Plant Hormones and Growth

Control and Coordination

• As the complexity of organisms increases, different cells and organs become more separated, requiring a system for coordinating their functions as a single unit.

Coordination in Animals

• Animals possess a nervous system and a hormonal (endocrine) system for control and coordination
• Complex animals depend on the nervous system for control and coordination

Coordination in Plants

• Plants lack a nervous system
• Plants utilize chemical substances known as plant hormones or phytohormones for control and coordination
• Plant growth is divided into three stages: cell division, cell enlargement, and cell differentiation

Stimuli and Response

• Organisms respond to changes in their environment via stimuli
• Response to a stimulus often involves movement of a body part
• Homeostasis is the maintenance of constant internal body conditions
• Homeostasis involves maintaining the conditions of the body by controlling the physiology of the organism
• Plants, like animals, must control and coordinate their various functions

Plant Hormones

• The plant hormones control aspects of growth, dormancy, stomata control etc
• Four major types include: auxins, gibberellins, cytokinins, and abscisic acid, and ethylene gas
• Auxins, gibberellins, and cytokinins promote plant growth
• Abscisic acid inhibits growth

Auxins

• Promotes cell enlargement and differentiation
• Promotes fruit growth
• Responsible for phototropic and geotropic movements
• Made at stem and root tips
• Moves away from light and towards gravity
• Accelerates stem growth but slows root growth
• Synthetic auxins (e.g., indole-3-acetic acid, 2,4-D) are used in agriculture and horticulture

Gibberellins

• Promotes cell enlargement and differentiation when auxins are present
• Functions in stem elongation, breaking dormancy in seeds and buds, and promoting germination
• Promotes fruit growth
• Gibberellic acid (GA3) induces parthenocarpy (seedless grape development)

Cytokinins

• Promotes cell division, breaks dormancy in seeds and buds, and delays leaf aging
• Promotes stomata opening and fruit growth

Abscisic Acid

• Functions as a growth inhibitor, promoting dormancy in seeds and buds
• Causes stomata to close
• Promotes wilting and the falling/detachment of leaves, flowers, and fruits

Ethylene Gas

• Plays a key role in stimulating fruit ripening
• Ripe fruit releases ethylene gas, which can accelerate ripening in nearby raw fruits
• Calcium carbide reacts with water to produce acetylene gas, which can also ripen fruits quickly

Plant Movements

• Divided into tropic and nastic movements

Tropic Movements

• Geotropic: Growth movement in response to gravity.
  • Roots show positive geotropism (growth in the direction of gravity)
  • Stems show negative geotropism (growth against the direction of gravity)
• Phototropic: Growth movement in response to light.
  • Stems show positive phototropism
  • Roots show negative phototropism
  • Higher cell division rate occurs on the side of the stem away from sunlight due to auxin
• Hydrotropic: Growth movement in response to water
  • Roots usually exhibit positive hydrotropism, growing towards water sources
• Thigmotropic: Growth movement in response to touch
  • Seen in climber tendrils that coil around supports
  • Differential cell division in tendrils is mediated by auxin
• Chemotropic: Movement in response to a chemical stimulus.
  • Positive chemotropism is growth towards the chemical
  • Negative chemotropism is growth away from the chemical
  • Pollen tube growth towards the ovule is an example of positive chemotropism

Nastic Movements

• Movements not dependent on the direction of the stimulus
  • Seismonastic/Thigmonastic: Caused by mechanical stimuli (e.g., touch)
    • Mimosa pudica leaves droop when touched due to changes in water balance in cells
  • Photonastic: Induced by light intensity fluctuations
    • Flowers opening and closing in response to light
  • Thermonastic: Brought about by temperature changes
    • Flower movements in response to temperature
  • Nyctinastic: 'Sleeping movements' induced by alternation of day and night
    • Leaves of some plants droop and close toward evening, then rise again in the morning

The Nervous System

• Composed of nervous tissue, with the neuron (nerve cell) as the functional unit
• Responsible for control and coordination in complex animals
• Receptors: Specialized nerve fiber tips that collect information
  • Located in animal sense organs
  • Classified by stimulus type: phono-, photo-, thermo-, olfactory-, gustatory-receptors

Neuron Structure

• Consists of dendrites, cyton/soma/cell body, and axon
  • Dendrites receive impulses
  • Cyton/soma processes impulses
  • Axon transmits impulses to another neuron or to muscles/glands
  • Axons can be myelinated or non-myelinated
  • Impulse transmission is faster in myelinated neurons

Neuron Types

• Sensory neurons: Receive signals from sense organs
• Motor neurons: Send signals to muscles or glands
• Association/relay neurons: Relay signals between sensory and motor neurons

Synapse

• The contact point between the terminal branches of one neuron's axon and another neuron's dendrite

Neuromuscular Junction (NMJ)

• Point where a muscle fiber meets a motor neuron carrying nerve impulses from the central nervous system

Nerve Impulse Transmission

• Nerve impulses travel from - Dendrites to cell body
  • Axon
    • Nerve endings at axon tip
    • Synapse
    • Dendrite of the next neuron
• Chemicals released from an axon tip cross the synapse or NMJ to reach the next cell
• Acetylcholine is an example of a neurotransmitter

Human Nervous System Organization

• Central Nervous System (CNS): Composed of the brain and spinal cord
  • Brain controls body functions
  • Spinal cord relays signals between the brain and peripheral nervous system
• Peripheral Nervous System (PNS): Composed of cranial and spinal nerves
  • Cranial nerves (12 pairs) originate from the brain and innervate the head region
  • Spinal nerves (31 pairs) originate from the spinal cord and innervate regions below the head
  • Visceral nerves connect internal organs to the spinal cord and brain
• Autonomous Nervous System:
  • Composed of a chain of nerve ganglia along the spinal cord
  • Controls involuntary actions and is divided into the sympathetic and parasympathetic systems

Sympathetic Nervous System (SNS)

• Controls the "fight or flight" response
  • Prepares the body to fight a threat or run away
  • Directs energy away from non-essential functions towards survival functions
• Adrenaline is released
  • Causes several physiological changes
• Heart rate and blood pressure increase
  • Boosting oxygenated blood flow to muscles
• Bronchial tubes dilate
  • Increasing airflow to improve alertness.
• Pupils dilate
  • Allowing more light for seeing

Parasympathetic Nervous System

• Slows down organ activity, creating a calming effect
• Breathing and heart rates slow down during sleep
• Aids energy conservation
• Maintains steady heart rate and blood pressure
• Stimulates digestion and sexual function

Human Brain

• Complex organ composed of nervous tissue folded to maximize surface area and minimize space
• Covered by the meninges (three-layered membrane system)
• Cerebrospinal fluid (CSF) cushions the brain against shocks
• Divided into: forebrain, midbrain, and hindbrain

Forebrain parts

• Olfactory lobes: Involved in the sense of smell
• Cerebrum: Largest brain part, divided into hemispheres connected by the Corpus callosum
  • Corpus callosum transmits messages
  • Each hemisphere controls the opposite side of the body
  • Consists of cortex and inner medulla
  • Hemispheres split into 4 lobes: Frontal, Parietal, Occipital and Temporal
• Diencephalon: Lies between the cerebrum and midbrain

Cerebrum function

• Controls voluntary motor actions
• Site of sensory perceptions (tactile, auditory)
• Seat of learning and memory

Diencephalon function

• Link between the nervous and endocrine systems
• Receives signals from nerves, interprets them, and the pituitary gland responds by secreting hormones
• Thalamus: Relay center for pain and pressure
• Hypothalamus: Controls sleep/wake cycles, urges for eating/drinking, body temperature, controls pituitary gland and blood pressure. Lies below the thalamus

Midbrain

• Connects the forebrain and hindbrain
• Relays neuronal transmissions from the peripheral to the central nervous system
• Integrates sensory information from eyes and ears with muscle movements

Hindbrain

• Formed by the pons, medulla oblongata, and cerebellum
• Structures govern autonomic body systems for heart, breathing, sleep and bladder
• Pons: Relays impulses between the cerebellum, spinal cord, cerebrum and midbrain. Regulates respiration
• Medulla: Forms the brain stem, regulating heartbeat, respiration, blood pressure, salivation, and vomiting
• Cerebellum: Coordinates motor functions, posture, balance, and precision of voluntary actions

Spinal Cord

• Extends from the medulla oblongata
• Consists of nerve fibers running through the vertebral column
• Segmented with nerve fiber roots to form spinal nerves
• Gray matter (butterfly-shaped) surrounded by white matter
  • Gray matter contains CSF -filled central canal
  • White matter consists of axons for CNS communication

Spinal Cord Function

• Link between the brain and PNS, providing structural support
• Facilitates flexible movements
• White matter myelin acts as electrical insulation
• Communicates messages to different parts of the body
• Coordinates reflexes, receives and sends sensory information for processing

Reflex Action

• A special case of body's involuntary movement
• A sensory organ that senses danger pulls itself away immediately
• Examples include pulling the hand away from hot electric iron, or other hot object

Reflex Arc

• The nerve signal path in a reflex action
• Pathway: receptor → sensory neuron → relay neuron → motor neuron → effector (muscle)
• The sensory neurons pick sends signals from a danger receptor to the relay neuron
• Relay neuron is in the spinal cord
• The spinal sends signals to the effector
• Effector moves receptor from danger

Reflex Arc Properties

• Passes at the spinal cord level
• Signals don't travel to the brain
• All actions are ultimately controlled by the brain. Reflex actions are controlled mainly in the spinal cord

Protecting the Brain and Spinal Cord

• Brain is protected by fluid and enclosed in the cranium (brain box)
• Spinal cord is enclosed in the vertebral column

Muscular Movements

• Muscle tissue contains actin and myosin filaments
• Nerves trigger the events in muscle movement
• Ions enter muscle cells
• Actin and myosin slide to contract muscle

Endocrine System

• Endocrine glands secrete hormones directly into the bloodstream
• Hormones, mainly composed of protein, control body functions such as: hunger, body temp, mood, growth, metabolism
• Assist the nervous system in control/coordination in parts of the body the nervous system does not react to
• Hormonal is a slower control system compared to the nervous system

Hormones

• Chemical messengers secreted by ductless endocrine glands
• Reach target sites, stimulating or inhibiting physiological processes
• target cell needs to have specific protein molecule (receptor) in order to be receptive
• Around 20 major hormones regulate physiological processes
• Hormone levels are influenced by stress, infection, and mineral levels
• Gland defined as a cell, tissue, or organ that secretes chemical compounds for requirement for a particular function

Gland Types

• Endocrine: Secretes hormones directly into the bloodstream (e.g., pituitary, thyroid)
• Exocrine: Secretes directly or via ducts onto a surface (e.g., salivary, sweat)
• Heterocrine: Glands contain both endocrine and exocrine tissues (e.g., pancreas)
  • The pancreas produces insulin/glucagon and secretes digestive enzymes

Hypothalamus

• Small region near the pituitary gland at the brain's center
• Link between the nervous system and the pituitary gland
• Main function: maintaining homeostasis (internal balance)
• Connects the endocrine and nervous systems, controlling pituitary hormone release
• Stimulates or inhibits activities to maintain homeostasis (body temperature, sleep, appetite)
• Controls circadian rhythm, sexual behavior, and reproduction

Pituitary Gland

• Small is gland at the brain's base
• Master gland controlling other glands
• Secretes GH (Growth Hormone)
  • causes dwarfism/gigantism/acromegaly with under/over-secretion
• Secretes thyroid-stimulating hormone, MSL, LH & FSH

Thyroid Gland

• Butterfly shaped gland in the throat that produces hormones
• Secretes thyroxine
  • which regulates metabolism
• Role in bone growth, brain development, and the nervous system
• Requires iodine to synthesize thyroxine
• Iodine deficiency causes goiter
• Children suffer from cretinism from iodine deficiency

Parathyroid Gland

• It releases parathormone
• Parathormone regulates calcium and phosphorus levels in the bone

Pineal Gland

• Produces melatonin
• Melatonin hormone regulates sleep patterns

Pancreas

• Leaf-like gland behind the stomach
• Both endocrine and exocrine gland
• Insulin and glucagon - antagonist hormones regulating blood sugar levels
• Exocrine releases enzymes to break proteins, lipids, carbohydrates, and nucleic acids

Adrenal Gland

• Hormone releasing gland near each kidney
• Decreases size overtime
• Releases adrenaline/epinephrine during flight or fight
• Secretes glucocorticoids, mineralocorticoids, and cortisol
• Regulates metabolism and blood pressure
• Epinephrine increases heart & breathing rates, cardiac contractions and glucose

Others

• Norepinephrine (noradrenaline) increases body actioning
  • release of above is via neural impulse from sympathetic nervous system
• Thymus is gland behind breastbone where thymosin promotes an immune response
• Gonads produces gamete and release estrogen/progesterone/testosterone for repro and 2ndary sex characteristics

Feedback Mechanism

• Precise quantification is desired
• The timing and amount of release is constructed in the body to monitor glucose
• Example, high levels after carbohydrates causes insulin by the pancreases to store sugar
• If levels go to low, it's glucagon that helps break down glycogen to maintain levels

Explore plant hormones such as gibberellins and cytokinins. Understand their functions in growth, aging, and environmental responses. Learn about tropic and nastic movements and neural signals in plants.