Signal Transduction Pathways

What happens inside of a cell when a signal has been received?

Why?
When cells release signal molecules (ligands) to send a message, and the message is received, a whole
host of events take place inside the cell. The ultimate goal is a response—a gene is turned on, a protein is
manufactured, an enzyme is activated, the cell divides or dies, etc. There are many responses that could
occur, but the pathways to those responses are very similar.

Model 1 – Basic Signal Transduction Pathway

Ligand e
an
br Cytoplasm
em
m
Cell Activated
relay Response
protein 2

Activated Activated
relay relay Response
protein 1 protein 2
Receptor
protein
Activated
relay Response
protein 2

1. According to Model 1, does the signal transduction pathway occur inside or outside of a cell?
inside of a cell

2. Compare the shape of the ligand in Model 1 to the shape of the receptor protein.
the ligand fits perfectly into the receptor protein, much like a key in a lock.

3. The four steps in the signal transduction pathway are listed below. Label the diagram above with
the Roman numerals to indicate where on the diagram each step is taking place.
I. Signaling
II. Reception done

III. Transduction
IV. Response

Signal Transduction Pathways 1

 4. Based on the diagram in Model 1 and your labels, propose a definition for “transduction” in the
context of the signal transduction pathway.

5. Amplification often occurs during the transduction step in the signal transduction pathway.
a. Define “amplification” as it is used in everyday language.
the increase of something, such as a sound, to make it stronger or greater

b. Explain how the signal in Model 1 was amplified.

the signal was passed from one protein into several proteins being activated

6. List several possible responses that could occur due to a signal being received by a cell. (Hint:
There are several listed in the Why? box.)

7. Once the response is achieved in a cell, what would need to occur to stop the response?
A relay protein would have to be deactivated

Read This!
Signal transduction pathways are as varied as the species on Earth, but each of them follow the same basic
steps. Reception may occur by the ligand attaching to a receptor protein or the ligand may travel through
the cell membrane by diffusion. Transduction may occur by one protein or by several proteins activating
each other in a cascade. Keep this in mind as you continue working through this activity.

2 POGIL™ Activities for AP* Biology

Model 2 – Phosphorylation Cascade
Nonamplification step

Amplification step response

Inactive
e
an protein
br Cytoplasm kinase 1
em Nucleus
ll m
Ce Active
P
P Active
protein
protein Active
kinase 2 Inactive
kinase 1 Inactive transcription
1 protein
transcription factor
kinase 2
factor ATP
2 7
reception Active relay
protein
P ADP
Inactive relay Active
Active
protein protein
3 protein
kinase 3
kinase 1
PP
P ATP Active
P
Inactive 4 protein
PP
Active protein kinase 2 P
protein kinase 1
ADP
kinase 3 Inactive Active 6
ATP
protein 5 protein
kinase 2 kinase 3
ADP
Inactive
protein Inactive
kinase 3 protein
transduction kinase 3

8. Refer to Model 2. Describe the event that sets off a phosphorylation cascade inside of a cell.
A ligand binds to a receptor protein to send a signal to the cell

9. Phosphorylation is a process that adds a phosphate group onto a protein to “activate” it—that
is, to change its shape enough that it can function properly.
a. Which step(s) in the phosphorylation cascade illustrated in Model 2 include phosphorylation?
4, 5, 7

b. Where do the phosphate groups come from that are added to the proteins during
phosphorylation?
it comes from ATP which later is turned into ADP

10. According to Model 2, what class of enzymes performs phosphorylation?

kinase protein

11. Identify the steps in Model 2 that represent reception, transduction, and response for the
phosphorylation pathway. done

Reception Transduction Response

Signal Transduction Pathways 3

 12. What is the cell’s response to the signal received in Model 2?

transcription of a DNA segment

13. In Model 2, steps 3, 4, and 5 are shown as amplification steps. Describe what that means in
terms of this signal transduction pathway example.
each kinase activates one or more of the next kinase enzymes in the sequence

14. What advantage would there be to an organism if the signal transduction pathway had several
amplification steps?
more amplification = more responses and faster

15. Describe what would occur in the cell if the activated protein kinase enzymes continued to be
active for a long period of time.
transcription process would continue to happen

16. What would need to occur in the cell to deactivate the protein kinase enzymes?
the phosphate group must be removed from the protein kinase enzymes

17. Protein phosphatases are enzymes that remove phosphate groups from proteins. Complete the
illustration in Model 2 by adding at least two protein phosphatases (PP) to show how the cell is
returned to inactive status.
done

18. Although signal transduction pathways vary among species, there are several common elements.
Explain how a biologist might use details about signal transduction pathways used in different
species as evidence for evolutionary relatedness.

Find similarities in the transduction process to see if they relate to an ancient ancestor
relatedness in ligand, receptor, and relay proteins

4 POGIL™ Activities for AP* Biology

Extension Questions
Model 3 – Secondary Messengers
1

Transport protein
ne
Cell membra
Active relay
Cytoplasm protein
2

3
Secondary
messenger

4
Response 5
Active relay
protein

19. Consider the signal transduction pathway in Model 3. What event begins the process of produc-
ing a cellular response?
reception of a ligand in the active site

20. Locate the secondary messenger molecules in Model 3.

a. Describe the type of diffusion that is used to get the secondary messengers into the cell.
facilitated diffusion

b. What activates or opens the transport protein channel that allows the secondary messengers to
enter the cell?
active relay protein, that attaches to the active site of the transport protein

Signal Transduction Pathways 5

Read This!
Relay molecules and secondary messengers have essentially the same jobs in signal transduction pathways.
However, relay molecules are almost always proteins that require activation. They are large and do not dif-
fuse through the cell quickly. Secondary messengers on the other hand are small, water soluble molecules
that can diffuse quickly. They may or may not need activation before they are able to move the signal on
to the next step in the process. Cyclic AMP (cAMP) and calcium ions (Ca2+) are common secondary mes-
sengers in human systems.

21. Within an organism it is critical that signals between cells are very specific. For example, if
ligand A is meant to activate immune system cells to reproduce in response to an infection, it
should not also cause other cells to grow as if they had received a growth hormone. When a
ligand is released, what prevents all of the cells in the body from being affected?
the ligand fits perfectly into a specific receptor. the receptors for ligand A would only be found in immune cells ,
not in other types of cells, so cells that are not immune cells would be exposed to but not bind and form a
reaction to ligand A

6 POGIL™ Activities for AP* Biology