Plant and Animal Processes: Tropism and Phototropism

Learning Competency

• Compare and contrast processes in plants and animals, including reproduction, development, nutrition, gas exchange, transport/circulation, regulation of body fluids, chemical and nervous control, immune systems, and sensory and motor mechanisms.

Plant Movement in Response to Environmental Stimuli

• Plants respond to external stimuli through directional growth, where the growth direction depends on the stimulus direction.
• Tropism is defined as an irreversible directional growth response.
• Positive tropic responses result in growth toward the stimulus.
• Negative tropic responses result in growth away from the stimulus.

Phototropism

• This involves the bending of growing stems and other plant parts toward light sources.
• Stems exhibit positive phototropism.
• Most roots do not respond to light or show a weak negative phototropic response.

Gravitropism

• This is the response of a plant to Earth's gravitational field.
• Shoots bend and grow upwards when a potted plant is tipped over.

Thigmotropism

• This represents a plant's response to touch from objects, animals, other plants, or even the wind.
• Tendrils curl around objects within 3 to 10 minutes due to specialized epidermal cells promoting uneven growth upon contact.

Other Tropisms

• Electrotropism involves responses to electricity.
• Chemotropism involves responses to chemicals.
• Traumotropism involves responses to wounding.
• Thermotropism involves responses to temperature.
• Aerotropism involves responses to oxygen.
• Skototropism involves responses to darkness.
• Geomagnetotropism involves responses to magnetic fields.
• Hydrotropism is a growth movement of roots following a water diffusion gradient, though the true tropism status is debated.

Signal Transduction Pathways

• Hormone or stimulus interaction with a receptor protein triggers the activation of relay proteins and production of second messengers.
• The signal is passed along, bringing about cellular responses.
• Receptors can be on the cell surface or inside the cell.

Steps in Information Processing

• Receptor cells perceive an external stimulus and transduce it into an internal signal.
• A hormone (cell-cell signal) is released by the receptor cell and travels throughout the body.
• Receptor cells receive the hormonal signal, transduce it to an internal signal, and change activity.

Growth, Development, and Responses to Stimuli

• A hormone is a signaling molecule produced in tiny amounts, transported to other parts, binding to a specific receptor, and triggering responses in target cells and tissues.
• Plant hormones influence roots and shoot growth, fruit formation and ripening, seed germination, flowering time, leaf fall, bud formation, and disease resistance.
• Promoters such as auxins, cytokinins, gibberellins, and brassinosteroids cause faster growth.
• Inhibitors such as ethylene, abscisic acid and jasmonic acid reduce growth.

Animal Sensory and Motor Mechanisms

• Sensory receptors detect changes in the internal or external environment in most animals.
• Receptors consist of specialized neuron endings or cells in close contact with neurons.
• Sensory receptors and other cells form complex sense organs like eyes, ears, nose, and taste buds.
• Sensory processing involves sensory reception, energy transduction, signal transmission, and brain interpretation.

Sensory Pathway to the Brain

• Reception is the detection of stimuli by sensory receptors.
• Transduction is the conversion of physical/chemical stimulus energy into a change in the membrane potential of a sensory receptor.
• Transmission involves some sensory cells with axons extending into the central nervous system (CNS) generating transmission of action potentials (nerve impulses) to the CNS after energy transduction.
• Perception construct stimuli in the brain (color, smells, sounds, tastes).

Classification of Receptors by Type of Energy Transduced

• Thermoreceptors detect heat and are found in blood-sucking insects/ticks, pit organs of pit vipers, and skin/tongues of many animals.
• Electroreceptors and electromagnetic receptors sense electrical differences and magnetic fields for navigation and orientation in various species.
• Nociceptors (pain receptors) respond to mechanical (physical force such as strong touch, pressure; heat, temperature extremes and damaging chemicals) with neuron endings in skin.
• Mechanoreceptors respond to mechanical change and shape as a result of being pushed or pulled; with the use of tactile receptors.
• Proprioceptors respond to movement and body position with muscle spindles, Golgi tendon organs and joint receptors.
• Statocysts are found in invertebrates and have hair cells that respond to gravity lateral line organs in fish; it detects vibrations in water and respond to waves and currents.
• Hair cells in the vestibular apparatus respond to gravity and acceleration.
• Hair cells in semicircular canals respond to angular acceleration.
• Hair cells in the organ of Corti in the cochlea respond to pressure waves (sound).
• Chemoreceptors detect specific chemical compounds with the use of taste buds and olfactory epithelium.
• Photoreceptors detect light in eyespots, ommatidia of arthropods, and in rods and cones in the retina of vertebrates.

Overview of Vertebrate Skeletal Muscle

• Vertebrate skeletal muscle moves bones and body and has a hierarchy of smaller units.
• Typical skeletal muscle contains long fibers in parallel with multiple nuclei.
• Muscle cell contains myofibrils (thin and thick filaments).
• Myofibrils contain repeating sections called sarcomeres (basic contractile units).
• Sarcomere borders line up forming light and dark bands (striations visible with a microscope).

Sliding-Filament Model of Muscle Contraction

• Muscle fibers contract using the sliding-filament theory.
• Myosin filaments use ATP energy to "walk" along the actin filaments with cross bridges.
• Actin filaments are pulled closer together.