PRINCIPLES & MODES OF CELL SIGNALLING

Why is signal termination important?

Prevents overactivation and preserves homeostasis

What happens if negative regulation fails?

Constant activation leads to disease states such as cancer or inflammation

Why is timing critical?

Cells must respond transiently and appropriately to environment changes

How is signalling tied to survival?

Cells must continually “prove” survival status via receptor stimulation

Why do disease processes often hijack signalling?

Mutations create inappropriate growth or survival signals

What does dysregulated signalling contribute to?

Cancer

Why do only some cells respond to a given signal?

Only cells expressing the matching receptor can interpret the message

Can one signal cause different outcomes?

Yes — different cell types may respond differently

Give examples of varied responses to the same signal

Proliferation vs differentiation vs apoptosis

Why do responses vary?

Downstream pathways differ between cell types

How does a cell process multiple signals?

Integration through signalling networks and transcriptional control

What defines output type?

Combination of receptors

What is endocrine signalling?

Hormones travel long distances via the bloodstream

What produces endocrine signals?

Specialised endocrine glands (e.g.

What carries hormones to target cells?

The bloodstream

Why can endocrine signals affect multiple tissues?

Receptors may be expressed across many cell types

Give an example of endocrine hormone

Insulin

Why are endocrine signals slower?

Transport and receptor activation require circulation and transcriptional change

What is neuronal signalling?

Electrical impulses transmitted along neurons triggering chemical release at synapses

What is released at synapses?

Neurotransmitters

Is neuronal signalling long- or short-range?

Long in physical distance via axons

Why is neuronal signalling fast?

Electrical depolarisation triggers near-instant neurotransmitter release

What is autocrine signalling?

Cells secrete signals that act back on themselves

Give an example of autocrine signalling

Tumour cells secreting growth factors for their own proliferation

What immune example uses autocrine signalling?

IL-1 released by monocytes acting on the secreting cell

What is paracrine signalling?

Short-range signalling to neighbouring cells

Give two examples of paracrine signalling

Wnt signalling

Why is paracrine signalling short-range?

Signals diffuse through extracellular fluid and degrade quickly

What biological context uses paracrine control?

Tissue patterning and immune cell recruitment

How do tumours exploit paracrine signals?

Paracrine loops guide cell migration and invasion

What determines the signalling type?

Physical distance between sender and target cell

What is direct contact signalling?

Cells must physically contact each other for communication

Give two examples of direct contact

GAP junctions and antigen-presenting cell–T cell interaction

What are gap junctions?

Cytoplasmic channels linking adjacent cells

How do gap junctions affect cell coordination?

They synchronise electrical and metabolic activity

Name a tissue that uses gap junctions

Cardiac muscle or epithelial sheets (both acceptable)

What is an APC?

Antigen-presenting cell

What do APCs present?

MHC class II–bound antigen peptides

What recognises APC presentation?

T helper cells via CD4 receptors

What does APC–T cell contact trigger?

Activation of B cells

What is cell signalling?

The system of communication that coordinates cell actions and responses

Why is signalling essential?

Cells depend on signals to survive

What happens to cells without signals?

They initiate apoptosis

What determines a cell’s fate?

The combination of signals received at the same time

Why do cells receive multiple signals simultaneously?

To integrate environment

Do cells respond to all signals they encounter?

No — they selectively respond to a limited set

What is meant by specificity?

Cells only react when they have the correct receptor and pathway

What does integration mean?

Cells combine multiple signalling inputs to form a coordinated response

What does amplification mean?

One activated receptor produces a large downstream response

Why is feedback important?

To fine-tune responses or shut pathways off

What is positive feedback?

Output amplifies the incoming signal to strengthen it

What is negative feedback?

Output suppresses further signalling to control pathway activity

What concept describes loss of signalling?

Cells requiring constant input undergo apoptosis if signals stop

Where does signalling occur in development?

Between differentiating cells that coordinate organ and tissue structure

How does signalling relate to disease?

Many diseases arise from dysregulated signalling pathways

What field uses signalling extensively?

Developmental biology

What is a signalling pathway?

A sequence of molecular interactions that transduces information from outside to inside a cell