The protist kingdom is very diverse. All protists are eukaryotes that cannot be classified as animals, plants, or fungi. All live in moist surroundings. Most
are unicellular, but some are multicellular. Some are heterotrophs, some are autotrophs, and some are both. Protists can be divided into three categories:
animal-like, funguslike, and plantlike protists.
Like animals, animal-like protists are heterotrophs, and most are able to move from place to place to obtain food. Animal-like protists are also
called protozoans. Protozoans can be divided into four types: sarcodines, ciliates, flagellates, and those that are parasites. Sarcodines move and feed
by using pseudopods. Pseudopods are temporary bulges of the cell.
Pseudopods form when cytoplasm flows toward one location and the rest of the organism follows. Protozoans that live in fresh water, such as amoebas,
have a contractile vacuole, which collects the extra water and expels it from the cell. Ciliates have structures called cilia, which are hairlike projections
that move with a wavelike motion. Flagellates move using whiplike flagella.
Some flagellates live inside the bodies of other organisms in a state of symbiosis. Symbiosis is a close relationship between two species where at
least one of the species benefits. Sometimes, flagellates harm their hosts. In other cases, their relationship is one of mutualism, in which both partners benefit. Protozoans that are parasites feed on their hosts’ cells and body
fluids.
Plantlike protists are called algae. Like plants, algae are autotrophs.
Algae can exist in a variety of colors because they contain many types of pigments—chemicals that produce color. Plantlike protists include diatoms,
dinoflagellates, euglenoids, red algae, green algae, and brown algae.
Diatoms have beautiful, glasslike cell walls. Dinoflagellates are covered by stiff plates and move using two flagella. Euglenoids can be heterotrophs
when sunlight is not available. Red algae and brown algae live in the oceans.
Green algae live in fresh water, salt water, and moist places on land.
Like fungi, funguslike protists are heterotrophs, have cell walls, and use spores to reproduce. Spores are tiny cells that are able to grow into new
organisms. All funguslike protists are able to move at some point in their lives. The three types of funguslike protists are slime molds, water molds,
and downy mildews. Slime molds live in moist soil and on decaying plants.
Water molds and downy mildews grow as tiny threads in water or moist places.
7th Grade
The Inner Planets
Mercury, Venus, Earth, and Mars are more similar to one other than they are to the five outer planets. The four inner planets are small and dense and
have rocky surfaces. These planets are often called the terrestrial planets, from the Latin word terra, or “earth.”
Earth is unique in our solar system in having liquid water at its surface. Earth has a suitable atmosphere and temperature range for water to exist as liquid, gas, or solid. Earth has an atmosphere that is rich in oxygen.
Nearly all of the remaining atmosphere consists of nitrogen, along with small amounts of other gases such as argon and carbon dioxide. The atmosphere also includes water vapor.
Mercury is the smallest terrestrial planet and the planet closest to the sun. Mercury is smaller than Earth’s moon and has no moons of its own. The
planet’s interior is probably made of iron, and its surface has many plains and craters. Because the planet is so close to the sun, the side facing the sun
reaches temperatures of 430°C. However, the temperature drops to –170°C at night.
Venus is similar in size and mass to Earth. Venus’ density and internal structure are similar to Earth’s. But in other ways, Venus and Earth are very
different. Venus rotates from east to west, the opposite direction from most other planets and moons. The pressure of Venus’s atmosphere is 90 times
greater than the pressure of Earth’s atmosphere. The atmosphere is mostly carbon dioxide, with clouds partly made up of sulfuric acid. The carbon
dioxide in the planet’s atmosphere traps the sun’s heat, causing the surface temperature of Venus to be about 460°C. This trapping of heat by the
atmosphere is called the greenhouse effect. Venus is covered with rock, similar to many rocky areas on Earth. Venus also has many volcanoes and
broad plains formed by lava flows. Mars is called the “red planet.” Its surface is covered with red dust. The planet Mars has a very thin atmosphere that is mostly carbon dioxide.
Temperatures on the surface range from –140ºC to 20ºC. Images of Mars show a variety of features that look as if they were made by ancient streams, lakes, or floods. Scientists think that a large amount of liquid water flowed on Mars’s surface in the distant past. At present, liquid water cannot exist for long on Mars’s surface. However, some water is frozen in the planet’s
two polar ice caps. A large amount of water may be frozen underground.
Like Earth, Mars is tilted on its axis, so its seasons change. Some regions of Mars have giant volcanoes. Mars has two very small moons, Phobos and Deimos.
The Outer Planets
The four outer planets—Jupiter, Saturn, Uranus, and Neptune—are much larger and more massive than Earth, and they do not have solid surfaces. Because these four planets are all so large, they are often called the gas giants. Pluto, is small and rocky, like the terrestrial planets, but it not more consider such.
Like the sun, the gas giants are composed mainly of hydrogen and helium. Because they are so massive, they exert a much stronger gravitational force than the terrestrial planets. This prevents their gases from
escaping, so they have thick atmospheres.
All of the giants have many moons and are surrounded by a set of rings. A ring is a thin disk of small
particles of ice and rock.
Jupiter is the largest and most massive planet. Jupiter has a thick atmosphere made up mainly of hydrogen and helium. An interesting feature
of Jupiter’s atmosphere is its Great Red Spot, a storm that is larger than Earth. Jupiter probably has a dense core of rock and iron at its center,
surrounded by a thick mantle of liquid hydrogen and helium. Galileo discovered Jupiter’s four largest moons: Io, Europa, Ganymede, and Callisto.
Saturn is the second-largest planet in the solar system. Its average density is less than that of water. The rings around Saturn are made of chunks of ice and rock. Saturn has the most spectacular rings of any planet.
Uranus is about four times the diameter of Earth and is twice as far from the sun as Saturn. Uranus looks blue-green because of traces of methane in
its atmosphere. Uranus’s axis of rotation is tilted at an angle of about 90 degrees from the vertical. It rotates from top to bottom instead of from side
to side.
Neptune is a cold, blue planet. Its atmosphere contains visible clouds.
Neptune was discovered as a result of a mathematical prediction. Astronomers have discovered at least 13 moons orbiting Neptune.
Pluto has a solid surface and is much smaller and denser than the other outer planets. Pluto has a single moon, Charon more than half the size of Pluto, Pluto revolves around the sun only once every 248 Earth years.
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