Chapter 11 Portfolio

• Describe how signals are received by cells.

Cells are able to communicate with each other via chemical signals. Through a signal transduction pathway, cells are able to send signals step by step that will lead to a specific cell response. Both animal and plant cells contain cell junctions that connect the cytoplasm of cells that are next to each other. This then allows for what is knows as local signaling, or the cells ability to communicate via direct contact or cell-cell recognition. 

Cells may able communicate long-distance with the use of hormones. Hormones will stimulate a cell only if the cell has the receptor specific to that signal. This is further demonstrated in the first step of cell signaling, reception. The signaling molecule will have to bind to the specific receptor protein in order for the cell to recognize the specific message. This binding between a signal molecule and a receptor is very specific and is known as a ligand. It is also important to note that the majority of signal receptors are plasma membrane protein, meaning that signals are usually received on the membranes of cells.

• Use an example to explain how a receptor protein recognizes signal molecules, causing the receptor protein’s shape to change, which initiates transduction of the signal.
  

A type of signal receptor is a ligand-gated ion channel. This type of receptor acts as a type of gate and changes shape. For example, when a signal molecule acts as a ligand by binding the the receptor, the ligand-gated ion channel will then allows only certain ions, like NA+ or Ca+ to pass through the channel found in the receptor. However, in order to open the gate, the ligand-gated ion channel must change shape to open the pathway. Once the ions pass the channel, transduction occurs.

• Describe the process of signal transduction - include a model in your discussion.

Signal transduction is when a signal molecule binds to a receptor protein and changes the shape of that receptor proteins, thus initiating the steps necessary for the cell signal to take a form that will cause a cellular response. For example, epinephrine will bind to a receptor protein found in the plasma of a liver cell. Once this binding occurs, it will lead to the activation of glycogen phosphorylase, which causes the response. Though in this example, transduction occurs in  a single step, it is more common for transduction to occur in a series of steps that causes changes in varis different molecules. This series of steps in knowns as a transduction pathway, and the molecules in the pathway are referred to was relay molecules.

• Explain the concept of signaling cascades - include a model in your discussion.
  

It usually takes multiple steps for a cell signal to initiate a response. Even though the signaling molecule is the molecule that initiates the entire process of cell signalling, that molecule usually does not even enter the cell. What occurs instead is that it begins a cascade, or a series of events with each one leading to the next. As the signal is passed along the pathway, at each step the signal is transduced into a different form, usually due to a change in shape of the next protein. This change of shape is many times brought about by phosphorylation. The entire series of many steps and the pathway in which the signal travels after being delivered by the signaling molecule is a signaling cascade.

For example, in a phosphorylation cascade, the series of proteins in the signal pathway are phosphorylated in order. This occurs because each pervious protein added a phosphate group to the next one in line as a way to carry along the signal by activating the necessary proteins. As these steps occur and the signal is passed on, dephosphorylation will return each activated protein to its inactive state.

Source: 2011 Pearson Education, Inc. 

• Use an example to explain how second messengers are often essential to the function of a signaling cascade.

Second messengers are small, non-protein molecules or ions that are soluble in water. One of the most commonly used ones is cyclic AMP, also known as cAMP. In order to understand the role and importance of cyclic AMP it is important to note that is has been found that epihpihprine will lead to the breakdown of glucose without penetrating the plasma membrane of liver or muscle cells. This is because, even though the signal is initiated by epinephrine it is cyclic AMP that carries the signal to the interior of the cell and eventually brings about the breakdown of glucose. The role of epinephrine is to begin the reaction that will cause the synthesis of many molecules needed for the creation of cAMP. Yet, it is cAMP, a second messenger, that carries the signal started by epinephrine and leads to glucose breakdown.

• Explain the key features of a model that illustrates how changes in a signal pathway can alter cellular responses.

An important function of signaling pathways is to sometimes regulate protein synthesis. This means that signaling pathways have the potential to turn certain genes on or off. For example, for an activated steroid receptor, the last activated molecule in that signaling pathway may be a transcription factor, which activates transcription, where mRNA is translated into specific proteins.

However, some pathways activate cell division. If the pathways were to malfunction, either a mutation in one of the genes that is turned off or on  by the way in which mRNA is transcribed then the pathways could lead to uncontrolled cell division, also known as cancer and have fatal effects.

- Use any of the examples below to explain how it relates to cell communication over short distances.

Neurotransmitters
Plant immune response
Quorum sensing in bacteria
Morphogens in embryonic development

Neurotransmitters are cells that use short distance communication. It begins with an electrical signal traveling down along a nerve cell. This electric signal is signal to the nerve cell to release a neurotransmitter. This neurotransmitter will then diffuse in the synapse, the space between two nerve cells. Once it crosses the synapse the neurotransmitter will stimulate the new nerve cell.

Source: 2011 Pearson Education, Inc. 

- Use any of the examples below to explain how endocrine signals are produced by endocrine cells that release signaling molecules, which are specific and can travel long distances through the blood to reach all parts of the body.

Insulin
Human growth hormone
Thyroid hormones
Testosterone
Estrogen

Whenever hormones are used for cell signaling, it is known as endocrine signaling. This type of signaling is classified as long distance signaling because the signal  will travel long distances in the body instead of just traveling to the adjacent cell. It will be able to do this because the hormone carrying the signal will enter the bloodstream and travel via this method.

In the case of insulin, it is a protein composed of thousands of atoms. It will enter the bloodstream and travel all the way down to the liver, will it will stimulate it to regular the sugar levels in the blood.

Source: 2011 Pearson Education, Inc. 

- Use any of the below examples to explain how, in multicellular organisms, signal transduction pathways coordinate the activities within individual cells that support the function of the organism as a whole.

Epinephrine stimulation of glycogen breakdown in mammals
Temperature

determination of sex in some vertebrate organisms
DNA repair mechanisms

Epinephrine will first bind to the plasma membrane of a liver cell. Once tis occurs, it will trigger elevated levels of cyclic AMP within the cell. This is done because when epinephrine binds to the specific receptor protein, the receptor protein will then activate adenylyl cyclase. The adenylyl cyclase will then catalyze the synthesis of the molecules necessary to create cAMP. The cAMP molecules will then tell the rest of the cell to break down glucose. However, it is important to note that the signal transmitted by cAMP does not last long enough, due to the enzyme phosphodiesterase, which converts cAMP to AMP. Therefore, another round of epinephrine is necessary to once again increase elves of cAMP in the liver cell.

-List the types of signals involved in communication and where they come from.

There are several different types of signaling that a cell may use. The two broad types are long distance and short distance. Short distance signaling involves the use of cell junctions and cell-cell recognition. Long term signaling involves synaptic signaling and endocrine signaling.

Short Distance Signaling:

For cell junctions, the substances that carry the signal between cells have the ability to pass freely between adjacent cells by dissolving in the cytosol.

Source: 2011 Pearson Education, Inc. 

For cell-cell recognition, this is more of a direct contact method. Cells will recognize other molecules on the membranes of other cells This type of cell communication is important for embryonic development and immune responses.

Source: 2011 Pearson Education, Inc. 

Long Distance Signaling :

Synaptic signaling is used primarily by animal nervous system. Electrical signals traveling along the nerve cells will cause the secretion of neurotransmitters. Neurotransmitters then act as chemical signals and travel across synapses to each next nerve cell.

Endocrine signaling involves hormones acting as chemical signals. Specialized cells will secrete these hormones. The hormones will then travel via the bloodstream to the target cells in order to deliver their message.

-Describe why signal transduction pathways that are under strong selective pressure.

The pressure to be very selective and be specific is due to the large amount of things that could go wrong. Changes in the pathways can lead to terrible consequences. For example, if a signaling pathway were to go wrong, an incorrect gener may be turned off and on. This mutation in genes is known to cause serious illness, including cancer that can be fatal.

- Use an example to explain how signal transduction pathways influence how the cell responds to its environment in unicellular organisms.

Cell signaling is very important to the microbial world. Bacteria cells will secrete molecules detectable for other neighboring bacterial cells. The neighboring bacteria will monitor its environment for the amount of these specific signaling molecules in it, a technique that is known as quorum sensing. This will aid bacteria in the production of biofilm, an aggregation of bacteria on a surface, of which they will derive the majority of their food.

- Using an example to explain how signal transduction pathways coordinate the activities within individual cells that support the function of the organism as a whole in multicellular organisms.

It is important to note that for some kinds of signals, it is possible to trigger responses in one or two different organs. This is because each different organ cell is equipped with its individual collection of signal receptor proteins, relay proteins, and proteins needed to carry out the response. The coordination in the body as a whole that this allows for can be seen in the effects of epinephrine on the heart and the liver. Epinephrine is a hormone that can stimulate a response in both the heart and the liver. However, the response will be different in each organ. For the liver, epinephrine will stimulate the breakdown of glucose. For the heart, epinephrine stimulates contractions, which then lead to an increased heart rate. In this way, epinephrine affects the activity of individual cells, but of different organs, helping the organism function as a whole. Again, this is due to the heart and the liver each having its different set of signal transduction pathways that will respond differently to the epinephrine.