CONCEPTS (continued)
Invertebrate Nervous Systems:
Single-celled protists do not have "systems", but are sensitive to the environment. Amebas move away from light, but have no photodetectors or eyes. The paramecium has no specialized sensory structures, but will back up and move away from extreme temperatures and toxic chemicals.
Euglena have an eyespot that acts as a light sensitive receptor. It prefers moderate light and moves away from darkness and bright light. Euglena probably use their receptor to keep themselves in light which they use for photosynthesis.
Sponges are the only multicellular animals without a nervous system. They do not have any nerve cells or sensory cells. However, touch or pressure to the outside of a sponge will cause a local contraction of its body.
The hydra has a nerve net, a collection of separate, but "connected" neurons. Communication between neurons can be in both directions at the synapse. The nerve net is concentrated around the hydra's mouth. Unlike higher animals, the hydra does not have groupings of nerve cell bodies (ganglia).
The hydra does have specialized cells for touch and chemical detection.
Flatworms, like the planarian, also have a nerve net. However, their nerves are connected by long nerve cords. These cords are connected to cerebral ganglia located in the head region. Their central nervous system has been described as "ladder-like" because of the nerves connecting the nerve cords. Flatworms have auricles that project from the side of the head, containing chemoreceptors used to find food. Flatworms also have eyespots called ocelli. The ocelli are sensitive to light and are connected to the cerebral ganglia. Generally, the flatworm avoids light.
The earthworm has a segmented nervous system, just like the rest of the body. The brain is located above the pharynx and is connected to the first ventral ganglion. Each segmented ganglion gets sensory information from only a local region and controls muscles only in this local region.
Earthworms have touch, light, vibration and chemical receptors all along the entire body surface.
The grasshopper has a brain located between its eyes, just above the esophagus. The brain is connected to the 1st ventral ganglion by a pair of ventral nerves that surround the gut. The brain is used to relay sensory information to other parts of the body and to help with movement. The first ventral ganglion is used primarily to control movement of the mouth. The segmental ganglia throughout the length of the grasshopper are connected to the first ventral ganglion by a double nerve cord and serve to coordinate local activities.
Insects have a compound eye containing many different lens units; composed of a lens, cone cell, retinula cell, and nerve cell. Each unit samples a small part of the visual field. There can be thousands of these units in a single insect eye.
Movies often show an insect seeing hundreds of identical images of the entire visual field. An insect sees only ONE image because each lens unit sees only a small part of the entire field. Some insects are sensitive to ultraviolet light and others can detect infrared wavelengths of light.
An octopus has the most complicated brain of all invertebrates.
It is estimated to have 300,000,000 neurons, arranged in lobes and tracts more specialized than simple ganglia. An octopus has a "good" memory and can learn.
The octopus also has a statocyst located next to the brain. The statocyst is used to detect changes in gravity and respond to acceleration.
The eye of the octopus is very similar to that of vertebrates, with a cornea, lens, iris and retina. However, the octopus eye is different from vertebrates in that it focuses light by moving the lens closer and further away from the retina. The vertebrate eye focuses by changing the shape of the lens. Octopi can perceive shape, color intensity and texture. Another difference is that the eye of the octopus has NO blind spot because the nerve cells leave from the outside of the eyeball.
Plant hormones are chemical messengers that affect a plant's ability to respond to its environment.
These chemicals are usually synthesized in one part of the plant and transported to another location. They interact with specific tissues to causes changes such as growth and fruit ripening. Because hormones stimulate or inhibit plant growth, they are referred to as growth regulators.
Five groups of plant hormones:
- Auxins - a group of hormones that promote plant-cell elongation, apical dominance, and rooting.
- Gibberellins - a group of hormones that primarily stimulate elongation growth.
- Ethylene - the hormone responsible for the ripening of fruit.
- Cytokinins - a group of hormones that promote cell division.
- Abscisic acid - a hormone that generally inhibits other hormones.
Tropisms are plant movements toward or away from an environmental stimulus.
- Phototropism - a growth response to light. Solar tracking is the phototropism of leaves or flowers as they follow the sun's movement across the sky.
- Thigmotropism - a growth response to contact with a solid object. Thigmotropism allows vines to climb. It is thought that an auxin or ethylene are involved in this response.
- Gravitropism - a growth response to gravity. Roots are positively gravitropic, usually growing downward and stems are negatively gravitropic, usually growing upward. Auxins are probably responsible for this growth.
- Chemotropism - a response to chemicals. The growth of a pollen tube is in response to chemicals produced produced by the plant ovary.
- Hydrotropism - a response to water. Most plants have a positive response to water.
Nastic movements: occur in response to environmental stimuli, but are independent of the direction of the stimuli. These movements are regulated by changes in water pressure (turgor pressure).
- Thigmonastic movements occur in response to touching or shaking a plant. Many of these movements are quite rapid. This movement is caused by the rapid loss of turgor in certain cells. Physical stimulation of the plant causes potassium ions to be pumped out of the cells. Water then moves out of the cells by osmosis. As the cells shrink, the plant leaves move.
- Nyctinastic movements are plant movements in response to the daily cycle of light and dark. Nyctinastic movements involve the same type of osmotic mechanism as thigmonastic movements, but the changes in turgor pressure are more gradual.
Photoperiodism: plants responding to changes in the length of days and nights.
- Long-day plants flower only when exposed to day lengths longer than their critical length. These are usually late spring and early summer flowers.
- Short-day plants flower only when exposed to day lengths shorter than their critical length. These are usually early spring and fall flowers.
- Day-neutral plants are not affected by the length of days and nights.
Pressure receptors in the skin provide this sense. Different areas of the body have different numbers of touch receptors.
Similar to pressure receptors, the three parts of the ear interpret vibrations in matter, distinguishing more than 300,000 tones.
Chemical receptors in the olfactory region of the nose detect minute concentrations of particles in the air.
Chemical receptors on the tongue and cheek lining.
Certain wavelengths of electromagnetic radiation are recognized by the rods and cones of the retina.
