Signalling Molecules

Week 10

Neurotransmitters in action

Comparison of hormone types

Types Size Movement Examples

Fatty-acidhormones

Small Lipid-soluble, so pass directly throughplasma membranes

Steroid hormones:testosterone,oestrogen, progesterone

Amino-acid

hormones

Larger Water-soluble, so bind to receptors on plasma membranes. This activates second messenger mechanism, cyclic AMP, which causes the change within the cell

Insulin, glucagon,adrenaline, thyroxine, oxytocin,ADH (anti-diuretichormone), GH(growth hormone)

Hormones in action

Pheromones• A female moth,

Estigmene acraea, releasing pheromones from brush-like scent organs at its tail

• In this scanning electron micrograph (SEM), the dark pit in the centre is a smell receptor among the feathery-type structures on a male moth’s antenna.

Making use of pheromones

•Pheromone traps used in glasshouses detect the presence of pest insects.

•An appropriate ‘non-pest’ or ‘crop friendly’ species can be used in biological control against the pest insect.

•A pheromone-baited trap in an orchard.

•The artificial pheromone used here mimics that of the codling moth, Carpocapsa pomonella, an important pest of apple and pear orchards.

Plant hormonesHormone Produced

in:Transport in

plantEffects

Auxins Apical tip, young leaves

Tip to base through parenchyma cells

Elongation of cells – tropisms, apical dominance, growth of fruit and delay in ripening

Gibberellins

Young leaves, roots, shoots and embryo in seed

Xylem and phloem

Promotes growth by stimulating cell division and cell elongation. Stimulates germination and flowering.

Cytokinins

Roots Xylem Promotes cell reproduction and differentiation and hence growth. Breaks dormancy.

Abscisic acid

Older leaves, root cap and stems

Vascular tissue

Inhibits germination of seeds, promotes seed dormancy and seasonal dormancy. Causes abscission of leaves and fruits. Stimulates stomatal closure.

Ethylene Stems, ripening fruit, damaged and senescing tissue, seeds

Diffusion throughout intercellular spaces.

Promotes fruit ripening, senescence and abscission of leaves and flowers. Promotes germination

Plant respons

es

• Plant hormones control many aspects of growth and development in flowering plants.

• Some typical hormone actions are shown in the diagram.

Auxins

Auxins are watersoluble chemicals produced in growing tips and they promote the elongation of

cells below.

Tropisms and Auxins

• Growing coleoptiles bend towards a light source.• Bending does not occur if the tip is shielded from

the light.• What conclusion can you draw from the

observation that a coleoptile without its tip fails to respond to the light?

Auxin experimentsExperiment Conclusion(s)

The tips of young shoots are covered with:1.an opaque cap2.a transparent capThe region below the tip is covered with:1.an opaque sleeve

Only the shoot with the opaque cap fails to bend towards the light. We can conclude that the light receptor is in the tip. Covering the bending region does not prevent bending so the tip must transmit information to the bending region.

The tips of young plants are removed and:1.replaced2.replaced, with a layer of porous gelatin3.replaced with an impervious mica layer

A chemical produced in the tip causes the bending response. It flows downwards from the tip to the elongation region. (The movement of the chemical is blocked by the mica, but it passes through the permeable gelatin.)

The region below the tip is partially blocked by a piece of impervious mica on:1.the side facing away from the light source2.the side facing the light source

The plant with the mica sheet on the side facing away from the light fails to bend towards the light. The substance produced in the tip flows down the stem on the side away from the light source. Greater cell elongation on this side of the shoot results.

Abscisic acid

• Abscission zone in a leaf.

• Separation of a leaf from the plant occurs across the abscission zone.

• What is the function of the protective layer?

Guard cells

Tropisms and Auxins

a)An even distribution of light results in an even distribution of auxin in the coleoptile which grows straight.

(b) If light shines from one side, auxin moves to the shaded side and causes the elongation of cells in that area.

The different rates of growth of cells on each side of the shoot result in bending.

Distribution of auxin influences bending in a plant.

Plant responsesResponse Stimulus Visible effect

Abscission Ethylene concentrations in fruitand leaves

Leaves and fruits drop

Apical dominance

Concentrations of cytokinins and auxins

Lateral branch growth inhibited

Geotropism Position of starch filled amyplasts within cells

Roots grow down, shoots grow up

Phototropism

Direction of light source andconcentration of auxins

Growth towards light source

Photoperiodism

Length of dark periods in 24 hours

Season-specific flowering

Fruit ripening

Ethylene concentrations in fruitand leaves

Starch converted to sugars

Thigmotropism

Physical contact Growth altered by contact, e.g. tendrils of vines bend

Cell communication

Cell communication

Principles of cell communication

Cell communication

• Different hormones travel different distances.(a) Some hormones act on the cell that produces them.(b) Some hormones diffuse to nearby cells.(c) Endocrine signals – carried by the bloodstream

Signal Transduction• differences in how the message is passed to the

receiver• involves relaying a message to various proteins

that then relay it to the target proteins• activation of target proteins will lead to the cellular

response:– activate enzymes and other proteins within the

cell– changing the expression of genes within the cell

so that certain proteins are produced, or not produced.

• after the cell has responded the signal must be switched off:

– enzymes breakdown a molecule in the signalling pathway

– switching off the signal itself through feedback mechanisms.

Signal Transduction

Signal transduction pathway through the cell.

Signal Transduction• Signals act

collectively to influence cell activity.

• Different combinations of signals activate different responses for maintenance and survival of the cell, cell division and differentiation.

• Withdrawal of some signalling molecules can induce programmed cell death

Cell receptors

StimulusHormone

ReceptorProtein in or on

cell

Response

Change in cell

function

EffectorIntracellular chemicals

Transmission and amplification

Receptors exteroceptors (cells) are highly specialised to receive

signals from the external environment receive information and convert it to a chemical signal

that can then be relayed between the cells of the body. the five main categories of receptors are:

– Chemoreceptor•detect molecules (smell = nose, taste = mouth)

– Mechanoreceptor•detect pressure, touch, sound vibrations (ear, skin)

– Photoreceptor•detect light (eyes)

– Thermoreceptor•detect temperature (skin)

– Pain receptor•detect pain (skin)

Different hormones, different kinds of chemicals

Hormones, classified on the basis of their chemical nature

Different hormones, some differences in cell signalling

Key differences in the types of hormones

Different hormones, some differences in cell signalling

•Polypeptide-based hormones use cell membrane receptors

•Signal transduction leads to the activation of intracellular proteins or the alteration of gene transcription and protein synthesis.

•Steroid hormones bind with receptors in the cytoplasm of target cells.

Lipid soluble hormones (steroid)

Steroid hormone action

Directly to the boss (lipid soluble hormones)

• Steroid hormones are able to enter the target cell and bind with specific proteins, which transport them to the nucleus.

• There, they switch on or off specific genes in the DNA.

Peptide hormones (water soluble)

Peptide hormones (cont.)

Second messenger (peptide hormones)

• The cyclic AMP second messenger mechanism of signal transduction.

• The hormone binds to a receptor on the plasma membrane.

• This then activates adenylate cyclase, which causes ATP to be converted into cyclic AMP.

• Cyclic AMP then activates a number of enzymes in the cytoplasm.

Positions of cell receptors

Positions of cell receptors

Hormones and signal transduction summary

Receptor proteins and signal transduction methods of

different types of hormones

Action of hormones on target cells

Body temperature

Water balance and ADH

• Vasopressin and renin both play a role in water conservation

Blood Glucose Regulation

Blood glucose

Example of negative feedback

Blood glucose

A digital glucometer and other equipment used by diabetics to measure their blood glucose level and inject the required amount of insulin

A portable insulin pump can be used to inject insulin slowly and continuously into the bloodstream

Blood pressur

e• Under

nervous and hormonal control.

• If a change in blood pressure occurs, events counteract the change.

Chapter 5 and 6 Reviews• Chapter 5

– Biochallenge (p.162), all questions– Chapter Review (p.163), Q. 2-6, 8-9

• Chapter 6– Biochallenge (p.196), all questions– Chapter Review (p.197), Q. 2-3, 6-8