Cell Signaling and Neurons

Cell

cell interaction

Ligand

Molecule that binds to a specific receptor site.

Ligand receptor complex

Complex formed by ligand binding to receptor.

Induced fit

Receptor changes shape upon ligand binding.

Positive feedback

End product amplifies the initial signal.

Negative feedback

End product inhibits further production of itself.

Oestradiol

Regulates hormone release in ovaries and uterus.

Intracellular receptors

Receptors that bind hydrophobic hormones inside cells.

Hormone receptor complex

Complex that activates gene expression in nucleus.

Kinase

Enzyme that adds phosphate groups to molecules.

Phosphorylation

Process of adding phosphate groups to proteins.

Transmembrane receptors

Receptors that span the cell membrane.

Transduction pathway

Series of events triggered by receptor activation.

Neurotransmitters

Convey signals between neurons, and between neurons and muscle fibers.

They are released into the synaptic gap and bind to receptors in these membranes of the postsynaptic neuron or the muscle fibre.

Synaptic gaps

Spaces between neurons where neurotransmitters diffuse.

G

proteins

G protein coupled receptors

Receptors that activate G

Axons

Long fibers that transmit impulses over long distances away from neurons.

Acetylcholine

Neurotransmitter that opens sodium channels, used as a neurotransmitter in many synapses, including those between neurons and muscle fibres.

Action potential

the change in electrical potential associated with the passage of an impulse along the membrane of a muscle cell or nerve cell.

ex: When acetylcholine binds to the binding site on an acetylcholine receptor, the conformation (shape) of the receptor changes. A channel opens, allowing sodium ions to pass into the cell. This leads to a local depolarization that triggers an action potential

Secondary messenger

Molecule that relays signals inside cells.

FADS1 gene

Gene increased by androgen receptor binding testosterone.

Draw a labelled diagram to show the fluid mosaic model of the plasma membrane.

Drawing may include:

a. two correctly orientated layers of phospholipids/phospholipid bilayer shown with heads facing in opposite directions

b. phospholipids shown with two parts labelled hydrophilic/phosphate head AND hydrophobic/hydrocarbon tail

c. protein (any) shown as a globular structure embedded in one/both layers of phospholipid

d. peripheral protein shown as globular structures at the surface of the membrane AND integral protein shown as embedded globular structures

e. glycoprotein shown as embedded globular structure with antenna

like carbohydrate protruding

OR

carbohydrate shown as branched/antenna

like structure attached either to a protein or to a phospholipid

OR

channel protein(s) shown with a pore passing through it

OR

pump protein shown as a transmembrane globular structure

f. cholesterol shown in between adjacent phospholipids

Do not award the mark unless the structure is labelled

membrane channel protein

when open, will allow anything through that has the right size and charge.

membrane transporter protein

transfer only molecules that fit into specific binding site within the protein similar to the specificity between enzyme and substrate.

In the Na+/K+ pump...

3 Na+ are removed for every 2 K+ brought in

The the uneven transport paired with the high concentration of negatively charged membrane proteins along the interior side of the membrane creates an electrochemical gradient where the outside of the membrane is (+) and the inside is (

)

There are three types of endocytosis

Phagocytosis ("cellular eating")

Pinocytosis ("cellular drinking")

Receptor

mediated endocytosis

Outline four types of membrane transport, including their use of energy.

a. simple diffusion is passive movement of molecules/ions along a concentration gradient ✔

b. facilitated diffusion is passive movement of molecules/ions along a concentration gradient through a protein channel «without use of energy» ✔

c. osmosis is the passage of water through a membrane from lower solute concentration to higher ✔ OWTTE

d. active transport is movement of molecules/ions against the concentration gradient «through membrane pumps» with the use of ATP/energy ✔

e. endocytosis is the infolding of membrane/formation of vesicles to bring molecules into cell with use of energy

OR

exocytosis is the infolding of membrane/formation of vesicles to release molecules from cell with use of energy ✔

f. chemiosmosis occurs when protons diffuse through ATP synthase «in membrane» to produce ATP ✔

Active transport requires mention of the use of energy.

Ion channels

Ion selectivity depends on the diameter and shape of the ion. Each ion in aqueous solution is surrounded by a small shell of water molecules, most of which have to be shed for the ions to pass, in single file, through the selectivity filter in the narrowest part of the protein channel.

An ion channel is narrow enough in places to force ions into contact with the channel wall so that only those ions of appropriate size and charge are able to pass

Ion channels open only briefly and then immediately close again.

As we see in our next presentation that most ion channels are gated: a specific stimulus triggers them to switch between a closed and an open state by a change in their conformation.

Over 1,000,000 ions can pass through an ion gate each second it is open which is a 1000x faster than any other transporter.

Osmosis

the net movement of water molecules across a semipermeable membrane from a region of low solute concentration to a region of high solute concentration (until equilibrium is reached)

Quorum sensing

This ability to sense when enough of the population is is present to change the collective behavior of bacteria

How does quorum sensing work?

signalling molecules (called autoinducers) are secreted at a low rate by all bacteria cells in the population.

When there has been sucient binding of the signalling molecules to receptors in a cell, gene expression is changed.

Bacteria not only sense how many others are around them but also who their neighbors are.

The existence of an autoinducer quorum sensing system is universal to all bacterial however the specific shape and chemistry of each autoinducer molecule is unique to the type of bacteria that produce them.

Quorum sensing in Vibrio cholerae

When cholera bacteria enter the host, they express genes for adhesion, including pili (TCP) and mucus biofilm production, helping them form aggregates. As the population grows, they reach quorum and shift gene expression to escape mechanisms, releasing cholera toxin (CT). CT activates chloride ion transport in gut cells, causing electrolyte imbalances and massive water loss. This leads to watery diarrhea, which spreads the bacteria to new hosts.

A signalling chemical must

be small and soluble enough to be transported.

Hormones as an example of a signaling molecules

Hormones are signalling chemicals produced by a group of specialized cells in the body and transported by the bloodstream. Most hormones are secreted into blood capillaries in the gland tissue. Because of this internal secretion, glands that secrete hormones are called endocrine glands.

Insulin, thyroxin and testosterone are example of hormones.

Endocrine glands

secrete their product (hormones) directly into the bloodstream, rather than through a duct (e.g. exocrine gland)

Major endocrine glands

pancreas, adrenal gland, thyroid gland, pineal gland and the gonads (ovaries and testes)

long distance signalling through the blood

The hypothalamus links the nervous and endocrine systems to maintain homeostasis. It receives input from the body and brain, then releases neurochemicals (releasing factors) that signal the pituitary gland to release hormones.

The pituitary gland

controls the secretion of a number of other endocrine glands; pituitary hormones will often target endocrine glands in other organs (e.g. gonads, pancreas, thyroid, mammary gland)

Pituitary hormones hence control many vital body processes, including:

Metabolism (e.g. TSH activates thyroxin)

Adult Development (e.g. LH / FSH trigger puberty)

Reproduction (e.g. LH / FSH control menstruation)

Growth (e.g. growth hormone promotes growth)

Equilibrium / Homeostasis(e.g. ADH and water balance)

Cytokines

Cytokines are small signaling chemicals. Molecules that act locally, on the cell that produced them or nearby cells, through paracrine signaling. They bind to plasma membrane receptors, triggering intracellular cascades that alter gene expression and cell activity.

Example: erythropoietin (EPO), interferon, and interleukin, play key roles in inflammation, immune responses, cell growth, proliferation, and embryo development. Some can bind to multiple receptors, leading to various effects. Certain ones can be secreted by almost all cells in the body.

Calcium ions

Calcium ions play a key role in cell signaling in muscle fibers and neurons.

In muscle fibers, calcium is stored in the sarcoplasmic reticulum.

When a nerve impulse is received, calcium channels open, allowing calcium to bind to proteins that prevent contraction, enabling muscle contraction. If no further nerve impulses are received, the calcium is pumped back into the sarcoplasmic reticulum, reversing the process.

General Cell Signaling Pathway

1. Ligand Reception

2. Signal Relay Transduction

3. Response

1. Ligand Reception

The first step involves the (ligand) binding with the protein receptor. Binding causes a sequence of events called a signal transduction pathway.

Signalling chemicals (ligands) can two groups:

1. Ligands that diffuse through the plasma membrane to enter the cell to bind with their receptor inside the cell. These ligands are typically nonpolar, lipid molecules such as the steroid hormone: oestradiol

2. Ligands that can not cross the membrane and therefore bind with a receptor on the surface of the cell. These ligands are typically polar, peptide molecules such as the protein hormone:insulin

Surface Receptors

Also called transmembrane receptors, have a section (domain) of their protein surface that is hydrophobic / nonpolar which allows them span the membrane an interact with the nonpolar phospholipid tails in the core of the membrane.

Intracellular Receptors

Located in the cytoplasm or nucleus of the cell and are largely globular receptors made up of hydrophilic amino acids so they remain dissolved in the aqueous environment.

They often have a hydrophobic core that will accept the nonpolar hormone ligand

Proteins signaling molecules

they bind with receptors on the surface of the cell. This binding results in a signal transduction pathway that then activates a secondary messenger that initiates a cellular response.

Common receptors include: G

protein coupled receptors or tyrosine kinase receptors