THE ENDOCRINE SYSTEM Option 2

HOW IT WORKS Hormones are secreted by ductless glands Hormones travel through the blood stream from glands to target cells The hormone will only bind to specific receptors on (or in) the target cells The hormone acts a messenger for target cells to produce a response For example: The message may activate a gene to produce a specific protein, such as an enzyme, to fuel metabolic reactions When a hormone has completed its task it is broken down by cellular enzymes and excreted

TYPES OF SIGNALS Autocrine hormones act on the cell that produces them Paracrine hormones diffuse to nearby cells Endocrine hormones are carried through the bloodstream to distant cells

TYPES OF HORMONES Amino Acid Derivatives Similar to amino acids Travel through blood stream Water soluble (Not lipid soluble) Steroid hormones Similar to cholesterol Transported by carrier proteins (via blood stream) Lipid soluble (Not water soluble) Peptide hormones Chains of amino acids Travel through blood stream Water soluble (Not lipid soluble)

TYPES OF SIGNALING MOLECULES Steroid based hormones Can pass directly through the plasma membrane Binds to a receptor in the cytosol Amino acid/Peptide based hormones Cannot enter the cell Binds to a receptor on the extracellular side of the membrane, which starts a reaction inside the cell Lipid solubleWater soluble

LIPID SOLUBLE HORMONES

WATER SOLUBLE HORMONE

SIGNAL TRANSDUCTION The hormone binds to an extracellular receptor This creates a change to the receptor on the intracellular side of the membrane Other proteins (eg. G proteins) are activated and act as a second messenger to create a signal pathway Messengers (such as cAMP) can be involved in this pathway This sends a message to the nucleus to create a response

PHEROMONES Species-specific messenger molecules Acts as a communication mechanism between individuals of the same species Commonly found in insects Secreted by specialised glands Pheromones can be effective over hundreds of meters

PLANT HORMONES

USES Plants use hormones to regulate growth and respond to the environment There are multiple categories of plant hormones which can influence growth in various ways Auxins Gibberellins Abscisic Acid Ethylene Cytokinins Plant hormones work together and influence each other to produce different effects

AUXINS Roles Accumulates to elongate (or enlarge) cells IAA (Indolacetic acid) promotes apical dominance, inhibiting lateral buds. Making fewer, longer growing shoots. Stimulates lateral growth in roots Promotes growth of flowers/fruits Contributes to the differentiation of vascular cells Inhibits growth in roots when in high concentrations Auxin is produced in growing tips and moves down the plant

AUXIN

GIBBERELLINS Promotes cell elongation and reproduction Initiate seed germination and bud development Water activates gibberellins, creating gibberellic acid. In turn, DNA creates amylase to digest the starch in the seed, creating glucose to fuel early growth

ABSCISIC ACID Abscisic Acid reduces water loss It interferes with the uptake or retention of ions in guard cells This causes the stomata to close, reducing water loss through transpiration

ABSCISIC ACID Inhibits growth Assists in the creation of an abscission zone This allows plants to drop leaves and/or fruit The presence of auxin prevents the formation of the abscission zone

ETHYLENE Gas which promotes the ripening of fruit Increases the rate of respiration High concentrations of ethylene can cause flowers to die Promotes the formation of the abscission zone in older leaves

CYTOKININS Promotes cell reproduction Found in high concentrations in young cells in fruits, shoots and roots Promotes rapid growth in these areas

THE ACTION OF HORMONES The hormones move through plants via either the xylem or the phloem Tropisms refer to the growth of a plant in response to external stimuli Positive tropism involves growth towards a stimulus Negative tropism involves growth away from a stimulus External stimuli can include Light Temperature Gravity Day length

PHOTOTROPISM Growth in response to light, caused by auxins It is positive phototropism, as growth is towards the stimulus If light distribution is even, then auxin is evenly diffused across a growing shoot. Therefore, cells are equally elongated, this causes the shoot to grow straight up. If light distribution is uneven. Auxin diffuses away from light, therefore the cells on the “dark” side will elongate, causing the growing shoot to bend towards the light This allows plants to gain maximum light exposure for photosynthesis

PHOTOTROPISM

GEOTROPISM This can be positive or negative Positive geotropism occurs in roots, as they towards the stimulus. Auxin accumulates on the lower side of horizontal root tips, this inhibits growth in the lower-side cells. This causes the root tip to bend downwards, as the top cells continue to grow Negative geotropism occurs in shoots, as they grow away from the stimulus Auxin accumulates on the lower side of the horizontal shoot tips, this promotes faster growth in the lower- side cells. This causes the root tip to bend upwards.

THIGMOTROPISM Growth in response to contact or touch Occurs in some species in growing shoots. This aides in support the growing plant The Venus Flytrap also uses this to gain nutrients