ap bio 4.1-4.3

n/a

cell to cell communication

is very important for cell survival and function/ also responsible for cell growth and development especially in multicellular organisms

three main types of cell-to-cell communication are:

direct contact, local signaling/regulators, long distance signaling

direct contact

communication between neighboring cells through gap junctions

signaling substances and materials dissolve in what to freely pass between adjacent cells

cytoplasm 

animal cell: direct contact location

gap junctions

plant cells: direct contact location

plasmodesmata

example of direct contact

APCs = teachers → They show the germ.

T cells = soldiers → They act based on what the APCs show them.

The direct contact between APCs and T cells is like a handshake that passes critical information. Without this handshake, T cells wouldn’t know which enemy to fight.

Immune cells

■ Antigen presenting cells (APCs) communicate to T cells through direct contact

local regulators

a secreting cell can send messages to nearby cell using tiny chemicals called local regulator or ligands

they only travel a short distance through the extracellular fluid around the cell

when the chemicals reach the target cell, the cell responds by changing something about how it works

ligand

any molecule that binds to a receptor on a cell to send a signal (so local regulator is a type of ligand)

two types of local signaling

paracrine signaling & synaptic signaling

paracrine signaling

secretory cells release local regulators (like growth factors) via exocytosis (a cell pushes things out of itself by using a vesicle (a tiny bubble)) to a nearby cell (short distance)

synaptic signaling

happens in animal nervous system

neurons secrete neurotransmitters(chemical messages)These neurotransmitters travel across a tiny space called the synaptic cleft.

The synaptic cleft = the small gap between the neuron and the cell it's sending the message to.

Depending on the length of the neuron, can be classified as long distance but mainly this is local signaling 

long distance signaling

animals and plants use hormones this

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

Animals use endocrine signaling (Specialized cells release hormones into the they reach target cells circulatory system where they reach their target cells)

endocrine signaling 

how animals release hormones

specialized cells release hormones to reach the specific target cell that they need to go to through the circulatory system which is pretty long

example of endocrine signaling

the pancreas release hormones called insulin through the bloodstream—> again through a long distance to get to target cell

how to cells process signals and do something with the communication and message they receive

signal transduction

the three stages/steps to signal transduction are:

reception, transduction, response (happens in this order)

reception

the detection and receiving of a ligand by receptor in the target

receptor 

macromolecule (usually a protein) binds to signal molecule (ligand)

all receptors have an area that interacts with the ligand & another area that transmits/sends a signal to another protein

binding btwn a ligand and receptor is really specific 

they are located in the target cell not the signal sending cell

receptor vs ligand

ligand sends a signals vs a receptor receives the signal

The ligand binds to the receptor’s binding site, causing a shape change that activates the receptor’s signal-transmitting area, which then triggers a chain reaction inside the cell leading to a response.

ligand purpose in reception

The ligand’s job is just to start the signal; once it binds and activates the receptor, it can detach or stay bound, but the receptor continues sending the signal inside the cell through a chain reaction (signal transduction) without needing the ligand to be directly involved anymore.

two types of receptors

plasma membrane receptors & intracellular receptors

plasma membrane receptors

most common type of receptor in signal pathways

binds to ligand that are polar, large & water-soluble

examples of plasma membrane receptors

g protein coupled receptors & ligand-gated ion channels

gpcrs

g protein coupled receptors- a receptor on the cell surface that activates G protein inside the cell when a ligand bind

largest category of cell surface receptors

important to animal sensory system

ligand-gated ion channels

• located in the plasma membrane and important in the nervous system

• A receptor that is also an ion channel (a pore in the cell membrane).

• When a ligand binds, the channel opens, letting ions flow into or out of the cell.

• Ligand = key, channel = gate that opens to let ions through

intracellular receptors

found in the nucleus or cytoplasm of target cell

binds to ligand that can pass through the plasma membrane (hydrophobic molecules like steroid and thyroid hormone or gasses like nitric oxide)

transduction- 2nd stage

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 aka a Signal Transduction Pathway

extracellular vs intracellular signal

• Extracellular: ligand stays outside, receptor on surface → message goes in

• Intracellular: ligand goes inside, receptor is inside → message is directly received

signal transduction pathway

regulates protein activity by:

• Phosphorylation by the enzyme

protein kinase (switch on protein)

■ Relays signal inside cell

• Dephosphorylation by the

enzyme protein phosphatase (switch off the protein)

■ Shuts off pathways

Transduction—>what happens to the signal

the signal is amplified (multiplies or grows bigger) using second messengers and really create an effect with the signal

second messengers

small, non-protein molecules and ions help pass the message from one molecule to the next inside the cell and make the response bigger

The ligand is someone ringing your doorbell.

The receptor is the doorbell button.

The second messengers are the kids inside the house who start yelling, “Someone’s here!!” and run around telling everyone (amplification)

example of second messengers

Cyclic AMP (cAMP) is a common second messenger

Response—> last stage

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

so, the cell actually does something, and a process is changed because of the signal/ activates a real action in the cell

examples of response stage

protein alters membrane permeability, enzyme that will change a metabolic process, & protein that turns genes on or off (ligands can be proteins and signaling molecules can be proteins but not all of them have to be)

Three Stages of Signal Transduction 

Reception = catch the signal

transduction = pass and amplify it

response = cell does something

Signal Transduction Effects if mutation

can influence how a cell responds to its environment

can result in changes in gene expression and cell function

can alter phenotypes or result in cell death

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

Normal: cell understands the message.

Mutation: cell misunderstands, ignores, or overreacts to the message.

More info on GPCRs

GPCR, G protein and enzyme( signal booster there) are all “off” until signal arrives

The ligand (signal) attaches to the GPCR on the outside of the cell.

This changes the shape of the GPCR on the inside (cytoplasmic side).

Part 2 of GPCR

The G protein can now attach to the GPCR.

This activates both the GPCR and the G protein.

The G protein swaps GDP for GTP → now it’s “on” and ready to send the signal.

Part of the G protein with GTP binds to the enzyme.

The enzyme is turned on, creating second messengers inside the cell.

amplifies signal

simple steps GPCR

Ligand binds to GPCR on the outside of the cell.

The GPCR changes shape on the inside, allowing the G protein to bind.

G protein swaps GDP for GTP → now the G protein is activated.

The activated G protein (with GTP) then binds to and activates the enzyme.

The enzyme produces second messengers, which amplify the signal.

The second messengers trigger the cellular response.

signal transduction—> dominoes

a signal passes through many molecules (cuz its extensive) in a cell and any mutation or change in the signal can have a large effect on the whole pathway and on a system

pathways can be activated or inhibited at many points (can be turn the pathway on and off by some molecules like fine-tuning)

inhibition—> last slide

Block any step in the pathway or slow it down (inhibition) → the cell might not respond correctly or at all

if a step is blocked the signal might not reach to final molecule and this can weaken, change or completely stop the cell’s response