic vesicle 4
Uncoated vesicle fuses with a sorting vesicle called an endosome.
Transport vesicle containing membrane components moves to the plasma membrane for recycling. 5
Figure 3.12 step 5

28. Coated pit ingests substance. Extracellular fluid Plasma membrane1
Extracellular fluid
Plasma membrane
Protein coat (typically clathrin)
Cytoplasm 2
Protein- coated vesicle detaches. 3
Coat proteins detach and are recycled to plasma membrane.
Transport vesicle
Endosome
Uncoated endocytic vesicle
Uncoated vesicle fuses with a sorting vesicle called an endosome. 4
Transport vesicle containing membrane components moves to the plasma membrane for recycling. 5
Lysosome 6
Fused vesicle may (a) fuse 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)
(a)
Figure 3.12 step 6

29. Summary of Active Processes
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
Secretion of hormones and neurotransmitters
Phagocytosis
White blood cell phagocytosis
Pinocytosis
Absorption by intestinal cells
Receptor-mediated endocytosis
Hormone and cholesterol uptake

30. Membrane Potential
A membrane potential is caused by the separation of oppositely charged particles (ions) across a membrane. (The potential energy is 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+)

31. Generation and Maintenance of RMP
The Na+- K+ pump continuously ejects Na+ from cell and carries K+ back in
Some K+ continually diffuses down its concentration gradient out of cell through K+ leakage channels
Membrane interior becomes negative (relative to exterior) because of large anions trapped inside cell
Electrochemical gradient begins to attract K+ back into cell
RMP is established at the point where the electrical gradient balances the K+ concentration gradient
A steady state is maintained because the rate of active transport is equal to and depends on the rate of Na+ diffusion into cell

32. Generation and Maintenance of RMP
K+ diffuse down their steep
concentration gradient (out of the cell)
via leakage channels. Loss of K+ results
in a negative charge on the inner
plasma membrane face. 1
Extracellular fluid
K+ also move into the cell
because they are attracted to the
negative charge established on the
inner plasma membrane face. 2
A negative membrane potential
(–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. 3
Potassium
leakage
channels
Protein anion (unable to
follow K+ through the
membrane)
Cytoplasm
Figure 3.15

33. Cell-Environment Interactions
Involves glycoproteins and proteins of glycocalyx
Cell adhesion molecules (CAMs)
Membrane receptors

34. Roles of Cell Adhesion Molecules (CAMs)
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

35. 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

36. Mechanism Of Action Of G Protein-Coupled Receptor2 1 3 4
Plasma membrane
G protein (inactive)
CYTOPLASM
Enzyme
Activated receptor
Signaling molecule
Inactive enzyme
Activated enzyme
Cellular response
GDP
GTP
P i

37. Cascade of cellular responses (metabolic and structural changes)
Mechanism Of Action Of G Protein-Coupled Receptor 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 4
Activated effector
enzymes catalyze reactions
that produce 2nd
messengers in the cell
Inactive 2nd
messenger
G protein 5
GDP
Active 2nd
messenger
Second messengers
activate other enzymes
or ion channels 6
Activated
kinase
enzymes
Kinase enzymes transfer
phosphate groups from ATP to
specific proteins and activate a
series of other enzymes that
trigger various cell responses.
Cascade of cellular responses
(metabolic and structural changes)
Intracellular fluid
Figure 3.16