AP Bio Cell Signaling, Homeostasis and Feedback

Cell to cell communication

critical for the function and survival of cells

• responsible for the growth and development of multicellular organisms

Direct contact

communication through cell junctions

• signaling substances and other materials dissolved in the cytoplasm can pass freely between adjacent cells

• Animal cells: gap junctions

• Plant cells: plasmodesmata

ex. immune cells

Local regulators

a secreting cell will release chemical messages (local regulators/ligands) that travel a short distance through the extracellular fluid

• the chemical messages will cause a response in a target cell

• examples: paracrine signaling and synaptic signaling

Paracrine signaling

secretory cells release local regulators (ie growth factors) via exocytosis to an adjacent cell

Synaptic signaling

• occurs in animal nervous systems

• neurons secrete neurotransmitters

  • diffuse across the synaptic cleft which is the space between the nerve cell and target cell

Long distance signaling

animals and plants use hormones for long distance signaling

• plants release hormones that travel in the plant vascular tisuse (xylem and phloem) or through the air to reach target tissue

• Animals use the endocrine signaling

  • specialized cells release hormones into the circulatory system where they reach target cells

• ex. insulin

Reception

ligand binds to receptors

• the detection and receiving of a ligand by a recepter in the target cell

receptor

macromolecule that binds to a signal molecule (ligand)

• all receptors have an area that interacts with the ligand and an area that transmits a signal to another protein

  • binding between ligand and receptor is highly specific

• When the ligand binds to the receptor activates (via a conformational change)

• Allows the receptor to interact with other cellular molecules

  • initiates transduction signal

• Receptors can be in the plasma membrane or intracellular

Plasma membrane receptors

• Most common type of receptor involved in signal pathways

• binds to ligands that are: polar, water-soluble, large

  • examples: GPCRS, Ligand-gated ion channels

Intraceullar receptors

found in the cytoplasm or nucleus of target cell

bind to ligands that can pass through the plasma membrane

ie. hydrophobic molecules

• steriod and thyroid hromones

• gasses like nitric acid

Transduction

• Signal is converted

• the conversion of an extracellular signal to an intracellular signal that will bring about a cellular response

  • requires a sequence of changes in a series of molecules known as a signal transduction pathway

• The signal transduction pathway regulates protein activity through

  • phosphorylation by the enzyme protein kinase

    • relays signal inside cell

  • Dephosphorylation by the enzyme protein phosphates

    • shuts off pathways

  • during transduction the signal is amplified

Second messengers

small, non-protein molecules and ions help relay the message and amplify the response

• Cyclic AMP (cAMP) is a common one

Response

A cell process is altered

• The final molecule in the signaling pathway converts the signal to a response that will alter a cellular process

• examples:

  • Protein that can alter membrane permeability (cytoplastic reponse)

  • Enzyme that will change a metabolic process (cytoplastic response)

  • Protein that turns genes on or off (nuclear response)

Signal Transduction Pathway

Signal transduction pathways can influence how a cell responds to its environment

• they can result in changes in gene expression and cell function

  • can alter phenotypes or result in cell death

Changes in Signal Transduction Pathways

Mutations to receptor proteins or to any component of the signaling pathway will result in a change to the transduction of the signal

GPCRs

largest category of cell surface receptors

important in animal sensory systems

Binds to a G protein that can bind to GTP, which is an energy molecule similar to ATP

The GPCR, enzyme, and G protein are inactive untul ligand binding to GPCR on the extracellular side

Ligand Binding causes cytoplasmic side to change to shape

• Allows for the G protein to bind to GPCR

  • Activates the GPCR and G protein

    • GDP becomes GTP

• Part of the activated G protein can then bind to the enzyme

  • Activates enzyme

  • Amplifies signal and leads to a cellular response

Ligand gated ion channels

Located in the plasma membrane

Important in the nervous system

Receptors that act as a “gate” for ions

• When a ligand binds to the receptor, the “gate” opens or closes allowing the diffusion of specific ions

  • Initiates a series of events that lead to a cellular response

Set point

values for various physiological conditions that the body tries to maintain

• this set point has a normal range for which it can fluctuate

Homeostasis

the state of relatively stable internal conditions

• organisms detect and respond to a stimulus

• the body maintains this through feedback loops

Stimulus

a variable that will cause a response

Receptor/sensor

sensory organs that detect a stimulus. this information is sent to the control center (brain)

Effector

muscle or gland that will respond

Response

changes (decreases or increases) that effect the stimulus

Negative feedback

the most common feedback mechanism

this type of feedback reduces the effect of the stimulus

• examples:

  • sweat

  • blood sugar breathing rate

Postive Feedback

This type of feedback increases the effect of a stimulus

examples:

• child birth

• blood clotting

• fruit ripening

Homeostatic Imbalances

There are many reasons for why the body may not be able to regulate homeostasis

for example:

• genetic disorders

• drug or alcohol abuse

• intolerable conditions (ie extreme heat or cold)

Disease

when the body is unable to maintain homeostasis

Cancer

the body cannot regulate cell growth

Diabetes

the body cannot regulate blood glucose levels

Cell Signaling as a Means of Homeostasis

In order to maintain homeostasis, the cells in a multicellular organism must be able to communicate

Communication occurs through signal transduction pathways