Unit 4: Cell Communication and Cell Cycle

Homeostasis

Steady state or internal balance

When do cells maintain homeostasis?

Maintain a relatively constant internal environment even when the external environment changes significantly

Set point

Internal conditions typical state

Stimulus

Fluctuations in that condition above or below the set point serve

Sensor

A receptor or sensor detects the stimulus and triggers a response

Response

Activity that returns the condition to the set point

Negative Feedback

Response reduces the stimulus

Example of negative feedback

When you exercise, you produce heat, which increases your body temperature.

Your nervous system detects this increases and triggers sweating. As your sweat evaporates, your skin cools, returning your body to its set point.

Positive Feedback

Stimulus is amplified in order to complete a process; then the condition returns to the set point

Example of positive feedback

During childbirth, the pressure of the baby’s head against the uterus stimulates contractions.

The contractions result in greater pressure, thereby stimulating more contractions and then more pressure. This continues until the baby is born.

Methods Used by Cells to Communicate

Autocrine, Juxtacrine, Paracrine, Endocrine

Autocrine signaling

A cell sends a signal to itself

Juxtacrine signaling

Cells communicate with adjacent cells through direct contact

• Paracrine signaling - cells communicate to nearby cells by releasing chemical messengers

  • Ex. neurotransmitters released into a synapse

• Endocrine signaling - cells communicate to cells far away by releasing chemical messengers that are carried to the target cell

  • Ex. adrenaline is produced by adrenal glands, released into the bloodstream, and carried to the heart and other muscles

Two examples of juxtacrine signalling

• Ex. plasmodesmata connect plant cells and gap junctions connect animal cells

• Ex. glycoproteins on one cell interact with glycoproteins on another cell

Paracrine signalling

Cells communicate to nearby cells by releasing chemical messengers

Paracrine signalling example

Ex. neurotransmitters released into a synapse

Endocrine signalling

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

Endocrine signalling example

Adrenaline is produced by adrenal glands, released into the bloodstream, and carried to the heart and other muscles

In all forms of cell signaling, a signal is converted to a cellular response in three steps:

1. Reception

2. Transduction

3. Response

Reception

Signaling molecule binds to the receptor protein

Transduction

The signal is converted into a form that can produce a cellular response

Response

The transduced signal triggers a cellular response

Reception process

A signal molecule, a ligand, binds to a receptor protein in a lock and key fashion, causing the receptor to change shape.

Where are receptor proteins found?

Most receptor proteins are in the cell membrane but some are inside the cell.

Locations of hydrophilic and hydrophobic ligands

1. Hydrophilic ligands bind to plasma membrane receptors

2. Small or hydrophobic ligands can pass through the membrane and attach to intracellular receptors (ex. steroid hormones like testosterone)

The three most common types of membrane receptor proteins:

• G-protein coupled receptors

• Receptor tyrosine kinases

• Ion channel receptors

The binding of ligands is how specific?

Highly specific (must be the right shape).

G-Protein Coupled Receptors

• G proteins bind the energy-rich GTP (very similar to ATP- source of energy)