Cell growth is controlled in multicellular organisms. Cells in parts of the body like the heart and liver rarely divide. These cells are unlike skin cells that divide rapidly through a person’s lifetime. Controls on cell growth can be turned on and off like a light switch. If a bone or skin is broken, cells divide in order it repair the damage that needs to be fixed.
Uncontrolled cell growth can be very harmful to multicellular organisms. Cancer is a disorder when cells have lost the ability to control their growth. Cancer cells keep growing and growing until the supply of nutrients shuts off. Cancer is a very serious disease that shows the importance of controls on cell growth.
Eukaryote cells divide in order to slow down cell growth. Cell division is the process in which a cell divides to form two daughter cells. The first stage of cell division is called mitosis. Mitosis is the process when the nucleus of a cell is divided into two nuclei, and both have the same number and type of chromosomes as the parent cell. Mitosis can be split into four parts.
Interpahse occurs before mitosis can begin. It is the period in between cell division and is the longest part of the cell cycle. The cell cycle is the process when a cell grows, prepares for division, divides, and begins a new cell cycle. Interphase itself is divided into three phases: G1, S, and G2. G1, called growth 1, or gap 1, is the stage in which a cell grows. The S stage is called the DNA synthesis stage. During this stage of interphase the DNA is replicated in DNA replication. Proteins are also synthesized in the S phase. G2, or growth 2, takes place when the S stage is finished. During G2 the synthesis or organelles and other materials happens furthermore preparing the cell for division. While interphase is taking place the nucleus is busy in synthesizing messenger RNA to direct all the steps.
The first phase in mitosis is called prophase. Prophase takes the longest time in mitosis, consuming 50-60% of the time it takes mitosis to occur. In prophase the chromosomes in a cell condense and coil up, making them more visible. The centrioles separate and go to opposite sides of the cell. Centrioles are small structures in the cytoplasm that contain tubulin, a microtubule protein. Plant cells don’t contain centrioles. The condensed chromosomes become attached to fibers in the spindle. The spindle is a mesh-like structure that helps move the chromosomes apart. At the end of prophase the chromosomes condense tighter, the nucleolus disappears, and the nuclear envelope begins to break down.
Metaphase is the second phase of mitosis, and is the shortest as well. During this phase the chromosomes line up across the center of the cell.
Anaphase is the next phase in mitosis. It begins when the sister chromatids split. Chromatids are the identical parts that form the chromosome. The chromatids become individual chromosomes and continue to split until they reach the opposite poles. Anaphase ends when the new chromosomes stop moving.
Telophase is the fourth and final stage of mitosis. The chromosomes begin o uncoil into a tangle of chromatin. Chromatin is the material that makes up chromosomes and itself is made from protein and DNA. All of this takes place where the two new daughter cells are taking shape. Two nuclear envelopes begin to reappear around the chromatin. The spindle begin to break apart and the nucleolus forms around the nucleus of the daughter cells. Mitosis is over but there is still one more step.
Cytokenesis follows quickly after mitosis is finished. In cytokenesis the cytoplasm of the parent cell splits into two to form the daughter cells. In animals, the cell membrane moves together and pinches the cells, giving making the daughter cells have their own nucleus and organelles. In plants the cell plate appears and forms a barrier between the two daughter cells. The cell plate then forms into a cell membrane, then the cell wall develops. (See Figure 4)
Tissues and Organs
In multicellular organisms, cells are organized in specialized groups, known as tissues. A tissue is a group of similar cells that preform similar functions. Different tissues form many different tasks. For example, a kind of tissue is made up of cells that produce digestive enzymes in the pancreas, and the cells in an eye respond to light. Most multicellular organisms have four main types of tissues: muscle, epithelial, nerve, and connective.
Some tasks in the body are too complicated to be preformed by only one type of tissue. So, organs preform these duties. An organ is a group of tissues that work together to preform a specific function. Many types of tissues may be used to form one organ. For example, a muscle in an organism is classified as an organ because not only muscle tissue makes up the muscle. There is nerve tissue, connective tissue, as well as a special tissue that connects the muscle with certain parts of the body. All the tissues in an organ work together to preform one common function.
Sometimes not just one organ can complete one task, so an organ system is needed. An organ system is a group of organs that work together to preform one function. There are many organ systems in our body. We have a muscular system, skeletal system, nervous system, and circulatory system.
Multicellular Organisms
A multicellular organism is a living thing that is made up of more than one cell. These organisms can contain hundreds, thousands, even billions of cells or more. We see multicellular organisms everyday: people, plants, and house pets.
To describe a multicellular organism, we have to put them into levels of organization. The levels of organization in multicellular organisms include cells, tissues, organs, and organ systems. The first level is cells, the second is tissues, next is the organs, and finally the fourth level is the organ system.
Multicellular organisms start off with one basic unit, the atom, and build up to make bigger things. Atoms combine to form compounds which then form organelles. Organelles then come together to make a cell. Cells then form tissues, which could then make organs. After organs are formed, then organs can be in an organ system.
Eagle
The eagle is sometimes referred to as the “king of flight” because of the power it shows while in flight. The eagle has been a symbol or strength and courage since ancient times. In 1782, Congress chose the American bald eagle to be the symbol of our nation. The national seal was the bird with its wings spread outward. It holds an olive branch in one claw and arrows in the other. The eagle appears in many places today in the United States.
Only two species of eagles are found in North America today: the American bald eagle, and the golden eagle. The bald eagle is more common than the golden eagle. This extraordinary bird has white tail feathers and white plumes on its head and neck. The bald eagle lives in open areas, or forests, near water. The bald eagle is usually 35-40 inches in length, and have a wingspan of 7.5 feet. The female bald eagle is more ferocious than the male, and is a couple inches larger. A bald eagle migrates only if the water it feeds in freezes in the winter months. It returns every year to the same nest and the same mate. The nests are built in trees or on cliffs, and sometimes on the ground. The eagle adds to it every year, making it bigger and bigger as time goes on. The nests can weigh up to one thousand pounds. The nests are made from sticks, weeds, and dirt. Bald eagles eat carrion, waterfowl, and especially fish.
The golden eagle was more common than the bald eagle when settlers first came here, but this is not the case today. It’s found in the western portion of North America, from Alaska, south to Mexico. The golden eagle is about the same size as a bald eagle. It’s feathers are much darker than that of its famous counterpart. There are feathers on the head and the neck of the bird that shine like gold when they’re in the sun. The toes and claws of the golden eagle are feathered, where as the bald eagle has no feathers on its legs. With their claws, golden eagles eat squirrels, prairie dogs, and rabbits. The golden eagle is very brave and can attack large animals such as deer, but can’t carry them away. They build nests in trees and rocky cliffs with sticks. The golden eagle has been known to defend its nest up to 75 square miles.
As you can see, the two types of eagles in North America are similar and different in many ways. Both of the eagles are very powerful birds. One thing is for sure, the eagle is a very beautiful bird that is extremely interesting. (See Figure 5)