AP Biology Chapter 11 Lecture Notes

1
AP BiologyChapter 11 Lecture Notes

• Cell Communication

2
Evolution of Cell Signaling

• Signal transduction pathway series of steps by
  which a signal on a cells surface is converted
  into a specific cellular response
• Pathway similarities suggest ancestral signaling
  molecules evolved in prokaryotes and modified
  later in eukaryotes
• Concentration of signaling molecules allows
  bacteria to detect population density (quorum
  sensing) may form ? biofilms

3
Local and Long-Distance Signaling

• Cells in multicellular organisms communicate via
  chemical messengers
• Animal and plant cells have cell junctions that
  directly connect cytoplasm of adjacent cells
• Gap junctions
• Plasmodesmata
• Contact-Dependent animal cells may communicate
  by direct contact (cell-cell recognition)
• Immune Response
• In other cases, animal cells
  communicate using messenger
  molecules that travel only short
  distances
• Paracrine signals (example growth
  factors)
• Neurotransmitters
• In long-distance signaling, plants and animals
  use chemicals called hormones (endocrine signals)

?
?
?
?
4
Three Stages of Cell Signaling

• Cells receiving signals go through 3 processes
• Reception
• Transduction
• Response

Animation Overview of Cell Signaling
EXTRACELLULAR FLUID
CYTOPLASM
Plasma membrane
Transduction
Response
Reception
1
2
3
Receptor
Activation of cellular response
Relay molecules in a signal transduction pathway
Signaling molecule
5
Reception Signal Molecule Binds to a Receptor
Protein, Causing it to Change Shape

• Most receptors are membrane proteins b/c most
  signal molecules are very polar and cant pass
  through membrane
• 3 types of membrane receptors
• G protein-coupled receptors (C)
• Receptor tyrosine kinases (B)
• Ion channel receptors (A)
• Another type of receptor is intracellular (D)
• Binding between signal molecule (ligand) and
  receptor is highly specific
• Shape change in receptor is often initial
  transduction of signal

6

• G protein-coupled receptor
• Plasma membrane receptor that works with help of
  a G protein (a peripheral protein)
• Acts as On/off Switch If GDP is bound to the G
  protein, the G protein is inactive
• Many medications affect cells in this manner

Plasma membrane
Inactive enzyme
G protein-coupled receptor
Signaling molecule
Activated receptor
GDP
GDP
GTP
Enzyme
G protein (inactive)
CYTOPLASM
2
1
Activated enzyme
GTP
GDP
P
i
Cellular response
4
G Protein Animation
3
7

• Receptor tyrosine kinases (enzymes)
• Membrane receptors attach phosphates to tyrosines
  (peripheral proteins)
• Can trigger (On/off switch) multiple signal
  transduction pathways at once

Signaling molecule (ligand)
Ligand-binding site
Signaling molecule
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyrosines
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Receptor tyrosine kinase proteins
Dimer
CYTOPLASM
1
2
Tyrosine Kinase (Enzyme) Animation
Activated relay proteins
Cellular response 1
P
P
Tyr
Tyr
P
Tyr
Tyr
Tyr
Tyr
P
Tyr
Tyr
P
P
Tyr
Tyr
P
Tyr
Tyr
P
Cellular response 2
P
P
Tyr
Tyr
P
Tyr
Tyr
Tyr
Tyr
P
6
6 ADP
ATP
Activated tyrosine kinase regions
Fully activated receptor tyrosine kinase
Inactive relay proteins
3
4
8

• Ligand-gated Ion Channel Receptor
• acts as a gate when receptor changes shape
• allows ions, such as Na or Ca2, through a
  channel in the receptor
• Important in neurons

1
Signaling molecule (ligand)
Gate closed
Ions
Plasma membrane
Ligand-gated ion channel receptor
2
Gate open
Cellular response
3
Gate closed
Ion Channel Animation
9
Intracellular Receptors
Hormone (testosterone)
EXTRACELLULAR FLUID

• Found in cytosol or nucleus of target cells
• Small hydrophobic chemical messengers (i.e.
  steroid and thyroid hormones) readily cross
  membrane and activate receptors
• Activated hormone-receptor complex can act as a
  transcription factor, turning on specific genes

Plasma membrane
Receptor protein
Hormone- receptor complex
DNA
mRNA
NUCLEUS
New protein
CYTOPLASM
Intracellular Receptors and Transcription Video
10
(No Transcript)
11
Transduction Cascades of Molecular Interactions
that Relay Signals from Receptors to Target
Molecules in the Cell

• Signal transduction usually involves multiple
  steps
• Multistep pathways can amplify a signal
• Multistep pathways provide more opportunities for
  coordination and regulation of the cellular
  response
• The molecules that relay a signal from receptor
  to response are mostly proteins
• Like falling dominoes, one receptor activates a
  protein, which activates another, and so on,
  until the the response is activated
• At each step, the signal is transduced into a
  different form, usually a shape change in a
  protein

12
Protein Phosphorylation and Dephosphorylation

• In many pathways, the signal is transmitted by a
  cascade of protein phosphorylations
• Protein kinases transfer phosphates from ATP to a
  protein (phosphorylation)
• Protein phosphatases remove phosphates from
  proteins (dephosphorylation)
• Phosphorylation and dephosphorylation system acts
  as a molecular switch, turning activities on/off

Amplification of Signals Video
13
Fig. 11-9
Signaling molecule
Receptor
Activated relay molecule
Inactive protein kinase 1
Active protein kinase 1
Inactive protein kinase 2
ATP
Phosphorylation cascade
ADP
P
Active protein kinase 2
PP
P
i
Inactive protein kinase 3
ATP
ADP
P
Active protein kinase 3
PP
P
i
Inactive protein
ATP
P
ADP
Active protein
Cellular response
PP
P
i
14
Comprehension CheckIndicate where each of the
labels should appear in the figure.
Transduction
Response
Reception
Activation of Cellular Response
Relay Molecules
Receptor
Signaling Molecule

• Receptor
• Relay molecules
• Transduction
• Activation of cellular response

• Signaling molecule
• Response
• Reception

15
Small Molecules and Ions as Second Messengers

• The extracellular signal molecule (ligand) that
  binds to the receptor is a pathways first
  messenger
• Second messengers small, nonprotein,
  water-soluble molecules or ions that spread
  throughout a cell by diffusion
• Second messengers participate in pathways
  initiated by G protein-coupled receptors and
  receptor tyrosine kinases (not needed for
  ligand-gated ions)
• Cyclic AMP and calcium ions are common second
  messengers

Animation Signal Transduction Pathways
Second Messengers Video
16
Fig. 11-11
First messenger
Adenylyl cyclase
G protein
GTP
G protein-coupled receptor
ATP
Second messenger
cAMP
Protein kinase A
Cellular responses
17
Fig. 11-13-3
EXTRA- CELLULAR FLUID
Signaling molecule (first messenger)
G protein
DAG
GTP
G protein-coupled receptor
PIP2
Phospholipase C
IP3
(second messenger)
IP3-gated calcium channel
Endoplasmic reticulum (ER)
Various proteins activated
Cellular responses
Ca2
Ca2 (second messenger)
CYTOSOL
18
Response Cell Signaling Leads to Regulation of
Transcription or Cytoplasmic Activities

• Ultimately, signal transduction pathway leads to
  regulation of cell activities
• Cells response to extracellular signal is called
  output response
• may occur in cytoplasm or involve action in
  nucleus
• Many signaling pathways regulate synthesis of
  enzymes or other proteins, usually by turning
  genes on/off in nucleus
• Final activated molecule may function as
  transcription factor
• Other pathways regulate the activity of enzymes
• Signaling pathways can also affect physical
  characteristics of cell, for example, cell
  shape/cell movement

19
Termination of the Signal

• Inactivation mechanisms are essential to cell
  signaling
• Occurs when signal molecules leave the receptor
  and the receptor reverts to its inactive state

20
Apoptosis (programmed cell death) integrates
multiple cell-signaling pathways
Ced-9 protein (active) inhibits Ced-4 activity

• Apoptosis programmed or controlled cell suicide
• Cell is chopped and packaged into vesicles that
  are digested by scavenger cells
• Prevents enzymes from leaking out of a dying cell
  and damaging neighboring cells
• Important during embryonic development

Mitochondrion
Ced-4
Ced-3
Receptor for death- signaling molecule
Inactive proteins
(a) No death signal
Ced-9 (inactive)
Cell forms blebs
Death- signaling molecule
Active Ced-4
Active Ced-3
Other proteases
Nucleases
Activation cascade
(b) Death signal
21
Comprehension CheckWhich of the following best
describes a signal transduction pathway?

• A. binding of a signal molecule to a cell protein
• B. catalysis mediated by an enzyme
• C. sequence of changes in a series of molecules
  resulting in a response
• D. binding of a ligand on one side of a membrane
  that results in a change on the other side
• E. the cells detection of a chemical or
  mechanical stimulus

22
Comprehension CheckA steroid hormone binds to an
intracellular receptor. When it does, the
resulting complex is able to do which of the
following? Why?

• A. open channels in the membrane for other
  substances to enter
• B. open channels in the nuclear envelope for
  cytoplasmic molecules to enter
• C. mediate the transfer of phosphate groups
  to/from ATP
• D. act as a transcription factor in the nucleus
• E. make water-soluble molecules able to diffuse
  across membranes

23
Comprehension CheckIn reactions mediated by
protein kinases, what does phosphorylation of
successive proteins do to drive the reaction?

• A. make functional ATP
• B. change a protein from its inactive to its
  active form
• C. change a protein from its active to its
  inactive form
• D. alter the permeability of the cells membranes
• E. produce an increase in the cells store of
  inorganic phosphates

24
Comprehension CheckWhich of the following is an
example of signal amplification?

• A. catalysis of many cAMP molecules by several
  simultaneously binding signal molecules
• B. activation of 100 molecules by a single signal
  binding event
• C. activation of a specific gene by a growth
  factor
• D. activation of an enzyme molecule
• E. utilization of a second messenger system

25
Comprehension CheckWhich of the following is not
a usual part of the process of apoptosis?

• A. cell shrinkage and blebbing
• B. destruction of the cells DNA
• C. formation of numerous vesicles to be digested
• D. damage to all cells in the immediate vicinity
• E. activation and deactivation of specific
  proteins