cell - lecture 1

what are the 2 types of receptros?

cell-surface receptors and intracellular receptors.

what are the 4 types of intercellular communication?

• contact-dependent

• paracrine

• synaptic (neurotransmitter)

• endocrine (hormone inside bloodstream)

What is Contact-dependent signalling + when is it important?

Requires direct cell-to-cell contact, which can be bidirectional and is relatively slow. It uses membrane bound ligand + receptor. Important for cell differentiation, immune response + cell guidance.

What is Paracrine + autocrine signalling?

Signal acts on nearby cells (acts locally). autocrine is when the cell signals to itself.

What are some limitations of Paracrine signalling?

• Diffusion

• Uptake by cells

• Degradation

• Immobilisation of the signal

What is Synaptic signalling + how fast is it?

Occurs at specialised synapses between neurons and target cells. Very rapid → occurs within milliseconds. (Usually triggered by an action potential)

How specific is Synaptic signalling?

It has very high spatial and temporal precision.

Give some examples of Neurotransmitters?

• Glutamate

• GABA

• Glycine

• Acetylcholine (ACh)

What is Endocrine signalling + how fast is it?

Produces a systemic signal → can affect distant parts of the body. Signal (hormone) travels via bloodstream + only cells with specific receptors respond to it. Usually slow compared to synaptic signalling.

Signalling rates vary, compare the speed of Voltage-dependent signals, Second messengers / modification of existing proteins and Transcription-dependent responses?

• Voltage-dependent signals → very rapid (~milliseconds)

• Second messengers / modification of existing proteins → relatively fast (~seconds)

• Transcription-dependent responses → slow

  • Require changes in gene expression and protein production

How many responses can one transmitter produce?

Many

What does the response depend on?

• Receptor type → which receptor the cell has

• Receptor coupling → which signalling pathway the receptor activates

• Effector molecules → the molecules inside the cell that produce the response

Key idea:
Same transmitter + different receptor/pathway → different cellular response.

What are Cell-surface receptors?

proteins on the outside of a cell that detect chemical signals from other cells and trigger a response inside the cell.

What are Ligand-Gated Ion Channels?

cell-surface receptors that open or close an ion channel when a specific signalling molecule (ligand) binds to them.

What does this do?

Allows ions to move across the cell membrane, changing the membrane potential + producing an electrical signal.

Give some examples of what Ligand-Gated Ion Channels allow?

Synaptic communication and Voltage/Ca2+ -mediated effects

What are the key characterisations of Ligand-Gated Ion Channels?

• Located postsynaptically (on the receiving cell).

• Very rapid because neurotransmitters travel a short distance and are quickly removed.

• Produce electrical effects through ion movement.

• Provide high spatial and temporal precision.

• Effects can involve changes in membrane voltage and intracellular Ca²⁺.

Another example of cell-surface receptors are GPCRs, what does that stand for?

G-protein-coupled receptors.

How many subunits do GPCRs have + what are they called?

3, alpha, beta and gamma.

What is the order of events when GPCRs are activated?

• Ligand binds to the GPCR.

• The receptor changes shape (conformational change).

• Gα exchanges GDP for GTP, activating the G protein.

• The Gα subunit separates from the Gβγ complex.

• Gα and Gβγ can both activate downstream targets, such as enzymes or ion channels.

• These targets produce a cellular response.

• ***** attach!!!!

How does GTP affect the G-protein?

GTP acts as an on/off switch for G proteins.

Activation:

1. A ligand binds to a GPCR, activating the receptor.

2. The Gα subunit exchanges GDP for GTP.

3. Gα becomes active and initiates a signalling cascade (signal transduction).

4. Gα and the βγ complex can regulate different target proteins.

Inactivation:

1. Gα hydrolyses GTP into GDP.

2. Gα becomes inactive.

3. The inactive Gα subunit re-associates with the βγ complex.

4. The G protein returns to its inactive state.

Key idea: GDP = OFF; GTP = ON.

What is cAMP + it’s role?

cyclic adenosine monophosphate is a second messenger that carries signals inside the cell.

How is cAMP produced?

Made from ATP by the enzyme adenylyl cyclase. 

What are cAMP levels regulated by?

G proteins

So what is Gs and Gi?

Gs = stimulatory G-protein = increases cAMP

Gi = inhibitory G-protein = decreases cAMP

What does cAMP phosphodiesterase do?

breaks down cAMP, helping terminate the signal.

what does cAMP activate + how? 

PKA - cAMP binds to regulatory subunits of PKA 

• PKA catalytic subunit dissociates

• Now active + phosphorylates cytosolic targets

How does this relate to gene regulation?

Active PKA enters the nucleus + phosphorylates CREB (activating it)

• Activated CREB promotes the transcription of specific target genes

What are AKAPs?

A-kinase anchoring proteins - Proteins that anchor PKA to specific locations inside the cell.

Why is this beneficial?

It keeps PKA close to its target proteins, so it can act quickly and phosphorylate the correct proteins, making cell signalling more efficient and specific.

what are olfactory receptors?

proteins (specifically G-protein-coupled receptors) on olfactory sensory neurons in your nose that detect odor molecules (smells).

explain the process that takes place from the olfactory receptors to the brain for smell to be recognised?

Olfactory receptors (which are GPCRs) detect odor molecules → they are coupled to G-protein Golf → which activates adenylyl cyclase → which produces + increases the levels of cAMP → cAMP opens cyclic nucleotide-gated ion channels, causes depolarisation, triggers an action potential + smell signal is sent to the brain.

what are Photoreceptor cells?

specialised cells in the retina of your eye that detect light.

what are the 2 types of Photoreceptor cells?

Rods + cones.

what do the stacks of discs in the Photoreceptor cells contains?

rhodopsin

what is rhodopsin?

a light-sensitive receptor protein.

what happens In the dark?

• cGMP levels are high.

• cGMP binds to CNG (cyclic nucleotide-gated) cation channels, keeping them open.

• Na⁺ and Ca²⁺ enter the photoreceptor → the cell remains relatively depolarised.

what happens When light enters?

• Light activates rhodopsin.

• Activated rhodopsin activates the G-protein transducin (Gα-transducin).

• Transducin activates cGMP phosphodiesterase (PDE).

• PDE breaks down cGMP → cGMP levels decrease.

• ↓ cGMP means CNG channels close.

• Na⁺ and Ca²⁺ entry decreases → the photoreceptor becomes hyperpolarised.

• This change in membrane potential allows the photoreceptor to signal that light has been detected.