Area of Study 2: Detecting and Responding

Detecting and Responding

1. Homeostasis

2. The Nervous System

3. The Endocrine System

4. Pathogens & The Immune System

The Endocrine System

Chemical Communication: The Endocrine System

The endocrine system is a series of ductless glands that secrete hormones directly into the bloodstream.

* Endocrine hormones can affect distant cells and persist in their effect over a prolonged period.

* The endocrine system contributes to homeostasis.

* It is also involved in growth and development and the regulation of sexual reproductive processes.

Endocrine Hormones

Chemical Communication: Endocrine Hormones

Gland or Organ Hormone Effect

Hypothalamus Dopamine Diverse psychological effects

Pituitary Follicle Stimulating Hormone (FSH)

Ovary maturation in females

Thyroid Thyroxine Cellular differentiation and growth, metabolism

Stomach Histamine Gastric acid secretion

Liver Insulin-like Growth Factor (IGF) Cell growth regulation

Pancreas Insulin Cellular uptake of blood glucose

Adrenal Epinephrine (Adrenaline) Fight or flight responses

A wide variety of hormones are released by the each of the different glands (and some organs) of the endocrine system. Each hormone has a specific effect.

A few examples:

Defining Signal Transduction

Chemical Communication: Defining Signal Transduction

Previously, with the introduction of cellular communication, I dropped the term signal transduction.

Signal transduction relates to the biochemical pathways by which messages are delivered and acted upon at cellular level. Now is a good time to narrow our definition of this term.

Signal transduction refers to the movement of signals from outside the cell to inside.

The movement of signals can be simple, like that associated with receptor molecules for acetylcholine: these receptors are found on ion channels which, upon ligand interaction, control the passage of Na+ into or out of the cell. These ion movements result in an action potential relays the signal along the cell.

More complex signal transduction, however, involves the coupling of ligand-receptor interactions to many intracellular events.

Ligand-Receptor Interactions

Binding Specificity: Ligand-Receptor Interactions

Signalling molecules are called ligands and they will bind to specific receptors that have a complementary shape. The shape of a receptor is defined by its tertiary and/or quaternary structure.

This binding specificity is reflected in the interactions of antigens with white blood cells and also substrates with the active sites of enzymes.

Ligands can be hydrophilic or hydrophobic.

That means, importantly, that some hormones can cross the cell membrane and others cannot.

Hydrophilic Hormones Bind Membrane Receptors

Signal Transduction: Hydrophilic Ligands

Hormones that are hydrophilic ligands; * are often derived from amino-acids or polypeptides* are stored in vesicles for secretion by exocytosis * cannot cross the cell membrane * must bind to membrane receptors

Signal transduction is initiated by three types of membrane receptors

* Ion Channel Receptors

* Kinase-linked Receptors

* G protein-coupled Receptors

This activity shows how the latter two work in two specific examples.

Click on picture

Membrane Receptors for Hydrophilic Hormones

Signal Transduction: Hydrophilic Ligands

Ion Channels(milliseconds)

Kinase-linked(can take hours)

G protein-coupled(seconds)

Binding of ligand causes shape change and opens

the ion channel.

eg. Acetylcholine

Binding of ligand causes shape change that activates

intracellular enzymes

eg. Insulin

Binding of ligand activates a G protein which stimulates secondary

messenger production.

eg.Epinephrine/Adrenaline

Glucose Homeostasis: A Closer Look

Hydrophilic Hormones: Insulin and Glucagon

Glucose is crucial to living organisms; * it is broken down by cellular respiration to provide energy* surplus glucose is converted to glycogen and for short term energy storage in liver and muscle cells* when glycogen stores are saturated, glucose is then converted into fat as triglycerides for longer term energy storage.

Glucose concentrations in the blood are generally kept within a very narrow range by negative feedback.

Blood Glucose Levels

Pancreas CellCell Signalling:

Hormone Receptor Type

Cell ResponseBlood

Glucose Levels

Increase Release of insulin Kinase-linked receptorAbsorption of

glucose stimulated Decrease

Decrease Release of glucagon G protein-coupled

receptor

Metabolism of glycogen and release

of glucoseIncrease

Hormones often exist in pairs whereby the action of one counteracts the other; the balancing of effects of two hydrophilic protein hormones, insulin and glucagon, are used to regulate glucose homeostasis.

Insulin production by

pancreas

Stimulus-Response Model

Homeostasis: Stimulus-Response Model

Stimulus (Input)Chemical & Internal

ReceptorMembrane Receptor

ProcessingChemical

EffectorChannels open up in

the membrane

Response (Output)Absorption of glucose

from the blood

FeedbackNegative

It is much harder to put hormone effects into a stimulus-response model because so many messages are involved. Let’s look at a very small part of glucose regulation.

Chemical

Chemical

Chemical

Chemical

Glucose Homeostasis: Diabetes I (Video here)

Signal Failure in Glucose Homeostasis: Diabetes 1

Glucose Homeostasis: Diabetes II (video here)

Signal Failure in Glucose Homeostasis: Diabetes 1I

Glucose Homeostasis: Diabetes Mellitus

Signal Failure in Glucose Homeostasis: An Overview of Diabetes Mellitus

Complications that result from fluctuations in blood glucose concentrations are usually the result of diabetes mellitus.

Blood Glucose Levels

Condition CausePrimary

Treatment Untreated

High Hyperglycaemia

Diabetes I (5-10%) Insulin not produced

Insulin injections Eye, nerve, kidney damageComaDeath

High Hyperglycaemia

Diabetes II (90-95%)Insulin Resistance

Management of lifestyle

Eye, nerve, kidney damageComaDeath

Low HypoglycaemiaUsually triggered by

over-administration of insulin.

Glucose Intake

Feel poorly, unconsciousness,

neurological damage, death

Rational drug design question: What would you target for the development of a drug to treat Type 2 Diabetes?

Hydrophobic Hormones Bind Intracellular Receptors

Direct Signaling: Hydrophobic Ligands

Hormones that are hydrophobic ligands; * are often derived from lipids (eg. steroids)* require carrier proteins for transport in the blood* can cross directly through the cell membrane and so do not require signal transduction* bind to specific receptors inside the cell

Steroid hormones regulate gene expression.

They bind to intracellular receptors

The ligand-receptor complex moves into the nucleus to bind

DNA

mRNA is transcribed and translated into protein.

Video here

Thyroid Control of Metabolism: A Closer Look

Hydrophobic Ligands: Thyroxines

Thyroxines are hormones secreted by the thyroid. Despite being derived from an amino acid and iodine they are hydrophobic. Thyroxines are responsible for regulating metabolism and, like steroids, are able to pass directly across the cell membrane to stimulate gene expression.

Thyroxine production is regulated by 2 other glands:

Thyroid Stimulating Hormone (TSH) produced by the pituitary gland

Thyrotrophin-Releasing Hormone (TRH) produced by the hypothalamus.

Your body responds to excess thyroxine levels by decreasing TSH and TRH production in a

negative feedback loop.

Thyroid Control of Metabolism: A Closer Look

Thyroxine Homeostasis

Diseases of the thyroid can lead to hyperthyroidism or hypothyroidism.

Thyroxine Levels Condition Symptoms

High Hyperthyroidism

Feel hotWeight lossRestlessness

Difficulty sleepingShortness of breath

Chest painMuscle Weakness

Low Hypothyroidism

FatigueWeight gain

Cold sensitivityDepression

Poor concentrationGOITRE!

The biggest goitre I’ve ever seen!

Thyroid Disease: A Closer Look (Video here)

Thyroxine Homeostasis

The Endocrine System and Homeostasis (Video here)

Some Random Guy

This guy does a good job of looking at a range of endocrine processes

Alternative Hormone Communication Pathways

Alternative Forms of Chemical Communication: Hormone Classification

The endocrine hormones are secreted into the bloodstream and are distributed widely throughout the body. Not all hormones, however, act in this way.

Hormone Type Effective Range Examples

Endocrine Hormones

Distributed widely through the body

Insulin & GlucagonThyroxines

Paracrine Hormones

Act on neighbouring cells

NeurotransmittersCytokines*

Autocrine Hormones

Act on the same cell that secretes them

Some Growth Factors

Don’t be fooled by the short explanation; knowing these hormone types is very important (they are just easy to explain).

What is a Hormone?

Chemical Communication: What’s a Hormone?

Hmmppfffhh....Isn’t it a little late for that?

Biology hasn’t really agreed on a definition yet.

Does that include cytokines like me?

Errr.....sure...... :)

Let’s go with this definition:

A hormone is a chemical signal that is released for the purposes of

communication.

Chemical Communication in Plants

Chemical Communication: Plants

Plants do not have an endocrine system but they rely on signaling molecules too; these phytohormones are usually growth regulators

You studied these in Yr11 and they remain an important part of Unit 3.

Auxins

Stimulate cell elongation, cell division, differentiation and organisation of phloem and xylem cells, phototropism, geotropism, hydrotropism, delay leaf senescence, delay fruit ripening, stimulate growth of flower parts, suppress growth of lateral buds, regulate ethylene production.

GibberellinsPromote cell division and elongation in plant shoots; influence germination, dormancy, flowering, leaf and fruit senescence

Ethylene Fruit ripening, leaf fall (abscission), senescence, stress responses

Abscisic Acid Stomatal closure, dormancy stimulation

Cytokinins Cell division

Phytohormones and Plant Tropisms (Video here)

Chemical Communication: Plant Tropisms & Phytohormones

Chemical Signaling Between Organisms: Pheromones

Chemical Signaling Between Organisms: Pheromones

Pheromones are chemicals used by organisms to send messages to individuals of the same species. They are most widely studied in insects.

Social Pheromones are used to communicate messages to a community.

Ants, for instance, leave pheromone trails to indicate the location of food or to send an alarm to members of their community.

Click on the picture!

Sexual Pheromones are used as sex attractants by individuals seeking a mate. Some plants even mimic the sex pheromones of certain insects to attract them and assist pollination.

Stimulus and Receptors: A Backward Glance

Detecting Stimulus: An Overview

Animals

Internal orexternal signals

Photoreceptors Vision & Light

Animals

Internal orexternal signals

Chemoreceptors Olfactory (Scent) & Taste

Animals

Internal orexternal signals

Thermoreceptors Temperature

Animals

Internal orexternal signals

Pain Receptors Ouch!Animals

Internal orexternal signals

Mechanoreceptors Pressure

Animals

Internal signals

Membrane-bound receptors Extracellular signaling

Animals

Internal signals Intracellular receptors Intracellular Signaling

PlantsExternalsignals

Phytochromes (protein) LightPlants

Externalsignals Cryptochromes (protein) Light

The detection of stimuli is described by a broad definition of receptor categories.

* Chemical communication focuses on the binding specificity of receptors that receive molecular messages (yellow).

* Electrical communication relies on sensory nerve endings (blue).

* Plants detect light using light-sensitive proteins (green)