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Biology Chapter 7
Cell signaling and Communication

7.1 What are Signals, and how do cells respond to them?
- Signal transduction is a process of a cell reacting to the influx of a signal from extracellular sources by its receptors

- a Signal transduction pathway is the process of signal transduction

- Chemical signals that reach cells by diffusion/circulation includes: a) Autocrine – diffuse cells that make them
b) Paracrine – diffuse & affect nearby cells
c) Hormones(circulating signals) – signals to distant cells, travel through circulatory organs

- cells do not react unless there IS a receptor that is SPECIFIC to the signal

- Signal Transduction pathway involves 3 actors:
a) signal
b) receptor
c) response

- Signals:
a) are necessary in order for a cell to maintain homeostasis b) binds to a receptor and activated and causes changes within the cell

- Receptors(e.g.EnvZ):
a) form a complex within the membrane
b) react to signals such as:
i) temperature
ii) concentration difference
c) change their conformation/dimentional structure when stimulated

- Domains:
a) are sequences of amino acids folded into a particular shape b) acts as an enzyme, opening the active site of the receptor, forming a Protein Kinase that: i) is an enzyme that catalyzes the transfer of a phosphate group from ATP to another molecule ii) leads the receptor proteins to go through phosphorylation, leading to function & shape change

EnvZ + ATP EnvZ-P + ADP (by Protein Kinase)

- Responders(e.g.OmpR)
a) are proteins that the phosphorylated protein binds to
b) are important in three functions:
i) ‘extracellular’ signals lead to ‘internal’ change of the cell through transduction ii) the altered responder has function (e.g. OmpR has relation to gene expression – on OmpC) the OmpC(Protein) discretes out, clogging the pore and re-balancing the concentration rate iii) The signal is amplified in the process – from a single enzyme to many molecules

* Summary on steps of signal transduction
1. as the signal meets the receptor protein(EnvZ), it changes form 2. the protein kinase activity increases & transmits Phosphate group from ATP to responder(OmpR) 3. Phosphorylation changes the shape & function of the protein(OmpR) 4. the signal is amplified

5. the phosphorylized Protein binds to the DNA, turning on/off the expression of specific genes(OmpC) 6. Cell activity changes, reaching homeostasis

7.2 How do Signal Receptors Initiate a Cellular Response?

- the Specificity of Binding indicates that only specific receptors react to specific signals

- Ligands:
a) are molecules that binds to a receptor site (signals)
b) does not have extra/secondary function other than altering the dimentional shape of the receptor c) can be classified if they are diffusible or not

- The Law of Mass Action:
a) indicates that the binding of Ligands and Receptors can be done reversibly b) although is open to reversible function, mostly favors binding c) indicates that the ligand is later released

- Inhibitors/Antagonists (e.g.caffeine) can also bind to a receptor, hindering the transduction

- Receptors can be classified according to their location in the cell: a) cytoplasmic receptors – can be diffused (e.g.estrogen)
b) (Plasma)membrane receptors – cannot be diffused (e.g. insulin)

- 3 types of plasma membrane receptors exist:
a) ion channel receptors
b) protein kinase receptors
c) G-protein receptors

- Ion channel receptors (e.g. acetylcholine receptor – sodium channel): a) are gated channels that function as allowing ions to enter and exit b) go through shape alteration from ligand binding

- Protein Kinase Receptors (e.g. insulin receptors – insulin response substrates): a) become protein-kinases when activated
b) catalyze the phosphorylation of themselves & other proteins

- G-Protein linked receptors:
a) are a group of receptors each composed of single protein with 7...
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