Fertilization is the combining of male and female sex cells. When a pollen grain touches the top of the pistil, called the stigma, it forms a tube through which male sex cells travel downward into the ovary. An egg is fertilized inside the ovary.
In the process of photosynthesis, plants take in carbon dioxide and water and using light from the sun, they produce sugar and oxygen.
The amniotic egg is watertight, and consists of several layers of membranes. Among other structures it contains a food source for the developing embryo in the form of a yolk.
Amphibians possess a three-chambered heart that pumps blood throughout the body via a network of blood vessels.
Nerve impulses occur when the electrical charge across a neuron cell membrane reverses. Nerve impulse is also referred to as "action potential" and involves the reversal and restoring of charges across a neuron cell membrane.
Neurons are one kind of nerve cell that transmit signals throughout the body.
The brain has three main parts-- the brain stem, the cerebellum, and the cerebrum. Let's start with the brain stem. It consists of the midbrain, pons, and medulla oblongata. Located just above the brainstem are the thalamus and hypothalamus.
When you respond involuntarily to a hot object, a reflex arc begins when sensory neurons in your hand detect pain. Sensory neurons then carry information in the form of a nerve impulse to the spinal cord.
The backbone encases the spinal cord, a bundle of nerves that play a key role in the central nervous system.
Nerve impulses move across spaces called "synapses" via specific chemical messengers referred to as "neurotransmitters." Special structures located on the ends of axons release neurotransmitters into the synapse when an impulse is received.
A black hole is created when an extremely large star has a supernova explosion and the remaining core is swallowed by its own gravity. The gravitational forces within a black hole are so strong light is not able to escape.
Temperatures can reach 15 million degrees Celsius at the center of the sun.
As a comet orbits the sun, solar wind pushes the gases outward in a trailing tail.
Constellations are groups of stars that look like animals, people, or objects.
Distances in the solar system are great.
Giant stars have a diameter of 10 to 100 times the size of the sun. White dwarfs are smaller stars that can be smaller than Earth.
A meteor is a meteoroid that strikes Earth's surface.
The sun is the center of the solar system around which Earth and the other planets orbit. The planets travel in an ellipse around the sun.
The sun is the center of the solar system around which Earth, the other plants, and their moons revolve. Inertia causes planets to remain in motion. The pull of gravity between the sun and planets keeps the planets from flying off into space.
A supernova is a huge explosion that results when a massive star has burned up most of its fuel and breaks apart. The resulting core can form a smaller, dense star called a neutron star.