Feedback Loops

Homeostasis

• Steady or internal balance

• Cell maintain a relatively constant internal environment even when the external environment changes significantly 

  • The internal condition’s typical state is the set point

  • Fluctuations in that condition above or below the set point point serve as the stimulus

  • A receptor or sensor detects the stimulus and triggers a response and activity that returns the condition to the set point.

    

Negative Feedback

• The response reduces the stimulus

• It is pretty much having too much of something and reducing it

• Example:When you exercise,you produce heat which increases your body temperature. Your nervous system defects this increases and triggers sweating. As you sweat evaporates you skin cools,returning your body to its set point.

Positive Feedback

• This stimulus is amplified in order to complete a process; then the condition returns to the set point

• Adding more to stimulus

• Example: During childbirth, the pressure of the baby’s head against the uterus stimulated contractions. The contractions result in greater pressure thereby stimulating more contractions and then more pressure. This continues until the baby is born.

Cell Communication

Signal Transduction Pathways

• Autocrine Signaling

  • A cell sends a signal to itself

• Juxtacrin Signaling

  • Cells communicate with adjacent cells through direct contact

    • Example:plasmodesmata connect plant cells and gap junctions connect animal cells

    • Example:glycoproteins on one cell interact with glycoproteins on another cell

• Paracrine Signaling

  • Cells communicate to nearby cells by releasing chemical messengers

    • Example: Neurotransmitters released into a synapse

• Endocrine signaling

  • Cells communicate to cells far away by releasing chemical messengers that are carried into the target cell

    • Example:adrenaline is pumped into the blood streams.

1.Reception-Signaling molecule binds to the receptor protein

2.Transduction-The signal is converted into a form that can produce a cellular response

3.Response-The transduced signal produces a cellular response

• A signal molecule is called a ligand which binds to a glycoprotein molecule like a lock and key fashion

• Most receptor proteins are in the cell membrane but some are in the cell membrane

  • Hydrophilic ligands bind to plasma membrane receptors

  • Small or hydrophobic ligands can pass through the membrane and attach to intracellular receptors

• The 3 most common types of membrane receptor proteins

  • G proteins coupled receptors

  • Receptor tyrosine kinase

  • IOn channel receptors

• G-Proteins bind the energy rich GTP(very similar to ATP)

• G-Proteins are all very similar in structure

• GPCR systems are widespread and diverse in function

Receptor Tyroskine Kinases

• Receptor Tyroskin Kinases(RTKs) are membrane receptors that transfer phosphate groups from ATP to another protein

• A RTK can trigger multiple signal transduction pathways at once

Ion Channel Receptors

• An ion channel receptor acts as a gate that opens and closes when the receptor changes shape

• When a signal molecule binds as a ligand to the receptor,the gate allows specific ions,such as Na+ or Ca2+ through a channel in the receptor.

Step 2 Transduction

• Molecular interactions relay signals from receptors to target molecules in the cell

• Multistep pathways

  • Can amplify a signal

  • Provide more opportunities for coordination and regulation.