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4.2 Introduction to Signal Transduction

5 min readdecember 28, 2022

Annika Tekumulla

Annika Tekumulla

Haseung Jun

Haseung Jun

Annika Tekumulla

Annika Tekumulla

Haseung Jun

Haseung Jun

Attend a live cram event

Review all units live with expert teachers & students

Transduction is the conversion of a signal into a cellular response. Transduction occurs in one step but the majority of the time it takes multiple changes. Think of it as the translation of the ligand into a response process with instruction manuals.

These multiple steps are called the signal transduction pathway. The signal transduction pathway is when a small collection of signal molecules produce a large response across the cell. The large response across the cell is called the cascade effect. The response of a cascade can be cell growth, gene expression, or secretion of molecules.

Memory Tip: A good way to think about the signal transduction pathway is like a waterfall or a line of falling dominoes.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-Ycxph6P67zG7.gif?alt=media&token=f2f4a002-9caf-45de-814c-39b1d1764f15

Image courtesy of Giphy.

Regardless of the distance between the cells, cell communication always follows the same 3 steps: reception, transduction, and response

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2Fcell-to-cell-communication-9-728.jpg?alt=media&token=b798c7e7-4973-45a5-9766-8985e76330f1

Image courtesy of Winnacunnet Biology.

Reception

Cell surface receptors are receptors that cover the entire membrane. These receptors are important because most signaling molecules are too big to cross the plasma membrane

The plasma membrane contains a phospholipid bilayer. The heads of the phospholipid are “water-loving” which are called hydrophilic. The tails of the phospholipid are “water-hating” which are called hydrophobic.

The phospholipid bilayer is very important because it creates a barrier between the inside and outside of the cell.

Examples of cell surface receptors

Ion Channel Receptors involves a channel that opens and closes. The channel is used to allow the passage of ions across the membrane.

Memory Tip: Think of ion channel receptors as a toll booth. Just like a channel receptor, a toll both opens and closes after an action is made.

G-Protein-Coupled Receptors: Through this process, the ligand binds to the outside part of the cell and changes the form of the inside cytoplasm. This causes a G protein to bind and activate the enzyme adenylyl cyclase. Adenylyl cyclase converts ATP (energy) to cyclic-AMP.

Finally, cyclic-AMP activates other molecules inside the cell which finally leads to the cellular response.

Transduction

Phosphorylation

We have a message. We were able to receive it. So how are proteins exactly activated? Proteins are activated in a process called phosphorylation. Essentially, a phosphate group is added to an already-existing protein to activate it. This phosphate group just comes from the cytoplasm ATP (adenosine triphosphate). When this is added to the protein, the protein will get "powered on" and start doing its job, while the ATP turns into ADP (adenosine diphosphate). After its job is over, the phosphate group will be taken off, powering off the protein (it's a waste of energy to have everything on right? 😉)

Amplification

There's a process within transduction called amplification. Not every signal leads to amplification, but it is a step that can be included. Amplification steps are a sequence of turning on communication pathways to amplify the response, or a process to make the transduction process faster. An example of this is the cAMP or the G-protein, which can be activated with GTP.

Transduction has a link to evolution

Since transduction occurs in an animal, bacterial, plant, and yeast cells, it shows that the signal transduction pathway has evolved from a common ancestor.

Isn’t it crazy that all these different organisms could have something in common?!

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2Fgiphy2.gif?alt=media&token=5d51110e-8a9f-4e89-bd5e-ebfabb8fc3c2

Image courtesy of Giphy.

Response

Depending on the ligand, the response can be anything. This can be turning on an enzyme, transporting a molecule, etc.

Special Cases

Like everything, the signal-transduction pathway has special cases! Two examples are lipid hormones and secondary messenger.

Lipid Hormones

Lipid hormones are a special type of ligand that are made out of...well... lipid. And since the cell membrane is made up of lipid, it doesn't need a receptor on the cell surface. Instead, it can pass through the cell membrane without a transport protein, and the receptor protein is actually inside the cell.

Then, this reception actually produces a transcription factor, and an entirely new protein is synthesized from DNA to mRNA to ribosome. Because the protein is made from scratch in response to the ligand, it can take up to an hour to create a response to the ligand! But on the bright side, slower response means the protein is more long-lasting, so a more functional (?) protein will be created.

Secondary Messenger

As the name suggests, a secondary messenger is just a second messenger. A receptor at the cell's surface receives a ligand, and this activates the transport channel. The transport channel is then opened, where the secondary messenger is allowed to enter the cell. After entering the cell, the secondary messenger binds to the appropriate protein to produce a messenger.

Key Terms to Review (27)

Adenylyl Cyclase

: Adenylyl cyclase is an enzyme that converts ATP to cyclic AMP (cAMP) in response to a signal.

ADP (adenosine diphosphate)

: ADP is an organic compound that plays a crucial role in metabolism. It's converted to ATP for storage of energy during cell metabolism.

Amplification

: In biology, amplification refers to the increase in number or expression of genes or gene products (like proteins).

ATP (Adenosine Triphosphate)

: ATP is a high-energy molecule that stores and provides energy for many biochemical reactions in the cell.

cAMP or cyclic Adenosine Monophosphate

: cAMP is a second messenger important in many biological processes; it transmits signals from hormones outside the cell to its interior.

Cascade Effect

: The cascade effect refers to an ordered sequence of reactions triggered by an event, each of which triggers the next.

Cell Communication

: Cell communication is the process by which cells interact and respond to signals from their environment or other cells. This involves a series of steps including signal reception, transduction, and response.

Cell Surface Receptors

: Cell surface receptors are proteins located on the cell membrane or within cells that bind to external signaling molecules such as hormones or neurotransmitters. They trigger changes inside the cell when activated.

Cyclic-AMP

: Cyclic-AMP (cAMP) is a second messenger used for intracellular signal transduction, such as transferring into cells the effects of hormones like glucagon and adrenaline, which cannot pass through the plasma membrane.

G-Protein-Coupled Receptors

: G-protein-coupled receptors (GPCRs) are large protein complexes that respond to signals outside the cell and activate internal signal transduction pathways and cellular responses by interacting with G-proteins inside the cell.

GTP

: GTP, or Guanosine Triphosphate, is a molecule similar to ATP that provides energy for protein synthesis and signal transduction in cells.

Hydrophilic

: Hydrophilic substances are those that have an affinity for water, meaning they can mix with or dissolve in water.

Hydrophobic

: Hydrophobic refers to substances that repel or do not mix with water.

Ion Channel Receptors

: Ion channel receptors are proteins that span the cell membrane and allow specific ions to pass through when they are open. They open or close in response to a signal molecule binding to the receptor.

Ligand

: A ligand is a molecule that binds to another (usually larger) molecule. In cell communication, it's often the signal molecule that binds to a receptor.

Lipid Hormones

: Lipid hormones are hormones derived from lipids such as sterols and fatty acids. They are typically non-polar and can pass through cell membranes easily.

Phospholipid Bilayer

: The phospholipid bilayer is a two-layered arrangement of phosphate and lipid molecules that form a cell membrane, the hydrophobic lipid ends facing inward and the hydrophilic phosphate ends face outward.

Phosphorylation

: Phosphorylation is a biochemical process where a phosphate group is added to an organic molecule. It's crucial in many cellular processes, including metabolism, transcription, and cell signaling.

Plasma Membrane

: The plasma membrane is a thin layer that separates the inside of cells from their environment. It controls what enters or leaves the cell.

Reception

: Reception is when a signal molecule binds to a receptor protein on or within the target cell, initiating transduction pathways that lead to cellular responses.

Receptor Protein

: A receptor protein is a molecule found on the surface of cells that binds to specific substances, triggering a specific response in the cell.

Response

: In biology, response refers to how an organism changes its behavior or physiology in reaction to changes in its environment.

RNA Polymerase

: RNA polymerase is an enzyme that synthesizes RNA from DNA during transcription.

Secondary Messenger

: Secondary messengers are molecules that relay signals received at receptors on the cell surface — such as the arrival of protein hormones, growth factors, etc. — to target molecules in the cytosol and/or nucleus.

Signal Transduction Pathway

: A signal transduction pathway is a series of molecular events through which cells respond to external signals. These pathways convert these signals into cellular responses.

Transcription of a Gene

: Transcription of a gene is the process by which the DNA sequence in a gene is copied into mRNA (messenger RNA). This mRNA then serves as a template for protein synthesis.

Transduction

: Transduction is the process by which a cell converts one kind of signal or stimulus into another. In biology, this often refers to the transfer of genetic material from one bacterium to another through a virus.

4.2 Introduction to Signal Transduction

5 min readdecember 28, 2022

Annika Tekumulla

Annika Tekumulla

Haseung Jun

Haseung Jun

Annika Tekumulla

Annika Tekumulla

Haseung Jun

Haseung Jun

Attend a live cram event

Review all units live with expert teachers & students

Transduction is the conversion of a signal into a cellular response. Transduction occurs in one step but the majority of the time it takes multiple changes. Think of it as the translation of the ligand into a response process with instruction manuals.

These multiple steps are called the signal transduction pathway. The signal transduction pathway is when a small collection of signal molecules produce a large response across the cell. The large response across the cell is called the cascade effect. The response of a cascade can be cell growth, gene expression, or secretion of molecules.

Memory Tip: A good way to think about the signal transduction pathway is like a waterfall or a line of falling dominoes.

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-Ycxph6P67zG7.gif?alt=media&token=f2f4a002-9caf-45de-814c-39b1d1764f15

Image courtesy of Giphy.

Regardless of the distance between the cells, cell communication always follows the same 3 steps: reception, transduction, and response

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2Fcell-to-cell-communication-9-728.jpg?alt=media&token=b798c7e7-4973-45a5-9766-8985e76330f1

Image courtesy of Winnacunnet Biology.

Reception

Cell surface receptors are receptors that cover the entire membrane. These receptors are important because most signaling molecules are too big to cross the plasma membrane

The plasma membrane contains a phospholipid bilayer. The heads of the phospholipid are “water-loving” which are called hydrophilic. The tails of the phospholipid are “water-hating” which are called hydrophobic.

The phospholipid bilayer is very important because it creates a barrier between the inside and outside of the cell.

Examples of cell surface receptors

Ion Channel Receptors involves a channel that opens and closes. The channel is used to allow the passage of ions across the membrane.

Memory Tip: Think of ion channel receptors as a toll booth. Just like a channel receptor, a toll both opens and closes after an action is made.

G-Protein-Coupled Receptors: Through this process, the ligand binds to the outside part of the cell and changes the form of the inside cytoplasm. This causes a G protein to bind and activate the enzyme adenylyl cyclase. Adenylyl cyclase converts ATP (energy) to cyclic-AMP.

Finally, cyclic-AMP activates other molecules inside the cell which finally leads to the cellular response.

Transduction

Phosphorylation

We have a message. We were able to receive it. So how are proteins exactly activated? Proteins are activated in a process called phosphorylation. Essentially, a phosphate group is added to an already-existing protein to activate it. This phosphate group just comes from the cytoplasm ATP (adenosine triphosphate). When this is added to the protein, the protein will get "powered on" and start doing its job, while the ATP turns into ADP (adenosine diphosphate). After its job is over, the phosphate group will be taken off, powering off the protein (it's a waste of energy to have everything on right? 😉)

Amplification

There's a process within transduction called amplification. Not every signal leads to amplification, but it is a step that can be included. Amplification steps are a sequence of turning on communication pathways to amplify the response, or a process to make the transduction process faster. An example of this is the cAMP or the G-protein, which can be activated with GTP.

Transduction has a link to evolution

Since transduction occurs in an animal, bacterial, plant, and yeast cells, it shows that the signal transduction pathway has evolved from a common ancestor.

Isn’t it crazy that all these different organisms could have something in common?!

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2Fgiphy2.gif?alt=media&token=5d51110e-8a9f-4e89-bd5e-ebfabb8fc3c2

Image courtesy of Giphy.

Response

Depending on the ligand, the response can be anything. This can be turning on an enzyme, transporting a molecule, etc.

Special Cases

Like everything, the signal-transduction pathway has special cases! Two examples are lipid hormones and secondary messenger.

Lipid Hormones

Lipid hormones are a special type of ligand that are made out of...well... lipid. And since the cell membrane is made up of lipid, it doesn't need a receptor on the cell surface. Instead, it can pass through the cell membrane without a transport protein, and the receptor protein is actually inside the cell.

Then, this reception actually produces a transcription factor, and an entirely new protein is synthesized from DNA to mRNA to ribosome. Because the protein is made from scratch in response to the ligand, it can take up to an hour to create a response to the ligand! But on the bright side, slower response means the protein is more long-lasting, so a more functional (?) protein will be created.

Secondary Messenger

As the name suggests, a secondary messenger is just a second messenger. A receptor at the cell's surface receives a ligand, and this activates the transport channel. The transport channel is then opened, where the secondary messenger is allowed to enter the cell. After entering the cell, the secondary messenger binds to the appropriate protein to produce a messenger.

Key Terms to Review (27)

Adenylyl Cyclase

: Adenylyl cyclase is an enzyme that converts ATP to cyclic AMP (cAMP) in response to a signal.

ADP (adenosine diphosphate)

: ADP is an organic compound that plays a crucial role in metabolism. It's converted to ATP for storage of energy during cell metabolism.

Amplification

: In biology, amplification refers to the increase in number or expression of genes or gene products (like proteins).

ATP (Adenosine Triphosphate)

: ATP is a high-energy molecule that stores and provides energy for many biochemical reactions in the cell.

cAMP or cyclic Adenosine Monophosphate

: cAMP is a second messenger important in many biological processes; it transmits signals from hormones outside the cell to its interior.

Cascade Effect

: The cascade effect refers to an ordered sequence of reactions triggered by an event, each of which triggers the next.

Cell Communication

: Cell communication is the process by which cells interact and respond to signals from their environment or other cells. This involves a series of steps including signal reception, transduction, and response.

Cell Surface Receptors

: Cell surface receptors are proteins located on the cell membrane or within cells that bind to external signaling molecules such as hormones or neurotransmitters. They trigger changes inside the cell when activated.

Cyclic-AMP

: Cyclic-AMP (cAMP) is a second messenger used for intracellular signal transduction, such as transferring into cells the effects of hormones like glucagon and adrenaline, which cannot pass through the plasma membrane.

G-Protein-Coupled Receptors

: G-protein-coupled receptors (GPCRs) are large protein complexes that respond to signals outside the cell and activate internal signal transduction pathways and cellular responses by interacting with G-proteins inside the cell.

GTP

: GTP, or Guanosine Triphosphate, is a molecule similar to ATP that provides energy for protein synthesis and signal transduction in cells.

Hydrophilic

: Hydrophilic substances are those that have an affinity for water, meaning they can mix with or dissolve in water.

Hydrophobic

: Hydrophobic refers to substances that repel or do not mix with water.

Ion Channel Receptors

: Ion channel receptors are proteins that span the cell membrane and allow specific ions to pass through when they are open. They open or close in response to a signal molecule binding to the receptor.

Ligand

: A ligand is a molecule that binds to another (usually larger) molecule. In cell communication, it's often the signal molecule that binds to a receptor.

Lipid Hormones

: Lipid hormones are hormones derived from lipids such as sterols and fatty acids. They are typically non-polar and can pass through cell membranes easily.

Phospholipid Bilayer

: The phospholipid bilayer is a two-layered arrangement of phosphate and lipid molecules that form a cell membrane, the hydrophobic lipid ends facing inward and the hydrophilic phosphate ends face outward.

Phosphorylation

: Phosphorylation is a biochemical process where a phosphate group is added to an organic molecule. It's crucial in many cellular processes, including metabolism, transcription, and cell signaling.

Plasma Membrane

: The plasma membrane is a thin layer that separates the inside of cells from their environment. It controls what enters or leaves the cell.

Reception

: Reception is when a signal molecule binds to a receptor protein on or within the target cell, initiating transduction pathways that lead to cellular responses.

Receptor Protein

: A receptor protein is a molecule found on the surface of cells that binds to specific substances, triggering a specific response in the cell.

Response

: In biology, response refers to how an organism changes its behavior or physiology in reaction to changes in its environment.

RNA Polymerase

: RNA polymerase is an enzyme that synthesizes RNA from DNA during transcription.

Secondary Messenger

: Secondary messengers are molecules that relay signals received at receptors on the cell surface — such as the arrival of protein hormones, growth factors, etc. — to target molecules in the cytosol and/or nucleus.

Signal Transduction Pathway

: A signal transduction pathway is a series of molecular events through which cells respond to external signals. These pathways convert these signals into cellular responses.

Transcription of a Gene

: Transcription of a gene is the process by which the DNA sequence in a gene is copied into mRNA (messenger RNA). This mRNA then serves as a template for protein synthesis.

Transduction

: Transduction is the process by which a cell converts one kind of signal or stimulus into another. In biology, this often refers to the transfer of genetic material from one bacterium to another through a virus.


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© 2024 Fiveable Inc. All rights reserved.

AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.