Essential knowledge 3.D.1: Cell communication processes share common features that reflect a shared evolutionary history.

a. Communication involves transduction of stimulatory or inhibitory signals from other cells, organisms or the environment. [See also 1.B.1]

b. Correct and appropriate signal transduction processes are generally under strong selective pressure.

c. In single-celled organisms, signal transduction pathways influence how the cell responds to its environment.

To foster student understanding of this concept, instructors can choose an illustrative example such as:
• Use of chemical messengers by microbes to communicate with other nearby cells and to regulate specific pathways in response to population density (quorum sensing)
• Use of pheromones to trigger reproduction and developmental pathways
• Response to external signals by bacteria that influences cell movement

d. In multicellular organisms, signal transduction pathways coordinate the activities within individual cells that support the function of the organism as a whole.
To foster student understanding of this concept, instructors can choose an illustrative example such as:

• Epinephrine stimulation of glycogen breakdown in mammals 
• Temperature determination of sex in some vertebrate organisms 
• DNA repair mechanisms

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Essential knowledge 3.D.2: Cells communicate with each other through direct contact with other cells or from a distance via chemical signaling.

a. Cells communicate by cell-to-cell contact.

To foster student understanding of this concept, instructors can choose an illustrative example such as:

• Immune cells interact by cell-cell contact, antigen-presenting cells (APCs), helper T-cells and killer T-cells. [See also 2.D.4]

• Plasmodesmata between plant cells that allow material to be transported from cell to cell.

Cells communicate over short distances by using local regulators that target cells in the vicinity of the emitting cell.
To foster student understanding of this concept, instructors can choose an illustrative example such as:

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

Signals released by one cell type can travel long distances to target cells of another cell type.

Evidence of student learning is a demonstrated understanding of the following:
1. 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.
To foster student understanding of this concept, instructors can choose an illustrative example such as:

• Insulin 
• Human growth hormone 
• Thyroid hormones 
• Testosterone 
• Estrogen

Study of the nervous and immune systems is required for concepts detailed in 3.E.2 and 2.D.4.

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Essential knowledge 3.D.3: Signal transduction pathways link signal reception with cellular response.

Signaling begins with the recognition of a chemical messenger, a ligand, by a receptor protein.

Evidence of student learning is a demonstrated understanding of each of the following:

1. Different receptors recognize different chemical messengers, which can be peptides, small chemicals or proteins, in a specific one-to-one relationship.

2. A receptor protein recognizes signal molecules, causing the receptor protein’s shape to change, which initiates transduction of the signal.

To foster student understanding of this concept, instructors can choose an illustrative example such as:
• G-protein linked receptors 
• Ligand-gated ion channels 
• Receptor tyrosine kinases

b. Signal transduction is the process by which a signal is converted to a cellular response.
Evidence of student learning is a demonstrated understanding of each of the following:
1. Signaling cascades relay signals from receptors to cell targets, often amplifying the incoming signals, with the result of appropriate responses by the cell.
2. Second messengers are often essential to the function of the cascade.
To foster student understanding of this concept, instructors can choose an illustrative example such as:

• Ligand-gated ion channels
  
• Second messengers, such as cyclic GMP, cyclic AMP calcium ions (Ca2+), and inositol triphosphate (IP3)
  

3. Many signal transduction pathways include:
i. Protein modifications (an illustrative example could be how methylation changes the signaling process)
ii. Phosphorylation cascades in which a series of protein kinases add a phosphate group to the next protein in the cascade sequence

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Essential knowledge 3.D.4: Changes in signal transduction pathways can alter cellular response.

a. Conditions where signal transduction is blocked or defective can be deleterious, preventative or prophylactic.
To foster student understanding of this concept, instructors can choose an illustrative example such as:

• Diabetes, heart disease, neurological disease, autoimmune disease, cancer, cholera
  
• Effects of neurotoxins, poisons, pesticides
  
• Drugs (Hypertensives, Anesthetics, Antihistamines and Birth Control Drugs)

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